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|Title: ||Tree Species Occurrence Related to Dry Season Duration, Panama Canal Area|
|Authors: ||Condit, Richard|
|Issue Date: ||4-Feb-2013|
|Abstract: ||Woody plant species with free-standing stems >=1 cm diameter were surveyed at 72 locations near the Panama Canal, each with an area <0.5 square kilometer. Sites were arranged to span geological formations and the rainfall gradient; all were within 65 km of one other. Most were low elevation, with just two sites >600 m above sea level, the highest at 888 m. All were in closed-canopy forest, including undisturbed old-growth and secondary stands 60-100 years old. Forty surveys were permanent census plots in which every individual tree was located, measured, and identified: a 50-ha plot at Barro Colorado, a 5.96-ha plot at Fort Sherman, and 38 1-ha plots. At 32 other sites, surveys were one-day inventories in which all tree species were noted until we could find no more, without counting individuals. Nearly every tree was identified, either on the spot or later after comparing against keys or guides and herbarium specimens (Smithsonian Tropical Research Institute and the University of Panama).
The Meteorology and Hydrology Branch of the Panama Canal Authority, Republic of Panama, maintains a network of meteorological stations throughout the Panama Canal Watershed. Daily rainfall at the 46 of those stations having 3-47 years of uninterrupted data since 1960, plus daily records from Barro Colorado Island station, were acquired by Smithsonian Tropical Research Station Physical Monitoring Program.
The mean annual extreme of dry-season moisture at the 47 rainfall gauges was calculated as the cumulative moisture deficit (precipitation minus potential evapotranspiration). This dry season measure was then interpolated at 72 forested sites across the Panama Canal area using an optimized spatial kernel fitted to the results at the 47 gauges.
Eight environmental predictors were used to model of tree distributions: dry season moisture plus soil Al, Ca, Fe, K, P, N, Zn. Soil concentrations were log-transformed, then each predictor was standardized to mean 0 and standard deviation 1. Species occurrences were modeled against the eight factors simultaneously using Gaussian logistic regression. All species were included at once using a hierarchical Bayesian approach (a multilevel regression. The hierarchy consists of a lower level of species-level response parameters beneath an overarching community-wide distribution of those responses. The upper level --- the hyperdistribution and particularly its standard deviation --- measures the variability of responses across a community and thus reveals whether a resource differentiates species. The lower level provides a separate measure for the response of every species to each environmental factor. The hierarchical aspect was critical, preventing over-fitting in rare species but weighting all species according to frequency. Parameters were fitted with a Gibbs sampler based on Metropolis updates, producing credible intervals for all statistics.
These results were published in Condit R, Engelbrecht BMJ, Pino D, Pérez R, Turner BL (2013) Species distributions in response to individual soil nutrients and seasonal drought across a community of tropical trees. Proc Natl Acad Sci USA, 10.1073/pnas.1218042110.|
|Description: ||File TreeCommunityDrySeasonDailyRain.txt: Daily rainfall over 3-47 years at 47 rainfall stations: date, rainfall in millimeters, site name; the file includes a full year of records for all site-year combinations, with 0 meaning no rainfall. Note, however, that there are cases, especially weekends, where Saturday and Sunday have 0, then Monday's record has the entire weekend's rain. Rainfall totals are thus complete, but estimating the number of days with rain would require care.|
File TreeCommunityDrySeasonStationSite.txt: Calculated mean extreme dry season moisture deficit at the 47 rainfall stations, with site name, years of data, UTM northing (zone 10), UTM easting (zone 10), and elevation (meters). The extreme moisture deficit of any one dry season is defined as millimeters of cumulative precipitation minus evapotranspiration at its most negative over any period of days spanning the dry season. The table includes a single average at each site, the average of all years for which data were available at the site.
File TreeCommunityDrySeasonSurveySite.txt: Interpolated mean extreme dry season duration at 72 forested sites where tree species were surveyed, with site name, UTM northing (zone 10), UTM easting (zone 10), and elevation (meters).
File TreeCommunityDrySeasonSpeciesOccurrence.txt: Occurrences of 550 tree species at the same 72 sites, with taxonomic name (genus, species, family), site name, and occurrence (=1 if species was observed, =0 if not observed)
File TreeCommunityDrySeasonSpeciesResponse.txt: Response parameters for 550 species occurrences from model using eight climatic and soil predictors in an hierarchical Gaussian logistic regression: Latin (Family), number of occurrences at 72 sites, then the model's 17 parameters per species, which are one intercept plus first and second order responses (the second order parameter is labelled ^2; 'Moist' means dry season moisture deficit; other parameters labelled with soil element).
|Appears in Collections:||CTFS Data Sets|
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