Nitrogen and phosphorus resorption efficiency of forests in North China

Nutrient cycling is one of the basic functions of forest ecosystems. As two of the main nutrition elements, nitrogen and phosphorus are critical for proper metabolism and growth processes of plants. In order to reduce the dependence on external nutrient, plants can resorb nutrients from senescing leaves prior to abscission and store them into other plant tissues for reuse. Resorption efficiency, defined as the percent nutrient reduction between live and senescent leaves, is used to quantify the resorption capacity of a plant. Nutrient resorption is a key process of plant nutrient utilization strategy. Through nutrient resorption, plants can reduce nutrient loss from the ecosystem and lower the dependence on the environment. This is crucial for the stability of plant population and community, and for ecosystem nutrient cycling. Based on published papers on forests in mountain regions of North China (including the Loess Plateau, Taihang Mountain, North Beijing Mountain Area, etc.), the systemization of nitrogen and phosphorus contents in mature and senescent leaves of forests was conducted depending on the life forms and management methods of forests. All in all, we collected data on nitrogen and phosphorus at 13 sites in the three regions and conducted a comprehensive assessment on nutrient resorption efficiencies. We compared nitrogen and phosphorus resorption efficiencies between trees and shrubs, as well as between natural and plantation forests. In order to explore the factors regulating nutrient resorption efficiency, we also analyzed the relationships between the contents of nitrogen and phosphorus against nutrient resorption efficiency. The results showed that the ranges of nitrogen and phosphorus resorption efficiencies for forests were respectively 24.5%-71.3% and 18.1%-75.4%, with averages of 45.5% and 47.4%, both slightly lower than global average. Average nitrogen content in mature leaves of plantation forests was 21.6 g·kg^-1, significantly higher than that of natural forests (11.6 g·kg^-1). This indicated that in the process of growth of plantation forests, there could be excessive consumption of nutrients. In Taihang Mountain, nitrogen content in mature and senescent leaves and nitrogen resorption efficiency for shrubs were significantly higher than those for trees. However, phosphorus resorption efficiency was relatively lower, demonstrating that plant growth in the region was mainly limited by nitrogen supply. Comparisons among different areas showed that nitrogen and phosphorus resorption efficiencies in North Beijing Mountain Area were higher than those in the Loess Plateau and Taihang Mountain. Correlation analysis showed that both nitrogen and phosphorus resorption efficiencies were significantly correlated with nitrogen content in senescent leaves, indicating that the nitrogen content of senescent forest leaves (also called nutrient resorption proficiency) in North China was the main limiting factor of nitrogen and phosphorus resorption efficiencies. Based on the comparison of nutrient resorption efficiencies of different vegetation species, nutrient resorption efficiencies were greater for conifer forests than that for broad-leaved species. This suggested that in nutrient-barren soils in mountain terrains, nutrient resorption in conifer forests was more efficient, and conifer trees were probably more adaptable to such barren soil environment. In the process of development of artificial plantations in mountain regions, it was recommended to consider high nutrient use efficiency of conifer forests.