Modelling Tree Height-Diameter Relationships and the Effect of Climate

  • Author / Creator
    Nozohourmehrabad, Pembegul
  • Tree size and tree height are two essential quantities to measure structure and productivity of forest stands. As such, they encourage predictability in changes among tree growth patterns and are fundamental to developing forest management plans. Tree height and size (commonly measured as diameter at breast height, DBH) vary within and between species. Monitoring their variations is necessary to understand how forest productivity changes and how forest can be managed accordingly. Moreover, recent climate change may affect tree height and diameter and their relationship, because tree growth is subject to change in climate. This thesis contributes to improving forest harvesting and silvicultural plans and tree height-diameter modelling by analyzing variation in tree diameter and height, and climate effect on their relationships.
    I first started modeling tree size distribution for a wide range of forest communities and derived a Weibull distribution from stochastic tree recruitment, growth and mortality. I tested the Weibull distribution model using tree size data collected from six continents and also from Alberta, Canada. I found that the Weibull distribution provided a good description of the curvilinear relationships of the tree size data. I further showed how to use size distribution for predicting growth with the data from a 50-ha long-term forest dynamics plot on Barro Colorado Island, Panama. The result showed that variation in recruitment time and growth is sufficient to explain the differentiation in tree size.
    Second, I modeled tree size (DBH) and tree height relationship and their derivative forms. Traditional tree height-diameter equations model tree height for a given size. However, the traditional models are cumulative functions and do not provide information about incremental rate in relative to tree size. The latter is more informative about tree growth, hence useful for forest management and planning silvicultural practices. Using 57,772 individual living trees from 575 permanent sample plots across Alberta boreal forest, I identified models (from an inclusive list of 19 candidate models) that best described tree height-DBH relationship for seven major tree species of Alberta. I then explored the derivatives of the best fitted tree height-DBH models to describe tree height increment in relative to diameter increment and associated the derivatives with life history traits of species. I found that although the cumulative height-DBH relationships all had similar shape, the derivatives of tree height-diameter relationship between early successional (deciduous) species were distinctly different from that of late successional (coniferous) species. This indicates that the derivative of height-diameter relationships is more informative then the cumulative equations in revealing tree growth patterns.
    Third, I studied climate effects on tree height-DBH relationships. The need to incorporate the effects of climate into existing tree height-DBH models has become clear, as tree height and diameter growth are inevitably sensitive to climate changes, and consequently so are tree height-DBH relationships. I incorporated a climate moisture index (CMI) and maximum temperature (Tmax) into seven most common tree height-DBH models to improve the predictability of traditional height-DBH models and to reveal whether species have specific response to climate change conditions. I discovered that CMI had negative effects on height growth of some species, where Tmax had significantly positive effects on height growth of all species. All climate-based tree height-DBH models increased the predictive ability of the original models. This suggests that climate effects need to be incorporated into models to predict tree growth. Also, the climate-based models showed that tree species would become taller with increasing temperature although their responses to climate moisture index would be different.
    The results of my thesis contribute to improving modelling tree diameter and tree height, understanding tree growth patterns by analyzing derivative curves, and increasing predictability of height-DBH models by including climate effects. The models derived and parameterized from this thesis should be useful in guiding forest harvesting and silvicultural planning.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.