Understory plant diversity assessment of Eucalyptus plantations over three vegetation types in Yunnan, China

摘要

Biodiversity in managed plantations has become an important issue for long-term sustainability of ecosystems. The environmental effects of plantations comprised of fast-growing introduced trees have been vigorously debated. On one hand, monocultures have been said to exhaust resources, resulting in decreased biodiversity. Conversely, it has been stated that monocultures may favor regeneration of undergrowth plants from surrounding forests, increasing biodiversity. In order to clarify the effects of planting Eucalyptus trees on species composition, diversity, and functional type of understory vegetation in Yunnan province, a field trial was implemented to compare Eucalyptus plantations (EPs) with two other local current vegetation types (secondary evergreen forests (SEs), and abandoned farmlands (AFs)). Each vegetation type was sampled in each of three elevational ranges (low = 1,000-1,400 meters above sea level (masl), medium = 1,400-1,800 masl, and high = 1,800-2,200 masl). Sample sites within each elevational range had similar environmental characteristics (slope, aspect, etc.). Thus, we sampled three vegetation types at each of three sites at each of three elevations for a total of 27 plots. We calculated relative abundance and importance value of species and diversity indexes to evaluate differences among local current vegetation types and elevational ranges, employing multivariate ordination analyses and other methods such as Analyses of Variance (ANOVA) and Indicator Species Analysis. We found that fast growing introduced Eucalyptus plantations led to reduced plant diversity in the study area, and that rare or threatened species were recorded almost exclusively in the SE plots, being essentially absent from the EP and AF plots. The understory plant diversity did not correlate with the altitude gradient significantly. Eucalyptus plantations (EPs) have a simpler community structure than that of either secondary evergreen forests (SEs; similar to natural state) or abandoned farmlands (AFs). No variable significantly explained variation of the understory shrub layer, but soil moisture-holding capacity and overstory coverage were significant in explaining variation of the understory herb layer, suggesting that the study of soil physical properties is necessary for better understanding of their importance in Eucalyptus plantations and other local current vegetation types.