Download Climatic warming strengthens a positive feedback between alpine

bioRxiv preprint first posted online Mar. 16, 2016; doi: http://dx.doi.org/10.1101/043919. The copyright holder for this preprint (which was not
peer-reviewed) is the author/funder. It is made available under a CC-BY 4.0 International license.
Climatic warming strengthens a positive feedback
between alpine shrubs and fire
James S Camaca,b , Richard J Williamsc , Carl-Henrik Wahrenb , Ary A Hoffmannd , and Peter A Veska
a
The Centre of Excellence for Environmental Decisions, School of BioSciences, The University of Melbourne, Melbourne, Victoria 3010, Australia; b Research Centre for
Applied Alpine Ecology, La Trobe University, Melbourne, Victoria 3086, Australia; c CSIRO Land and Water, Tropical Ecosystems Research Centre, PMB 44 Winnellie, Northern
Territory 0821, Australia; d Bio21 Institute, School of BioSciences, The University of Melbourne, Victoria 3010, Australia
This manuscript was compiled on November 12, 2016
Abstract
Climate change is expected to increase fire activity and woody plant encroachment in arctic and alpine landscapes. However,
the extent to which these increases interact to affect the structure, function and composition of alpine ecosystems is largely
unknown. Here we use field surveys and experimental manipulations to examine how warming and fire affect recruitment,
seedling growth and seedling survival in four dominant Australian alpine shrubs. We found that fire increased establishment of
shrub seedlings by as much as 33–fold. Experimental warming also doubled growth rates of tall shrub seedlings and could
potentially increase their survival. By contrast, warming had no effect on shrub recruitment, post-fire tussock regeneration,
or how tussock grass affected shrub seedling growth and survival. These findings indicate that warming, coupled with more
frequent or severe fires, will likely result in an increase in the cover and abundance of evergreen shrubs. Given shrubs are one
of the most flammable components in alpine and tundra environments, warming is likely to strengthen an existing feedback
between woody species abundance and fire in these ecosystems.
Introduction
Alpine and arctic vegetation are considered to be particularly vulnerable to the effects of changing climate (Engler
et al., 2011; Dullinger et al., 2012; Elmendorf et al., 2012a). In
these ecosystems, field manipulative experiments have shown
that climate directly influences plant phenology (Hoffmann
et al., 2010; Dorji et al., 2013), reproduction (Klady et al.,
2011), morphology (Hudson & Henry, 2009), growth (Hollister
et al., 2005), floristic composition (Elmendorf et al., 2012a)
and biotic interactions (Klanderud, 2005). However, most
studies in these ecosystems have focused on mature plant responses in undisturbed vegetation (Briceño et al., 2015). Few
have included disturbance as a factor (but see: Munier et al.,
2010; Graae et al., 2010; Camac et al., 2015), or examined the
influence of climate change on seedlings in post-disturbance
environments. As a consequence, little is known about how
climate affect seedling demographic rates (Briceño et al., 2015),
particularly in alpine and tundra post-disturbance conditions.
This is despite mounting evidence that seedling regeneration
is important in alpine and arctic ecosystems (Venn & Morgan,
2009; Briceño et al., 2015), particularly for woody species (Camac et al., 2013; Williams et al., 2014). Seedlings are the life
stage that determines the long-term persistence of a species
as well as its capacity to establish in new areas (Walck et al.,
2011). As such, in order to accurately predict future trajectories of vegetation change in alpine and arctic ecosystems, it
RA
Accurately forecasting the effects of climatic warming on vegetation dynamics requires an understanding of the mechanisms
by which climate and vegetation interact. Most forecasting
models include the direct effects of climatic conditions on
species distributions, but largely ignore disturbances, particularly their type, frequency and severity (Guisan & Thuiller,
2005; Tucker et al., 2012). Recurrent disturbance is integral to
all ecosystems, strongly influencing fundamental demographic
processes such as recruitment and mortality, and thus the
composition and structure of plant communities and biomes
worldwide (Bond et al., 2005; Turner, 2010). Disturbance
regimes are already changing as a consequence of climate
change (Turner, 2010; Westerling et al., 2011; Bradstock et al.,
2014). It is therefore imperative we understand how the effects
of disturbance will interact with climate change, and whether
such effects amplify or diminish how vegetation responds to
changing temperature and moisture (Post & Pedersen, 2008;
Camac et al., 2015; Enright et al., 2015)
FT
Keywords: Bayesian models, Biotic interactions, Gap dynamics, Growth, Mortality, Recruitment
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Understanding the feedback between climate, vegetation
and fire will be particularly important in order to accurately
predict the trajectory of ecosystem change in coming decades
(Bowman et al., 2009). In addition to the direct effects of
warming on fire weather, changing climate may also indirectly
alter fire regimes by altering vegetation productivity, structure
and composition (Keeley et al., 2012; Matthews et al., 2012;
Bowman et al., 2014). Such feedbacks may be positive or
negative, and have been documented in tropical savanna (Hoffmann, 2003), boreal forests (Krawchuk & Cumming, 2011),
grasslands (Flannigan et al., 2009) and alpine and arctic environments (Goetz et al., 2007; Wookey et al., 2009).
doi:10.1111/XXX.XXXXX
J.C. conceived, designed and performed the experiments, field surveys and analysis; C-H.W, R.W,
A.H and P.V. supervised the development of this work, aided in data collection and provided statistical advice. All authors contributed to the writing of this manuscript.
The authors declare that they have no competing financial interests.
1
To whom correspondence should be addressed. E-mail: [email protected]
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bioRxiv preprint first posted online Mar. 16, 2016; doi: http://dx.doi.org/10.1101/043919. The copyright holder for this preprint (which was not
peer-reviewed) is the author/funder. It is made available under a CC-BY 4.0 International license.
is impertative we understand how seedlings respond to both
changing climate and disturbance regimes (Walck et al., 2011;
Briceño et al., 2015).
Does warming alter the post-fire
regeneration of tussock grass?
a
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Fire
Mo
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Feedback between climatic warming, fire and shrubs
Increased ambient
temperature
Bare ground
Does warming alter the
recruitment of shrubs post-fire?
Tussock grass
Does fire stimulate
shrub recruitment?
Landscape More flammable fuel
Shrub
flammability
establishment
Does warming alter the growth
and survival of shrub seedlings?
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Shrub seedling
growth & survival
use
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Does proximity to tussock
affect the growth & survival of
shrub seedlings?
Bare ground
fire intervals
< 20 years
Increased ambient
temperature
Tussock grass
Landscape More flammable fuel
Shrub
flammability
establishment
Shrub seedling
growth & survival
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Fig. 1. Conceptual diagram illustrating positive warming-shrub-fire feedback. (a)
Hypothesised positive feedback loop between fire, climatic warming and landscape
flammability. Solid lines = known mechansims; Dashed lines = mechansims that may
exacerbate or diminish this feedback but which we have a paucity of information on.
(b) Feedback updated based on new evidence obtained from experimental and field
surveys conducted in this study. Also includes an example of a potential, but unlikely,
effect (red dashed line) that could break this feedback. Blue = positive relationships,
Red = negative relationships.
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In alpine and arctic ecosystems, warming experiments and
long-term monitoring have documented significant increases
in the growth and cover of woody species (Sturm et al., 2001;
Walker et al., 2006; Myers-Smith et al., 2011). The frequency
and extent of wildfires in these environments have also increased over recent decades, a trend expected to continue
(Westerling et al., 2006; Flannigan et al., 2009; Qiu, 2009;
Bradstock et al., 2014). Current evidence from these ecosystems indicate that shrub recruitment and encroachment is
highest in disturbed areas and lowest in areas with minimal
bare ground cover (Williams & Ashton, 1987; Batllori et al.,
2009; Frost et al., 2013). Evidence also suggests that climatic
warming is likely to increase growth rates of woody species
(Arft et al., 1999; Elmendorf et al., 2012b; Myers-Smith et al.,
2011) and that shrubs are potentially the most flammable
vegetation component in these ecosystems (Williams et al.,
2006; Higuera et al., 2009; Fraser et al., 2016). Thus, increases
in the cover of woody species in alpine and arctic environments
may increase the flammability of these ecosystems.
The results of these studies suggest a positive feedback
could exist between warmer temperatures, woody species and
fire in alpine and tundra environments (Fig. 1a). Specifically,
warmer temperatures may lead to more frequent and severe
fire, which in turn, may increase recruitment opportunities
(i.e. more bare ground) for woody species and increase shrub
thickening both within and beyond shrub boundaries (Racine
et al., 2004; Lantz et al., 2013). If this effect is coupled with an
increase in the growth and survival of shrub seedlings, highly
flammable fuels will accumulate at a faster rate, have higher
landscape connectivity, and ultimately lead to increases in
the likelihood of fire. The consequence of which will further
increase shrub recruitment opportunities. Thus, warming
could strengthen an existing climate-disturbance feedback,
that not only has the potential to cause rapid changes in the
composition and structure of alpine and arctic vegetation, but
also has serious social, biodiversity and carbon sequestration
consequences (Mack et al., 2011).
While paleoecological studies have indicated that such a
feedback may exist in the arctic (Higuera et al., 2008, 2009),
there is a paucity of information on what may strengthen,
mitigate or break this feedback. For example, we have little
information on whether fire will stimulate shrub seedling recruitment in alpine or tundra ecosystems, nor do we know
how rates of seedling growth and survival will be affected
under warmer, more exposed, post-fire conditions. Furthermore, we do not know how tussock grasslands will respond to
warmer post-fire conditions, how grasses affect vital rates of
shrub seedlings, or whether such effects are altered by warmer
post-fire conditions.
Here, we examine these unknowns and their impacts on
this proposed feedback between climatic warming, shrubs and
fire in Australian alpine vegetation. As with other tundra
ecosystems, Australian alpine landscapes consist of a mosaic
of vegetation types dominated by either herbaceous or woody
species (Williams et al., 2014). They have also experienced
significant changes in climate. Since 1979, mean growing
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season temperatures have risen by approximately 0.4°C and
annual precipitation has fallen by 6% (Wahren et al., 2013),
with a consequent decline in snow pack depth (Sánchez-Bayo
& Green, 2013). These climatic changes have been correlated
with a 10 to 20% increase in shrub cover and a 25% decline in
graminoids cover (Wahren et al., 2013). Much of the Australian
Alps has also been burnt by recent (2003 and 2006) wildfires,
the frequency and severity of which are expected to increase
as a consequence of further climate change (Bradstock et al.,
2014; Williams et al., 2014).
In this study we took advantage of recent fires in alpine
open heathland, a plant community that occupies ca. 25% of
the Australian alpine landscape (Williams et al., 2014). Under
global warming, open heathland is likely to encroach upon
grasslands, wetlands and herbfields (including the nationally
endangered snow patch herbfields; Williams et al., 2015), and
is itself susceptible to shrub thickening (McDougall, 2003).
To identify some of the biotic and abiotic factors that affect shrub establishment and how warmer post-fire conditions
affect shrub seedling vital rates, we combined field observations of post-fire seedling densities with a warming experiment
that used seedlings of four dominant Australian evergreen firekilled shrubs: Grevillea australis (Proteaceae; a tall shrub),
Asterolasia trymalioides (Rutaceae; a prostrate shrub), Phebalium squamulosum (Rutaceae; a tall shrub) and Prostanthera
cuneata (Lamiaceae; a tall shrub).
This combination of data allowed us to quantify the following unknowns in the hypothesised climate-fire-shrub feedback
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bioRxiv preprint first posted online Mar. 16, 2016; doi: http://dx.doi.org/10.1101/043919. The copyright holder for this preprint (which was not
peer-reviewed) is the author/funder. It is made available under a CC-BY 4.0 International license.
1. Does fire stimulate shrub recruitment?
2. Does warming alter the recruitment of shrubs post-fire?
3. Does warming alter the growth and survival of shrub
seedlings?
4. Does proximity to tussock grass affect the growth and
survival of shrub seedlings?
5. Does warming alter the post-fire regeneration of tussock
grass?
Methods
Below, we provide a summary of the experimental design
and analysis. Full details are presented in the Supporting
Information.
Study Sites. The Australian Alps are an ideal place to examine
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the proposed fire-climate-vegetation feedback because they
have all the elements needed to demonstrate such a feedback.
The Alps are subject to recurrent landscape fire (approximately
every 50 to 100 years), and were burnt extensively in 2003
(Williams et al., 2006, 2014). The vegetation consists of a
range of life forms (graminoids, forbs and shrubs), of which
shrub abundance strongly determines landscape flammability
(Williams et al., 2006; Fraser et al., 2016). There is also
evidence that both fire regimes (Bradstock et al., 2014) and
the relative abundance of life forms in the Australian Alps have
changed as a consequence of recent climate change (Wahren
et al., 2013; Camac et al., 2015).
We investigated shrub-grass dynamics in open heathland
on the Bogong High Plains, Australia. Open heathland is a
common Australian alpine vegetation type, occupying approximately 25% of the treeless landscapes above 1600m in the
Australian Alps. It is an ecotone between closed heathland
(>70% shrub cover) and tussock grassland (shrub cover <
20%). Open heathland is dominated by the shrub Grevillea australis (shrub cover generally 20–60%) with inter-shrub
regions occupied by snow grasses (Poa hiemata and Poa costiniana) and other herbaceous species. Grevillea australis is
an obligate seeding shrub, and establishment of seedlings is dependent on disturbance creating bare ground in the grass–herb
sward (Williams & Ashton, 1987). Relative to tussock grassland, open heathland is a highly flammable plant community
(Williams et al., 2006; Fraser et al., 2016). A consequence of
this differential flammability led to the 2003 wildfires on the
Bogong High Plains burning approximately 60% of open heathland and only 13% of tussock grasslands. Following these fires,
seedling regeneration of Grevillea australis and other shrubs
was prolific (Williams et al., 2014).
Grevillea australis (Proteaceae; a tall shrub) and Asterolasia trymalioides (Rutaceae; a prostrate shrub), a dominant
closed heathland species Prostanthera cuneata (Lamiaceae;
a tall shrub) that typically grows on warmer aspects and a
species common to both open and closed heathland Phebalium squamulosum (Rutaceae; a tall shrub). We focused on
these four species because they are common in the Australian
Alps; are fire-killed and thereby re-establish via seed (the
dominant shrub post-fire strategy in the Australian mainland
Alps; Walsh & McDougall, 2004); and under climatic warming, have the potential to increase in cover and height within
heathlands, and invade non-shrubby plant communities such
as alpine grasslands and herbfields.
A total of 640 seedlings, 256 Grevillea (half used in Poa
inter-tussock experiment—see below) and 128 for each of Asterolasia, Prostanthera and Phebalium were used. Four seedlings
per species were randomly selected and transplanted into a
4×4 square grid in the center of each burnt patch, with 14
cm between individuals and the edge of the patch, which was
dominated by resprouting tussock grass Poa hiemata. To examine interactions between tussock grass and shrub seedlings
we also randomly transplanted four additional Grevillea australis seedlings into various sized bare gaps between burnt Poa
hiemata tussocks that were immediately surrounding the 4×4
bare ground square grid (Fig. S1). The experimental site was
fenced to prevent grazing by deer and horses. We detected
no obvious signs of herbivory by invertebrates or hares within
our plots.
To simulate near-term warmer conditions indicated by the
IPCC (2013), we randomly assigned Open Top Chambers
(OTCs) to half (16) the plots, with the remainder treated as
unwarmed controls. The chambers were constructed following
the International Tundra Experiment (ITEX) protocols (Molau
& Mølgaard, 1996). OTCs were placed over plots, ensuring
all seedlings (including inter-tussock shrub seedlings) occurred
within the 1.1 m2 open top to minimise edge effects. OTCs
were placed out at the start of the growing season (October)
where they remained until first snowfall (early June). This
procedure was repeated for six growing seasons from May 2010
to May 2016.
Microclimatic conditions were measured hourly using Onset
Micro Stations (Onset Computer Corporation, Bourne, MA,
USA) at four control and four OTC plots. Across six growing
seasons (1281 growing season days), OTCs simulated warmer
conditions at the lower end of IPCC (2013) projections (Fig.
S6-S8). OTCs passively increased average ambient and soil
temperatures by 0.9°C. Minimum and maximum temperatures
were also raised in both ambient air (min: 1.1°C; max: 2°C)
and soil (min: 2°C; max: 0.1°C). Chambers only marginally
decreased soil moisture by 0.1% and relative humidity by 0.7%.
Seedling survival, maximum height and stem diameter
(nearest mm measured with Vernier calipers) were initially
recorded in May 2010 and then subsequently re-measured at
the end of each growing season (May-June). At the same
time, we recorded the distance to the nearest tussock or
grass seedling in each of four cardinal directions for Grevillea seedlings growing in inter-tussock gaps. Thus, temporal
changes in inter-tussock gap size could be due to either vegetative growth of resprouting tussocks, or the establishment of
grass seedlings. We did not measure individual characteristics
(e.g. height and basal diameter) of surrounding tussock grass
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loop (Fig. 1a):
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Open Top Chamber Experiment. In March 2010, at 1750 m
a.s.l, we burnt 32 randomly selected mature (60 cm tall and
1.5 m2 ) Grevillea australis shrubs in an open heathland site
that was not burnt by wildfire in 2003 or 2006. This created patches of bare ground approximately 0.7 m2 surrounded
by burnt tussock grass, simulating disturbance of individual
shrubs in open heathland burnt by wildfire. We collected
seedlings of dominant alpine shrub species from a nearby (<2
km) site of similar altitude burnt by 2006 wildfires. We collected seedlings of two dominant open heathland shrub species
Camac et al.
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bioRxiv preprint first posted online Mar. 16, 2016; doi: http://dx.doi.org/10.1101/043919. The copyright holder for this preprint (which was not
peer-reviewed) is the author/funder. It is made available under a CC-BY 4.0 International license.
Seedlings / m2
Unburnt
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Fig. 2. Mean Grevillea australis seedling density as a function of burning
(burnt/unburnt), fire severity (twig diameter), altitude, Topographic Wetness Index
(TWI) and adult density. (a) Centered and standardized model coefficients (on the
log scale); and effects of: (b) burning, (c) fire severity, (d) altitude, (e) topographic
wetness and (F) adult density, in areas burnt by the 2003 fires. All bars and shaded
areas indicate 95% Bayesian Credible Intervals.
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Data analysis. We built multiple hierarchical models to ex-
b
●
Intercept
Seedlings / m2
Seedling gradient study. We used 40 open heathland monitoring sites established after the 2003 fires (Williams et al., 2006;
Camac et al., 2013). These sites consisted of 30 burnt sites and
10 sites known to be unburnt for over 70 years. The disparity
in sample number between burnt and unburnt treatments was
due to very few open heathland sites of sufficient size (0.25
ha) remaining unburnt after both the 2003 and 2006 fires.
In the summer of 2011-12, at each site, seedling density/m2
was estimated using 40 quadrats, each 1 m2 , that were evenly
distributed in groups of 10 along four 50 m transects, with
10 m between transect lines, subsampling an area of 2000 m2 .
Within plots we recorded the number and maximum height
of Grevillea and Asterolasia seedlings. For unburnt sites we
counted the number of mature Grevillea plants (>0.5 m2 )
within 5 m of each transect. In burnt sites, this required
counting the number of skeletons (there were no living adults
at any burnt site) that still persisted post-fire. We were unable
to estimate numbers of adult Asterolasia because this species
does not have a persistent woody skeleton post-fire. Site
level data, elevation and Topographic Wetness Index (TWI; a
measure of plant available water, Moore et al., 1993) were obtained from a 30 m resolution digital elevation model. Lastly,
for burnt sites, fire severity was estimated by twig diameters
(Whight & Bradstock, 1999), collected immediately after the
2003 fires (Williams et al., 2006).
a
Seedlings / m2
because we could not distinguish individuals, and because
height varied substantially throughout the season. Numbers
of natural Grevillea australis recruits establishing within the
plots were also recorded and identified for the first two years
of the experiment.
amine how increased temperature and other factors influenced shrub seedling recruitment, growth and mortality, as
well as tussock-grass gap dynamics. For each model we
used Bayesian inference and fitted models in R 3.3.1 (R
Core Team, 2016) using package rstan 2.12.1 (Stan Development Team, 2016). Detailed information about experimental design and analysis is provided in the Supporting Information. Data and source code are available at:
https://github.com/jscamac/Alpine_Shrub_Experiment. In order
to aid in the reproducibility of this work, our code was written
using a remake framework (FitzJohn, 2015), such that others
can readily reproduce our entire workflow from data processing, through to producing a pdf of this manuscript by calling
remake() in R.
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Results
Drivers of shrub seedling establishment. We first investigated
how altitude, Topographic Wetness Index (TWI), adult density,
fire and fire severity (as measured by post-fire twig diameters—see Supporting Information) influenced the density of
Grevillea and Asterolasia seedlings (the two dominant shrubs
of alpine open heathland). Across the 40 alpine sites surveyed
in 2011-12 we found that the abundance of Grevillea (Fig. 2)
and Asterolasia (Fig. S2) seedlings was strongly influenced by
the occurrence of fire. Sites burnt in 2003 had seedling densities between 15 and 33 times higher than unburnt sites. The
mean seedling density of Grevillea, was 1.31/m2 at burnt sites
and 0.04/m2 at unburnt sites. Asterolasia had similar mean
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densities: 1.65 and 0.11 seedlings/m2 at burnt and unburnt
sites, respectively. For both species, seedling density increased
with increasing fire severity (i.e. sites with larger post-fire
twig diameters). As hypothesised, pre-fire adult density also
positively influenced Grevillea seedling density. For both shrub
species, we detected no change in seedling density along a 190
m elevational gradient (equivalent to a 1.5°C change in mean
temperature; Brown & Millner, 1989). This is consistent with
the field warming experiment (see below) which indicated rates
of recruitment (Fig. S3) and mortality in Grevillea is largely
insensitive to a 0.9°C change in temperature. We detected no
strong effect of Topographic Wetness Index for either species.
Experimental warming and shrub seedling growth. Given that
seedlings were more abundant in burnt vegetation, we investigated how warmer post-fire conditions affected seedling
growth and mortality rates. Seedlings of Grevillea, Asterolasia,
Phebalium and Prostanthera emerging after a wildfire were
transplanted into experimentally burnt plots. These plots
were either subjected to ambient conditions (i.e. controls),
or enclosed in Open Top Chambers (OTCs) which increased
temperature by 0.9°C.
After 2182 days (1281 growing season days) or 6 years’
growth, mean seedling heights of the tall shrubs (Grevillea,
Phebalium and Prostanthera) growing in post-fire bare ground
were greater in warmed plots relative to controls by 11.8, 4.5
and 14.9 cm, respectively (Fig. 3a). Warming increased heights
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seedling heights in either species.
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of both Grevillea and Prostanthera seedlings in all years, while
Phebalium did not respond to the warming treatment until the
second growing season. By contrast, seedlings of the prostrate
shrub, Asterolasia, showed no difference in growth rate between
warmed and control plots in any year. For each species, similar
treatment effects were observed for stem diameter growth
(Fig. S4). Accounting for initial height and assuming logistic
growth, the rates of change in mean annual predicted height
of Grevillea, Phebalium and Prostanthera were 2.5, 1.6 and 2
times that observed in control plots, respectively. According
to this model, a 6 cm seedling (the mean initial height of
seedlings used in this experiment) attains maximum height 42
(Grevillea) or 18 years sooner (Phebalium and Prostanthera)
when warmed by 0.9°C (Fig. 3b). Asterolasia was predicted
to reach its maximum height in approximately 23 to 27 years,
irrespective of warming treatment.
FT
Fig. 3. Observed and projected growth trajectories of four dominant Australian alpine shrubs. (a) Observed height growth: Thick lines with error bars represent mean (±
95% confidence intervals) observed heights at each May census. Thin lines represent individual growth trajectories. (b) Mean (± 95% Bayesian Credible Interval) projected
growth trajectories. In all cases, red and orange lines = seedlings growing in warmed (OTC) conditions and blue and light blue lines = seedlings growing in control conditions.
Projections were based on an logistic growth model using initial seedling size of 6 cm (the mean initial height observed in the OTC experiment) and mean maximum heights
observed in long-unburnt sites (i.e. 34, 67, 66 & 70 cm for Asterolasia, Grevillea, Phebalium, & Prostanthera, respectively). Vertical lines delimit year in which maximum height
is obtained
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The growth responses of both Grevillea and Asterolasia
observed in experimentally warmed plots were validated by the
maximum heights of post-fire recruits across 30 open heathland sites burnt in 2003. Here, a 190 m altitudinal range
is equivalent to a mean ambient temperature difference of
approximately 1.5°C (Brown & Millner, 1989), which is comparable to that observed between experimentally warmed and
control plots (0.9°C). In response to shifts in temperature, the
maximum height of Grevillea post-fire recruits was expected to
decrease with elevation, whereas Asterolasia seedlings were not
expected to show this pattern. Our experimental predictions
were verified (Fig. S5). Mean maximum height of Grevillea
seedlings in burnt open heathland were 8 cm taller at 1670
m a.s.l compared to seedlings at 1860 m a.s.l. (22 cm vs 14
cm; a difference comparable to our experimental findings). In
Camac et al.
Experimental warming and shrub seedling mortality. After six
years and across all plots, 36% (185 out of 511) of all seedlings
transplanted into the 4×4 bare ground square grid had died.
Most deaths occurred in Asterolasia (67) and Phebalium (67),
followed by Grevillea (36) and Prostanthera (15). Prostanthera
showed the largest treatment effect (Fig. 4), with annual
mortality rates estimated to be near 0% in warmed plots and
4% in control plots. This significant decrease in mortality may
be a consequence of OTCs reducing the severity of spring frosts
by rising minimum ambient and soil temperatures by 1.1°C
and 2°C, respectively (Fig. S6-S8). Warming also reduced
mean seedling mortality in Grevillea and Phebalium (Fig. 4);
however, for both species, the effect was highly uncertain (i.e.
credible intervals overlap). By contrast, annual mortality rates
in the prostrate shrub, Asterolasia, were marginally higher in
warmed plots, but again this effect was highly uncertain (Fig.
4).
Effects of tussock grass proximity on shrub seedling growth
and mortality rates. Climate change is expected to alter bi-
otic interactions because their strength and direction depend
strongly on climatic conditions, particularly in alpine and
arctic ecosystems (Callaway et al., 2002; Klanderud, 2005).
Here, we assess the interactive effects of warming and grass
proximity on the growth and survival of Grevillea seedlings
transplanted into various sized inter-tussock gaps. We detected
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bioRxiv preprint first posted online Mar. 16, 2016; doi: http://dx.doi.org/10.1101/043919. The copyright holder for this preprint (which was not
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ground gaps (whether by vegetative growth or seedlings) faster
under warmer conditions. Using six years of post-fire intertussock gap size changes in warmed and unwarmed plots, we
found that gaps were being infilled by tussock grasses (Fig. 6).
However, the rate at which this occurred was very slow, with
a 10 cm radius gap predicted to decrease by approximately
3 cm over a ten year period. We also detected no significant
effect of a 0.9°C temperature rise on the rate of infilling.
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Fig. 4. Mean (± 95% Bayesian Credible Interval) annual probability of death for each
species growing in control (blue) and warmed (red) conditions.
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Fig. 5. Experimental warming and inter-tussock gap size effects on Grevillea australis
seedlings. Top rows = Growth rate effects, Second row = Mortality effects. (a &
c) Centered and standardized model coefficients; (b & d) growth and mortality rate
response curves along an inter-tussock gap size gradient in warmed (red) and control
(blue) conditions. All error bars and shading are 95% Bayesian Credible Intervals.
Experimental warming and rates of gap infilling by tussock
grass. Despite having little impact on shrub seedling growth
and mortality rates, tussock grass may still limit shrub recruitment, and thus, shrub expansion, by infilling post-fire bare
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log coefficients
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Fig. 6. Effects of warming on rates of gap infilling by tussock grass. (a) Model
coefficients and (b) projected temporal change in size for an average 10 cm intertussock gap in warmed (red) and unwarmed (blue) conditions. All error bars and
shading are 95% Bayesian Credible Intervals.
Discussion
0.3
Growth coefficients
0.00
FT
b
−0.05
In this study we have quantified several unknown interactions
between fire, shrub-grass relationships and climate to extend
a conceptual model of alpine shrub dynamics under climate
change (Fig. 1b). In particular, we have shown how these interactions strengthen a hypothesized feedback that can rapidly
increase shrub cover in the Australian Alps, and potentially,
other alpine and tundra ecosystems (Higuera et al., 2008).
While high cover of tussock grass is known to inhibit the establishment of shrub seedlings (Williams & Ashton, 1987),
fire can create bare ground neccesary for shrub seedling establishment. Our results highlight that fire can increase shrub
seedling establishment by as much 33–fold. Our results also
indicate that if a shrub seedling manages to establish in a
bare ground patch, its rate of growth and survival are not
affected by tussock grass proximity, irrespective or warming.
More importantly, our findings indicate that for tall shrubs, a
1°C increase in mean growing season temperature will result
in a doubling of growth rates and a potential increase in survival. The consequence of these demographic effects will likely
manifest as shrub thickening within and at shrub community
boundaries. This in turn, will facilitate shrub establishment,
via increased seed pools, into non-shrubby communities.
Landscape flammability, and thus fire, are also likely to
increase as a result of warming effects on shrub seedling vital rates. Evidence from field studies of burning patterns
(Williams et al., 2006), quantification of fuel mass and architecture (Fraser et al., 2016) and paleoecological studies
(Higuera et al., 2008, 2009) indicate that shrubs are the most
flammable component of alpine and tundra ecosystems (significantly more than tussock grassland; Fraser et al., 2016).
In the Australian Alps, this differential flammability resulted
in approximately 60% of open heathland and only 13% of
RA
a
logistic growth rate parameter (R)
a strong positive effect of warming treatment on growth rates
and a marginally non-significant decrease in mortality (Fig.
5). However, we did not detect significant inter-tussock gap
size effects or an interaction between gap size and warming
treatment on either growth or mortality rates (i.e. coefficient
credible intervals overlap zero).
Annual probability of death
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bioRxiv preprint first posted online Mar. 16, 2016; doi: http://dx.doi.org/10.1101/043919. The copyright holder for this preprint (which was not
peer-reviewed) is the author/funder. It is made available under a CC-BY 4.0 International license.
Ecological Research Network. Funding was also received from the
Centre of Excellence for Environmental Decisions (CEED) and
Holsworth Research Committee. J.S.C was a recipient of an Australian Postgraduate Award. Monica Camac, Brad Farmilo, Monica
Hursburgh, Paulius Kviecinskas, Karen Stott, Imogen Fraser, Sarah
Bartlett, Paul Smart, Steven Rumbold, Lachlan Yourn and Chris
Jones provided assistance with field measurements. William Morris
and John Baumgartner provided coding advice and Warwick Papst
provided field logistic advice. Daniel Falster and Richard Fitzjohn
for developing software that enabled these analyses to be readily
reproducible. Special thanks to John Baumgartner, David Bowman,
Manuel Esperón-Rodríguez, Anaïs Gibert, John Morgan, Kimberley
Millers, Michaela Plein and Inka Veltheim for their comments on
this manuscript.
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FT
tussock grasslands being burnt by the extensive 2003 wildfires. Consequently, our results indicate that flammable fuel
loads will accumulate twice as fast under a warmer climate.
Ultimately this will further strengthen the feedback between
shrubs and fire by increasing the frequency and severity of
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more shrub recruitment opportunities both within and beyond
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could persist for decades (Williams et al., 2014).
While we have addressed several unknowns between climatic warming, shrubs and fire, there are others we have not
addressed that may also strengthen, weaken or break this
feedback. The most obvious mechanism that will break this
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reaching reproductive age and thereby exhausting the seedbank
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and disturbance in unpredictable ways.
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communities will become shrubbier and more flammable, the
effects of which, will have significant consequences for carbon
sequestration, water supply and biodiversity.
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ACKNOWLEDGMENTS. This research was funded through Australian Research Council Linkage Grants, partnered through the
Department of Sustainability and Environment, Parks Victoria,
Commonwealth Scientific and Industrial Research Organisation
(CSIRO). Plot infrastructure was supported by the Long Term
Camac et al.
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