Aim: By moving beyond traditional biome‐centric approaches to sample continuously across a massive ecotone, we investigate how species richness, community composition, and dominance of woody plants change across a strong longitudinal precipitation gradient and identify thresholds in water availability that drive shifts in community assembly between tropical dry forests and rainforests. Location: A longitudinal precipitation gradient within and beyond the Marañon Valley, a biogeographic corridor linking Pacific dryland forests, Andean highland forests, and Amazon rainforests in northern Peru. Time Period: Contemporary forests sampled between 2016 and 2020. Major Taxa Studied: Woody tree species (angiosperms). Methods: We modelled species richness, community composition, and species dominance in response to climatic and edaphic factors using floristic data from the first National Forest Inventory in Peru. Results: Alpha diversity increased from the coastal drylands to the Andes, peaking at a 1299 mm annual precipitation threshold. Beyond this threshold in the Amazon, climatic factors no longer explain diversity patterns. Species composition shifted gradually, exhibiting sharp regional differences at 1265 mm. Species dominance patterns changed markedly along the longitudinal axis, from locally dominant, stress‐adapted species in drylands to regionally widespread low‐dominant species in the Amazon Forest. Main Conclusions: Our findings indicate that major transitions in woody plant biodiversity are tightly linked to rainfall patterns up to a critical threshold, beyond which other factors likely govern community structure, highlighting the vulnerability and potential reorganization of these ecosystems under ongoing climatic change. Ultimately, this demonstrates that assessing the landscape as a continuum, rather than exploring isolated biomes, is crucial for capturing the non‐linear environmental thresholds that redefine community assembly rules on a global scale.

Neltuma pallida is a multi-purpose tree species of the seasonally dry tropical forests of northern Peru, where it provides essential ecological and socioeconomic functions. However, recurrent defoliation associated with the cecidomyiid gall midge Enallodiplosis discordis may compromise early seedling establishment and the success of forest restoration programs. This study evaluated the effects of larval infestation on foliar integrity and established quantitative damage thresholds in N. pallida seedlings under dry forest conditions. Insects collected from naturally infested plants were identified using an integrative taxonomic approach that combined classical morphological diagnosis with COI-based DNA barcoding obtained by Sanger sequencing. Morphological assessment assigned the defoliating dipteran to E. discordis, while BLASTn v2.17.0. analysis of the 576-bp partial COI sequence showed 92.6% identity and 100% query coverage with Cecidomyiidae records, supporting its taxonomic placement within this family. Field bioassays conducted over a 17-week period, in which 25 individual seedlings were evaluated (N = 25), revealed a strong and significant positive correlation between larval density and foliar damage percentage (r = 0.872; p < 0.001), with moderate damage levels predominating throughout the evaluation period. Despite sustained larval presence, seedlings did not reach severe damage categories, suggesting potential relative tolerance to partial defoliation under the evaluated field conditions. Temperature and relative humidity were not significantly associated with infestation intensity or foliar damage during the study period. Overall, these findings indicate that E. discordis-associated foliar damage represents a relevant, although not necessarily lethal, biotic constraint for the early regeneration of N. pallida under the field conditions assessed. The quantitative thresholds reported here provide useful criteria for dry forest restoration programs, phytosanitary monitoring, and integrated pest management strategies in the Peruvian dry forest.