Root biomass and soil δ13C in C3 and C4 grasslands along a precipitation gradient

Abstract:

Many studies have investigated the distribution of C₃ and C₄ grasses along climatic gradients 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 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. Results showed that δ¹³C underpredicted C₄ dominance in wetter sites. Indeed, the relationship between % C₄ cover and soil δ¹³C became more negative with increasing mean annual precipitation (MAP) based on a linear mixed-effects model (F _1,34 = 12.25, P < 0.01). Soil δ¹³C in wetter sites indicated a larger C₃ contribution than estimated by aboveground cover, which was in part due to C₃ root biomass increasing (P < 0.05) whereas C₄ root biomass did not change along the precipitation gradient. C₃ and C₄ grasses appear to allocate disproportionately belowground, thus a different understanding of C₄ ecological dominance may emerge when considering both above and belowground components. Our results show that belowground allocation and interpretation of soil δ¹³C need to be more carefully considered in global vegetation and carbon models and paleoecological reconstructions of C₄ dominance.