【作者】 李文金；

【导师】 王刚； 李金花； Jean Knops；

【作者基本信息】 兰州大学 ， 生态学， 2010， 博士

【摘要】 随着人口的增长,粮食问题的出现,人类为了自身生存的需要对自然资源进行掠夺性开发,其强度远远超出自然生态系统的承载力,这样自然生态系统遭到严重破坏,导致其结构和功能退化,生产力下降,生物多样性丧失。因此,保护自然生态系统、恢复和重建退化的生态系统已成为人类面临的重要课题。弃耕地演替是指耕地弃耕撂荒后所发生的次生演替。研究表明：如果弃耕地不超过其恢复的生物阈值(植物散布体、土壤种子库和自然物种库等)和非生物阈值(土壤结构,肥力等),给与它们足够的时间,让其自然演替,经过几十年,大多可以恢复或接近顶极植被的状态(特别对于生物多样性而言)。然而,当弃耕地超过了恢复阈值,它们的恢复和重建可能需要一个相当漫长的过程并且不同生态系统下的弃耕地恢复没有一种固定的演替模式。另外,在弃耕地演替恢复过程中,什么因素决定着群落的演替模式?不同生态系统下的弃耕地恢复机制是什么?对这些问题还不清楚,特别对于高海拔地区的生态系统,耕作干扰后草地的恢复是否与其它低海拔生态系统的弃耕地具有一致的恢复模式、速率及其恢复机制呢?为此,2003年采用了空间序列替代时间序列的方法,在甘肃省合作市郊(海拔3000米左右)选择了一个弃耕演替梯度,研究亚高寒草甸弃耕地演替恢复过程中植物群落的种类群成、结构、及其生态系统的功能演变过程,阐明植物群落、功能群和物种之间及其与各主要环境因素之间的相互关系,以期揭示亚高寒弃耕地生态恢复机制,为高海拔地区退化草地的恢复重建提供生态学依据。研究表明：(1)在弃耕地恢复过程中,随着弃耕时间的增加,物种丰富度,多度和地上生物量显著增加。非禾草(Forbs)植物增加速率远超过禾草(Grasses)和豆科功能群(Legumes)。非禾草植物可以解释总植物物种丰富度,多度和地上生物量的65%-85%以上。因此,在亚高寒弃耕地生态系统中非禾草植物对生态系统的恢复起着重要的作用,经过15-20年的弃耕恢复,弃耕地逐渐演替为一个物种丰富的非禾草植物群落(forblands)。(2)在弃耕地恢复过程中,弃耕15年后,灭绝率和迁入率相当,随后,迁入率低于灭绝率,从而导致了恢复演替过程中,物种丰富度增加,物种周转率(Species Turnover Rate)下降。(3)对演替恢复过程中不同功能群种子大小的变化进行比较后发现：各个功能群的种子大小随弃耕时间的增加而减小。一方面,邻近的自然植被可以散布种子到这些小面积的弃耕地；另一方面,高寒草地生态系统中,大多数莎草功能群和非禾草功能群植物具有良好的克隆繁殖能力,这样弃耕后,植物群落将利用种子繁殖和克隆繁殖来恢复植被。(4)在弃耕地恢复过程中,植被恢复较快,而土壤恢复相对缓慢,具有明显的滞后性。2006年和2007年数据表明：土壤有机碳和全氮随着弃耕时间是显著地线性下降。土壤微生物碳MBC (0-20 cm),土壤微生物碳与土壤有机碳的百分比MBC/Corg (%) (0-20 cm),土壤微生物碳与全氮的百分比MBC/TN (0-20 cm),土壤有机碳与全氮的比值C/N (20-40 cm)和土壤微生物氮MBN(0-20 cm)随着弃耕时间是“U”型的变化模式。而2008年的数据表明：0-10cm和10-20cm的土壤碳和土壤全氮表现出不显著增加趋势,20-40 cm的土壤碳、全氮随着弃耕时间表现出不显著地下降趋势。(5)在弃耕地恢复过程中,豆科和禾草的一些关键种对生态系统过程中的土壤碳氮的积累有更重要的作用,特别是对土壤氮的成份。禾草物种甘青针茅(Stipa przewalskyi Roshev),藨草(Scirpus tripueter),洽草(Koeleria cristata)与豆科物种黄花棘豆(Oxytropis ochrocephala),披针叶黄华(Thermopsis lanceolate)和多枝黄芪(Astragalus polycladus)相比,对MBN, MBN/TN有更大的影响；‘在演替后期,矮嵩草(Kobresio humilis)比野豌豆(Vicia sepium)与NH4-N有更显著的正相关关系。这样植被群落恢复过程中,豆科和禾草的关键种的协同作用对土壤氮的成份有更大的影响。总之,在低氮水平下,豆科物种丰富的草地群落中,随着弃耕时间的增加,土壤氮成份控制着植物群落的演替。(6)弃耕地恢复过程中,随着弃耕时间的增加,地上地下生物量显著地增加。地上总的生物量从弃耕1年的188 g/m2增加到弃耕30年的516 g/m2。地下根的生物量,从弃耕初期的402 g/m2增加到弃耕30年的2002 g/m2,再到天然草地的3000g/m2。地下地上生物量之比从最低的1倍到最高的10倍。所以,弃耕有助于植被群落的恢复,特别是有助于植被生产力的提高。(7)弃耕地植被恢复过程中,生态系统氮库均有增加的趋势。地上氮的范围为：2-10 g/m2；而地下氮的范围为：500-1800 g/m2。地上植物活体和凋落物的氮库低于根系。地上地下氮库大小的顺序为：土壤1007 g/m2>根13.76g/m2>植物地上活体3.68 g/m2>地上凋落物3.20 g/m2。相对于生态系统氮的增加趋势而言,生态系统磷库表现出一些不同的趋势：植物体增加,而土壤下降。地上植物活体和凋落物的磷库也低于根系。地上地下磷库大小顺序为：土壤179 g/m2>根1.16 g/m2>植物地上活体0.31 g/m2>地上凋落物0.27 g/m2。(8)在亚高寒草甸,小面积弃耕地由于具有持久的土壤种子库和来自于邻近的天然草甸的种子散布,在弃耕管理的方式下,这些弃耕地生态系统具有很高的自我修复能力和重建能力,且大多数情况下,经过15年至20年的弃耕恢复,’植物群落能够恢复并接近演替顶极的天然草甸。更多还原

【Abstract】 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.更多还原