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 betwee n white spruce (Picea glauca [Moench] Voss) and green alder (Alnus crispa [ Ait.] Pursh) grown together in Nelder plots were assessed through measureme nt of spruce in situ shout photosynthetic potential, stomatal conductance a nd 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 sap lings grown at low densities at the end of the summer. suggesting that spru ce saplings benefited from the microhabitat modification provided by both a ider and spruce neighbors at higher densities. Although temperature and pre cipitation were normal for interior Alaska, low soil volumetric water conte nt and low needle water content suggested that saplings in all treatments w ere draught stressed. Path analysis was used to relate above- and below-gro und abiotic conditions to photosynthetic rates and stomatal conductances to estimate the relative importance of plant-modified environmental condition s in explaining photosynthetic tares. We found that although both airier an d spruce decreased soil water, there were not strong relationships between plant modified soil water content and needle water content, or stomatal con ductances Data from nearby canopy and open plots suggested that lower ma ma ximum temperatures and higher minimum humidity; under the canopy might impr ove 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 tha t increased biomass resulted in below-ground competition for nitrogen. We f ound a direct negative effect of light on maximum net photosynthesis late i n the summer suggesting that neighbors could prevent photoinhibition after cool nights in the fall.