Due to fundamental differences in the biogeochemistry of nitrogen (N) and phosphorus (P), fire has the potential to alter the relative availability of N versus P both immediately following fire and over inter-fire cycles. To investigate whether fire causes shifts in N versus P limitation of primary productivity, we established plots in scrubby flatwood sites 1, 8, and 20 years since fire, which we fertilized with N, P, or N and P together, or left as controls. We examined the effects of fertilization on stem basal diameter, apical shoot growth, and litterfall of the dominant species, scrub oak (Quercus inopina), and on root production of the plant community.

Results/Conclusions

Fertilization treatments did not affect Q. inopina basal diameter, apical shoot growth, or litterfall; however, there were differences in these measures with time since fire. In sites 1 year since fire, Q. inopina had the fewest and longest apical shoots, the greatest increase in basal diameter, and the least litterfall. Root production, by contrast, did respond to fertilization: root production in the N+P treatment was greater than the control in sites 8 and 20 years since fire, but not in sites 1 year since fire. There were no differences, however, in root production with time since fire. In this series of scrubby flatwood sites, aboveground productivity was not responsive to nutrient addition over the initial year of the experiment, while belowground productivity increased with nutrient addition. Our results suggest that belowground productivity is co-limited by N and P in older sites, but fire ameliorates this limitation either via effects on soil nutrient availability or effects on plant nutrient demand, at least in the first year after fire.