Size-Class Effect Contributes to Tree Species Assembly through Influencing Dispersal in Tropical Forests

<div><p>We have investigated the processes of community assembly using size classes of trees. Specifically our work examined (1) whether point process models incorporating an effect of size-class produce more realistic summary outcomes than do models without this effect; (2) which of three selected models incorporating, respectively environmental effects, dispersal and the joint-effect of both of these, is most useful in explaining species-area relationships (SARs) and point dispersion patterns. For this evaluation we used tree species data from the 50-ha forest dynamics plot in Barro Colorado Island, Panama and the comparable 20 ha plot at Bubeng, Southwest China. Our results demonstrated that incorporating an size-class effect dramatically improved the SAR estimation at both the plots when the dispersal only model was used. The joint effect model produced similar improvement but only for the 50-ha plot in Panama. The point patterns results were not improved by incorporation of size-class effects using any of the three models. Our results indicate that dispersal is likely to be a key process determining both SARs and point patterns. The environment-only model and joint-effects model were effective at the species level and the community level, respectively. We conclude that it is critical to use multiple summary characteristics when modelling spatial patterns at the species and community levels if a comprehensive understanding of the ecological processes that shape species’ distributions is sought; without this results may have inherent biases. By influencing dispersal, the effect of size-class contributes to species assembly and enhances our understanding of species coexistence.</p></div

The p-values of pairwise Wilcoxon rank sum tests on SAR and <i>g(r) AIC</i> values among the three point process models for the three DBH classes.

<p>Note: See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108450#pone-0108450-t001" target="_blank">Table 1</a> for the abbreviations.</p><p>The p-values of pairwise Wilcoxon rank sum tests on SAR and <i>g(r) AIC</i> values among the three point process models for the three DBH classes.</p

Boxplots of <i>AIC</i> value distributions of the SARs generated by the three process models with and without the size-class effect at Class 0 (that is: size-classes 1, 2 and 3 combined) forthe Bubeng plot and the BCI plot.

<p>HT.l = the homogeneous Thomas process with size-class effect; HT.w = the homogeneous Thomas process without size-class effect; IP.l = the inhomogeneous Poisson process with size-class effect; IP.w = the inhomogeneous Poisson process without size-class effect; IT.l = the inhomogeneous Thomas process with size-class effect; and IT.w = the inhomogeneous Thomas process without size-class effect.</p

Boxplots of <i>g(r) AIC</i> value distributions generated by the three processe models with and without the size-class effect at Class 0 (that is: size-classes 1, 2 and 3 combined) of the Bubeng plot and the BCI plot.

<p>Boxplots of <i>g(r) AIC</i> value distributions generated by the three processe models with and without the size-class effect at Class 0 (that is: size-classes 1, 2 and 3 combined) of the Bubeng plot and the BCI plot.</p

Comparison of observed <i>g(r)</i> of species at DBH Class 0 (that is: size-classes 1, 2 and 3 combined) with <i>g(r)</i> simulated by three point process models with and without size-class effect.

<p>Abbreviations as for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108450#pone-0108450-g004" target="_blank">Figure 4</a>.</p

The p-values from Kruskal-Wallis rank sum tests on SAR and <i>g(r) AIC</i> values across the point process models with and without size-class effect at DBH class 0 (that is: size-classes 1, 2 and 3 combined).

<p>Note: IP = the inhomogeneous Poisson process, HT = the homogeneous Thomas processt, IT = the inhomogeneous Thomas process.</p><p>The p-values from Kruskal-Wallis rank sum tests on SAR and <i>g(r) AIC</i> values across the point process models with and without size-class effect at DBH class 0 (that is: size-classes 1, 2 and 3 combined).</p

Cell connectivity at each of the four scales of cell size for <i>Sloanea tomentosa</i> in DBH class 4.

<p>Cell connectivity at each of the four scales of cell size for <i>Sloanea tomentosa</i> in DBH class 4.</p

Boxplots of <i>AIC</i> value distributions of the SARs generated by the three process models for size-classes 1, 2 and 3 of the Bubeng plot and the BCI plot.

<p>HT = the homogeneous Thomas process; IP = the inhomogeneous Poisson process; IT = the inhomogeneous Thomas process.</p

Principal component analysis ordinations (based on matrices of transformed p-values from the SAR models) of the 14 explanatory variables and the spatial autoregressive parameter λ for each of the 5 DBH classes at the 20-m scale, obtained with basal area data.

<p>Classes 0 to 4 are defined as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038247#pone-0038247-g001" target="_blank">Figure 1</a>. The abbreviations are defined as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038247#pone-0038247-g003" target="_blank">Figure 3</a>.</p

Results of the partitioning variation between environmental variables and spatial effects for each of the 20 combinations of DBH and cell size using basal area data.

<p>Note: Adjusted R-squared statistics are shown. Fractions [a] – [d] are as follows: [a]  =  variation explained by the environmental variables and not spatially structured; [b]  =  variation explained by the environmental variables and spatially structured; [c]  =  spatially structured variation not explained by the environmental variables; [d]  =  residual variation. Fraction [b] is the intersection of the variation explained by linear models of the two groups of explanatory factors. Topographic and edaphic variables were used to compute fractions [a] and [b]. Principal coordinates of neighbor matrix eigenfunctions were used as explanatory variables to compute fractions [b] and [c]. class 0 (DBH ≥1 cm), class 1 (1 cm ≤ DBH <5 cm), class 2 (5 cm ≤ DBH <10 cm), class 3 (10 cm ≤ DBH <25 cm), class 4 (DBH ≥25 cm).</p