SCOPE Project on Tree-Grass Dynamics

Document 2, March 1999

Workshop 2 Summary Report:

Models, data and experiments, Santa Barbara, May 1998

The second SCOPE tree-grass workshop was held in May 1998, the first of two funded by NCEAS (National Centre for Ecological Analysis and Synthesis, USA), attended by an international group of 17 modellers and data holders representing a broad range of models and savanna regions. As a follow up to the initial SCOPE-funded meeting in Paris 1996, it identified in more detail the format of the data synthesis and availability of further data sets, the nature of the data analysis and modelling experiments, how these will be carried out, and what kind of product(s) would result. NCEAS has provided a web site ( for general, public access, information about the project and a password protected area onto which the group can place and access the common data sets. Once the group has finished their analyses, the data sets and results will also become public access. NCEAS normally maintains these sites for two years, after this the data will be stored on the Oak Ridge DAAC (Data Archive and Analysis Centre) web site.

The Data

This group has identified a large amount of data sites available across a variety of topo-edaphic and management regimes (including fire, grazing and tree harvesting experiments) and at various levels of detail. All of the sites have data on either tree biomass or grass biomass at least above ground, fewer sites have both tree and grass biomass recorded above and belowground. Some sites have data on the effects of different levels of trees on grasses, or grasses on trees due to different treatments or environmental gradients. Other sites have a range of basal areas for similar climate and soils. Some sites just have measurements at one point in time, but others have measurements for a number of years with climatic and seasonal differences. Several sites have patches of different vegetation structure due to climate, soils or disturbance, and where trees are highly clumped, these can be considered as unique patches compared to the surrounding vegetation. This range of data sets available can be viewed in a hierarchical structure. Not all sites will have data available at all levels, and not all statistical/modelling tests will wish to use data at all levels. Therefore it is proposed that the data sets be organised into distinct file types within this hierarchical structure.

Level 1 = basic structural data (at a point in time or over time): basic site information, climate, soil & biomass data.

Level 2 = Annual NPP, community/patch scale: includes the above plus NPP data for trees and grasses preferably over a range of soils/climate, over time, or under different treatments, data on tree clumping, under/outside canopy data.

Level 3 = Monthly NPP, detailed community or plant scale: data at a finer temporal scale, more detailed spatial data, plant allometry, chemistry, physiology, phenology and population dynamics, litterfall and turnover, roots, water balance, PAR, fire/grazing/removals details.

The Modelling

The modelling activity will be a major component of the project. Tree-grass systems are extremely complicated, with non-linear interactions, and often highly dynamic, yet many global models do not address the interactions and dynamics within these systems. The current generation of broad-scale terrestrial ecosystem models tend to be either grassland models or forest models and few are able to handle the mixed life-form case with any confidence. This weakens the predictions of global vegetation models, especially under global change, when mixed tree-grass systems could be an important transitional feature. Several models have been developed specifically for mixed tree-grass systems. They fall on a continuum of detail, from highly-validated empirical formulations, such as the GRASP model developed for Queensland, Australia (Scanlan & Burrows), to the mechanistic, spatially-explicit, individual tree modelling framework, MUSE-TreeGrass (Noble, Menaut, Gignoux). In between are the tree-grass nitrogen allocation rules in the savanna version of CENTURY (Parton), in which the Queensland relationships are applied to a mechanistic soil process model, and the mechanistic but semi-spatially explicit approach taken in the SAVANNA model (Cougenhour) on African savannas. These models will be used as tools for integrating and synthesising existing knowledge and as exploratory/investigative tools. This exercise will lead to the evolution and improvement of existing individual models, and may also improve DGVMs and other global models thus benefiting global change predictions and land and environmental management decisions.

Savanna Theory, Issues and Analysis Approaches

The issue, theories and questions to be examined can be addressed within the same hierarchical structure as the existing data:

Level1 Questions: Can tree biomass/NPP be predicted from climate/soils? What equilibrium theories apply? Where are the domains of different tree-grass mixtures, the position of end-point envelopes and disturbance thresholds for given environmental conditions and disturbance regimes?

Level 1 Analysis: These questions can be studied statically using statistical analyses, and dynamically using the models with more basic data requirements (GRASP, CENTURY). The statistical approach would be to fit surfaces for tree BA based for several variables using multivariate, least-square optimisations (analysis of (co-)varience). Once the statistical surfaces have been plotted, the outliers can be identified and the reasons for each outlier examined. Sensitivity to different variables can be tested. This approach can be carried out with very basic data as this optimises the number of sites available. It is also a test of the models if they can predict this surface and account for the outliers given enough information, and whether they can represent the dynamic effects (rate and outcome) of changes.

Level 2 Questions: Can grass NPP be predicted as a function of tree NPP for different climate/soils/treatments? How does the tree-grass mixture effect total system NPP? How do tree-grass interactions impact on NPP underneath and outside tree canopies? How does clumping effect the above?

Level 2 Analysis: This will be a modelling exercise that can incorporate all the models, several model experiments are initially envisaged: run the models at points along transects with varying climate and woody cover to get a good handle on the water budget and itís role in tree-grass interaction, and the effects of changes in soil texture along these gradients; examine the effect of trees (individuals) on soils and vegetation; the ecosystem-scale tree effect on herbaceous NPP; the landscape level effect of trees on pasture production; the effects of patches and clumping; and disturbance effects.

Level 3 Questions: What mechanisms are needed to explain the observed relationships? What factors control tree-grass growth? What are the effects of plant spatial distribution and demography? How are water and nutrients allocated between plant types? What is the spatial and temporal separation of resource allocation and production?

Level 3 Analysis: This will be a modelling test to look at what mechanisms are needed to explain the observed relationships from level 1 and 2 analyses. Only SAVANNA and MUSE have sufficient detail to explore these questions. It will look at the processes behind the way the tree grass balance is affected by resource partitioning (by depth, lateral spread and phenology) and disturbance. At least eight sites were identified within the group for which this should be possible incorporating a range of precipitation, clay content and nutrient availability, and under different treatments. The importance of different site specific factors in the models can be examined.