Secondary successions in abandoned fields

abandoned old fields: succession to forest in North America .

... but to grassland in China

Successions on old fields have been studied particularly along the eastern part of the USA where many farms were abandoned by farmers who moved west after the frontier was opened up in the 19th century (Tilman, 1987, 1988). Most of the precolonial mixed conifer-hardwood forest had been destroyed, but regeneration was swift. In many places, a series of sites that were abandoned for different, recorded periods of time are available for study. The typical sequence of dominant vegetation is: annual weeds, herbaceous perennials, shrubs, early successional trees and late successional trees.

Old-field succession has also been studied in the productive Loess Plateau in China, which for millennia has been affected by human activities so that few areas of natural vegetation remain. The Chinese government has launched some conservation projects focused on the recovery of damaged ecosystems. A big question mark is whether the climax vegetation of the Plateau will prove to be grassland steppe or forest. Wang (2002) studied the vegetation at four plots abandoned by farmers for known periods of time (3, 26, 46 and 149 years). He was able to age some of his plots in an unusual manner. Graveyards in China are sacred and human activities are prohibited in their vicinity - gravestone records indicated how long ago the older areas had been taken out of agricultural production. Of a total of 40 plant species identified, several were considered dominant at the four successional stages (in terms of relative abundance and relative ground cover). In the first stage (recently abandoned farmland) Artemesia scoparia and Seraria viridis were most characteristic, at 26 years Lespedeza davurica and S. viridis dominated, at 46 years Stipa bungeana, Bothriochloa ischaemun, A. gmelinii and L. davurica were most important, while at 149 years B. ischaemun and A. gmelinii were dominant (Figure 16.11). The early successional species were annuals and biennials with high seed production. By 26 years, the perennial herb L. davurica, with its ability to spread laterally by vegetative means and a well-developed root system, had replaced A. scoparia.

Artemesia scoparia

-■- Seraria viridis

Lespedeza davurica

-*- Stipa bungeana

Artemesia gmelinii

-*- Bothriochloa ischaemun

26 46

Successional stages

26 46

Successional stages

Figure 16.11 Variation in the relative importance of six species during an old-field succession on the Loess Plateau in China. (After Wang, 2002.)

The 46-year-old plot was characterized by the highest species richness and diverse life history strategies, dominated by perennial lifestyles. The dominance of B. ischaemun at 149 years was related to its perennial nature, ability to spread clonally and high competitive ability. As in Tilman's (1987, 1988) North American studies, soil nitrogen content increased during the succession and may have facilitated some species in the succession. Wang concludes that the grass B. ischaemun is the characteristic climax species in this Loess Plateau habitat, and thus the vegetation seems likely to succeed to steppe grassland rather than forest.

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