Abstract
Tree species have good tolerance to a range of environmental conditions, though their ability to respond and persist to environmental changes is dramatically reduced at the rear-edge distribution limits. At those edges, gene flow conferring adaptation is impaired due to lack of populations at lower latitudes. Thus, trees mainly rely on phenotypic changes to buffer against long-term environmental changes. Interspecific hybridization may offer an alternative mechanism in the generation of novel genetic recombinants that could be particularly valuable to ensure persistence in geographically isolated forests. In this paper, we take advantage of the longevity of a temperate-submediterranean mixed-oak forest to explore the long-term impact of environmental changes on two different oak species and their hybrid. Individual trees were genetically characterized and classified into three groups: pure Quercus petraea (Matt.), Liebl, pure Q. pyrenaica Willd, and hybrids. We calculated basal area increment and intrinsic water-use efficiency (iWUE) from tree-ring width and δ13C per genetic group, respectively. Tree-growth drivers were assessed using correlation analyses and generalized linear mixed models for two contrasting climatic periods: (1880–1915, colder with [CO2] < 303 ppm; and 1980–2015, warmer with [CO2] > 338 ppm). The three genetic groups have increased radial growth and iWUE during the last decades, being the least drought-tolerant QuPe the most sensitive species to water stress. However, no significant differences were found among genetic groups neither in mean growth rate nor in mean iWUE. Furthermore, little differences were found in the response to climate among groups. Genetic groups only differed in the relationship between δ13C and temperature and precipitation during the earlier period, but such a difference disappeared during the recent decades. Climate change may have promoted species-level convergence as a response to environment-induced growth limitations, which translated in synchronized growth and response to climate as well as a tighter stomatal control and increased iWUE across coexisting oak species.
Highlights
Forest persistence largely depends on the ability of individual species to adapt their hydraulic strategy to regimen shifts by changes in genotype and phenotype (Potvin and Tousignant, 1996; Aitken et al, 2008; Kremer et al, 2012)
As fixed effects we considered intrinsic water-use efficiency (iWUE), seasonal climate variables and individual tree features such as current Diameter at breast height (DBH) and tree height
Comparing size and growth rate among the three genetic groups revealed no statistical differences on basal area increment (BAI) anomaly and DBH (Figure 2)
Summary
Forest persistence largely depends on the ability of individual species to adapt their hydraulic strategy to regimen shifts by changes in genotype and phenotype (Potvin and Tousignant, 1996; Aitken et al, 2008; Kremer et al, 2012). The question is whether phenotypic changes are able to buffer plants against longterm environmental changes on forests at the tolerance limits, considering that genotypic changes do not have time to take place unless rear-edge populations hybridize with other species. In this context, interspecific hybridization could be valuable in long-lived forests and populations geographically isolated due to the limited accumulation of novel genetic variations for an adaptative response as a consequence of generation length, reduced mutation rates and limited migration across heterogeneous environments (Davis and Shaw, 2001; Petit and Hampe, 2006; Kremer et al, 2012). Interspecific hybridization has been recognized as a relevant mechanism in the generation of novel genetic forms that may ensure populations persistence in a changing climate (Anderson and Stebbins, 1954; Rius and Darling, 2014; Janes and Hamilton, 2017)
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