Climate change projections for the Mediterranean basin predict a continuous increase in extreme drought and
heat episodes, which will affect forest dynamics, structure and composition. Understanding how climate influences
the maximum size-density relationship (MSDR) is therefore critical to designing adaptive silvicultural
guidelines based on the potential stand carrying capacity of tree species. With this aim, data from the Third
Spanish National Forest Inventory (3NFI) and WorldClim databases were used to analyze climate-related variations
of the maximum stand carrying capacity for 15 species from the Pinus, Fagus and Quercus genera. First,
basic MSDR were fitted using linear quantile regression and observed size-density data from monospecific 3NFI
plots. Reference values for maximum stocking, expressed in terms of the Maximum Stand Density Index (SDImax),
were estimated by species. Then, climate-dependent MSDR models including 35 annual and seasonal climatic
variables were fitted. The best climate-dependent models, based on the Akaike Information Criteria (AIC) index,
were used to determine the climatic drivers affecting MSDR, to analyze general and species-specific patterns and
to quantify the impact of climate on maximum stand carrying capacity. The results showed that all the selected
climate-dependent models improved the goodness of fit over the basic models. Among the climatic variables,
spring and summer maximum temperatures were found to be key drivers affecting MSDR for the species studied.
A common trend was also found across species, linking warmer and drier conditions to smaller SDImax values.
Based on projected climate scenarios, this suggests potential reductions in maximum stocking for these species.
In this study, a new index was proposed, the Q index, for evaluating the impact of climate on maximum stand
carrying capacity. Our findings highlight the importance of using specific climatic variables to better characterize
how they affect MSDR. The models presented in this study will allow us to better explain interactions
between climate and MSDR while also providing more precise estimates concerning maximum stocking for
different Mediterranean coniferous and broadleaf tree species.
