Remote sensing of vegetation-topography relationships in a mixed C3-C4 temperate grassland

Parolari, A.; Sivandran, G.; Bras, R. L.

American Geophysical Union, Fall Meeting 2009, abstract #B31B-0341

Species identity and abundance have important consequences for biogeochemical cycling and land-atmosphere fluxes. This is particularly true for mixed C3-C¬4 communities due to considerable differences in water and nitrogen use efficiency between the two photosynthetic pathways. In a mixed C3-C4 temperate grassland in northeastern Kansas, field observations suggest C3 and C4 species percent cover varies with disturbance regime and topography-modulated resource availability. Here, C4 grasses are the dominant vegetation, yet factors reducing their viability (i.e. soil moisture stress, low frequency fire, and grazing) lead to increased abundance of sub-dominant C3 grasses and forbs. To date, however, field campaigns have contrasted upland and lowland sites only. The use of remotely sensed observations would contribute to the understanding of vegetation-topography relationships in this setting, providing data for a broader range of topographic niches. Soil moisture is the factor that varies most strongly with topography. Its distribution is driven by lateral redistribution and differential solar radiation, such that north-facing hillslopes and lowlands tend to be wetter than south-facing hillslopes and uplands. Therefore, it is anticipated C4 grasses are least dominant in uplands and south-facing hillslopes. In this research, a combination of remotely sensed NDVI and ground observations of species cover is used to identify the relationship between topography and species percent cover at Konza Prairie Biological Station LTER, KS. A classification technique is applied to growing season phenology derived from Landsat TM imagery at 30m resolution. Remotely sensed estimates of relative C3 and C4 cover are regressed with topographic indices: slope, aspect, and contributing area. Inferences are then made regarding the spatial distribution and scaling properties of percent cover, transpiration, and carbon and nitrogen storage on the landscape.