Examples of forest change

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9.4.1 Wistman's Wood, Devon

This ancient ecosystem (see Box 1.3, p.36) has changed considerably since Hansford Worth recorded a 0.06 ha sample of a typical area in 1921. Further systematic records of the Worth site were made in 1965 (when it was relocated and enlarged to 0.33 ha), 1987 and 1997. The latter two records were of a 0.45 ha near-rectangular area covering much of the southern end of South Wood which included nearly all the 1965 plot. The wood has been much studied by ecologists, so many aspects of its history over the last century are well known. Following the Worth survey came a period when grazing intensity diminished, allowing tree regeneration. This ended soon after the prolonged snow cover of 1962/3, when much of the rowan was debarked and some stems killed by starving sheep. Considerable damage, particularly to old trees, resulted from heavy snowstorms in February 1978. Branches and trunks were snapped, while many boughs were weighed down and subsequently remained prostrate.

Oaks present at the beginning of the twentieth century, that had long been subjected to grazing and severe weather conditions, were low in height and often multi-stemmed. Transplantation to a garden at low altitude demonstrated that the dwarf form did not have a genetic basis. A suggestion that the stunted condition of the oaks was the result of repeated defoliation by winter moth Operophtera brumata has not been generally accepted, as attacks by this species were only sporadic. While the defoliation checked growth it did not usually kill entire branches. Some individual trees show remarkable sequences of growth, degeneration and recovery. A tree photographed in 1892 with a low but full and apparently healthy crown, had degenerated substantially by 1921, but is shown with a new regenerated crown in a photograph of 1973.

The oaks now present belong to three generations. The A generation consists of the oldest trees; these were present as stunted wind-cut trees in

1900. The B generation consists of the erect-trunked individuals which developed after 1900, while the C generation is formed by all individuals present as saplings below 1.3 m in height in 1965 and subsequent recruits. The 1997 plot map given by Mountford et al. (2001) shows oaks of all three generations, these having been distinguished on the basis of size, vigour and growth form. Past records, including those of trees which have died, enabled rates of increase in canopy height to be calculated for the periods 1965-79, 1979-87 and 1987-97. The A generation did well for the middle period, but the total rate of height increase by the B generation trees from 1965-97 was much greater. Wistman's Wood has greatly changed in the past century. An oak population which from 1600 until the mid-nineteenth century consisted of 'large bushes', and whose canopy trees had in 1900 an average height of less than 4 m, had individual trees 12 m high and an average canopy tree height of around 8 m a century later. Climate amelioration commencing as early as the mid-nineteenth century appears to be the primary cause of these changes.

9.4.2 Forests of New England

The eastern seaboard of the USA, from Connecticut up to southern Maine, has a number of old-growth forests interspersed with large tracts of what looks like old forest. But in many cases these forests date back less than a century and, as described by Foster (1999) and Foster & Aber (2004), are the latest version of the landscape in a long series of human-induced changes.

Early indigenous people of the eastern seaboard were comparatively few in number but given their skills in manipulating fire, it is highly likely that they had a significant effect on the forest. Certainly, around their habitations, they cleared areas that are recorded as being a few tens of miles in width. It is also highly likely (though not universally agreed) that their influence on the landscape was much wider, resulting in widespread alteration of the landscape, changing the density and species of trees, and creating more clearings over large areas. Archaeological remains of the Iroquois in eastern North America are associated with high charcoal levels and a change from beech Fagus grandifolia and sugar maple Acer saccharum to oaks and white pine Pinus strobus.Thiscreated the first 'artificial' landscape of New England.

The arrival of Europeans took this change much further with a rapid and larger-scale conversion of the forest into agricultural land. By the early 1800s more than 60% of the land was in open fields, with the forest remaining as scattered small woodlands. This taming of the landscape was possible (and necessary) due to the small, evenly spread population, having to live off the landscape. So much woodland was removed that by the mid 1850s, wood for everyday use and heating was in short supply and highly sought-after (Foster, 1999).

Beginning in 1830 the industrial revolution arrived, leading to a steady abandonment of the hard-won agricultural land. People moved to the growing urban areas, others migrated westwards to better land or the lure of the gold fields even further west. We have a unique window into this time of abandonment, as Foster (1999) shows, through the prolific journals of Henry Thoreau (1817-1862) who lived in New England as agriculture was on the wane. Land that over two centuries had been made suitable for productive agriculture was now abandoned. This led to an inevitable successional change as weeds, scrub and finally trees invaded the former farmland. White pine, pitch pine P. rigida, red cedar Juniperus virginiana were the commonest pioneers, with red maple Acer rubrum or birches on ploughed land. With time, oak came to dominate these pine forests, and they acquired the look of antiquity, yet old walls, piles of rocks moved from fields, ruined buildings and other artefacts in these forests attest to their former agricultural background (Fig. 9.16).

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