Red maple, a tree species that originated in swampy habitats, is taking over eastern forests, says a researcher in Penn State's College of Agricultural Sciences. And its lowly, ordinary nature may be determining its success. "A century ago -- because of its sensitivity to fire -- red maple was relegated to the swamps," says Marc Abrams, professor of forest ecology and physiology. "In fact, it used to be called swamp maple. But now that we suppress forest fires, red maple has emerged from the swamps and taken over upland sites, and can be found on just about every landscape in the eastern deciduous forest." Travelers driving along highways this time of year can see the tell-tale profusion of red maple's silvery branches in Pennsylvania's mountainous landscape.
This change in our forests, Abrams says, may have profound economic and ecological consequences. "Forest regeneration is a huge concern," he says. "Trees that historically dominated this region -- the pines, oaks, hemlocks and hickories -- no longer regenerate very well. Red maple is replacing trees that have high economic value. Its soft wood, color and grain aren't as highly valued as that of black cherry, oak and walnut. Also, many wildlife species depend on the trees that are being replaced." Oaks and hickories supply many small mammals and birds with nuts and acorns. And the oak's rough bark -- unlike the maple's smooth bark -- houses bark-dwelling insects for insect-eating birds. A shift in wildlife populations is likely to parallel this shift in tree species, Abrams believes.
Red maple's proliferation also poses a biodiversity concern, says Abrams. "Very diverse forests -- with six to 12 different species in the overstory -- all may be changing to red maple-dominated stands. And stands of single species are more susceptible to total devastation by insects and disease." So how does the unassuming red maple outcompete the hardy oak? In a May 1998 article in Bioscience, Abrams describes the scientific detective work behind this mystery. "Through the 1700s and 1800s, the charcoal industry cut a lot of forests and burned them into charcoal," he says. "There were a lot of escaped wildfires. So we had these dramatic disturbances going on that were encouraging growth of the fire-resistant oak. "But now that we're cutting forests less and suppressing forest fires, forests are going through a natural succession or aging process," he says. "Red maple, because it grows well in shade, is a key late-successional species. But red maple also has the incredible ability to act as an early-successional species and invade disturbed sites as well." It's red maple's low requirements for water, nutrients and light -- the key resources for plants -- that allow it to grow on really poor sites, or really good ones.
Physiologists call such organisms "supergeneralists" -- they do fairly well over a broad array of conditions, rather than have super abilities in a few specialized areas. "By invading sites in all kinds of conditions -- sunny or shady, high or low nutrients, dry or moist -- red maple really is an ecological marvel," Abrams says. Red maple also has taken its uniquely wetland reproductive strategy to the uplands. "After spring floodwaters recede, wetland plants drop their seeds on the fresh soil layer," Abrams explains. "Upland species, on the other hand -- which aren't tied to spring flooding -- drop their seeds in the fall. So red maple can spend most of the growing season establishing seedlings, giving them a three to four month advantage over other plants. "Also, white-tailed deer don't browse red maple strongly during the summer, yet browse oaks all year round," he says. "In the wintertime, when food is scarce, deer will browse red maple. But it's not as harmful then, because the trees are dormant."
Red maple also may have benefited from the ill affects of acid rain and global warming in this century -- problems that have particularly affected Pennsylvania, Abrams adds. Like the athlete who isn't the biggest, strongest or fastest, but brings other skills to the playing field, red maple prevails. "That's the real paradox," Abrams says. "Ecologically, red maple is one of the most aggressive tree species. But physiologically, it's one of the most conservative." Normally, scientists associate aggressive behavior with robust physiology, Abrams explains. They assume plants that photosynthesize faster, for example, will outcompete other plants. Or plants with higher nutrient levels in their leaves or better drought adaptation will have the advantage. Red maple breaks the mold. "The red maple has changed my thoughts on some basic principles of physiological ecology. Ranking species based on the usual physiological parameters may not be a good way to rank their ecological success."