Growth and establishment of species with a range of shade tolerance within a strip shelterwood in the Interior Cedar Hemlock Zone of British Columbia

• Author / Creator

  Hossain, Kazi

• Strip or group shelterwood and other continuous cover forestry practices are commonly advocated for managing mixed species forests of Interior-Cedar Hemlock (ICH) Biogeoclimatic zone in British Columbia. In strip-shelterwood systems, narrow openings are created in the canopy to facilitate regeneration of a mixture of species. Successful regeneration of mixed species stands using the shelterwood method requires knowledge of how the various component species respond to the range in light levels created as a function of opening size and location within openings. However, the effectiveness of these silvicultural systems in regenerating mixed species stand for various forest ecosystems, particularly for the ICH are not well investigated. Available information suggests that for several species with different shade tolerances to coexist in a forest stand, species must show evidence of growing at different rates along the light gradient (niche partitioning). In particular, to achieve a competitive hierarchy, shade intolerant species need to grow faster than tolerant species in areas with high light levels.
  To address this issue, I conducted this study in two small (50 m × 150 m) clearings in a mixed stand of the interior cedar hemlock zone of southern British Columbia.
  Obtaining reliable estimates of the light gradient within shelterwood strips was an essential part of this study. Thus, in the first study, I evaluated several commonly used light estimation techniques including: 1) hemispherical photography; 2) LAI-2000 plant canopy analyzer; and, 3) midday measurements of % photosynthetic photon flux density (hourly average PPFD) under sunny and overcast skies. The core finding of this chapter was that light indices that integrated direct beam contribution gave the best characterization of the existing light gradient. On the other hand, indices that accounted primarily for diffuse light were poorly correlated with the growing season light. Canopy conditions (degree of heterogeneity), stand orientation and gap sizes are important considerations in selecting any indirect light index for studies, especially in high latitude forests.
  In the second study, I compared growth rates of nine species with varying shade tolerance along the light gradients within the strip-shelterwood. I also examined how gap-position influences vary between species and draw inferences on the sensitivity of each species to edge proximity. I found evidence of species’ niche partitioning along the light gradient, and also for changing positions toward the north – south direction (separation into different clusters and niches not occurring at the same position within the gap). Two highly shade intolerant species (Ponderosa pine and lodgepole pine) exhibited the highest growth rates along the light gradient and achieved a discrete growth hierarchy by separating from the other species. In general, the overlapping species were mostly close in shade tolerance. Furthermore, shade tolerant species were relatively less sensitive to changes in gap position than intolerant species. Therefore, my conclusion from this chapter is, to maximize the potential of mixed species establishment, species also need to be matched to their suitable niches within strip-shelterwood gaps.
  As an extension to my second study I investigated how shade tolerant and intolerant species differ in distribution of height and diameter growth during the growing season across gap environments, and if differences in growth distribution would confer competitive advantages to certain species. In general, shade intolerant species adopted a competitive strategy (higher height surge at the beginning of the growing season followed by sustained diameter growth in the later season) across all gap environments. Between the two moderately tolerant species (Douglas-fir and white pine), white pine appears to have adopted a more conservative growth strategy across the gap due to continuity in its diameter investment throughout the growing season. Douglas fir, Engelmann spruce and subalpine fir appear to have adopted a growth strategy that is gap-environment specific. Finally, the range of light conditions created by strip-shelterwood harvesting combined with differences between shade tolerant and intolerant species in resource exploitation strategies suggests that regenerating a diverse mixture of species through strip-shelterwood systems is a viable option in Interior Cedar Hemlock forests.

• Subjects / Keywords

  • Niche theory
  • Adaptation strategy
  • Shade tolerance
  • Mixed conifer
  • Strip shelterwood

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-4g6g-fe61

• License

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