Responses of belowground bud banks and their relationships with aboveground vegetation to experimental drought along an aridity gradient in temperate grasslands

1. Climate change is expected to increase the magnitude and frequency of extreme drought in most grassland ecosystems. Exploring the responses of the belowground bud banks and their relationships with aboveground plant parts to drought is of great significance in explaining impacts of climate change on grasslands. However, studies on the response patterns of community-scale bud and shoot densities and their ratios to experimental drought along an aridity gradient are rare. 2. We experimentally removed 66% of growing season precipitation for 4 years in three temperate grasslands that spanned an aridity gradient in northern China, and tracked the legacy effects of drought on community belowground bud density, community aboveground shoot density, and the ratio of bud to shoot density one year following treatment. 3. Belowground bud density was the lowest at the highest aridity site for the entire community, while aboveground shoot density was the highest at the medium aridity site. Belowground bud and aboveground shoot densities were the lowest at the high aridity site for grasses, but the highest for forbs at this site. Bud:shoot ratios decreased with increasing aridity for grasses, yet remained constant for forbs along the aridity gradient. Experimental drought reduced community belowground bud density a year following drought at each site. Experimental drought did not alter belowground bud bank for grasses, but decreased forb bud banks across sites. Experimental drought had little legacy effects on aboveground shoot density and bud:shoot ratios for the entire community or each functional type (grasses and forbs) at each site. 4. We found that grasses and forb bud banks differ in their responses to both multi-year drought and long-term climate patterns, and that community meristem limitation may increase as grasslands get drier. Bud bank responses to climate will likely alter future plant communities and functioning, and incorporating bud bank dynamics may improve projections of grassland ecosystems under future climate changes.

1. 气候变化预计将加剧多数草原生态系统极端干旱的强度与发生频率。探究地下芽库（belowground bud banks）及其与地上植株组分对干旱的响应关系，对于阐释气候变化对草原生态系统的影响具有重要科学意义。然而，目前针对沿干旱梯度开展的群落尺度芽密度、枝条密度及其比值对实验性干旱的响应模式的研究仍较为匮乏。2. 本研究在中国北方三条沿干旱梯度分布的温带草原样地中，开展了为期4年的生长季降水移除实验（移除生长季降水量的66%），并在实验结束一年后，追踪了干旱处理对群落地下芽密度、群落地上枝条密度以及芽-枝条密度比的遗留效应。3. 就整个群落而言，地下芽密度在干旱程度最高的样地最低，而地上枝条密度在中等干旱程度的样地最高。对于禾本科功能群而言，地下芽密度与地上枝条密度均在高干旱程度样地最低，但该样地的杂类草功能群的这两项指标却最高。沿干旱梯度，禾本科的芽-枝条密度比随干旱程度升高而降低，而杂类草的该比值则保持稳定。在各实验样地中，实验性干旱均会在干旱结束一年后降低群落地下芽密度。实验性干旱未对禾本科的地下芽库产生显著影响，但却降低了所有样地中杂类草的芽库规模。此外，实验性干旱对各样地的整个群落或各功能群（禾本科、杂类草）的地上枝条密度以及芽-枝条密度比几乎无遗留效应。4. 本研究发现，禾本科与杂类草的芽库对多年干旱与长期气候格局的响应模式存在显著差异，且随着草原趋于干旱化，群落分生组织限制可能会加剧。芽库对气候的响应或将改变未来植物群落的组成与生态系统功能，将芽库动态纳入考量将有助于提升未来气候变化情景下草原生态系统的预测精度。