agriculture is limited only by nature or, where nature is momentarily bowed, by sporadic natural disaster; and the pastoralists have been reduced to areas where nothing else works.

In 1991 the focus of attention along this frontier returned to its source, the environment. As indicated, the pastoral frontier has been defined by a difference in climate, principally rainfall, and its history by the different and changing human adaptations to this reality. Now, for the first time, the dominant issue has become the human impact on the environment and the degree to which that impact can be limited, directed, or controlled. The members of this panel are encouraged by the fact that our Chinese colleagues are addressing these problems and seeking solutions to them.

ATMOSPHERE-BIOSPHERE INTERACTIONS

Scientists have long recognized that climate influences the vegetation species composition of landscape, but only in recent years have they begun to explore the feedback of landscape to local and regional weather and climate. Our understanding of climate change, due to either natural or anthropogenic causes, can profit from further study of the response of landscape composition to perturbations in climate and the ways landscape affects changes in climate.

The distribution of photosynthetically active plants over the grasslands of northern China by using a Normalized Difference Vegetation Index (NDVI) is expected to show considerable spatial structure such as observed over the grasslands of the United States (Pielke et al., 1991). During winter, of course, this area would be dormant. Such imagery demonstrates, first, that there is a large seasonal response of the grasslands to the changing weather; and second, that there is considerable spatial structure in this landscape.

Pielke and Avissar (1990) and Pielke et al. (1990a,b) review observational and modeling evidence to show that atmosphere boundary layer structure and the generation of local wind circulations as strong as sea breezes can occur over grasslands in the United States, when located between irrigated land and adjacent prairie. Influences of regions of different species composition have also been observed elsewhere (e.g., Andre et al., 1990, for southwest France).

Similar responses should be expected for the grasslands of northern China. Such an evaluation is critical not only to understanding local and regional climate and weather, but also as input into global circulation models. Analysis of this type could help answer a number of interesting questions: Do intense grazing and wildfire cause changes in grassland climate due to major alterations in albedo and the portioning of latent and sensible heat fluxes, compared to what might be expected in the absence of such activities? When material- or man-caused global climate changes occur, what is the expected influence on the seasonal and spatial landscape of the northern China grasslands?



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