Plant life-form distribution patterns in a tropical mountain region: Effect of climate, topography, and human disturbance

Question: Understanding the specific distribution patterns of vascular plants on different spatial scales is central in ecology and conservation. We evaluate the distribution patterns of five plant life forms (climbers, epiphytes, herbs, shrubs, and trees) along the elevation gradient and geographic space of a mountain system, analyzing climate, topography, and human disturbance to explain variation in the richness of each life form, and determine the contribution of each life form to total richness along the elevation gradient.Location: Sierra Madre of Chiapas, southeastern Mexico.Methods: We used linear models to evaluate the elevational pattern of richness for each life form, as well as total richness, and analyzed the effects of climate (water-energy dynamics, thermal range), topography (topographic heterogeneity), and disturbance (Human Influence Index) on variation in total richness as well as of each life form using Generalized Linear Models (GLM). We used Non-Metric Multidimensional (Distance) Scaling to determine variation in species composition along the elevation gradient. Finally, the contribution of each life form to total richness was evaluated using GLM.Results: We recorded 235 families, 1,439 genera, and 5,196 species. Total richness and the richness of each life form increased with increasing elevation. GLM explained a high proportion of the variation in the richness of each life form (34.3% for total richness; climbers 17.2%, epiphytes 44.9%, herbs 20.4%, shrubs 33.5%, and trees 24.9%). The proportion of richness of herbs and trees to total richness decreased, and the proportion of shrubs, epiphytes, and climbers increased with elevation. Climate largely determined species richness for all life forms, whereas disturbance was significant only for epiphyte richness.Conclusions: Results suggest that mechanisms driven by climatic variables (especially thermal range) contribute to maintaining the richness of each life form. However, human disturbance modifies distribution patterns and leads to a decrease in epiphyte richness. The differential contribution of each life form to total richness along the elevation gradient presents a challenge for designing conservation strategies applicable to all plant groups.