Aspen density dynamics in western boreal mixedwood stands

• Author / Creator

  Kweon, Deog Kyu

• Trembling aspen (Populus tremuloides Michx.) is a common species in Canadian boreal forests. As a shade intolerant and dynamic species (i.e., stand density and structure change over time), it naturally regenerates by root suckering and grows rapidly. Aspen has a short life compared to spruce or pine species, and experiences severe mortality derived from intraspecific competition, fungal diseases, and insect attacks. As a result, aspen stands often develop an irregular structure with age. In addition, if spruce is present in the stand, mixedwood stands may develop. In this thesis, I examine three aspects relating to aspen dynamics in pure and mixed stands. The first study examined maximum size-density relationships (MSDR). I developed static and dynamic self-thinning lines for pure aspen stands in the boreal forest region of Alberta and Saskatchewan. Climate variables were included to examine effects of climates on both self-thinning lines. The results demonstrate that climate influences the slope and intercept of self-thinning lines, suggesting that increasing summer dryness related to climate change may decrease the carrying capacity and productivity of aspen stands in portions of this region. For the second study, I investigated how age structure influences tree survival in aspen dominated stands and examined effects of stand factors, climate, and tree age on survival probability. Based on field data, I found a positive relationship between stand age and stand structure (i.e., structural diversity or stand age structure). Using data from 199 permanent sample plots, I examined effects of tree size, intraspecific competition, tree size diversity, and climate on survival probability. Results indicate that structural diversity and its effects on survival probability varied between stand age classes. In particular, deciduous basal area larger (i.e., sum of basal area of deciduous trees larger than a target tree) and tree size diversity were negatively related to tree survival and the magnitude of the negative impact varied between age groups (young, mature, and old stands). Tree size diversity was related to increasing stand level competition while deciduous basal area larger was effective as a measure of individual tree level competition. Under warmer conditions, competition variables (e.g., deciduous basal area larger and tree size diversity) had stronger effects on tree mortality, with magnitude of effects varying between age groups.The third study examined whether overyielding occurs in young aspen-spruce mixtures and investigated which factors influence productivity of these mixtures. Overyielding is occurring in these young boreal mixedwood stands in western Canada. Relative productivity total (overyielding rate) varied from 0.921 to 1.537 among mixedwood treatments, and thinned aspen stands show higher productivity than unthinned aspen stands. Initial stand density (basal area) and initial quadratic mean diameter of aspen positively influenced mixing effect while initial quadratic mean diameter of spruce negatively influenced mixing effect. Consequently, a mixed species stand that has space partitioning and size inequality between species, which reduces competition and favours expression of the functional traits (e.g., shade tolerance) of each species, tends to have better productivity. The magnitude of overyielding declined with increasing growing season length and the relative productivity of aspen decreased with increasing site quality.

• Subjects / Keywords

  • Trembling aspen
  • Stand dyanmics
  • Maximum size-density relationship
  • Survival probability
  • Mortality
  • Mixedwood stands
  • Overyielding

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-4jcy-td35

• License

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