GROWTH, ALLOCATION, AND MORPHOLOGICAL RESPONSES OF BETULA-PUBESCENS AND BETULA-PENDULA TO SHADE IN DEVELOPING SCOTS PINE STANDS

The growth, biomass allocation, crown architecture, and leaf morphology of 50 to 200 cm tail Betula pubescens Ehrh. and Betula pendula Roth seedlings growing under eight Scots pine (Pinus sylvestris L.) stands varying in age from 7 to 105 years were studied in southern Finland. The main objective of the study was to assess the degree of morphological plasticity of birch in response to understory conditions in stands of varying structure. In each of these stands, the light, water, and nutrient availability were measured. No clear trend in nutrient and water availability was found from the 7- to 105-year-old stands. Light availability decreased from the 7-year-old stands (45% and 82% of full sunlight in nongap and gap locations, respectively) to the 20-year-old stands (5% and 12% of full sunlight), and then increased to the mature stands (21% and 26% of full sunlight). Relative branch growth rate and height increment decreased with increasing shade. The ratios of leaf area to branch length and to branch weight did not vary significantly among the eight stands. The ratios of height to diameter and fine-root biomass to leaf biomass were the highest under the most shaded stands (20-year-old). Increase in shading decreased leaf thickness and increased specific leaf weight. No major differences were found in any of the growth parameters measured between the two birch species, indicating that they do not differ in their juvenile stage. These results indicate that these two birch species have relatively little morphological plasticity in response to shade. They appear to be able to persist in shade by minimizing carbon demand for growth and by modifying leaf morphology to presumably optimize photosynthesis.