Frequency and extent of water limitation to primary production in a mesic temperate grassland

The frequency and extent of water limitation to aboveground net primary pro duction (ANPP) in a mesic grassland in NE Kansas (Konza Prairie, USA) was a ssessed with an 8-year irrigation experiment. Since 1991, transects spannin g upland and lowland sites in annually burned, ungrazed tallgrass prairie w ere provided with supplemental water to satisfy evapotranspirational demand s. This protocol minimized water limitations during the growing season, as well as interannual variability in water stress. Irrigation of this mesic g rassland increased ANPP in 6 of 8 years by an average of 26% when compared to control transects. Although interannual variation in ANPP was greater in uplands than lowlands at nominal levels of precipitation, reducing interan nual variability in water availability via irrigation eliminated topographi c differences; the irrigation protocol also reduced interannual variability in ANPP by as much as 40%. The addition of supplemental water enabled us t o extend the relationship between annual precipitation and ANPP in grasslan ds to precipitation levels (average, 1153 mm; maximum, 1346 mm) similar to those experienced by more mesic grasslands that today exist only as remnant s several hundred kilometers east of Kansas. This relationship was linear ( r(2) = 0.81), with maximum ANPP (738 g/m(2)) similar to values reported for sites in Illinois and Wisconsin. After 8 years of irrigation, production o f the C-3 forb component was twice that in control sites. These results ind icate that water limitations in grasslands at the western edge of the prese ttlement extent of tallgrass prairie affect ANPP in most years and that thi s high frequency of water limitation may lead to greater dominance of the C -4 grasses than is seen in more eastern grassland sites.