The photosynthetic response of a high-altitude spruce forest to nitrogen amendments with implications for gross primary productivity

Abstract

Characterizing the capacity of the terrestrial biome to absorb CO2 is one of the most pressing topics in climate science. One of the key players in this arena is also one of the most poorly understood: the role of nitrogen deposition. While N deposition seems to fertilize some forests, in others it has been blamed for forest decline and tree mortality. In order to further understand the role of N deposition in forest primary productivity, an in situ N-amendment study was conducted in a relatively high N deposition region of the CO Rocky Mountains over 3 yr.Chlorophyll fluorometry is a non-invasive technique that allows for the characterization of photosynthetic efficiency and photoinhibition in plants. Analysis of chlorophyll fluorometry data showed that in this tree line forest, moderate amounts of additional N significantly lower the photosynthetic efficiency of individual branches when compared to control branches, while significantly improving the non-photochemical dissipation of excess absorbed energy. This suggests that in high-altitude spruce forests receiving relatively high N inputs, GPP may be impaired by continued chronic additions of N. One potential cause of this is an increase in the light harvesting capacity of N amended branches without a concurrent increase in photosynthesis.