Impacts of elevated nitrogen inputs on oak reproductive and seed ecology

ABSTRACT

The effects of increased anthropogenic inputs of reactive nitrogen (N) have been studied at the Harvard Forest Chronic N Experiment, where NH₄NO₃ has been applied experimentally since 1988 to increase atmospheric deposition rates ∼6- and ∼18-fold above ambient. This paper asks whether conditions favorable to primary production also resulted in plastic increases to flower, fruit, and seedling traits, and focuses primarily on the oaks that dominate the hardwood stands of the Harvard Forest experiment. Litterfall samples collected between 1996 and 2001 revealed that flowers and fruits were significantly more abundant in N-treated plots, and an analysis of oak tree abundance found significant variation both among and within plots. Acorn samples collected during 2003 and 2004 (a mast and a postmast year) were therefore analyzed using ancova models that included an estimate of oak tree abundance. This tree abundance estimate was the only significant driver of increased acorn production during the mast year, and in both years it was a significant factor on plots receiving the highest levels of N. In the postmast year, acorn production was increased in direct response to N-related factors other than tree abundance. Our comparisons of control and N-treated plots for acorn quality traits (e.g. rates of acorn damage, germination percentage, seedling growth) revealed negligible or only transient differences. Shifts in overall acorn abundance – particularly disproportionate N-mediated increases during nonmast years – could have a wide range of ecological consequences beyond the more frequently examined impacts of N deposition on primary production and carbon sequestration.