Aims: Across tropical regions, large forest areas have been converted to different agricultural land uses. These uses impose ecological disturbances affecting forest regeneration potential after field abandonment. Finding ways to identify those agricultural land uses limiting forest regeneration is a critical issue for conserving biodiversity in human-modified landscapes. Here, we developed a fast and inexpensive index, useful for quantifying ecological disturbance regimes associated with agricultural land uses, and tested its power to predict forest regeneration potential.Location: Municipality of Marqu es de Comillas, southeast Mexico.Methods: Interviews were conducted with local farmers to quantify disturbance components (size, duration and severity) associated with agricultural land uses. The scaled values of these disturbance components were added in a simple ecological disturbance index (EDI). In each one of nine recently abandoned fields representing a wide range of EDI values, two 10-m 2 plots, one close to and one far from nearby forest remnants, were established. On each plot, all woody plants of 10-100 m in height were counted, identified and measured in four 1m 2 subplots, at the time of field abandonment and 2 yr later. In addition, at each plot, 18 site condition (microclimate and soil) attributes were quantified at the time of abandonment. Plant density, biomass, species richness and species diversity were used as regeneration variables, and EDI and site condition attributes as independent ones.Results: Two years after abandonment, most regeneration variables declined exponentially with EDI. Biomass was not explained by EDI but changed positively with light availability. EDI was strongly correlated to vapour pressure deficit, which also predicted regeneration potential (except biomass).Conclusions: EDI is a cheap and easy tool for quantifying the ecological disturbance produced by a wide range of agricultural land uses. The index predicted several regeneration variables as well as or better than direct measurements of the site condition at the time of abandonment. EDI can be used to identify biodiversity-friendly agricultural land uses in human-modified landscapes.

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Questions What is the magnitude of the differences in environment and chronic human disturbance between contrasting slope aspects and topographic positions in a seasonally dry tropical forest? What is the effect of such topography‐related differences on composition, structure and diversity attributes of the tree community of this forest? Location Tziritzícuaro, Michoacán State, southern Mexico. Methods Vegetation was sampled in 36 100‐m2 plots evenly distributed among three topographic positions (lower, middle and upper parts of a slope) and two slope aspects (N‐ and S‐facing). Environment at these sites was described through modelling incoming solar radiation and in situ recording of temperature during 1 yr. Disturbance was visually assessed in the field to calculate a Chronic Disturbance Index. Vegetation structure and diversity were compared among the resulting combinations of slope and topographic position. PERMANOVA and CCA were used to examine the multivariate relationship among vegetation, topography and disturbance. Results Slope aspects and topographic positions differed in terms of annual mean temperature, potential energy income and evapotranspiration. Conversely, disturbance was not so clearly related to topography. Regarding vegetation structure, significant differences were only found for individual sizes and abundance; these values increased towards the upper portion of S‐facing slopes, but decreased with elevation in N‐facing slopes. Species diversity (S, Jacknife 1 and Fisherʼs α) was higher in S‐facing slopes and increased from lower to upper topographic positions. PERMANOVA showed that vegetation structure and diversity were influenced by topographic position (12.9%) and the interaction between soil moisture and chronic disturbance with slope aspect (8.2% and 8.3%, respectively). CCA showed that a modest proportion (21%) of variation in species composition is explained by the combination of environmental and disturbance variables. Conclusions Slope aspect and topographic position represent axes of environmental and disturbance differentiation. Although vegetation attributes respond to these ecological factors, they do not show homogeneous responses. Floristic composition is clearly linked to environmental heterogeneity, while structural attributes and α‐diversity appear to be more closely related to human disturbance and soil moisture, particularly on S‐facing slopes. Integrating environmental heterogeneity and human disturbance with topographic variability enhances our understanding of large variation in tree community attributes in seasonal dry tropical forests.

In seasonal plant communities, it is recognized that topography-related variation in water availability and solar radiation determine vegetation structure and community composition; however, the effects on functional structure, particularly through changes in resource use strategies of plants are still poorly understood. This study examines the effects of slope aspect and topographic position on functional trait dominance in a tropical dry forest landscape and explores whether strategies for coping with drought (avoidance vs. tolerance) segregate spatially along the water stress gradient created by the interaction of these two topographic factors. The study was conducted in the Balsas river basin in south-central Mexico. Functional traits were evaluated in 63 species of trees (≥ 2.5 cm diameter at breast height) dominant in plots located at three topographic positions (low, medium and high) and on two slope aspects (north and south). Eight leaf and four stem functional traits, relating to the plants' ability to avoid or tolerate water and temperature stress, were measured. Community-level functional traits were strongly affected by topographic position while only a weak signal was detected by the slope aspect. Contrary to our expectations, attributes associated with drought tolerance predominated on the lower topographic positions of the slopes, (moister and warmer sites), while on the upper parts with drier soil, but cooler air, attributes associated with water stress avoidance dominated. In addition, variation in the dominance of leaf pulvini and trichomes along the topographic gradient suggests environmental filtering by elevated air temperatures and water stress, respectively. Overall, our results suggest that the upper topographic positions that generate a shorter and more fluctuating water-availability window, favor readily-deciduous plants with high levels of water storage in their tissues, traits allowing for a rapid avoid of water stress, whereas on the lower topographic positions, where the soil remains moist for longer periods of time, tardily-deciduous species thrive with dense, low-water content tissues, attributes that are associated with physiological tolerance to drought.

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1. Predicting plant community assembly is challenging in part because the influence of environmental conditions via plant functional strategies and the relevance of mechanisms of community assembly change across habitats and these changes remain poorly studied.

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The regeneration of many tropical trees is threatened by forest fragmentation because it produces major physical, biological and ecological changes that limit seed germination and seedling establishment. We analyzed the regenerative potential of an old growth forest tree species-Ampelocera hottlei (Ulmaceae)-in three contrasting habitats located in the Lacandona rain forest, southeastern Mexico: continuous forest, fragments occupied by black howler monkeys (Alouatta pigra) and fragments unoccupied by howlers. We tested if germination of A. hottlei seeds among habitats was affected by understory temperature, light incidence and ingestion by A. pigra. We compared seedling survival and relative growth rate in height (RGR H ) for 20 d among habitats and between ingested and control seeds (from mature fruits). Germination was higher in continuous forest than in fragments (occupied or not), with higher germination rates for ingested seeds in fragments. Temperature and light incidence were lower in continuous forest than in fragments. Germination decreased with increasing temperature and light incidence with this relationship being significantly higher for ingested seeds. Seedling survival was higher in continuous forest than in fragments, whereas RGR H did not differ among habitats. In addition, survival and RGR H were higher in seedlings originating from ingested seeds. Overall, our results suggest that the populations of A. hottlei can be limited in fragments where changes in the understory physical environment and the extirpation of A. pigra will likely have deleterious consequences for the regeneration of A. hottlei and possibly for other tree species, ultimately affecting forest composition and structure.