Ecological stoichiometry is an important approach to understand the nutrient cycling and balance through the leaf-litter-soil system of Mongolian pine among different stand origins in desert regions. To reveal the variations in Mongolian pine carbon (C), nitrogen (N), and phosphorus (P) stoichiometry and stoichiometric homeostasis among different stand origins, we measured C, N, and P concentrations of leaves, litter, and soil, and analyzed the nutrient resorption efficiencies of leaves in differently aged plantations and natural forests from semi-arid and dry sub-humid regions. The results showed that (a) the stand origin had a significant effect on the C-N-P stoichiometry, and also significantly affected leaf N and P reabsorption efficiencies. Leaf N/P ratios indicated that Mongolian pine was co-limited by N and P in the NF, HB and HQ, and was mainly limited by P in MU. (2) With increasing stand age, C concentrations in the leaf-litter-soil system initially increased and then decreased, the N and P concentrations and reabsorption efficiencies in the leaf-litter-soil system were gradually increased. Overall, stand age had a significant effect on N concentrations, C/N and C/P ratios in the leaf-litter-soil system. (3) The C and N elements between the leaf-litter-soil system had a strong coupling relationship, and the P element between litter-soil had a strong coupling relationship. In addition, plantations exhibited greater N/P homeostasis than natural forests, and N/P exhibited greater homeostasis than N and P alone, which may be a nutrient utilization strategy for forests to alleviate N or P limitation. (4) Environmental factors have a significant influence on C-N-P stoichiometry in the leaf-litter-soil system, the most important soil properties and meteorological factors being soil water content and precipitation, respectively. These results will be essential to provide guidance for plantation restoration and management in desert regions.

Drought is one of the most significant natural disasters in the arid and semi-arid areas of China. The growth stages of populations or plant organs often differ in how they respond to drought and other adversities. At present, little is known about size- and leaf age-dependent differences in the mechanisms of shrubs-related drought resistance in China’s deserts. We studied Artemisia ordosica Krasch to evaluate its photosynthesis responses to drought stress. A field experiment conducted in Mu Us Desert, Ningxia, China. Rainfall was manipulated by installing outdoor shelters, with four rainfall treatments applied to 12 plots (each 5  5 m). There were four precipitation levels CK (ambient), -30%, -50%, -70%, each with three replications. Taking individual crown size as the dividing basis, the responses of the plants’ photosynthetic systems to drought were measured at different growth stages, i.e. large-sized (＞1 m2), medium-sized (0.25-1 m2), small-sized (＜0.25 m2). In the meanwhile, leaves were divided into mature leaves and new ones for separately measurement. Our results showed that (1) under drought stress, the transfer efficiency of light energy captured by antenna pigments to the PSII reaction center decreased, and the heat dissipation capacity increased simultaneously. To resist the photosynthetic system damage caused by drought, A. ordosica enhanced free radical scavenging by activating the antioxidant enzyme system. (2) The threshold for a reduction in rainfall was 70%; beyond this value, the adaptive regulation of the photosynthetic system in A. ordosica failed. (3) The growth stages and leaves age led to differences in the photosynthetic system reaction to drought. Small A. ordosica plants could not withstand severe drought stress (70% rainfall reduction), whereas large A. ordosica individuals could absorb deep soil water to ensure their survival. Under mild drought stress, tender (younger) leaves had a greater ability to resist drought than older leaves, whereas the latter were more resistant to drought under severe stress.