Functional diversity and composition of Caatinga woody flora are negatively impacted by chronic anthropogenic disturbance

Tropical plant assemblages can be taxonomically and phylogenetically impoverished by chronic anthropogenic disturbance (CAD), such as firewood collection and extensive grazing. However, to what extent the functional dimension responds to CAD is still unclear. Such knowledge is urgently required for predicting, preventing or even reversing the impacts of CAD.
Chronic anthropogenic disturbance may operate as an ecological filter by selecting functional trait values (e.g. low wood density), thereby altering the functional composition and diversity of plant assemblages. We tested this hypothesis using 29 woody plant assemblages across three ontogenetic stages (seedlings, saplings and adults) in a 220‐km2 landscape of the Caatinga, northeast Brazil. We adopted a CAD index consisting of four indicators (proximity to urban centre and houses and the density of both people and livestock) and tested how well it explained the functional diversity and effect sizes (richness, evenness and dispersion) and composition (community‐weighted mean).
Chronic anthropogenic disturbance affected several functional metrics across the three ontogenetic stages. However, CAD effects were stronger in adult communities by negatively affecting functional richness, dispersion and their effect sizes. CAD also altered the functional composition of leaf mass per area, woody density and leaf area of adult assemblages. Sapling communities were affected in terms of functional composition (leaf area, leaf dry matter and wood density), with positive and negative effects, while seedling assemblages responded positively to CAD only in terms of functional evenness and its effect size. Some changes in functional metrics were influenced by dominant Euphorbiaceae species across ontogenetic stages, especially in terms of leaf area and woody density.
Synthesis. Chronic anthropogenic disturbance is an important driver of plant‐community functional organization across ontogenetic stages in the Caatinga. Adult assemblages are particularly sensitive and tend to lose functional niche space and support more acquisitive rather than conservative strategies as chronic anthropogenic disturbance increases. The proliferation of Euphorbiaceae disturbance‐adapted species can explain part of the community responses to chronic anthropogenic disturbance. Our findings highlight the ecological effects of chronic anthropogenic disturbance and show that it is a key influence on tropical biotas. Changes in plant functional traits associated with plant resource use are likely to affect ecosystem functioning and services provided by Caatinga.

热带植物群落在长期人为干扰（chronic anthropogenic disturbance，CAD，包括薪柴采集与过度放牧）的作用下，其分类学与系统发育多样性会出现衰减。然而，目前尚不清楚功能维度对长期人为干扰的响应程度究竟如何。该领域的研究数据对于预测、防控乃至逆转长期人为干扰的影响至关重要。
长期人为干扰可作为一种生态过滤机制，通过筛选功能性状（functional trait）值（例如低木材密度），进而改变植物群落的功能组成与多样性。本研究以巴西东北部卡廷加植被（Caatinga）一片220平方千米的景观为研究区域，选取了涵盖幼苗、幼树与成年植株三个个体发育阶段的29个木本植物群落，对上述假说进行了验证。本研究采用由四项指标构成的长期人为干扰指数，包括距城镇与居民点的距离、人口密度与牲畜密度，并检验该指数对功能多样性、效应量（涵盖功能丰富度、功能均匀度与功能离散度）以及以群落加权平均值（community-weighted mean）表征的功能组成的解释能力。
长期人为干扰对三个个体发育阶段的多个功能指标均产生了影响。但长期人为干扰对成年群落的影响更为显著，会对功能丰富度、离散度及其效应量产生负面影响。长期人为干扰还改变了成年植物群落的比叶质量、木材密度与叶面积的功能组成。幼树群落在功能组成（叶面积、叶干物质含量与木材密度）方面受到正负双向影响，而幼苗群落仅在功能均匀度及其效应量方面对长期人为干扰产生正向响应。在各个体发育阶段，部分功能指标的变化受到优势大戟科（Euphorbiaceae）物种的调控，尤其是在叶面积与木材密度方面。
综合分析。长期人为干扰是卡廷加植被各个体发育阶段植物群落功能组织的重要驱动因子。成年植物群落尤为敏感，随着长期人为干扰程度加剧，其功能生态位空间会逐渐缩减，且群落会愈发偏向资源获取型策略而非资源保守型策略。适应干扰的大戟科物种的增殖，可部分解释群落对长期人为干扰的响应模式。本研究结果凸显了长期人为干扰的生态效应，证实其是影响热带生物群落的关键因子。与植物资源利用相关的植物功能性状变化，可能会影响卡廷加植被的生态系统功能与服务。