Abstract
In drylands all over the world, chronic anthropic disturbances are relevant factors for woodland degradation. Different disturbances can potentially interact to change vegetation structure. Knowing woodland structure is crucial for better predicting forest dynamic because trees of different sizes significantly affect the regeneration processes, number of saplings and young trees, crown traits and competition attributes. Throughout the Monte biogeographic region, grazing and wood extraction are the main anthropogenic disturbances on Prosopis dry woodlands that can generate modifications in key ecosystem processes. First, we aimed to assess woodland structure at sites under chronic anthropic disturbances. Subsequently, we explored how the combination of grazing and wood extraction affected the population structure of P. flexuosa considered by classes: seedlings, saplings and adults. We found that sites under disturbances presented a high density of trees of smaller sizes. Besides, the interaction effect between grazing and wood extraction depended on the intensities of each of the disturbances. The combination of high intensities of both grazing and wood extraction negatively affected the abundance of all classes, but low or moderate grazing intensities combined with high wood extraction had a positive effect on the abundance of saplings and adults. High intensities of co-occurring disturbances decreased seedling abundance affecting forest recruitment which also was negatively reflected in sapling and adult abundance. However, the combination of low or moderate grazing and high wood extraction generated a woodland structure characterized by few seedlings, and many saplings and adult trees of small size. Application of management options (e.g., spatial or temporal separation of human use, programmed management of grazing and wood extraction intensities or planned practices as wood removal for local use) should contribute to enhance the provision of ecosystem services in Prosopis sp. woodlands without risking regeneration persistence and improving the woodland structure.
Highlights
The combination of high intensities of both grazing and wood extraction negatively affected the abundance of all classes, but low or moderate grazing intensities combined with high wood extraction had a positive effect on the abundance of saplings and adults
Chronic anthropogenic disturbances derive from the human use of woodlands to support livelihoods; for example, the extraction of deadwood and non-timber woodland products, grazing or hunting (FAO 2016)
We explored the potential interactive effects of grazing and wood extraction including the interaction between wood extraction index and grazing index in the generalized linear mixed models (GLMM)
Summary
Interactive effects of chronic anthropogenic disturbances on Prosopis woodland structure in the Central Monte, Argentina. Once woody plants reach the sapling stage, they become much less vulnerable, but competition, nutrient limitations and disturbances such as herbivory can combine to slow down sapling development and prolong the time they require to reach the adult stage (Archer et al 2017) Likewise, disturbances such as grazing or wood extraction can cause the destruction of the apical bud in P. flexuosa saplings, favoring vegetative regeneration (regrowth) and generating individuals with multiple stems (Alvarez et al 2011a). Considering what has been stated so far, we hypothesize that chronic disturbances, despite representing a low intensity disturbance, are determinant of P. flexuosa woodland structure in the Monte of Argentina, and their effects could be differential among stage classes, specially affecting the most vulnerable stages, as regeneration (Villagra et al 2004). The final goal of this study is to generate information for the regional management of Prosopis woodlands and to contribute to the knowledge of the functioning of dryland ecosystems in the face of low intensity disturbances sustained over time
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