| Afforestation is adopted as the main strategy to meet the demand for wood.The impact of human activity on environment is given high priority for research.Meanwhile environmental issues caused by human activities like ecosystem degradation,loss of biodiversity and global warming are worldwide concerned.Southern subtropical areas in China are often severely degraded,but also important timber production bases.Conducting afforestation and degraded rain forest rehabilitation experiments using indigenous species was an effective way to meet timber demand and to restore degraded ecosystem. Betula alnoidis is a fast-growing tree with very high value,widely planted in tropical and southern subtropical mountain areas.This study is to assess the adaptability of Betula alnoidis as a plantation species and suitable silviculture model,and to find out the impacts of human activities on regional ecosystems through a comparison between half-rotation aged Betula alnoidis plantations and the regional vegetation—mountain rain forests with its focus on the aspects of plantation biodiversity, community structure and soil etc.Community plot survey method was adopted to investigate species composition and structure. Braun-Blanquet system,aunkiaer system,S index,Shannon-Wiener H index and Simpson index were used to study the plant biodiversity.Flora composition of both plantation and nature forest was studied based on the seed plant genera distribution type in China by Wu-Zhengyi.Community similarity was estimated by common species and diversity index of the plantations.Biomass was assessed using methods of analytical wood,clear cutting and tree harvesting.Conventional method was used for soil analysis.Results showed that compared with nature community,impacts brought by planting Betula alnoidis were not remarkable in terms of life form spectrum,leaf form spectrum,flora composition and plant diversity in the plantation.Small differences in above aspects might be more likely caused by microenvironments.However remarkable impacts were observed in the plantation in terms of community physiognomy,community structure,species composition,species richness,and biomass accumulation.1) Planting trees made community structure simple.Betula alnoidis plantation had three layers including a dominant arbor layer,a complex and well developed shrub layer and a poorly developed vine layer.The arbor layer contained few species and had a single layer.The mountain rain forest had 4 layers including the dominant arbor layer,shrub layer,grass layer and vine layer.The vine layer was well developed and very rich in species composition,and the arbor layer contained as many as 38 species that could be grouped into 3 layers.2) Sequenced by the important value,main species for the plantation and the nature forest were totally different,and the plantation had one dominant species while the nature forest contained many dominant species.3) Planting trees changed the community physiognomy.The plantation defoliated in dry seasons while the mountain rain forest was tall and dense evergreen broadleaved forest and kept green in dry season.The mountain rain forest's arbor layer contained 3 layers.Relatively open space under the canopy,rare shrub,sparse grasses,very tall wood vines and rich epiphytes were found in it.4) Afforestation did not change the life form spectrum significantly.The Betula alnoidis plantation and the compared mountain rain forest had similar life form spectrum with phanerophyte accounted for the biggest percentage of above 80%,similar percentages for both hemicryptophyte and cryptophytes.However,some differences were also observed that the biggest part of life form spectrum for the plantation was small phanerophyte,followed by the middle phanerophyte,and that of the mountain rain forest was converse.5) The leaf form spectrum was not significantly changed by afforestation.The Betula alnoidis plantation and the mountain rain forest showed similar leaf form spectrum with some differences that the mountain rain forest had a few small leaves while the plantation did not.6) Species richness and species diversity were changed by afforestation.The rain forest had greater species richness in all layers.The fact indicated the rain forest was rich in species composition.The rain forest also had more layers in arbor layers.Although the two vegetation types had different species richness,the Simpson index and Shannon-Wiener index did not differ remarkably.7) Afforestation changed the flora components slightly.The plantation and the nature rain forest had similar basic components but with slight differences.8) Biomass assessed in the plantation was much lower than that of the mountain rain forest.Biomass in the rain forest was far more than that in the plantation of 12 years,especially for the shrub layer.But the biomass of litterfall collected in the plantation was greater than that of the rain forest for the reason that Betula alnoidis defoliated in dry seasons.9) Four years observation found that soil bulk density tended to increase first, then decrease after planting trees;total soil porosity and non-capillary porosity tended to decrease and after few years they recovered to different degrees;the soil cation exchange capacity for all forest types increased;in the soil of 0-30cm depth,soil salt saturation tended to decrease;nutrient content in the soil of 0-60cm depth in the young stand tended to decline;soil effective P,effective K and N contents all tended to decrease.Based on the above findings,the followings were generalized:1) Mixed species plantations established with indigenous species could maintain biodiversity and produce timber and non-timber products when they were sustainably managed.2) Plantations could also achieve both protection and production functions found in most nature forests.However replacing nature forests with plantations may simplify the ecosystems.3) Although planting trees shortened the time for the nature vegetation to get restored and brought some economic benefits,the plantation was still different from the nature vegetations.
|
PDF Full Text Request