The effect of chronic disturbance on the woody plant diversity in a tropicaldry forest of Central Mexico

After deforestation, forest fragments can be impacted by chronic disturbances (e.g., small-scale constant wood extraction), which can alter the forest’s richness and composition, favoring a few generalist species while forest specialist species decline. We tested the effect of chronic disturbance along a disturbance gradient in a tropical dry forest (TDF) of central Mexico, including mature, disturbed, and secondary forests. In the Río Apatlaco subbasin,
central Mexico, we randomly chose 73 fragments (23 mature, 25 disturbed, and 25 secondaries). Along a 0.1-ha transect within each fragment, we identified and measured the diameter at breast height of all plants (DBH ≥ 1 cm, height >1.30 m) and the number of dead trees (snags), stumps, trees with scars of machete/axe, and trees with carbonized trunks. We also quantified the number of cowpats within the transect. We calculated true diversity metrics and pairwise Morisita-Horn similarities. Disturbance diminished from the mature to the secondary forest, while diversity followed the opposite pattern. Along this gradient, distance to the nearest town directly increases plant diversities. An ordination based on floristic composition showed a mixture of forest types, mainly because the disturbed forest became split between secondary and mature forests. This divergence split can be explained because, in some disturbed forests, disturbance-prone pioneer species were favored, and these species were also common in secondary forests. In contrast, the most abundant species in some other disturbed forests are late-successional species. The data suggest that the main drivers of forest change are wood extraction and cattle ranching. Wood extraction generates gaps in the forest, and the cattle can then disperse seeds of pioneer species within these gaps.