Relationship between degradation and the structural-functional complexity of subtropical xerophytic forests in the Argentine Wet Chaco

Abstract

Globally, forests are severely compromised by land use change and anthropogenic degradation. Forests' structural and biotic homogenization leads to the loss of ecosystem processes that sustain their functionality and determine their contributions to people. Resilience is a key property that expresses the capacity of an ecosystem to tolerate, recover, and/or adapt to disturbances without drastically changing its structure or ecological functions. Once this capacity is exceeded beyond the threshold limit, resilience is lost, and degradation occurs. The Structural – Functional of State and Transition Models (SFSTM) provide a conceptual framework to address ecosystem resilience and the identification of degradation thresholds. In this work, we sought to describe and quantify structural degradation and its relationship between vegetation structural complexity, proxies of ecological processes, and species composition of the quebrachales, a threatened xerophytic subtropical forest of great environmental and socio-economic value in the Wet Chaco of Argentina. For this purpose, a set of forest sites were selected to represent the different histories of uses, where vegetation and soil samplings were carried out. A Structural Degradation Index (SDI) was constructed based on a set of structural variables using multivariate techniques, and the sites were ordered and classified into two structural groups. Linear (generalized) and segmented models were performed to analyze the responses of vegetation heterogeneity and proxies of ecosystem process to structural degradation and to identify thresholds. In addition, species composition was analyzed based on comparing the coefficients of beta diversity, nestedness, and concordance between sites. The structural degradation of the quebracho forest was negatively related to vegetation complexity and ecological processes, and there were breakpoints or non-linear responses between structural groups. Biological diversity was negatively related to anthropogenic degradation due to an increase in beta diversity between structural groups, as well as a process of species divergence between degraded sites. This work shows a clear approach to studying the resilience of subtropical xerophytic forests with concrete results on structural homogenization, loss or decrease of ecological processes, and biotic simplification due to anthropogenic degradation of these ecosystems. In the context of global climate change and rapid human-induced alterations, addressing forest ecosystem resilience from a structural and functional perspective could be a novel approach to its medium- and long-term management.