Climate, nitrogen mineralization, and productivity in contrasting montane forests
In temperate forests, it is unclear whether nitrogen availability or climate is the most important influence on aboveground net primary productivity (ANPP), partially because the influence of climate is expressed on both ANPP and nutrient availability itself. The relationships among climate, nitrogen availability, and forest productivity were examined in two study area representing opposite extremes in mineral N availability: Isla de Chiloe (CP) in southern Chile and Whiteface Mtn. (WF), in New York. Mineral N at WF is abundant and not correlated to climate due to atmospheric deposition of anthropogenic N forms. Mineral N is the logical growth-limiting resource in CP because of low levels of atmospheric inorganic N input and internal N cycling. By accounting for the effect of climate in both above- and below-ground processes, and by incorporating sites where mineral N availability is known not to be controlled by climate, I separated the effects of mineral N availability and climate on the productivity of a range of temperate montane forests.
Net N mineralization (the primary process whereby N is converted to plant-available inorganic forms) was measured using the in situ resin-core technique. Lignin, N, and C contents of litterfall and soil organic matter were measured in order to gauge substrate quality. ANPP on an annual basis was measured as the sum of litterfall quantity and wood production. Growing season degree days (GSDD or soil GSDD), an index of air or soil temperature and growing season length, was calculated for each site.
Rates of net N mineralization in CP were indeed low compared to both WF and other similar forests. Substrate quality and soil temperature exerted the most influence on net N mineralization, though soil temperature appears to be a more important control at cooler sites.
Regression analyses showed GSDD accounted for the most variance in ANPP at both sites. Neither net N mineralization nor mineral N availability explained any variance in ANPP at either site. By decoupling the direct influence of temperature on N mineralization and on ANPP, this study showed that productivity responds most strongly to air temperature, regardless of N availability.