Regulation of leaf life-span and nutrient-use efficiency of Metrosideros polymorpha trees at two extremes of a long chronosequence in Hawaii

Leaf traits related to life-span and nutrient-use efficiency were studied in the dominant Hawaiian tree species, Metrosideros polymorpha, at both ends of a natural fertility gradient, from young, nitrogen-poor soils to older, phosphorus-poor soils. The main objective of this study was to understand how nutrient limitations affect leaf-level attributes that ultimately play a mechanistic role in regulating whole-ecosystem function. Different types of adjustments to removal of nutrient limitation by long-term fertilization (9-15 years) with nitrogen (N), phosphorus (P), and a combined treatment of N plus P were observed at each site. Nitrogen fertilization at the young, mostly N-limited site did not significantly affect net CO2 assimilation (A), foliar N content, or N resorption. The primary response to N fertilization was a decrease in average leaf life span to approximately 553 days compared with 898 days in the control plot. Significantly shorter average leaf life-span coupled with constant A and foliar N content resulted in reduced integrated photosynthetic nitrogen-use efficiency (PNUE: A summed over the life-span of a leaf divided by foliar N) in the fertilized plots. In contrast, removal of nutrient limitations at the old, mostly P-limited site resulted in increased A, and increased foliar P concentration which also resulted in reduced integrated photosynthetic phosphorus-use efficiency (PPUE). P resorption was also reduced at this site, yet leaf life-span remained constant. When results from both sites and all treatments were combined, statistically significant relationships between leaf life-span, and A, leaf mass per area (LMA), and the cost of leaf construction per unit carbon gain (cost of construction determined by combustion of leaf samples divided by A) were found. As leaf life-span increased, A decreased asymptotically, and LMA and the carbon cost per carbon gain increased linearly. It appears that the balance between leaf carbon cost and carbon uptake is a major determinant of leaf longevity in M, polymorpha despite contrasting responses to removal of N and P limitations by long-term fertilization. Removal of the main nutrient limitations at both sites also resulted in reduced integrated nutrient use efficiency. However, the regulatory mechanisms were different depending on the site limitations: a shorter leaf life-span in the young, N-limited site and substantially higher foliar P concentration in the P-fertilized plots at the old, P-limited site.