Are the combined effects of warming and drought on foliar C:N:P:K stoichiometry in a subtropical forest greater than their individual effects?

Abstract

Carbon (C), nitrogen (N), phosphorus (P), and, more recently, potassium (K) have been examined in ecological stoichiometry because they are the most abundant elements in organic matter. An increasing number of studies are investigating the potential responses of plants to future global climate change scenarios, and although warming and drought may greatly influence ecosystem function and services, their combined effects on C:N:P:K stoichiometry remain unknown. In the present study, we assessed the stoichiometry of a major subtropical tree species Cunninghamia lanceolata (Lamb.) Hook, in southern China in terms of its responses to warming and drought stresses. To measure C, N, P, and K concentrations, C:N:P:K stoichiometric ratios, and stable isotope abundance, we sampled C. lanceolata needles from seedlings subject to four treatments: control (CT), warming (W, with 5 °C increase in temperature), drought (D, with 50% decrease in precipitation), and the abovementioned warming and drought conditions combined (WD). The warming and drought conditions applied simulated the climate changes predicted for the next decades in southern China. We found that variations in foliar C:N:P:K stoichiometric ratios were driven by changes in nutrient concentration, season, and treatment. Unexpectedly, differences in foliar intrinsic water use efficiency across sampling seasons were not significant. The WD conditions increased foliar N concentration (57%) and the δ15N value (111%) across the different seasons, and significant interactive effects between warming and drought on foliar N concentration and δ15N were clearly demonstrated. The N cycle is likely to accelerate in subtropical forests, with more of the “heavier” N being mobilized for plant use, and thus the combination of warming plus drought may alleviate plant N restrictions in the future. Thus, foliar δ15N might be used as an indicator of the changes in N cycling in C. lanceolata forests.