Photosynthetic responses of white spruce saplings (Picea glauca) to controlled density gradients of spruce and green alder (Alnus crispa)

Physiological responses to competitive and facilitative interactions between white spruce (Picea glauca [Moench] Voss) and green alder (Alnus crispa [Ait.] Pursh) grown together in Nelder plots were assessed through measurement of spruce in situ shout photosynthetic potential, stomatal conductance and measurements of needle chemistry. Neither proximity to aider nor overall plant density were found to impact spruce maximum net photosynthetic rates during midsummer. However, photosynthetic potential declined in spruce saplings grown at low densities at the end of the summer. suggesting that spruce saplings benefited from the microhabitat modification provided by both aider and spruce neighbors at higher densities. Although temperature and precipitation were normal for interior Alaska, low soil volumetric water content and low needle water content suggested that saplings in all treatments were draught stressed. Path analysis was used to relate above- and below-ground abiotic conditions to photosynthetic rates and stomatal conductances to estimate the relative importance of plant-modified environmental conditions in explaining photosynthetic tares. We found that although both airier and spruce decreased soil water, there were not strong relationships between plant modified soil water content and needle water content, or stomatal conductances Data from nearby canopy and open plots suggested that lower ma maximum temperatures and higher minimum humidity; under the canopy might improve the water status of spruce growing under the canopy. Increasing density of both spruce and alder increased the soil nitrogen slightly, but needle nitrogen was negatively related to the density of neighbors, suggesting that increased biomass resulted in below-ground competition for nitrogen. We found a direct negative effect of light on maximum net photosynthesis late in the summer suggesting that neighbors could prevent photoinhibition after cool nights in the fall.