Charral Project -- initial woody plant colonization

Sampling in 1998 showed that there was wide variation in the density, basal area, and species composition of colonists among our five sites, and that the presence of rotting log microsites may have pivotal influence on initial successional patterns.

One potential influence on tree establishment in pastures is the local availability of seeds. This map of the woody plants colonizing Site 1 shows the location relative to forest of seedlings and saplings in the fenced area. Color symbols show common colonizing species Cecropia insignis and Croton draco. Note that large individuals of Croton along the forest edge are shown in red, while smaller, colonizing Crotons are shown in green. The concentration of Croton colonists near the forest strongly suggests that proximity to sources of propagules is an important determinant of establishment probability for this species.

This map shows Site 3, with fenceposts as white squares and woody colonizing plants > 1 m tall as black circles. Note the overall lower density of colonists compared to Site 1. However, as in Site 1, woody plants establish more abundantly near the forest. In this site, the most common colonizing woody plants are Cecropia insignis, Gonzalagunia rosea, two species of Miconia, Solanum ferrugineum, and a species in the Asteraceae (perhaps Vernonia sp.). The low density of woody plant establishment is puzzling, given the presence of adults of several potential pioneer species in along the edge of the forest.

Here is a map of Site 5. In this site, density of woody colonists is higher than at Site 3, but not as high as at Site 4. The forest is at that top of the figure; the concentration of colonists near the forest is not as pronounced here as at Site 1. In addition to proximity to forest, another potentially important influence on initial woody plant establishment is visible in this figure, and is especially pronounced in Site 4 as well: rotting logs and stumps provide establishment microsites for several of the common colonizing species, especially Miconia. Here, notice the line of colonists in center of the fenced area, sloping up and to the left. All of this group of plants is growing on the same rotting log. The tendency for establishment on rotting wood microsites is statistically significant in the two species of Miconia.

This figure shows total density and basal area of woody plants > 1 m tall, as measured in the summer of 1998. Note that there are two distinct vertical axes. Site 4 clearly has the most abundant regeneration, in terms of both density and basal area, and most of this is made up of two species of Miconia. However, those same species are not the predominant species in the other sites; in Site 1, for example, Croton draco is one of the most abundant.

Here we show the relative importance of the various woody plant species in our five study sites. Relative importance is the average of relative density and relative basal area. Basal area is simply the sum of the circular areas calculated from trunk diameters for each woody stem; it is relativized for each species by dividing the species total basal area by the grand total basal area for that site. Relative density is determined in an analogous manner. Reinforcing the importance of rotting log microsites, we can see that species composition varies substantially among sites, with the Miconia species especially important in Sites 4 and 5, where there is the highest abundance of rotting wood microsites. While causes of other aspects of the variation in species composition remain to be shown experimentally, we suspect that local propagule availability or limitation explains much of the variation in initial woody plant species composition in these sites. Analyses of the seed rain into the five sites is currently underway.

This figure shows the distribution of woody plants among rotting log microsites, vs ordinary soil, in Site 4. Included are the most abundant species in this site, although this figure does not show the overwhelming dominance by Miconia A. It does, however, indicate the statistically highly significant tendency for woody plants of these species to occur on the logs. The horizontal line labeled "Expected" indicates how many individuals would be expected to occur on logs if these species' distributions were unrelated to log microsites. It is very clear that many more occur on logs than expected, indicating some sort of favorable microclimate, preferential propagule input, or perhaps higher germination and/or survival.

Here we present the species accumulation curves for each site, to show how species diversity differs. Species accumulation curves represent the number of species encountered (vertical axis) in samples of increasing number of individuals (x axis). Error bars in the main figure are one standard deviation. Because Site 4 has so many more stems, we present the full species accumulation curves in the inset, without error bars for clarity. The figure clearly shows that while density of individuals is lower in Sites 1 and 2, they are more diverse per capita than Sites 4 and 5.