The Effects of an Initial Extreme Drought and Chronic Change in Precipitation on Plant Biomass Allocation in a Temperate Grassland.

Drying climate may strongly affect plant productivity and standing biomass and, thus, ecosystem carbon cycling. Whilst drought effects on grassland aboveground plant biomass (AGB) have been widely studied and are generally negative, reports on belowground plant biomass (BGB) are much fewer, and results are more variable. In a full factorial experiment in a temperate grassland, we studied the legacy effect of an initial extreme drought, conducted in 2014, and the effect of four levels of chronic precipitation change: water addition, ambient precipitation, moderate drought and severe drought between 2015 and 2018, on AGB, BGB and BGB/AGB in 2019. AGB was negatively affected by chronic drought and also by extreme drought within each level of chronic precipitation change except severe drought. Total BGB was not influenced by extreme drought but was negatively impacted by chronic severe drought. Responses were the strongest in the shallow soil layer (0-10 cm), where both extreme and chronic droughts decreased belowground biomass, and weak in the deep soil layer (10-20 cm). BGB/AGB was not altered by extreme drought but increased in chronic drought plots because the decrease of AGB was much greater than the decrease in BGB. Our results indicate that the legacy of previous extreme drought and chronic change in precipitation are both important in shaping biomass pools and allocation in grasslands, sometimes with interactive effects.