Case Studies On Adaptations Of Vegetation To Disturbance And Roles Of Disturbance In Determining Vegetation Processes

Effects of disturbance on vegetation can be summarized in two categories: 1) adaptations of species or vegetation to disturbance; and 2) roles of disturbance in determining vegetation processes. Plastic responses of plant species to disturbance, tradeoffs between variables within populations in response to disturbance, differentiation of plant functional types in plant communities (or vegetation) in relation to disturbance, and roles of disturbance in dispersing diaspores, determining seed composition, species composition and above-ground biomass have attracted extensive attentions of ecologists in recent years. To determine the trait variations associated with disturbance history and current disturbance regime as well as the interactions between these factors, global-scale comparisons of numerous individual studies are required. Therefore, several case studies concerning 6 aspects are presented in this thesis.Plastic responses of plant species to disturbance. 20 species from habitats of widely varying moisture status, ranging from marsh to desert in three countries, were selected to compare the morphological responses of their seedlings' roots to drying soil.Trade-offs between variables in populations in response to disturbance. A study on the roles of an altitudinal gradient, sand burial and cutting in Sophora moorcroftiana population characteristics was conducted.Differentiation of plant functional types in plant communities in relation to disturbance. According to existed literature, an analysis on vegetation dynamics concerning shifting sand reversion and habitat drying at Shapotou was carried out with reference to plant functional types.Role of disturbance in dispersing diaspores. Based on the surveys of seed bank patterns and seed dispersal ways of Sophora moorcroftiana, the roles of dispersal agents such as wind force, flowing water, animals and gravity, which are sources of disturbance in dispersing seeds of S. moorcroftiana, were discussed.Role of disturbance in determining seed composition. In the Horqin Sandy Land, soil seed bank characteristics of sandy steppe under different grazing regimes were explored.Roles of disturbance in determining species composition and above-ground biomass.Based on the investigation of shifts of species richness, above-ground biomass and importance value shift of Artemisia intramongolica and A. frigida at Bashang in Hebei Province, the vegetation process, and especially, the indicating roles of the two Artemisia species in the desertification process were discussed. The conclusions in specific adaptations to disturbance and roles of disturbance on vegetation processes derived from these case studies are listed here.(1) The comparison of the morphological responses in seedlings' roots to drying soil showed that 1) morphological plasticity in seedlings' roots is one of the adaptive mechanisms of plants to drying soil; and 2) seedlings of species from dry habitats make more appropriate morphological responses to the onset of drought than do seedlings of species from wet habitats.(2) The study on performances of Sophora moorcroftiana showed that trade-offs between rapid and slow growth, sexual and vegetative reproduction in populations are important adaptations of plants to disturbances.(3) The study on vegetation dynamics at Shapotou showed that 1) with the changes of disturbance types, vegetation processes are characterized as the shift of primary plant functional types; and 2) compared with species composition and lifeforms, plant functional types have more universal significance in describing vegetation dynamics.(4) The study on seed dispersal of Sophora moorcroftiana showed that 1) wind force, flowing water, animals and gravity all contributed to the diaspore dispersal and the formation of population patterns although their effects on seed distribution were different; 2) disturbance has positive effects on vegetation.(5) In the Horqin Sandy Land, with the increase of grazing intensity, seed composition and seed number in the soil seed bank changed in four aspects: 1) seed bank density declined; 2) seed composition proportion of annuals increased; 3) seed number proportion of annuals decreased, but that of leguminous species increased; 4) the correlation between seed number in the seed bank and species frequency in the vegetation became poorer. The influence degree of grazing on soil seed bank is one of indexes in determining grazing intensity.(6) At Bashang, with desertification occurring, the vegetation structure was modified in two aspects: 1) invasion of shrubs; 2) the increase in the proportion of annuals. However, the above-ground biomass did not change linearly. A conclusion is that degree of species dominance is more sensitive to desertification than above-ground biomass, and that speciescomposition and indicating species, rather than biomass, seem to be the more important factor in determining desertification intensities.These case studies are significant in ecological restoration and vegetation management.(1) To successfully establish artificial vegetation on sand dunes in arid area through seeding is greatly relies on the emergence and survival of seedlings. It might be helpful to select species, which can adjust their root morphologies and exploit deeply buried water resources in the seedling phase in drying soil, as sand binders.(2) The characteristics of Sophora moorcroftiana (such as trade-offs in growth and reproduction, morphological characteristics which may have drought-resistant significance, long life span and slow growth) in response to disturbance mean that it can be selected as a sand binder in its distributed area.(3) In sand-fixing practice, psammophytes, ruderals and drought tolerators have different functions. An expected result can be realized if vegetation management protocol is established based on the analysis of roles of different plant functional types in vegetation process.(4) In the Horqin Sandy Land, because the treatment of 6 sheep unit/ha had caused the evident decrease in seed density of soil seed bank and strikingly poor relativity between seed number in the seed bank and species frequency in the vegetation, it should be taken as intense grazing regime.(5) At Bashang, Artemisia intramongolica is an indicator of serious desertification. A. frigida is an indicator of grassland degradation due to over-grazing, or of vegetation restoration after serious desertification. Therefore, when carrying out restoration projects of desertified land at locations like Bashang, Artemisia frigida could be introduced to prevent desertified lands from further deterioration, Artemisia intramongolica could be introduced to rapidly restore the desertified land.