Predicting Population Trends from Size Distributions: A Direct Test in a Tropical Tree Community

Abstract

Forest ecologists often evaluate how well the species composition of saplings in the understory matches that of the canopy: absence of juveniles suggests that a tree species is suffering population decline. Here we offer a theoretical and empirical test of this assertion using data from a 50-ha census plot in Panama. Theory indicates that higher rates of population change, lambda, lead to more steeply declining size distributions (more juveniles relative to adults). But other parameters also affect the size distribution: lower growth rate of juveniles and lower survival at any size produce more steeply declining size distributions as well. Empirical evaluation of 216 tree populations showed that juvenile growth was the strongest predictor of size distribution, in the direction predicted by theory. Size distribution did correlate with population growth, but weakly and only in understory species, not canopy species. Size distribution did not correlate with the growth rate of larger individuals nor with survival. Results suggest that static in formation on the size distribution is not a good predictor of future population trends, while demographic information is. Fast-growing species will have fewer juveniles in the understory than slow growing species, even when population growth is equal.

Microhabitat associations and seedling bank dynamics in a neotropical forest

“…For many species, particularly those with large seeds, recruitment events may be sporadic due to supra-annual fruiting events or low production years (DeSteven 1994;Condit et al 1998;Connell and Green 2000). Such was the case for several of the species in this survey, and we thus did not measure new recruits into the seedling bank.…”

Estimating tree abundance from remotely sensed imagery in semi-arid and arid environments: bringing small trees to the light

“…This is a realistic assumption, since the exponential distribution is consistently detected in forest studies regardless of location or species identity (Harper 1977;Harcombe 1987;Niklas et al 2003). The slope of the distribution generally varies depending on several demographic factors such as density, mortality, and growth rates (Condit et al 1998), as well as on disturbances due to climatic and anthropogenic causes (Coomes et al 2003;Niklas et al 2003). While the negative exponential is a good model for uneven-aged forest stands, this assumption is not applicable to over-grazed, heavily disturbed, or selectively logged areas.…”

“…sufficiently small) trees grow only to the next size class and not to a larger one (Condit et al 1998). The general problem though is that in many trees growth is sizedependent, meaning that the transition probability from S i to the sequential one S i+1 generally depends on S i (Condit et al 1998).…”

Recruitment dynamics in a rainforest seedling community: context-independent impact of a keystone consumer

“…Planchonella, Syzygium, and Cryptocarya) are widespread and common across the island as large trees (Mitchell 1985). It is not unusual, even in continental forests, to Wnd species with low juvenile-adult ratios (Condit et al 1998), but on Christmas Island this occurs in the majority of canopy and understorey species.…”

“…First, low seedling/ adult sapling ratios, typical of the island rainforest, are not necessarily indicative of decline (Condit et al 1998). Fast juvenile growth rates could compensate for low rates of recruitment.…”