BIOMASS ALLOCATION AND MULTIPLE RESOURCE LIMITATION IN TREE SEEDLINGS

We conducted a greenhouse experiment to determine how differences amon g tree species in allocation of biomass to roots versus shoots affecte d their responses to different combinations of light, water, and soil nutrients. Across a full range of light levels, we were specifically i nterested in examining the sensitivity of tree seedlings to additional reductions in aboveground growth due to soil resource limitation, and the relative sensitivity of seedlings to water versus nutrient stress under different light regimes. The four tree species used in our expe riment included two species that are the most common initial tree inva ders of abandoned agricultural lands (old fields) in the Hudson Valley (red maple (Acer rubrum L.) and white pine (Pinus strobus L.)) and tw o species that are less frequently found in old fields, but that are d ominant forest species, and are noted for their tolerance of either li ght stress or water stress (sugar maple (Acer saccharum Marsh.) and re d oak (Quercus rubra L.), respectively). At the lowest light levels (2 % of full sun), there was no effect of variation in soil resource avai lability on shoot growth of any of the four species. At the 9% full su n light level, red maple seedlings showed clear evidence of simultaneo us limitation by light, water, and nitrogen. At higher light levels (2 0-100% of full sun), all four species responded to variation in at lea st one of the two soil resources. The four species showed two contrast ing patterns of allocation of biomass to roots. Red maple and white pi ne responded to an increase in soil resource availability by reducing relative allocation to roots and increasing aboveground growth. Sugar maple and red oak had much more conservative root allocation patterns: root allocation was high (58-75% of added biomass allocated to roots) and did not vary in response to soil resource availability. Allocatio n to roots was affected more strongly by variation in soil nitrogen av ailability than it was by soil moisture availability.