Tropical forest dynamics across a rainfall gradient and the impact of an El Niño dry season

Abstract

Abstract:Tropical forest demography and dynamics were examined in three inventory plots across a precipitation gradient in central Panama. The harsh dry season of 1998 that accompanied the 1997-98 El Niño was spanned by censuses at all three sites. The wet and intermediate plots were similar in total species richness, the dry site somewhat lower in diversity; all three sites differed substantially from each other in species composition. Forest-wide growth of large trees was higher at the wet and intermediate sites than at the dry site, but sapling growth was highest at the dry site and lowest at the intermediate site. Forest-wide growth differences were reflected by individual species, for example, saplings of species at the dry site grew faster than saplings of the same species at the intermediate site. Forest-wide mortality was lowest at the dry site and highest at the wet, and this difference was also reflected by individual species. We suggest that low mortality and growth in the drier forest was due to the longer annual dry season and higher deciduousness, and that high sapling growth at the dry site was due to greater light penetration to the forest floor. Growth rates were elevated at aU three sites during 1998, possibly due to reduced cloud-cover during the El Niño. Contrary to expectation, mortality during 1998 was not elevated at wet and intermediate sites during the El Niño drought, but was at the dry site. Finally, we found that some species performed poorly at one site and declined in abundance, while having stable or increasing populations at another site, demonstrating that the communities are not at equilibrium.

Extensions and evaluations of a general quantitative theory of forest structure and dynamics

“…10 and 3 implies that mortality rate, , should be directly proportional to rates of limiting resource supply, Ṙ. Several recent field studies appear to support a positive relationship between rates of resource supply and rates of mortality (38)(39)(40)(41). Similarly, our model can be extended to understand how plant metabolism, allometry, and resource supply can influence maximum tree size, total stand biomass, and other ecosystem level processes (see SI Text).…”

Species abundance patterns in an ecosystem simulation studied through Fisher’s logseries

“…Originally it was not much employed, perhaps because its calculation was quite tedious without a computer [46] (even with the help of Williams's nomograms [21,70]). Nowadays, a is often used to compare communities in terms of species richness, by eliminating the effect of sample size [8,12,13,45,46,50]. In that context, it has shown a strong positive correlation with habitat heterogeneity (expressed in terms of its effect on the fauna) [8,45].…”

“…We have chosen to focus on relative species abundance because this is an issue that ecologists have been studying for several decades [23,40,46,70], and that still remains a central theme of ecology [3,12,73]. We can thus benefit from an extensive expertise, and from numerous well established results.…”

Patterns of tree growth in relation to environmental variability in the tropical dry deciduous forest at Mudumalai, southern India

“…However, longer term studies (without dendrometers) have suggested that on an annual basis the relationship between rainfall and growth might become negative as the average annual rainfall increases. Thus, a four year study in a dry evergreen rainforest of Ghana (Swaine et al 1990;annual rainfall: 570-910 mm, at least eight months with less than 100 mm) found that faster growth occurred during higher rainfall years as expected based on seasonal patterns, whereas studies lasting up to eight years in moist forest sites of Panama (Condit et al 1993a(Condit et al , 2004annual rainfall: 2030-2892.5 to 4.5 months with less rainfall than potential evapotranspiration) found higher growth rates in saplings and several large trees during a drought year, and similarly a six-year study in a wet forest of Costa Rica (Clark and Clark 1992; annual rainfall: 3859 mm, no months with less than 100 mm) found highest median annual growth rates in the smallest and largest size classes during the lowest rainfall years and lowest growth during the highest rainfall years in most of the species studied. Similarly, a short twoyear dendrometer study in a wet evergreen forest of India (Pelissier and Pascal 2000; annual rainfall >4000 mm, five months with less than 100 mm) also showed higher average annual growth during the relatively drier year.…”

“…Similarly, a short twoyear dendrometer study in a wet evergreen forest of India (Pelissier and Pascal 2000; annual rainfall >4000 mm, five months with less than 100 mm) also showed higher average annual growth during the relatively drier year. It has been hypothesized for moist forests that increased growth during relatively dry years may be due to reduced cloud cover resulting in increased photosynthetically active radiation Clark 1992,1994;Wright et al 1999;Condit et al 2004) or due to increased atmospheric temperatures during droughts (Condit et al 2004). In this context it is interesting to examine the long term relationship between rainfall and tree growth in the comparatively dry forest of Mudumalai.…”