COS 73-3
Belowground ecosystem function of C3 and C4 grasslands in response to climate
The distributional patterns of C3 and C4 grasses in relation to climate have attracted much attention because they illustrate complex interactions between abiotic and biotic controls on ecosystem functions. Yet few studies have examined belowground components of these distributions, which may present very different patterns compared to aboveground measures. In this study, we surveyed aboveground grass species cover and collected soil and root samples from field plots at 100 - 150 m elevation intervals along a climatic gradient in Hawai‘i. We examined how the relationship between soil carbon isotopic composition (δ13C), a proxy for C4 productivity and dominance, and % C4 cover changed along a climatic gradient using linear regression models. Across sites, the relationship between % root biomass (C3 and C4), % cover (C3 and C4), and climate (MAP and MAT) were also examined using linear regressions.
Results/Conclusions
The relationship between % C4 cover and soil δ13C changed with mean annual precipitation (MAP) (F 1,34 = 12.25, P < 0.01). Soil δ13C became more negative as % C4 cover increased, which was in part due to C3 root biomass increasing (P < 0.05), whereas C4 root biomass did not change. There was a tradeoff in above and belowground dominance—when % C4 cover was high, C4 root biomass was low. C3 and C4 grasses appear to allocate disproportionately belowground, thus a different understanding of C4 ecological dominance emerges when considering both above and belowground components. Our results furthermore show that belowground allocation and interpretation of soil δ13C need to be more carefully considered in global vegetation models and paleoecological reconstructions of C4 dominance.