Читать книгу Ecology - Michael Begon - Страница 99

Changes to farming practices mean that there are an increasing number of cases of agricultural land being abandoned. Whenever this happens, there is an understandable hope that the natural habitat that had been destroyed to make way for farming can be restored. One example is the swampland dominated by bald cypress trees, Taxodium distichum, lying along the rivers and streams of the Gulf coastal floodplain of North America, running from south‐eastern Texas north and east to the Atlantic Ocean, and including a study site along the Cache River in southern Illinois (Middleton, 2003). Agricultural development expanded there in the 1950s, and by the late 1980s only about half of the forested swampland remained. However, the process has been halted and reversed, in part by a crash in the soybean market, and the emphasis now is on plans to restore the original swamplands, with the provision of habitat for hunting being a particular commercial driver, though there are Nature Preserve areas where hunting is not allowed.

In fact, restoration has proved difficult. The original species‐rich forests supported up to 60 or so species of trees, shrubs and vines, many of them with seeds dispersed in the seasonal floods, but the forests developing following agricultural abandonment tend to be dominated by a few species with wind‐dispersed seeds. An important question, therefore, is what potential seed banks have in promoting more natural restoration. To address this, Middleton (2003) assayed the seeds from nine sites in intact bald cypress swamps and 51 sites in the area that had been farmed for between one and 50 years. She found not only that there was no relationship for the dominant swamp species between the length of time farming had been practiced and seed abundance – but actually, the seeds of many of those species, including bald cypress itself, were absent from the seed banks altogether in both the farmed and intact sites. Instead these were composed mostly of seeds from large numbers of herbaceous species. It seems, therefore, that in this case seed banks can have little part to play in habitat restoration, and that abandonment alone, even of land that has been farmed for a relatively short time, offers little prospect of a return to natural habitat. Rather, the short‐lived seeds of the dominant woody species of bald cypress swamps are likely to return and promote successful restoration only if flood pulsing across the landscape is re‐engineered, reconnecting pristine to abandoned sites.