Seasonally dry tropical forest temporal patterns are marked by floristic stability and structural changes

ABSTRACT Background: Seasonally Dry Tropical Forests (SDTF) are recognized by their great biological diversity, but little is known about their temporal patterns, which may be crucial in a changing world. Here we aimed to characterize the SDTF dynamic patterns in floristic and structural changes, and also to evaluate shifts in dynamic rates and structural attributes such as richness, number of trees and biomass. Our hypothesis is that SDTF tree communities have their dynamic characterized by temporal fluctuations related to an instability pattern, in according to the already found for others tropical forests. For this we used a data of 42 400 m² plots in three fragments in the Brazilian Arboreal Caatinga, measured in 2005, 2010 and 2015. We evaluated temporal changes in richness, species composition, trees density, biomass and rates of tree mortality and recruitment; and also rates of biomass gain and loss. Results: We found a pattern of floristic composition stability, but with changes in structure and dynamics. There was a tree density decrease driven by constant mortality and by recruitment decrease, and biomass increase due to decrease in biomass loss and to increase in biomass gain. The biomass increase was main related to a small set of dominant species that are the most representative in the community dynamics. Conclusion: SDTF dynamics are related to stability in species composition, but with structural changes towards higher biomass stocks. The results are main related to community dynamics and to SDTF attributes such as climatic seasonality that drives the local ecological processes.

摘要：研究背景
季节性干旱热带森林（Seasonally Dry Tropical Forests, SDTF）以极高的生物多样性著称，但针对其时间动态格局的相关认知仍较为匮乏，而在全球变化的当下，这些格局的重要性愈发凸显。本研究旨在阐明季节性干旱热带森林在区系组成与结构层面的动态格局，并评估其动态速率及物种丰富度、树木数量、生物量等结构属性的变化趋势。我们提出假说：与其他热带森林的已有研究结论一致，季节性干旱热带森林的木本植物群落动态以与不稳定模式相关的时间波动为核心特征。
为此，我们使用了巴西乔木型卡廷加（Brazilian Arboreal Caatinga）3个生境片段内共计42块400㎡样地的调查数据，调查时间分别为2005年、2010年与2015年。本研究评估了物种丰富度、物种组成、树木密度、生物量以及树木死亡率与种群补充率的时间变化，同时分析了生物量的积累与损失速率。
结果：我们发现，季节性干旱热带森林的区系组成整体呈现稳定格局，但群落结构与动态均发生了显著变化。树木密度因持续的高死亡率与种群补充率下降而降低；生物量则因生物量损失减少、积累速率提升而实现增长。生物量的提升主要由少数在群落动态中占据主导地位的代表性物种驱动。
结论：季节性干旱热带森林的动态表现为物种组成保持稳定，但群落结构朝着更高生物量储量的方向发生改变。本研究结果主要与群落动态以及驱动当地生态过程的季节性干旱热带森林气候季节属性相关。