Patterns of and controls over nitrogen inputs by green alder (Alnus viridis ssp. fruticosa) to a secondary successional chronosequence in interior Alaska

Abstract

Patterns of and controls over N₂ fixation by green alder (A. viridis) were studied in post-fire, mid-succession, and white spruce upland forests in interior Alaska during 1997-2000, focusing on the hypothesis that ecosystem-level nitrogen (N) inputs decrease with successional development. Across all stands, alder created islands of elevated soil N and carbon, depleted soil phosphorus (P), and more acidic soils, effects which translated to the stand-level in response to greater alder stem density. Rates of N₂ fixation (measured by acetylene reduction = ARA) closely tracked plant phenology during the 1997 (a drought year) and 1998 (a year of normal precipitation) growing seasons. During 1998, stands with higher maximum ARA had higher %N in the O, A, and C soil horizons. N₂-fixation rates were influenced by soil P, as evidenced by the findings that maximum ARA was positively correlated with foliar N:P ratios, and with subcanopy %P in the O and A soil horizons. During the drought year, alder leaf %P and leaf N resorption were lower and leaves were thinner when compared to 1998. Drought effects were most pronounced in mid-succession where alder exhibited reduced ARA ( -76%), leaf %P ( -14%), leaf thickness ( -40%), and lower leaf resorption of P ( -66%) and N ( -78%). Although ARA and nodule biomass did not differ among stand types, increases in alder densities with successional time translated to increasing ecosystem-level N inputs across the chronosequence. These results contradict established theory predicting a decline in N₂-fixation rates and N₂-fixer abundance during successional stand development.