I am looking out across a broad river valley here in Western Pennsylvania. At least one third of the trees I see are red maples (the photo to the left was taken about a month after the photo in last week’s blog). Last week I wrote about the amazing reproductive potential of the red maple and the readiness of its seeds to germinate and survive as seedlings in almost any natural and human generated habitat. Now let’s talk about the ultimate forest sculpting force in Pennsylvania today: the white-tailed deer.
Red maple seedlings are extensively browsed by deer in the winter. So, there is a large advantage for a seedling to grow as fast as possible in order to stretch itself above the browse level. Growth rates of red maples are prodigious: one foot height increases in the first year, two feet per year after that. Red maples, though, are also able to sprout after the above ground stem has been damaged. After a seedling has been browsed by a deer, for example, that individual can actually re-sprout at a growth rate of three feet per year! Sprouts, initially, have a faster growth rate than seedlings! There may actually be a selective advantage for a seedling to be browsed so that this accelerated growth spurt can be achieved!
The symbiosis between deer and red maples, then, is complex and, on the surface, seemingly contradictory. Deer eat red maples voraciously in the winter. But this winter browsing may actually stimulate those red maple individuals that are capable of the most robust sprouting response, to attain dominance in their ecosystem! Further, the sheer numbers of red maple seedlings in a forest floor community make it likely, especially in years of abundant browse, that at least some of the red maple seedlings will be passed over by the deer and, thus be able to survive to grow. Tree species that generate smaller numbers of seedlings are less likely to survive this intense survival filter of deer browsing.
Many insects feed on red maples, but few of these insects are serious, lethal pests. Gypsy moth larvae, for example, feed preferentially on oak leaves over red maple leaves possibly because of certain alkaloid chemicals that are present in the red maple leaf tissues. These alkaloids may also be an important in the red maple’s resistance against excessive summer leaf browsing by deer.
Red maples are extremely sensitive to fire. All of its life stages (seedling, sapling, “pole” tree, and mature tree) are easily damaged and killed by forest fires. Fire is a very effective way by which a forested ecosystem can be scoured clean of red maples! The minimal presence of red maples in pre-settlement forests and their predominant confinement to very wet habitats may be explained by the regular occurrence of fires in the drier habitats. Some of these fires, in fact, are thought to have been set by Native Americans in a management strategy that increased the dominance of oaks and sculpted the physical structure of the forest itself into a configuration more conducive to hunting and the support of deer populations. The wide-spread suppression of forest fires throughout the twentieth century may be an extremely significant factor in understanding the expanding distribution and abundance of red maples.
The pre-settlement forests of Pennsylvania were dominated by oaks (especially white oak), American chestnut, eastern hemlocks, and white pines. All of these tree species were extensively cut for timber, mine props, railroad ties, firewood, charcoal, tannins and other wood chemicals, and paper pulp. The oaks in the re-growing, secondary forests were then exposed to intense browse pressures by the re-surging white-tailed deer populations, and defoliated often fatally by the explosive outbreaks of the introduced gypsy moths. The suppression of forest fires also removed an on-going environmental factor that favored the dynamic persistence of the oak forest. The American chestnut was functionally exterminated as a potential canopy tree by the twentieth century expansion of the introduced fungus that caused chestnut blight. Eastern hemlocks were logged into near extinction in the state and, most significantly, the moist, highly shaded forest floor microhabitat in which their extremely specialized seeds and seedlings could germinate and grow was destroyed by the clear cutting and subsequent soil and litter erosion and drying exposure to direct sunlight. White pines thrived on sites with coarse, well drained soils and relied on periodic fires to reset their successional sequences and maintain a site consistency of white pines. Fire suppression, then, also limited the ability of white pines to reestablish their “pineries.”
The pre-settlement forests, then, were densely populated with highly specialized trees that had established and maintained themselves in their habitats. These tree species modified the characteristics of their habitats to favor their growth and survival. These species were also adapted to respond to the catastrophic but regularly occurring disturbance events that beset their ecosystems. Settlement, then, changed everything. The suppression of fire, the destruction of the ancient, persisting seed beds of the forest floor, and the introduction of exotic insects and diseases altered the stress and selection matrix of these forests possibly forever. Tree species with broader tolerance ranges especially with regard to reproductive processes and stages were now favored. And, the red maple is acknowledged to be the “ultimate generalist.”
So, the question I have been pondering here in the shade of this wonderful red maple in the company of generation after generation of red maples, is “why are they here?” and “why are they everywhere I look?”
The destruction of the pre-settlement forest opened many niches. The failure of the hemlocks and white pines to reproduce, the loss of the American chestnut, and the on-going failure of the oak forest to re-establish itself reduced competition, and the incredibly fecund and adaptable red maple swept into the available ecological spaces.
Will other tree species succeed the red maple? Will northern red oaks, sugar maples, or American beeches (all slow growing but very shade tolerant species) become established in the understories of the red maples and eventually grow into the canopy? Or will the red maples maintain their dominance via their staggering numbers and ecological tenacity? Check back in hundred years or so to see!