Intensity and number of thinning operations affect the response of Eucalyptus grandis to water availability and extreme drought events

Abstract

Climate change has induced an increase in the frequency and intensity of droughts and heat waves, reducing growth and increasing the mortality risk of both natural and planted forests. Management practices are expected to affect the capacity of stands to tolerate these new and fluctuating environmental conditions. Particularly, the effect of thinning on growth and mortality responses to climatic variation is highly variable among species and environmental conditions, and very few studies have been carried out in broad-leaved species. The objectives of this study were to assess the effect of forest thinning on (1) the growth sensitivity of planted Eucalyptus grandis to typical interannual variation in water availability and (2) its response to extreme drought events. We analyzed data collected in the period 1999–2016 from three thinning trials installed in sites with normal growth conditions of the species in a subtropical, humid and warm region of South America (Mesopotamia region, Argentina), where it is the most planted Eucalyptus species. Different intensities (final densities between 300 and 1250 plants/ha) and modalities (single- versus two-stage) of thinning operations were applied in each trial. First, we used a hierarchical linear model to relate basal area increment to water balance, and from this analysis we obtained the mean growth and sensitivity to water balance at individual, treatment and site levels. Results at the treatment level show that the mean growth of E. grandis increases with thinning intensity while its sensitivity to water balance decreases, consistently across all sites. At the individual level, using Hegyi’s intraspecific competition index we observed that higher competition induces lower mean growth and higher sensitivity to water balance, regardless of the number of thinning stages. Second, we selected an extreme drought event at each site and computed resistance and resilience indices, as well as the probability of tree mortality associated with the event. Within the single-stage thinning treatments, those with the highest intensity showed the highest resistance (i.e. the lowest growth decrease during the drought event), while stand density did not affect their resilience (i.e. growth recovery after the drought event). The effect of thinning intensity on the probability of death due to the drought event was not conclusive. On the other hand, the two-stage treatments presented much higher resistance and resilience values than the other treatments, suggesting that this modality of treatment could be effective in improving the adaptability of E. grandis to extreme drought events. We conclude that high-intensity thinning interventions could increase the ability of this species to cope with climate change and benefit solid wood production, where the associated decrease in stand-level growth may be compensated by an increase in the individual-tree growth. For other industrial purposes, where final stand-level production is a key driver, medium intensity two-stage thinning is recommended, in particular when genetic materials selected for drought resistance are not available and there is high climatic risk.