Acclimation to light in planted and naturally regenerated populations of white spruce seedlings

Physiological responses to sun and shade were examined in white spruce (Pic ea glauca (Moench) Voss) seedlings collected from three naturally regenerat ed (N1-N3) and three planted (P1-P3) stands in the boreal forest of Saskatc hewan. Seedling survival was greater in the sun than in the shade pretreatm ent periods. Dark respiration declined by 70% in shade- compared with sun-a cclimated seedlings; however, the decline was not statistically significant because of the large within-population variation. Quantum yield, total chl orophyll content, specific leaf area, and absolute water content of needles increased by 45, 33, 32, and 50%, respectively, in response to shade. At l ow light levels, shade-acclimated populations showed greater photosynthetic rates and steeper light-response curves than seedlings growing in full sun . Seedlings of P3, N2. and N3 saturated at about 200 mu mol.m(-2).s(-1) PAR in the shade, with no increase in photosynthesis at higher light intensiti es. At light saturation, populations P1 and N3 showed similar photosyntheti c rates to both light acclimation regimes; populations P2, P3, and N2 had l ower light-saturated photosynthesis in sun, compared with shade pretreatmen t. Only in N1 was photosynthetic rate greater after sun than shade pretreat ment; this population behaved like a "sun" and "shade" population depending on pretreatment. Differences in physiological responses to light among pop ulations suggest the presence of more than one ecotype. N1 showed the great est plasticity in response to light pretreatment. The remaining populations behaved more like shade-adapted populations, with little adjustment to lig ht conditions. Intrapopulation variation was large for both regeneration ty pes. It appears that selection pressure during reforestation was not great enough to cause a decline in intrapopulation variation in planted compared with naturally regenerated white spruce seedlings.