Land Care Report: Why We Are Seeing More Woody Species in Curtis Prairie

Much of Curtis Prairie’s edge comprises tall trees, especially on the south side. The trees cast dense shade onto the southern part of the prairie creating a cooler, damper environment more hospitable to woody species as shown here.

Much of Curtis Prairie’s edge comprises tall trees, especially on the south side. The trees cast dense shade onto the southern part of the prairie creating a cooler, damper environment more hospitable to woody species as shown here.

Over the last few years we have fielded many questions regarding woody vegetation on Curtis Prairie. Native shrubs such as smooth sumac (Rhus glabra) and gray dogwood (Cornus racemosa) seem to be increasing exponentially and covering the prairie at an alarming rate.

Unfortunately, the increase is not one unique to Curtis Prairie, but a global trend that has become apparent to ecologists and land managers over the last 10–15 years.

Why the sudden and rapid increase? Regarding Curtis Prairie, it’s often thought that a lack of fire over the years is the culprit. However, Curtis has been burned fairly frequently for over 50 years. Fire only does so much in preventing woody species from establishing—once they become established, research at the Arboretum and elsewhere has shown that fire can actually accelerate their expansion.

So, what’s going on out there? We think there are a number of additional factors at play, including but not limited to:

Size. Curtis is only 70 acres, not very large when you consider there were once over 2 million acres of prairie in Wisconsin.

Edge. Curtis is long and narrow with lots of edge. More edge means more opportunity for undesirable species to enter the prairie and spread.

Shade. Much of the prairie’s edge comprises tall trees, especially on the south side. The trees cast dense shade onto the southern part of the prairie, creating a cooler, damper environment more hospitable to woody species.

Geography. Wisconsin lies in the transition zone between the true prairie to the west and south, the boreal forest to the north, and the hardwood forest to the east. In this region there has always been a give-and-take between forest and prairie as the dominant vegetation type.

History. According to the surveyor’s notes from 1834, the pre-settlement vegetation of Curtis Prairie was actually oak savanna (at least partially), not open prairie, so woody vegetation (likely including shrubs) on the site is not new.

Location. Given its location in the middle of a highly urban landscape, Curtis is receiving inputs of carbon dioxide, nitrogen and stormwater that exceed the historic norm. Research indicates prairie plants may be at a disadvantage in these new conditions.

Precipitation. Over the last several years Madison has experienced some of its wettest weather on record. The greater amounts of precipitation and associated stormwater inputs give advantages to woody vegetation and other non-prairie plants.

Age. Curtis Prairie is close to 80 years old. The changes observed could simply be related to the amount of time that has passed since restoration was initiated.

Ecosystems are dynamic by nature and always changing. Curtis Prairie is no different. Our challenge is to understand the factors driving the change and balance it with a certain level of management that allows conservation goals to be met.

Currently, the woody vegetation on Curtis is managed with a combination of prescribed fire, cutting/mowing, and herbicide. As mentioned above, fire is only effective to a point, so more frequent and larger-scale cutting/mowing and herbicide application, and perhaps other methods, will likely be needed to suppress the woody expansion and swing the pendulum back towards favoring the prairie species.

—Michael Hansen, Arboretum land care manager
Reprinted from NewsLeaf, Friends of the Arboretum newsletter, September 2014

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