The objective of this study was to dynamically simulate the response of a
complex landscape, containing forests, savannas and grasslands to potential
climate change. It was thus essential to accurately simulate the
competition for light and water between trees and grasses. To accurately
represent water competition requires simulating the appropriate vertical
root distribution and soil water content. The importance of differential
rooting depths in structuring savannas has long been debated. In simulating
this complex landscape we examined alternative hypotheses of tree and grass
vertical root distribution and their impacts on savanna dynamics under
historical and changing climates. MC1, a new dynamic vegetation model, was
used to estimate the distribution of vegetation and associated carbon and
nutrient fluxes for Wind Cave National Park, SD. MC1 consists of three
linked modules simulating biogeography, biogeochemistry, and fire
disturbance. This new tool allows us to document how changes in rooting
patterns may affect production, fire frequency and trace gas emissions, and
if current vegetation types and lifeform mixtures can be sustained at the
same location or replaced by others. Since climate change may intensify
resource deficiencies, it will likely affect allocation of resources to
roots and their distribution through the soil profile. By manipulating the
rooting depth of two lifeforms -trees and grasses - that are competing for
water, and running MC1 for historical climate (1895-1994) and a
GCM-simulated future scenario (1995-2094), we document its impact on
ecosystem processes and vegetation distribution. Deeply rooting trees
causes higher tree productivity, lower grass productivity, and longer fire
return intervals. When trees are shallowly rooted, grass productivity
exceeds that of trees even if total grass biomass only represents a third to
a fourth that of trees. Deeply rooted grasses develop extensive root
systems that increase N uptake and the input of litter into soil organic
matter pools. Shallowly rooted grasses produce smaller soil carbon pools.
Under the climate change scenario, NPP and live biomass increase for
grasses and decrease for trees, and total soil organic matter decreases.
However, differences between alternative rooting patterns remain similar.
Deeply rooted grasses grow larger than shallowly rooted ones and deeply
rooted trees outcompete grasses for resources. Consistent changes in fire
frequency and intensity are simulated; more fires occur during the climate
change scenario, because temperatures are higher, which results in decreased
fuel moisture. Fire also increases in the deeply-rooted grass
configurations, because grass biomass, which serves as a fine fuel source,
is relatively high.
Figure 1: Simulated total tree (deep rooted) and grass (shallow rooted) biomass (g C m-2)
over our study area. 5,000 grid cells were averaged for each year
between 1895 until 1994. The date and the extent (number of grid
cells that sustained a fire) of simulated natural fires are also
presented.
