Past precipitation alters current year plant growth

Plant biomass production tends to increase with precipitation. However, this relationship is not always straightforward. Frequently, previous conditions that the plants have experienced will impact their responses to current water availability. For instance, plants that had grown under wet conditions may be better able to survive and grow under drought than plants that had grown under drier conditions. Similarly, plants that experienced drought previously may not be able to respond as vigorously to a year of high rainfall as plants that had grown under wetter conditions. These lagged effects, or legacies of past precipitation, may have large impacts on biomass production. 

To better understand the importance of legacies of past precipitation on plant performance, we tested the impact of year-to-year changes in precipitation on current plant responses with the candidate biofuel species Panicum virgatum (switchgrass). To do this, we performed three distinct experiments: 

1. A rainout shelter study, where plants grew under different precipitation regimes for multiple years
2. A transplant study, where plants that had been growing under different precipitation regimes in a rainout shelter were moved to an unmanipulated common area
3. A mesocosm study, where the impact of switching precipitation regime (wet-to-dry or dry-to-wet) was studied on plants growing alone.

P. virgatum experienced stronger effects of prior wet conditions on current-year production than of prior dry conditions. This is evidence of a legacy effect of prior wet conditions resulting in larger plants than those grown under consistent, longer-term drought. The opposite response—legacies of prior drought when grown under wet conditions—were not detected. Prior year tiller number was an important determinant of legacy effects on plant production. When prior conditions allow plants to become larger and capable of producing more tillers, they may be better able to withstand drought conditions.