Philip G. Comeau

Aboveground biomass and nutrient accumulation in an age sequence of aspen (Populus tremuloides) stands in the Boreal White and Black Spruce Zone, British Columbia

Abstract A chronosequence of Populus tremuloides Michx. stands ranging from 5 to 95 years of age was measured to document changes on aboveground biomass and nutrient (N, P, K, Ca, Mg) contents for three site classes (good, medium and poor) in the Boreal White and Black Spruce (BWBS) zone in northeastern British Columbia. Allometric equations relating dry weights of stemwood, branches and leaves to tree diameter at breast height were developed to estimate aboveground tree biomass. Total aboveground tree biomass increased with stand age from 34.8 t ha −1 in the 5-year-old stand to 359.7 t ha −1 in the 95-year-old stand on good sites, and from 8.3 t ha −1 to 267.7 t ha −1, respectively, on medium sites. On poor sites, the total aboveground tree biomass ranged from 33.9 t ha −1 in a 15-year-old stand to 245.4 t ha −1 in a 95-year-old stand. As stands aged, an increasing proportion of the aboveground biomass was allocated to stems. Nutrient accumulation in aboveground tree biomass increased with stand age and was in the order of N> Ca> K> Mg> P. Temporal patterns of nutrient accumulation followed the pattern of aboveground biomass accumulation. The nutrient concentrations in different tree tissues decreased in the order leaf> branch> stembark> stemwood, except for K. Understory vegetation contributed little to the nutrient pool of aspen ecosystems. Mineral soil contained the greatest proportion of nutrient capital of the various ecosystem components.

Aspen competition affects light and white spruce growth across several boreal sites in western Canada

The effectiveness of competition indices for predicting light transmittance and white spruce (Picea glauca (Moench) Voss) growth were examined across trembling aspen (Populus tremuloides Michx.) density gradients using sites from a long-term study of mixedwood growth and development in Alberta and Saskatchewan. Competition indices based on density (number of trees, basal area, and spacing factor), distance-dependent and -independent size ratio (Hegyi’s and Lorimer’s), and crown characteristics (crown volume, surface area, and cross-sectional area) were tested. Transmittance was effectively predicted by crown competition indices followed closely by aspen basal area and size ratio indices. Models of spruce growth indicated better results for stem volume compared with diameter or height. Competition alone accounted for less than 60% of stem growth variation, with basal area and transmittance providing some of the best models. The predictive ability of spruce growth was increased up to 93% by adding initial s...

Temporal variations in the understorey photosynthetic photon flux density of a deciduous stand: the effects of canopy development, solar elevation, and sky conditions

The effects of canopy development, solar elevation, and sky conditions on temporal variation in photosynthetic photon flux density (PPFD) were examined within a 9-year-old bigleaf maple stand on Vancouver Island (Canada). PPFD was measured every second and stored as 10-min averages from 18 May to 14 October 1996, at 52 microsites characterised according to their growing season %PPFD (GSP). PPFD and %PPFD variability was examined at three different temporal levels. Specific days in May, July, and September with clear and overcast sky conditions were selected to separate the effects of canopy development and solar elevation on diurnal and seasonal light variability. Diurnal light variability expressed as the mean of the difference between two consecutive 10-min averages of PPFD and %PPFD decreased with increasing GSP on clear days in May. For clear days in July and September, variability was characterised by arc-shaped relationships with high variability for microsites receiving between 20 and 80% GSP and lower variability for microsites below 20 and above 80% GSP. On overcast days, diurnal variability in PPFD increased with increasing GSP while diurnal variability in %PPFD showed an arc-shape relationship. The coefficient of variation of PPFD and %PPFD decreased with increasing GSP on clear days and sunflecks decreased with decreasing GSP and from May to September. Day-to-day light variability expressed as the mean difference between consecutive daily PPFD increased with increasing GSP while the mean difference between consecutive daily %PPFD was higher for microsites receiving between 20 and 80% GSP. The coefficient of variation for the daily PPFD and %PPFD was higher for microsites receiving <20% GSP compared to other microsites. Seasonal light variability showed that microsites with <50% GSP received up to eight times more light in May than in July on both clear and overcast sky conditions because of canopy development. From July to September in clear sky conditions, decrease in light was variable for microsites receiving <40% GSP; probably because of the position of microsites in relation to the solar track. On overcast days, mean daily PPFD above canopy and in the understorey was 2 to 3 times higher in July than in September while mean daily %PPFD remained stable. The possible effects of the types of diurnal and day-to-day light variability on physiological and morphological responses of understorey plants are discussed.

Modelling the interactions between moisture and nutrients in the control of forest growth.

Abstract Three major approaches to stand-level forest growth-and-yield modelling are briefly described: an empirical predictive approach; a process-based explanatory approach; and a predictive approach which combines both the empirical and the process approaches (the ‘hybrid simulation’ approach). The degree to which models representing these three approaches include an explicit representation of moisture and nutrients is reviewed, and the relationship between modelling objective, time-scale, and the inclusion of representation of moisture and nutrients is explored. There is a brief consideration of the nutrient and moisture-related processes that might be represented in a process-based, rotation-length, explanatory model, and the major processes and parameters that are affected by changes in these factors. The paper concludes with a suggestion as to how the inherent complexity of a process-based approach to simulating moisture might be simplified for use in a predictive hybrid simulation model. The way in which moisture and nutrients are represented in a forest stand or ecosystem model should be determined by the objectives of the model. There is no single ‘ideal’ approach, but for both explanatory and predictive models used over longer time-scales (one or more rotations), both moisture and nutrients should be explicitly represented. Because of the different time-scales of variation in moisture and nutrient parameters, and because management has a greater potential to affect nutrient parameters than moisture parameters over rotation-length time-scales, moisture can be treated in a more highly aggregated manner than nutrients in long-time-scale models. In contrast, nutrients can probably be omitted from shorter-time-scale models without significantly impairing their performance unless they are to be used to simulate events that effect nutrient availability significantly (e.g., fertilization). The relative importance of representing moisture and nutrients will also depend on the type of site and on the adaptations of the species being simulated. Simulation of moisture should probably take procedence on dry sites and in climates which have significant growing-season moisture-deficits, whereas a detailed simulation of nutrients may be essential for accurate growth-and-yield prediction on fresh to moist sites and in humid climates. Any simulation of the effects of long-term climatic change on forest productivity should include the interactive effects of both moisture and nutrients.

Early vegetation control for the regeneration of a single-cohort, intimate mixture of white spruce and trembling aspen on upland boreal sites

In Canada’s boreal forest region, there is increasing demand for practical regeneration strategies that will recreate mixed stands of white spruce (Picea glauca (Moench) Voss) and trembling aspen (...

Competitive effects of woody and herbaceous vegetation in a young boreal mixedwood stand

The influence of aspen (Populus tremuloides Michx.) and herbaceous (forb and grass) vegetation on resource availability and white spruce (Picea glauca (Moench) Voss) growth were examined as part of a long-term experiment established in 2002 near Whitecourt, Alberta, Canada. During the 2005 growing season, we examined the effects of herbicide treatments designed to control only woody (triclopyr ester) or both woody and herbaceous (glyphosate) vegetation on leaf area index (LAI) of both the woody and herbaceous components and relationships among LAI and light, soil moisture, air temperature, soil temperature, nitrogen availability, and spruce growth. Treatments reduced LAI and increased light, soil nitrogen availability, and white spruce growth. There were no apparent effects of the treatments on soil moisture in 2005. Both the woody and herb–grass layers appear to be competing for light and soil nitrogen in this young plantation. Controlling only woody vegetation resulted in an increase in herbaceous and t...

Simulation of Mixedwood Management of Aspen and White Spruce in Northeastern British Columbia

FORECAST, an ecosystem simulation model, was calibrated for aspen (Populus tremuloides Michx) and white spruce (Picea glauca (Moench) Voss) stands using data collected in the Boreal White and Black Spruce biogeoclimatic zone in northeastern British Columbia and published data. Simulations were undertaken to examine the effects of initial density of aspen on yield of white spruce in an aspen and spruce mixedwood stand, and to compare the predicted stemwood biomass yields of aspen, white spruce and mixedwood stands. Results of the simulations suggest that mixedwood management regimes on the same medium quality site should have higher stemwood yield compared to pure white spruce stand. Simulated stemwood biomass yield of pure aspen stands over 240 years on medium site varied from 682.5 Mg ha−1 to 239.1 Mg ha−1 for different rotation lengths (30 to 120 years). Repeated rotations of monoculture white spruce produced much less stemwood biomass, simulated yields over 240 years ranging from 877.3 Mg ha−1to 248.4 Mg ha−1 for rotation lengths of 60 to 240 years. Simulated aspen and white spruce mixedwood stands produced higher stemwood biomass yields than the pure white spruce stands, but less than the pure aspen stands; from 217.4 Mg ha−1 to 292.8 Mg ha−1 over 240 years. Variations in initial densities of aspen did not affect spruce stemwood biomass yield over the simulation period. This model shows potential for comparing the relative effects of different management strategies on harvestable volume and variety of other ecosystem variables. A calibrated version of the model should be useful as both a management simulator and a research tool. However, shortcomings in the representation of the canopy architecture of mixed species stands suggested the need to develop an individual tree version of this ecosystem management model for application to mixed species stands.

Modelling growth-competition relationships in trembling aspen and white spruce mixed boreal forests of Western Canada.

We examined the effect of competition on stem growth of Picea glauca and Populus tremuloides in boreal mixedwood stands during the stem exclusion stage. We combined traditional approaches of collecting competition data with dendrochronology to provide retrospective measurements of stem diameter growth. Several competition indices including stand basal area (BA), the sum of stem diameter at breast height (SDBH), and density (N) for the broadleaf and coniferous species, as well as similar indices considering only trees with diameters greater than each subject (BAGR, SDBHGR, and NGR), were evaluated. We used a nonlinear mixed model to characterize the basal area increment over the past 5, 10, 15, 20, 25, 30, and 35 years as a function of growth of nearby dominant trees, the size of the subject trees, deciduous and coniferous competition indices, and ecoregions. SDBHGR and BAGR were better predictors for spruce, and SDBHGR and NGR were better for aspen, respectively, than other indices. Results showed strongest correlations with long-term stem growth, as the best models integrated growth for 10–25 years for aspen and ≥25 for spruce. Our model demonstrated a remarkable capability (adjusted R2>0.67) to represent this complex variation in growth as a function of site, size and competition.

Nitrogen-15 uptake by Pinus contorta seedlings in relation to phenological stage and season

This study measured the amount of uptake of labeled nitrogen (15N) of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm) seedlings, at three different phenological stages, in a growth chamber experiment. Thirty days after 15N application, the amount of 15N recovered in seedlings as a percentage of the total 15N fertilizer applied was 4% in early spring, 43% in summer and 33% in autumn. The total 15N recovered in the plant–pot system ranged from 80 to 96%, and is higher than reported in other studies. Total 15N recovered from the pot compartment alone ranged from 48 to 95%, suggesting that substantial pools of N remain in the soil. Results suggest that low 15N uptake in the spring was associated with limited development of new root as a result of low spring soil temperatures. The lack of unsuberized roots in spring could be a key factor decreasing the effectiveness of early spring fertilization in the boreal forest.

Influence of Populus tremuloides density on air and soil temperature

Abstract Density manipulation of overstory aspen provides increased light levels to promote growth of understory spruce, while it may also impact the air and soil temperature. We monitored temperature for three growing seasons in young mixedwood stands of variable density at two boreal locations in Alberta, Canada. Results indicate that partial aspen cover provides frost protection that may be location specific. Mean air temperature was similar or increased slightly across aspen densities during the growing season, while mean soil temperature significantly increased with decreasing overstory cover. The amount of accumulated air and soil heat over the growing season was inversely related to aspen density. The differential impact of density on temperature contributes to variation in growth relationships and may explain regional differences in competition effects.