|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. More recently, Kobe (2006) was able to describe foliar N-light relationships on radial growth for a number of species in Michigan, based on natural ecosystem gradients, which nicely demonstrated the purpose of this investigation. An experimental approach to light/nitrogen interactions would help separate and quantify 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. We will also examine soil N measures, light availability and foliar N concentrations in more detail using natural productivity gradients of oldgrowth boreal forests. Natural soil fertility, as determined under the biogeoclimatic system of ecosystem classification, could lend legitimacy to our experimental results and clearly establish relationships between soils and foliar attributes. |
Carter, R.E., and Klinka, K. 1992. For. Ecol. Manage. 55: 87-105. Drever, C.R., and Lertzman, K.P. 2001. Can. J. For. Res. 31: 2124-2133. Finzi, A.C., and Canham, C.D. 2000. For. Ecol. Manage. 131: 153-165. Kranabetter, J.M., and Coates, K.D. 2004. Can. J. For. Res. 34: 800-809. Kobe, R.K. 2006. Oecologia 147: 119-133.