Controls on use efficiency of plant nutrients along subtropical to alpine gradients on the Tibetan Plateau

QuestionKnowledge of how nutrient use strategies differ between forest trees and alpine shrubs/grasses is important to understand the mechanisms of vegetation changes from montane forests to alpine shrubs/grasslands along altitudinal gradients. We tested the hypothesis that, to maximize the nitrogen use efficiency (NUE) of canopy production, forest trees tend to have a higher mean residence time (MRT) of nitrogen in the plants through increased leaf life span, whereas alpine shrublands and grasslands tend to have higher nitrogen productivity through increased below-ground biomass fraction. We further tested whether similar patterns are found in phosphorus use efficiency (PUE).LocationTwenty-one sampling sites along Tibetan Alpine Vegetation Transects (TAVT) at altitudes from 1900 m to 4900 m.MethodsWe measured the maximum biomass of new canopy leaves and twigs and the concentrations of their nutrients N and P and associated ecosystem variables along the TAVT. NUE (PUE) was calculated as the product of nutrient productivity (dry matter production per unit N or P in new canopy leaves and twigs) and MRT (the ratio of foliage nutrient pool to annual nutrient uptake).ResultsWith increasing altitude, leaf life span increased in forest trees but decreased in shrublands and grasslands, while below-ground fraction increased when vegetation changed from montane forests to alpine shrubs/grasslands. In forest trees, higher N-MRT and P-MRT and lower P productivity were associated with longer leaf life span and lower below-ground fraction, while N productivity varied little. In alpine shrublands and grasslands, N-MRT, P-MRT and P productivity varied little with leaf life span and below-ground fraction, while N productivity was positively correlated with below-ground fraction.ConclusionsOur data supported the hypothesis, suggesting that NUE of canopy production would be a measure of changes in ecosystem functioning from montane forests to alpine shrublands and grasslands along altitudinal gradients. The findings provide an insight into the linkage between biogeochemistry and phytogeographic processes across ecosystems. The data along Tibetan Alpine Vegetation Transects indicate that, to maximize nitrogen use efficiency of canopy production, forest trees tend to have a higher mean residence time of nitrogen in the plants through increased leaf life span, whereas alpine shrubs and grasslands tend to have higher nitrogen productivity through increased below-ground fraction of biomass. Similar patterns are found in phosphorus use efficiency.image