Kenneth J. Stadt

MIXLIGHT: a flexible light transmission model for mixed-species forest stands

We describe the calibration and structure of a multi-species, two-scale light transmission model, and demonstrate its effectiveness for predicting instantaneous light availability at the stand scale across a wide range of forest stand compositions. The model, MIXLIGHT, calculates light transmission through the forest overstory at the stand or microsite scale using standard forest inventory data and two other parameters, foliage area density and foliage inclination. Since MIXLIGHT simulations on both scales are based on a list of individual tree characteristics, it allows for simple manipulation of stand structure to study the effects of silvicultural options on light availability. The two scales allow the input of various kinds of data, allowing predictions from data of different sources and completeness. A simple and rapid method of calibrating the foliage parameters for the species of interest is presented, using measurements of direct-beam light transmission measurements made in the shadows of newly isolated trees. In an independent validation, MIXLIGHT predicted light transmission at the stand level closely for 17 forest inventory plots with a wide range of density and species composition, during leaf-on and leaf-off seasons and under sunny and cloudy conditions. A sensitivity analysis indicated that the influence of the parameters in the model on stand level light transmission predictions was (highest to lowest): foliage area density, crown radius, crown length, and foliage inclination. Nonetheless, a test of the common assumption that the foliage is spherically inclined caused significant underestimation of light transmission. With this flexibility and demonstrated accuracy, we believe MIXLIGHT will provide an accessible and effective tool for forest stand management and regeneration modeling.

Evaluation of competition and light estimation indices for predicting diameter growth in mature boreal mixed forests

A series of conventional distance-independent and distance-dependent competition indices, a highly flexible distance-dependent crowding index, and two light resource estimation indices were compared to predict individual tree diameter growth of five species of mature trees from natural-origin boreal mixed forests. The crowding index was the superior index for most species and ecosites. However, distance-independent indices, such as basal area of competing trees, were also effective. Distance-dependent light estimation indices, which estimate the fraction of seasonal photosynthetically-active radiation available to each tree, ranked intermediate to low. Determining separate competition indices for each competitor species accounted for more variation than ignoring species or classifying by ecological groups. Species’ competitive ability ranked (most competitive to least): paper birch ≈ white spruce ≈> trembling aspen > lodgepole pine > balsam poplar. Stratification by ecosite further improved model performance. However, the overall impact of competition on mature trees in these forests appears to be small.RésuméCe travail a évalué la capacité d’indices de compétition à prédire la croissance radiale individuelle d’arbres adultes de cinq espèces de forêts mixtes boréales. Ont ainsi été comparés : (1) une série d’indices conventionnels de compétition indépendants ou dépendants de la distance, (2) un indice très flexible d’encombrement dépendant de la distance et (3) deux indices d’estimation de l’éclairement. L’indice d’encombrement a été le plus efficace dans la plupart des stations et des espèces. Cependant, les indices indépendants de la distance tels que la surface terrière des arbres en compétition, ont été également efficaces. Les indices dépendants de la distance, d’estimation de l’éclairement, qui estiment la fraction saisonnière du rayonnement photosynthétiquement actif disponible pour chaque arbre, se sont classés en position intermédiaire. L’identification d’indices de compétition spécifiques de chaque espèce compétitrice a mieux rendu compte de la diversité des stations qu’un indice non spécifique ou qu’un classement des espèces par groupes écologiques. L’aptitude à la compétition des espèces a été classée de la manière suivante (de la plus à la moins compétitive) : Betula papyrifera, Picea glauca, Populus tremuloides, Pinus contorta, Populus balsamifera. La stratification par station améliore encore la performance du modèle. Cependant, l’impact général de la compétition sur les arbres adultes dans ces forêts semble être faible.

Modelling the interactive effects of aluminum, cadmium, manganese, nickel and zinc stress using the Weibull frequency distribution

A modified version of the Weibull frequency distribution was used to determine if phytotoxic concentrations of aluminum (Al), cadmium (Cd), manganese (Mn), and zinc (Zn) affected the response of Triticum aestivum to nickel (Ni) stress. Results indicated that Al had no effect on the response of plants to Ni, the nature of the interaction was adequately described by a multiplicative model (relative yield under conditions of multiple metal stress was equal to the product of the relative yields produced by the metals in isolation). In contrast, data from the Ni × Cd, Ni × Mn, and Ni × Zn experiments indicated potential antagonistic effects (relative yields under conditions of multiple metal stress were greater than predicted by the multiplicative model). The nature of these antagonistic effects was consistent with potential effects on metal ion speciation, competitive interactions affecting membrane transport, or competitive interactions at sites of toxic lesions. The degree of antagonism appeared to increase in the order Mn < Cd < Zn, which is consistent witt the similarity of these metals to Ni in terms of their affinity for donor atoms in organic ligands. The Weibull function appears to provide a more realistic interpretation of the nature of metal-metal interactions than other commonly used techniques, including ANOVA of primary growth data. Use of the Weibull function overcomes conceptual problems inherent with the use of the additive model to define interactions, as well as difficulties associated with data transformation. These experiments clearly demonstrate the utility of the Weibull function for identifying interactions between phytotoxic metals. Furthermore, the technique can be applied to a wide variety of toxic substances that produce a sigmoidal dose response.

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.

Control of relative growth rate by application of the relative addition rate technique to a traditional solution culture system

The relative addition rate (RAR) technique allows the nutritional control of plant relative growth rate (RGR) by the provision of nutrients at exponential supply rates. The technique, however, was developed with technologically sophisticated aeroponic systems. In this paper, we report on experiments used to adapt the RAR technique to a conventional solution culture system. A background concentration requirement of 36 μM nitrogen (N), with other nutrients supplied in proportion to N, was necessary to produce a constant RGR of Triticum aestivum L. (wheat) at a low RAR. Solution pH changes were reduced by increasing the percentage of NH4 in the nitrogen supply, but the plants exhibited dry weight reductions and symptoms of toxicity above 30% NH4. For wheat, a ratio of 25/75 NH4/NO3 was optimum for minimizing pH changes within the nontoxic range. A test of the effectiveness of the RAR technique using this background concentration and NH4/NO3 ratio showed that RGR increased with RAR with a linear slope of 0.55 and an intercept of 0.07 d-1. Although the relationship between growth rate and nutrient supply was less than the one-to-one dependence of RGR on RAR that has been obtained with more sophisticated apparatus, application of the RAR technique to a conventional solution culture system still affords considerable control of RGR and presents a simple method for growing plants at different levels of nutrient stress and at distinct RGRs.

Comparing PAR transmission models for forest understorey vegetation

Abstract Question: Do Beers Law models, multi-layer scattering models, and a semi-empirical model for predicting PAR transmission through understorey vegetation give comparable results? Do different driving variables (LAI, PLAI and percentage cover) give different results? How do the models vary when fit with species-specific, species-average and the ‘default’ parameters recommended in the literature? Location: Upland boreal forests of western North America. Methods: In calibration and validation plots, PAR transmission was measured, total cover visually estimated, and leaf dispersion, PLAI and cover estimated for each species using a point-frame. Leaf inclination was measured by clinometer. PAR transmission was modelled using empirically-fit Beers Law models, a semi-empirical model based on hemispherical gap fraction and first-order scattering, and a multi-layer model allowing multiple scattering. All models were modified to use leaf area index (LAI), vertically projected leaf area index (PLAI), or percentage cover data. Results: The empirical Beers Law models had the least bias and best precision in predicting PAR transmission. The semi-empirical model also had little bias and good precision, since the scattering coefficient compensated for problems in the estimation of gap fraction. The multi-layer model consistently underestimated transmission. There was little benefit in accounting for species separately. LAI and PLAI-based models were the most precise, but percentage cover models also provided reasonable predictions of PAR transmission. Conclusions: PAR transmission through forest understories can be simply modelled with Beers Law using one empirical coefficient representing the average understorey species. More complex scattering models are less effective, likely because they fail to account for the complexity of the dispersion of this vegetation layer and its effect on radiation scattering. Nomenclature: Abbreviations: PAR = Photosynthetically active radiation (400–700 nm); LAI = Leaf area index; PLAI = Vertically projected leaf area index.

Effects of competition, drought stress and photosynthetic productivity on the radial growth of white spruce in western Canada

Understanding the complex interactions of competition, climate warming-induced drought stress, and photosynthetic productivity on the radial growth of trees is central to linking climate change impacts on tree growth, stand structure and in general, forest productivity. Using a mixed modeling approach, a stand-level photosynthetic production model, climate, stand competition and tree-ring data from mixedwood stands in western Canada, we investigated the radial growth response of white spruce [Picea glauca (Moench.) Voss] to simulated annual photosynthetic production, simulated drought stress, and tree and stand level competition. The long-term (~80-year) radial growth of white spruce was constrained mostly by competition, as measured by total basal area, with minor effects from drought. There was no relation of competition and drought on tree growth but dominant trees increased their growth more strongly to increases in modeled photosynthetic productivity, indicating asymmetric competition. Our results indicate a co-limitation of drought and climatic factors inhibiting photosynthetic productivity for radial growth of white spruce in western Canada. These results illustrate how a modeling approach can separate the complex factors regulating both multi-decadal average radial growth and interannual radial growth variations of white spruce, and contribute to advance our understanding on sustainable management of mixedwood boreal forests in western Canada.

Measuring the effect of an abiotic stress on competition

Using recently developed solution culture techniques, the effect of a non-resource abiotic stress, nickel toxicity, was tested on intraspecific nutrient competition among wheat. The choice of an appropriate statistical model was of paramount importance in interpreting these effects. We argue that a multiplicative model is more appropriate for experiments on interactions of competition and abiotic stress. By such an analysis, nickel had no relative effect on the ability of competition to reduce plant size in two experiments, and caused a small reduction in competition in another. These results are contrary to other reports of the effect of a non-resource abiotic stress on competition and appear to be due to an increased demand for nutrients in the presence of toxic levels of nickel. The effects of an abiotic stress on competition may thus be specitic to the stress and not generalized across all abiotic stresses.

Interspecific variation in growth responses to tree size, competition and climate of western Canadian boreal mixed forests

Tree growth of boreal forest plays an important role on global carbon (C) cycle, while tree growth in the western Canadian boreal mixed forests has been predicted to be negatively affected by regional drought. Individual tree growth can be controlled by many factors, such as competition, climate, tree size and age. However, information about contributions of different factors to tree growth is still limited in this region. In order to address this uncertainty, tree rings of two dominant tree species, trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench.) Voss), were sampled from boreal mixed forest stands distributed across Alberta, Canada. Tree growth rates over different time intervals (10years interval, 1998-2007; 20years interval, 1988-2007; 30years interval, 1978-2007) were calculated to study the effects of different factors (tree size, competition, climate, and age) on tree growth. Results indicated that tree growth of two species were both primarily affected by competition or tree size, while climatic indices showed less effects on tree growth. Growth of trembling aspen was significantly affected by inter- and intraspecific competition, while growth of white spruce was primarily influenced by tree size, followed by competition. Positive relationship was found between growth of white spruce and competition index of coniferous group, suggesting an intraspecific mutualism mechanism within coniferous group. Our results further suggested that competition driven succession was the primary process of forest composition shift in the western Canadian boreal mixed forest. Although drought stress increased tree mortality, decline of stem density under climate change released competition stress of surviving trees, which in turn sustained growth of surviving trees. Therefore, climatic indices showed fewer effects on growth of dominant tree species compared to other factors in our study.