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Studies Research Theme 1: Hypothesis A
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METHODS
Research Theme 1: Cumulative effects of Hurricane Hugo and Georges and Legacies of Human Disturbance
"Census of the Luquillo Forest Dynamics Plot (LFDP)" Jill Thompson
The census will follow standard Center for Tropical Forest Science (CTFS) methods for the international network of large forest plots (Condit 1998). All free standing woody stems ?1 cm D130 will be measured for diameter, identified and mapped to 0.5 m within each 5 m x 5 m subplot in the 16 ha Forest Dynamics Plot. Our method is slightly modified as we tag all stems on multiple stemmed individuals while CTFS only requires one tag per individual woody plant. All stems tagged in the previous censuses will be located by tag number, and all new stems which have grown since the last census will be tagged with an aluminum number tag. Data on stem and species survival, mortality, growth and recruitment will be calculated in relation to soil type, previous land use history and the distribution of damage from Hurricanes Hugo and Georges.
In 150, 1 m x 2 m seedling plots in the LFDP all seedlings with stems ?10 cm in height and ? 1 cm D130 will be tagged and measured for height and root collar diameter. The growth, mortality and survival of these seedlings will be assessed in relation to parent tree distribution, soil type, previous land use history and the distribution of damage from Hurricanes Hugo and Georges.
"The reforestation of tropical pastures will lead to an increase in surface soil carbon derived from forest vegetation, but this will be offset by a loss of pasture soil carbon from deeper soil depths. Over time, the amount of soil carbon lost from the previous land use will approximate the amount of soil carbon gained through reforestation." Whendee Silver, Ariel Lugo.
Methods will follow Silver et al (in review) who estimated the rate of C accumulated following reforestation at one site in the LEF. We will use two sets of chronosequences of secondary forest recovering from pasture and current pasture within and near the LEF, where the land use history, plant species composition, and aboveground biomass have been well documented (Aide et al. 1995, Zimmerman et al. 1995). Eight sites ranging from 1 to 80 years since abandonment will be selected along each chronosequence, and paired with a pasture of approximately the same age. At each site, we will excavate five 0.5 x 0.5 x 1 m depth soil pits and collected two quantitative soil cores at 10 cm depth increments. One core will be used to estimate bulk density and the second will be used for total C and C isotope analyses.
Cores for bulk density will be dried at 105 oC and weighed to determine mass per unit volume. Soils for C analyses will be ground to a fine powder. Previous analyses with 5 % HCl have indicated no evidence of carbonates in this area. Triplicate 45 mg subsamples will analyzed for d 13C on an Europa 2020 continuous-flow mass spectrometer at U.C. Berkeley. The C isotope ratio will be expressed in d units relative to a PDB standard such that the d 13C = (Rsample – Rstandard) x 1000.
We will estimate the proportion of C3- and C4-C in the soil using a modified version of the standard mixing equation proposed by Vitorello et al. (1989):
Equation 2: % C4 = (d - d L / d g - d L)*100
% C3 = 100 - % C4
Where d = the d 13C of the soil sample, d L is the d 13C of a composite sample of forest litter and roots, and d g is a composite sample of pasture grass. The percentages of C3- and C4-C will then be multiplied by the total C pool to estimate the proportion of C derived from the forest or pasture by depth. Values will be corrected for bulk density by depth (Veldkamp 1994). We will estimate a net rate of C accumulation or loss following reforestation as the difference in the C pools in the reforested sites by depth and the adjacent pasture.
Research Theme 2: The Interactive Roles of Disturbances and the Biota in Controlling Community and Ecosystem Processes (e.g., Nutrient Availability and Organic Matter Processing)
"Consumer populatins in tabonuco forest are limited by predation. Exclusion of predators will have cascading effects on the abundance of detritivores and rates of decomposition." Paul Klawinski
Because rates of litter decomposition can be significantly increased by the presence of micro- and macro-arthropods (Heneghan et al. 1998, 1999), we propose to examine the cascading effect of top predators on arthropod detritivores and rates of leaf litter decomposition. We have established 32 exclosures, 3.3 m in length, width and approximate height, which have been distributed over eight spatial blocks in tabonuco forest with a history of little human disturbance. Each exclosure is covered with 1 cm square plastic mesh which preliminary studies showed excluded all Anolis lizards and Eleutherodactylus frogs except juvenile Eleutherodactylus size classes. Each block contains an open control (to test for the effect of added understory structure caused by the exclosures) and four exclosures: Closed Control (netted exclosure with field densities of Anolis and Eleutherodactylus); Anolis Exclusion (field densities of Eleutherodactylus; no Anolis); Eleutherodactylus Exclusion (field densities of Anolis; no Eleutherodactylus); Total Exclusion (neither Anolis nor Eleutherodactylus). We have used these exclosures to examine the effect of frog and lizard predation on arthropod herbivores and the resulting rates of herbivory and propose to continue to use these exclosures after the herbivory experiment is completed in order to examine the effect of frog and lizard predation on arthropod detritivores and rates of litter decomposition. We are currently monitoring flying, foliar and litter arthropods within these exclosures as well as measuring rates of herbivory. Also, as leaf litter accumulates on the roof of the exclosures, it is removed (every 2 weeks) and evenly spread in the exclosures. When we shift the focus of these experiments to studies of decomposition, we will discontinue flying and foliar arthropod monitoring and will begin weighing the amount of litter collected on the roof prior to placing it in the exclosure. At regular time intervals, random quadrats of litter (0.25 m2) will be collected (each quadrat sampled only once) from within each exclosure, placed in Tullgren funnels for litter arthropod extraction, dried to constant mass, weighed, and analyzed for nutrient content. Leaf litter decomposition will be measured through the use of leaf litter decomposition bags. These bags will be constructed of 2.5 cm plastic mesh to allow the entrance of macroarthropods and will be placed at the litter/humus interface on 1 mm nylon mesh sheets which will effectively capture decomposed fragments of litter. Each litter bag will contain 5 g of air dried tabonuco (Dacryoides excelsa) leaves. Twelve litter bags will be distributed in each plot (480 bags total) and one bag will be collected from every plot each month, be placed in Tullgren funnels for arthropod extraction, oven dried, weighed, ground and analyzed for nutrient content. All arthropods will be identified to species and counted to test for the effects of predation regime on arthropodd abundance and the effects of arthropod abundance on rates of decomposition. This experiment will give us information on the relative effects of both frog and lizard predation (separate and combined) on litter arthropod communites and what effect this, in turn, has on litter decomposition.
Because Anolis lizards seldom forage in leaf litter (Reagan 1996), we expect that Eleutherodactylus frogs will be the dominant predator in leaf litter communities, especially juvenile size classes which primarily occupy the litter layers (Stewart and Woolbright 1996). Therefore, we propose a second experiment that will specifically examine the effect of juvenile frogs on litter arthropods and rates of litter decompostion. This experiment will involve smaller exclosures (2 m x 1 m) which will be covered in 1 mm nylon mesh which should effectively control the movements of even juvenile frogs. Because the mesh will also control the movement of many litter arthropods, the plots will be made long and thin as well as relatively low to the ground (ca. 1 m in height) such that the entire plot can be sampled from the outside of the plot. Each exclosure will have a removable roof which is also covered in 1 mm nylon mesh. We propose three treatments: Control (exclosures containing field densities of juvenile frogs (3 frogs/m2); Frog Exclusion (litter hand sifted and frogs removed); Total Exclusion (frogs removed and arthropods repelled by the addition of naphthalene). These treatments will allow us to quantify the rate of decomposition in the presence of frogs (low arthropod abundance), absence of frogs (high arthropod abundance) and in the absence of frogs and arthropods (decomposition due to microbial activity). As above, litter collecting on the surface of the roof will be removed, weighed, and spread within each plot. Arthropod abundance will be assessed as above using small quadrats of leaf litter extracted with Tullgren funnels. These litter samples will be returned to the plots after extraction due to the small size of the plots being used. Litter decomposition will also be assessed using leaf litter decomposition bags as described above (12 bags/exclosure; 360 total). Plots will be arranged in 10 blocks on undisturbed ridge tops as previous data on litter arthropod communities has revealed small-scale spatial variation which will be assessable through spatial blocking.
"Earthworms improve phosphorus availability in highly weathered soils through increasing the solubility of inorganic phosphorus and accelerating the mineralization of organic phosphorus. Experimental removal of earthworms from tropical pastures will reduce soil phosphorus availability more than in tabonuco forest because of differences in earthworm abundances between the two habitats." Xiaoming Zou
Soil samples will be collected from each plot at the depth of 0-10, 10-25, 25-50 cm. Each sample will be analyzed for resin extractable organic and inorganic phosphorus and the potential rates of phosphorus transformations. The resin extractable inorganic and organic phosphorus will follow the procedures proposed in the LTER standard soil method book (Lajtha et al. 1999).
Potential rates of phosphorus transformations will be estimated using the irradiation-autoclaving-incubation procedures developed by Zou et al. 1992 and 1995. Three treatments will be applied to each fresh soil sample: control, irradiation, and irradiation plus autoclaving. Soils will then be incubated with resin bags under aerobic conditions. The control treatment allows for the occurrence of three processes: net inorganic P solubilization, microbial immobilization of orthophosphate, and the mineralization of organic phosphorus. The irradiation treatment will kill microbes, thus terminate microbial immobilization process. The irradiation plus autoclaving treatment will kill microbes and denature phosphatase enzymes, thus terminate both microbial immobilization and organic P mineralization processes. Difference in resin extractable P after the incubation among the three treatments will give estimates of net mineral P solubilization, microbial P immobilization, and the mineralization of organic P.
Research Theme 3: The Influence of Climate and Physical Constraints on the Distribution and Abundance of Organisms and Related Ecosystem Processes in the Luquillo Mountains
Hypothesis 3A:
"Top Down Versus Bottom-Up Control
of Food Web Dynamics" D. Jean Lodge
Workplan - Given that most habitats are spatially patchy, pools along the same reach often differ in resources and predators. These differences may influence energy flow within headwater streams. Some pool habitats are likely to have higher primary and secondary productivity than others. Since the recent impact of Hurricane Georges on Puerto Rico in September, 1998, we have identified some large, persistent riparian gaps that allow us to test new hypotheses regarding both detrital and algal resource enhancement and predator control (sensu Osenberg and Mittelbach 1996). In some pools, the riparian canopy was only slightly damaged (or quicky grew back) so that shading has persisted and algal growth has been minimal. In other pools there have been major gaps opened by tree falls and tree mortality so that algal growth increased rapidly in response to intense light (and an initial pulse of nutrients from leaf leachates). We can manipulate the presence or absence of different predators and nutrients in these pools and comprehensively determine the relative importance of top-down or bottom-up regulation. Because of the large-scale disturbances from the recent hurricane, we are in a position to test hypotheses regarding individual-, population- and community-level responses. These predictions are based both on current theory regarding top-down versus bottom-up control in aquatic communities and on observations following Hurricane Hugo in 1989 (Covich et al. 1991; 1996; 1998, Crowl et al., In review, Johnson et al. 1998).
To investigate the interactions between
fish, shrimp, insect larvae, and algal production and detrital loading,
we will perform an in situ experiment in the Bisley watershed. A two by
two cross-classified design will be employed in which fish and shrimp presence
and absence will be manipulated in three replicate pools. Replicate stream
pools will receive: no shrimp or fish; shrimp (5 m-2) and no
fish; fish (2 m-2) and no shrimp; and fish and shrimp (2 and
5 m-2 respectively). Fish and shrimp densities are based on
pre-hurricane densities and will be maintained by fencing off the pools
(using 5 mm mesh plastic screens) to prevent migration. Tiles will placed
into the pools and will be harvested weekly over a 3-6 week period for
algal and benthic organic matter standing crop and amounts of carbon and
nitrogen in benthic organic matter (BOM). The quality and quantity of BOM
will be compared to pre-hurricane levels (Pringle et al. 2000). Drift nets
and Surber samplers will be used weekly to measure the responses of mayflies
and caddisflies in all pools. Monitoring of nutrient pools (dissolved organic
carbon, nitrogen and phosphorus) have been monitored weekly in this stream
and will be continued. All response variables will be analyzed using a
repeated measures, multiple ANOVA model. We will also include a nested
experiment in which we use electrified hoops to exclude shrimp and/or fishes
from small patches within the pools. This will allow us to determine the
spatial extent of the shrimp/fish effects and more directly test the hypothesis
that shrimp grazing is the primary cause of algal and detrital BOM decreases
in the absence of fish.
References cited
Carpenter, S.R. and J.F. Kitchell. 1988. Consumer control of lake productivity. BioScience 38: 764-769.
Covich, A.P. and W.H. McDowell. 1996. The stream community. Pp.433-459, IN: D.P. Reagan and R.B. Waide (eds.), The Food Web of a Tropical Rain Forest. University of Chicago Press, Chicago.
Covich, A.P., T.A. Crowl, S.L. Johnson, and D.L. Certain. 1991. Post-Hurricane Hugo increases in atyid shrimp abundances in a Puerto Rico montane stream. Biotropica 23: 448-454.
Covich, A.P., T.A. Crowl, S.L. Johnson, and M. Pyron. 1996. Distribution and abundance of tropical freshwater shrimp along a stream corridor: response to disturbance. Biotropica 28: 484-492.
Covich, A.P. , M.A. Palmer, and T.A. Crowl.
In press. The role of benthic invertebrate species in freshwater ecosystems.
BioScience 49.
Covich, A.P., T.A. Crowl, and F. N. Scatena.
In press. Linking habitat stability to floods and droughts: effects on
shrimp in montane streams, Puerto Rico. Verhandlungen der Internationale
Vereinigung fur Theoretische und Angewandte Limnologie 27.
Crowl, T.A., C.R. Townsend, N. Bouwes, and H. Thomas. 1997. Scales and causes of patchiness in stream invertebrate assemblages: top-down predator effects? Journal of the North American Benthological Society. 16:277-285.
Crowl, T.A., McDowell, W.H., A.P. Covich and S.L. Johnson. In review. Species-specific effects of freshwater shrimp on detrital processing and localized nutrient dynamics in a montane tropical rain forest stream. Ecology.
Cummins, K.W., C.E. Cushing, and G.W. Minshall. 1995. An overview of stream ecosystems. Pp.1-8, IN: C.E. Cushing, K.W. Cummins and G.W. Minshall (eds.), River and Stream Ecosystems. Ecosystems of the World, vol. 22. Elsevier, Amsterdam.
de la Rosa, C.L. 1995. Middle American streams and rivers. Pp.189-218, IN: C.E. Cushing, K.W. Cummins, and G.W. Minshall (eds.), River and Stream Ecosystems. Ecosystems of the World, vol. 22. Elsevier, Amsterdam.
Friberg, N. and M.J. Winterbourn. 1996.
Interactions between riparian leaves and algal/microbial activity in streams.
Hydrobiologia 341: 51-56.
Johnson, S.L., A.P. Covich, T.A. Crowl,
A. Estrada-Pinto, J. Bithorn, and W.A. Wurtsbaugh 1998. Do seasonality
and disturbance influence reproduction in freshwater atyid shrimp in headwater
streams, Puerto Rico? Verhandlungen der Internationale Vereinigung fur
Theorestiche und Angewandte Limnologie 26: 2076-2081.
Hershey, A.E. and B. J. Peterson. 1996. Stream food webs. Pp.511-530, IN: F.R. Hauer and G.A. Lamberti (eds.), Methods in Stream Ecology, Academic Press, San Diego.
Hildrew, A.G. 1992. Food webs and species interactions. Pp.309-330, IN: P. Calow and G.E. Petts (eds.), The Rivers Handbook, vol. 1. Blackwell, Oxford.
Hildrew, A.G. and P.S. Giller. 1994. Patchiness, species interactions and disturbance in the stream benthos. Pp.21-62, IN: P.S. Giller, A.G. Hildrew, and D.G. Raffaelli (eds.), Aquatic Ecology: Scale, Pattern and Process, Blackwell, Oxford.
Wallace, J.B. and J.R. Webster. 1996. The role of macroinvertebrates in stream ecosystem function. Annual Review of Ecology and Systematics 27: 567-594.
"Consumer populations in tabonuco forest
are limited by predation. Increases of pivotal predators will affect productivity
and biomass of lower trophic levels." Robert Waide, Michael Willig
Workplan - The food web in tabonuco forest has four trophic
levels and is characterized by the absence of large consumers and predators
(Reagan and Waide 1996). The top predators are birds which consume lizards
and frogs, which have the greatest animal biomass. An experiment using
predator exclosures that was initiated at the end of the last LTER cycle
will be completed. The exclosure experiments had been dropped because of
flat funding in the LTER budget but were initiated recently using other
funds. These exclosures were designed to include or exclude coqui frogs
and anolis lizards, which are thought to play a pivotal role in controlling
herbivory as well as nutrient mineralization by detritivores (primarily
fungus gnats). While nitrogen is generally abundant in undisturbed tabonuco
forest, immobilization of N by microbial biomass following Hurricane Hugo
(Zimmerman et al. 1995) suggests that frogs could play a pivotal role in
controlling N-avialability after disturbance. New long-term experiments
on predator augmentation will be initiated during this funding cycle.
"Variation in climate as a function of elevation in the Luquillo Mountains affects patterns of soil carbon via effects on soil processes" Charles A.S. Hall, Hongqing Wang and Wei Wu (SUNY-ESF, Syracuse NY 13210)
Further development of simulation models:
We have built a spatially explicit tropical ecosystem model base on
the FOREST-BGC (Running and Coughlan 1988) and the CENTURY (Parton et al.
1987, 1988) ecosystem models for the Luquillo Experimental Forest (Marley
1998, Wang et al. Submitted). Our model is process-based and is already
a good predictor of spatial pattern of photosynthesis and soil carbon in
the Luquillo Mountains. What we need to do is to examine the spatial
reliability of the model. We need spatially selected field data to
calibrate and validate the model. For simulation of photosynthesis,
we need to measure the physiological response of vegetation (e.g.. stomatal
conductance) to changing environmental gradients over the Luquillo Mountains.
For soil carbon pools, we will develop techniques for direct measurement
of soil carbon pool sizes in order to initialize each pool, monitor the
fluxes of carbon through each pool and then validate simulations.
We will incorporate soil oxygen concentration and biogeochemical cycling
into the model structure as determinants of soil decomposition and mineralization
rates. Meanwhile, we will incorporate models of hurricane influence
on landscapes in the Luquillo Mountains, e.g. EXPOS, HURRECON models (Boose
et al. 1994) and RECOVER model (Everham, 1996) to simulate the long term
hurricane impacts on the spatial and temporal patterns of soil carbon over
the Luquillo Mountains. Finally, we will combine GIS tools and graphic
techniques into our simulation to determine and display the scale and degree
of the spatial and temporal dynamics of carbon cycling in this tropical
forest ecosystem.
Model validation
a) Soil sampling and field measurements
We will collect soil samples from different depths (0-10, 10-30, 30-100
cm) from 7 randomly selected sites along an elevation gradient over the
entire Luquillo Mountains. At or near each elevation site, we will
select 4 subsites that represent topographic gradients (i.e. ridge, slope,
upland valley and riparian valley in this tropical mountain area) (Garcia-Montiel
and Scatena, 1994; Scatena and Lugo, 1995). For representative purposes,
we will mix samples at each site. In order to examine the variability at
each site due to laboratory analysis, quality control (i.e. duplication
of samples and blanks) will be conducted (Anderson and Ingram 1989).
We will obtain monthly air and soil temperature and rainfall data from
nearby climatic stations or by field measurements.
b) Field measurement of photosynthetic rates
We will select 10 trees for 3 dominant species at each sampling site
to measure photosynthetic rates using Lci (the Ultra Compact Photosynthesis
Measurement System), ADC2250 Advance Gas Exchange Management System and
DEX Electronic Dendrometers (www.dynamax.com). Measurements will
be conducted on a monthly basis.
c) Soil chemical analysis:
Total soil organic carbon (SOC) will be determined on air-dried soils
using the Modified Walkly-Black method (Anderson and Ingram, 1989).
Microbial biomass carbon will be determined by the CHCl3 fumigation-direct
extraction method (Motavalli et al., 1994; Vance et al., 1987). Carbon
contained in a 0.5 M K2SO4 extract of unfumigated soils is considered a
measure of soluble carbon. The sum of microbial biomass carbon plus
soluble carbon will be treated as an estimate of the active carbon pool.
The slow carbon pool will be determined using the suspension method (Motavalli
et al., 1994). The passive carbon pool is calculated as the total
organic carbon minus the active and slow carbon pools. Monthly soil
oxygen concentration will be measured using an oxygen electrode (Farrell
et al., 1993). Measurements of fluxes of CO2 and CH4 will be measured
monthly using plastic chamber method (Steudler et al. 1991). In the
meantime, we will analyze for gravimetric moisture content (g H2O/ 100
g dry soil), bulk density (g dry soil/ cm ^3), soil pH (using pH meter)
and exchangeable cations (Anderson and Ingram, 1989).
d) Statistical analysis of data:
We will apply spatial statistical analysis technique (Griffith and
Layne 1999) to determine the spatial dependence of soil carbon storage
and fluxes and the spatial relation between soil carbon and other soil
physical and chemical properties in order to make precise prediction of
patterns in soil carbon dynamics. In addition, we will use conventional
statistical techniques (e.g. SAS) to examine the basic statistical features
of variables used in our simulations.
References:
Anderson, J.M., and J.S.I.Ingram (Ed.). Tropical soil biology and fertility:
a handbook of methods. 1989.
C.A.B International.
Boose, E.R., D.R.Foster, and M.Fluet. 1994. Hurricane impacts to tropical
and temperate forest landscapes.
Ecological Monographs 64: 369-400.
Everham, E.M. 1996. Hurricane disturbance and recovery: An empirical
and simulation study of vegetation
dynamics in the Luquillo Experimental Forest, Puerto Rico. Ph.D. Dissertation
of the College of
Environmental Science and Forestry, State University of New York, Syracuse
NY.
Farrell, R.E., J.A.Elliott, and E. de Jong. 1993. Soil Air. In: Carter,
M.R. (Ed.): Soil sampling and methods
of analysis. Lewis Publishers.
Garcia-Montiel, D.C., and F.N. Scatena. 1994. The effect of human
activity on the structure and
composition of a tropical forest in Puerto Rico. Forest Ecology
and Management 63:57-78.
Griffith, D.A., and L.J.Layne. 1999. A casebook for spatial statistical
data analysis. Oxford University
Press.
Marley, D.P. 1996. Spatial modeling of climate and photosynthesis in
the Luquillo Mountains, Puerto Rico.
MS thesis of State University of New York, College of Environmental
Science and Forestry,
Syracuse, New York.
Motavalli, P.P., C.A.Palm, W.J. Parton, E.T. Elliott and S. D. Frey.
1994. Comparison of laboratory and
modeling simulation methods for estimating soil carbon pools in tropical
forest soils. Soil Biol.
Biochem. 26(8):935-944.
Parton, W. J., D. S. Schimel, C. V. Cole, and D. S. Ojima. 1987.
Analysis of factors controlling soil
organic matter levels in Great Plains Grasslands. Soil Society
of America Journal. Vol. 51 (5): 1173-1179.
---------, J. W. B. Stewart, and C. V. Cole. 1988. Dynamics of C, N,
P and S in grassland soils: a model. Biogeochemistry 5: 109-131.
Running, S.W., and J.C.Coughlan. 1988. A general model of forest ecosystem
processes for regional
applications. I. Hydrologic balance, canopy gas exchange and primary
production processes. Ecol.
Modeling 42:125-154.
All samples will be filtered with a pre-combusted Whatman GF/F glass fiber filter prior to analysis; a subsample for silica and cations will be held refrigerated, and a subsample for other analyses will be held frozen until analysis. Dissolved organic carbon will be measured as non-purgeable organic carbon using automated high temperature platinum-catalyzed combustion (Shimadzu TOC 5000 with autosampler). Total dissolved N will be measured using high temperature platinum-catalyzed combustion followed by analysis of total NO in the combustion gas stream using an Antek Model 720C chemiluminescent detector (Merriam et al. 1996). Ion chromatography (Dionex micromembrane chemical suppression and conductivity detection) anions (sulfate, chloride, and nitrate). Single column non-suppressed ion chromatography will be used to measure base cations (sodium, calcium, magnesium, and potassium). Ammonium (phenol hypochlorite method), orthophosphate (ammonium molybdate method), and total dissolved P (persulfate digestion followed by ammonium molybdate for orthophosphate) will be measured with a flow injection analyzer (Lachat QuikChem). Dissolved organic N will be estimated as the difference between total dissolved N and DIN.
References:
Merriam, J., W.H. McDowell, and W.S. Currie. 1996. A high-temperature
catalytic oxidation technique for determining total dissolved nitrogen.
Soil Science Society of America Journal 60: 1050-1055.
Riparian Studies
"Relationship between primary gradients, disturbance and riparian
species distribution along Sonadora Creek, El Verde" E. Meléndez-Ackerman,
R. Tremblay, J. Sharpe.
Little is known about what regulates the distribution of lithophitic plant species in riparian zones. One posibility is that their populations are mainly controlled by primary gradients in environmental factors (i.e. light, temperature, humidity) with dominant species exhibiting either different or analogous responses to changes in these factors along elevational gradients. Aside from changes in elevation, changes in the microenvironment along streams may also be regulated by the frequency and intensity of disturbance events (e.g. floods, droughts, hurricanes) on forest streams. Thus, disturbances may also play an important role in the distribution of plant species in riparian zones.
We will explore the role of primary gradients and disturbances by monitoring 3 dominant lithophitic species (Lepanthes rupestris (Orchidaceae), Pitcarnia angustifolia (Bromeliaceae), , (Polypodiaceae) along Sonadora Creek near the El Verde Field Station Research Area. We will monitor populations of these species along a 400 m stretch on both sides of the Sonadora creek between 300 m and 700 m in elevation. As part of another project we have already marked all patches (i.e. rocks and trees) containing Lepanthes rupestris plants for a total of 200 occupied sites. Since occupied patches have on average a number of 45 plants growing on them (Tremblay, 1997) our total number of plants is likely to be in the order of 9,000. In addition, we have randomly selected and marked 4 unocupied patches in the vecinity of every occupied site for an additional 800 marked sites. We will complete surveying and marking on the remaining strech of Sonadora this summer so that the area surveyed on both sides of the creek is symetrical. We will also use marked patches to census the fern species. We will survey the area to determine the locations of P. angustifolia populations which tend to occupy larger areas than our study orchid and fern populations.
For each species we will initially tag or map all individuals regardless of life history stage in marked patches as well as all newly recruited plants throughout the duration of the study. Every 4 months we will census all sites to obtain individual data on survival, life history stage (seedling, juvenile, non-reproductive adult and reproductive adult) and reproductive output. We intent to count the number of new recruits and as well as the presence or absence of live plants in all marked sites. At every census and for every site we will also collect data on environmental variable that may influence population performance (i.e. temperature, % relative humidity, light intensity). Additional censuses will be performed after disturbance events (i.e. floods, droughts, hurricanes) and rainfall data from the El Verde weather Station will be recorded for each census period. For every flood event we will obtain data on stream flow from the Sonadora US Geological Survey water gauge.
The above data will serve various purposes. First it will allow us to
construct life tables that will be used to estimate the intrinsic growth
rates of individual patches using a matrix-based demographic analysis (Tremblay,
1997) which will be used to determine relationship between the primary
gradients and disturbance events with the degree of local population persistence.
Second, it will help us estimate the rates of colonization and extinction
of patches and how these relate to changes in environmental parameters.
With these data we hope to determine the extent by which riparian lithophitic
populations behave as metapopulations where persistence is likely to be
achieved at the landscape level.
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