Growth, photosynthesis, and needle structure of silver fir (Abies alba Mill.) seedlings under different canopies

As part of fir reintroduction program in the Karkonosze Mountains (S. Poland) a study was undertaken to investigate growth and ecophysiological performance of young firs (Abies alba) under varying degrees of natural shade. Threeyear-old fir seedlings were planted in five forest stands dominated by Picea abies, Fagus sylvatica, Betula pendula, Pinus sylvestris or Larix decidua. Over four growing seasons, annual apical increments became progressively greater under the light permeable canopy of Larix, but remained stunned under the deep shade of Picea, while intermediate irradiance at other sites resulted in intermediate growth responses. Similarly, apical dominance ratio was positively influenced by irradiance. These two contrasting sites were subsequently used for ecophysiological comparisons. Fir seedlings under the Larix canopy had consistently higher photosynthetic rates throughout the day compared to those under Picea, but in both stands light adapted quantum yield of PSII remained mostly above 0.8 indicating lack of photoinhibition. Quantum yield declined transiently only during sunflecks, which were more frequent under Larix. Light dependence of apparent electron transport rates in both sites was linear showing lack of light saturation, while slope differences between sites were only slight. In contrast, depression of PSII quantum yield by light was less pronounced under Larix, showing an acclimation to higher irradiance. Fir needles produced under Larix were thicker and contained more chlorophyll, carotenoid and nitrogen on area basis (but, except for nitrogen, not on dry mass basis) suggesting that structural modification is a major component of acclimation to light. On anatomical levels, needles produced under Larix had the traits typical for sun-acclimated ones, while those under Picea for shade-acclimated. The magnitude of modifications was, however, less than that reported in literature for sun and shade needles. Young seedlings were able to acclimate to higher irradiance under Larix without showing light stress symptoms. This suggests that success of stand regeneration may be enhanced by adjusting light penetration through the overstorey. Fir seedlings found best growth conditions under the relatively light permeable crowns of light-demanding tree species (Larix decidua, Pinus sylvestris) suggesting that stands dominated by these trees are more suitable for transformation using silver fir seedlings than those with shade-tolerant tree overstoreys.