The impact of broadleaved woodland on water resources in lowland UK: III. The results from Black Wood and Bridgets Farm compared with those from other woodland and grassland sites

摘要

In the United Kingdom the planting of broadleaved woodland has led toconcerns about the impact on water resources. Comparative studies, typicallyusing soil water measurements, have been established to compare water use ofbroadleaved woodland and grassland. The diversity of outcomes from thesestudies makes it difficult to make any consistent prediction of thehydrological impact of afforestation. Most studies have shown greater dryingof soils under broadleaved woodland than under grass. However, two studies ina beech wood growing on shallow soils above chalk at Black Wood, Micheldever,Hampshire showed little overall difference between broadleaved woodland andgrass, either in soil water abstraction or in evaporation. Two factors arethought to contribute to the different results from Black Wood. It is knownthat evaporation can be considerably enhanced at the edges of woodlands or insmall areas of woodlands. The studies at Black Wood were made well within alarge area of fairly uniform woodland. Other studies in which a differenceoccurred in soil drying between broadleaved woodland and grass usedmeasurements made in small areas of woodlands or at woodland edges. Anotherimportant difference between comparison of woodland at Black Wood andgrassland growing nearby, also on shallow soils above Chalk, compared toother broadleaved woodland/grass comparisons, growing on other geologies, isthe influence of the Chalk. Although vegetation such as grass (and woodland)does not populate the chalk profusely with roots, water can be removed fromthe Chalk by the roots which proliferate at the soil/chalk interface andwhich can generate upward water movement within the Chalk. Published workshowed that only in a very dry summer did the evaporation from grass growingon shallow soils above chalk fall below potential. In broadleavedwoodland/grass comparisons on non-chalky soils it is possible that moisturedeficits in the soil below the grass may reach critical levels and reduceevaporation below that of the woodland with which it is being compared.