Accurate and unbiased radiative energy transfer models are critical to our understanding of ecosystem primary productivity, carbon cycling, and climate change. Much of the current research in this area is based on models parameterized for grasslands and broadleaf forests. However, many temperate montane and boreal forests are dominated by conifers, which present unique challenges to modellers. We propose two fundamentally different strategies by which plant canopies optimize solar radiation interception. Laminar canopies (e.g., grasslands, broadleaf trees) are .solar panels. that directly intercept incoming radiant energy. By contrast, conifer canopies are conical anechoic (.without echo.) surfaces that intercept radiant energy by scattering it through the canopy. The properties of anechoic surfaces are well known in acoustical and electrical engineering, but have not been applied in environmental biophysics. We discuss the physical principles of anechoic surfaces, and demonstrate how these principles apply to conifer trees and canopies. A key feature of anechoic interception is low radiance over all wavelengths, which is an emergent property of the system. Using empirical data from boreal forest stands in Riding Mountain National Park (Manitoba, Canada), we demonstrate that conifer canopies have very low near-infrared radiance compared to laminar broadleaf canopies. Vegetation index values for conifers are thereby reduced, resulting in underestimates of primary productivity and other biophysical parameters. We also discuss the adaptive significance of boreal conifer geometry, and consider factors driving selection of laminar versus anechoic canopy architectures.
We used multivariate analysis to model boreal forest stand structure and dynamics at Riding Mountain National Park, Manitoba based on data from 202 sampled stands. Eight forest stand-types were recognized based on canopy composition: black spruce on peat substrates, jack pine - black spruce, bur oak, eastern deciduous (green ash - American elm - Manitoba maple), balsam fir, trembling aspen - paper birch - mountain maple, trembling aspen - balsam poplar, and white spruce. The first four stand-types occur in edaphically distinct environments, while the four remaining boreal mixedwood stand-types occur in edaphically similar environments. We found that the composition and abundance of advance regeneration were best predicted by current canopy composition (redundancy = 54.4%); this reflects both the limited dispersal of conifer seeds and the strong vegetative reproductive capacity of hardwoods. Biotically-controlled site factors such as bareground, herb and shrub cover, ungulate browsing intensity, and stand age were also reasonably good predictors f advance regeneration (redundancy = 31.7%). Edaphic variables such as soil pH, conductivity, particle size, organic horizon depth and slope proved to be poor predictors of advance regeneration, however (redundancy = 18.1%). Size-class ordination indicated that many stand-types have relatively short successional trajectories, suggesting limited change in forest canopy composition over time. There are two exceptions: in the jack pine - black spruce stand-type, black spruce will increase over time, and in the trembling aspen - paper birch - mountain maple stand-type, eastern deciduous species (green ash, American elm, Manitoba maple, and bur oak) are forecast to become increasingly dominant. We also describe a synoptic model of mixedwood boreal forest stand dynamics for the Riding Mountain area. The model includes a number of factors that we consider to be critical determinants of forest dynamics, such as seed source availability, small and large-scale disturbances, species life-history characteristics, and environmental gradients. Our succession model is more similar those described for eastern than western Canada, which may reflect the lower frequency of catastrophic fires in the Riding Mountain area compared to boreal forests further west. Our model emphasizes that successional trajectories do not converge towards a single self-perpetuating "climax". Instead, successional vectors may diverge, converge or remain cyclical, and multiple potential pathways are possible for each stand-type. Our results also illustrate that species assemblages, and the propensity for canopy change in the absence of fire, are governed by the cumulative and synergistic effects of climate, topography, disturbance frequency, size and intensity, edaphic conditions, and the proximity of parental seed sources. Fire suppression in the southern boreal forest has resulted in a paradigm shift in disturbance regime, from large, synchronous catastrophic fires to small-scale, asynchronous gap formation. A major challenge for boreal forest ecologists is to determine the long-term consequences of this paradigm shift on the composition, structure and health of boreal forest stands and landscapes.
Landscape complexity in the boreal forest is a function of physiographic complexity (spatial processes) and post-fire successional (temporal) processes operating across scales. In this study we examine the role of succession and topographic complexity in determining the landscape complexity of Riding Mountain National Park, Manitoba, Canada. Landscape complexity is assessed by using multifractal analysis to quantify landscape patterns from Landsat TM imagery. To determine whether complexity changes with age, . young. sites (post-fire stand ages = 11 and 30 years) were matched with adjacent . old. sites (post-fire stand ages ≯ 95 years). The influence of physiography on landscape complexity is examined by comparing sites having . simple. and . complex. physiographies (as determined by fractal surface analysis). The scaling properties of landscape complexity are determined by calculating the multifractal spectrum (Dq) for each site. Landscape complexity increases during early succession; multifractal profiles of 11 year old sites are lower than those of adjacent older stands. However, the multifractal profiles of 30 year old and adjacent older stands are indistinguishable, suggesting that change in landscape complexity occurs within 30 years following fire. Physiographically . complex. sites have consistently greater landscape complexity than adjacent . simple. sites. We conclude that landscape complexity increases over time as succession proceeds, and in space along a gradient from . simple. to . complex. physiographies. It follows that landscape complexity is lowest in early-successional, physiographically . simple. sites and highest in late-successional, physiographically . complex. sites.
This study investigates the relationship between seed dispersal and spatial pattern in a population of wild oats (Avena fatua L.), a common annual weed of arable fields in western Canada. Fractal analysis revealed that wild oats has an aggregated spatial pattern with statistically self-similar properties. Wild oats seed dispersal was successfully modelled using the fractal inverse power law. The estimated fractal dimension of the seed dispersal curve (D = 1.912) is strikingly similar to that of the population's spatial pattern (D =1.881), indicating that the observed spatial pattern is characteristic of the dispersal distribution. This result suggests that the dispersal curve of a given weed species may be used to successfully predict its pattern of invasion.
The relationship between species diversity and ecosystem functions has generated considerable debate among ecologists. Ecosystem functions (e.g. productivity, nutrient retention) are often positively correlated with species richness in experimental plant assemblages, but little or no correlation exists in natural communities.We examined the effects of species richness on productivity and available soil nitrate by experimentally manipulating richness using random draws from a pool of ten perennial grasses. Species richness had no significant effect on aboveground productivity or soil nitrate availability, suggesting that functional diversity may be more important than species richness in determining ecosystem functions. The relationship between diversity and ecosystem functions may also depend on resource limitation. A positive relationship is expected when below-ground resources are limiting, but the relationship is expected to weaken when below-ground resource supply rates are higher and competition for light becomes more important. Further experiments are required to determine the mechanisms underlying diversity-productivity relationships.