The heterogenous and dynamic hardwood forest understory contains the bulk of the future regeneration potential and thus have important influences on tree species recruitment. Light is a crucial resource for regeneration success in the understory. However, high variability during the early phases of recruitment highlight the importance to understand whether other factors like water and nutrients affect survival, especially for shade tolerant species. In this study, we asked the following questions- (i) what influences seedling recruitment, and does environmental change, through various effects on nutrient availability influence recruitment? And (ii) If alleviating nutrient limitation favorably shifts allocation aboveground, would improved nutrition encourage herbivory? We tested the above questions in mature (>100 years age) northern hardwood forest stands where plots in each stand had been fertilized with either 30 kg N ha-1 yr-1, 10 kg P ha-1yr-1, the same amounts of N+P, or neither (control) since 2011. We followed a single cohort of beech (Fagus grandifolia Ehrh.) and sugar maple (Acer saccharum Marsh.) seedlings in these forest plots from 2012 and monitored survivorship and allocation responses (biomass, root to shoot, leaf area, root length and architecture) to light, soil moisture, and soil nutrients for three years.
Results/Conclusions
Seedling survivorship in both species increased with light availability. Nutrient additions decreased sugar maple survivorship and there was no effect of nutrients on beech seedling survivorship. Seedling survivorship showed a strong decline over time and seedling mortality at these earlier stages appeared to be mostly stochastic. Biomass growth was not water or nutrient limited in the first three years, and adding nutrients did not increase the relative allocation aboveground. However, shallow rooting by sugar maple caused a higher soil moisture sensitivity, whereas more deeply rooted beech was less sensitive to drying of surface soil which could have contributed to relatively higher beech seedling survival. Also, sugar maple leaf damage increased with nutrient addition which suggests higher herbivory in this species, but not in beech. Beech seedlings were relatively non-plastic with respect to the environmental variables measured. It is uncertain in the earlier stages of seedling establishment whether higher aboveground allocation increases seedling survivorship or if improved nutrition encourages herbivory.