The historic peat record: Implications for the restoration of blanket bog.

摘要

Peaty soils such as those found in blanket bogs lay down a history of their vegetation composition, along with the relative humification of these remains, which can be used to reconstruct bog surface wetness. These records highlight the variability of vegetation cover over time at blanket bog sites and could therefore be used to provide context to inform targets for the outcome of any restoration work. Because the vegetation present currently or in the recent past on the surface of a bog might be due to a relatively recent change, a longer-term history would give a different baseline to aim for. This should be relatively easy to achieve at some level for sites that have had cores taken, for which a database already exists. Historical records show a general picture of natural cycles in peat wetness and vegetation cover, and highlight a large level of variability that is sometimes found over even quite small areas in the timing of peat initiation and the types of vegetation present. This local variability suggests that topography (which varies more within a small spatial extent than climate or edaphic conditions) is a strong determinant of peat growth. However, climatic factors are also important. At some sites, past recovery of Sphagnum has been associated with wetter, cooler phases in the climate. Because rainfall and temperature are likely to change in future with continued climatic change, reconstructed climatic conditions at blanket peat sites could be used to determine whether the projected future climatic conditions at a potential restoration site are likely to fall within the range that supported peat formation in the past. This would help determine whether any restoration work has a chance of succeeding in the long term, and a database of sediment cores has been assembled for England that could be used as a starting point for this. One potential management strategy would be to accept that some vegetation that is currently considered undesirable has been present on blanket bogs in the past, and allow enough time and space for natural cycles to occur. However, what is not known is how much space would be needed to allow such natural cycles to continue, which sites are undergoing change as part of a natural cycle and which are not, or whether these natural cycles might be expected to continue in the same way in the future under climate change. There is some evidence for human impacts on blanket bogs in the past; the use of marginal lands for grazing and agriculture appear to have stopped the spread of peat in some areas, and in some other areas deforestation by humans appears to have been instrumental in initiating peat production. This demonstrates that there is considerable potential for management actions to impact the character of blanket bogs – based on inductive reasoning it seems likely that the reversal of some negative factors (such as blocking drains and removing plantation forestry) will result in the restoration of hydrological function, which could drive further changes in vegetation away from dry heath and trees to Sphagnum-dominated blanket bog that actively forms peat. However, care should be taken when choosing species to re-vegetate bare peat, since Calluna is associated with drier areas. What is not currently known is whether this is a cause or effect of drier conditions, but it is possible that the introduction of Calluna to the bog surface might result in drying of the peat, which would slow the rate of peat formation. Sphagnum is associated with lower emissions of methane and is a main peat forming species, so its suitability for re-vegetating bare peat is likely to be higher if it can become established. The timing of the initiation of erosion at some sites is well before the nitrogen deposition that is often implicated in this, and erosion has not always been caused by human activity. Indeed, some bogs have switched between erosion and accumulation over time, and others have recovered naturally from human impacts. However, what makes some bogs more resilient than others or causes the switch between erosion and accumulation is not known, and further research here would be beneficial to inform restoration management.