Crown function and dimensions in boreal conifers

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The size of tree crowns is linked to the productivity, water use and health of forests. In this proposal my students and I will explore the factors that control the development of crowns in trees. Put simply, crown size is controlled by the vertical and lateral extension of shoot and branches and the self-pruning of branches at the bottom of the crown. In terms of self-pruning, we plan studies that will test the hypothesis that as the lower branches of shade intolerant species enter zones of low light, these branches develop xylem with lower hydraulic conductivity and therefore they tend to restrict the water supply to the leaves of these lower branches. Shade tolerant species are proposed to have less change in hydraulic conductivity in branches and therefore slower self-pruning. Secondly, I propose that shade intolerant species will tend to allocate more of the sugars that they produce to upper branches and tend to remove mobile nutrients such as N from lower branches once they begin to experience shading. In terms of lateral extension of branches, I want to explore the linkage of branch extension with nutrient supply and changes in hydraulic conductivity of branches. Trees from field fertilization trials will be used to examine crown width and stem hydraulics. Crown friction from the collision of trees during their sway in wind is also thought to cause breakage of twigs and buds, thereby limiting lateral expansion of crowns. This will be explored by a regional study of crown width of trees from fully-stocked conifer forests in zones of different wind speeds. We will also study the effects of wind on stem shape and crown development in trees growing in zones of different wind speed. This work will provide new insights into the way that crowns develop and help managers control this important feature of tree growth under management or climate change.