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Abstract:
Drought has been frequently discussed as a trigger for forest decline. Today, large-scale forest decline is
observed at the rear edge of Mediterranean forests, with drought identified as the most likely driver. The
vulnerability of Mediterranean mountain plantations to regional climatic variations; however, is poorly
understood. In this paper, we analyze the impact of biotic and abiotic factors on the growth and vigor
of two pine species in drought-prone areas. We assess the main factors influencing crown defoliation
and radial growth to develop a predictive model of forest decline risk for pine plantations at the dry edge
of the species range. Dendrochronological data were collected on 50 plots for Pinus nigra subsp. salzmanii
and 40 plots for Pinus sylvestris.We examined tree size, competition, site characteristics and climate variables
related to decline of pine plantations. Correlation and principal component analysis (PCA) were
used to identify the correlates of decline with crown condition and growth, separately. Logistic regression
and generalized linear models were used to study the relationship between canopy defoliation and
growth, respectively, for P. nigra and P. sylvestris. Explanatory variables were introduced in a stepwise
selection. The PCA revealed three main axes, associated with climate, competition and physiographic
variables. Those three axes were associated with crown damage and basal area growth, respectively.
Probability of crown damage was associated with tree size, competition and climate conditions. P. sylvestris
was more sensitive than P. nigra to summer potential evapotranspiration effects. Also, climate and
competition were the two main drivers affecting basal area growth. Unlike crown defoliation, physiography
had an important effect. Within each species, there was a divergence between healthy and damaged
trees, mainly related to competition factors. On the other hand, growth trends as an independent variable
were not included in the crown damage probability model in a stepwise selection. However, declining
growth found in trees at higher crown defoliation and drier sites may imply a greater vulnerability to
decline, suggesting an enhanced die-off risk. The sharp growth reduction and widespread defoliation
in declining pine plantations make their future persistence in xeric sites subject to frequent and severe droughts unlikely under expected warmer and drier conditions in the future.
