Seed fate, seedling establishment and the role of propagule size in forest regeneration under climate change conditions

Climate change can affect tree recruitment altering post-dispersal seed fates and impairing seedling development, and these effects will be stronger at man-disturbed areas than beneath forest canopies. Seed biomass, however, positively influences tree recruitment processes and its relevance increases with environmental harshness. Therefore, this seed trait may become critical for forest regeneration under climate change scenarios. We experimentally tested these hypotheses with two Mexican oaks that dominate forests at sites where temperature will increase and rainfall will decrease in the next 20 years. At both experimental sites, we sowed acorns of varying biomass at forest understories and deforested areas under the current climate and simulated climate change conditions. We measured microclimate, incidence of seed parasitism by insects and fungi, germination, shoot emergence, and seedling survival and growth over a year. Temperature in climate change simulations was increased 2 degrees C and rainfall was reduced 17-18%, and this concurred with the near-term changes expected for both variables at the experimental sites. Climate change conditions intensified insect parasitism and decreased fungal infestation on seeds, but these effects were only found within forests and they did not relate with propagule size. Increased warming and drought reduced germination, shoot emergence, and seedling survival and growth at forests and deforested areas, but all these processes were positively related with seed biomass. Indeed, in an oak species, these relationships displaced towards bigger-sized seeds under climate change conditions. Our findings suggest that climate change can alter the intensity of seed-parasite interactions and impair tree establishment processes, also suggesting that this phenomenon will favor large-sized over small-sized seeds during recruitment of some tree species.