Fine Root Growth Increases in Response to Nitrogen Addition in Phosphorus-limited Northern Hardwood Forests

Resource allocation theory posits that increased soil nutrient availability results in decreased plant investment in nutrient acquisition. We evaluated this theory by quantifying fine root biomass and growth in a long term, nitrogen (N) × phosphorus (P) fertilization study in three mature northern hardwood forest stands where aboveground growth increased primarily in response to P addition. We did not detect a decline in fine root biomass or growth in response to either N or P. Instead, fine root growth increased in response to N, by 40% for length (P = 0.04 for the main effect of N in ANOVA), and by 36% for mass, relative to controls. Fine root mass growth was lower in response to N + P addition than predicted from the main effects of N and P (P = 0.01 for the interaction of N × P). The response of root growth to N availability did not result in detectable responses in fine root biomass (P = 0.61), which is consistent with increased root turnover with N addition. We propose that the differential growth response to fertilization between above- and belowground components is a mechanism by which trees enhance P acquisition in response to increasing N availability, illustrating how both elements may co-limit northern hardwood forest production.