Changes in the Amount of Forest on the Great Plains

At the end of the last ice age,some 8,000 to 10,000 years ago, the climate was cool and damp,and a dense coniferous (evergreen) forest occupied the Great Plains,the vast region stretching from Alberta,Saskatchewan,and Manitoba in Canada south through Texas.A dark mantle of spruce trees extended from what are now the Canadian prairies south through the Dakotas into the central states of the United States,while pinewoods appear to have flourished on the southern plains of the United States.But as the chill of the glaciation gradually lifted,the climate eased into a drier and warmer phase, marked by more frequent droughts,and the forest was forced to retreat to the north.Within a remarkably short time,the forests of the Great Plains either surrendered directly to grasses or else gave way first to deciduous trees (trees that lose their leaves seasonally)and then to prairie grasses.

By the time the transformation was over,coniferous forests could be found in the central United States only on the crowns of the tallest breaks and ridges.High enough to catch the rain and snow,and cooler than the grasslands below,these uplands created a microclimate in which the trees could retain a toehold.The thin mineral soils of the ridges-more suitable for conifers than for grasses or other plants-probably also gave the trees an advantage. The factor that made the biggest difference to the scarp forests (forests on a line of cliffs),however,was their high,isolated location. Historically speaking,the greatest threat to prairie woodlands,apart from prolonged drought,was the fierce heat of grass fires.Where better to find refuge than atop a natural firebreak?The present distribution of scarp woodlands therefore probably represents the limits of past wildfires.

For the woody plants that grew down on the grasslands,by contrast,there was no chance of escape.Despite a variety of adaptations for withstanding occasional burns-corky,fire-resistant bark (bur oak trees);seeds that are stimulated by heat (ponderosa pine trees);roots that put out new shoots to compensate for fire damage (many poplars and oaks)-few species of trees can survive frequent,intense fires.Grasses,by contrast,are basically born to burn.Not only do they produce a tinder-dry thatch of dead foliage that lights with the slightest spark,but they are equipped to rise from their own ashes.The buds,or meristems,from which they put forth new growth,are tucked down at the surface of the ground,where they are protected from serious harm.But woody plants,which are inclined to reach for the light,hold their buds on the tips of their branches,where they are exposed to the flames and are sorely vulnerable to fire damage.

In the days when vast herds of buffalo roamed the prairie,frequent grass fires conspired with severe drought to limit the spread of woody plants.In the sometimes-lengthy interludes between the killing droughts,trees and shrubs were often able to take advantage of cool, moist weather to extend their reach onto the grassy plains.But no sooner were the trees established than some random bolt of lightning would set the prairies aflame,killing the shrubs and most trees except in the humid streambeds and river valleys.On the tall-grass prairies, in particular,where catastrophic droughts were relatively infrequent, trees and shrubs might have taken over completely if it had not been for the erratic but inevitable return of lightning.

Prairie fires were also set by Native people,who used burning as a tool to hold back the growth of woody plants and maintain the grasslands as pasture for buffalo.But with the introduction of intensive farming,prairie fires were suppressed and trees began to make a slow but steady advance over the last 200 years.This expansion of woody vegetation has now been documented in most ecoregions across the Great Plains.Some researchers have suggested that overgrazing of grasslands by cattle in the nineteenth century created openings where woody plants could grow. Alternatively,global rather than local forces may be at work.Recent evidence suggests that rising concentrations of atmospheric carbon dioxide may be giving woody plants a competitive advantage over nonwoody plants.But whatever other factors may be involved,the suppression of natural fires has certainly played a central role in the intrusion of woody plants into the grasslands.?

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?At the end of the last ice age,some 8,000 to 10,000 years ago, the climate was cool and damp,and a dense coniferous (evergreen) forest occupied the Great Plains,the vast region stretching from Alberta,Saskatchewan,and Manitoba in Canada south through Texas.A dark mantle of spruce trees extended from what are now the Canadian prairies south through the Dakotas into the central states of the United States,while pinewoods appear to have flourished on the southern plains of the United States.But as the chill of the glaciation gradually lifted,the climate eased into a drier and warmer phase, marked by more frequent droughts,and the forest was forced to retreat to the north.Within a remarkably short time,the forests of the Great Plains either surrendered directly to grasses or else gave way first to deciduous trees (trees that lose their leaves seasonally)and then to prairie grasses.?

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