Abstract
Abstract. The savanna complex is a highly diverse global biome that occurs within the seasonally dry tropical to sub-tropical equatorial latitudes and are structurally and functionally distinct from grasslands and forests. Savannas are open-canopy environments that encompass a broad demographic continuum, often characterised by a changing dominance between C3-tree and C4-grass vegetation, where frequent environmental disturbances such as fire modulates the balance between ephemeral and perennial life forms. Climate change is projected to result in significant changes to the savanna floristic structure, with increases to woody biomass expected through CO2 fertilisation in mesic savannas and increased tree mortality expected through increased rainfall interannual variability in xeric savannas. The complex interaction between vegetation and climate that occurs in savannas has traditionally challenged terrestrial biosphere models (TBMs), which aim to simulate the interaction between the atmosphere and the land surface to predict responses of vegetation to changing in environmental forcing. In this review, we examine whether TBMs are able to adequately represent savanna fluxes and what implications potential deficiencies may have for climate change projection scenarios that rely on these models. We start by highlighting the defining characteristic traits and behaviours of savannas, how these differ across continents and how this information is (or is not) represented in the structural framework of many TBMs. We highlight three dynamic processes that we believe directly affect the water use and productivity of the savanna system: phenology, root-water access and fire dynamics. Following this, we discuss how these processes are represented in many current-generation TBMs and whether they are suitable for simulating savanna fluxes.Finally, we give an overview of how eddy-covariance observations in combination with other data sources can be used in model benchmarking and intercomparison frameworks to diagnose the performance of TBMs in this environment and formulate road maps for future development. Our investigation reveals that many TBMs systematically misrepresent phenology, the effects of fire and root-water access (if they are considered at all) and that these should be critical areas for future development. Furthermore, such processes must not be static (i.e. prescribed behaviour) but be capable of responding to the changing environmental conditions in order to emulate the dynamic behaviour of savannas. Without such developments, however, TBMs will have limited predictive capability in making the critical projections needed to understand how savannas will respond to future global change.
Highlights
Savanna ecosystems are a diverse and complex biome covering approximately 15 to 20 % of the global terrestrial surface (Scholes and Hall, 1996) and are important in providing ecosystem services that maintain biodiversity and support the majority of the global livestock (Van Der Werf et al, 2008)
We examine whether terrestrial biosphere models (TBMs) are able to adequately represent savanna fluxes and what implications potential deficiencies may have for climate change projection scenarios that rely on these models
There is a large degree of uncertainty as to what impact climate change may have on the structure and function of savanna ecosystems given their complex interaction with climate
Summary
Savanna ecosystems are a diverse and complex biome covering approximately 15 to 20 % of the global terrestrial surface (Scholes and Hall, 1996) and are important in providing ecosystem services that maintain biodiversity and support the majority of the global livestock (Van Der Werf et al, 2008). The role of fire in modulating vegetation structure allows savannas to occur across a broad demographic continuum, where the density of woody biomass is coupled to the annual amount of rainfall (Hutley et al, 2011; Lehmann et al, 2011) These environmental traits and behaviours mark savannas as one of the most complex terrestrial biomes on the planet, and understanding the vegetation dynamics and underlying processes of this ecosystem type (especially in response to future global change) has proven a challenging task for the ecosystem modelling community (House et al, 2003; Scheiter et al, 2013; Scheiter and Higgins, 2007). Terrestrial biosphere models (TBMs) are defined here as bottom-up modelling approaches that simulate coupled dynamics of water, energy, carbon and, in some cases, nutrients in vegetation and soils These models have mostly underperformed when modelling fluxes from savanna ecosystems (Whitley et al, 2016). We conclude with a discussion on model evaluation and benchmarking for this ecosystem and show that eddy flux measurements in combination with observations from multiple data sources (PhenoCams, remote sensing products, inventory studies) are essential to capture the seasonality of fluxes from soil, grasses and tree components and to capture the high frequency of disturbance events that perturb the carbon cycle in savanna ecosystems
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