Herbicide application often is used with partial cutting to reduce the influence of dense interfering plants on recruitment of tree seedlings. Short-term interfering plant cover reductions, followed by rapid recovery of some species often is observed. Herbicide affects microclimates through increased light levels, higher soil temperature, and temporary nutrient fluxes. Higher soil temperatures increase microbial activity. A northcentral Pennsylvania study of herbicide impacts on understory plant recovery found the seed bank to play a substantial role in recovery of Carex spp., Danthonia compressa, Prunus pensylvanica, Rubus allegheniensis, and Viola mackloskeyi. To identify environmental cues that elicit a seed bank response, I measured understory light, soil temperature and soil nitrate levels one year after herbicide application. Mean temperatures increased by 5° C and nitrate levels increased, but with high variability. A dose response fertilization study in the region showed a seed bank response with 50 kg/ha ammonium nitrate. Using these data, a growth chamber experiment was installed using seventy-two soil cores collected with minimal disturbance. Growth chambers maintained daytime temperatures of 23° C or 28° C, with 15° C night time temperatures and a 12 hour daytime length. Four chambers (two of each temperature) were maintained at temperature for 45 days, then germinants were counted by taxon. Half of the samples in each growth chamber received a 50 kg/ha ammonium nitrate treatment.
Results/Conclusions
Significantly higher seed bank species germination was observed with nitrogen addition, but only at the higher temperature treatment. Temperature or nitrogen alone did not result in significant germination differences. Carex spp., Danthonia compressa, Prunus pensylvanica, and Rubus spp. dominated the growth chamber germinants. Several seed bank species have been shown to break dormancy with increased nitrate nitrogen. Herbicide and partial cutting combine to create conditions favorable for the seed bank to respond. The most likely location within herbicide treated forest stands to observe a seed bank response is in an overstory gap with disturbed soil.
