Download Vegetative Response to Long-Term Resource

University of Colorado, Boulder
CU Scholar
Undergraduate Honors Theses
Honors Program
Spring 2017
Vegetative Response to Long-Term Resource
Manipulations in the Alpine Tundra
Evelyn Beaury
[email protected]
Follow this and additional works at: http://scholar.colorado.edu/honr_theses
Part of the Biochemistry Commons, Biodiversity Commons, Biology Commons, Ecology and
Evolutionary Biology Commons, and the Plant Sciences Commons
Recommended Citation
Beaury, Evelyn, "Vegetative Response to Long-Term Resource Manipulations in the Alpine Tundra" (2017). Undergraduate Honors
Theses. 1289.
http://scholar.colorado.edu/honr_theses/1289
This Thesis is brought to you for free and open access by Honors Program at CU Scholar. It has been accepted for inclusion in Undergraduate Honors
Theses by an authorized administrator of CU Scholar. For more information, please contact [email protected].
Vegetative Response to Long-Term Resource Manipulations in the Alpine Tundra
By
Eve Beaury
Ecology & Evolutionary Biology, University of Colorado at Boulder
March 13th, 2017
Thesis Advisor:
Dr. Timothy Seastedt, Ecology & Evolutionary Biology
Defense Committee:
Dr. Timothy Seastedt, Ecology & Evolutionary Biology
Dr. Pieter Johnson, Ecology & Evolutionary Biology
Dr. Daniel Doak, Environmental Studies
1 Abstract
Considering their sensitivity to change, alpine plant communities are useful
systems in studying the indirect effects of anthropogenic activities on the environment.
Climate change is increasing variability of temperature and precipitation, shifting wind
patterns, and altering nutrient composition and cycling (especially deposition of nitrogen
(N) and phosphorus (P)). Therefore, it is becoming increasingly important to understand
how climate change impacts vegetation. This study continues efforts of a Long Term
Ecological Research program in the Colorado Rocky Mountains by surveying plant
community composition in response to nutrient additions (N and P) and changing
moisture regimes mimicking potential climate shifts. In addition to updating such
surveys, species-specific responses to nutrients and functional differences between
treatments were considered in order to expand our understanding of how alpine
ecosystems function. Over time, there has been a shift towards species that capitalize on
added N and P rather than deposition of one or the other. This indicates that P deposition
may neutralize the effects of N deposition, favoring generalist species. Additionally,
alpine vegetation appears to be relatively resistant to changes in moisture caused by
snowmelt. A longer growing season, as evidenced by cover increases in areas with earlier
snowmelt, may increase primary productivity in the alpine tundra. Neither N nor P
additions alone related to any single functional trait, but both leaf area (typically smaller
in stressful environments) and chlorophyll content (indicating photosynthetic rates)
appear to increase in the presence of N and P. Overall, these vegetative communities
remain resistant to certain climatic alterations and may even mitigate the impacts of shifts
such as N deposition and changing precipitation patterns.
2 Keywords: alpine plant communities; nitrogen; phosphorus; snowmelt; climate change
Introduction
The alpine tundra presents a unique study system classified by low temperatures,
a short growing season, and often, a dependence on winter snowpack as a water resource
during the summer. In this study, I focus on the herbaceous plants in the alpine, given this
ecosystem’s dependence on vegetation to support higher trophic levels (organisms higher
on the food chain), drive productivity (energy production), and withstand drastic weather
changes throughout the year. In addition, the short growing season in the tundra creates a
narrow window within which to collect vegetative data, and the alpine experiences
change that may predict environmental response in lower-elevation ecosystems, thus
providing a system to study the effects of climate change from season to season. In this
study, I focus on community and functional trait (plant characteristics) responses to longterm resource manipulations in order to evaluate change over time and predict future
conditions of a site in the Colorado Rocky Mountains. Specifically, I address whether or
not plant community structures uphold over time in response to added nitrogen and
phosphorus as well as different timings of snowmelt.
Background
I. On overview of research on Niwot Ridge
Over the last several decades, Niwot Ridge—located 5.6 kilometers east of the
Continental Divide in the Colorado Rocky Mountains—has participated in a Long-Term
Ecological Research (LTER) program aimed at understanding the functioning of high-
3 elevation ecosystems. As climate change centralizes many ecological considerations,
Niwot Ridge has focused its research on exploring the changes that mountain
environments are likely to face under warming temperatures and shifting precipitation
patterns. The alpine faces a unique balance between resilience, as an ecosystem relatively
untouched by direct human impact during its evolution under naturally harsh conditions,
and fragility, in that it now experiences the indirect effects of anthropogenic activities
(Williams et al. 2015b). However, these factors are influenced by climate, and under the
changing conditions against which the environment continues to battle, the alpine has
emerged as a likely “bellwether” of climate change (Smith et al. 2009; Wookey et al.
2009; Williams et al. 2015b). That is, trends in alpine ecosystems may reflect, and help
us anticipate, how areas at lower elevations will respond to global change.
Under these considerations, Niwot Ridge LTER documents long-term changes
due to a warming climate—in addition to other measurements—in order to predict future
conditions. Thus far, research has found that the alpine is experiencing an overall
increase in precipitation, a modest shift in wind patterns carrying the precipitation, a
higher presence of nitrogen (N) due to human activity, and evidence of phosphorus (P)
deposition as well (Bowman et al. 2015; Kittel et al. 2015; Niwot LTER unpublished
data). These changes likely result in an increase in winter snowpack and a change in the
timing of snowmelt, which consequently affect the length of the growing season, increase
soil moisture throughout the growing season, increase microbial respiration (promoting
decomposition and nutrient cycling), and cause a higher presence of shrubs in
comparison to other vegetation types (Spasojevic et al. 2013; Kittel et al. 2015; Suding et
al. 2015). For areas that sit below alpine ecosystems, this affects the amount and quality
4 of snow that melts down into lakes, rivers, and sources of drinking water (Williams et al.
2015a).
Alpine plant species are strongly dependent on snowmelt and nutrient
composition during the short growing season. In response to the shifts in these factors,
plant dynamics are likely to change (Billings and Bliss 1959; Bowman 2000; Gasarch
and Seastedt 2015a,b; Schmidt et al. 2015; Suding et al. 2015). Vegetation of the Niwot
Ridge alpine tundra includes many species of shrubs (small woody plants), graminoids
(grasses, sedges, and rushes), and forbs (herbaceous flowering plants), which typically
have slow growth rates that limit their ability to quickly adjust to change (Bowman
2000). A myriad of studies through the Niwot Ridge LTER program have compiled
information regarding alpine species’ responses to nutrient changes, snowpack, microbial
relationships, productivity, and functional diversity, which is the set of a plant’s traits that
describe its role within an ecosystem (Venn et al. 2011; Gasarch and Seastedt 2015a,b;
Suding et al. 2015). These factors alter community composition, species’ dominance, and
even allow new species to show presence in high-elevation ecosystems. Although alpine
plant communities are susceptible to change, they have also shown resilience to date
(Spasojevic at al. 2013). Whether or not this resilience will continue under ongoing
climate change is a question left unanswered.
II. Current theory explaining vegetative responses to resource drivers
The alpine tundra of Niwot Ridge, as a naturally low-nutrient system and one
greatly affected by snow, emerges as a biome thought to be especially susceptible to the
effects of climate change (Ernakovich et al. 2014). For example, this area is sensitive to
changes in atmospheric chemistry such as nutrients coming in from dust and changing
5 source winds, as well as N deposition through rainfall increasing from human activity and
enhanced by global warming (Bowman et al. 2015; Rouston et al. 2016). These added
nutrients have resulted in various plant responses dependent on community type, life
form, and nutrient uptake (Bowman et al. 2015; Gasarch and Seastedt 2015a,b; Suding et
al. 2015). For example, in response to high levels of N, alpine species richness and
diversity tend to decrease while species that respond to N and graminoids may increase
(Suding et al. 2005; Bowman et al. 2015; Gasarch and Seastedt 2015a). There is currently
a gap in research involving response to P. However, individual species likely show
varying effects to N, P, or the combination of the two, and many species seem to be both
N- and P-limited (Theodose and Bowman 1997; Gasarch and Seastedt 2015a; Gough et
al. 2016). In a changing alpine environment, the varying and species-specific responses to
nutrients show one of the complicated resource interactions happening in these plant
communities.
As mentioned, the moisture coming from snowmelt provides an invaluable
resource for alpine plant communities during the short summer growing season. An
earlier snowmelt brought on by warmer temperatures, as shown by Venn et al. (2011),
may result in plant functional trait responses favoring a high leaf area and plant height,
indicating species that maintain water balance and compete for light. Pickering et al.
(2014) supports these trends in functional traits and also finds that early snowmelt favors
alpine graminoids and generalists over other life forms. Graminoids are typically taller
and more competitive species, so resource shifts lengthening the growing season provide
an ideal opportunity for these species to take over alpine communities (Matteodo et al.
2016). Snowmelt also triggers phenological responses, which are the events in an
6 organism’s life cycle. Although most snowbed species are familiar with, and able to
adjust to, inter-annual variation, earlier flowering times may alter plant development,
allocation of resources, and community distribution (Carbognani et al. 2016). These
resource shifts in terms of moisture provided by snowmelt enhance competition and have
the potential to affect plant processes on a species-specific level, which then change the
overall distribution of plant species and their interactions.
To summarize, the effects of warming, resource distributions, and timing and
distribution of snowmelt have varying impacts depending on community type and
functional patterns (Debinski et al. 2010; Spasojevic and Suding 2012). Species in highelevation systems, as evidenced by their persistence, have spent endless years adjusting to
severe conditions. The recent warming and shift in precipitation due to climate change
present trends new to most alpine species. Consequently, current literature emphasizes
the importance of alpine resources and their susceptibility to the conditions brought on by
environmental change. The countless interactions involved in plant communities and the
individuality of species’ responses to resource shifts emphasize the need to understand
the dynamics in alpine vegetation in order to predict future assemblages and to better
manage resources to maintain diverse, stable, and productive ecosystems.
III. Research questions and hypotheses tested here
Considering the many factors discussed, the vegetation of the alpine tundra
provides a unique study system to explore the effects of environmental stress in an area
where time seems to move faster than at lower elevations. Therefore, this study continues
efforts of the Niwot Ridge LTER program by comparing plant communities through time
to monitor shifts in response to climate, evaluate the impact of snow manipulation, and
7 continue studying the effects of long-term nutrient additions. In doing so, these data aim
to confirm patterns observed in previous studies, monitor the resilience of alpine
environments, and make predictions about environmental response in lower-elevation
ecosystems. In addition, this study adds functional trait responses into the analysis of
community composition, creating a more complete idea of the biotic interactions across
varying resource and landscape gradients, thus encompassing the dynamics in
communities facing environmental change (Dıaz and Cabido 1997; Spasojevic and
Suding 2012; Venn, Pickering, and Green 2014).
In doing so, these data test why, under different treatments, certain species find
success over others, why these communities are arranged as such, which traits dominate
in alpine environments, and how the alpine will respond to climate change. These are
important measures with the potential to disrupt alpine stability by affecting larger-scale
ecosystem processes such as nutrient cycling, primary productivity, and relationships
between trophic levels (Wookey et al. 2009).
Methods
I. Study site
This study was conducted in the alpine of Niwot Ridge in the Colorado Rocky
Mountains, specifically in the Saddle area (Figure 1) at an average elevation of 3500
meters (m) and 5.6 kilometers east of the Continental Divide (0° 03ʹ N, 105° 35ʹ W). The
climate here is typical of an alpine tundra environment with long and cold winters and a
short summer growing season lasting from mid-May until August. The average annual
temperature is about -1° Celsius (C), and around 2000 millimeters of precipitation fall
8 each year (calculated from Niwot LTER public data). About 80% of this precipitation
falls as snow (Williams et al. 2015a) and is redistributed across the landscape by strong
winds sourcing from the northwest, thereby generating large moisture and snow-free
length differences among the alpine plant communities (Kittel et al. 2015).
This study area represents a diverse landscape containing several community
types and a snow fence that manipulates snow depths, creating a drift as pictured in
Figure 2. From 1993-2015, the fence stood 60m long and 2.6m high. For the winter of
2015-16, it was halved to 30m in length in order to measure a recovery response (i.e. the
snowdrift is smaller, so the vegetation behind the removed area of the fence faced a
longer growing season with less available moisture than in the 1993-2015 interval).
On the Saddle, 86 2m2 plots are located within and around the snow fence—each
classified as residing in a dry, mesic (moist), or wet meadow community based on the
vegetation type and the natural moisture gradient of the area. In addition, these plots are
subject to various nutrient enrichments as part of LTER started in 1993. The treatments
include control (no added nutrients), added phosphorus (P), added nitrogen (N), or added
nitrogen and phosphorus (N+P). The frequency and amounts added to each plot have
been detailed in Gasarch and Seastedt (2015b), but current additions are 10 grams (g) of
N per m2 as calcium nitrate and 2 g of P per m2 as superphosphate. Every other year,
nutrients are distributed evenly on the soil surface across each plot. The most recent
additions occurred at the beginning of the 2016 growing season.
Of the 86 plots on the Saddle, only the dry and mesic meadow plots—of which
there are 70—participated in the current snow manipulation study. This analysis focused
on these 70 plots. Of this subset, 38 plots sit in the area originally impacted by the 60m
9 snow fence (i.e. within the manipulated snowdrift). Now that the fence has been halved to
30m, 15 of the original 38 plots sit in the recovery area, while the other 23 of the 38 plots
remain within the footprint of snow generated by the fence. Therefore, to summarize, this
study focused on 23 plots having added snow, 15 recovery plots, and 32 reference plots
(outside of the snow fence) to total 70 plots. Each of these plots also experience one of
the four nutrient treatments and consist of species common in dry or mesic meadow
communities. The remaining 16 of the 86 plots are found in a wet meadow community,
which were not represented within the snow fence area and were not included in this
study.
Figure 1: Locations in the Niwot Ridge LTER study area. Figure from Williams et al.
2015b.
10 Figure 2: Aerial image of the Saddle area of Niwot Ridge, Colorado. This image of the
snow fence, standing 60 meters long and 2.6 meters high, was taken in June of 2008. This
image shows accumulated snow due to the presence of the snow fence. The dry and
mesic meadow plots sit in the area to the right of the snow fence both underneath and
around the snowdrift currently pictured.
II. Data collection and analysis
Data were collected in the summer of 2016 to record community composition,
which included a combination of graminoids, shrubs, forbs, bare ground, lichen, moss,
rocks, and plant litter. Composition was recorded using the point intercept method with a
1m x 1m quadrat placed in the northwest corner of each plot. The quadrat was made of
PVC pipe, stood about a foot above the ground using legs, and was strung with line to
create a 10x10 grid (i.e. 100 points to collect data per plot). At each intersection of string,
a pin flag was used to point from the quadrat directly down to the ground. I then recorded
the plant species touching the pin flag. If more than one plant species occurred directly
underneath a point, the second and third species were included. Plants were identified
11 down to the species level. Checking 2016 findings with those obtained in 2012 resulted in
minor corrections to the raw counts of Deschampsia cespitosa and Festuca brachyphylla
in seven dry meadow plots. Dry meadow plots were sampled first, from about June to
July, and then the mesic meadow plots were surveyed during July.
Species richness (the number of species present) was also recorded within the
1m2 area designated by the quadrat. Using these data, I calculated species diversity per
plot using the Shannon Diversity Index (H’ = -Σ pi *ln(pi) where H’=diversity and
pi=proportion of each species relative to total species). Plants that occurred in each plot,
but were not present in composition, were given a count of 0.1 and included in species
diversity. I also calculated species evenness per plot by dividing diversity by species
richness to give an image of the relative distributions of species in a community.
Functional trait data for plant species in the alpine were collected and reported by
Spasojevic and Suding (2012) according to protocols outlined by Cornelissen et al.
(2003). Traits selected for this analysis included height, stomatal conductance,
chlorophyll content, leaf area (LA), specific leaf area (SLA), and percent N content. Plant
height indicates competition for light, stomatal conductance correlates with water
retention, N and chlorophyll content relate to photosynthetic rates, and both LA and SLA
relate to a plant’s energy and water balance (Spasojevic and Suding 2012). In this study, I
focused on the average traits of the ten most abundant species that dominated relative
vegetative cover. Trait values for these species are reported in Table 1.
Data were analyzed using R (v. 3.3.1) and Statistical Analysis Institute (SAS v.
9.4) programs. To evaluate change from 2012 to 2016, the absolute cover and relative
cover per species per plot were averaged and compared between years using paired t-
12 tests. Absolute cover is the average hits of vascular plants per plot per treatment. Relative
cover is given as the percent of vegetation of a certain type out of the total vascular plants
per plot per treatment. This analysis combined cover for Carex scopulorum and Carex
aquatilis for the 2016 values, for data from 2012 did not differentiate between these two
while data from 2016 did.
Community composition data were slightly skewed from an expected poisson
distribution for count data, so I used generalized linear mixed-effects models to account
for random effects between plots when analyzing individual species and life form (either
graminoid, forb, or shrub) responses to treatments. Total cover, species richness, and
species diversity each represented a normal distribution and were analyzed using
generalized linear models. Species evenness data were slightly skewed, so I took the
natural log of these data before analyzing them with generalized linear models.
Dependent variables were tested against the presence of N, the presence of P, and the
interaction between the two.
When analyzing patterns in functional traits, I evaluated community trait response
by focusing on the values of the ten most abundant species. Since functional trait data
were measured prior to this study, for each plot, I multiplied the proportion of each
species by its corresponding trait value and then summed the values for the ten most
abundant species to simulate community trait response to treatments. For example, if
there was a higher abundance of a taller species, this brought up the community height
for a plot. I then took the natural log of these data to transform them into a normal
distribution, averaged trait values across treatments, and compared them using a one-way
ANOVA to specifically look at N+P versus N and P treatments (instead of testing the
13 presence of N or P on a dependent variable, which does not exclude N+P treatments
when testing each nutrient individually). N content for Carex aquatilis was not recorded
in the original functional trait data set, so it was excluded when analyzing this trait.
Lastly, I analyzed correlations between the traits using linear models. The output of each
analysis is described in the Results section, and the Appendix displays some of these data
as well.
Table 1: Functional trait data for plant species found on Niwot Ridge, Colorado. These
data were collected and described in Spasojevic and Suding (2012). Average trait values
of the ten most abundant plant species from this analysis are included, and below they are
ordered from most to least abundant in terms of relative cover: Geum rossii,
Deschampsia cespitosa, Carex scopulorum, Artemisia scopulorum, Trifolium parryi,
Kobresia myosuroides, Carex aquatilis, Lloydia serotina, Erigeron simplex, Oreoxis
alpina. Nitrogen content for C. aquatilis was not recorded and was therefore not included
in %N analyses. Stomatal conductance and chlorophyll content are measured in units
specific to the devices used in Spasojevic and Suding (2012).
Plant Species
Height
(cm)
Stomatal
Chlorophyll Leaf Specific Nitrogen
Conductance
Content
Area Leaf Area
(%)
(cm2)
(cm2/g)
G. rossii
12.46
199.89
540.91
7.45
95.68
1.40
D. cespitosa
40.76
207.12
339.18
3.20
76.61
1.21
C. scopulorum
27.93
180.70
500.34
5.64
140.63
2.69
A. scopulorum
16.01
239.30
246.46
2.98
154.24
2.37
T. parryi
8.08
172.28
684.47
5.18
137.30
2.65
K. myosuroides
7.74
178.90
339.55
0.48
43.25
2.67
C. aquatilis
24.40
34.34
607.94
6.98
146.53
NA
L. serotina
10.51
151.83
120.55
1.62
88.12
3.90
E. simplex
9.77
205.77
541.46
1.00
171.72
2.04
O. alpina
3.84
189.90
310.44
0.94
29.00
2.13
14 Results
I. Overview
This study observed 47 species of vascular plants, areas of bare ground, as well as
ground cover by lichen, moss, rocks, and plant litter. Across all plots, absolute cover
ranged from 72 to 187 hits of vascular plants, averaging to 126.6 hits per plot. Vegetative
hits correlated with productivity (Niwot LTER unpublished data). Species richness
ranged from 4 to 20 species with an average of 12.9 species per plot. Richness was not
correlated with plant cover.
Overall, plots in the mesic meadow had significantly higher plant cover than plots
in the dry meadow (p<0.000), although species richness (and diversity, but not
significantly) was higher in the dry meadow (p=0.011, Figure 4). Nutrient treatments in
both meadow types had fairly similar cover (Figure 3). Graminoids favored the mesic
meadow (p<0.000), but forb and graminoid cover were similar within each meadow type
(Figure 8). When looking at snow manipulation, the recovery plots had the highest
absolute cover per plot (although there was visible evidence of earlier senescing in this
area), plots unaffected by the snow fence had the second highest, and the areas remaining
in the snowdrift had the lowest absolute cover per plot.
To examine patterns of individual species, I focused on measures from the ten
most abundant species in this analysis, which accounted for 78% of total cover of
vascular plants across all plots. These included the following, in order of most to least
abundant: Geum rossii (forb), Deschampsia cespitosa (graminoid), Carex scopulorum
(graminoid), Artemisia scopulorum (forb), Trifolium parryi (forb), Kobresia myosuroides
(graminoid), Carex aquatilis (graminoid), Lloydia serotina (forb), Erigeron simplex
15 (forb), and Oreoxis alpina (forb). These species accounted for 68% of cover in the dry
meadow and 81% in the mesic meadow. Four of the most abundant species had higher
cover in the mesic meadow, Oreoxis alpina (p=0.000) and Kobresia myosuroides
(p=0.008) preferred the dry meadow, and cover of the remaining four most abundant
species did not differ between meadow types (Figure 9).
2016 Cover
Absolute Cover (total hits)
160
* 140
120
100
80
60
40
20
0
Control
N
N+P
P
Control
N
Dry
N+P
P
Mesic
Treatment per Meadow Type
*p<0.05
Figure 3: Absolute cover for the 2016 growing season averaged across all plots in the
Saddle area of Niwot Ridge, Colorado. Absolute cover is measured in total hits of
vascular plants per plot, therefore excluding cover of bare ground, lichen, moss, rocks,
and plant litter. Data are divided by meadow type—either dry or mesic—and secondarily
by nutrient additions, which include control (no nutrients added), added nitrogen (N),
added phosphorus (P), or added N and P (N+P). The asterisk (p<0.05) indicates
significance between meadow types. Error bars represent standard error.
II. Change through time
In order to evaluate change over the last four years, data collected over the
summer of 2016 were compared against data collected in 2012, as published by Gasarch
and Seastedt (2015a,b). In general, canopy cover did not increase over this time period,
but there were significant changes in community distribution and structure. For example,
16 over time, species richness decreased in both meadow types and all nutrient treatments
except for the N+P plots in the dry meadow. Species diversity measures were also lower
across all treatments, although evenness remained stable between years. When comparing
trends between treatments, species richness and diversity both showed less of a negative
response to N in 2016, especially in the dry meadow. Species evenness showed less of a
negative response to P in the dry meadow and to N in the mesic meadow. All 2012
measures were significantly affected by N additions and meadow type. In 2016, only
richness differed between meadow types (p=0.011), N affected all measures, and
diversity and evenness also differed in the presence of P and N+P. Figure 4 displays these
patterns for 2016. Lastly, the ratio of forbs to graminoids was similar between years.
Patterns in individual species also changed over time (Figure 5). The perennial
forb and most abundant species of this site, Geum rossii, showed a significant increase in
cover (p<0.000) from 2012 to 2016 when ignoring individual treatments. The perennial
grass Deschampsia cespitosa was the second most abundant species in both years of data
collection, but it decreased significantly in absolute cover over time (p=0.027). As for the
rest of the dominant species of 2016, the two species of Carex, Artemisia scopulorum,
and Kobresia myosuroides showed significant increases in absolute cover (Carex
p=0.000, A. scopulorum p=0.008, K. myosuroides p=0.004). The four remaining
species—Oreoxis alpina, Trifolium parryi, Lloydia serotina, and Erigeron simplex—did
not change in cover over the time period. Although Trifolium parryi did not increase,
when grouping together the three species of Trifolium found on the Saddle (known Nfixers), cover significantly increased between years (p<0.000). Cover of the dominant
alpine shrubs—three species of the genus Salix—increased between 2012 and 2016.
17 To evaluate the snow manipulation study over time, I compared plots unaffected
by the snow fence with all plots affected by the snow fence. Areas impacted by the snow
fence continued to have lower cover than areas outside of the snow fence.
(b) Species Diversity
20
15
Average Species Diversity
* 10
5
0
Control
N
P
N+P
Control
Dry
N
P
N+P
Mesic
2
1.9
1.8
1.7
1.6
1.5
1.4
Control
N
P
N+P
Control
N
Dry
P
Mesic
Treatment per Meadow Type
Treatment per Meadow Type
(c) Species Evenness
Average Species Evenness
Average Species Richness
(a) Species Richness
0.8
0.6
Control
N
P
N+P
Dry
Control
N
P
N+P
Mesic
Treatment per Meadow Type
*p<0.05
Figure 4: Species richness (a), diversity (b), and evenness (c) for plots in the Saddle area
of Niwot Ridge, Colorado. Diversity per plot was calculated using the Shannon Diversity
Index (H’ = -Σ pi *ln(pi) where H’=diversity and pi=proportion of each species relative to
total species). Evenness per plot was calculated by dividing species diversity by species
richness. The values per plot were then averaged together by treatment. Treatments
include meadow type—dry or mesic—and nutrient additions, which are control (no
nutrients added), added nitrogen (N), added phosphorus (P), or added N and P (N+P). An
asterisk indicates significance (p<0.05) between meadow types. Error bars represent
standard error.
18 N+P
Change from 2012 to 2016
Absolute Cover (hits/plot)
30
25
*
20
15
* * * * 10
2012
2016
5
0
Plant Species
*p<0.05
Figure 5: Change in absolute cover from 2012 to 2016 of the ten most abundant species
found in the Saddle area of Niwot Ridge, Colorado. Absolute cover is averaged per plot.
Cover from 2012 was collected and reported by Gasarch and Seastedt 2015a,b. Carex
combines absolute cover for Carex scopulorum and Carex aquatilis because data from
2012 did not differentiate between these two while data from 2016 did. Error bars
represent standard error.
II. Snow fence
For the snow manipulation study of 2016, I compared recovery plots with the
plots remaining in the imprint of snow (snow plots) after the fence was halved in the
winter of 2015-16. I therefore measured a first-year response to change in snowmelt and
soil moisture. Overall, recovery plots had more cover per plot than snow plots (p=0.019).
Despite the difference in cover, richness and diversity were similar when comparing
treatments. Evenness was higher in the recovery area, but only in the N+P plots
(p=0.009). Snowmelt caused differences between nutrient treatments in terms of absolute
cover (Figure 6). However, the disparity in sample sizes between treatments (there is only
one recovery plot receiving N compared to seven snow plots receiving N) affected
19 significance between snowmelt and nutrients, which was also evident in species-specific
responses to snow.
Neither life form nor any of the most abundant species were significantly affected
by snowmelt alone (Figure 7). Species’ responses to snow were often confounded by
nutrient additions, meadow type, or sample size. For example, Carex scopulorum favored
the recovery area in the N plots (p=0.049), and Carex aquatilis responded to moisture—
cover was low in dry meadow recovery plots and high in mesic meadow snow plots
(p=0.033). Functional trait values of plots were not affected by snow treatments despite
the growth response of the recovery area.
Absolute Cover (hits/plot)
Effect of Snow Fence
145
*p<0.05
* 140
135
130
125
120
Control
N
N+P
P
Control
N
Recovery
N+P
P
Snow
Nutrient per Snow Fence Treatment
Figure 6: Effect of the halved snow fence on absolute cover in the Saddle area of Niwot
Ridge, Colorado. Absolute cover is measured in total hits of vascular plants per plot,
therefore excluding cover of bare ground, lichen, moss, rocks, and plant litter. Nutrient
additions are control (no nutrients added), added nitrogen (N), added phosphorus (P), or
added N and P (N+P). The second designation—either recovery or snow—refers to
moisture manipulated by the snow fence. “Snow” refers to the area behind the snow
fence that sits in a manipulated snowdrift. “Recovery” refers to the area that sits behind
the part of the fence that was removed for the winter of 2015-16, and therefore the area
that had an earlier snowmelt than in the previous time period. The asterisk (p<0.05) refers
to significance between snow treatments. Error bars represent standard error.
20 Snow Fence and Abundant Species
Relative Cover (%)
30
25
20
15
10
Recovery
Snow
5
0
Plant Species
Figure 7: Effect of the halved snow fence on the ten most abundant species in the
Saddle area of Niwot Ridge, Colorado. Data are reported in terms of relative cover
(percent of a species relative to vascular vegetation), ignoring nutrient additions and
meadow type. Error bars represent standard error.
III. Nutrient treatments
Total cover was not significantly affected by nutrient additions. Species richness
was lower in the presence of N (p<0.000). Species diversity was significantly affected by
each of the nutrient treatments (N: p<0.000, P: p=0.032, N+P: p=0.028), decreasing in the
presence of P. Species evenness also differed between nutrient treatments (N: p=0.005, P:
p=0.026), although the moisture regime filtered these effects for N+P plots.
Life form showed varying responses to nutrients (Figure 8). Both relative cover
and absolute cover are given in these analyses to give an idea of how certain life forms
favored treatments overall (absolute), and how they responded in comparison to the other
life form (relative). For example, absolute cover of forbs responded negatively to the
presence of N (p=0.006), did not respond to P, and responded to N+P differently based
21 on meadow type (p=0.016), favoring the dry meadow. Absolute cover of graminoids was
significantly higher in the mesic meadow (p<0.000). Considering this, meadow type
affected how graminoids responded to N (p=0.022) and P (0.007), but not to N+P. In
terms of relative cover, forbs dominated over graminoids in the P and N+P dry meadow
plots as well as the P mesic meadow plots. Graminoids dominated the N+P plots in the
mesic meadow. Neither life form significantly dominated the other nutrient treatments
(Figure 8).
There were also species-specific responses to nutrients, as shown in Figure 9.
Cover of Geum rossii was higher in the presence of P across all treatments (p=0.028) and
was affected by N+P depending on meadow type, showing higher cover in the dry
meadow (p=0.002). Deschampsia cespitosa did not respond differently based on the
nutrient additions, but cover was lower in the presence of P. Carex scopulorum
responded positively to P overall (p=0.010) and to N in the recovery area (p=0.049).
Despite patterns in Figure 9j, variation in cover of Carex aquatilis between plots resulted
in a lack of significant response to nutrients. Artemisia scopulorum was unaffected by P
and favored N and N+P in the dry meadow (N: p=0.015, N+P: p=0.042), although
variation between plots also affected significance of these interactions. N additions
decreased cover of Trifolium parryi and Erigeron simplex (T. parryi: p=0.028, E.
simplex: p=0.000). Both Kobresia myosuroides and Oreoxis alpina showed much higher
cover in the dry meadow, and this preference affected their responses to nutrients, which
were insignificant despite the patterns in Figures 9e and 9h. Lastly, Lloydia serotina was
affected by each nutrient addition (N: p<0.000, P: p=0.001, N+P: p=0.027). Despite these
varying responses, Geum rossii had an overwhelming dominance in the P and N+P
22 treatments, and in plots with added N alone, Geum rossii and Deschampsia cespitosa
codominated.
Cover of Forbs vs. Graminoids
80
*
*
Relative Cover (%)
70
*
* 60
50
40
Forbs
30
Graminoids
20
10
0
Control
N
N+P
P
Control
N
Dry
N+P
P
Mesic
Treatment per Meadow Type
*p<0.05 Figure 8: Relative plant cover in terms of life form found in the Saddle area of Niwot
Ridge, Colorado. These data include vascular plants and exclude shrubs. Data are divided
by meadow type—either dry or mesic—and secondarily by nutrient additions, which
include control (no nutrients added), added nitrogen (N), added phosphorus (P), or added
N and P (N+P). Error bars represent standard error.
50
40
30
20
10
0
* Control
N
N+P
Dry
(b) Deschampsia cespitosa
* P
Control
Relative Cover (%)
Relative Cover (%)
(a) Geum rossii
N
N+P
P
Mesic
30
25
20
15
10
5
0
* Control
N
N+P
Dry
Treatment per Meadow Type
P
Control
N
N+P
Mesic
Treatment per Meadow Type
*p<0.05
23 P
(d) Trifolium parryi
* 15
Relative Cover (%)
Relative Cover (%)
(c) Artemisia scopulorum
* 10
5
0
Control
N
N+P
P
Control
N
Dry
N+P
P
20
* 15
10
5
0
Control
N
Mesic
N+P
Relative Cover (%)
Relative Cover (%)
15
10
5
N
N+P
P
Control
N
Dry
N+P
8
6
4
2
0
Control
P
N
N+P
Dry
P
Control
Relative Cover (%)
N+P
P
Control
N
Dry
Mesic
N+P
P
Mesic
Treatment per Meadow Type
(h) Oreoxis alpina
Relative Cover (%)
N
Mesic
10
(g) Erigeron simplex
Control
P
12
Treatment per Meadow Type
10
8
6
4
2
0
N+P
(f) Lloydia serotina
* Control
N
Treatment per Meadow Type
(e) Kobresia myosuroides
0
Control
Dry
Treatment per Meadow Type
20
P
N
N+P
P
Mesic
10
8
6
4
2
0
* Control
N
N+P
Dry
Treatment per Meadow Type
P
Control
N
N+P
Mesic
Treatment per Meadow Type
*p<0.05
24 P
60
50
40
30
20
10
0
(j) Carex aquatilis
* Control
N
N+P
P
Control
Dry
N
N+P
Relative Cover (%)
Relative Cover (%)
(i) Carex scopulorum
P
15
10
5
0
Mesic
Control
N
N+P
P
Control
N
Dry
Treatment per Meadow Type
N+P
Mesic
Treatment per Meadow Type
*p<0.05
Figure 9: Relative cover distributions for each of the ten most abundant species (a-j)
found in the Saddle area of Niwot Ridge, Colorado. Percentages were calculated as cover
of each species divided by total cover within each treatment. Total cover included
vascular plants, therefore excluding hits of bare ground, lichen, moss, rocks, and plant
litter. Data are divided by meadow type—either dry or mesic—and secondarily by
nutrient additions, which include control (no nutrients added), added nitrogen (N), added
phosphorus (P), or added N and P (N+P). Asterisks refer to significance (p<0.05) of a
nutrient treatment within a meadow type, and an asterisk in the middle refers to
significance between meadow types. Error bars represent standard error.
IV. Functional traits
In these analyses, I evaluated community trait response to treatments. Each
functional trait was significantly affected by meadow type, showing higher community
values in the mesic meadow (p<0.004 for all traits). However, this response was likely
related to the fact that relative cover of the ten most abundant species formed a higher
percentage of plant cover in the mesic meadow. In general, community functional
response tended to increase in the presence of N+P, although only values for LA and
chlorophyll content responded significantly (p<0.050, Figure 10). For the other traits—
height, stomatal conductance, SLA, and %N—nutrient treatments did not significantly
affect community response. There were strong positive correlations between all traits
25 P
except for LA and %N, meaning that treatments with high measures for one trait mostly
had high measures for all traits.
Leaf Area
0.7
Chlorophyll Content
60
* 0.5
0.4
0.3
0.2
40
30
20
10
0.1
0
* 50
Logged Trait Value
Logged Trait Value
0.6
0
Control
N+P
Control
N+P
*p<0.05
Figure 10: Significant effects of nitrogen and phosphorus on functional traits in the
Saddle area of Niwot Ridge, Colorado. Trait values source from Spasojevic and Suding
(2012) and include the natural log of average values from the ten most abundant species
in this analysis: Geum rossii, Deschampsia cespitosa, Kobresia myosuroides, Carex
aquatilis, Carex scopulorum, Lloydia serotina, Trifolium parryi, Artemisia scopulorum,
Erigeron simplex, Oreoxis alpina. Traits include leaf area (cm2) and chlorophyll content
(units specific to the device used). Data are divided by nutrient treatment—control plots
receiving no nutrient additions and N+P plots receiving both N and P. Error bars
represent standard error.
26 Discussion
After exploring the patterns predicted by previous studies, including those
specific to Niwot Ridge and the plots measured on the Saddle, these results confirm
several key trends but also contradict important predictions about the trajectory of alpine
plant communities.
Although community composition and cover differed between treatments and
meadow types, the lack of change in total cover over time points to resilience in the
alpine, continuing essential ecosystem services such as primary productivity and biomass
production to support higher trophic levels. On the other hand, declining richness and
diversity over time indicate a loss of species and an increase of cover of the dominant
species in most treatments. Specifically, there is greater potential for dominance in wetter
areas due to a stronger community trait response and higher cover of the ten most
abundant species in the mesic meadow. As for the dry meadow, N+P plots saw
consistency in richness over time, showing that if both N and P were added into the
alpine, these communities may preserve diversity. Other alpine studies have seen an
increase in diversity due to uphill encroachment of subalpine species in response to
climate change, so the loss of diversity may only be relative to this study site considering
all vegetative plots were located within the same geographical area. Nonetheless, shifts in
diversity can affect an ecosystem’s ability to buffer disturbance and provide services, so
monitoring these shifts improves understanding of the stability of the alpine.
Forbs and graminoids have continued to maintain their relative cover
contributions as reported in Gasarch and Seastedt (2015b). Balance in cover between life
forms reinforces findings in Wookey (2009) and Spasojevic et al. (2013) and contradicts
27 Pickering et al. (2014) and Bowman (2000)—studies that predicted increasing graminoid
cover driven by a few species. Graminoids are often viewed as more competitive species
than forbs and theorized to capitalize on N deposition, but forbs have shown positive
responses to the combination of N and P (Theodose and Bowman 1997). The significant
increase of Geum rossii at the expense of Deschampsia cespitosa in the N+P plots show a
changing pattern in community dominance over the last four years, signifying Geum
rossii as a generalist species and supporting forb response to N and P. Furthermore, the
decline of Deschampsia cespitosa in response to N+P, despite its superior rates of
nutrient cycling (Steltzer and Bowman 1998), show that P deposition appears to
neutralize the benefits of increasing N, indicating the value of a species’, or life form’s,
ability to capitalize on N and P rather than just N. If these nutrients are both in abundance
in the alpine, the vegetative communities can mitigate soil acidification and nutrients
leaching into water sources, as well as maintain efficient rates of decomposition and
nutrient cycling. In addition, the amount of N sequestered in an environment relates to the
amount of carbon (C) (Fisk et al. 1998). If P deposition facilitates N cycling, which then
relates to C sequestration, the alpine has the potential to reduce harmful effects of climate
change by sequestering C as well as sequestering N in soil organic forms.
The variations between the other of the dominant species portray the complexity
of community response to nutrients. There does not appear to be a uniform pattern in how
nutrients affect dominance of life form, individual species, or cover in the alpine, but in
several cases, N in the absence of P causes lower cover and community measures such as
richness, diversity (in the mesic meadow), and evenness. Species of Trifolium are known
N-fixers and have previously capitalized on P more so than N+P on Niwot Ridge
28 (Gasarch and Seastedt 2015b), which is consistent with the results found here and once
again shows the importance of P deposition in cycling N. In addition, the increase of
Trifolium species supports the burst of N-responders, as predicted by Bowman (2000)
and Bowman et al. (2015). The increase in shrub cover over time reflects patterns across
several alpine environments, including observations on Niwot Ridge (Wookey et al.
2009; Debinski et al. 2010; Spasojevic et al. 2013; Suding 2015). Therefore, there is a
likelihood of shrubs threatening to invade alpine meadows, although the mechanism(s)
for this increase remain unknown.
Community composition and functional traits between snow regimes varied only
slightly, indicating a production and cover response but not a compositional response to
snowmelt. Although recovery plots had higher cover than snow plots, the consistent
difference in overall cover through time between areas affected by the snow fence and
areas not affected by the snow fence indicates that added snow during the entire time
period of the study (beginning in 1993) overshadowed the first-year response to less
moisture. Higher cover in the recovery plots may have merely been a consequence of
surveying plots at different times during the season—a difference in phenology rather
than an actual difference in cover. However, several studies predict that an earlier
snowmelt lengthens the growing season, allowing plants a longer time to grow and
increase productivity as a result (Bowman 2000; Pickering et al. 2014; Kittel et al. 2015).
The growth of the recovery area reflects this, but otherwise, it does not seem as though
one year of earlier snowmelt and less moisture greatly impacted the plant communities of
the tundra. Alpine sites, and even different areas within alpine sites, observe dissimilarity
when it comes to snowmelt. Niwot Ridge has received more precipitation through time
29 but also more rapid snowmelt, so the lack of community change in the recovery plots
may not directly relate to the future of Niwot Ridge, but considering other sites are seeing
a lengthening growing season, these results support the contention that the alpine tundra
exhibits resilience to some variability in precipitation patterns through time.
When discussing functional diversity, each of the dominant species varied
between individual traits (i.e. a species with a high value for one trait did not necessarily
have high values for all traits). Considering the positive trait correlations, it does not
seem that the alpine selects for dominance in certain traits over others, but rather
functional traits exhibit codominance within a community, even under varying
treatments. Leaf area (LA) is typically smaller in stressful environments so that a plant
can conserve energy and retain water. The positive response of LA to N+P indicates that
N and P in unison reduce stress by relieving nutrient limitation. This is somewhat
predictable considering the alpine is a naturally low-nutrient system, but it also indicates
some level of adaptability in alpine plant communities considering their capacity to cycle
increased nutrient inputs. Furthermore, larger leaves may provide more food for higher
trophic levels during longer growing seasons in the alpine, and larger LA is more typical
of forbs than grasses, so there is potential for forbs to increase in dominance in the
presence of N and P. Chlorophyll content relates to photosynthesis, and since this trait
was also higher in response to N+P, plants receiving these nutrients may grow more
throughout the season. Cover of vegetation (and thus biomass) correlated positively with
productivity (Niwot LTER unpublished data), so the increase of traits—not only those
that are significant—in response to N+P likely point to an increase in primary
productivity in the presence of both nutrients. Spasojevic and Suding (2012) also saw
30 increasing trait values in areas of resource abundance, but since traits used in this study
were not measured as a real-time response to the nutrient additions, including functional
trait measurements in future surveys of plots on the Saddle would reinforce these
predictions.
Conclusions and Future Directions
The alpine of the Colorado Rocky Mountains remains resistant to, and potentially
responds positively to, changing precipitation and increased nutrient presence as a result
of climate change. Although several trends upheld through time, the change observed
over the four-year time period reflects the dynamism in this alpine ecosystem. Vegetative
responses to resources indicate the variability of alpine ecosystems and the importance of
species-specific responses, although Geum rossii has emerged as the dominant force in
the alpine tundra under most conditions. The continued encroachment of shrubs presents
a threat for alpine meadows, emphasizing the importance in monitoring these species in
the future. The potential for P deposition to neutralize N deposition may mitigate the N in
snowmelt and precipitation that melts into rivers and high-elevation lakes, and may even
promote a growth response in several alpine plant species. However, since these
responses hinge on P deposition balancing N deposition, focusing future studies on
incoming P compared to N would ease concerns of the impacts of N deposition on alpine
plant communities.
If the alpine acts as a “bellwether” of change, the observed resistance to climatic
simulations may provide a positive outlook on lower-elevation ecosystems, although
once again, this does appear to depend on P deposition, which may be less apparent at
31 lower elevations. In addition, these responses may only maintain ecological integrity up
to a point—when vegetation can no longer cycle the influx of nutrients or prolong
productivity throughout a lengthening growing season with less moisture. Looking
forward, confirming incoming P though dust, observing phenological changes in response
to snowmelt, measuring functional trait response to nutrients in real time, and
continuously updating the vegetative surveys of this site in the Colorado Rocky
Mountains will enhance our understanding of environmental responses to climate
change—not only in alpine sites, but also across all environments.
Acknowledgements
First and foremost, I would like to thank Dr. Tim Seastedt for his help and
expertise throughout this process. I would also like to thank Dr. Piet Johnson for his
invaluable input and Dr. Dan Doak for his involvement. Thank you to all three for
serving on my committee. Next, I would like to thank Wren Kelmen for accompanying
me in the alpine and assisting with data collection. Thank you to my fellow honors
students for their feedback and constant encouragement. Another thank you goes to the
Marion and Gordon Alexander Memorial Fellowship for supporting me in the field.
Finally, I would like to thank Dr. Katie Suding, Dr. Jane Smith, Dr. Hope Humphries,
and the remaining of the Niwot Ridge community for allowing me to take part in such an
in depth research endeavor. These parties have been incredibly influential in my
development as an ecologist, and for that I am extremely grateful!
32 Literature Cited
Billings, W.D., and L.C. Bliss. (1959) An alpine snowbank environment and its effects
on vegetation, plant development, and productivity. Ecology 40:388-397.
Bowman, W.D. (2000) Biotic controls over ecosystem response to environmental change
in alpine tundra of the Rocky Mountains. AMBIO: A Journal of the Human
Environment 29:396-400.
Bowman, W.D., Nemergut, D.R., McKnight, D.M., Miller, M.P., and M.W. Williams.
(2015) A slide down a slippery slope – alpine ecosystem responses to nitrogen
deposition. Plant Ecology & Diversity 8:727-738.
Carbognani, M., Bernareggi, G., Perucco, F., Tomaselli, M., and A. Petraglia. (2016)
Micro-climatic controls and warming effects on flowering time in alpine
snowbeds. Oecologia 182:573-585.
Cornelissen, J.H.C., Lavorel, S., Garnier, E., Diaz, S., Buchmann, N., Gurvich, D.E.,
Reich, P.B., ter Steege, H., Morgan, H.D., van der Heijden, M.G.A., Pausas, J.G.,
and H. Poorter. (2003) A handbook of protocols for standardized and easy
measurement of plant functional traits worldwide. Australian Journal of Botany
51:335-380.
Debinski, D.M., Wickman, H., Kindscher, K., Caruthers, J.C., and M. Germino. (2010)
Montane meadow change during drought varies with background hydrologic
regime and plant functional group. Ecology 91:1672-1681.
Diaz, S., and M. Cabido. (1997) Plant functional types and ecosystem function in relation
to global change. Journal of Vegetation Science 8:463-474.
Ernakovich, J.G., Hopping, K.A., Berdanier, A.B., Simpson, R.T., Kachergis, E.J.,
33 Steltzer, H., and M.D. Wallenstein. (2014) Predicted responses of arctic and
alpine ecosystems to altered seasonality under climate change. Global Change
Biology 20:3256-3269.
Fisk, M.C., Schmidt, S.K., and T.R. Seastedt. (1998) Topographic patterns of above- and
belowground production and nitrogen cycling in alpine tundra. Ecological Society
of America 79:2253-2266.
Gasarch, E.I., and T.R. Seastedt. (2015a) Plant community response to nitrogen and
phosphorus enrichment varies across an alpine tundra moisture gradient. Plant
Ecology & Diversity 8:739-749.
Gasarch, E.I., and T.R. Seastedt. (2015b) The consequences of multiple resource shifts on
the productivity and composition of alpine tundra communities: inferences from a
long-term snow and nutrient manipulation experiment. Plant Ecology & Diversity
8:751-761.
Gough, L., Bettez, N.D., Slavik, K.A., Bowden, W.B., Giblin, A.E., Kling, G.W.,
Laundre, J.A., and G.R. Shaver. (2016) Effects of long-tern nutrient additions on
Arctic tundra and lake ecosystems: beyond NPP. Oecologia 182:653-665.
Kittel, T.G.F., Williams, M.W., Chowanski, K., Hartman, M., Ackerman, T., Loseleben,
M., and P.D. Blanken. (2015) Contrasting long-term alpine and subalpine
precipitation trends in a mid-latitude North American mountain system, Colorado
Front Range, USA. Plant Ecology & Diversity 8:607-624.
Matteodo, M., Ammann, K., Verrecchia, E.P., and P. Vittoz. (2016) Snowbeds are more
affected than other subalpine–alpine plant communities by climate change in the
Swiss Alps. Ecology and Evolution 6:6969-6982.
34 Pickering, C.M., Green, K., Barros, A.A., and S.E. Venn. (2014) A resurvey of late-lying
snowpatches reveals changes in both species and functional composition across
snowmelt zones. Alpine Botany 124:93-103.
Rouston, C.C., Overpeck, J.T., Woodhouse, C.A., and Kenney, W.F. (2016) Three
millennia of southwestern North American dustiness and future implications.
Plos One 11:e0149573.
Schmidt, S.K., King, A.J., Meier, C.L., Bowman, W.D., Farrer, E.C., Suding, K.N., and
D.R. Nemergut. (2015) Plant-microbe interactions at multiple scales across a
high-elevation landscape. Plant Ecology & Diversity 9:703-712.
Smith, W.K., Germino, M.J., Johnson, D.M., and K. Reinhardt. (2009) The altitude of
alpine treeline: a bellwether of climate change effects. The Botanical Review
75:163-190.
Spasojevic, M.J., and K.N. Suding. (2012) Inferring community assembly mechanisms
from functional diversity patterns: the importance of multiple assembly processes.
Journal of Ecology 100:652-661.
Spasojevic, M.J., Bowman, W.D., Humpries, H.C., Seastedt, T.R., and K.N. Suding.
(2013) Changes in alpine vegetation over 21 years: are patterns across a
heterogeneous landscape consistent with predictions? Ecosphere 4:117.
Steltzer, H., and W.D. Bowman. (1998) Differential influence of plant species on soil
nitrogen transformations within moist meadow alpine tundra. Ecosystems 1:464474.
Suding, K.N., Collins, S.L., Gough, L., Clark, C., Cleland, E.E., Gross, K.L., Milchinas,
D.G., and S. Pennings. (2005) Functional- and abundance-based mechanisms
35 explain diversity loss due to N fertilization. Proceedings of the National Academy
of Sciences of the United States of America 102:4387-4391.
Suding, K.N., Farrer, E.C., King, A.J., Kueppers, L., and M.J. Spasojevic. (2015)
Vegetation change at high elevation: scale dependence and interactive effects on
Niwot Ridge. Plant Ecology & Diversity 8:713-725.
Theodose, A.T., and W.D. Bowman. (1997) Nutrient availability, plant abundance, and
species diversity in two alpine tundra communities. Ecological Society of America
78:1861-1872.
Venn, S.E., Green, K., Pickering, C.M., and J.W. Morgan. (2011) Using plant functional
traits to explain community composition across a strong environmental filter in
Australian alpine snowpatches. Plant Ecology 212:1491-1499.
Venn, S.E., Pickering, C.M., and K. Green. (2014) Spatial and temporal functional
changes in alpine summit vegetation are driven by increases in shrubs and
graminoids. AoB Plants 6:1-15.
Williams, M.W., Hood, E., Molotch, N.P., Caine, N., Cowie, R., and F. Liu. (2015a) The
‘teflon basin’ myth: hydrology and hydrochemistry of a seasonally snow-covered
catchment. Plant Ecology & Diversity 8:639-661.
Williams, M.W., Seastedt, T.R., Bowman, W.D., McKnight, D.M., and K.N. Suding.
(2015b) An overview of research from a high elevation landscape: the Niwot
Ridge, Colorado Long Term Ecological Research programme. Plant Ecology &
Diversity 8:497-605.
Wookey, P.A., Aerts, R., Bardgett, R.D., Baptists, F., Brathen, K.A., Cornelissen, J.H.C.,
Gough, L., Hartley, I.P., Hopkins, D.W., Laborel, S., and G.R. Shaver. (2009)
36 Ecosystem feedbacks and cascade processes: understanding their role in the
responses of Arctic and alpine ecosystems to environmental change. Global
Change Biology 15:1153-1172.
37 Appendix
Table 2: Statistical analyses and general output for vascular plant cover, species richness, species
diversity, and adjusted species evenness in response to the treatments explored in this study. Each value
corresponds to the average measure per plot of each community trait (cover, richness, diversity, and
adjusted evenness respectively). Treatments are meadow type, snow manipulation, and nutrient additions.
The statistical distribution describes the data and the statistical test refers to the type of analysis used to
evaluate the data (glm stands for generalized linear model). The interaction column indicates whether any
treatments significantly interacted, meaning that the output depends on more than one treatment. If there is
a significant interaction, it is mentioned, but the outputs are not. Asterisks indicate a significant effect of a
treatment (p<0.05). Meadow type
Snow manipulation
Nutrient additions
Statistical
distribution
Statistical
test
Dry
Mesic
Snow
Recovery
Presence
N
Presence
P
Presence
N and P
Interaction
Average
cover per
plot
normal
glm
110.57
142.71*
116.70
135.20
122.56
132.31
132.00
no
Average
richness
per plot
normal
glm
13.91*
11.85
11.78
12.80
10.88*
12.44
10.12
no
Average
diversity
per plot
normal
glm
1.84
1.70
1.75
1.72
1.57*
1.67*
1.42*
no
Average
adjusted
evenness
per plot
left-skewed
glm using
| ln(evenness) |
0.29
0.30
0.27
0.31
0.34*
0.33*
0.40
yes
snow~N~P
38 Table 3: Statistical analyses and average cover per plot in response to treatments for each of the ten most
abundant species in the Saddle area of Niwot Ridge, Colorado. Treatments are meadow type, snow
manipulation, and nutrient additions. The statistical distribution describes the data and the statistical test
refers to the type of analysis used to evaluate the data (glmer stands for generalized linear mixed-effects
model). The interaction column indicates whether any treatments significantly interacted, meaning that the
output depends on more than one treatment. If there is a significant interaction, it is mentioned, but the
outputs are not. Asterisks indicate a significant effect of a treatment (p<0.05). Meadow type
Snow
manipulation
Nutrient additions
Plant species
Statistical
distribution
Statistical
test
Dry
Mesic
Snow
Recovery
Presence
N
Presence
P
Presence
N and P
Interactions
G. rossii
poisson
glmer
27.37
36.14*
31.70
30.73
34.95
37.03*
44.38*
yes
meadow*N*P
D. cespitosa
poisson
glmer
12.69
28.71*
21.57
20.80
20.03
16.75
13.38
no
C. scopulorum
poisson
glmer
3.74
25.97*
10.39
24.87
23.91
23.44*
35.06
yes
snow*N
A. scopulorum
poisson
glmer
8.80
10.37
9.35
12.47
7.75*
9.94
7.19
yes
meadow*N
meadow*N*P
T. parryi
poisson
glmer
4.31
12.74*
8.96
15.27
4.78*
8.59
2.38
no
K. myosuroides
poisson
glmer
10.06*
1.17
1.00
0.00
3.03
4.97
1.00
no
L. serotina
poisson
glmer
4.89
3.74
5.26
4.87
1.28*
1.91*
0.06*
no
E. simplex
poisson
glmer
5.17
3.17
5.57
6.73
1.63*
2.13
0.88
no
39 Table 3 continued
Meadow type
Snow
manipulation
Nutrient additions
Plant species
Statistical
distribution
Statistical
test
Dry
Mesic
Snow
Recovery
Presence
N
Presence
P
Presence
N and P
Interactions
O .alpina
poisson
glmer
5.63*
1.69
1.00
3.73
3.41
4.47
5.75
no
C. aquatilis
poisson
glmer
4.66
4.77
10.57
5.80
8.00
6.59
10.13
yes
meadow*snow
40