From Plant Properties To Forest Function In Temperate Mixed Angiosperm-conifer Old Growth

Ecosystem functions are the processes that cycle matter among vegetation, soils and the atmosphere. These processes are conventionally modelled as the physiological response of homogeneous plant cover driven by abiotic conditions over large areas. However, efforts invested in understanding how structural complexity within these homogeneous pixels influences ecosystem functions, especially the abundance and distribution of tree species, have led to substantial improvements in predictive accuracy. The ecosystem consequences of adaptive variation among plants were explored in a 13 ha old-growth plot located in Central Ontario's mixed conifer-angiosperm forest. Topography, soil moisture and soil nitrogen were found to structure the distribution of tree species: Conifers dominated the less fertile, xeric lake margin and tolerant hardwoods dominated the richer mesic interior. Patterns in functional composition reflected the trade-offs that have influenced the evolution of plant traits, especially the Leaf Economics Spectrum. Functional composition based on species-level traits, more than acclimation of growth rates along environmental gradients, translated into variability in stand structure and consequently biomass and productivity. Functional diversity was positively correlated with productivity. Rates of litter production and decomposition were quantified along the conifer-angiosperm gradient. Functional composition of litter assemblages accurately predicted decomposition rates and dominated temperature and moisture effects. Organic-horizon dynamics that resulted from coevolved green-leaf and litter properties have presumably acted to stabilize relative abundances in these plant-soil systems, through cultivation of nutrient levels more beneficial to existing assemblages than invaders. Functional diversity was negatively correlated with decomposition, which has likely favored conifers in mixed-species communities. Impacts of tree ontogeny on soil respiration were investigated for the hyper-dominant species, Acer saccharum Marsh. Following an intense rainfall, soil respiration was especially amplified in tree root zones. Post-rain root-zone fluxes increased with tree size, implying that ontogenetic processes have contributed soil-carbon dynamics. A trait-based perspective has linked adaptive variation among trees to ecosystem function. Important diversity effects on productivity and decomposition were detected, implying that increased representation of community structure in ecosystem models can lead to improved predictions. Investigating relationships between community and ecosystem processes in remaining old-growth forests is necessary to evaluate the potential functioning of degraded landscapes.Ph.D