Crown Structure and Light Interception Studies

The light model in SORTIE-ND predicts incident radiation at any given location within a forest as a function of (1) species-specific light transmission coefficients, (2) variation in crown geometry as a function of tree size, (3) the identities, sizes, and spacing of trees in the immediate neighborhood, and (4) the local sky brightness distribution The modeling approach is designed to predict understory light levels experienced by individual seedlings and saplings at any point in the understory, in the same units that are used in the field to measure growth of seedlings and saplings as a function of light level. The power of the SORTIE-ND light model is largely a function of the simplicity of the input required for accurate predictions of spatial variation in understory light levels.

Managing understory light conditions in boreal mixedwoods through variation in the intensity and spatial pattern of harvest: A modelling approach
Beaudet, Marilou, Brian Harvey, Christian Messier, David Coates, Julie Poulin, Daniel Kneeshaw, Suzanne Brais & Yves Bergeron. 2010.

This project from Quebec provides light extinction coefficients for eastern boreal species and an example of light dynamics in boreal forests.

In the context of partial harvesting, adequately managing post-harvest light conditions are essential to obtain a desired composition of tree species regeneration. The objective of this study was to determine how varying the intensity and spatial pattern of harvest would affect understory light conditions in boreal mixedwood stands of northwestern Quebec using the spatially explicit SORTIE-ND light model. The model was evaluated based on comparisons of observed and predicted light levels in both mapped and un-mapped plots. In mapped plots, reasonably accurate predictions of the overall variation in light levels were obtained, but predictions tended to lack spatial precision. In un-mapped plots, SORTIE-ND accurately predicted stand-level mean GLI (Gap Light Index) under a range of harvest intensities. The model was then used to simulate nine silvicultural treatments based on combinations of three intensities of overstory removal (30%, 45% and 60% of basal area) and three harvest patterns (uniform, narrow strips, large gaps). Simulations showed that increasing overstory removal had less impact on light conditions with uniform harvests, and a more marked effect with more aggregated harvest patterns. Whatever the harvest intensity, uniform cuts almost never created high light conditions (GLI > 50%). Gap cuts, on the other hand, resulted in up to 40% of microsites receiving GLI > 50%. Our results suggest that either a 30% strip or gap cut or a 45-60% uniform partial harvest could be used to accelerate the transition from an aspen dominated composition to a mixedwood stand because both types of cut generate the greatest proportion of moderately low light levels (e.g., 15-40% GLI). These light levels tend to favour an accelerated growth response among shade-tolerant conifers, while preventing excessive recruitment of shade-intolerant species. A better understanding of how spatial patterns of harvest interact with tree removal intensity to affect understory light conditions can provide opportunities for designing silvicultural prescriptions that are tailored to species' traits and better suited to meet a variety of management objectives.

Competition and tree crowns: A neighborhood analysis of three boreal tree species
Thorpe, H.C., Astrup, R., Trowbridge A., Coates, K.D. . 2010.

This paper builds on our early work to quantify crown openness and understory light levels in interior cedar-hemlock and boreal forests. Here we use non-linear models to quantifiy crown radius and length of lodgepole pine, interior spruce and subalpine fir as functions of tree size and neighbourhood competition in sub-boreal spruce forests.

Competition for canopy space is a fundamental structuring feature of forest ecosystems and remains an enduring focus of research attention. We used a spatial neighbourhood approach to quantify the influence of local competition on the size of individual tree crowns in north-central British Columbia, where forests are dominated by subalpine fir, lodgepole pine and interior spruce. Using maximum likelihood methods, we quantified crown radius and length as functions of tree size and competition, estimated by the species identity and spatial arrangement of neighbouring trees. Tree crown size depended on tree bole size in all species. Given low levels of competition, pine displayed the widest, shortest tree crowns compared to the relatively long and narrow crowns found in spruce and fir. Sensitivity to crowding by neighbours declined with increasing tree height in all but the pine crown radius model. Five of the six selected best models included separate competition coefficients for each neighbouring tree species, evidence that species generally differ in their competitive effects on neighbouring tree crowns. The selected crown radius model for lodgepole pine, a shade-intolerant species, treated all neighbours as equivalent competitors. In all species, competition from neighbours exerted an important influence on crown size. Per-capita effects of competition across different sizes and species of neighbours and target trees varied, but subalpine fir generally displayed the strongest competitive effects on neighbours. Results from this study provide evidence that species differ both in their response to competition and in their competitive influence on neighbours, factors that may contribute to maintaining coexistence.

A system of nonlinear simultaneous equations for crown height and radius
Sattler, Derek. 2008.

This study is part of Derek Sattler's masters thesis at UBC. To use SORTIE-ND in dense dry pine forests of the central interior region of British Columbia a crown structure behaviour was developed that explicitly accounts for the effect of crowding (neighbourhood density) on crown depth and crown radius of lodgepole pine. A paper is in preparation.   

Variability of species-specific crown openness for aspen and spruce in western boreal Canada
Astrup, R. and B.C. Larson. 2006.

This paper quantifies crown openness of aspen and spruce across the Canadian boreal mixedwood region of western Canada using methods similar to those developed by Canham et al. (1999).

Species-specific crown openness is defined as the fraction of sky that can be seen through the crown of an individual tree of a given species. Species-specific crown openness is of general interest for understanding light transmission through forest canopies, and is an essential part of the light submodel in SORTIE. The main objective of this study was to investigate whether regional differences in mean species-specific crown openness exist for trembling aspen and white spruce in western boreal Canada. To ensure a robust comparison of regional mean species-specific crown openness, we investigated the underlying assumption that crown openness is unaffected by dbh and angle of view. Both aspen and spruce crown openness were found to be independent of angle of view. Crown openness was also independent of dbh in aspen, while weak indications of a correlation between crown openness and dbh was found for white spruce. However, this relationship has little actual effect on crown openness and its effect on predicted understory light level is judged to be small. We found significant regional differences in mean crown openness for both aspen and spruce. However, these regional differences are small and are likely to have relatively little effect on understory light levels predicted with SORTIE. The results from this study indicate that, although previous estimates of aspen and spruce crown openness in western boreal Canada varied greatly, this is more likely the result of different methodologies than actual differences in crown openness.

Gap disturbances in northern old-growth forests of British Columbia, Canada
Bartemucci, P., Coates, K.D., Harper, K., Wright, E.F. 2002.

This study was part of efforts to better quantify the role and importance of small-scale disturbances in northern latitude forests. Gap disturbances have been largely ignored in northern latitude forests, especially those with frequent large stand-destroying fires. In this study, we sampled in four major forest types of northern British Columbias with varying fire return intervals, stand composition and climate. We determined the amount of gap disturbance in northern latitude forests, identified the gap-forming processes, determined the spatial patterning of gaps, and characterized understory light regimes.

Gap sizes in northern British Columbia forests were at the upper end of ranges reported for other high latitude forests and were mostly larger than those in tropical and temperate deciduous forests. Abundant gaps, large gap sizes, high numbers of gap makers per gap, and frequent gap expansion events suggest that gaps have long tenure in these forests. Snapped stems and standing dead mortality were the most common modes of mortality in all forest types resulting in little forest floor disturbance, creating few germination sites for seedling establishment. We found high mean light levels (16-27 % full sun), and little difference between non-gap and gap light environments. Our results suggest that gap dynamics in these forests differ fundamentally from those in temperate and tropical forest ecosystems. Old-growth forests and small-scale disturbances were prevalent in the high-latitude forests of British Columbia, which were traditionally considered fire-driven, large-scale disturbance systems.

Measurement and modeling of spatially explicit variation in light transmission through interior cedar???????hemlock forests of British Columbia
Canham, C.D., K.D. Coates, P. Bartemucci, and S. Quaglia. 1999.

This paper is another core SORTIE-related study from the Date Creek Experiment. Field data from interior cedar hemlock forests were used to parameterize a spatially explicit model of light transmission through mixed-species forests. We simplified the approach developed for eastern deciduous forests (Canham et al. 1994) by focusing on sampling individual tree crowns. The SORTIE-ND light model is an extraordinarily simple representation of the underlying complexity of light transmission through forest canopies and gaps. the parameters required by the SORTIE-ND light model are operationally defined, and can be readily estimated directly from field data. 

In the paper, we characterized canopy geometry and light transmission of the nine dominant conifer and broadleaved tree species at Date Creek. Canopy openness varied significantly among the 9 species and ranged from a low of 5.8% in paper birch to a high of 20.6% in trembling aspen. To tested the model against independent datal, we took 67 fisheye photographs distributed in 5 stands at Date Creek in which all trees > 15 cm DBH had been previously stem-mapped. The 5 validation sites were physically separate from the sites where calibration photos were taken. The parameterized light model, using cylindrical crowns with radii set at 50% of maximum crown radius, produced excellent fits to the test data. The model slightly underestimated canopy openness when observed openness exceeded 50%. The ability of The SORTIE-ND light model to accurately predict spatial variation in light levels within mixed-species stands suggests that the most important factors regulating spatial variation in understory light levels are the basic geometry of tree crowns, combined with information on the sizes and distribution of nearby trees, and the local sky brightness distribution.