Hydrological impacts of reafforestation with eucalypts and indigenous species: a case study in southern China

Abstract

Restoration of forest ecosystems on degraded lands in the subtropical area of southern China may be achieved by planting exotic pioneer species followed by enrichment planting with indigenous species and promotion of understorey establishment. A 10-year study was conducted in a degraded area of coastal western Guangdong Province to quantify the effectiveness of eucalypts and subtropical mixed forest for ameliorating microclimate and reducing surface runoff and erosion, and to examine the importance of understorey and litter layer development in achieving these effects. Tree growth, climate, litter biomass and understorey cover, throughfall and stemflow, surface runoff, soil erosion, soil moisture and watertable depth were monitored in three unreplicated catchments containing bare land, Eucalyptus plantation and mixed forest, respectively. Litter was regularly removed from most of the Eucalyptus catchment, but allowed to accumulate in a section fenced to limit access. Both forest types increased absolute humidity and reduced maximum temperature near the ground compared to bare land. The mixed forest canopy intercepted more rainfall and generated less stemflow than the Eucalyptus forest. Surface runoff was greatest from bare land, and least from mixed forest. Annual runoff from the forested catchments (but not the bare land) decreased over 10 years of measurement. Runoff from the protected area of the Eucalyptus catchment also decreased as litter accumulated and an understorey developed. Soil erosion from bare land was greater than from the forested catchments, and its surface runoff carried a higher proportion of coarse sediments. Soil moisture content was highest in the bare land catchment, but did not show a long-term trend in the vegetated catchments. Watertable depth averaged 30 cm deeper beneath mixed forest and 80 cm deeper beneath Eucalyptus forest, compared with bare land. The results of this case study support the use of eucalypts as a pioneer species in the rehabilitation of degraded lands in southern China, but demonstrate the importance of allowing litter accumulation and understorey development beneath the tree canopy and the additional hydrological benefit of encouraging succession to a mixed forest ecosystem.