I must admit, when I first encountered an Oak Opening, I had a hard time initially wrapping my brain around what was unique about the habitat. I looked up and could recognize that it contained oak trees, standing high in their far-reaching, craggy-branched splendor. “Ok, they’re oak trees,” I shrugged. Then one day it dawned on me: an Oak Opening possesses a vast amount of open space compared with what I understood a forest to look like.
A great distance of space can exist between trees, sometimes 100-feet from one another. This is why these habitats are also known as “Oak Openings,” and is the name for this one-acre area at Riveredge Nature Center overlooking the Milwaukee River. This portion of the property also boasts uniquely untilled original soil and a rarely seen guild of native plants. But what else is unique about Oak Openings?
Oak Openings have become incredibly rare
The numbers of Oak Savanna (a somewhat similar habitat with less tree density than an oak opening) previously standing and currently in existence are staggering. Of the 5.5 million acres that once existed, according to the Natural Heritage Inventory, less than 500 acres exist that had plant assemblage similar to the original Oak Savannas. Similar to savannas, Oak Openings are one of the rarest and most threatened habitats in the world. Summarily, many of the plant and animal species that flourished in these systems have perished, or their populations have taken hits as they struggled to find other, less suitable habitats.
Pre-settlement, wildfires and fires set by Native Americans took place across the US throughout the year, burning off smaller trees and invigorating understory plant seeds to sprout. Oaks have thick bark and a deep taproot, which equips them uniquely to tolerate fires more than other woody species. After a fire the only plants that stood throughout the charred landscape were oak trees, such as Bur Oak.
What happened to Oak Openings?
Prior to settlement, about half of Wisconsin was covered in Oak ecosystems (such as oak woodland, oak savanna, oak opening). When settlers moved west into this territory, these oak ecosystems appeared, and proved to be ideal areas for farmland and more readily cleared than a dense forest. Many of the soils were rich in nutrients after centuries of plants and animals had built up the soil. The removal of indigenous peoples, their customs, and traditional ecological knowledge, as well as the removal of fire fuel continuity by turning over original ground with the plow, worked to suppress fires that had been previously afforded greater affect on the landscape.
Oak trees provided ideal building material for houses and barns, and if any was left over it became firewood. Millions of linear board feet would be shipped to become furniture, tool handles, and flooring throughout cities such as Milwaukee. While the wood lasted, logging was a bustling business in Wisconsin.
In areas of Oak Savannas that still stood, without fire management or grazing by wild or domesticated animals, smaller trees would begin to grow up between oaks, competing for sunlight and rain. Invasive species such as Buckthorn would begin to fill in areas that were previously the domain of native plants that grow more slowly. When we picture a forest, this, comparably more cluttered, landscape is likely what we imagine.
Today, an oak opening gives the same reprieve from a forest’s overstory as it always has; however, it now represents some of the best of what many areas have lost. For ecologists, oak openings and other similar rare habitats now act as living libraries of species and their interconnected assemblages, to reconstruct in our restoration efforts.
This summer, experience Wisconsin’s natural heritage by visiting the Oak Opening at Riveredge Nature Center, and continue visiting throughout the seasons. This location is also one of our most picturesque locations from which to view the Milwaukee River. In this now uncommon location, you can experience the tranquility that can only be found within trees that live for hundreds of years.