Convergence and contingency in production-precipitation relationships in North American and South African C4 savanna grasslands.
Knapp, Alan*,1, Burns, Catherine2, Fynn, Richard3, Smith, Melinda 2, 1 Colorado State University, Ft. Collins, CO2 Yale University, New Haven, CT3 University of KwaZulu-Natal, Pietermaritzburg, South Africa
ABSTRACT- Mesic savanna grasslands of North America (NA) and South Africa (SA) are structurally convergent with C4 grasses dominating the herbaceous layer and coexisting with a C3 shrub and tree overstory. Growing season temperature and precipitation amounts (annual and growing season) for these two ecosystems can be quite similar, thus, strong convergence in the control of aboveground net primary production (ANPP) by water limitation is expected. However, high potential evapotranspiration during the warm, dry dormant season in subtropical SA typically results in soils drying severely in the winter, requiring soil water recharge prior to initiation of the growing season. This contrasts with soils in NA savanna grasslands where water is stored in the cold dormant season prior to green-up, and soil water is fully recharged at the beginning of the growing season. This difference in soil water status at the beginning of the growing season represents a potential contingent factor that may alter precipitation-ANPP relationships. To assess convergence vs. contingency in these savanna grassland ecosystems, we compared precipitation-ANPP relationships with two long-term data sets from the Konza Prairie (KNZ) LTER site in NE Kansas (USA) and the Ukulinga Research Farm (URF) near Pietermaritzburg, South Africa. At KNZ, 15 years of ANPP data from sites burned annually in the spring were contrasted with 24 years of data from similar plots at URF. Precipitation patterns and amounts, and growing season temperatures were virtually identical at these sites. Both sites had positive precipitation-ANPP relationships, but the relationship between interannual variability in precipitation and ANPP was stronger at URF (greater slope and r2 values), particularly for early season precipitation-ANPP relationships. However, ANPP and precipitation use efficiency (production/precipitation), were greater at KNZ. Finally, the coefficient of variation of ANPP in URF was higher than at KNZ despite less variable precipitation patterns in SA. These results indicate that differences in patterns of dormant season soil moisture strongly impact precipitation-ANPP relationships during the growing season, reducing convergence in ecosystem processes.
Key words: productivity, grassland, savanna, precipitation
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