Biomass allocation and multiple resource limitation in tree seedlings

We conducted a greenhouse experiment to determine how differences among tree species in allocation of biomass to roots versus shoots affected their responses to different combinations of light, water, and soil nutrients. Across a full range of light levels, we were specifically interested 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 experiment included two species that are the most common initial tree invaders of abandoned agricultural lands (old fields) in the Hudson Valley (red maple (Acer rubrum L.) and white pine (Pinus strobus L.)) and two species that are less frequently found in old fields, but that are dominant forest species, and are noted for their tolerance of either light stress or water stress (sugar maple (Acer saccharum Marsh.) and red oak (Quercus rubra L.), respectively). At the lowest light levels (2% of full sun), there was no effect of variation in soil resource availability on shoot growth of any of the four species. At the 9% full sun light level, red maple seedlings showed clear evidence of simultaneous limitation by light, water, and nitrogen. At higher light levels (20-100% of full sun), all four species responded to variation in at least one of the two soil resources. The four species showed two contrasting patterns of allocation of biomass to roots. Red maple and white pine 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. Allocation to roots was affected more strongly by variation in soil nitrogen availability than it was by soil moisture availability.