Stoichiometric homeostasis, physiology, and growth responses of three tree species to nitrogen and phosphorus addition

Stoichiometric homeostasis, indicated by the homeostasis regulation coefficient (H), is closely related to evolutionary stage, and can reflect physiological and biochemical adaption of organisms to environmental changes. However, stoichiometric homeostasis is seldom studied in trees. In this case study, three tree species, two angiosperms (Quercus acutissima and Sapium sebiferum) and one gymnosperm (Metasequoia glyptostroboides), were chosen to calculate the H of nitrogen (H-N), phosphorus (H-P), and nitrogen:phosphorus (H-N:P) after soil N and P addition. The responses of physiology, growth, and biomass to nutrients addition were also determined. The value of H ranged from 5.03 to 21.28 for all organs of the three tree species. H was significantly different among tree species (P=0.010), and was all higher than that of herbs, which may result from phylogenetic differences (angiosperms:gymnosperms) and life form differences (trees:herbs). While having the largest H value, Q. acutissima had the smallest response ratio of physiology (effective quantum yield of PSII), growth (height, root collar diameter), and biomass to soil N and P addition, which indicated that species with stronger stoichiometric homeostasis showed better adaption to environment changes. Our results successfully expanded the stoichiometric homeostasis theory to trees, and provided a link between stoichiometric homeostasis and response of plant traits to environmental changes.