Climate change is expected to cause greater stress on forests by superimposing more extreme droughts on elevated evaporative demand (VPD) due to warming. While nitrogen is abundant on the surface of the Earth, less than 2% is available to organisms and although the supply of nitrogen is expected to increase due to human activities, forest nutrition could be severely impaired by a warmer and drier climate. There is clear evidence that altered water availability and temperature rise have multiple effects on the nutrient supply of plants, although the mechanisms are far from being understood.
In a semi-arid woodland, we subjected mature piñon pine and juniper trees to multi-year precipitation reduction, atmospheric warming and their simultaneous effects. We report here the response of tree nitrogen use and analyze how soil nitrogen cycling, tree nitrogen uptake and allocation patterns in response to long-term drying and warming conditions influence these patterns.
This work shows that warmer conditions superimposed on more extreme precipitation regimes will likely modify the nutrition of trees in the future. Changes in soil nitrogen cycling (e.g. microbial activity, nitrification and ammonification rates, soil pH) occured in response to higher temperature and reduced precipitation. Consequently, the ability of trees to take up nitrogen could be impaired and modify nitrogen allocation patterns between aboveground and belowground compartments. Although no additive effect of warming and drying were found for the two studied species, contrasting responses to warming and drying were observed between the two functional types. Overall, our results show that higher temperature and reduced precipitation regimes will alter the nutrition of forest ecosystems in the future with potentially large consequences for forest function, structure and biodiversity.