Ecological Processes And Mechanisms Of The Recovering Succession Of Abandoned Farmland In The Hilly And Gully Loess Region Of North Shaanxi

Taking the abandoned farmland in the hilly and gully Region of North Shaanxi as its subject, the study investigated the variations in the structural characteristics, species diversities and niches of communities on abandoned farmland during its recovering successions by the substitution method of time sequence for space sequence and the field survey and lab combined analysis, and examined the responses of recovering succession patterns and soil qualities of abandoned farmland to vegetation recoveries in combination with the dynamic variation in its soil physical chemical and biological properties so as to probe into the relations between community structure and environmental factors and then the mechanisms of vegetation recoveries. Its main conclusions are as follows:1.Dynamic characteristics of the communities on abandoned farmland during its recovering successionDuring the whole succession, the plant communities were dominated mainly by dicotyledons, secondarily by monocotyledons, and thirdly by gymnosperms pteridophytes. In the region under study, the large plant classes and groups had relatively simple structures and relatively small numbers of plant species. Composite, Gramineae and Leguminosae species played the most important role in the vegetation recoveries. L.davurica, Stipa bungeana, Artemisia gmelinii and Bothriochloa ischaemum existed through the whole succession, occupying an important position in the community successions and being of great value in maintaining the communities stable. During the succession of abandoned farmland, the dominant species alternatively and repeatedly appeared and the community successions tended to conform. The life types of the species were perennial herbs in most cases, the sub-shrubs and under-shrubs occupied certain proportions, and the shrubs and arbors occupied very small proportions. The life types of the species were perennial herb dominated and contained certain proportions of sub-shrubs and under-shrubs.The life types mainly embodies their variations in their reproduction strategies, 1-2 year old herbs taking R-strategy during their life history and perennial herbs, sub-shrubs and under-shrubs basically taking K-strategy during their life history.The numbers of the communities tended to increase as their successions prolonged, but their biomasses tend to decline with the farmland-abandoning time. The underground biomasses mainly distributed in 0-60cm soil and no longer distribute below 100cm deep. As the soil depth increased, the root biomasses gradually decreased.During the vegetation recoveries, the species diversities presented different patterns. Some communities hadα-species diversity indexes tending to increase with the farmland-abandoning time, some hadα-species diversity indexes tending to stabilize within certain limits and others hadα-species diversity indexes tending to rise. The analysis of theβ-species diversity indexes showed that although the quadrats under study were far away from one another in time sequence and thus their communities were isolated in microhabitat and lie differently in space sequence, their species isolations appeared not remarkable. Therefore, the species diversity variations were dynamic during the recovering succession of abandoned farmland and their mechanisms of the variations need to be further studied.2. Responses of soil properties of abandoned farmland to its vegetation recoveriesIn the vertical soil profile, as the soil depth increased, the soil water contents tended to gradually increase. The soil water contents of the different sites ranked in the decreasing order of north-facing slopeland > semi-north facing slopeland > south-facing slopeland >semi-south-facing slopeland. The monthly soil water contents were limited by many factors and thus did not presented a consistent pattern. The year to year variations of the soil water contents showed that farmland abandoning did not recover soil water content.According to the study, the variations of the soil water contents between the different years, months and the sites did not present very consistent patterns, but their general tendencies were that during the succession of abandoned farmland, the soil water content did not evidently recover. The succession of abandoned farmland rendered its soil bulk weight to decrease.The soil nutrients mainly distributed in topsoil, thus varying only slightly in deeper soil and having higher contents in upper soil than in lower soil. in the early farmland abandoning period the soil organic matter contents, nitrogen totals, available nitrogen, potassium and phosphorous contents, and organic carbon contents of the communities were higher and with the farmland-abandoning time they tended to increase but their absolute contents increased slightly .The soil biological properties presented following several tendencies: with the succession time, some of the soil microbial parameters tended to increase, some of them tended to stabilize, and several of them tended to decline, and some of them tended to fluctuate.Because soil biological properties sensitively indicated soil quality variations, they performed better than soil chemical and physical properties in indicating soil quality responses to dynamic succession of abandoned farmland. Thus it was proposed that soil chemical, physical and biological properties be employed as the indicators for assessing soil quality responses in research.3. Succession direction of abandoned farmlandThe dynamic analysis of the numbers and structures of the communities showed that with the time, the structures of the communities became complicated and their natural resource-exploiting capacities gradually increased. With the succession, the soil physical and chemical indicators and some microbial indicators tended to increase. Ecological recovery involves two respects, vegetation recovery and soil recovery, and taking into account a full range of all its relevant respects, the succession of abandoned farmland in the hilly and gully loess region was concluded as a progressive succession.4. Vegetation-recovering mechanism of the succession of abandoned farmlandIn most of the quadrats, the species with the widest niches were dominant or subdominant species, which played a dominant role in determining the natures and environments of the communities to which they belonged, so that they could more fully exploit their environmental resources, better adapt themselves to their environments and take advantageous positions in competition. The different major populations had highly overlapped niches and the dominant and subdominant species in particular had highly overlapped niches; and these overlaps made the different species compete with one another for more resources and as a result the communities inevitably succeeded towards the directions in favor of their resource exploitations, thus resulting in their niches differentiating. This was the driving force of the species replacement in the vegetation-recoveries of abandoned farmland as well as the ecological mechanism of its vegetation successions.The successions of the plant communities were the processes whereby the plant communities and soils mutually interacted with and acted on each other, the soil fertilities affecting the colonization and replacement of the dominant species and the vegetation recovery promoting the soil development. In this co-succession, the different soil property indicators took different effects and among them, the farmland-abandoning times, slope aspects and gradients, nitrogen totals, organic matter contents, soil respirations and phosphatase activities were the key factors that affected the vegetation successions, and the microbial-biomass nitrogen contents and soil respiration quotients had stronger effects on the vegetation recoveries.The successions of the plant communities were the dissimilating processes of their original forms whereby they embodied their variations not only in their species compositions but also in their environments, and the plant successions were the processes whereby the species continuously adapted themselves to and improved the soil fertilities, and the different species competed and replaced one another at different fertility gradients, so that the soil fertilities were the driving force of the plant succession.