Understanding the importance of intrapopulation functional variability and phenotypic plasticity in Quercus suber

Climate is one of the main abiotic factors influencing natural selection patterns. Year-to-year variation in climate is postulated to elicit temporal shifts in the direction and form of selection. Here, we examine the importance of trait means and plasticities for fitness under interannual variation in rainfall and assess the shifts in selection in cork oak. We performed selection analyses using the progeny of 45 mother trees established in a common garden experiment across two consecutive years that differed in rainfall. Growth and seven functional traits (specific leaf area, leaf size, leaf shape traits, 13-carbon isotope discrimination, and leaf nitrogen) related to drought tolerance were measured. Selection analyses showed fitness benefits of reduced specific leaf area (SLA) in a dry year and increased leaf size in a mesic year, indicating that they are key traits for this evergreen oak to cope with different water availabilities. SLA and leaf size were also particularly plastic traits, but the adaptive significance of plasticity could not be confirmed. The absence of correlation between growth across years using family means and the absence of correlations between SLA and leaf size suggested that fluctuating selection over time favored different maternal families under different annual weather conditions, which could promote functional diversity within populations in this long-lived species.