Education & Training

Effects of Shrub Islands on Forest Patch-clearcuts in Alpine Region

Update time: 07/01/2010

As a bridge from herbs to the woody stage, pioneer shrubs would play important role in the process and trajectory of secondary succession after forest cutting. In the present study, therefore, we selected isolated shrubs from four shrub species (Cerasus trichostoma, Ribes glaciale, Rosa omeiensis and Salix sphaeronymphe) on cutovers in Alpine and Plateau, eastern Tibetan Plateau, to evaluatethe effects of shrub islands size or species on microclimate, soil, herbaceous community and woody seedlings regeneration by field investigations and laboratory testing. Key results and conclusions are as follows:

1)    By lowering light intensity and surface air temperature, and enhancing air relative humidity, shrub island formed a different microclimate from open field (CK). With increasing shrub area, the light intensity and the surface air temperature significantly decreased, while the air relative humidity increased. Differences in microclimate effect among four shrub species were found: in a given shrub area, R. glaciale had the lowest light intensity and air temperature, and the highest air relative humidity; Contrary to R. glaciale, S. sphaeronymphe had the highest light intensity and air temperature,and the lowest air relative humidity. Ameliorated microclimate on shrub islands would facilitate plantation success since failure of plantation in this region is attributed to severe climate.

2)    Contents of soil organic matter, total organic carbon and total nitrogen in soil surface layer(0–20cm) were higher under shrub islands than in open field, and increased with increasing shrub island area, although shrub islands did not affect total phosphorus. There were significant differences in soil nutrients (except total phosphorus) among four shrub species were found: in a given shrub area, the content of the soil organic and the total nitrogen were the following order: R. glaciale> R. omeiensis≈ C. trichostoma> S. sphaeronymph. Shrub islands facilitate restoration of fertility on cutovers. Moreover, spatial heterogeneity of fertility can be of great importance to evaluate the dynamics of soil nutrients after forest clear-cutting.

3)    Shrub islands changed the composition of water stable aggregate and related total organic carbon and total nitrogen in soil surface layer (0–20 cm). With the increasing shrub area, mass proportion of microaggregate(0.25-0.053 mm) and silt+clay(<0.053 mm) gradually decreased, macroaggregate(2-0.25 mm) significantly increased, megaaggregate(8-2 mm) did not vary. As a result, shrub islands improved value of mean weight diameter compared to open field (CK), then enhanced ability of soil and water conservation. Content of total organic carbon and total nitrogen related to aggregate obviously increased, but total phosphorus did not vary with the increasing shrub island area. It was concluded that shrub islands improve soil and water conservation and soil fertility by affecting component of soil water aggregates and relevant nutrients.

4)    Presence of shrub islands increased richness of herbaceous plants, but decreased cover and biomass. Two herb functional groups showed reverse trends with shrub island area: cover and biomass of grasses decreased while forbs increased. It suggested that shrub island facilitate herbaceous community succession, and that reduction of biomass and cover is attributed to combination of forb and grass groups.

5)    Number and species richness of woody seedlings increased with island area for all shrub types except Ribes glaciale, and were higher than open field. In a give area, both the number and species richness under S. sphaeronymphe were higher than those under C. trichostoma, R. glaciale and R.omeiensis. It indicated that shrub island facilitate natural regeneration of woody plants, and accelerate pace of translating herbaceous stage into woody stage.

6)    Shrub islands improved standing ratio of woody seedlings in artificial regeneration, but inhibited growth of current seedlings by reducing biomass of leaf, stem and root. For both early-succession woody species(Spiraea myrtilloides and Potentilla glabra) and late-succession ones(Picea likiangensis var.balfouriana and Picea asperata), most of characteristics of morphology and physiology did not performance regularity with increasing shrub area. It implied that shrub islands facilitate artificial regeneration of woody plants.

“microhabitat islands”, “fertility islands” and “nucleation succession island” of shrubs occurred on forest cutovers in alpine region. With respect of effects of shrubs on natural succession and artificial restoration, therefore, protection and utilization for shrub islands can be crucial part of management on cutovers in the alpine region, eastern Tibet Plateau.