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 andanthropogenic degradation. Forests´ structural and biotic homogenization leadsto the loss of ecosystem processes that sustain their functionality anddetermine their contributions to people. Resilience is a key property thatexpresses the capacity of an ecosystem to tolerate, recover, and/or adapt todisturbances without drastically changing its structure or ecologicalfunctions. Once this capacity is exceeded beyond the threshold limit,resilience is lost, and degradation occurs. The Structural – Functional ofStates and Transitions Models (SFSTM) provide a conceptual framework to addressecosystem resilience and the identification of degradation thresholds. In thiswork, we sought to describe and quantify structural degradation and its relationshipbetween vegetation structural complexity, proxies of ecological processes, andspecies composition of the quebrachales, a threatened xerophyticsubtropical forest of great environmental and socio-economic value in the Wet Chacoof Argentina. For this purpose, a set of forest sites were selected torepresent the different histories of uses, where vegetation and soil samplings werecarried out. A Structural Degradation Index (SDI) was constructed based on aset of structural variables using multivariate techniques, and the sites wereordered and classified into two structural groups. Linear (generalized) andsegmented models were performed to analyze the responses of vegetationheterogeneity and proxies of ecosystem process to structural degradation and toidentify thresholds. In addition, species composition was analyzed based oncomparing the coefficients of beta diversity, nestedness, and concordancebetween sites. The structural degradation of the quebracho forest wasnegatively related to vegetation complexity and ecological processes, and therewere breakpoints or non-linear responses between structural groups. Biologicaldiversity was negatively related to anthropogenic degradation due to anincrease in beta diversity between structural groups, as well as a process ofspecies divergence between degraded sites. This work shows a clear approach to studyingthe resilience of subtropical xerophytic forests with concrete results onstructural homogenization, loss or decrease of ecological processes, and bioticsimplification due to anthropogenic degradation of these ecosystems. In thecontext of global climate change and rapid human-induced alterations,addressing forest ecosystem resilience from a structural and functionalperspective could be a novel approach to its medium- and long-term management.