Modelling biomass of mountainous grasslands by including a species composition map

High mountain grasslands offer multiple goods and services to society but are severely threatened by improper land use practices such as abandonment or rapid intensification. In order to reduce abandonment and strengthen the common extensive agricultural practice a sustainable land use management of high mountain grasslands is needed. A spatially detailed yield assessment helps to identify possible meadows or, on the contrary, areas with a low carrying capacity in a region, making it easier to manage these sites. Such assessments are rarely available for remote and inaccessible areas. Remotely sensed vegetation indices are able to provide valuable information on grassland properties. These indices tend, however, to saturate for high biomass. This affects their applicability to assessments of high-yield grasslands.The main aim of this study was to model a spatially explicit grassland yield map and to test whether saturation issues can be tackled by consideration of plant species composition in the modelling process. The high mountain grassland of the subalpine belt (1800 – 2500 m a.s.l.) in the Kazbegi region, Greater Caucasus, Georgia, was chosen as test site for its strong species composition and yield gradients.We first modelled the species composition of the grassland described as metrically scaled gradients in the form of ordination axes by random forest regression. We then derived vegetation indices from Rapid Eye imagery, and topographic variables from a digital elevation model, which we used together with the multispectral bands as predictive variables. For comparison, we performed two yield models, one excluding the species composition maps and one including the species composition map as predictors. Moreover, we performed a third individual model, with species composition as predictors and a split dataset, to produce the final yield map.Three main grassland types were found in the vegetation analysis: Hordeum violaceum-meadows, Gentianella caucasea-grassland and Astragalus captiosus-grassland. The three random forest regression models for the ordination axes explained 64%, 33% and 46% of the variance in species composition. Independent validation of modelled ordination scores against a validation data set resulted in an R² of 0.64, 0.32 and 0.46 for the first, second and third axes, respectively. The model based on species composition resulted in a R² = 0.55, whereas the benchmark model showed weaker relationships between yield and the multispectral reflectance, vegetation indices, and topographical parameters (R² = 0.42). The final random forest yield model used to derive the yield map resulted in 62% variance explained and an R² = 0.64 between predicted and observed biomass. The results further indicate that high yields are generally difficult to predict with both models.The benefit of including a species composition map as a predictor variable for grassland yield lies in the preservation of ecologically meaningful features, especially the occurrence of high yielding vegetation type of Hordeum violaceum meadows is depicted accurately in the map. Even though we used a gradient based design, sharp boundaries or immediate changes in productivity were visible, especially in small structures such as arable fields or roads (Fig. 6b), making it a valuable tool for sustainable land use management. The saturation effect however, was mitigated by using species composition as predictor variables but is still present at high yields.     

退化高寒草甸关键生态属性对多途径恢复措施的响应特征

高寒草甸是青藏高原的主体植被类型，但退化态势较为严峻，严重威胁青藏高原生态屏障的战略地位。退化高寒草甸的复健是世界性难题，治理效果也因退化状态、恢复措施及气候环境而异。以春季休牧、秋季休牧、畜群结构优化、减畜轮牧、围栏封育及翻耕改建等典型多途径恢复措施下的退化高寒草甸为对象，系统探讨主要生态要素和生态功能的响应特征及潜在过程。结果表明，典型恢复措施下退化高寒草甸的植被生产力、土壤有机碳密度及土壤饱和持水量等生态要素都得到一定程度的提升，而恢复效果与实施年限及恢复措施密切相关。围栏封育和翻耕改建下土壤有机碳密度及饱和持水量随恢复年限均表现为对数饱和型的响应特征，退化高寒草甸固碳持水功能的基本恢复年限约为6—10年。春季休牧、秋季休牧、畜群结构优化、减畜轮牧、围栏封育等放牧管理恢复措施应适用于轻度退化至重度退化的高寒草甸，而翻耕改建则是极度退化高寒草甸的适宜治理措施。由于多途径恢复措施的关注目标不同，今后研究应集中在恢复措施的组合优化和综合评价等方面。     

New method for rapid assessment of the functional composition of herbaceous plant communities

Abstract We propose a rapid sampling method to assess the functional composition of herbaceous plant communities without prior knowledge of the floristic composition. To determine the community‐level value of traits (‘aggregated trait values’) for a plant community, a standardized population‐centred method exists, but requires substantial manpower and reliable botanical knowledge. We tested an alternative method, the trait transect, using four subalpine pastures in the Beaufortain region (Northern French Alps) selected along a fertility gradient. We applied both methods to measure five commonly used ‘soft traits’ known to be responsive to soil nutrient availability: plant vegetative and reproductive height, specific leaf area, leaf dry matter and nitrogen contents. We tested whether the variation of these traits along the gradient detected with the population‐centred method was also detected with the trait transect. Both methods detected expected trends in the traits in response to the fertility gradient. The trait transect method was as efficient as the population‐centred method and is recommended as an appropriate tool for monitoring ecosystem changes in response to environmental conditions and management, especially in species‐rich communities.     

A study of the abundance and patchiness of cicada nymphs (Homoptera: Tibicinidae) in a New Zealand subalpine shrub-grassland

Abstract

Cicada emergence skins in a subalpine shrub grassland have been sampled during 1969–75 to determine the abundance and spatial distributions of nymphs feeding on plant roots. A guild of six cicada species is primarily associated with two forms of vegetation: shrubs (Dracophyllum and Cassinia) and tall tussock (Chionochloa). Skin locations were mapped relative to dominant vegetation species, litter zones, and soil and rock pavements over a range of aspects, altitudes, and vegetation types, and sampling methods were scaled at four levels: the locality, plot, quadrat, and individual plant. There were significant differences in skin counts over four years, and different measures of mean skin densities are given for the four sampling scales. The two primary vegetation types had cumulative 1969–72 mean densities of 5.2 ± 4.0 and 12.9 ± 10.0 skins/quadrat (2.3 m²) , and the 1969–72 mean productivities of the upper 25% of quadrats (adjusted for percent ground cover) were, respectively, 5.5 and 35.5 skins/m². These productivities are believed to be conservative estimates of the maturing nymph numbers per individual host plant over the span of one cicada generation. Over a 17-year span, such productivities lie within the upper range of mean densities recorded for 17-year periodical cicadas in the United States. As the dominant subalpine vegetation species are very slow-growing,it is suggested that high densities of nymphs feeding on root sap may affect plant vitality, although 1971/1987 comparisons of vitality in 52 Chionochloa tussocks could not positively demonstrate a correlation across all data. Skin dispersion analyses indicated significant levels of patchiness, in agreement with other nymphal studies and with known cicada oviposition behaviour. No single dispersion model fitted the data comprehensively, and it is suggested that a gradual shifting of the centres of cicada aggregation may occur over a cumulative period of several generations.     

川西亚高山不同海拔森林土壤活性氮库及净氮矿化的季节动态

研究了川西理县毕棚沟不同海拔梯度(3600 m、3300 m和3000 m)森林群落土壤活性氮库及土壤净氮矿化速率的季节动态.结果表明: 研究区森林土壤活性氮库(铵态氮、硝态氮、微生物生物量氮和可溶性有机氮)及净氮矿化速率存在明显的季节变化,但不同形态土壤活性氮库的季节动态有一定差异.4个采样时期(非生长季与生长季初期、中期及末期)各海拔土壤硝态氮浓度(8.38～89.60 mg·kg^-1)均显著高于铵态氮浓度(0.44～8.43 mg·kg^-1).生长季初期各海拔梯度的土壤净氮矿化速率均表现为负值(-0.77～-0.56 mg·kg^-1·d^-1),而非生长季、生长季中期和末期均为正值.除硝态氮外,不同海拔的土壤铵态氮、微生物生物量氮和可溶性有机氮浓度的差异极显著,海拔对它们的影响与季节变化有关.该区土壤净氮矿化以硝化为主,且氮矿化过程不受海拔梯度的影响.冬季土壤净氮矿化明显(0.42～099 mg·kg^-1·d^-1),早春高的土壤无机氮可能为植物生长提供基础养分,也可能通过淋溶方式从系统中丢失.     

川西亚高山不同森林类型土壤呼吸和总硝化速率的季节动态

川西亚高山原始林及其采伐后通过不同恢复措施形成的不同类型森林土壤呼吸和总硝化速率的对比分析及其耦合关系的研究相对匮乏。采用气压过程分离系统(Ba PS)技术研究了川西亚高山岷江冷杉原始林及其砍伐后恢复的粗枝云杉阔叶林、红桦-岷江冷杉天然次生林和粗枝云杉人工林土壤呼吸和总硝化速率的季节动态及其影响因素。结果表明:生长季内平均土壤呼吸速率和总硝化速率分别以粗枝云杉阔叶林和粗枝云杉人工林较高,均以岷江冷杉原始林较低。土壤呼吸和总硝化速率在生长季内具有明显的季节动态,呈以7月份最高的单峰趋势。土壤呼吸和总硝化速率与土壤温度显著相关,而与土壤水分相关性不显著,表明土壤温度是调控呼吸和总硝化作用季节动态的主要因子。土壤呼吸的温度敏感性(Q_(10))介于2.59—4.71,以岷江冷杉原始林最高,表明高海拔的岷江冷杉原始林可能更易受到气候变化的影响。林型间土壤呼吸和总硝化速率主要受凋落物量、p H和有机质的影响。不同林型间土壤呼吸和总硝化速率显著正相关,表明土壤呼吸和总硝化速率存在耦合关系。     

践踏干扰对碧塔海高寒草甸植被茎叶性状、物种多样性和功能多样性的影响

高寒草甸具有重要的生态服务功能,然而固有脆弱性使其极易遭受气候变化和人为干扰等多重因素的影响。作为滇西北旅游资源中重要的组分之一,高寒草甸吸引了大批游客前往开展徒步旅行活动,但伴随着的践踏干扰作用会不可避免地对高寒草甸生态系统带来负面影响。然而,目前关注践踏干扰对滇西北高寒草甸植被的影响,特别是植被功能性状和功能多样性如何发生变化方面的研究还十分欠缺。以云南省香格里拉市碧塔海自然保护区内的典型高寒草甸生态系统为研究对象,采用实验践踏的方式(一共5种不同强度的践踏处理)来模拟旅游活动对草甸植被的干扰作用,并以草甸植被的茎叶性状特征为切入点,重点探讨践踏干扰对茎叶性状的平均大小和变异程度的影响,以及物种丰富度(以物种形态分类为基础)和功能丰富度(以功能性状为基础)之间的关系。研究结果显示,随着践踏强度的增加,植株高度和叶片大小的平均值,而不是茎叶性状的变异程度,出现明显下降趋势。此外,物种丰富度和功能丰富度均随践踏强度的增强而减小,且两者之间呈现显著正相关关系。然而,较之轻度践踏实验组,重度践踏实验组中的功能均匀度和功能分离度水平均有所增加,表明践踏干扰可能会在短期内打破优势种对资源的绝对占有格局和减少物种间的生态位重叠程度。尽管高寒草甸对人类践踏活动有一定的承受能力,但气候变化和人为干扰等多重因素势必会改变和影响高寒草甸群落的结构和功能可持续性,这也对高寒草甸的保护与管理工作提出了更加紧迫的要求。     

Relative sea‐level change regulates organic carbon accumulation in coastal habitats

Because coastal habitats store large amounts of organic carbon (C_org), the conservation and restoration of these habitats are considered to be important measures for mitigating global climate change. Although future sea‐level rise is predicted to change the characteristics of these habitats, its impact on their rate of C_org sequestration is highly uncertain. Here we used historical depositional records to show that relative sea‐level (RSL) changes regulated C_org accumulation rates in boreal contiguous seagrass–saltmarsh habitats. Age–depth modeling and geological and biogeochemical approaches indicated that C_org accumulation rates varied as a function of changes in depositional environments and habitat relocations. In particular, C_org accumulation rates were enhanced in subtidal seagrass meadows during times of RSL rise, which were caused by postseismic land subsidence and climate change. Our findings identify historical analogs for the future impact of RSL rise driven by global climate change on rates of C_org sequestration in coastal habitats.     

Climate change reduces resilience to fire in subalpine rainforests

Climate change is affecting the distribution of species and the functioning of ecosystems. For species that are slow growing and poorly dispersed, climate change can force a lag between the distributions of species and the geographic distributions of their climatic envelopes, exposing species to the risk of extinction. Climate also governs the resilience of species and ecosystems to disturbance, such as wildfire. Here we use species distribution modelling and palaeoecology to assess and test the impact of vegetation–climate disequilibrium on the resilience of an endangered fire‐sensitive rainforest community to fires. First, we modelled the probability of occurrence of Athrotaxis spp. and Nothofagus gunnii rainforest in Tasmania (hereon “montane rainforest”) as a function of climate. We then analysed three pollen and charcoal records spanning the last 7,500 cal year BP from within both high (n = 1) and low (n = 2) probability of occurrence areas. Our study indicates that climatic change between 3,000 and 4,000 cal year bp induced a disequilibrium between montane rainforests and climate that drove a loss of resilience of these communities. Current and future climate change are likely to shift the geographic distribution of the climatic envelopes of this plant community further, suggesting that current high‐resilience locations will face a reduction in resilience. Coupled with the forecast of increasing fire activity in southern temperate regions, this heralds a significant threat to this and other slow growing, poorly dispersed and fire sensitive forest systems that are common in the southern mid to high latitudes.