Juvenile and Adult Tree Mortality Studies

There are several mortality functions in SORTIE-ND. Juvenile mortality is based on the well-documented empirical relationship between growth rates and survival of understory seedlings and saplings. The probability of survival for each juvenile tree is predicted by species as a function of recent radial growth rates. The user can also specify species-specific random background mortality rates for juvenile or adult trees, and there is a separate senescence mortality function for old-age mortality. We have also developed adult tree mortality functions for sub-boreal spruce tree species based on the relationship between growth rates and survival using methods similar to those developed for juvenile trees. For dense even-aged and single-species stands, self thinning functions can be implemented for specific model simulations. Parameter values for random, senescence and self-thinning mortality should be estimated from locally available data.

Models of individual tree mortality for trembling aspen, lodgepole pine, hybrid spruce and subalpine fir in northwestern British Columbia
Pedersen, S.M. 2007.

This thesis was initiated in the summer of 2005 for the project "Modelling individual tree mortality for northern mixed-species stands" (Forest Science Program Project Y061012) by the Department of Forest Sciences at the University of British Columbia. The study used sub-boreal spruce forest sampling sites near Smithers. The objective of this study was to examine density dependent mortality in adult trees with methods that had been previously been used for modelling density dependent mortality for saplings with good results. Mortality was predicted as a function of recent diameter growth and we tested if incorporating tree size into the mortality model improved it. The models were parameterized from field data using a maximum-likelihood method. Field data was gathered from 16 stands comprised of 337 live and 345 recently dead trees in total. Incorporating tree size into the mortality models gave better fits to the field data. Tolerance to low growth decreases to a minimum at intermediate trees size for all species except for subalpine fir, where it decreases and remains low as trees growth larger. Testing the mortality models in SORTIE-ND showed that they contribute to realistic thinning patterns in simulations of both pure even-aged stands and complex stands. However, it was evident that the performance of the mortality models is highly dependent on the underlying growth models as well as mortality models accounting for random mortality. Discrepancies in modelling results were linked to over- and underestimation of growth or inappropriate random mortality rates. Overall the tested method provides a straight forwards approach to parameterizing growth based mortality models from field data which is relatively easy to obtain.

Sapling size influences shade tolerance ranking among southern boreal tree species
Kneeshaw, Daniel D., R.K. Kobe, K.D. Coates, and C. Messier. 2006.

This paper presents results from boreal forests in Quebec. Part of the analysis uses models developed by Kobe and Coates (1997) - see below - and Kobe et al. (1995). The models presented in this paper are part of behaviours developed for the Quebec version of SORTIE-ND. See Our Collaborators and Links of Interest for more information on SORTIE-ND in Quebec.  

Traditional rankings of shade tolerance of trees make little reference to individual size. However, greater respiratory loads with increasing sapling size imply that larger individuals will be less able to tolerate shade than smaller individuals of the same species and that there may be shifts among species in shade tolerance with size. We tested this hypothesis using maximum likelihood estimation to develop individual tree-based models of the probability of mortality as a function of recent growth rate for seven species: trembling aspen, paper birch, yellow birch, mountain maple, white spruce, balsam fir and eastern white cedar.

Models of sapling mortality as a function of growth to characterize inter-specific variation in shade tolerance of eight tree species of northwestern British Columbia
Kobe, R.K., K.D. Coates. 1997.

This is the original juvenile mortality study undertaken in the interior cedar hemlock forests (ICH) at Date Creek as part of our overall SORTIE-related studies. This study builds on the work of Kobe et al. (1995). The results from this study were incorporated into SORTIE/BC (Coates et al. 2003). We developed species-specific models that characterized an individual's probability of mortality as a function of recent growth (a surrogate for whole-plant carbon balance) and provides a quantitative measure of the effective shade tolerance of each tree species. We found significant interspecific variation in juvenile mortality among the major tree species. The species exhibited a broad and continuous range of variation in the functional relationship between mortality and recent growth. More shade tolerant species exhibited more rapid decays in mortality probability with increased radial growth. Our mortality models were consistent with previous qualitative categorizations into shade tolerance classes and paralleled the dominance of different species in post-disturbance succession for ICH forests. Species-specific differences in non-catastrophic juvenile tree mortality are critical to understanding and predicting forest community dynamics and succession in this region.