Hydrological effectiveness and rural community perceptions of upland afforestation in Yunnan Province, Southwest China
Date
2010
Authors
Willson, Andrew James
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Abstract
Many studies have reported changes in the infiltration properties of soils after deforestation, whereas studies on the effects of reforestation or afforestation are scarce. On a watershed scale, it is often assumed that planting trees will increase infiltration, reduce flooding, increase dry season river flows and decrease erosion. In China, despite massive government-promoted land conversion programs occurring over the last 11 years, there is insufficient research on changes in fundamental soil infiltration properties which underpin the premise that afforestation will improve important hydrologic functions. This research, located in a poor upland farming region in Yunnan Province, southwest China, aimed to investigate the effects of monoculture plantation-style afforestation on key soil infiltration properties. This was primarily achieved by measuring hydraulic conductivity (with a tension infiltrometer) and bulk density at two depths across a mix of land uses -plantations of three different age classes, cultivated land and semi-natural 'primary' forest. Topsoil organic matter was also measured, as were plantation structural characteristics. Despite measurement uncertainties caused by heavy clay soil, soil water repellency and probable variable disturbance (past and present) of the soil's A-horizon, the results show that recovery of hydrological functions via improved infiltration properties cannot be assumed through plantation-style afforestation with Pinus species. Previous topsoil degradation was probably considerable, significantly inhibiting recovery potential especially in poorly managed and over-utilised plantations. It appears recovery is highly dependent on management-induced plantation structural characteristics. Intensely utilised thirty year old plantations with very sparse understories had inferior hydraulic characteristics and higher compaction then the prior land use of fallow-cultivation. Conversely, less-utilised and less-thinned 18-20 year old plantations with dense shrub understories and thick diverse leaf litter layers had enhanced infiltration characteristics compared to cultivated land uses and other plantation age classes. The 'primary' forest had significantly superior hydraulic properties than any other land cover, though topsoil measurements were also confounded by water repellency. The flat shape of the hydraulic conductivity curves and the relatively low saturated conductivities suggests that the heavy clay soil is poorly aggregated and that soil macropores are few in number and/or are poorly connected. In general it would appear that runoff is potentially generated quite readily even under moderate rainfall intensities and durations across most land uses. However, interception of rainfall by vegetation was not accounted for in this research. Hydraulic conditions may improve after initial wetting during rainfall events as the effects of water repellency decreases. A second component of the research explored the farming community's experiences and perceptions of afforestation, and the motivation for converting large areas of cultivated land to plantation without government support. On average, each household has converted 43 per cent of their cultivated lands to plantation in the last 10 years. The sole purpose for afforesting, from the farmer's perspective, is to increase future cash income by selling timber. However, there are numerous other important secondary uses of maturing plantations, such as sources of material for organic fertiliser and firewood, and grazing for domestic livestock. While the majority of farmers perceived positive general environmental benefits from converting large areas of arable land to plantations in their village, they were more divided on whether the plantations had positive or negative local hydrological effects. Assuming off-farm work opportunities continue to exist so that farmers can maintain increased cash income to supplement reduced crop production caused by afforestation, there are likely to be long-term socio-economic benefits to villagers.
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