Growth of Acer saccharum seedlings in deeply shaded understories of northern Wisconsin: Effects of nitrogen and water availability

Availability of soil nitrogen, soil moisture, and light were examined, along with the growth, biomass allocation, and leaf nitrogen concentration of naturally established Acer saccharum Marsh. seedlings, in the understories of 12 forest sites in northern Wisconsin. The sites represented a nutrient and moisture gradient (poor to rich) according to a habitat classification system. We asked (1) Are seedling growth rates, biomass allocation patterns, and leaf nitrogen related to soil water and nitrogen availability? and (2) Do soil resource rankings predicted by habitat classifications mirror our direct observations? Across sites compared in a low-light data set (plots with <5% canopy openness), rich sites had 2- to 4-fold higher seedling growth, percent leaf nitrogen, nitrogen mineralization rates, and nitrification rates than poor sites. Seedling growth in low light correlated (P ≤ 0.05) positively with nitrification, total nitrogen mineralization, percent leaf nitrogen, soil moisture, and organic carbon, and negatively with fine root density. In multiple regression, soil moisture (P = 0.033) and nitrification (P = 0.015) together explained 79% of the variation in growth. Thus, seedling growth in shade was enhanced on richer sites in part because of higher nitrate N and water availability. This has potential implications for forest dynamics, since the probability of sugar maple becoming the dominant woody regeneration in any given understory may be partially dependent upon the level of soil resources.