Data from: Phenology differences between native and novel exotic-dominated grasslands rival the effects of climate change

1. Novel ecosystems can differ from the native systems they replaced. We used phenology measures to compare ecosystem functioning between novel exotic-dominated and native-dominated grasslands in the central US.

2. Phenology, or timing of biological events, is affected by climate and land use changes. We assessed how phenology shifts are being altered by exotic species dominance by comparing remotely sensed Normalized Difference Vegetation Index (NDVI) within growing seasons at exotic- and native-dominated sites along a latitudinal gradient. Exotic species were dominated by the C3 species functional group in the north and the C4 species functional group in the south.

3. Date of senescence was an average of 36 days later in exotic than native-dominated grasslands, and this effect was consistent across latitudes.

4. Exotic-dominated grasslands greened up an average of 10.7 days earlier than native-dominated grasslands, but this effect was highly dependent on latitude and the plant functional group that dominated at that latitude. Greenup differed between native and exotic sites the most in central and northern regions that had dominant C3 grasses.

5. We estimated the effects of an increase in global temperatures on green-up and senescence with a space-for-time substitution, and by comparing growing degree day differences between historical average temperatures and +2.5° C. Green-up was significantly earlier and senescence was significantly later with a 2.5 ° C increase in temperature. The native-exotic difference was significantly greater than the difference due to increased temperature for senescence, but not for green-up.

6. Synthesis and applications. Native to exotic plant conversions in central US grasslands have led to highly altered phenology, especially in terms of senescence, and this effect should be considered along with global warming in models moving forward. This conversion will have to be considered in developing estimates of how global change will affect phenology in locations where exotics are present, especially in cases where their abundance is increasing concurrent with climate change. Global change models and policy should consider exotic species invasion as an additional widespread factor behind changes in phenology.

1. 新型生态系统（novel ecosystems）与其所替代的原生生态系统存在显著差异。本研究以美国中部地区以外来物种占优与本土物种占优的草原为研究对象，通过物候（phenology）观测指标对比二者的生态系统功能。
2. 物候，即生物事件发生的时间节点，受气候变化与土地利用变化影响。本研究沿纬度梯度选取外来物种占优与本土物种占优的样地，对比生长季内遥感获取的归一化植被指数（Normalized Difference Vegetation Index, NDVI），以此评估外来物种优势度如何改变物候偏移。北部外来物种群落以C3物种功能群为主，南部则以C4物种功能群为主。
3. 外来物种占优草原的衰老期平均比本土占优草原晚36天，且该效应在所有纬度梯度上均保持一致。
4. 外来物种占优草原的返青期平均比本土占优草原早10.7天，但该效应高度依赖于纬度以及该纬度占优的植物功能群。本土与外来样地的返青期差异在以C3草本为优势种的中部与北部地区最为显著。
5. 本研究采用空间替代法（space-for-time substitution），通过对比历史平均气温与升温2.5℃情境下的生长度日数差异，估算全球气温升高对返青期与衰老期的影响。结果显示，气温升高2.5℃时，植物返青期显著提前，衰老期显著延后。对于衰老期而言，本土与外来样地间的差异显著大于升温带来的差异；但返青期则无此规律。
6. 结论与应用：美国中部草原由本土群落向外来物种群落的转换已大幅改变了当地物候，尤其是在衰老期方面；未来的模型构建中应将该效应与全球变暖一并纳入考量。在预测全球变化对存在外来物种区域的物候影响时，必须考虑此类群落转换的作用，尤其是在外来物种丰度随气候变化同步增加的情境下。全球变化模型与相关政策应将外来物种入侵视为物候变化背后又一广泛存在的驱动因素。