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Development and structure of three high-elevation old spruce-fir stands in the quesnel highland of east-central British Columbia

Author(s) or contact(s): O.A. Steen, R.A. Coupe, H.M. Armleder, and R.J. Dawson
Source: Research Branch
Subject: Stand Structure
Series: Research Report
Other details:  Published 2005. Hardcopy is available.
 

Abstract

This study describes the composition, size and age structure, and development of three old Engelmann spruce-subalpine fir stands at high elevations (>1500 m) in the Quesnel Highland in east-central British Columbia. The descriptions provide a basis for designing alternative silvicultural systems that will maintain usable old-forest habitat for mountain caribou and other old-growth-dependent species. Subalpine fir, which comprised 86% of all stems in the three stands, had inverse-J-shaped size and age profiles. Engelmann spruce included the largest trees in each stand but had bimodal size and age profiles. Age structure interpretations suggest that the stands have developed over periods of about 290-450 years since the last major stand-level disturbances (probably wildfires). Current stand age structures contain no clear evidence of post-establishment stand-level disturbances, although the data do not exclude the possibility of such disturbances. Approximately 29% of the fir trees and 13% of the spruce trees were dead. Age analyses of the tree seedling banks in the three stands indicate stable populations with continuous recruitment during the last 40 years. Young seedlings occurred preferentially on woody debris in advanced stages of decay, suggesting that seedling density differences among stands may be due at least partially to differences in the amount of woody debris.

The stand that apparently had the shortest time (about 290 years) for development since major disturbance had a structure that was still adjusting to disturbance. Evidence of this adjustment includes a bimodal fir age distribution and a high density of small trees that are apparently undergoing thinning. In addition, this stand had very few large (>60 cm dbh) live or dead trees and low volumes of coarse woody debris, especially in large and well-decomposed pieces, compared to the other two stands. A comparison of the three stands in this study suggests that naturally established high-elevation forests of the ESSFwc3 may not achieve some old-growth attributes, such as large dead trees and large, well-decomposed woody debris, until 400 or more years following initiation. The two stands in this study with the longest period of development (at least 450 years) had balanced size and age structures and well-represented old-growth attributes such as large dead trees and large, well-decayed woody debris.

Planted stands may achieve some ESSFwc3 old-growth features several decades sooner than natural stands, due to a shorter stand initiation period. The alternative silvicultural system that has been proposed for maintaining suitable mountain caribou habitat following harvesting in this area (group selection system with 240-year rotation) will likely result in stands with many old-growth features, including abundant arboreal lichens. Some features that may not be present in these stands include a uniform inverse-J age distribution, many large (>60 cm dbh) live and standing dead trees, and large-diameter, well-decayed woody debris.

As old stands in the ESSFwc3 continue to age without outside disturbance, the spruce component of the stands will likely decline. Partial harvesting would help to maintain a significant component of spruce in these stands.

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Updated October 24, 2008