Phenology, or the timing of events within ecosystems such as spring green-up or late-season senescence, can be important to higher trophic levels if mismatches occur between timing and animal and microbial activities. Phenology is changing due to global temperature increases, and green-up is estimated to occur 2.5 – 5 days earlier with each degree C increase. However, there have been other concurrent changes over the last century, such as changes in species composition due to invasion and planting of exotic species. Exotic species were found to green up earlier than paired native species in previous common garden experiments. We tested for differences in phenology between 21 native and 21 novel ecosystems along a latitudinal gradient in the central U.S. with NDVI data from Landsat satellite measurements for green-up, midseason dips in activity, and senescence. Data were from 2004-2011. Exotic fields were 89.6% exotic, and native fields were 91.8% native biomass. Our predictions were: 1) exotic-dominated fields have earlier green-up and later senescence dates than natives, 2) differences will be greater in the north due to photosynthetic type differences, and 3) altered phenology will be similar or greater in magnitude compared to changes expected with increased temperatures.
Results/Conclusions
Green-up and senescence dates differed significantly between native- and exotic-dominated (novel) grasslands. Exotic-dominated grasslands greened up an average of 12 days earlier than native-dominated grasslands (P < 0.01), but this effect was highly dependent on latitude (origin x latitude interaction P<0.05). Differences in green-up date between native and exotic grasslands were greater in northern latitudes and smaller in southern latitudes. Using mean minimum temperatures of native sites along the latitudinal gradient, we estimated that green-up date was 5.5 days earlier with each 2.5° increase in temperature (linear regression, P < 0.01). Date of senescence, was on average, 23 days later in exotic- than native-dominated grasslands (P<0.05). Date of senescence was unaffected by temperature. Green-up and senescence dates remained significantly different between native and exotic grasslands after accounting for soil differences. Our results indicate that green-up and senescence dates are altered more by biotic conversions from native to exotic species than by temperature changes associated with climate change.