¹ Dept. of Forest Resources and Ecology Center, Utah State University, Logan, UT 84322-5215, USA;
² Swiss Federal Institute of Forest Snow and Landscape Research, CH-8903 Birmensdorf, Switzerland;

Project Summary

We propose to test a series of specific, nested hypotheses regarding individual tree and ecosystem leaf area distributions, individual tree and ecosystem productivity, and the distribution and dynamics of vegetation composition and structure as determined by environmental variability in climate, landform, and soils. These hypotheses form an explicitly hierarchical model of the behavior of individual organisms as determined by fundamental carbon acquisition and allocation patterns at the lower level balanced against ecosystem-level constraints on vegetation structure and function.  In turn, community composition and structure is determined by the performance of individual organisms at the lower level as constrained by landscape and regional patterns of environmental variability. We will test the individual hypotheses and the integrated ecological implications through a combination of site-specific field tests and an integrated simulation model.

We believe that the distribution and dynamics of forest vegetation are driven by fundamental relations of energy, water, and nutrient resource distribution in the environment and the patterns of resource capture and carbon allocation by woody plants.  Environmental energy, water, and resource availability are determined by the interaction of regional geomorphology, landform, and lithology with regional climate. Given a spatially- and temporally-complex pattern of resource availability, vegetation distribution and dynamics are determined by the aggregate response of the reproduction, growth, and mortality of individual plants responding to environmental variability at a range of spatial and temporal scales.  Individual plant life histories are governed by genetic constraints, but are determined physiologically by the interaction of resource capture and carbon allocation as described below.

The framework for the conceptual model and the specific hypotheses is built on the extensive theoretical and empirical research on plant water relations and carbon allocation.  In recent years forest science has achieved a comprehensive view of ecological plant water relations, including specifically the relationships between: (1) individual tree sapwood cross-sectional area and leaf area and (2) stand-level site water balance and ecosystem leaf area.  In addition, recent work on plant carbon gain and allocation has led to a detailed understanding of plant energy balance, which ultimately governs growth, reproduction, and mortality.  The key to integration of these research areas, and the central concept of the proposed research, is to analyze and model the distribution and dynamics of  leaves.  Leaf area distributions along environmental gradients (determined by resource availability and environmental limitations) determine the relative productivity and density of vegetation.  Leaf area distribution among individual stems in a single stand, limited by total stand leaf area, determines individual tree carbon gain, and therefore ultimately vegetation composition, structure, and diversity.

Individual plant leaf area and growth will be predicted as an interaction between basic species-specific physiological parameters and the environment experienced by the plant, determined by the local physical environment and the relative size and position of the plant in the canopy. This complex, heterogeneous environment will be modelled in a spatially-explicit system that calculates or incorporates the time series of: (1) direct and diffuse solar radiation, (2) precipitation, (3) potential evapotranspiration, (4) soil water availability, (5) heat, and (6) relative nutrient availability. Community composition and structure will be predicted based on ecological optimization principles scaled up from the individual plant level to the community and landscape-levels by simulation modelling of the underlying lower-level processes as constrained by the ecosystem-level resource availability.

We propose to develop a new, spatially-explicit, physiologically-mechanistic forest simulation model based on the roles of leaf area dynamics in photosynthetic carbon gain, net carbon balance, soil water and nutrient requirements, and the environmental roles of climate, landform, and soils to test specific predictions regarding the distribution and dynamics of forest vegetation based on the principles outlined above. The Leaf Area Allocation SIMulation model (LAASIM) is an individual tree-based simulation model that is physiologically-mechanistic in a way that is sensitive to climatic and environmental variability, based on fundamental research in woody plant biology, suitable for implementation in a spatially-explicit modelling framework, and suitable for the analysis of basic questions regarding the interaction of environmental variability and population- and community-level response of complex ecosystems.

Research will be conducted on approximately 1 million hectares of extremely heterogeneous mountainous terrain in Wyoming.  Specifically, we will predict: (1) individual tree and forest growth and leaf area dynamics for a range of forests from open savanna woodland through closed forest to alpine forest; and (2) the appropriate species composition and successional dynamics for all stages of succession for the same range of forests. The research will clearly highlight our understanding of the interaction of environmental variability and basic organism- and community-level biological and ecological mechanisms, and test a number of hypotheses concerning these mechanisms.

Keywords: Forest dynamics, Forest modelling, Hydraulic model, Hydrologic equilibrium theory, Leaf area index, Pipe model theory, Trade-off model

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