Climate change in forest ecosystems: A field experiment addressing the effects of raising temperature and reduced rainfall on early life cycle stages of oaks

Higher temperatures and reduced rainfalls that are expected with the advance of climate change can impair the emergence and establishment of tree seedlings in forest ecosystems. These climatic changes can also decrease the availability of soil resources and reduce the performance of seedlings. We evaluated these effects in a temperate forest from Mexico with two native oak species (Quercus crassifolia and Quercus eduardii). As recently emerged oak seedlings are highly sensitive to changing environmental conditions, our field experiment was conducted across the season in which seedling emergence occurs (October-February). In the field, we used open-top chambers to increase temperature and rainout shelters to reduce rainfall, while controls were exposed to the current climate. Experimental plots of both treatments were established beneath the forest canopy because most oaks recruit in understory habitats. In these plots, we sowed acorns of both species in October 2015 and recorded seedling emergence and survival until February 2016, also monitoring temperature, precipitation and contents of water and nitrogen in the soil. On seedlings that survived until the end of the experiment we measured their growth, photosynthetic efficiency and foliar contents of water, carbon and nitrogen. Both the emergence and survival of Q. crassifolia seedlings were lower in climate change plots than in controls, but no differences were found for Q. eduardii. However, seedlings of both species had lower growth rates, photosynthetic efficiencies and contents of water, nitrogen and carbon in climate change simulation plots. These results indicate that climate change can impair tree seedling establishment in oak forest, also suggesting that their development will be constrained by reduced water and nitrogen availability.