Phenology differences between native and novel exotic-dominated grasslands rival the effects of climate change

Abstract

Novel ecosystems can differ from the native systems they replaced. We used phenology measures to compare ecosystem functioning between novel exotic-dominated and native-dominated grasslands in the central U.S. Phenology, or timing of biological events, is affected by climate and land use changes. We assessed how phenology shifts are being altered by exotic species dominance by comparing remotely sensed Normalized Difference Vegetation Index within growing seasons at exotic- and native-dominated sites along a latitudinal gradient. Exotic species were dominated by the C3 species functional group in the north and the C4 species functional group in the south. Date of senescence was an average of 36 days later in exotic than native-dominated grasslands, and this effect was consistent across latitudes. Exotic-dominated grasslands greened-up an average of 10.7 days earlier than native-dominated grasslands, but this effect was highly dependent on latitude and the plant functional group that dominated at that latitude. Green-up differed between native and exotic sites the most in central and northern regions that had dominant C3 grasses. We estimated the effects of an increase in global temperatures on green-up and senescence with a space-for-time substitution, and by comparing growing degree day differences between historical average temperatures and +2.5°C. Green-up was significantly earlier and senescence was significantly later with a 2.5°C increase in temperature. The native–exotic difference was significantly greater than the difference due to increased temperature for senescence, but not for green-up. Synthesis and applications. Native to exotic plant conversions in central U.S. grasslands have led to highly altered phenology, especially in terms of senescence, and this effect should be considered along with global warming in models moving forward. This conversion will have to be considered in developing estimates of how global change will affect phenology in locations where exotics are present, especially in cases where their abundance is increasing concurrent with climate change. Global change models and policy should consider exotic species invasion as an additional widespread factor behind changes in phenology.