Earlier Flowering in a Restored Wetland–Prairie Correlated with Warmer Temperatures (Ohio)

Phenological patterns could reflect the effect of climate change (Root et al. 1973). A 30–year study of garden plants has revealed significantly earlier flowering in southwestern Ohio (McEwan et al. 2010), which might be due at least in part to warmer temperatures associated with global warming. Here, we share evidence in support of this hypothesis using wild plants in their natural habitats.

The Shaker Trace Wetlands is a restored wetlands/prairie complex area at Miami Whitewater Forest in southwestern Ohio. Vascular plant surveys were conducted there from 1992 to 1996 and again from 2005 to 2008 via weekly visits during the growing season (March through October) and monthly visits from November through February. Some of the changes that took place in the wetlands/prairie complex between the two periods are described by Conover and Klein (2010). During the first survey, 527 species in 84 plant families were identified, while during the second survey, 478 species in 84 plant families were identified. During these surveys, the month that a species was first observed in flower was recorded. Of 269 species that were observed in flower during both surveys, 39% bloomed earlier during the second time period (Table 1), 45% bloomed at the same time, and 16% bloomed later.

The mean annual temperature (11.88°C) during the first survey was significantly lower than the mean annual temperature (12.93°C) during the second survey (Figure 1). Of the 149 species that changed their flowering time in ten years, 105 flowered earlier. This proportion, 0.70, is significantly higher (exact binomial test, p < 0.0001) than the 50% that would be expected if there were no temperature effect.

Figure 1.

Mean annual temperatures (open circles) for the period from 1992 to 2008 at Shaker Trace Wetlands in Ohio. The average values (dotted lines) during the plant surveys (1992–1996 and 2005–2008) were significantly different (1–sided Welch's t-test: t = −3.11, df = 5.36, p = 0.012).

This observation is not unique to the Shaker Trace Wetlands. From 1997 to 2000, one of us (Conover) conducted a plant survey near Lawrenceburg, Indiana, in the Oxbow Area of southeastern Indiana/southwestern Ohio. This survey is being repeated during 2010 to see what changes have taken place. The survey is still underway, but it is clear that a great many species are flowering earlier now than they did during the previous survey ten years ago. Another change is the presence of new invasive species in the Oxbow Area such as Callery pear (Pyrus calleryana), fig buttercup (Ranunculus ficaria), and Japanese stiltgrass (Microstegium vimineum).

Table 1.

Vascular plant species in the Shaker Trace Wetlands/Prairie Complex that bloomed earlier during the second survey (2005–2008) than during the first survey (1992–1996). Woody species are in boldface, and shaded rows indicate species not indigenous to southwestern Ohio.