Evidence of Species-Specific Neighborhood Effects in the Dipterocarpaceae of a Bornean Rain Forest

Abstract

Abstract. Although accumulating evidence indicates that local intraspecific densitydependent effects are not as rare in species-rich communities as previously suspected, there are still very few detailed and systematic neighborhood analyses of species-rich communities. Here, we provide such an analysis with the overall goal of quantifying the relative importance of inter-and intraspecific interaction strength in a primary, lowland dipterocarp forest located at Danum, Sabah, Malaysia. Using data on 10 abundant overstory dipterocarp species from two 4-ha permanent plots, we evaluated the effects of neighbors on the absolute growth rate of focal trees (from 1986 to 1996) over increasing neighborhood radii (from 1 to 20 m) with multiple regressions. Only trees 10 cm to Ͻ100 cm girth at breast height in 1986 were considered as focal trees. Among neighborhood models with one neighbor term, models including only conspecific larger trees performed best in five out of 10 species. Negative effects of conspecific larger neighbors were most apparent in large overstory species such as those of the genus Shorea. However, neighborhood models with separate terms and radii for heterospecific and conspecific neighbors accounted for more variability in absolute growth rates than did neighborhood models with one neighbor term. The conspecific term was significant for nine out of 10 species. Moreover, in five out of 10 species, trees without conspecific neighbors had significantly higher absolute growth rates than trees with conspecific neighbors. Averaged over the 10 species, trees without conspecific neighbors grew 32.4 cm 2 in basal area from 1986 to 1996, whereas trees with conspecific neighbors only grew 14.7 cm 2 in basal area, although there was no difference in initial basal area between trees in the two groups. Averaged across the six species of the genus Shorea, negative effects of conspecific larger trees were significantly stronger than for heterospecific larger neighbors. Thus, high local densities within neighborhoods of 20 m may lead to strong intraspecific negative and, hence, density-dependent, effects even in species-rich communities with low overall densities at larger spatial scales. We conjecture that the strength of conspecific effects may be correlated with the degree of host specificity of ectomycorrhizae.

Forest tree neighborhoods are structured more by negative conspecific density dependence than by interactions among closely related species

“…This is further supported by the fact that the proportion of neighbors that were conspecific declined significantly with focal tree size at all sites, particularly for sapling-sized neighbors. Numerous studies have reported strong conspecific negative density-dependent mortality in both tropical and temperate forests (Augspurger 1984, Terborgh et al 2008, Uriarte et al 2004a, Stoll and Newbery 2005, Chen et al 2010, Swamy and Terborgh 2010, Bai et al 2012, Comita et al 2014), particularly at early life stages (e.g., seedling and sapling stages; Zhu et al 2015). Our results are consistent with these other studies and demonstrate that such density-dependent mortality patterns are strong enough to structure forest composition.…”

How individual species structure diversity in tropical forests

“…Note that this approach is based on a separation of scales. Several studies using individual-based analyses of local neighborhood effects on growth and survival have shown that direct plant-plant interactions may operate only at local plant neighborhoods Ͻ20-30 m, fading away at larger scales (12)(13)(14)25), and the parameter of dispersal kernels at BCI typically range at approximately Ͻ 40-50 m (2 2 is the mean square dispersal distance from parent to surviving offspring) (38). On the other hand, habitat conditions for trees, i.e., elevation, orientation, or soil nutrients, vary typically at larger scales along environmental gradients that are often related with topographical features such as slope and elevation (22,23,29,36).…”

“…Studies investigating neighborhood effects on tree growth and survival showed that direct plant-plant interactions are strong at local plant neighborhoods (of, say, Ͻ30 m) but fade away at larger scales (12)(13)(14)25). We therefore expect strong departures from a neutral ISAR at local neighborhoods but mostly neutral ISARs at larger scales.…”

“…Neutral theory (6)(7)(8) suggested that species-specific differences are unimportant for certain community attributes, whereas niche theory outlines the importance of species characteristics and trade-offs (9,10). It is also clear that species-specific differences affect the spatial distribution of populations (11)(12)(13)(14)(15)(16). Surprisingly, although plant-plant interactions should play a major role in structuring tropical forests, the resulting spatial patterns in diversity have not been analyzed from the viewpoint of individual species.…”

Local neighborhood competition following an extraordinary snow break event: implications for tree-individual growth

“…It has been found that conspecifics are stronger competitors than heterospecifics (Stoll & Newbery 2005, Massey et al 2006, Pretzsch & Schütze 2009. This may be due to high competitive equivalence between individuals of the same species, whereas individuals of different species may use resources in a complementary, and thus more efficient way, and, thereby, reduce competition.…”