Han, Juanjuan; Chen, Jiquan; Shi, Weiyu; Song, Jian; Hui, Dafeng; Ru, Jingyi; Wan, Shiqiang

Climate models project that temperature will increase by 1.1 to 5.4°C by 2100. Extreme precipitation events including severe droughts are likely to increase in some regions of the world, especially in semi-arid grasslands. However, how the current year and previous years’ precipitation will influence grassland ecosystems remain unclear.

Precipitation could influence grassland ecosystems via at least two possible ecological pathways: 1. Precipitation will alter current-year resource availability, such as soil water content and soil nitrogen content. These changes in resource availability may carry over to subsequent years, further affecting resource absorption by plants and changing resource use efficiency (RUE). 2. Precipitation may alter current-year plant community composition, which may affect RUE in subsequent years.

To test the legacy effects of previous-years’ precipitation on RUE, we established a precipitation gradient experiment with seven treatment levels: 20%, 40% and 60% decreases and 20%, 40% and 60% increases in the amount of natural rainfall, plus an ambient precipitation control. Plots receiving 60% less precipitation were representative of extreme dry years, while the other treatment levels fell within the normal year-to-year range in precipitation. We focused on the post-treatment period (2013-2015), where measurements continued as in the treatment period (2010-2012), but no precipitation treatments were applied in the post-treatment period.

We found water use efficiency and light use efficiency responded more strongly to previous normal dry than wet conditions, even though the same amounts of precipitation were removed or added to the plots. Importantly, they were more sensitive to a previous extreme dry year (represented by 60% precipitation reduction) than to a normal wet year (represented by 60% precipitation increment) in 2014-2015. Plant community biomass (dry matter), especially of grasses, rather than resource absorption drove these asymmetric responses of RUE. In other words, RUE has a memory of previous-years’ precipitation, and this memory arises from changes in both plant community biomass and composition.

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