Restoration Succession And Ecological Mechanism Of Old-fields In Subalpine Meadow

With the growth of population and the emergence of food problems, the human have exploited too much natural resources to meet their own survival and demand, but its intensity has exceeded the carring capacity of natural ecosystems. The natural ecosystems have been seriously damaged, which leading to degradation of its structure and function, reduced productivity, loss of biodiversity. Therefore, protection of natural ecosystems, restoration and rehabilitation of degraded ecosystems will pose significant scientific and policy challenges. For restoration of abandoned farmlands, if they don't exceed the biotic and abiotic threshold in the cultivation legacy, it will not take too much time. In most cases, after several decades they can restore historical vegetation state. However, intensification of agriculture and rapid environmental change will lead to increasing numbers of old fields that show little recovery towards an historic vegetation state and exist the possibility of multiple pathways and trajectories after disturbances. In addition, the succession of old fields in the natural recovery process, what factors determine the pattern and structure of community succession, what affects the community succession rate? Currently, most of studies are focusing on the low altitude region, little information are available in the high altitude. In contrast to temperate ecosystem, many alpine grasslands are often dominated by forb species. Therefore, predictions based on temperate vegetation succession may not be valid for alpine ecosystems. The chronosequence approach to studying vegetation dynamics has provided significant insights into the patterns and mechanisms of plant succession. A chronosequence of abandoned fields was established in the Research Station of Alpine Meadow and Wetland Ecosystems of Lanzhou University in the eastern part of the Qing-Hai Tibetan Plateau, China (N34°55', E102°53') in 2003. We monitored plant life histories, species composition, diversity, structure, productivity, dynamics and functioning of plant communities over a 6-year period to assess vegetation establishment and recovery after cessation of agriculture. Understanding the relationships among plant species diversity, plant productivity and resource availability in restored ecosystems are important for the management, preservation, and restoration of native communities and may also be crucial for successfully restoring species-rich ecosystem.A 6-year period study showed that:1 During secondary succession, a significant increase in species richness, abundance and aboveground biomass occurred over time. More interesting, whether the early fields or the late fields, the forbs increased faster than the other functional groups over time. Based on plot level, the forbs accounted for 65-85% of species richness, abundance and aboveground biomass in all fields, suggesting that forb species drive the entire plant communities'assembly and are a key factor to restoration.2 Species turnover rate generally declined while species richness increased over time, which supports the generally accepted successional rate hypothesis. Following 15 years, immigration rate and extinction rate converged, suggested that it will take at least 15-20 years to restore, which suggesting that spontaneous succession has higher potential in restoration of degraded ecosystem, particularly of ex-arable fields in the eastern Tibet Plateau.3 We examined seed size of different functional groups during succession and found that seed size did not increase over time. Instead we found a decreasing trend in seed size over time, which suggests that seed dispersal influenced by seed size is not a key factor driving succession in the subalpine ecosystem. We hypothesize that seed dispersal is rapid regardless of seed size, because these relatively small abandoned fields are located in a matrix of native vegetation and have a persistent soil seed bank. Second, late successional stages are dominated by forbs and sedges and clonal reproduction may be driving force in their increase in abundances.4. The data in 2006 and 2007 showed that soil microbial carbon (MBC) and nitrogen (MBN) in the upper layer (0-20cm) showed U-shaped patterns along the fallow time gradient. However, soil organic carbon (Corg.), total nitrogen (TN) and the percent of microbial carbon to soil total nitrogen (MBC/TN) in the soil of deep layer (20-40cm) showed significant patterns of linear decline along the fallow time gradient. The data in 2008 showed that soil C and N (0-10cm and 10-20cm) had a nonsignificant increase trend over fallow time. In contrast, soil C and N (20-40cm) decreased significantly over fallow time. These results indicated that fallow time had a greater influence on development of the plant community than soil processes in abandoned fields in sub-alpine meadow ecosystem. These results also suggested that although the succession process did not significantly increase soil C, an increase in microbial biomass at the latter stage of succession could promote the decomposability of plant litter. Therefore, abandoned fields in sub-alpine meadow ecosystem may have a high resilience and a stronger rehabilitating capability and soil restoration had seriously time-lag under natural recovery condition.5. During the succession, legume richness and aboveground biomass significantly increased and both were positively correlated with total species richness (S) and aboveground biomass (T-bio). This pattern suggests that legume richness increases community productivity. In addition, we found that aboveground biomass, legume and grass richness were positively correlated microbial nitrogen (MBN), and the ratio of microbial nitrogen to soil total nitrogen (MBN/TN), soil organic carbon and the ratio of soil total nitrogen (C/N) were negatively correlated with soil total nitrogen (TN), organic carbon (Corg), and microbial carbon (MBC). Contrary to our predictions grasses such as Stipa grandis, Scirpus tripueter, Koeleria cristata were more closely associated with MBN, MBN/TN than legumes such as Oxytropis ochrocephala, Thermopsis lanceolate and Astragalus polycladus. The late-successional grass Kobresio humilis had a stronger positive correlation with NH4-N as compared to the legumes and NO3-N was not associated with any legume species. This suggests that the combination grasses and legumes have a synergetic positive influence on the ecosystem properties, especially nitrogen. Therefore, in this N-limited, plant community diversity of both legumes and grasses has a strong influence on ecosystem changes during succession.6. Along secondary succession, the aboveground and belowground biomass both significantly increased over time. The total aboveground biomass increased from 188 g/m2 in the 1-year fallow to 516 g/m2 in the 30-years fallow. The root biomass increased signicantly from 402 g/m2 in the early successionl stages to 2002 g/m2 in the 30-years fallow to 3000 g/m2 in the natural meadow. The rate of below and above biomass varied from 1 time to 10 times, which the average in the all old fields was 1.58 and the average in the nature meadow was 9.97. These suggested that spontaneous succession is helpful for the vegetation development and ecosystem productivity restoration of old fields in the eastern Tibet Plateau.7. During succession, there was an increasing trend for the ecosystem nitrogen pool, of which the aboveground was 2-10 g/m2 and the belowground was 500-1800 g/m2, This suggests that the N pool size in the soil was significantly higher than in the aboveground. The order of N pool size among the above and belowground was soil 1007 g/m2>root 13.76g/m2>live plant 3.68 g/m2>above litter 3.20 g/m2, suggesting that the N pool in the soil and root made 77% contribution to the whole N ecosystem. Compared to the ecosystem nitrogen pool, the ecosystem P pool had an opposite trend that the live plant P content increased and the soil P content decreased over time and the P pool size in the soil was significantly higher than in the aboveground. The order of P pool size among the above and belowground was soil 179 g/m2>root 1.16 g/m2>live plant 0.31 g/m2> above litter 0.27 g/m2, suggesting that the P pool in the soil and root made 99.7% contribution to the whole P ecosystem.8. In this subalpine ecosystem, forbs are the most important functional group driving biodiversity and ecosystem productivity. Succession is strongly influenced by the low cultivation intensity and a small scale of agricultural fields in this region, which leads to a persistent soil seed bank of native species and local seed dispersal into abandoned fields from surrounding native vegetation patches. Because of this seed bank and dispersal, plant communities can recover without seed additions within a timescale of decades. However, the oldest fields still differed significantly from the control, never cultivated field, which had higher sedge and lower legume diversity and abundances. In addition, species turnover rate stabilized after 15-30 years of succession.