Download 2001-2002 - Luquillo LTER

Luquillo LTER 2001-2002
ACTIVTIES AND FINDINGS
Project Activities and Findings
LUQ-LTER began in 1988. This report covers the period 2001-2002. Literature
cited are LUQ-LTER publications during the 2001-2002 period.
The Luquillo Long-Term Ecological Research (LUQ-LTER) program takes place
in the Luquillo Mountains of Puerto Rico. The major question of LUQ-LTER has been
how disturbance, the physical environment, and the biota interact to shape the structure
and function of a tropical ecosystem. Results help with management of ecosystem
services such as conservation of biodiversity, sequestration of carbon, and supply of fresh
water (J. Clark et al. 2001, Scatena 2001, Lugo 2002). LUQ-LTER is multi-disciplinary
and involves many researchers and students, using the tools of long-term field
experiments, short-term mechanistic experiments, monitoring of populations and
processes, and simulation modeling. Primary research locations are in 'tabonuco' forest at
El Verde Research Area and the Bisley Experimental Watersheds, in higher elevation
forests near Pico del Este, and in several streams in the Luquillo Mountains. LUQ-LTER
also participates in cross-site studies, comparing Luquillo with other LTER and tropical
forest sites.
From 1988 the focus of LUQ-LTER has been on natural disturbance and recovery
of forest and stream ecosystems. Disturbances are 'experiments', or large-scale
manipulations that can reveal mechanisms of recovery which ultimately control the
structure and function of an ecosystem. Natural disturbances during LUQ-LTER have
included two major hurricanes, landslides, droughts, and floods. Hurricane Hugo struck
in the second year of study and Hurricane Georges struck in the ninth year, providing
before-and-after perspectives on forest dynamics (changes in tree species composition
and forest structure), animal population dynamics and food webs, and ecosystem function
(energy flow and nutrient cycling).
The main question of LUQ-LTER studies on forest dynamics are: what factors determine
the current and changing distributions and abundances of tree species, the overall
diversity of tree species, and the three-dimensional vegetation structure in the tabonuco
forest? We study forest dynamics mainly in the 16-ha Luquillo Forest Dynamics Plot
(LFDP) at El Verde (Thompson et al. 2002) and in plots at Bisley. The LFDP is an
especially intensive project: all non-liana, woody stems  1.0 DBH on the plot are
identified, mapped, measured for DBH, and permanently tagged, totaling about 70,000
stems. We are conducted the third census of this plot in 2001-2002, with the help of
many students and financial support from the Andrew Mellon Foundation. This longterm, large-scale, and detailed study is needed to understand the patterns and dynamics of
tree species in a species-rich, heterogeneous tropical forest. Starting in 2001-2002, forest
dynamics on the plot are being modeled with the forest model SORTIE, as a separately
funded NSF project. A study on the effects of hurricane debris on seedling communities
showed that, broadly speaking, removing hurricane debris increased diversity and density
of woody seedlings and decreased mortality (Guzmán-Grajales, in prep.). We have also
continued in the past year to monitor vegetation changes on landslides and to carry out
experiments that explore the mechanisms of those changes (Myster 2001, Walker 2001,
Frizano et al. 2002). One experiment concerned the effects of nutrient additions.
Expectations about the responses of species in functional groups such as pioneer and nonpioneer species were not met. These results support the concept that individualistic
responses of species contribute to the promotion and maintenance of species diversity in
the tropics.
In a study of alien species in different forest types in the Luquillo Mountains,
preliminary results showed that the average number of species within the 10 x 10 plots in
the different forest types varies between 3 species in the mangrove forest to 35.44 in the
tabonuco forest (G. González, unpublished data). All forest types had high numbers
(between 88% and 100%) of native species. The amount of non-natives species are
represented by less than 3% in all forests. The number of native species fluctuates
between 22 and 35 for all the forests except the Pterocarpus and the mangrove forests
which have less than 8. The highest percentage of introduced species occurs in the
lowland moist forest with 8.5%. There were no introduced species in the palm,
Pterocarpus and mangrove forests.
The main question of LUQ-LTER animal studies is similar to that for forest
dynamics: What factors control the distributions and abundances of species and the
diversity of species? However, animals are more explicitly studied as part of terrestrial
and stream food webs, including both consumer and detritus pathways. We continue to
census animals regularly to reveal short- and long-term population changes, especially in
response to disturbance. Groups include insects (terrestrial and aquatic) (Bloch & Weiss
2002), birds, snails, frogs, bats (Jones et al. 2001) and shrimp in streams. For several
species we have exceptionally long-term records. Several terrestrial groups are
repeatedly censused at designated points on the LFDP, which puts the animal data into a
data-rich habitat context. Food web analysis is based on observation and experiment. In
2000-2001 we conducted terrestrial food web experiments by adding or excluding
animals to microcosms created with large cages in the forest (Beard 2001). Similarly,
stream food webs were studied using removal experiments in natural streams, and in
artificial streams in work funded by a separate NSF grant. Another grant is funded work
on herbivory by canopy insects and the effects of resulting frass on soil processes. Food
webs are paths of energy flow and nutrient cycling, linking LUQ-LTER plant, animal,
and ecosystem studies (Kilham & Pringle 2001, Covich & Crowl 2002).
The general question of LUQ-LTER ecosystem studies is: how do disturbance,
physical environment, and biota interact to control energy flow and nutrient cycling? The
varied disturbance regime; gradients in climate, soil, and topography; and high species
diversity make the Luquillo Mountains an appropriate setting for this question. A
foundation of this ecosystem work is long-term monitoring in LUQ-LTER of vegetation
biomass, climate, and biogeochemical flux in rain, throughfall, and stream water along
environmental gradients in the mountains.
Building on this foundation, in 2000-2001 LUQ-LTER studied long-term patterns in
litterfall productivity, tree growth, and decomposition in relation to climate (González
and Seastedt 2001, González et al. 2001) and nutrient and plant community
characteristics at sites along an elevational gradient (Cox et al. 2002, Pett-Ridge & silver
2002f). Variables of special interest were soil oxygen availability (typically low at high
elevations) and deposition of 'green' litter (leaves stripped from trees during a storm). We
studied the effects of green litter using reciprocal litter bag experiments to distinguish
litter versus site effects. A new topic in our research is the newly understood nitrogen
cycling pathway: dissimilatory nitrate reduction to ammonium (Silver et al. 2001).
Continuing topics were fungi and decomposition (Cantrell et al. 2001 a,b, Groffman et al.
2001, Lodge 2001a, b, Myster 2002); earthworms (Zou & González 2001, Lachnicht et
al. 2002, Liu & Zou 2002) and shrimps, important components of the decomposer guilds
in terrestrial and steam systems (Crowl et al. 2001, March et al. 2001, Peterson et al.
2001); controls on dissolved organic carbon and nitrogen fluxes in streams (McDowell
2001, Mulholland et al. 2001,); flood effects on litter inputs and stream organisms; and
residence time of coarse woody debris in streams (Crowl et al. 2002). Detrital
processing rates and export were compared among four streams within the LUQ that
differ in decapod abundance and community composition. These analyses show that
long-term detrital dynamics are significantly affected by the species composition and
abundance of the dominant shrimp species in these streams (Crowl et al. 2001). For a
comparative perspective on how biodiversity and climate affect decomposition and soil
processes LUQ-LTER participated in the LIDET cross-site studies (McDowell 2002.
LUQ-LTER also participated in global syntheses of primary production (D. Clark et al.
2001a, b).
Another project, sponsored by the both USFS and LUQ-LTER program, made
extensive use of the Luquillo LTER data base and demonstrated how deforestation of the
coastal plain can affect cloud and rainfall inputs to the upper Luquillo Mountains (Van
der Molem, in prep.).
During 2001-2002 LTER-LUQ progressed toward synthesis of ecosystem studies
at a landscape scale by developing models of spatial and temporal variation in forest
productivity, soil carbon, and nitrogen. We used geo-spatial statistical techniques to
examine the spatial variation and relation of soil organic carbon and soil moisture, and
we used TOPOPROD, a spatially explicit forest productivity model, to simulate spatial
patterns of gross primary production, net primary production, and respiration (Wang et al.
2002a, b).
Beginning in 1994 LUQ-LTER focused more on human disturbance, including,
among other things, logging, clearing for pasture, farming, and water diversion from
streams (López del Mar et al. 2001, Lugo et al. 2001, Zimmerman 2001). Some the
resulting impacts are apparent in a recent analysis of tree species composition in the
LFDP, where most land uses ceased c. 70 yr ago (Thompson et al. 2002). This past year
we also studied patterns in above and belowground C pools over 65 yr of forest regrowth,
the impacts of N saturation on tropical ecosystems, and the effects of water diversion on
stream organisms (Pringle 2001 a,b,c). The multiplicity of disturbance types and
frequencies, natural and human, in the Luquillo Mountains has increased appreciation of
interactions between disturbances.
Training and Development
One post-doctoral associate, John Barone, was supported by this project. He conducted a
study of plant community variation along the elevational gradient in the Luquillo
Mountains.
Eight graduate students at the University of Puerto Rico and mainland institutions were
supported by the project.
During the summers, 10 students conducted research in the Research Experience for
Undergraduate Program.
Ten teachers from six high schools participated in the Schoolyard LTER program.
During the year they attended workshops on measuring forest dynamics and sampling
and analysis of soils.
Outreach
In both 2001 and 2002 LUQ-LTER organized a symposium at the University of
Puerto Rico entitled “Long-Term Ecological Research in Puerto Rico”. About 120
scientists and students interested in ecological issues attended each year.
This year, the publication of the Science Teachers Association of Puerto Rico,
Acta Cientifica, published a special issue of research results from high schools in the
Schoolyard LTER Program. The special issue included seven articles with research
results ranging from stand nutrient cycling in secondary forests to stand dynamics in
mangrove forests.
People from John Thomlinson's lab have led a reforestation project on the island
of Vieques. This project, Sembrando Paz, had a major accomplishment in April when the
initial planting of over 100 trees was conducted on Monte Carmelo during the Earth Day
weekend. The activity was attended by local citizens, a state senator, Viequense
environmental activists, university and high school students, representatives of municipal
and state agencies, and members of the press. The project continues, having gained the
support of the PR Department of Natural and Environmental Resources.
With funding from USDA Forest Service (State and Private Forestry), we made
two environmental outreach posters on water quality and watershed protection for the
Luquillo Experimental Forest that are passed out to school children and sold at the Forest
Service's Visitors Center.
CONTRIBUTIONS
Contributions within discipline
The principal discipline of the Luquillo Long-Term Ecological Research (LUQLTER) program, in the Luquillo Mountains of Puerto Rico, is life sciences. More
specifically, the disciplines are: population biology and ecosystem ecology. Population
biology is broadly defined as the study of the distribution and abundance of organisms.
Ecosystem ecology is defined as study of the flow of energy and cycling of nutrients in
an environment. Our work in these areas has developed new information as well as new
methods in these fields. Most of these contributions result from work over many years.
In this report we describe this long-term progress, while highlighting contributions from
2000-2001.
Population Biology: Contributions
LUQ-LTER has made many contributions to population biology, in two main
areas. First, we have demonstrated the pervasive effects of disturbance (a discrete event
that disrupts ecosystem, community, or population structure and alters the environment)
on the distribution and abundance of organisms. Second, we have described and
experimented on the food web (array of feeding relationships among organisms) of an
ecosystem. Understanding the effects of disturbance is a key to managing any ecosystem
in the face of human impacts. Understanding food web relations is a key to managing
any species population.
The disturbance regime refers to the size, intensity, and frequency of different
types of disturbance that occur over a particular landscape. At our site we have
developed an unusually good record of this regime, which includes: hurricanes and lesser
windstorms, treefalls not associated with storms, floods, droughts, and landslides, as well
as several types of human-caused disturbance. In the past year we have increased our
understanding of the spatial variation of human impacts on streams, including dams,
water diversion, and fishing.
LUQ-LTER has amassed a detailed record of the population biology response to
disturbance, based on both observation and experiment. Among other things, we have
described mechanisms of primary succession (development of vegetation in a completely
unvegetated site) on landslides, leading to a better understanding of the many factors that
control this process and that could be used for forest restoration. In 2000-2001 we
experimentally removed some colonizing plant species from landslides, to determine if
those species were facilitating or inhibiting colonization by other species and affecting
overall vegetation development. To test methods for enhancing vegetation development
we added artificial bird perches to landslides, to see if perches increase the number of
seeds dispersed to landslides by birds, and we added various kinds of organic matter to
possibly stimulate growth.
In other studies of response to disturbance LUQ-LTER has helped pioneer
methods and models that can be used elsewhere. To describe changes in tree populations,
we established a large-scale (16 ha), long-term, intensive study plot in the forest, linking
our work to a network of such plots in the tropics coordinated by the Smithsonian
Institution. Our plot is the largest, long-term, intensive forest dynamics plot in the
Caribbean, and its disturbance regime, including hurricanes and well-known historical
land uses, make it unique in the Smithsonian plot network. In 2001 we continued our
third inventory of the plot, adding to our understanding of impacts from both natural and
human disturbance. This inventory is of special interest because it follows the second
hurricane in 10 years on the site and thus may help us predict the possible consequences
of the predicted increase of hurricane frequency with global warming. To understand
mechanisms of changes in tree populations, in the past year we began using the SORTIE
forest dynamics model. This promotes comparisons with other forests, since the model
has also be used in Connecticut and British Columbia.
The food web of an ecosystem is the total array of feeding relationships among
organisms. Since feeding has an obvious impact on survival and reproduction of
organisms, understanding feeding patterns gives insight into control of a species'
population, and, taking all species into account, insight into control of the distribution and
abundance of species in an ecological community. Last year saw completion partly under
LUQ-LTER auspices of a landscape-level analysis of fruit availability for the endangered
Puerto Rican Parrot.
This array of feeding relationships making up a food web can be complex,
especially in species-rich tropical systems. The web is composed of producers (plants),
consumers (animals that consume live or freshly killed organisms), and detritivores
(consumers of dead organisms), all in cross-linked chains. Based on long-term and
detailed field studies LUQ-LTER has constructed the most nearly complete picture of a
food web for any tropical forest (The Food Web of a Tropical Rain Forest, Douglas
Reagan & Robert Waide, editors) and thus provided a model and standard for such
studies elsewhere. We are building on this description of food web patterns with
experimental approaches to elucidate mechanisms that produce the patterns. One set of
experiments completed in the past year used cages to exclude or increase the density of
different consumers, thereby revealing their impact on the ecosystem. Another set of
experiments being planned in 2001 involves additions and subtractions of detritivores and
organic matter to reveal their respective roles in nutrient cycling, a subject of ecosystem
ecology. Indeed, both disturbance and food web dynamics, important in population
biology, relate strongly to ecosystem ecology, because organisms, nutrients, and energy
all interact.
LUQ-LTER scientists also presented overviews and syntheses of biological
diversity (Willig 2002a, b).
Ecosystem Ecology: Contributions
An ecosystem is all the interacting parts of the physical and biological worlds;
however, ecosystem ecology is usually considered to be the study of energy flow and
nutrient cycling in an ecosystem. Rates of ecosystem processes and storage of nutrients
in different parts of an ecosystem depend on physical conditions (e.g., climate, soil),
species composition (because species differ in nutrient and energy use), and disturbance
(which redistributes nutrients and energy). The results of ecosystem ecology help us
maintain 'ecological services' that are valued by society, such as conservation of
biodiversity, sequestration of carbon, and supply of fresh water.
As mentioned, the LUQ-LTER site has suffered two severe hurricanes, allowing
us to compare ecosystem states before and after a large-scale, major disturbance.
Hurricanes are a large-scale experiment that reveals the function of an ecosystem and that
would be impossible to carry out by design. Hurricanes redistribute large amounts of
biomass and nutrients (volumes of downed leaves and wood) and energy (light
penetrating the damaged forest). A striking previous result was how efficiently some
plants responded to the changes with growth that restored above-ground biomass and
prevented loss of nutrients. The third inventory of the forest dynamics plot, underway in
2001, as mentioned above, will allow us to compare some of these patterns through two
hurricanes.
Moving over the landscape, physical conditions, species composition, and
disturbance regime all change along the elevational gradient in the Luquillo Mountains.
LUQ-LTER studies along this gradient in the past year have begun to reveal how these
factors control ecosystem processes. We found, for example, that litterfall (production of
dead plant material), and thus productivity, decreases with elevation and is correlated
with a small decrease in temperature. Control of productivity by temperature has clear
implications for ecosystems that may be affected by global warming .
An important topic of LUQ-LTER research has been the flux of carbon (C) and
soil organic matter (SOM). Work in the past year in a secondary forest at our site showed
that soil C accumulated rapidly and was not affected by species composition, while
aboveground C accumulation was highly variable. Previously we used the CENTURY
model to describe disturbance effects on soil organic matter (SOM). SOM affects forest
recovery and productivity and has implications for the global carbon budget and thus
global warming. We recently developed a spatial version of CENTURY for the entire
Luquillo Mountains area to test how variation in climate, vegetation, and soils affects
SOM. Further, we then used the TOPOPROD to model gross and net primary
productivity on the gradient (Wang et al. 2002a, b). Thus, climate appears to strongly
influence, directly or indirectly, the way different communities in the LEF are able to
respond to a disturbance such as a hurricane.
Another issue in global change is elevated nitrogen (N) addition to ecosystems,
which can drastically alter productivity. Ecosystems process N in several ways. This
past year LUQ-LTER began work on dissimilatory nitrate reduction to ammonium
(DNRA) in upland forest soils, a process that is only recently been understood. We
found that DNRA in Puerto Rico is about five times greater than the better understood
process of denitrification. Also in the past year we continued participation in
comparative studies of N cycling in streams among LTER sites. This has shown that the
smallest streams are the most effective at retaining and processing N inputs. Work on
stream biota in 2000-2001 showed that nutrient losses in streams are related to the
presence of certain kinds of shrimp, which consumers detrital input from the forest. A
continuing contribution of LUQ-LTER are studies linking terrestrial and stream
environments.
Among LUQ-LTER ecosystem studies another area of comparative work that
advanced over the past year was review and synthesis of studies on the relationship
between biodiversity and net primary productivity. Given declines in biodiversity in
some habitats, this relationship is important to the human economy.
Contributions to other disciplines
Outside the life sciences LUQ-LTER has contributed in the past year to an
understanding of the economics and ecosystem services of the Luquillo Mountains
(Scatena et al. 2002), ethnobotany (Vogt et al. 2002), and the development of methods of
data acquisition and management(Meléndez-Colom & Baker. 2002). At our study site we
established and maintained, on an experimental basis, climatic monitoring devices that
transmit data by radio to central data banks. This wireless technology could greatly
improve quality and quantity of available field data in environmental biology.
There is also a need to develop ways of managing and synthesizing the increased
amount of data being collected (Meléndez-Colom & Baker. 2002). The cross-site
database on biodiversity and net primary productivity mentioned above is being used to
test new ways to manage ecological data. This work is LUQ-LTER's contribution to a
project undertaken by the LTER Network Office, the National Center for Ecological
Analysis and Synthesis, and the San Diego Supercomputer Center.
Contribution to human resource development
The University of Puerto Rico is the largest Hispanic institution of higher learning
in the U.S. system. Our project has created a magnet drawing Hispanic and
undergraduate students to study and conduct research in environmental sciences.
Through the Schoolyard LTER program, we expose local high school teachers and
students to ecological research.
In collaboration with Wheeling Jesuit University we continued developing
software that allows middle school students to learn tropical ecology using our research
program as an example &&& web site). Initial development of this project was
conducted using SGER funding of which the final report can be found in this reporting
system (proposal ESI-9908438).
Contributions to Resources for Research and Education
El Verde Field Station is one of the principal field sites for the project. The staff
and facilities maintenance are supported by matching funds to the LUQ-LTER grant from
the University of Puerto Rico. In addition to LUQ-LTER research, the station supports
student learning (field trips) as well as a location for research by investigators and
students from both Puerto Rican and mainland institutions.
Our web site is frequently used by educational institutions as a source of
information on tropical ecology. Our site in included as a link on many other
educationally-oriented web pages.
The analytical laboratory of the International Institute for Tropical Forestry
(USDA Forest Service) provides free analyses to undergraduate and graduate students
performing research projects in the Luquillo Experimental Forest.
Contributions Beyond Science and Engineering
LUQ-LTER studies of dams and water withdrawals on stream organisms by this
project caused the Puerto Rico Water Authority to modify the practices they use in
acquiring drinking water from local streams.