Abstract:
The spatial distribution of recruits around conspecific adults of woody species conveys detailed
insights on the mechanisms governing species co-existence in tropical rainforests. Hence, we studied
adult-recruit association of eight Shorea species (Shorea affinis, S. congestiflora, S. cordifolia, S.
disticha, S. megistophylla, S. trapezifolia, S. stipularis, S. worthingtonii) in the Sinahraja Forest
Dynamic Plot, using spatial point pattern analysis. The adult-recruit spatial patterns were extracted
from four censuses in 1996, 2001, 2006 and 2011 conducted at the plot. We used slope, elevation,
aspect, curvature, topographic wetness index and seventeen soil nutrients as habitat variables. To
assess the adult-recruit spatial relationship, we used two null models created by the pattern
reconstruction method. The null model of independence distributed recruits independently on
conspecific adults without considering habitat association, but maintained their observed aggregation.
In contrast, the Heterogeneous Poisson Process considered additionally the effect of topographic
variations. Departures from the null models were assessed by simulation envelopes and goodness-of-
fit tests using the bivariate distribution function of nearest neighbor distances and the bivariate pair
correlation function. S. trapezifolia and S. stipularis indicated independent placement of recruits
around adults for all distance intervals of 1-120m while other species showed significant positive
association, for all distance intervals of 1-50m. Thus, the majority of recruits of Shorea species
showed positive associations with conspecifics adults at medium spatial scale which implies high
interference of individuals of Shore species with conspecifics. Based on the fitted intensity functions
of recruits, slope, elevation and soil nutrients were identified as the main environmental factors
influencing the bivariate adult-recruit spatial distributions of Shorea species. Further, for S. stipularis,
the fitted Heterogeneous Poisson Process described the observed pattern well (goodness-of-fit test‟s
p-value = 0.15), which suggests that the adult-recruit pattern was mainly driven by habitat
heterogeneity. However, departures from the null hypothesis of habitat heterogeneity were shown for
all other Shorea species (goodness-of-fit tests‟ p-values < 0.05). We conclude that the heterogeneous
environment alone is insufficient to explain the observed patterns of recruits around conspecific
adults of Shorea species. Consequently, the impact of other mechanisms on conspecific adult-recruit
association should also be studied.
Keywords: Adult-recruit association; Intraspecific; Habitat heterogeneity; Heterogeneous Poisson
Process; Tropical rainforest
