Dynamics of frost tolerance during regeneration in a mixed (pine–oak–juniper) Mediterranean forest

Abstract

Key message The effect of unexpected low tempera- ture on coexisting species in inland locations was examined; frost tolerance simulation over a 2-year period was used to determine bottlenecks where com- petition can arise. Abstract The effect of frost events on natural regenera- tion of Mediterranean mixed forests under a scenario of climate change is not well known. The expected increase in mean annual temperature and the greater inter- and intra- annual variability in frosts occurrence can significantly damage natural regeneration leading to death, with a decisive effect on species composition and forest dynam- ics. We investigated seasonal variations in frost tolerance of three Mediterranean species (Pinus pinea, Quercus ilex and Juniperus oxycedrus) co-occurring in inland locations in Spain. Our aim was to assess seasonal inter-specific differences in frost sensitivity or tolerance. First, we applied the relative electrolyte leakage method (REL) to estimate the frost tolerance of these species every 3-4 weeks during the 9-month period in which frost events can occur in a year. Second, we explored the REL rela- tionship with environmental variables impacting frost acclimation/deacclimation, i.e., absolute minimum and maximum temperatures and photoperiod. Third, we built a REL model to predict seasonal variations in REL perfor- mance of young trees. We simulated REL through a 2-year period, thus defining seasonal bottlenecks in which the dynamics of regeneration can be affected by frost events. Our results showed contrasting behaviors between species, highlighting a greater sensitivity to cold of P. pinea com- pared to the other two species. The three species are most vulnerable in autumn; however, the different strategies to cope with unfavorable conditions adopted by Q. ilex and J. oxycedrus (e.g., stress tolerance and/or resprouting ability) give these species a competitive advantage over P. pinea.