Modelling approach and capability

Major limitations, assumptions or uncertainties

Macroclimate (weather) processes

Light

Known relationships for Sun/Earth geometry give rise to continuous empirical model of incident light intensity which acts as a driving variable for all other models.

If rainfall patterns are modified by climate change how will this effect atmospheric transmissivity, and the ratio of direct to diffuse light? The continuous nature of the model makes it difficult to explore transient phenomena. What are the consequences of short term (minutes) changes in incident solar radiation?

Temperature

Empirical model based upon continuous sine wave functions incorporating diurnal and annual means and ranges.

Difficult to explore transient temperature effects with this model. Are such effects important?

Humidity

Simple model with atmospheric humidity set to a constant or according to rainy/dry day status.

Stomatal conductance, transpiration, energy budget and photosynthesis are intimately related to humidity. Is such a simple approach satisfactory?

Rainfall

Stochastic rainfall model on a daily basis parameterised from 5 year monthly mean values.

Rainfall distribution may vary both temporally and spatially with climate change. Is a daily time step sufficiently small to adequately simulate water related plant/soil processes?

Wind speed

Assumed atmospheric constant.

Simple but unlikely to be an adequate representation, especially given the variable nature of windspeed naturally and the importance of wind speed in the calculation of transpiration and leaf temperature.

Leaf level processes

C₃ model is mechanistically based on limitation of photosynthesis by either rubisco activity, electron transport or the rate at which inorganic phosphate is recycled. C₄ model is semi-mechanistic and predicts photosynthetic rate as a function of temperature, light, C_i and O₂.

Acclimation/adaptation of vegetation to long term exposure to elevated CO₂ and temperature is uncertain. How do model parameters change for different species? What is the importance of limitations arising from inorganic P under natural conditions and are they likely to increase as CO₂ increases.

Rubisco activity

Activation state of rubisco is ignore - it is assumed to be fully activated

Activation state declines with short term exposure to elevated CO₂. Does it recover with time? If not how will this effect the relationship between rubisco activity and leaf nitrogen concentration?

Effects of leaf nitrogen concentration

Leaf photosynthetic properties are assumed to be linear properties of nitrogen concentration. Thus allocation of nitrogen to the leaf determines leaf photosynthetic response.

Is nitrogen allocation the primary means by which leaves alter their photosynthetic capacity in high CO₂. Are V_cmax, J_max and TPU always linearly related to L_N? Do all species utilise N with the same efficiency? How are C₄ plants different from C₃ in their use of nitrogen?

and water stress

Phenomenological model assumes that stomatal conductance is proportional to photosynthesis modified by CO₂, humidity and leaf water potential.

Is the Ball & Berry model applicable to a wide range of species? Is the model response to elevated CO₂ realistic? Do stomatal model parameters change with acclimation to elevated CO₂?

Photorespiration

Dealt with explicitly based upon Michaelis-Menten kinetics of competition between CO₂ and O₂.

Dark respiration is assumed to be incorporated in the growth/maintenance respiration model when this is used at the whole plant level.

Transpiration/energy budget

Physically based model of evapo/transpiration for each leaf class including boundary layer resistance term. Energy budget determines the leaf temperature.

The suitability of the Penman/Monteith model will depend on the performance of the stomatal model since this determines the conductance in the transpiration calculation.

Canopy level processes

Light interception

Calculates profiles of incident solar radiation in the canopy. Distinguished between direct and diffuse radiation and sunlit and shaded leaf classes. Stem interception of radiation is not directly considered.

Non random leaf distribution can be accounted for using advanced empirical models which include a cluster parameter or the use of detailed ray tracing models. Is it necessary to have more detailed structure in the canopy model?

Important distinction between sunlit and shaded leaves both in the single and the multiple layer canopy models. Each leaf class receives a different light, temperature and chemical microclimate.

When parameterising leaf nitrogen concentration and other properties such as _Vcmax, J_max and TPU of what importance are the transient changes in light intensity upon its leaf in which in the real canopy a leaf may change leaf class many times within a day?

Assumed to be the sum of photosynthesis for all leaf classes.

How will acclimation affect photosynthesis? What age affects will be apparent at the canopy level and should these be related to elapsed leaf thermal time rather than atmospheric thermal time?

(wind speed)

Calculates profile of wind speed in the canopy using either very simple exponential gradient or more complex empirical profiles based upon experimental observations.

Characteristics of turbulence within a canopy are complex and not well understood.

Leaf energy budget calculations determine temperatures for all leaf classes within the canopy.

Characteristics of heat and vapour transfer within canopies are not well understood and are complex to model.

Leaf transpiration rates determine water loss for all leaf classes within the canopy.

Same as above

Calculates vertical nitrogen distribution within the canopy using either a simple exponential distribution function or optimisation method

Little experimental validation for either approach. What affects will acclimation and climate change have on N allocation with in the canopy?

Plant level processes

Carbon partitioning table determining allocation on the basis of elapsed thermal time since germination/start of plant growth. Mechanistic model of partitioning is under development but is currently too complex for general use.

No generally accepted mechanisms are known. Partitioning to roots/shoots is more easily estimated that between the root types. Storage of C and N is poorly understood.

Leaf expansion determined by carbon allocation and specific leaf area. Leaf growth limited by availability of carbon when water stressed. Minimum temperature requirement for leaf expansion.

Specific leaf area is incorrectly assumed to be constant in the model. This should change dynamically with plant age but little experimental evidence is available to do this. A better method for predicting leaf area development should be found.

Growth dependent on the allocation of C and the specific root length. Vertical profile of root lengths set empirically..

Morphological changes and root exudation are uncertain. Allocation of C to fine or structural root pools is uncertain.

Seed production based upon C allocation to the seed pool.

More detailed model of seed production is require. Very little known about global change effects on seed production and fecundity.

Interactive effects of allocation, CO₂, light, temperature and nitrogen are represented by the inclusion of responses at leaf level physiology.

Semi-mechanistic model of stomatal closure due to water stress within wimovac is underdevelopment but is too complex for general use.

No responses to herbivory included at the present time.

Many effects of herbivory are unknown at either the plant or the ecosystem level.

Ageing and senescence model monitors elapsed thermal time since the construction of plant organs and removes them to the appropriate decomposition pool when there fixed thermal time to death is reached.

This may be an over simplification and with the exception of protein remobilization from senescent leaves no other ageing related processes are included. This is almost certainly an incorrect assumption.

Soil level processes

Detailed Century based model of carbon and nitrogen cycling in the soil. Empirical observations used in a semi-mechanistic manner with fast and slow turnover pools of C and N. P is not included in the current model.

Mechanistic understanding of all soil C,N,P processes does not yet exist and what mechanisms have been described are too complex to be generally useful.

Semi-mechanistic electrical analog of soil water content and potential. Linked to leaf water potential and to stomatal conductance/transpiration. Negative feedback effect of soil water potential on plant water loss.

Mechanistic understanding of effects of soil water potential on stomatal conductance and transpirational water loss remain elusive.

Physically based model of soil layer temperatures including conduction, convection and radiative heat transfers.

The potential importance of 3 dimension heat flows is ignored. The effects of plant root material on soil heat flow are not distinguishable from general soil terms.

Community level processes

Individual plants are not described per se. Growth competition within the model can be incorporated by competition for light as a function of LAI and self shading. Competition for nutrients and water on the basis of root length and distribution is possible.

The general pattern of plant morphological change in response to competition is not well known

Mortality rates as a function of competition are not considered in the model.

Little information for this exists

Not included

Limited information exists for most species especially in relation the effects of climate change

Ecosystem level processes

Responses of whatever the predominant species of the ecosystem are to environment change is possible. Complex interactions between multiple species are not currently possible.

Validation of long term responses is only possible for a very limited range of sites.

Not currently included

Difficult to conceive of a mechanistic model of this being developed in the near future. Very poor understanding of species diversity responses.

Not currently included

Some detailed models of succession do exist but these are highly empirical in nature and not currently suitable for inclusion into general vegetation models.

Not currently included

How will herbivory be affected by changes in plant species, forage quality and plant morphology.

Decomposition and nutrient cycling is assumed to be a function of the soil processes described by the Century model. These include both physical and microbial driven reactions and response to the C:N ratio and lignin content of plant litter.

Will microbial species composition change with enhanced CO₂, temperature and modified rainfall patterns? What affect would this have on decomposition rates>

Distribution of water in soils can be predicted from a combination of soil, plant and atmospheric factors which are fully couples in Wimovac. Runoff is determined empirically.

3 dimensional models which account for lateral movement of water should be used. Canopy water interception should be incorporated.

Responses at the leaf level photosynthetic property _Vcmax

Leaf level models of pollutant affects can be formulated and there affects investigated in conjunction with global climate change.