Four centuries of forestry
When European settlers started displacing the native peoples of the Americas, one of the biggest changes was in the forests. In the north-eastern US, the land use was carefully recorded by the colonists, after 10,000 years of modest management by under story burning. Natural communities ruled until 1650, when firewood, building projects and agricultural clearing took a toll for 200 years. Half of the forest remained but in a very human-influenced ecology. Interestingly, industrialisation caused the abandonment of many farms and some reforestation, interrupted only by such events as the 1938 hurricane that devastated forests and briefly increased the lumber effort. Meanwhile the deer and the insects were adapting, but also becoming extinct.
At the moment, forest cover is reducing once more, although agriculture is in decline. Currently about 80% of the region is forested, with a mere 1% old growth forest because of that mixed history. That must mean a vast change from the wonderful "greenwood expanse," with all of the consumers that it supplied. One-third of the earth's surface is now under agricultural use, but half of our expanding population live in cities.
While Europe and North Africa still lack tree cover, despite some reforestation, other, more tropical areas have completely reverted to forest and their natural communities after civilisations have collapsed. The key question is how resilient are forest species to the environmental changes. The American North-East gives a prime example of how a large forest expanse might have maintained some of the rich fauna and flora that you would expect in a pristine ecosystem.
This paper is expansive in quantifying this region's response to large-scale shifts in land use over a long period. Archives of colonial surveys in 9 US states give data on "witness trees," for example. These individual specimens recorded the corners of sample plots, although size wasn't recorded. Fortunately the genus of tree was written down, after which these "witness trees" were used by town proprietor records. The regular, 6 square-mile proprietary towns were granted by colonies and states to absentee persons to encourage settlers to colonise and improve more land from 1620, with the English arrival, to 1825 when the Erie Canal was dug. In this way, more than 150,000 tree records have been utilised in the survey!
In combination with very small scale ecological notes, the history of quite a large number of forests can be followed. The American chestnut, Castanea dentata, was lost from a fungal infection and the progress of an awful disease can be followed from the early 1900s. Genera such as oak have increased in some areas, too, while decreasing elsewhere. Modern data was naturally useful, but suffered from a pre-colonial lack of small tree records. The latest small tree records therefore couldn't sadly be used.
1280 towns initially studied produced 701 suitable sets of data that helped to show how the modern forest composition differs from the ancient. Beech had a distinct decline overall, dipping from 22% to 7%, except in the Adirondacks. Oaks declined from 18% to 11% while the chestnut is virtually extinct. Among conifers, spruce decreased while the group of firs increased.
The eastern forests described here are largely unchanged but very much transformed. All the main tree groups remain in place. There have been many opportunities for invasion or loss, but the resilience is obvious. The present level of forest cover is a good guide to how much change has happened, although northern areas have the least amount of change, where the tree composition is still linked to some extent to the local environment.
Oak and hickory dominate early southern towns. They have little in common with spruce and fir forests in the north, except for the ubiquitous and increasing maple trees. Nowadays, there is no longer any regional sign of climate-driven gradients of tree composition.
There is an interesting pocket of beech where the most old-growth and even primary forest remains in the Adirondacks, but the decline of beech was probably the most significant among the towns.
Deforestation, logging and fire apparently leave the beech with no defence. Over 10,000 years, hemlock has always declined when drought and dry periods dominate. Its tannins being popular with colonists, its double decline at the time of the first colonists has now been made worse by the wooly adelgid, Adelges tsugae! It is surviving still in northern New England due to the effect of cold winter temperatures on the insect. This hemipteran was accidentally brought from East Asia in 1924.
Acer rubrum, Prunus serotina and various aspen (Populus spp.) are pioneer species, so the authors expect some succession to the ancient forest species to take place (if human influence is negligible). However there is no sign of this yet, possibly due to climate change, although 150 years is perhaps not enough for a major successional change.
The loss of the chestnut and the great decrease in oaks indicate that the succession to these species has a long way to go. Pockets, such as those in eastern Pennsylvania, are again are the great hope for oaks. The increase in white tailed deer, Odocoileus virginianus, populations can be held responsible for consumption of large numbers of acorns, but human influences can't be discounted so easily.
Pines were a very resilient group, but also dynamic because of their pioneering ability and the relative weakness to wind effect (and human cabin-building!) As native species, they have changed their abundance frequently, but as most data shows, the early successional species have thrived, even though the pine is a great survivor - because of its great age.
The homogenous forest is the fate we have given the world generally, and the Americas are no exception. The natural disturbance regime of small gap-openings in the forest provided seedlings, insects, birds, mammals and others with opportunity. That has been replaced with land clearing on the grand scale, even with that magnificent 80% tree cover. The animals now have little opportunity. The modern forests are not a true representation of what once was. Many components are missing and the processes that took millions of years to evolve must have gone without our even knowing what they were!
At least it looks right and the native species still predominate, and that's a darn sight better than in most of the "balding" continents.
Jonathan R. Thompson of the Smithsonian Institution Front Royal in Virginia, and his co-authors from Harvard and the University of Wisconsin publish the paper and findings.