Reproductive functional organization of woody plant assemblages along regeneration in a Caatinga dry forest

Abstract

Tropical forest regeneration across old fields has been mainly described as a predictable sequence of functional plant assemblages in response to environmental filtering. However, the way plant reproductive diversity is organized along forest regeneration and how the reproductive profile of woody flora may impact regeneration have been poorly investigated. This paper examines the organization of plant assemblages throughout the regeneration of a Caatinga dry forest in the context of slash-and-burn agriculture with a focus on reproductive plant traits and strategies. We hypothesized a recovery and directional changes in specialist reproductive traits/strategies along forest regeneration but with aridity and chronic anthropogenic disturbances posing contrary forces to this recovery process. This study was carried out in a human-modified landscape of Caatinga dry forest in northeast Brazil. We compiled information on woody plant assemblages inhabiting a forest chronosequence consisting of 28 forest stands of varying age: 14 regenerating (4–50-yr old) and 14 old-growth forests. Woody plant assemblages along forest regeneration had higher occurrence of specialized traits and strategies, such as pollination by vertebrates (16.6%) and dispersion by animals (47.8%). Regenerating forest stands exhibited a higher occurrence of species with specialized floral size (19%) and pollination by vertebrates (26.5%) as compared to old-growth forests. Regenerating forest stands also supported a higher (30%) functional diversity of reproductive traits than old-growth stands. Biomass, aridity, and chronic anthropogenic disturbances affected a myriad of traits/strategies: pollination by birds and animal dispersion increased with increases in biomass while total functional diversity responded negatively; pollination by Sphingids and beetles negatively responded to aridity, while very large fruit size species negatively responded to chronic anthropogenic disturbances. Both forest biomass and aridity correlated negatively with reproductive trait diversity. The Caatinga dry forest regeneration suggests high resilience, no directional/predictable changes (although regenerating vs. old-growth differ in some traits) and aridity imposed to the successional mosaics as a driver. This pattern probably emerges conditioned by the reproductive traits posed by the plant species able to thrive in such stressful environments, including those species that persist through the entire regeneration process, i.e., the regeneration process is marked by high complexity and low predictive power. Although rapid recovery represents good news, the plant-assemblage reproductive profile imposed by disturbance-adapted species may drastically alter resource availability for a myriad of plant attendants (e.g., pollinators and thief) and, consequently, the complex/diverse plant-animal interactions typical of dry forests. This working hypothesis has both theoretical and applied implications in the context of dry forest regeneration in human-modified landscapes.