Response of Demographic Rates of Tropical Trees to Light Availability: Can Position-Based Competition Indices Replace Information from Canopy Census Data?

<div><p>For trees in tropical forests, competition for light is thought to be a central process that offers opportunities for niche differentiation through light gradient partitioning. In previous studies, a canopy index based on three-dimensional canopy census data has been shown to be a good predictor of species-specific demographic rates across the entire tree community on Barro Colorado Island, Panama, and has allowed quantifying between-species variation in light response. However, almost all other forest census plots lack data on the canopy structure. Hence, this study aims at assessing whether position-based neighborhood competition indices can replace information from canopy census data and produce similar estimates of the interspecific variation of light responses. We used inventory data from the census plot at Barro Colorado Island and calculated neighborhood competition indices with varying relative effects of the size and distance of neighboring trees. Among these indices, we selected the one that was most strongly correlated with the canopy index. We then compared outcomes of hierarchical Bayesian models for species-specific recruitment and growth rates including either the canopy index or the selected neighborhood competition index as predictor. Mean posterior estimates of light response parameters were highly correlated between models (<i>r</i>>0.85) and indicated that most species regenerate and grow better in higher light. Both light estimation approaches consistently found that the interspecific variation of light response was larger for recruitment than for growth rates. However, the classification of species into different groups of light response, <i>e.g.</i> weaker than linear (decelerating) vs. stronger than linear (accelerating) differed between approaches. These results imply that while the classification into light response groups might be biased when using neighborhood competition indices, they may be useful for determining species rankings and between-species variation of light response and therefore enable large comparative studies between different forest census plots.</p></div

Community-wide distribution of the light response of growth (<i>b_g</i>).

<p>Light is estimated either with the canopy index (log(CAI)), the interval-specific neighbordood competition index (NCI_ts) or the general neighborhood competition index (NCI_tg). The underlying histogram can lead to sharp edges of the density plot.</p

Species-specific light response of growth (<i>b_g</i>) in models with neighborhood competition index versus canopy index.

<p>Panels show posterior means of <i>b_g</i> in models with the interval-specific neighborhood competition index (NCI_ts, A, C) and the general neighborhood competition index (NCI_tg, B, D) compared to <i>b_g</i> in models with the canopy index (log(CAI)). The correlation between the coefficients (<i>r</i>) and the 1∶1 line are indicated.</p

Correlation between log(canopy index) and neighborhood competition indices with different values of <i>α</i> and <i>β</i>.

<p>The correlation is based on the canopy index and neighborhood competition indices (NCIs) for 16,544 5×5 m grid cells (A, B) and for 135,788 (C) and 139,625 trees (D) in the BCI forest plot. For grid cells, the highest correlation was achieved with <i>α</i> = 1.6 and <i>β</i> = 0.4 in the first, and <i>α</i> = 1.4 and <i>β</i> = 0.4 in the second census (highlighted). For individual trees, the highest correlation was achieved with <i>α</i> = 1.2 and <i>β</i> = 0.6 (highlighted) in both censuses. Colors indicate Pearson's <i>r</i>.</p

Species-specific light response of recruitment (<i>b_r</i>) in models with neighborhood competition index versus canopy index.

<p>Panels show posterior means of <i>b_r</i> in models with the interval-specific neighborhood competition index (NCI_ts, A, C) and the general neighborhood competition index (NCI_tg, B, D) compared to <i>b_r</i> in models with the canopy index (log(CAI)). The correlation between the coefficients (<i>r</i>) and the 1∶1 line are indicated.</p

Community-wide distribution of the light response of recruitment (<i>b_r</i>).

<p>Light is estimated either with the canopy index (log(CAI)), the interval-specific neighbordood competition index (NCI_ts) or the general neighborhood competition index (NCI_tg). The underlying histogram can lead to sharp edges of the density plot.</p

Posterior estimates of hyperparameters for the species-specific parameters of the recruitment model.

<p>Posterior estimates of hyperparameters for the species-specific parameters of the recruitment model.</p

Best neighborhood competition index (NCI, <i>α</i> = 1.6, <i>β</i> = 0.4) versus canopy index (CAI).

<p>Lines show quantile regressions to convert neighborhood competition index (NCI) into an interval-specific estimate of log(canopy index) (NCI_ts, dashed line) or into a general estimate of log(canopy index) which is based on pooled data from both census intervals (NCI_tg, dotted line).</p

Classification of tree species on the basis of their recruitment responses to light availability: negative (<i>b_r</i><0), decelerating (0<<i>b_r</i><1) and accelerating (<i>b_r</i>>1).

<p>Classification of tree species on the basis of their recruitment responses to light availability: negative (<i>b_r</i><0), decelerating (0<<i>b_r</i><1) and accelerating (<i>b_r</i>>1).</p