Spatially-Explicit, Mixed-Species Forest Dynamics Simulator

In summer 1995, Dave Coates, Phil LePage, Elaine Wright and other scientists from the Research Section of the British Columbia Forest Service in Smithers began a project with Charles Canham of the Institute of Ecosystem Studies, Millbrook, New York, to develop a spatially-explicit model of the dynamics of the interior cedar-hemlock (ICH) forests of northern BC.  The model is a descendent of a recently developed model (SORTIE) for mixed conifer/hardwood forests in eastern North America. A challenge for both ecology and silviculture is to understand how large-scale or long-term community and ecosystem dynamics result from interaction among individuals.  This is especially the case following natural and human disturbances.  Our understanding of the implications of varying frequency, intensity and pattern of tree death (by either natural agents or logging) on forest community dynamics and ecosystem processes at large-scales and over long time periods is limited. In order to develop more reliable predictions of the effects of management on long-term forest dynamics, we undertook an integrated program of modeling and empirical studies centred at Date Creek, but with field studies throughout northwestern B.C.

The SORTIE model consists of 4 submodels:  (1) seedling recruitment (reproduction) – function of parent tree proximity and seedbed substrate; (2) resource availability – predicts understory light dynamics as a function of species specific light extinction coefficients; (3) tree growth – function of light availability; and (4) tree mortality – function of recent growth rates. We are currently undertaking baseline assessments with the model with a view towards operational applicability.  A preliminary version is presently in use in the Prince Rupert Region and an updated version is expected to be ready by the summer of 2000.

Model predictions include:  (1) spatial distribution and sizes of all individuals in a simulated stand (of any specified size, limited only by computing resources); (2) DBH and height distributions, by species; (3) changes in basal area and density, by species, over time; and (4) tables of basal area and densities of both adult and juvenile trees.

One of our major objectives is to use SORTIE to simulate partial cutting strategies.  At the present time, there are few good tools to predict the long-term stand structure implications of partial cutting and this is proving to be a major impediment to its use.  We have developed a very flexible user-interface to allow incorporation of a wide range of partial cutting strategies into model simulations.  Future work on the model will involve an upgrade to the adult tree growth parameters to improve the growth and yield predictions.

Available Literature:

Canham, C.D., Coates, K.D., Bartemucci, P. and Quaglia, S.  1999.  Measurement and Modeling of Spatially-explicit Variation in Light Transmission Through Interior Cedar-Hemlock Forests of British Columbia. Can. J. For. Res. 29: 1775 - 1783.
Abstract: We have characterized canopy geometry and light transmission by the nine dominant conifer and hardwood tree species of the interior cedar-hemlock (ICH) forests of northern British Columbia.  Our field data were used to parameterize a spatially-explicit model of light transmission through mixed-species forests.  That model, a component of the forest dynamics simulator SORTIE, was developed for eastern deciduous forests, and this paper presents a test of that model in a very different ecosystem. Our results show that individual crowns of the ICH forests generally intercepted much more light than species of eastern deciduous forests, but that the canopy as a whole allowed much greater light penetration, largely because of openings between the relatively narrow, conical crowns of the western conifers. Light transmission by individual crowns was generally correlated with shade tolerance among the conifers
Fisheye photo of ICH forest
(as in eastern deciduous species, but crown depth was not (in contrast to eastern species).  Despite the fundamental differences in the nature of light transmission in the two ecosystems, the SORTIE light model developed for eastern deciduous forests was very effective at predicting spatial variation in understory light levels in these western coniferous forests.   The ability of such a simple model to accurately predict 3-dimensional variation in light levels within mixed-species stands suggests that the most important factors regulating spatial variation in understory light levels in forests are simply the sizes, shapes and distribution of nearby trees, and the local sky brightness distribution.  Discrete canopy gaps represent a special case in which a region of the canopy is not occupied by crowns.

Kobe, R.K. and Coates, K.D. 1997. Models of sapling mortality as a function of growth to characterize interspecific variation in shade tolerance of eight tree species of northwestern British Columbia. Can. J. For. Res. 27:227-236.
Abstract: We have developed models of sapling mortality for the eight dominant tree species of northwestern British Columbia in order to better understand forest community dynamics and succession in this important forest region.  The species-specific models characterize an individual’s probability of mortality as a function of recent growth (a surrogate for whole-plant carbon balance).  Interspecific comparisons of survival under low growth rates (i.e. suppression) provide a quantitative measure of the effective shade tolerance of these tree species.  In particular, more shade tolerant species exhibited more rapid decays in mortality probability with increased radial growth.  The broad and continuous range inshade tolerance ranking was: [Thuja plicata (Dougl. ex D. Don)] > [Tsuga heterophylla (Raf.) Sarg.] = [Abies lasiocarpa (Hook.) Nutt.)] > [Picea glauca (Moench) Voss x sitchensis (Bong.) Carr.] > [Pinus contorta var. latifolia Engelm.] > [Populus tremuloides Michx.]> [Populus balsamifera ssp. trichocarpa Torr. & Gray] = [Betula papyrifera Marsh].  At low growth rates, mortality varied between T. plicata and B. papyrifera by more than an order of magnitude.  For some species, the three replicate sites exhibited significant variation, suggesting that shade tolerance may vary with site conditions (presumably soil moisture in our study sites).  The mortality models are consistent with previous qualitative categorizations into shade tolerance classes and parallel the dominance of different species in post-disturbance succession.  Our results suggest that species differences in non-catastrophic mortality are critical to understanding and predicting forest dynamics.

LePage, P., Canham, C.D., Coates, K.D. and Bartemucci, P. 2000. Seed abundance versus substrate limitation of seedling recruitment in northern temerate forests of British Columbia. Can. J. For. Res. 30: 415 - 427.
Abstract: We examine the influence of (1) the spatial distribution and abundance of parent trees (as seed sources) and (2) the abundance and favourability of seedbed substrates, on seedling recruitment for the major tree species in northwestern interior cedar-hemlock forests of British Columbia, under 4 levels of canopy openness (full canopy, partial canopy, large gaps, and in clearcut sites).  Substrate distribution varied with canopy openness, and substrate favourability was a function of both canopy openness and seedling species.  Lack of suitable substrates was the predominant factor limiting seedling density under full canopies.  Partial canopy and gap sites provided a broad range of favourable substrates in close proximity to parent trees, resulting in the highest observed seedling densities.  There was much higher effective dispersion of seedlings away from parent trees in gaps than in the partially cut stands.  Seedling dispersion to clearcut sites was poor with seedlings being tightly restricted to a narrow band along the forest edge.  Thus, seedling recruitment in these forests was a reflection of the interaction between the abundance of seed and substrate favourability, and the relative importance of these factors varied significantly with canopy structure.

Wright, E.F., Coates, K.D., Canham, C.D. and Bartemucci, P. 1998. Species variability in growth response to light across a climatic gradient in northwestern British Columbia. Can. J. For. Res. 28:871-886.
Abstract: We characterize variation in radial and height growth of saplings of 11 tree species across a range of light levels in boreal, sub-boreal, subalpine and temperate forests of northwestern British Columbia.  Shade tolerant species had the greatest response to an increase in light at low light levels, but had low asymptotic growth at high light.  Shade intolerant species had weaker responses to increases at low light, but had the highest growth rates at high light.  The effects of climate on intraspecific variation in sapling response to light were also related to shade tolerance: across different climatic regions, the most shade tolerant species varied in their response to low light but not high light, while shade intolerant species varied only in their high-light growth.  Species with intermediate shade tolerance varied both the amplitude of growth at high light and the slope of the growth response at low light.  Despite the interspecific trade-offs between high and low-light growth, there was a striking degree of overlap in the light response curves for the component species in virtually all of the climatic regions.  Successional dynamics in these forests appear to be more strongly governed by interspecific variation in sapling survival than growth.

Wright, E.F., Canham, C.D. and Coates, K.D. 2000. Effects of suppression and release on sapling growth for eleven tree species of northern British Columbia. In preparation.
Abstract: There has been a long-term debate in forestry over the management of advance regeneration following cutting.  Saplings of different tree species clearly have different abilities to survive periods of suppression and different magnitudes of response to release. We examined the effects of the lengths of previous periods of suppression and release on the growth responses of eleven conifer and hardwood tree species of northern British Columbia  We were specifically interested in the degree to which increasing length of suppression had long-term effects on subsequent response to release in gaps or following partial cutting, and the degree to which the effects of previous periods of suppression were ameliorated with time following release.  We calculated the total number of years of suppression and release,  the number of distinct periods of suppression and release, and the length of the most recent (or current) period of suppression and release for each sapling.  The individual patterns of suppression and release varied enormously among saplings and species. Three of the 11 species - western redcedar, amabilis fir, and subalpine fir (all shade tolerant)- showed no effect of previous periods of suppression and release on current response to light.  Four species - hybrid spruce, lodgepole pine, cottonwood, and birch  - showed significant responses to the lengths of the most recent periods of both suppression and release.  The remaining 4 species had significant responses to either suppression or release (but not both) and ranged from shade tolerant (western and mountain hemlock) to intolerant (aspen).

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