Long term grassland productivity responses to changes in the amount and timing of rainfall are mediated via shifts in community stability

Rainfall is a major determinant of aboveground net primary productivity (ANPP) in grasslands, but warming-driven intensification of the hydrological cycle is changing the amount, timing and seasonality of rainfall, including more extreme events. The relative importance of these changes for biodiversity over decadal scales, and the plant community mechanisms underlying associated productivity responses, are currently unresolved. To address these knowledge gaps, we undertook a 10-year rainfall manipulation experiment in a mesic grassland in New South Wales, Australia. We used rain shelters to achieve five contrasting rainfall treatments: (i) ambient, (ii) ambient +50%, (iii) ambient -50%, (iv) reduced frequency (cumulative ambient rainfall applied once every 3 weeks) and (v) summer drought (no rain during the Austral summer).Inter-annual differences in ANPP were best explained by the amount of rainfall relative to atmospheric moisture demand (i.e. precipitation/potential evapotranspiration (P/PET), or aridity), with growing season aridity being a stronger predictor of ANPP (R²_adj 0.52) than either annual rainfall (R²_adj 0.26) or annual P/PET (R²_adj 0.44). Changes in the size, frequency and seasonality of rainfall events were associated with shifts in community composition, loss of diversity and a reduction in the proportion of native species. Reduced stability of ANPP in response to summer drought was mediated via reduced species asynchrony – reflecting a large decline in species richness - and a decline in dominant species stability. At the other end of the scale, greater ANPP and overall stability of productivity under increased rainfall was mediated via increased asynchrony and stability of dominant grasses, despite a reduction in species diversity. Our study uniquely emphasises the importance of accounting for seasonal drivers of moisture demand when predicting productivity responses to changes in rainfall regimes and highlights the critical importance of species asynchrony and dominant species’ stability for maintaining productivity in the face of climate change.

降水是草地地上净初级生产力（aboveground net primary productivity, ANPP）的核心调控因子，但气候变暖驱动的水文循环强化正在改变降水的总量、发生时序与季节节律，极端降水事件亦愈发频发。目前，学界尚未明确这些降水变化在十年尺度上对生物多样性的相对重要性，以及驱动伴随生产力响应的植物群落调控机制。为填补这一研究空白，我们在澳大利亚新南威尔士州的一处中生草原开展了为期10年的降水调控实验。本实验采用防雨棚设置了5种差异化降水处理：(i) 自然降水对照，(ii) 自然降水增加50%，(iii) 自然降水减少50%，(iv) 降水频率降低（每3周施加一次累积量等同于自然降水的水量），以及(v) 夏季干旱处理（南半球夏季无降水）。年际间ANPP的差异最能通过降水相对于大气水分需求的比值（即降水量/潜在蒸散量（precipitation/potential evapotranspiration, P/PET），又称干旱度）解释，其中生长季干旱度对ANPP的预测能力（校正决定系数R²_adj=0.52）优于年降水量（校正决定系数R²_adj=0.26）与年际P/PET（校正决定系数R²_adj=0.44）。降水事件的量级、频率与季节节律变化与植物群落组成的转变、多样性丧失以及本土物种占比下降显著相关。夏季干旱导致的ANPP稳定性下降，由物种异步性降低（反映物种丰富度大幅下降）与优势物种稳定性降低共同介导。而在降水增加的处理组中，尽管物种多样性有所下降，但ANPP提升与生产力整体稳定性的增强，通过优势禾草的异步性与稳定性提升得以实现。本研究首次强调，在预测降水格局变化下的生产力响应时，需纳入水分需求的季节驱动因子；同时也凸显了物种异步性与优势物种稳定性在气候变化背景下维持草地生产力的关键作用。