CONICET | Buscador de Institutos y Recursos Humanos

Climate change has induced an increase in the frequency and intensity of droughtsand heat waves, reducing growth and increasing the mortality risk of both natural andplanted forests. Management practices, particularly those affecting stand stocking andtherefore the competition of individuals for available resources, are expected to affectthe capacity of stands to tolerate these new and fluctuating environmental conditions.However, the effect of thinning on growth and mortality responses to climatic variationare highly variable among species and environmental conditions. Moreover, most ofthe studies focused on this topic have been carried out in temperate conifers, with veryfew -and constrasting- antecedents in broadleaf species. In this study we analize theeffect of forest thinning on the growth sensitivity of planted Eucalyptus grandis totypical interannual variation in water availability and its response to extreme droughtevents. We analyzed data collected in the period 1999 - 2016 from three thinning trialsinstalled in sites with normal growth conditions of the species in a subtropical, humidand warm region of South America (Mesopotamia region, Argentina), where it is themost planted Eucalyptus species. Different intensities (final densities between 300and 1250 plants/ha) and modalities (single- versus two-stage) of thinning operationswere applied in each trial. The diameter at breast height of all trees was measured witha frequency of approximately one measurement per year, and from these data wecomputed the individual basal area increment. Different climatic variables were alsorecorded during the trials and used to compute the water balance, defined as thedifference between precipitation and potential evapotranspiration. First, we used ahierarchical linear model to relate basal area increment to water balance, and from thisanalysis 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 ofE. grandis increases with thinning intensity while its sensitivity to water balancedecreases, consistently across all sites. At the individual level, using Hegyisintraspecific competition index we observed that higher competition induces lowermean growth and higher sensitivity to water balance, regardless of the number ofthinning stages. Second, we selected an extreme drought event at each site andcomputed resistance and resilience indices, as well as the probability of tree mortalityassociated with the event. Within the single-stage thinning treatments, those with thehighest intensity showed the highest resistance (i.e. the lowest growth decrease duringthe drought event), while stand density did not affect their resilience (i.e. growthrecovery after the drought event). The effect of thinning intensity on the probability ofdeath due to the drought event was not conclusive. On the other hand, the two-stagetreatments presented much higher resistance and resilience values than the othertreatments, suggesting that this modality of treatment could be effective in improvingthe adaptability of E. grandis to extreme drought events.

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