This study examined the composition of large trees in a lowland forest in the Panama Canal Watershed. In the context of this study, the term biodiversity will be used synonymously with the number of species present, often referred to as species richness.
Intermediate Disturbance Hypothesis
This hypothesis suggests that tropical tree species have partitioned the available resources very finely and that many species are extreme habitat specialists. Many habitat specialists have been detected, but the percentage appears to be too low to play a primary role in supporting the high levels of alpha and beta diversity seen in the lowland forest (Hubbell et al. 1999).
Climatic and temporal controls
The conclusion is that the diversity of forests is the result of the frequent relocation of taxa in space under the influence of the cyclically changing climate regime.
Biological Productivity
Dispersal Limitation
Harms concluded that dispersal limitation is likely to play a strong role in the distribution of individual tree species and ultimately community composition on the BCI. All studies of dispersal limitation in Panama to date (Hamill 1986, Harms 1997, Hubbell et al. 1999) have been conducted over very small geographic areas within the already isolated forest on the BCI.
Synthesis
BIOPHYSICAL ENVIRONM ENT
Climate
Geology
Soils
On sedimentary rocks, rivers fed by heavy rains during the rainy season cut deeply into weathered slopes, which then experience deep landslides (Dietrich et al. 1982). Previous work (Orians et al. 1996) has shown that longer dry season lengths correspond to increased fluctuations in soil microbial populations and nutrient release.
Vegetation
STRI has provided institutional support for a large body of research focused on and around the BCI field station (see Leigh et al.). Forest composition responds rapidly to climatic factors, such as drought resulting from El Nino events (Condit et al. 1995). ).
Overview
Vegetation and soil data
This restriction kept the majority of the 1-hectare plots either near the Panama Canal in lands historically administered by the US. I tried to select well locations that best represented most of the terrain in the plot.
Watershed floristic patterns
The complexity of the indirect gradient analysis suggested the need to independently test the relationships within the data set. Based on the sign of the test results, the alternative hypothesis states either a positive or negative correlation between the matrices.
Gradient dynamics along the Panama Canal
Ultimately, the product of the asymptotic approximation method is a standardized Mantel statistic, r, that can be used as the equivalent of a Z value in a significance test (McCune and Mefford 1997). Despite these potential complications, a gradient in precipitation is the most obvious environmental change occurring from the Caribbean to the Pacific sides of the Isthmus of Panama. Given the available station data and the lack of supplementary information, it seemed questionable to attempt to develop interpolations for the plots on the edge of the Channel Watershed.
It can be in excess during most of the year, but during the dry season it can become a critically limiting resource. Annual precipitation accumulation curves for the 20 meteorological stations in the Channel watershed indicate that the maximum departure between the individual stations occurs at the end of May. All information regarding surficial geology is based on the USGS geologic map of the Panama Canal and vicinity published in 1980.
The map was drawn at a scale of 1:100,000 and digitized as part of the United States Agency for International Development (USAID) Monitoring Project.
Tropical trees generally do not produce interpretable annual rings like their temperate counterparts (Mabberley 1992). Direct gradient analysis is most often applied to situations where a single known environmental control is well documented and used to describe observed responses in a plant community. I have discussed the mechanisms of NMD in an earlier section (see Methods ii. Non-metric multidimensional distance scaling of 54 plots in the Canal Watershed), and suffice it to merely point out that the main differences are with DCA.
NMDS attempts to place plots in the correct order of similarity in a two-dimensional ordering space, rather than pulling plots apart along a fixed hierarchical set of axes. This interpretation allows correlation between a pure indirect gradient analysis (i.e., the NMD scores) and measured environmental variables. The per-species-per-plot abundance matrix used in the analyzes discussed above was re-aggregated to each taxonomic level, and two new matrices were created for genera per plot and families per plot.
Downweighting of rare taxa was considered for each matrix; however, it did not produce significant changes in the prescriptions.
Floristic nestedness
To perform this analysis, I performed a principal component analysis (PCA) with a variance-covariance cross-product matrix on the NMDS scores for the 45 channel area plots. The difficulty of a species-level taxonomy using typically only sterile plant specimens is daunting, and some authors have evaluated crude patterns of forest composition using families rather than species (Terborgh 1997). Part of the debate centers on the nature of species increases in resource-rich areas.
One hypothesis posits that species accumulation along a gradient involves the addition of relatively closely adapted species to rich, productive areas with fewer limiting resources (Cutler 1994, Boecklen 1997). Limitations with the NestCalc program limited the analysis to the 192 most abundant species in the 45 Canal Zone plots. This packing involves shuffling the table until the plot containing the most species is at the top and the remaining plots are sorted below it in descending order of nesting.
According to Atmar and Paterson (1992), this program creates the best possible nested hierarchy for the given species-per-plot table.
RESULTS
If the outer watershed plots are added to the end of the main species area curve, again in any order, the slope increases dramatically, indicating the accumulation of many new species. An abrupt change in slope between these curves indicates a demonstrable change in species accumulation rates, ie. the outer watersheds contain many species that are not present at the canal. Using principal component analysis to center the biaxial NMDS scores provided information about the structure of the data.
Although the NMDS scores provided improved separation of the individual plots relative to the DCA output, relationships with categorical variables were still difficult to identify. These models suggest that the interpretation of the ordination scores is relatively insensitive to the choice of precipitation index. It is interesting to note that the flora seems somewhat embedded from the interior of the isthmus towards the coasts.
For a more complete description of the sorting procedure, see the main text in the Methods section.
Soil factors
It is worth noting that the three most extreme marginal plots in the data set (as identified by PC-ORD outlier analysis) can be explained by soil properties. The gradient analysis showed that the composition of the forest in plot L1 is relatively similar to the dry forest in Cocola. Condit (personal communication) suggests that thin, excessively well-drained soils at L1 create conditions that promote local drought stress, perhaps more similar to Cocoli than to the wet surrounding forest of Fort Sherman.
Plots m19 and m20 are relatively close to the mid-strait BCI, but have ordination estimates very close to the Fort Sherman wet forest. Plots Fort Sherman and m19-m20 are on different geologic substrates, but both have the most acidic soils in the plot network. These examples show that more subtle differences in soil properties can explain a large component of ordination remains.
A more extreme view might suggest that the first NMDS axis in the 45-plot ordination is not really precipitation, but soil pH/soil fertility.
DISCUSSION
Regional forest classification
Regional controls on forest composition
They believe that their data from Lake Yeguada (altitude 650 meters) correlates well with records from Lake Valencia, Venezuela, suggesting at least regional climatic synchronization. Deep-sea cores collected by Harris and Mix (1999) provide fascinating evidence that, at least in the Amazon, orbital dynamics can create. Recent ecological studies (Condit et al. 1992b, Condit et al. 1995, Condit et al. 1996b, Condit et al. 1996a) indicate that the Panamanian flora, as represented by forest dynamics on the BCI, is closely coupled to the decadal scale climatic forcing.
They note that a 25-year drying trend at BCI has clear consequences for the composition of the forests.
Maintenance of biodiversity
Conservation implications
Part of this pattern is predictable from classical biogeographic and sampling theory; however, the L1-Sn-Sn column (L1 dry forest-like area plus the average of any two of the nearby Fort Sherman plots) illustrates the potential to increase species accumulation by identifying local areas with contrasting soils.
FUTURE RESEARCH
ACKNOWLEDGEMENTS
LITERATURE CITED
Density and distance to adults effects of a canker disease of trees in a humid tropical forest. Monitoring seed dispersal in isolated standing trees in tropical meadows - Implications for the availability of local species. Short-term dynamics of a neotropical forest - Why ecological research is important for tropical conservation and management.
Physiographic controls on sediment composition derived from volcanic and sedimentary terranes on Barro Colorado Island. Effects of soil and topography on the distribution of tree species in a tropical riparian forest in southeastern Brazil. Strong density- and diversity-related effects help maintain tree species diversity in a Neotropical forest.
APPENDIX
Adding plots of moist forest plots in a random order produces the smooth curve to the left up to 40 on the x-axis. The addition of the rest of the watershed plots, again in random order, produces a jump in species accumulation as predicted by Condit et al. The dashed line at the top illustrates the shape of the species-area curve if all plots were added in random order.
Bold letters indicate statistical significance: B is significantly different from A at p < 0.01 based on a two-tailed two-sample t-test with unequal variances.
