Interaction of land management and araucaria trees in the maintenance of landscape diversity in the highlands of southern Brazil

In the southern Brazilian highlands, pre-Columbian societies created domesticated landscapes through the use and management of forests, including nurse Araucaria angustifolia trees, a common conifer in these regions. Nowadays, local smallholders still use traditional practices, such as burning, to promote vegetation for cattle grazing in highland grasslands. Even though burning is normally of small extent and low frequency, such management can slow down natural forest expansion and contribute to the maintenance of grasslands, by opposing the facilitative effect of nurse araucaria trees. To comprehend the interplay between human cultural management, species interactions and the environment, it is important to better understand how these relations affect diversity and composition. Our goal was to investigate how land management, biotic interactions and abiotic factors affect saplings species richness, abundance and composition. We hypothesized that (1) land management would decrease sapling richness and abundance and change sapling composition, (2) nurse araucaria trees would increase species richness and abundance and change sapling composition, and (3) the interactive effect between land management and nurse araucaria trees would shape sapling richness, abundance and composition. Data were collected in unmanaged and managed conditions, both beneath araucaria crowns and in nearby treeless areas. Our results indicate that abundance and species composition are affected by land management and araucaria crown influence. The highest values of sapling abundance were found beneath crowns in unmanaged areas. Species composition changed between all assessed combinations of land management and crown influence. Our study demonstrates the major roles of land management and facilitation in structuring communities, despite the effects of rock and grass cover. Moreover, our results clarify patterns and processes that may emerge in natural highland grasslands, such as the conversion of grasslands into forests and the loss of cultural landscapes when the main local management actions are excluded.

在巴西南部高地，前哥伦布时期（pre-Columbian）的社会通过利用与管理森林打造了驯化景观，其中包括保育型狭叶南洋杉（Araucaria angustifolia）——该区域常见的针叶树种。如今，当地小型农户仍沿用传统手段，例如火烧，以促进高地草原的植被生长，满足牲畜放牧需求。尽管火烧通常规模较小、频率较低，但这种管理方式会抵消保育型狭叶南洋杉的促进效应，进而减缓天然林的扩张进程，助力草原的维持。为厘清人类文化管理、生物互作（biotic interactions）与非生物因子（abiotic factors）之间的复杂互作关系，亟需深入理解这些关系如何影响群落多样性与组成结构。本研究旨在探究土地管理、生物互作与非生物因子如何影响幼苗的物种丰富度、多度及群落组成。我们提出如下假说：（1）土地管理会降低幼苗的物种丰富度与多度，并改变幼苗群落组成；（2）保育型狭叶南洋杉会提升物种丰富度与多度，并改变幼苗群落组成；（3）土地管理与保育型狭叶南洋杉之间的交互效应会塑造幼苗的物种丰富度、多度及群落组成。研究数据采集于未受管理与受管理的样地，且分别设置于南洋杉冠幅下与邻近的无树区域。研究结果表明，幼苗多度与物种组成同时受土地管理措施与南洋杉冠幅效应的调控。未受管理样地内的南洋杉冠幅下，幼苗多度达到峰值。所有评估的土地管理与冠幅效应组合下，物种组成均存在显著差异。尽管岩石与草本覆盖存在影响，本研究仍证实了土地管理与促进作用（facilitation）在群落构建中的核心作用。此外，本研究结果阐明了天然高地草原可能出现的格局与过程，例如当当地主要管理措施被移除后，草原将向森林转化，文化景观也随之丧失。