Determinants of Plant Community Assembly in a Mosaic of Landscape Units in Central Amazonia: Ecological and Phylogenetic Perspectives

<div><p>The Amazon harbours one of the richest ecosystems on Earth. Such diversity is likely to be promoted by plant specialization, associated with the occurrence of a mosaic of landscape units. Here, we integrate ecological and phylogenetic data at different spatial scales to assess the importance of habitat specialization in driving compositional and phylogenetic variation across the Amazonian forest. To do so, we evaluated patterns of floristic dissimilarity and phylogenetic turnover, habitat association and phylogenetic structure in three different landscape units occurring in <em>terra firme</em> (Hilly and Terrace) and flooded forests (Igapó). We established two 1-ha tree plots in each of these landscape units at the Caparú Biological Station, SW Colombia, and measured edaphic, topographic and light variables. At large spatial scales, <em>terra firme</em> forests exhibited higher levels of species diversity and phylodiversity than flooded forests. These two types of forests showed conspicuous differences in species and phylogenetic composition, suggesting that environmental sorting due to flood is important, and can go beyond the species level. At a local level, landscape units showed floristic divergence, driven both by geographical distance and by edaphic specialization. In terms of phylogenetic structure, Igapó forests showed phylogenetic clustering, whereas Hilly and Terrace forests showed phylogenetic evenness. Within plots, however, local communities did not show any particular trend. Overall, our findings suggest that flooded forests, characterized by stressful environments, impose limits to species occurrence, whereas <em>terra firme</em> forests, more environmentally heterogeneous, are likely to provide a wider range of ecological conditions and therefore to bear higher diversity. Thus, Amazonia should be considered as a mosaic of landscape units, where the strength of habitat association depends upon their environmental properties.</p> </div

Number of individuals and diversity metrics based on trees ≥10 cm DBH, for each of the six 1-ha plots established at the Mosiro-Itajura Caparú Biological Station (Colombian Amazon).

<p>Number of individuals and diversity metrics based on trees ≥10 cm DBH, for each of the six 1-ha plots established at the Mosiro-Itajura Caparú Biological Station (Colombian Amazon).</p

Correlation coefficients of Mantel test (R) relating floristic and environmental similarity, while controlling by distance, at three geographical scales: within plots, within landscape units (LU) and among landscape units.

<p>Open represent PC1 values and open represent PC2 values. <i>P</i>≤0.05 are indicated with asterisks.</p

Pearson’s correlations between Chao and PCD indices.

<p>The orange dots represent coefficients calculated for Hilly and Terrace plots, blue dots represent coefficients calculated for Terrace and Igapó plots, and violet dots represent coefficients calculated for Igapó and Hilly plots. Black dots represent the coefficients for plots from the same landscape unit.</p

Species rarefaction curves within each landscape unit for trees >10 cm DBH.

<p>The red line denotes Hilly forests, the green line denotes Terrace forests and the blue line denotes Igapó forests. The shaded region represents 95% confidence intervals.</p

Phylogenetic species diversity (PSV) for each landscape unit.

<p>Significant values (<i>P</i>≤0.05, one-tailed test) are indicated in bold. Based on our a priori hypotheses, for Hilly and Terrace forests, PSV scores are significant if higher than the 95% quantile of randomized PSV. For Igapó forests, PSV scores are significant if lower than the 5% quantile of randomized PSV.</p

CCA of all tree species occurring in the six 1-ha plots.

<p>The arrows correspond to the abiotic variables included in the analysis. Symbols represent species of 20×20 m quadrants and show their association with the abiotic variables. Red triangles correspond to Hilly 1, red circles to Hilly 2; green triangles to Terrace 1, green circles to Terrace 2; blue triangles to Igapó forests plot 1, blue circles to Igapó plot 2.</p

Fitted parameters (a, τ, <i>b</i>, <i>T</i>, β), AIC values of the two models predicting temporal autocorrelation in seed production along 30 months for each species, Pearson regression coefficient of observed against predicted values, and CV.

<p>Boldface indicates the best model for each species. If the periodicity <i>T</i> in model 2 was greater than <i>k</i>_max = 29 months, then model 2 was assumed non-biological and was not further considered as an option.</p

Left panels: observed seed production in the 2001–2006 period for 8 species, typical of the different fruiting strategies in the Nouragues forest.

<p>Right panels: observed and predicted values of the autocorrelation index <i>C(k)</i>.</p

Taxonomic description of the study species, life form, dispersal syndrome, and number of sampled seeds.

<p>Taxonomic description of the study species, life form, dispersal syndrome, and number of sampled seeds.</p