Positive outcomes between herbivore diversity and tree survival: Responses to management intensity in a Patagonian forest

Abstract

Sustainable forest management aims to both enhance biodiversity and tree productivity. However, trade-offs may exist between these objectives, e.g. arthropod diversity and associated arthropod herbivory can reduce tree growth and survival. Tree productivity and biodiversity may also show non-linear responses to management intensity, making applied recommendations even more challenging. We studied the effects of harvesting intensity for firewood extraction in a northern Patagonian forest (Argentina) on leaf damage diversity (as a proxy of arthropod folivore diversity), leaf damage frequency, growth and survival of planted trees of Nothofagus obliqua for timber purpose. We randomly applied four levels of harvesting intensity (0, 30, 50, and 70% of basal area removal) in experimental plots and we followed the responses on the focal planted tree species during two growing seasons. After harvesting, air temperature and photosynthetic active radiation increased, while relative humidity decreased, with harvesting intensity. Leaf damage diversity and frequency showed non-linear responses to harvesting intensity, both achieving its highest values at intermediate harvesting intensities (30% and 50% of basal area removal). Such responses were consistent for contrasting feeding guilds, using different diversity indices, and during the two study growing seasons. Interestingly, leaf damage diversity and frequency were positively and strongly correlated across the plots. Despite great leaf damage frequency (around 45% of the leaves were damaged), plant survival was also highest at intermediate harvesting intensities during the two years. Planted trees also grew more at intermediate harvesting intensities during the second year, but increased linearly with harvesting intensity during the first year. Hence, at intermediate harvesting intensity, no trade-off was observed between arthropod biodiversity and planted tree sapling survival and growth. In northern Patagonia, trees may face less competition for light and soil resources at intermediate harvesting intensities, while at high harvesting intensities water stress typical of dry Patagonian summers could reduce tree survival. Such benefits were not offset by the greater leaf damage (associated with enhanced damage diversity) observed at intermediate harvesting intensities. Therefore, intermediate levels of management intensity can provide the double service of increasing arthropod diversity and maximizing tree survival and growth, especially during the most critical establishment period.

Intermediate harvesting intensities enhance native tree performance of contrasting species while conserving herbivore diversity in a Patagonian woodland

“…Within this woodland, in May 2013, we applied four increasing levels of harvesting intensities in four plots (31.5 m × 45.0 m), meaning that we had one harvesting intensity level per plot (Nacif et al, 2020). The plots were aligned (east-west) and separated approximately 30 m from each other.…”

“…firewood, posts) instead of applying large clear-cut techniques. As a result, patches remaining within the original woodland have proved to enhance productivity, woodland health, resilience and biodiversity conservation, besides providing an initial economic income that helps cover the investment of planting trees (Gadow, 2006;Nacif, Kitzberger & Garibaldi, 2020;Goldenberg et al, 2020). Among plantation options, native tree plantations are a recommended option since they improve ecosystem and social value, conserve biodiversity and mitigate the risks associated to exotic species plantations such as fire propagation, potential invasion of plants, pathogens and/or other associated organisms (Cusack & Montagnini, 2004).…”

“…In this context, facilitation from wild trees and bushes to the planted saplings could be critical in some landscapes (Pretzsch, Forrester & Bauhus, 2017). Thus, plant response against a management stress gradient could be non-linear (Malkinson & Tielbörger, 2010;Nacif, Kitzberger & Garibaldi, 2020), meaning that plant performance does not simply increase or decrease with increasing harvesting intensities. Thus, even shade-intolerant species might be affected by high radiation levels, and plants that need shade protection cannot develop to their fullest (Bauhus et al, 2010;Liu et al, 2018).…”

Diversity, functionality, and resilience under increasing harvesting intensities in woodlands of northern Patagonia

“…Between 2013 and 2014 six plots were harvested in six strips of increasing width (1.5, 2.5 and 3.5 m; along the plots) with the two remaining plots serving as controls, resulting in 0, 30 %, 50 % and 70 % approximately of basal area removal, respectively. All stems with more than 4 cm of diameter were classified as firewood leaving smaller branches and leaves in the intervention strips (Coulin et al 2019;Nacif et al 2020;Carrón et al 2020). The design of the interventions were relatively conservative, as we performed a high number of harvesting strips of relatively low width (i.e.…”

“…Forest harvesting is a common silvicultural practice, which reduces tree density to improve tree growth, affecting biodiversity and community structure according to stand complexity and management strategies (Verschuly et al 2011;Duguid and Ashton 2013;Root and Betts 2016). In temperate forests, where light availability is a key driver of community composition, canopy opening can increase species diversity, as it promotes a variety of new habitat types and resources (Lencinas et al 2011;Chillo et al 2018;Nacif et al 2020). Yet disturbance effects on biodiversity are not independent of site productivity (Thomas et al 1999;Cingolani et al 2005), although few studies consider both factors simultaneously.…”