Plant Population Dynamics and Biotic Interactions in two Forest Herbs

Abstract: In Sweden today, deciduous woodlands are often restricted to small isolated remnants of what it once was. Managing practices have changed concerning both cattle grazing and logging. Thus, dispersal, habitat requirements and their importance for forest species distributions become a relevant issue. Most of the species found in the deciduous flora are perennial herbs. I have examined population dynamics in the forest perennial Sanicula europaea, and its relation to environmental factors such as grazing, competition and spatial and temporal variation in such factors. Moreover I examined species distributions in relation to dispersal and habitat suitability in S. europaea and Dentaria bulbifera. To understand mechanisms behind the observed patterns I performed a number of experiments.The results clearly demonstrated that the distribution and abundance of S. europaea was not dispersal limited, whereas the opposite held true for D. bulbifera. Moreover, mollusc exclusion increased recruitment thus influencing population dynamics in D. bulbifera. Leaf losses had negative effects on S. europaea individuals. These negative effects depended both on the extent, frequency and timing of the leaf losses and early losses were more severe than late. Population level effects of grazing were not negative as negative direct effects were counterblanced by positive indirect effects. The positive effects of grazing were mainly in terms of an increased recruitment. In S. europaea, high variation in a life cycle transition was always coupled to low elasticity, and traits that varied much due to the examined environmental factors had little importance to population growth rate. A population level perspective is required to assess total effects of environmental factors. In perennial organisms such evaluations need to calculate integrated measures of the effects over the entire life cycle. Field studies spanning several years and demographic models are important to achieve these objectives.