Photosynthetic acclimation of white spruce (Picea glauca) to canopy microhabitats

Abstract

Slow growing white spruce (Picea glauca) seedlings and saplings often become established early in succession and mature through several succession seres. During early succession, spruce often germinate in mineral soils and become established in alder (Alnus tenuifolia or A. crispa) thickets, with the potential for both competitive and facilitative relationships. Although competitive and facilitative plant interactions are often identified by changes in the growth or density of the interacting species, the result of the interaction will depend upon the individual plant's physiological acclimation to abiotic changes caused by neighboring plants. This study analyzes components of photosynthesis to provide information about the effects of alder on spruce. To isolate the responses of components of the photosynthetic process to neighbors, gas exchange techniques, needle chemical analysis, and observations of environmental parameters were utilized in growth chamber experiments, with individual plants in the field, and in controlled density plantations of alder and spruce. Growth at high light in all experiments resulted in lower maximum photosynthetic rates in current year shoots. Light response curves showed lower incident quantum yields in spruce seedlings growing at the high light levels typical on the floodplain. Increased soil nitrogen did not increase photosynthetic rates per gram needle in any of the experiments. However, increased seedling growth at high light in growth chamber experiments, and increased plant density in spruce/alder plantations, resulted in dilution of needle nitrogen. High needle nitrogen concentrations did not result in higher maximum net assimilation rates, although needle nitrogen was positively correlated with dark respiration rates. Concentrations of rubsico, a potentially rate limiting enzyme for photosynthesis at high light, was very responsive to changes in irradiance, but constituted only a small part of the needle nitrogen pool and did not appear to be limited by nitrogen availability. This work suggests that on a physiological level, spruce is a stress adapted plant with a low capacity to up-regulate photosynthetic physiological processes in response to increased light or nitrogen conditions.