Age-related changes in tree structure and function: mechanisms and consequences to forest ecosystems processes

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Young and old trees commonly differ greatly in terms of canopy structure, leaf gas-exchange and chemistry, and exposure to biotic factors such as parasites and pathogens. Such "age-related" changes may act as important drivers of ecosystem processes; they are also critical in a management context, since harvesting generally involves the removal of larger trees. The proposed research builds on a set of recent discoveries from my research group showing that some temperate trees show a long period of decline in intra-crown leaf area index, and that plant parasite loads can have large impacts on leaf physiology that partially explain "age-related" photosynthetic decline in some species. Four inter-related studies are proposed: (1) We will utilize ecophysiological techniques to evaluate the prevalence, age-dependence, and magnitude of impacts of gall-forming arthropods on foliage of adult canopy trees at one tropical and one temperate site. In cases of very large effects, the physiological mechanisms will be analyzed in detail making use of controlled inoculation studies coupled with diagnostic gas-exchange and fluorescence measurements. (2) We will examine effects of age-dependent changes in tree form and function on soil processes, specifically testing the hypotheses that older trees will show reduced autotrophic respiration, and reduced litter decomposition rates related to increasing C:N ratio of foliage. (3) We will examine trends in forest regeneration patterns beneath crowns of trees varying in age and size, testing the hypothesis that older trees will show enhanced recruitment of more light-demanding tree species due to increased light penetration. (4) The potential for remote sensing of tree age and senescence will be explored using data from airborne LiDAR, multi- / hyper-spectral reflectance, and leaf-level spectral reflectance and transmittance measurements -- specifically testing for consistent signals related to age-related reductions in within-canopy LAI, and for changes in leaf spectral signals related to age-related changes in leaf structure and chemistry.