Growth and Photosynthesis of Ash Fraxinus excelsior and Beech Fagus sylvatica Seedlings in Response to a Light Gradient following Natural Gap Formation

• Published in: Ecological bulletins 2007-01 (52), p.147-165
• Main Author: Einhorn, Katrina S.
• Format: Article
• Language: English
• Subjects: Biomass production; Forest soils; Growing seasons; Leaf area; Photoinhibition; Photosynthesis; Plants; Seedlings; Shade tolerance; Understory
• ISSN: 0346-6868

The photosynthetic and morphological responses of ash and beech seedlings were studied along the light gradient in a small, naturally-formed gap and into the surrounding forest in Suserup Skov. Potted seedlings were used to eliminate water and nutrient availability as co-varying factors. Seedlings were placed along perpendicular transects through the center of the gap for the first two years after gap formation. Survival, growth, biomass allocation, net photosynthesis and maximum quantum yield were measured. Differences in the plasticity, defined as the slope of the response, of ash and beech to the gradient of light through the gap were also investigated. Both ash and beech seedlings showed a significant increase in overall growth with increasing irradiance up to almost 8 mol$\text{m}^{-2}\ \text{d}^{-1}$averaged over the growing season. During the first growing season, however, the species had significantly different strategies for growth and biomass allocation. Ash seedlings invested primarily in belowground biomass, while beech seedlings had more typical shade-plant morphology, with high leaf area and leaf weight ratios. In the second growing season, ash seedlings grew rapidly in height, suggesting that explosive growth of gap-specialist species relies on the early establishment of a large root system to supply water to the rapidly growing shoot in subsequent years. Beech seedlings, on the other hand, showed significant branching of the crown. These results support the hypothesis that there is a trade-off between height growth and crown width in regenerating tree species. The plasticity of a number of morphological traits was higher in ash than in beech. There was no significant difference in the plasticity of photosynthesis; however, this may have been attributed to noisy data. Measurements of maximum quantum yield suggested that beech seedlings were experiencing mild chronic photoinhibition, which increased with increasing irradiance. It is, however, suggested that factors such as crown morphology and biomass allocation have a much more central role than photosynthesis in explaining the patterns of growth and survival seen in the early stages of gap capture by these two species.