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The effect of site type and stand structure on the relationship between growth and light availability for understory trees
Lilles, Erica B.
Complex stand management is becoming more prevalent in northern interior forests due to changing management practices and the mountain pine beetle (MPB) epidemic. The MPB epidemic is creating vast areas of complex multi-storied stands. To predict growth of complex stands it is essential to understand the factors controlling growth of understory trees. Considerable work has been undertaken on mesic (average) sites to predict understory tree growth as a function of light availability. Light availability is the primary driver of photosynthesis and is bound to have an effect on understory tree growth. Additionally, light availability has shown good correlation with understory tree growth on mesic sites in northern temperate, sub-boreal and boreal forests (e.g. Wright et al. 1998; Stadt et al. 2005; Astrup and Coates in review). It is important to expand this work to a broader range of site types to aid the further development individual tree based complex stand growth models (e.g., SORTIE-ND, TASS 3). Light availability is not the only resource affecting growth of understory trees in complex stands. Below-ground competition for moisture and nutrients is important for understory tree growth (Coomes and Grubb 2000) and landscape-level variation in nutrient and moisture availability has been shown to alter the relationship between light and growth for understory trees (Kobe 2006). Within a region of uniform climate, the ecological classification system in BC is based on identification of unique site types (site series) that cover a gradient from dry-poor to moist-rich sites (Banner et al. 1993). Different site types have different levels of available belowground resources. Thus, site type can be expected to affect the light-growth relationship for understory trees in the sub-boreal forests of BC. We intend to develop site-type specific relationships between light availability, tree size and growth for understory trees in sub-boreal BC. For most tree species in northern BC, the effect of site-type on the light-growth relationship has never been investigated. The composition, abundance and spatial distribution of canopy trees determines the availability understory light in forests (e.g. Canham et al. 1999) and affects the availability of below-ground resources through competition. For prediction of individual tree growth, below-ground competition is often represented with a competition index (e.g. Ledermann and Stage 2001). For canopy trees, it has been shown to be advantageous to use both light availability and a distance-dependent competition index in models of individual tree growth (Canham et al. 2004). We intend to explore if this also is the case for understory trees and will use a similar approach and analysis as described by Canham et al. (2004) for canopy trees. Specifically, we intend to explore if our initially developed site-type specific light-growth models can be improved by utilizing light availability in combination with a distance-dependent competition index as predictor variables. For understory trees in northern BC, the combined effect of light availability and below-ground competition (represented as a competition index) has never been investigated. Our primary objective is to understand the factors that control understory tree growth and to develop statistical models that can be used, in conjunction with simulation models, to predict understory tree growth. To meet this objective we propose to: (1) investigate how the relationship between light availability and growth differ between site types (site series) ranging from dry and poor to rich and moist, and (2) investigate if utilizing light availability, site quality and a competition index improves our predictive models of understory tree growth. The simple site-series and species-specific regression models can be readily used in combination with existing light models to predict the growth of understory trees on a specified site-type. Thus, the mode ...
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