Long-term forage production of North American shortgrass steppe

Evaluated the relationship between annual forage production and annual and seasonal precipitation and temperature at a shortgrass steppe site in N-central Colorado using a long-term data set (52 yr), and constructed a relationship between forage production and aboveground net primary production (ANPP). Precipitation flucutated randomly, but temperature had clear warming and cooling trends including a warming trend from 1974-1990. Forage production was significantly related to both annual and seasonal precipitation but not temperature. Precipitation events between 15 and 30 mm accounted for most of the variability in production because they account for most of the variability in precipitation and because they wetted the soil layers that have the largest effect on production. Forage production amplified variability in annual precipitation. Production showed time lags of several years in responding to increases in precipitation. Change in vegetation structure has a characteristic response time, which constrains production responses in wet years. Constrain caused by vegetation structure is the reason why regional ANPP-precipitation models have a steeper slope than long-term models. -from Authors