Supervisors: prof. Olevi Kull (1955-2007) ja dotsent Arne Sellin
Opponent: Dr. Rolf Lutz Eckstein (Gießeni Ülikool, Saksamaa)
Light- and nitrogen-use and biomass allocation along productivity gradients in multilayer plant communities
Plants need various resources in sufficient quantities for their growth, reproduction and survival. Light and nitrogen availability are two factors most frequently limiting plant growth. Nitrogen acquisition from soil is an expensive process. Consequently, it is vital that those resources are utilized efficiently. Which strategies enable plants to cope better in various conditions and competition? Can effective nitrogen users be concurrently effective in light use? To answer the questions, changes in resource-use at species and community levels were studied along productivity gradients in grassland and deciduous forest communities (Laelatu in West-Estonia and Aru in South-Estonia). A two-resource model was developed with available nitrogen and light as parameters, which describe vertical distribution of foliage mass by vegetation layers in a community. The overstory receives more light resource compared to lower layers, while the lower layers have greater nitrogen-use efficiency and/or a better capability to acquire soil nitrogen. The growth of the herbaceous layer is co-limited by light and nitrogen availability. The growth of the moss layer is limited primarily by light availability. On the basis of two grasslands, the hypothesis was established that an optimum of light acquisition efficiency exists along a productivity gradient. Further studies are needed to clarify if an analogous relationship exists in other community types? On the basis of the thesis and literature, it is possible to conclude that plants have to compromise between effective light- and nitrogen-use. Plants cannot be effective in utilizing both resources simultaneously. Comparison of grassland species revealed clear differences in acclimation patterns among species that become dominant and that remain as subordinates. More precisely, plasticity in aboveground growth patterns and nitrogen allocation varies among species, thus leading to substantially different strategies for survival.