Alien dominance of the parasitoid wasp community along an elevation gradient on Hawai’i Island

Through intentional and accidental introduction, more than 100 species of alien Ichneumonidae and Braconidae (Hymenoptera) have become established in the Hawaiian Islands. The extent to which these parasitoid wasps have penetrated native wet forests was investigated over a 1,765 m elevation gradient on windward Hawai’i Island. For >1 year, malaise traps were used to continuously monitor parasitoid abundance and species richness in nine sites over three elevations. A total of 18,996 individuals from 16 subfamilies were collected. Overall, the fauna was dominated by aliens, with 44 of 58 species foreign to the Hawaiian Islands. Ichneumonidae was dominant over Braconidae in terms of both diversity and abundance, comprising 67.5% of individuals and 69.0% of species collected. Parasitoid abundance and species richness varied significantly with elevation: abundance was greater at mid and high elevations compared to low elevation while species richness increased with increasing elevation, with all three elevations differing significantly from each other. Nine species purposely introduced to control pest insects were found, but one braconid, Meteorus laphygmae, comprised 98.0% of this assemblage, or 28.3% of the entire fauna. Endemic species, primarily within the genera Spolas and Enicospilus, were collected almost exclusively at mid- and high-elevation sites, where they made up 22.1% and 36.0% of the total catch, respectively. Overall, 75.9% of species and 96.0% of individuals are inferred to parasitize Lepidoptera larvae and pupae. Our results support previous data indicating that alien parasitoids have deeply penetrated native forest habitats and may have substantial impacts on Hawaiian ecosystems.     

Low oxygen pressure as a driving factor for the altitudinal decline in taxon richness of stream macroinvertebrates

The objective of this study was to explore the altitudinal decrease in local richness of stream macroinvertebrates. I compared the explicatory power of a mid-domain effect (MDE) null model and a number of selected contemporary ecological variables, with a special emphasis on the altitude-mediated decrease in temperature and oxygen availability as possible driving factors for the observed pattern. Benthic macroinvertebrates were sampled at 30 stream sites between 2,600 and 4,000 m a.s.l. in northern Ecuador. All four measures of local richness (total number of taxa, taxa in Surber samples, Fisher’s α index and rarefied richness) decreased with increasing altitude. The MDE null model, water temperature and dissolved oxygen also decreased with altitude, while other measured variables were uncorrelated with altitude. Minimum oxygen saturation had the highest explanatory power of the density-corrected Fisher’s α and rarefied richness (R = 0.48 and 0.52, respectively), but also minimum temperature (R = 0.48 and 0.41) and the MDE null model (R = 0.48 and 0.46) correlated significantly. Multiple regression analyses using several predictive variables showed that oxygen saturation had the greatest and only significant effect on density-corrected richness. The relationship between richness and oxygen corrected for the effect of altitude (using analyses of double residuals) was significant, whereas that of richness versus temperature was not. The results indicate that the decrease in richness with increasing altitude is mainly caused by a decrease in oxygen saturation rather than by a decrease in temperature. Levels of oxygen saturation such as those found at high altitudes do not appear to be lethal to any species, but could affect macroinvertebrates through long-term, sub-lethal effects. I suggest that low oxygen availability may limit biodiversity at high altitudes not only in the aquatic, but also in the terrestrial environment. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Trends in surface elevations of American Samoa mangroves

Rates of change in elevation of mangrove surfaces, determined from observations of changes in the height above the mangrove surface of stakes, generally inserted through the organic peat layer to reach consolidated substrate, were measured in one fringe and one basin mangrove wetland on Tutuila Island, American Samoa. Knowledge of trends in elevation change of coastal wetlands contributes to assessing vulnerability to projected relative sea level rise. The fringe and basin mangroves had rates of change in elevation of −0.6 mm yr⁻¹ (±2.0) and −2.2 mm yr^-1 (±5.6), where a negative result means lowering in elevation. These trends were not statistically significant (P > 0.05) and the error intervals around the point estimates of trends in change in elevation overlap zero for both study sites, meaning that it is not clear if the mangrove surfaces have been lowering, rising or not changing. Despite the large error intervals, likely due to short-term variability and cyclical patterns in sedimentation, results indicate that the fringe mangrove has been experiencing a rise in sea level relative to the mangrove surface as the relative sea level rise rate (+1.65 to +2.29 mm yr⁻¹) has been exceeding the rate of change in elevation of the mangrove surface (−2.6 to +1.4 mm yr⁻¹). It is unclear if the basin mangrove has been experiencing a rise in sea level relative to the mangrove surface. If upper projections for accelerated relative sea level rise in American Samoa occur over coming decades, American Samoa mangroves will migrate landward, where unobstructed, as a natural response to relative sea level rise.

Searching for a model for use in vegetation analysis

Summary Indirect gradient analysis methods require an explicit vegetation model which must be based on direct gradient analysis studies. Various vegetation models are reviewed. Field evidence for the models is discussed. Experimental studies of species response to environmental gradients are reviewed and discussed. Three types of gradient are recognized as important for development of models: indirect environmental gradients where the environmental factor has no direct physiological influence on plant growth e.g. elevation; direct environmental gradients where the factor has a direct physiological effect on growth but is not an essential resource, e.g. pH; resource gradients where the factor is an essential resource for plant growth. The behaviour of the ecological carrying capacity and the role of competition along such gradients are shown to be important for developing vegetation models.     

Concentration of ultraviolet-B radiation absorbing compounds in leaves of a range of fynbos species

Leaves from 38 species representing 17 families were collected from contrasting elevations (100 to 824 m above sea level) in sclerophyllous mountain fynbos vegetation near Hermanus, South Africa, and the leaf percentage dry mass (PDM), specific leaf mass (SLM) and ultraviolet-B (UV-B, 280 to 320 nm) absorbance properties determined. Leaf PDM, SLM and UV-B absorbance were generally high compared to mesophyllous plants. Leaves collected at high elevation exhibited higher absorbances per unit dry mass at both 280 and 320 nm than those from the same species at low elevation. No differences in absorbance per unit leaf area were observed. UV-B absorbance properties differed between families, with high values obtained for the Apiaceae, Asteraceae, Cyperaceae, Ericaceae, Penaeaceae and Proteaceae, but lower values for the Anacardiaceae, Fabaceae and Geraniaceae. A positive correlation was found between absorbance at 280 nm per unit leaf area and SLM. It was concluded that most fynbos species, on account of their highly sclerophyllous leaves and large accumulation of UV-B absorbing compounds (flavonoids and related phenolics) may be well protected against future increases in UV-B radiation.     

Vegetation of alpine marshland and its neighboring areas,northern part of Sichuan Province,China

To obtain the characteristics of Chinese alpine grassy marshland, the vegetation of Ruoergai marshland, which is the largest peatland in China, and of neighboring areas was surveyed along mountain slopes in three areas, Ouhailao, Waqie and Kaharqiao. Based on the clustering of 122 1 m × 1 m quadrats surveyed, the vegetation was grouped into 8 types. From bottom to top of mountain slopes, the dominant species, zonally established, were: 1) Carex enervis and Equisetum limosum; 2) Carex meyeriana; 3) Deschampsia caespitosa, Caltha scaposa and Anemone obtusiloba; 4) Koeleria cristata; 5) Kobresia tibetica and Caltha scaposa; 6) Blysmus sinocompressus; 7) Polygonum sphaerostachyum and Trollius ranuncloides; and 8) Ranunculus pedicularis. In this order, species richness increased gradually along the slopes. A principal component analysis demonstrated that water depth partly regulated the establishment of vegetation. The geographical difference of vegetation was also observed, in particular, in Waqie. Likewise, grazing is considered to modify this vegetation.     

郭岩山不同海拔丝栗栲细根功能性状及其与土壤因子的关系

为揭示丝栗栲(Castanopsis fargesii)细根功能性状对环境变化的适应机制,对郭岩山500、700、900 m海拔处丝栗栲细根功能性状及其与土壤因子的关系进行研究。结果表明,丝栗栲细根生物量与细根根长密度、表面积密度、组织密度及体积密度呈正相关,细根根长密度、体积密度、表面积密度和比根长4个性状间均呈极显著正相关关系,且均与细根组织密度呈显著负相关。根际土含水量、C和N含量与细根比根长、根长密度、体积密度、表面积密度均存在显著正相关关系,而土壤容重与细根组织密度呈正相关。海拔700 m的细根生物量、根长密度、表面积密度及体积密度显著大于海拔500和900 m的。500和900 m海拔的根长密度、表面积密度与土壤深度呈负相关,而500 m海拔细根的组织密度与土壤深度呈正相关。因此,郭岩山丝栗栲通过改变细根功能性状来适应海拔和土壤的变化。     

Below-ground response of Norway spruce to climate conditions at Mt. Brocken (Germany)—A re-assessment of Central Europe's northernmost treeline

Alpine treelines at medium high mountains are less abundant and thus have been less frequently studied than at high-elevation mountain ranges of the world. We studied mature Norway spruce stands along an extended elevation transect at Mt. Brocken (Harz Mountains, Central Germany) to analyse the altitudinal changes in climate-related growth conditions, and to evaluate the prevailing climate conditions at the treeline of medium high Mt. Brocken. A particular aim was to analyse the change in fine root biomass partitioning along the transect towards the treeline. Microclimate conditions at the treeline of Mt. Brocken were very similar to other treeline sites worldwide. Tree height and stem biomass strongly decreased from middle elevations towards the treeline. On the contrary, fine root biomass and the ratio of fine root/stem biomass strongly increased towards the treeline indicating a marked shift in carbon allocation in favour of the fine root system with elevation. A meta-analysis of literature data revealed that the elevation-related increase in dry mass partitioning to the fine root system is a general phenomenon for Norway spruce stands in northern and central European mountains. We conclude that the particularly large fine root system of Norway spruce at cold sites represents a mechanism to cope with unfavourable soil conditions such as reduced or temporally variable nutrient supply.     

Consistent growth of black cottonwoods despite temperature variation across elevational ecoregions in the Rocky Mountains

Globally, water and temperature provide the dominant environmental determinants of tree distribution and growth. In riparian or streamside zones, groundwater is abundant, and we consequently predicted that temperature would limit the growth of riparian cottonwoods in a cool climate northern mountain region. To investigate this association, we analyzed tree rings of 167 black cottonwoods, Populus trichocarpa, along two adjacent Rocky Mountain creeks in Alberta. Cottonwoods were sampled from 1700 m, near their upper elevational limit, down 500 m through three progressively warmer ecoregions, the montane, aspen parkland, and fescue prairie. Across these zones, June through September mean temperatures rose from 12.4 to 16.2°C (lapse rate = 0.67°C/100 m), and there was subsequently a 42% increase in growing degree days (base 5°C, GDD₅) from 900 GDD₅ at the trees’ upper limit. Despite this variation, growth rate of most trees was fairly consistent across the ecoregions; trunk diameter versus age associations were relatively similar (r ² = 0.85) with an estimated 14% increase in trunk sizes of 50 year-old trees with decreasing elevation. In all ecoregions, developmental patterns were prominent as annual radial increments increased up to about 20 years, and then progressively declined to an apparent lethal threshold of about 0.4 mm/year at about 100 years. Basal area increments also increased through the juvenile phase, but remained fairly constant thereafter. The weak association between growth and temperature suggests that other environmental factors limited growth rates or there were differences in temperature adaptation across these elevational ecoregions. The results suggest that predicted regional climate warming may not substantially promote the growth rates of these Rocky Mountain trees.