Weed invasion tracked long-term precipitation in native- and exotic-dominated grasslands
Precipitation is anticipated to vary greatly under global climate change. However, little is known about the role of precipitation in shaping weed invasion of established grassland ecosystems. Here, we explored how weed invasion was influenced by precipitation and how this pattern was modified by grassland origins (native- vs novel exotic-dominated perennial species) and species diversity under ambient and elevated summer precipitation in Texas in 2009. A common garden experiment was established in 2008 using 36 widely distributed native and exotic grassland species. A two-way factorial treatment arrangement (origin ´ irrigation) was applied to plots with a randomized block design, using random draws to vary species composition. We planted equal-mass transplants at a common density (72 individuals m-2) of either all native or all exotic species. Native and exotic species were planted in 9-species mixtures and in monocultures using a paired species approach that controlled for phylogeny and growth form between pairs of native and exotic species. Origin treatments (native vs. exotic) were crossed with irrigation treatments (128 mm or none). Weeds that invaded the native and the exotic plots were hand removed, transported to the laboratory, and dried at 70 oC six times around days 50, 100, 130, 180, 230, and 330.
Weed invasion tracked long-term dynamics of climatic precipitation more closely than the precipitation in 2009. Summer irrigation marginally decreased weed invasion and had little effect on the date at which the canopy of perennial species was estimated to have surpassed 50 % cover. In comparison to exotic communities, weeds invaded native communities more in the early growing season (February and March), although invasion did not differ significantly between native and exotic communities across the six sampling times in 2009. More weed invasion in natives was consistent with the phenomenon that exotics greened up and surpassed 50 % canopy cover earlier than natives by an average of 18 days. Weed invasion was negatively influenced by species richness in native communities but by dominant species in exotic communities. Weed invasion tracked long-term climatic precipitation. Our results indicate that matching between long-term climate and invasion may constrain responses to fluctuations in precipitation expected with climate change.