Invertebrate grazing during the regenerative phase affects the ultimate structure of macrophyte communities

1. Although the biomass of freshwater macrophytes consumed by invertebrate herbivores (excluding crayfish) is usually low, there is growing evidence that invertebrates do exert a structuring effect on macrophyte communities. To explain this, we postulated that the effect of invertebrates may be concentrated on macrophytes during their regenerative phase.
2. We tested this hypothesis by means of a mesocosm-based experiment, in which we investigated the effects of different densities of pond snails [ Lymnaea stagnalis (L.)] on macrophytes regenerating from the natural sediment propagule bank.
3. After 2 months, a diverse macrophyte community had established in the absence of snails, mainly from sexual propagules. Under moderate snail grazing (4 individuals m⁻²), the ultimate biomass of macrophytes was similar, but its species composition differed dramatically. Only a few unpalatable taxa, such as Ceratophyllum demersum and Nymphaeaceae, persisted. Moreover, the relative success of macrophytes regenerating from vegetative rather than sexual propagules improved. Under higher snail grazing (20 m⁻²), all macrophytes disappeared before the end of the experiment.
4. These results confirm that snails at natural densities can have a strong effect on the ultimate structure of macrophyte communities by selectively consuming some species at a juvenile stage. Therefore, the regenerative phase can be seen as a window of opportunity for invertebrate grazers, which can have a qualitative effect on communities that is disproportionate to the biomass consumed.     

Direct and indirect effects of rats: does rat eradication restore ecosystem functioning of New Zealand seabird islands?

Introduced rats (Rattus spp.) can affect island vegetation structure and ecosystem functioning, both directly and indirectly (through the reduction of seabird populations). The extent to which structure and function of islands where rats have been eradicated will converge on uninvaded islands remains unclear. We compared three groups of islands in New Zealand: islands never invaded by rats, islands with rats, and islands on which rats have been controlled. Differences between island groups in soil and leaf chemistry and leaf production were largely explained by burrow densities. Community structure of woody seedlings differed by rat history and burrow density. Plots on islands with high seabird densities had the most non-native plant species. Since most impacts of rats were mediated through seabird density, the removal of rats without seabird recolonization is unlikely to result in a reversal of these processes. Even if seabirds return, a novel plant community may emerge.     

哀牢山中山湿性常绿阔叶林两种优势幼苗C、N、P化学计量特征及其对N沉降增加的响应

N沉降对不同森林生态系统的影响是当今全球变化生态学研究的一个热点问题。山地湿性常绿阔叶林是我国西部高海拔地区重要的森林植被类型之一。该文以云南哀牢山中山湿性常绿阔叶林为对象, 研究了其林下优势树种多花山矾(Symplocos ramosissima)和黄心树(Machilus gamblei)幼苗不同器官中C、N、P含量和生态化学计量特征及其对N沉降增加的响应。结果表明: 两种幼苗C、N、P含量的差异均达到了显著性水平(p < 0.05), 多花山矾的C含量较低, N和P含量较高。N处理对植物幼苗元素含量及其比值影响极显著(p < 0.01), 且与物种和器官之间存在显著的交互作用。N处理提高了幼苗体内N含量, 导致不同器官N:P值有不同程度的增加。随N处理水平的升高, 多花山矾幼苗P含量下降, 黄心树幼苗P含量整体升高, 幼苗间P含量差异减小。在一定范围内, 植物幼苗N含量与土壤无机N含量之间存在极显著的相关性(p < 0.01)。不同器官之间相比, 植物幼苗根和茎的N内稳性比叶片更高, 即植物叶片对N沉降的响应更为敏感。     

Warming drives a front of white spruce establishment near western treeline,Alaska

Regional warming has led to increased productivity near the boreal forest margin in Alaska. To date, the effects of warming on seedling recruitment have received little attention, in spite of forecasted forest expansion. Here, we used stand structure and environmental data from 95 white spruce (Picea glauca) plots sampled across a longitudinal gradient in southwest Alaska to explore factors influencing spruce establishment and recruitment near western treeline. We used total counts of live seedlings, saplings, and trees, representing five life stages, to evaluate whether geospatial, climate, and measured plot covariates predicted abundance, using current abundance distributions as a surrogate for climate conditions in the past. We used generalized linear models to test the null hypothesis that conditions favorable for recruitment were similar along the environmental gradient represented by longitude, by exploring relationships between per‐plot counts of each life stage and the covariates hypothesized to affect abundance. We also examined the relationship between growing degree days (GDD) and seedling establishment over a period of three decades using tree‐ring chronologies obtained from cores taken at a subset of our sites (n = 30). Our results indicated that seedling, sapling, and tree abundance were positively correlated with temperature across the study area. The response to longitude was mixed, with earlier life stages (seedlings, saplings) most abundant at the western end of the gradient, and later life stages (trees) most abundant to the east. The differential relationship between longitude and life‐stage abundance suggests a moving front of white spruce establishment through time, driven by changes in environmental conditions near the species’ western range limit. Likewise, we found a positive relationship between periods of seedling establishment and GDD, suggesting that longer summers and/or greater heat accumulation might enhance establishment, consistent with the positive relationship we found between life‐stage abundance and temperature.     

酸雨胁迫对苦槠幼苗气体交换与叶绿素荧光的影响

常绿阔叶树苦槠(Castanopsis sclerophylla)是亚热带地带性顶极群落的建群种之一, 在区域森林资源保护和可持续利用方面具有重要的地位。在该区域日益严重的酸雨胁迫下, 研究其对于胁迫的生理生态响应具有重要的理论价值和实践意义。该文以苦槠幼苗为研究材料, 研究了酸雨胁迫对苦槠幼苗光合生理的影响。结果表明: (1)短时间内, pH 2.5处理下的幼苗叶绿素相对含量最低, 且与pH 5.6处理下的存在显著性差异(p < 0.05); 经过一段时间处理后, pH 4.0处理下的叶绿素相对含量最高, 表明低浓度的酸雨会促进叶绿素相对含量的增加。(2) 2007年4月, 光合速率(P_n)、PSⅡ最大光化学效率(F_v/F_m)和PSⅡ的潜在活性(F_v/F₀)在不同酸雨浓度处理下基本无变化。随着酸雨处理时间的延长, pH 2.5处理下的P_n、光饱和点、光补偿点和暗呼吸速率有显著降低, 且与pH 5.6处理下的存在显著性差异(p < 0.05)。pH 2.5处理组与pH 5.6处理组之间的F_v/F_m和F_v/F₀差异性逐渐减小。表观量子效率和气孔导度变化规律不明显。综合来说, 酸雨处理前期, 高浓度的酸雨胁迫对苦槠幼苗叶绿素相对含量、光合生理参数有显著影响, 但随着酸雨处理时间的增加, 酸雨胁迫对苦槠幼苗的影响逐渐减小, 表明其对外界不良环境具有一定的抵御能力和适应能力。     

Morphological and physiological differentiation of seedlings between dry and wet habitats in a tropical dry forest

A common observation in tropical dry forests is the habitat preference of tree species along spatial soil water gradients. This pattern of habitat partitioning might be a result of species differentiation in their strategy for using water, along with competing functions such as maximizing water exploitation and tolerating soil water stress. We tested whether species from drier soil conditions exhibited a tolerance strategy compared with that of wet-habitat species. In a comparison of 12 morphophysiological traits in seedlings of 10 closely related dry and wet-habitat species pairs, we explored what trade-offs guide differentiation between habitats and species. Contrary to our expectations, dry-habitat species showed mostly traits associated with an exploitation strategy (higher carbon assimilation capacity, specific leaf area and leaf-specific conductivity and lower water-use efficiency). Strikingly, dry-habitat species tended to retain their leaves longer during drought. Additionally, we detected multiple strategies to live within each habitat, in part due to variation of strategies among lineages, as well as functional differentiation along the water storage capacity-stem density (xylem safety) trade-off. Our results suggest that fundamental trade-offs guide functional niche differentiation among tree species expressed both within and between soil water habitats in a tropical dry forest.     

Influence of extremely low-frequency electric fields on the growth of Vigna radiata seedlings

The biological effects of extremely low-frequency electric fields (ELF) on living organisms have been explored in many studies, but the results are controversial and only a few studies investigated the influence of the intensity of the applied field on seedling growth. Here we assess the effects of a 50 Hz sinusoidal electric field on the early growth of Vigna radiata seedlings while varying the field intensity. Experiments performed in a dark, constant-climate chamber on several thousands of seedlings show that the field produces an inhibitory effect at a low field intensity and an enhancing one at a higher intensity. The maximum negative effect occurs at about 450 V/m, which is an intensity much lower than the exposure limits currently in force in the safety regulations.     

甘蔗健康种苗繁育技术研究进展

甘蔗健康种苗系指通过种茎温汤处理或腋芽茎尖培养等方法脱去甘蔗体内病菌的腋芽试管苗或种茎及其无性繁殖种茎苗,以及在不同隔离条件下逐代繁育并符合标准的原原种、原种和生产用甘蔗种苗。在生产甘蔗健康种苗过程中,利用间歇浸没式生物反应器(TIBs)进行甘蔗健康种苗的快繁,提高了增殖系数,降低了工作量。"甘蔗健康种子"的新技术,大大减少了甘蔗种植的用种量,并有利于机械化操作,降低了甘蔗种植成本。