Inter- not intra-annual precipitation variability determines invasion of a mesic grassland
Invasion by non-native plant species is occurring worldwide with serious consequences for ecological, economic, and social systems. Fluctuations in resource availability are thought to be a major factor governing the invasibility of ecosystems. With global climate change, resources like precipitation are expected to become more variable both within and across years. Here we addressed the relative roles of intra- vs. inter-annual rainfall variability in determining invasion success. We conducted a long-term (2004-2012) invasion experiment within the context of an existing climate change experiment – the Rainfall Manipulation Plots (RaMPs) at the Konza Prairie Biological Station, Kansas. The RaMPs experiment imposes a more variable intra-annual precipitation regime, by increasing the timing between growing season rainfall events by 50%, without changing total rainfall amounts. To assess invasion success, we added seeds of Bromus inermis to four 0.10-m2 plots per treatment plot (n = 6) in May of each year. Seedling establishment and survival were monitored throughout the growing season. We hypothesized that the altered rainfall treatment, which results in fewer, larger rainfall events, would increase invasion by creating resource pulses conducive to establishment. Thus, we expected increased intra-annual variability in rainfall would be as or more important than natural patterns of inter-annual precipitation.
We found inter-annual precipitation was more important than intra-annual precipitation. The altered rainfall treatment increased soil moisture variability on average by 17% compared to the ambient rainfall treatment. Despite this increase in intra-annual variability in soil moisture, establishment and persistence of the target invader was significantly lower in the altered rainfall treatment in most years. This is likely because soil moisture was on average drier and dry periods were longer in the altered rainfall treatment. However, invasion by B. inermis did vary significantly between years, with growing season precipitation explaining 80% of the variation in seedling persistence (p=<0.0001). Invasion success was highest for both treatments in 2008, the highest rainfall year of the study, but invasion remained significantly higher for the ambient treatment. This nine year invasion study shows that establishment is greatest when resources are abundant supporting the fluctuating resource hypothesis, but that the time scale of these fluctuations are important. Here, inter-annual variation in precipitation was the primary driver of invasion success. Increased inter-annual and intra-annual variability in precipitation regimes are forecast with climate change, and our long-term invasion study suggests that increases in inter-annual variation in precipitation regimes could enhance invasibility of ecosystems.