Sapling growth as a function of resources in a north temperate forest

Abstract

Radial and height growth are characterized for saplings of 10 dominant tree species in a transition oak–northern hardwoods forest in southern New England. Growth of saplings in the field is regressed against measures of whole-season light availability, soil moisture, and sapling size. Statistical tests show strong effects of light availability on growth, but no significant effects of soil moisture. Comparison of the light-dependent growth functions for the 10 species revealed three apparent interspecific trade-offs. (i) Species growing quickly at high light tended to grow slowly at low light and vice versa. The order of species from fast growing at high light to fast growing at low light did not correspond to traditional classifications of shade tolerance, and variation along this axis was approximately continuous. (ii) There was substantial variation off the species continuum defined in i. At any point along the continuum from fast growth at high light to fast growth at low light, some species grew faster than others, and these faster growing species had lower survivorship during periods of suppression than the slower growing species. (iii) Height growth at high light was inversely related to survivorship when suppressed. This variation was again continuous (species did not cluster into discrete categories), but the order of the species did correspond closely to a traditional ordering of shade tolerance. There was little correspondence between our estimated growth functions and the growth functions assumed in the JABOWA–FORET class of forest simulation models. These results raise serious concerns about the current practice of assigning growth functions to species in simulation models using traditional classifications of shade tolerance.