Introduction
Establishing a productive vegetation community is critical to ensuring the long term success of a cover system. A plant community will enhance cover system performance by increasing resistance to soil erosion, increasing organic matter content and structure, transpiring stored water, providing habitat for wildlife, and improving aesthetics, among many other benefits. While a productive vegetation community is an important contributor to cover system performance, the physical presence of a plant community will alter its environment, both above and below the cover system surface. The changes to the cover system as a result of vegetation establishment are non-linear and will have myriad effects on the transfer of mass and energy between the atmosphere, plants, and soil. Plants exist as a critical component in a plant-soil-atmosphere continuum, and consequently will drastically alter expected performance.
Numerical modelling investigations are often conducted in order to understand the effects that vegetation will have on cover system performance. However, many current modelling codes used in the cover system industry over-simplify the contribution of vegetation to cover system performance. A productive plant canopy serves many more functions than simply as a means of removing water stored in the cover system. Many modelling codes simplify the analyses by simulating soil water uptake by roots as a user-defined function, and ignoring the effect of the plant canopy on the water and surface energy balances altogether. Ignoring the physical mechanisms and physiological processes at work in plant transpiration over-simplifies the analysis, and the cover system designer risks misinterpreting modelling results that could have a critical effect on the overall performance of the system.
Objectives
Numerical modelling approaches that ignore the effect of a plant canopy and root dynamics do not fully capture the processes and mechanisms contributing to the performance of a cover system. Numerical model codes that do not account for plant physical and physiological processes and mechanisms are no longer appropriate for understanding how a cover system will behave. The objective of this report is to determine the preferred approach for numerical simulation of vegetation in cover system design. This objective was met by summarizing physical, physiological, and ecological properties of vegetation that are relevant to cover system design. A summary of current modelling codes and their deficiencies is then compiled, followed by examples of how models that accurately capture plant processes are formulated. Finally, a set of recommendations and conclusions is provided.
Summary of Plant Properties
Plant communities will evolve over time as the cover system develops. Cover system modelling efforts that incorporate vegetation performance parameters tend to remain constant throughout numerical simulations and thus are not representative of their dynamic nature in natural systems. Rather, the establishment of vegetation on soil covers undergo a series of community shifts until a mature community is reached many years after establishment has occurred. A recognition of plant community structure shifts should be incorporated into long-range modelling analyses.
Plants seek to maintain an internal water balance, and will extract water from the soil to replace water lost through transpiration. Root uptake of soil water is a function of the negative pressure gradient between roots and the soil. Plant roots continuously grow towards sources of water, and optimal use of available soil water requires continuous root growth. Root systems will grow in accordance with species specific morphology and in response to site specific conditions. Tree root patterns will depend on the species, and are generally categorized as heart (aspen), flat (spruce) and tap (pine). For example, over 80% of root biomass in the boreal forest root is concentrated in the upper 30 cm of soil, although fine roots will commonly penetrate to 2 m. Shrubs are characterized by shallower rooting systems with greater than 70% of root biomass being concentrated in the top 0.2 m of soil. Grass roots are concentrated near the surface, with more than 80% of root biomass concentrated in the top 0.3 m of soil.
Leaf area index (LAI) is an important variable in determining plant productivity, but is one that is frequently improperly used. Leaves and needles act as collectors of solar radiation for converting CO2 into carbohydrates. As the surface area of leaves increases, so too will plant productivity, until the point at which self-shading of leaves decreases marginal productivity. Typical modelling analyses take LAI to be the primary determinant of transpiration. While transpiration and LAI are correlated, LAI is a genetic factor that will determine productivity, and not simply water withdrawal from the soil as is implied in modelling analyses. Plants that have access to an unrestricted water supply will increase LAI up to the maximum for that species and leaf morphology. Using LAI as the main determinant of transpiration oversimplifies the parameter and ignores important physiological processes.
Transpiration as a physical process is governed by the total potential in the soil-plant-atmosphere continuum. Restrictions in the plant are analogous to electrical resistances, and the entire transpiration process can be conceptualized using Ohm’s law. Conceptualizing resistances in the roots, stems, and leaves will allow for an intuitive formulation when deriving mathematical equations describing transpiration. The primary resistance of interest when discussing transpiration is stomatal resistance, which is used by the plant to control transpiration rates. Transpiration rates will vary throughout the day and over a growing season in response to climatic, biotic, and abiotic controls.
Plant Root Models
Modelling the contribution of vegetation to cover system performance requires consideration of processes and mechanisms both above and below the soil surface. Modelling extraction of water by plant roots generally uses either a microscopic formulation that models the uptake of individual roots, or a macroscopic approach where a diffuse root mass is modelled for an individual soil layer. Microscopic approaches are often too detailed for a water balance model, while macroscopic models tend to oversimplify root geometry. The cover design modeller should be aware of the advantages and disadvantages of each approach and select the appropriate one based on the specific cover objectives.
Plant roots grow in order to exploit water and nutrient resources. Modelling root growth is desirable, especially in water balance models, where plant roots may grow deeper as the surface of the cover system desiccates. Root growth consists of a set of concurrent and sequential processes that include proliferation, extension, senescence, and death. Root growth in natural systems requires a partitioning of biomass from the areas of the plant that are actively photosynthesizing. Modelling of root growth would require estimates of photosynthetically active radiation, leaf area index, leaf-water content, stomatal opening, and atmospheric CO2, and biomass partitioning. The ROOTSIMU model combines root uptake and growth, and has been used to account for photosynthesis, respiration, transpiration, and soil hydraulic processes. Despite the availability of models, the estimation of carbon production in plants for biomass accumulation in roots has likely restricted the widespread adoption of root growth models for water balance monitoring.
Summary of Current Modelling Codes
Many numerical model codes exist for simulation of water balances and near-surface soil microclimates. The complexity and focus of each individual model varies depending on the intended application. Ecophysiological models, such as those developed for the forestry industry, focus primarily on the plant canopy. Models such as FOREST BGC and MAESTRO provide very detailed estimates of carbon and nitrogen budgets for vegetation. Soil water balances are computed as a model input for estimation of carbon balances.
Many current numerical codes that are used in the cover design industry treat vegetation as a simple sink term for soil water, and do not simulate turbulent transfer of mass and energy in the plant canopy. Common model codes such as UNSAT H and VADOSE/W oversimplify vegetation and in so doing, ignore the important effects that vegetation has within the soil-plant-atmosphere continuum. The HYDRUS model code incorporates a rigorous model of solute concentrations and uptake by plants as well as the effect of osmotic stress on plant water uptake responses. Nevertheless, plant water uptake is still modelled as a sink term in HYDRUS, and is not driven by atmospheric conditions.
The ideal approach to simulating vegetation in cover systems is to have the simulated plant canopy respond in a physically appropriate manner with respect to plant physiology. A plant canopy is also important in modifying the near surface atmosphere, and an appropriate model must account for changes in mass, energy, and momentum transfer caused by plants. The Fast All-Season Soil Strength (FASST) model is capable of estimating a rigorous coupled water and energy balance, and calculates the transfer of mass and energy at the air-canopy-snow-soil interface. The model specifically models radiation transfer through the plant canopy and into the underlying soil. Turbulent transfer induced by the plant canopy is explicitly captured by incorporating vegetation height and roughness length. Most importantly, transpiration is simulated as a component of the soil-plant-atmosphere continuum, and is governed by stomatal resistance in response to the vapor pressure deficit. Transpiration is linked to soil water content through a resistance term between soil and plant roots. The Simultaneous Heat and Water (SHAW) model is a detailed one-dimensional process model that simulates the transfer of heat and water through a plant-snow-residue-soil system to simulate a coupled water and energy balance. The SHAW model is unique in that transpiration is mechanistically linked to soil water by calculating a flow through roots and leaves within the soil-plant-atmosphere continuum, while satisfying a leaf energy balance. In contrast to VADOSE/W, SHAW calculates a rigorous fully coupled energy balance, and computes soil evaporation separately from transpiration. The SHAW and FASST models stand as excellent examples of how a numerical model should incorporate plants into an analysis, due to their physical treatment of transfer of mass and energy within the plant canopy. However, there is no single modelling code that accurately captures the detailed physics and physiology of the transfer of mass, energy, and momentum within the soil-plant-atmosphere continuum, and the cover system designer should account for this when selecting their model of choice. Nevertheless, modelling vegetation as a sink term is no longer appropriate, and it is recommended that future modelling analyses account for the physical and physiological drivers of transpiration.
