Chronic human disturbance and environmental forces drive the regeneration mechanisms of a Caatinga dry tropical forest

Abstract

It is now clear that dry forest regeneration relies on a set of regeneration mechanisms such as seed rain, seed and seedling banks, and resprouts that operate simultaneously, but their relative contribution and drivers remain poorly understood. In this paper, we examined the role of seed rain, seed soil bank, seedling, sapling, and resprout assemblages in the regeneration of a Caatinga dry forest in northeast Brazil. We also examined how these regeneration mechanisms respond to chronic disturbances, rainfall, soil fertility, and light in 18 plots located in old-growth forest stands. A total of 5239 seeds from seed rain, 932 seeds in the soil seed bank, 158 seedlings, 85 saplings and 416 resprouts were recorded. Overall regeneration mechanisms provided low-density and impoverished assemblages when examined isolated. Soil seed bank and seed rain provided up to 423 and 35.5 seeds per m², respectively. Seedling and sapling assemblages were dominated by a small set of species (mostly Jatropha mutabilis, Pityrocapa moliliformis, and Trischidium molle), which were both frequent and accounted for most of the individuals. Resprouting was the most important mechanism, accounting for 63.1% of all individuals in the seedling, sapling, and resprout assemblages. Chronic disturbances were identified as key drivers, negatively affecting several community-level attributes of seed rain, soil seed bank, seedlings, and resprouts. Soil fertility and leaf area index (i.e., light availability) were negatively related to seedling and sapling abundances, respectively. Furthermore, the effect of environmental factors on regeneration was more pronounced in disturbed areas. Species composition of saplings and resprouts were associated with chronic disturbances and environmental factors. Our results suggest that, in human-modified landscapes, Caatinga forest regeneration relies primarily on resprouting, while other mechanisms play a minor role in plant species recruitment and maintenance along gradients of disturbances and environmental factors. Given the predicted increase in aridity associated with increased human disturbance, the Caatinga dry forest may experience shifts in forest structure and regeneration patterns. We refer to an absolute prevalence of vegetative propagation leading to a low-statured forest dominated by multi-stem individuals, with unknown effects on long-term forest dynamics and resilience, including community-level homogenization.