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Interactions between light and nitrogen availability on juvenile tree growth in partial cut forests
Kranabetter, J. Marty
1.1 Complex stands Priority areas for research: species interactions, light level effects related to tree and stand growth in multi-storied stands, partial cutting and variable retention, mortality. Forest managers in British Columbia are under pressure to adopt more complex silvicultural regimes that retain live trees and add structural diversity to managed stands. The retention of forest canopy will reduce light levels by varying degrees, however, which can affect tree growth responses and species interactions within the regenerating stand. The resulting uncertainty in stand performance has prompted scientists to examine tree growth and light conditions that can occur under partial-cutting systems. These relationships have been incorporated into models, such as SORTIE, that predict juvenile tree growth. Currently these simulation models are based on species-specific growth relationships with light conditions, without yet allowing for differences in nitrogen uptake. This simplification may be adequate for modelling juvenile tree growth across constant site conditions, but might become less appropriate where tree nitrogen status varies temporally or spatially. These models may, for example, overestimate tree growth under partial canopies on N-poor sites, or underestimate growth of N-tolerant tree species in mixed stands. There has been some investigation into how significant the interaction between light availability and tree nitrogen status (or relative site quality) can be on growth within natural ecosystems. Carter and Klinka (1992) found shade tolerance of western redcedar, western hemlock and Douglas-fir varied to some degree by site quality. Drever and Lertzman (2001) found little effect of site quality (both nutrient and moisture availability) on growth of Douglas-fir at light levels below approximately 40% full sun. There was substantial variation in height growth at a given light level in this retrospective study, however, which would also make it more difficult to detect nitrogen effects. Finzi and Canham (2000) found variation in light availability explained most of the growth patterns for saplings, with only small effects from nitrogen availability on red maple and sugar maple. It was likely that the selection of sites did not encompass much range in nitrogen availability, however, which probably minimized the significance of nitrogen effects on tree growth. An experimental approach to light/nitrogen interactions is needed to better determine these relationships. At the Date Creek Research Forest, we have found soil N availability and conifer nutrition across canopy openings to be quite uniform, making it an ideal site for further experimental research (Kranabetter and Coates 2004). In the proposed experiment, juvenile trees (~ 12 years old) will be selected from under a range of canopy openings (e.g. 15% to 85% of available light). Nitrogen availability will be manipulated for these juvenile trees through either an addition of N as fertilizer, or an immobilization of N through sawdust (e.g. foliar N range of 0.8% to 1.6%). Four tree species will be tested across four blocks in all combinations of light and N availability (320 trees altogether). A particular focus will be on the cessation of growth at lower levels of N and light availability since survival is an important driver in stand development. The objectives of this study are as follows: 1. Quantify differences in growth (height and radial increment) of juvenile trees as a function of foliar nitrogen across a full range in light availability. 2. Test for differences in foliar N and light interactions across species of varying shade tolerance; western redcedar (shade tolerant), western hemlock (shade tolerant), hybrid spruce (moderately shade tolerant) and paper birch (shade intolerant). 3. Determine phenotypic plasticity in foliar N partitioning (chlorophyll and rubisco) and water use efficiency (natural C13 isotope abundance) across light levels and bet ...
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