Mesic grasslands in North America and South Africa share many structural attributes, but less is known of their functional similarities. We assessed the control of a key ecosystem process, aboveground net primary production (ANPP), by interannual variation in precipitation amount and pattern via analysis of data sets (15- and 24-year periods) from long-term research programs on each continent. Both sites were dominated by C4 grasses and had similar growing season climates; thus, we expected convergence in precipitation–ANPP relationships. Lack of convergence, however, would support an alternative hypothesis—that differences in evolutionary history and purportedly greater climatic variability in South Africa fundamentally alter the functioning of southern versus northern hemisphere grasslands. Neither mean annual precipitation nor mean ANPP differed between the South African and North American sites (838 vs. 857 mm/year, 423.5 vs. 461.4 g/m2 respectively) and growing season precipitation–ANPP relationships were similar. Despite overall convergence, there were differences between sites in how the seasonal timing of precipitation affected ANPP. In particular, interannual variability in precipitation that fell during the first half of thegrowing season strongly affected annual ANPP in South Africa (P & 0.01), but was not related to ANPP in North America (P = 0.098). Both sites were affected similarly by late season precipitation. Divergence in the seasonal courseof available soil moisture (chronically low in the winter and early spring in the South African site vs. high in the North American site) is proposed as a key contingent factor explaining differential sensitivity in ANPP to early season precipitation inthese two grasslands. These long-term data sets provided no support for greater rainfall, temperature or ANPP variability in the South African versus the North American site. However, greater sensitivity of ANPP to early season precipitation in the South African grassland suggests that future patterns of productivity may be more responsive to seasonal changes in climate compared with the North American site.
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机译：北美和南非的中型草原具有许多结构特征，但对其功能相似性知之甚少。我们通过对每个大陆长期研究计划的数据集（15年和24年期）进行分析，通过分析降水量和格局的年际变化，评估了关键生态系统过程（地上净初级生产力（ANPP））的控制。两个地点均以C4草为主，生长季节气候相似。因此，我们期望降水与ANPP之间的关系趋于一致。然而，缺乏融合将支持另一种假设-南非进化史的差异和据称更大的气候变异性从根本上改变了南半球和北半球草地的功能。南非和北美站点之间的年均降水量和平均ANPP均无差异（分别为838 vs. 857 mm /年，423.5 vs. 461.4 g / m2），并且生长季节降水与ANPP的关系相似。尽管总体趋同，但降水季节的时机如何影响ANPP之间在地点之间存在差异。特别是，生长季节上半年下降的年际降水变化强烈影响了南非的年度ANPP（P＆ 0.01），但与北美的ANPP无关（P＆nb = 0.098）。 。后期降雨对两个地点的影响相似。提出了土壤可利用水分的季节性变化的差异（南非站点冬季和春季早时较低，而北美站点则较高），这是解释这两个草原ANPP对早期降水敏感性差异的关键或有因素。 。这些长期数据集无法为南非和北美站点的更大降雨，温度或ANPP变异性提供支持。但是，ANPP对南非草原早期降水的敏感性更高，这表明与北美站点相比，未来生产力的模式可能对气候的季节性变化更敏感。
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