Growth, allocation and water relations of shade-grown Quercus rubra L saplings exposed to a late-season canopy gap

For understory saplings to exploit canopy gaps successfully, carbon gain must increase in the gap environment. We predicted that total biomass of shade-grown red oak saplings would increase after exposure to a late-season canopy gap, and that increased water and nutrient demand within the canopy gap would drive changes in the allocation of this carbon. Shade-grown red oak saplings acclimated to gaps by increasing biomass during the season of gap formation and increasing the potential for carbon gain in the following summer. Within-season carbon gain did not result from greater production of leaf area, so it most likely arose from higher photosynthetic rates of existing shade-developed foliage, which may be linked to accumulation of leaf nitrogen. During the season of gap formation, shade-gap plants increased allocation to storage of total non-structural carbohydrates (TNC), and to root growth. The increase in TNC storage suggests that shade-developed saplings exposed to gaps were also primed for fast growth and carbon gain in the following summer. The increase in root growth suggests that higher nutrient and water demand drove allocation shifts to enhance the capacity for nutrient and water uptake in the gap. Plant hydraulic conductivity (K-a) of shade-grown plants was limited upon exposure to the gap, possibly because of embolism formation resulting from the abrupt increase in water demand. Greater water potential gradients compensated for limitations to K-a, allowing saplings to maintain high transpiration rates, suggesting that actual water uptake of shade-gap plants was unaffected by gap exposure. (C) 1997 Annals of Botany Company.