Matching Species Traits And Microsites Improves Regeneration In Mixed Oak Woodlands

Question Worldwide, mixed oak woodlands regenerate poorly due to different anthropogenic impacts. Our question is whether spatial distribution (regeneration microsite) of oak recruits is determined by different biotic and abiotic stress agents and whether recruits of co-occurring oak species may respond differently to each source of stress.Location Mediterranean California mixed oak woodlands.Methods We recorded the main regeneration microsites of four co-occurring oak species (Quercus lobata, Q. douglasii, Q. kelloggii and Q. agrifolia) in 25 belt transects in a nature reserve and identified the main stress agent for each species and microsite. We also analyzed their response to stress through physiological parameters (chlorophyll, flavonol, anthocyanin and nitrogen).Results Q. agrifolia and Q. lobata seedlings mostly established under shrubs and trees, respectively. The other species did not have a prime regeneration microsite. Biotic stress occurred more frequently than abiotic (drought) stress for all species. Regarding biotic sources of stress, the deciduous Q. lobata, Q. douglasii and Q. kellogii showed high browsing damage; insect damage was light and occurred mostly in the evergreen Q. agrifolia whereas fungal damage was significantly lower in Q. douglasii. Physiologically, all oak species responded similarly to stress agents although Q. douglasii and Q. kelloggii showed higher chlorophyll content than Q. agrifolia and Q. lobata, which might be related to differences in resilience.Conclusions Deciduous oaks are particularly vulnerable to overgrazing whereas the evergreen Q. agrifolia seems more affected by fungal and insect damage. Management aimed at the co-existence of deciduous and evergreen oaks should consider the enhancement of microsites that best ameliorate the impact of predicted stress agents but also the oak physiological responses (i.e. resilience) to each stress agent. We suggest keeping low ungulate densities and carefully consider the species selection in assisted regeneration projects depending on the predominant stress agent and microsite.