Forest Ecology. Dan Binkley

pigeon. This large (30 cm) bird was a major consumer of large tree nuts, including acorns, beechnuts, and chestnuts (Halliday 1980; Johnson et al. 2010). Huge flocks contained tens of thousands (perhaps even millions) of birds, with large impacts on dispersal of tree seeds and nutrient cycling (with concentrated feces beneath favored roost and nest trees). Passenger pigeons were the most dominant species of bird in eastern North America, and perhaps the most numerous bird species in the world. Over the course of a few decades, this species spiraled to extinction as a result of massive hunting, and perhaps the effects of changing forest cover and even exotic diseases. How did the forests change in the absence of passenger pigeons? This important question is easily asked, but probably cannot be answered without stronger historical records.

      Two other human‐related events marked the 1800s in the Coweeta Basin. The second half of the century saw Euroamerican settlers occupying the land. Their major impacts included some logging, agricultural cropping on a few hundred hectares, widespread grazing of pigs and cattle, and hunting of wildlife for food. The prior inhabitants were Cherokee Indians, forcibly removed in the 1830s. Cherokee influences on the forests included some agriculture (maize, squash, and beans), extensive hunting (primarily deer, turkeys, and bears); food collection (including tree nuts); and frequent use of fire to clear the forest understory (Van Derwarker and Detwiler 2000; Gragson and Bolstad 2006).

Across Dozens of Generations of Trees, Almost Everything Changed at Coweeta

      The past 10 000 years have seen dozens of generations of trees and forests come and go in the Coweeta Basin, in response to fluctuations in climate, events such as hurricanes, and probably sizable fluctuations in populations of humans and other animal species that influence forest dynamics. The frequency of fires may have increased as people ignited forest fires (intentionally or unintentionally). Fires may have burned the tulip poplar/mixed broadleaf stands every 200 years or so over the millennium before European settlement (Fesenmeyer and Christensen 2010). Some notable events include a near‐disappearance of eastern hemlock throughout its range, between about 5500 and 6500 years ago (Calcote 2003; Heard and Valente 2009), followed by recovery. The cause of the decline is unknown, and speculations include some sort of novel disease. This also happened to be one of the coolest times in the past 10 000 years, so multiple factors may have been involved.

Continuing back to 12 500 years ago, the continent (and much of the globe) was undergoing rapid warming as the most recent Ice Age ended. Temperatures in the Coweeta Basin would have risen by more than 5 °C from conditions that prevailed for 100 000 years. Under colder conditions, the forests in the Basin would have resembled forests that are currently found farther north, with pines and spruces dominating even the lower elevations. During some periods, the assemblages of trees species across the region included combinations that have no modern analog in local forests, or in forests now found farther north (Jackson and Williams 2004). Assemblages of tree species change in response to interactions among temperature, precipitation, and biotic factors. Unlike organisms, the genotypes of forests change routinely as species come and go.

      The most notable difference in the forest at the end of the Ice Age would have been the presence of many large species of mammals in the region. The list of now‐extinct species includes tree‐browsing American mastodons; grass and tree‐browsing Columbian mammoths; woody‐plant browsing stag moose; tree‐eating giant beavers more than 2 m in length; and large predators such as dire wolves, sabretooth cats, and massive short‐faced bears. The now‐extinct mammals would have been joined by at least one now‐extinct tree species, Critchfield spruce (Jackson and Weng 1999).

The Futures of the Tree and the Forest Will Depend on Both Gradual, Predictable Changes and Contingent Events

      The future is largely unpredictable for individual trees, but some predictions may have a high probability of coming true. The dominant situation enjoyed by the tulip poplar featured in this chapter would generally predict steady growth into the future. Growth might even increase as neighbors are suppressed. Dominant trees of this species may live for more than two centuries, and such a long lifespan provides opportunities for dispersing millions of seeds.

      A long lifespan also increases the odds that the tree will experience rare weather events. For example, a severe drought with a probability of occurrence once in 100 years might severely challenge a tree's survival. A tree that lives only about five decades would have a 60% chance of never experiencing a 100‐year‐magnitude drought (if weather is random), whereas a tree that lived two centuries would have an 87% probability of experiencing at least one 100‐year drought.

      A host of other future factors are more difficult to assign probabilities. The death of a neighboring tree may suddenly increase the supplies of resources available to this tree, or the falling neighbor may collide and uproot this tree as well. Lightning tends to kill large trees more often than smaller trees. Outbreaks of insect populations and fungal diseases influence the long‐term development of many forests. The climate experienced by this tree (and its ancestors) may not continue into the future. Novel pests may arrive in the forest, as a result of widespread transport associated with world‐wide travel by people and materials. The future of the tree may also depend very heavily on choices made by people; a large tulip poplar tree can be transformed into thousands of dollars‐worth of furniture and other products.

      Some changes in a forest tend to be cyclic, with repeating patterns of species and growth rates following major events. The major recolonizing species will have predictably high tolerance for full sunlight and rapid early growth, whereas trees that remain after two centuries will likely grow slowly and the community will include trees that thrive under shady conditions. Other changes are clearly not cyclic, and lead us to expect that the future forests in the Coweeta Basin will not be simple analogs of past forests (Jackson and Williams 2004). The development of forests responds to changes in climate, and climatic patterns (and the responses of trees and species to these patterns) have long legacies (Kardol et al. 2010). Changes in future climates may have modest effects on the forests compared to novel insects and diseases. The chestnut blight removed the dominant tree species from the Coweeta forests, and the hemlock wooly adelgid decimated the population of eastern hemlock trees. What will be the legacies of the loss of almost all the chestnuts and hemlocks trees from Coweeta's forests? Might we be able to predict the response of surviving species to the disappearance of hemlock, based on the patterns from 6000 years ago when eastern hemlock experienced another decline, or will other factors (such as changing climate) limit the ability of the past to illuminate the future? We might speculate about how other species will take advantage of reduced competition from these species, but the actual impacts will include the ecological legacies of changes in soils and in animal communities. Forests often respond to more than one event; future forests develop from the combined legacies of historical events (such as losses and gains of species) in combination with current conditions. Warming climate, rising atmospheric concentrations of carbon dioxide, and other factors will influence future forests, shaping the legacies of the losses of chestnuts and hemlocks. Will new species of exotic insects arrive and remove other tree species from Coweeta's forests?

      The future development of a tree, and of a forest, derives from the gradual accumulation of routine changes, such as annual increases in height and mass of stems. Over limited periods, these gradual, expected trends are punctuated by contingent events that are largely unpredictable, such as hurricanes and invasions by exotic pathogens. Humans are another force for change in forests, through direct management (typically favoring some species over others, often limiting the opportunity for old trees to develop) and indirect activities (such as nutrient enrichment of rain, air pollution, and climate change).

Given all these forces of change, how can we predict future forests? The short answer is simply that we cannot predict future forests with much confidence. The longer (and more useful) answer is that we can indeed develop insights about the likely forests of the future, if we understand some of the basic features that have shaped forests in the past, and how ecological interactions will combine to shape future forest.

Ecological Afterthoughts: Is a Forest an Organism?

      A