Assessing carbon and nitrogen economics in temperate forests through the relationship between foliar nutrient resorption and root production.

Abstract

Plants both respond to and influence their immediate soil environment, which can yield divergent predictions regarding plant economics and trait coordination. Tree species with high foliar nitrogen (N) resorption efficiency (NRE)-an important N conservation strategy-may invest less carbon (C) belowground to acquire soil-derived N. This "tree perspective" hypothesis predicts a negative relationship between NRE and root production. Alternatively, high NRE reduces litter N concentrations, which can reduce soil N availability, requiring trees to invest more C belowground to get N. This "soil perspective" hypothesis predicts a positive relationship between NRE and root production. We test these hypotheses and then examine how NRE relates to foliar and litter N in three natural forests (~ 80-120-year-old trees; 12 species) and one common garden (~ 25-year-old trees; 9 species) in the eastern U.S. NRE was weakly and positively related to root production at the common garden, supporting our "soil perspective" hypothesis that litter-soil nutrient feedbacks drive a positive relationship between NRE and root production. There was no relationship between NRE and root production at the natural forest sites, providing no evidence for our "tree perspective" hypothesis, which purports that NRE is negatively related to root production given competition between roots and leaves for C. NRE was positively related to foliar N but negatively related to litter N, illustrating that NRE is an important physiological trait linking aboveground nutrient use with litter-soil nutrient feedbacks. These findings suggest that plant economics and the cost of soil N acquisition contribute to local-scale nutrient cycling in temperate forests.