Recovery of evergreen clonal dwarf shrubVaccinium vitis-idaea after simulated microtine herbivory in a boreal forest

The evergreen dwarf shrub lingonberry (Vaccinium vitis-idaea L.) was subjected to microtine herbivory simulations to investigate its recovery in a boreal forest. Beside the control (A), the study included three levels of herbivory: clipping of half the annual branches from 50% of ramets (B), removal of 50% of ramets (C), and removal of 100% of ramets (D). Density, growth, and fecundity of the ramets were monitored during the study. New ramets emerged rapidly from dormant basal buds at the base of the removed ramets in groups C and D. After three growth seasons, 60–117% and 42–112% of density and biomass, respectively, had been regained in the groups damaged relative to the control, the difference being insignificant between the treated groups apparently because of the small sample size. Survival, fecundity and net growth of ramets were not affected by the treatment. Mean dry weight of annual branches in new ramets was lower in the damaged groups than the control. The recovery of the lingonberry was unexpectedly good. The results suggest that, since natural herbivory seldom is as heavy as in this work, the time between population density peaks of microtine rodents is usually long enough for the lingonberry to recover in boreal ecosystems.     

Multiannual effects of induced plant defenses: Are defended plants good or bad neighbors?

Defenses induced by herbivore feeding or phytohormones such as methyl jasmonate (MeJA) can affect growth, reproduction, and herbivory, not only on the affected individual but also in its neighboring plants. Here, we report multiannual defense, growth, and reproductive responses of MeJA‐treated bilberry (Vaccinium myrtillus) and neighboring ramets. In a boreal forest in western Norway, we treated bilberry ramets with MeJA and water (control) and measured responses over three consecutive years. We observed the treatment effects on variables associated with herbivory, growth, and reproduction in the MeJA‐treated and untreated ramet and neighboring ramets distanced from 10 to 500 cm. MeJA‐treated ramets had fewer grazed leaves and browsed shoots compared to control, with higher effects in 2014 and 2015, respectively. In 2013, growth of control ramets was greater than MeJA‐treated ramets. However, MeJA‐treated ramets had more flowers and berries than control ramets 2 years after the treatment. The level of insect and mammalian herbivory was also lower in untreated neighboring ramets distanced 10–150 cm and, consistent with responses of MeJA‐treated ramets, the stronger effect was also one and 2 years delayed, respectively. The same neighboring ramets had fewer flowers and berries than untreated ramets, indicating a trade‐off between defense and reproduction. Although plant–plant effects were observed across all years, the strength varied by the distance between the MeJA‐treated ramets and its untreated neighbors. We document that induced defense in bilberry reduces both insect and mammalian herbivory, as well as growth, over multiple seasons. The defense responses occurred in a delayed manner with strongest effects one and 2 years after the induction. Additionally, our results indicate defense signaling between MeJA‐treated ramets and untreated neighbors. In summary, this study shows that induced defenses are important ecological strategies not only for the induced individual plant but also for neighboring plants across multiple years in boreal forests.     

Interactions between gray-sided voles (<Emphasis Type="Italic">Clethrionomys rufucanus</Emphasis>) and bilberry (<Emphasis Type="Italic">Vaccinium myrtillus</Emphasis>), their main winter food plant

We compared the abundance, population structure and palatability of bilberry ramets on vole-free islands, islands with voles but no predators (predator-free islands) and mainland sites with both voles and predators. As expected, bilberry biomass was strongly correlated with the herbivory pressure exerted by the voles, since it was significantly lower on the mainland, and much (>80%) lower on the predator-free islands, than on the vole-free islands. However, another finding, which conflicts with hypotheses postulating that herbivory generally induces plant defenses, was that voles preferred ramets from predator-free islands. Bilberry plants were fairly tolerant to grazing since they compensated for some of the lost tissue by producing more new ramets. This response should promote stability in the plant–herbivore interaction by reducing the impact of past grazing on current food production and thus minimizing time delays in the interactions that could potentially generate population cycles.     

Ungulate herbivory reduces abundance and fluctuations of herbivorous insects in a boreal old-growth forest

Ecological theory predicts the strongest ecosystem effects of herbivory when dominant and ecologically important species are consumed. Bilberry, Vaccinium myrtillus, is such a key plant species, attractive to many other species in the boreal forests, for example ungulate and invertebrate herbivores. Large herbivores may remove substantial biomass and alter plant quality and therefore affect abundance and populations of invertebrate animals sharing the same food plant. We combined experimental exclusion of ungulates with a browsing intensity gradient to investigate the 15-year effect of ungulate (Cervus elaphus and Ovis aries) browsing on bilberry plant size and on bilberry-feeding herbivorous larvae (Lepidoptera and Symphyta), in a Norwegian old growth boreal forest ecosystem. Bilberry ramets in exclosure plots had nearly nine times higher dry mass and three times higher abundance of invertebrates feeding on them than in ungulate-access plots. Sweep-netting data verified these findings as larval numbers were twice as high in exclosure plots. The pattern in the large herbivore effects on bilberry size and abundance of herbivorous larvae were identical along the browsing gradient. Differences in larval abundance between treatments, as indicated by leaf-chewing, increased during the 15-year study period, and the community fluctuations were larger when ungulate herbivores were excluded. The browsing effect was moderated by plant quality as larval densities were lowest on both heavily-browsed and non-browsed plants, and highest on ramets that had 50–74% of annual shoots browsed. Our study supports previous findings in that bilberry is relatively disturbance tolerant and may recover quickly, but that ungulates may compete with herbivorous larvae for food biomass. Additionally, our results strongly indicates that population insect community peaks and fluctuations are dampened by ungulate consumption. Our findings add to the understanding on how ungulates may structure forest ecosystems directly and indirectly.     

Patterns of phytophagous insect herbivory and abundance on juvenile regrowth of Eucalyptus subgenera coexisting in southeastern Tasmania

Abstract Eucalypts from the subgenus Monocalyptus tend to be more abundant and dominate species from the subgenus Symphyomyrtus where they occur together in the mixed species stands of southeastern Australia. The differential impacts of herbivory by phytophagous insects has been postulated as a causal mechanism in the creation and maintenance of such stands. This research aimed to quantify phytophagous insect abundance and herbivory in mixed species juvenile regrowth of Eucalyptus globulus, E. viminalis (Symphyomyrtus), E. obliqua and E. pulchella (Monocalyptus} southwest of Hobart, Tasmania. Monocalyptus experienced a higher level of herbivory than Symphyomyrtus. However, mean damage levels were relatively low at less than 11% throughout. Furthermore, due to the positively skewed nature of herbivory data the mean was an inappropriate measure of central tendency; median damage levels ranged from 4.9% to 8.4%. Patterns of herbivory tended to be different for each eucalypt species: E. obliqua was particularly prone to chewing damage, E. pulchella and E. globulus suffered higher levels of distortion while E. viminalis was least affected by insect attack. Even though some trends in insect community structure seemed apparent at the level of eucalypt subgenus, closer examination revealed patterns of abundance were characteristic of each Eucalyptus species. The composition of foliar damage corresponded with the prominence of particular insect groups. Sucking insects tended to dominate the fauna except on E. obliqua where chewing insects in general, and chrysomelids in particular, were most prevalent. Both the distribution and magnitude of herbivory suggested that phytophagous insects had a negligible effect on competition between coexisting juvenile eucalypts and were unlikely to be responsible for the dominance of Monocalyptus or the maintenance of mixed species stands.     

Recovery of the bilberry (Vaccinium Myrtillus L.) from artificial spring and summer frost

Vegetative and sexual recovery of bilberry (Vaccinium myrtillus L.) was monitored in the field for three growing seasons after artificially applied spring and early summer frost. Bilberry recovered vegetatively (density and biomass) by vigorous production of new ramets and by production of large shoots in the damaged ramets. Recovery did not occur sexually (production of flowers), however. Summer frost was slightly more detrimental than the spring frost, but the difference was significant only in the percentage of flowering ramets. The results showed that bilberry is buffered against moderate frost injury. The rapid vegetative recovery guarantees survival of the plants after frost damage, whereas sexual reproduction may need accumulation of resources for many seasons. Consequently, the ability of bilberry to recover from sporadic frosts increases its potentiality to persist in a changing environment, where increasing temperature leads to earlier snowmelt in the spring, and increases the risk of frost injuries in plants that overwinter under snow.     

Long-term suppression of insect herbivores increases the production and growth of Solidago altissima rhizomes

Summary Although insect herbivores have many well documented effects on plant performance, there are few studies that assess the impact of above-ground herbivory on below-ground plant growth. For a seven year period in which no large-scale herbivore outbreaks occurred, a broad spectrum insecticide was utilized to suppress herbivorous insects in a natural community dominated by Solidago altissima. Ramet heights, rhizome lengths, rhizome biomass, and the number of daughter rhizomes all were lower in the control plots than in the insecticidetreated plots. These effects should lead to a decrease in the fitness of genets in the control plots relative to the fitness of genets in the insecticide-treated plots. We also found that ramets in the control plots appear to have compensated for herbivory: the ratio of rhizome length to rhizome biomass was greatest in the control plots, which indicates that clones moved farther per unit biomass in these plots than in the insecticide-treated plots. Clonal growth models show that this shift in allocation patterns greatly reduced the magnitude of treatment differences in long-term clonal displacements.Previous work has shown, and this study verified, that clonal growth in S. altissima is well represented by random-walk and diffusion models. Therefore, we used these models to examine possible treatment differences in rates of clonal expansion. Although rhizome lengths were greater in the insecticide-treated plots, results from the models suggest that our treatments had little impact on the short- and long-term displacement of S. altissima ramets from a point of origin. This occurred because S. altissima ramets backtrack often, and thus, treatment differences in net displacements are less pronounced than treatment differences in rhizome lengths.     

酸雨和采食模拟胁迫下克隆整合对空心莲子草生长的影响

研究表明克隆整合可以显著提升异质环境中克隆植物的生长,然而当克隆植物遭受均质环境压力时,整合对植物生长影响的研究相对较少。本文以典型入侵克隆植物空心莲子草（Alternanthera philoxeroides）为例,研究均质环境压力酸雨和采食模拟胁迫对空心莲子草生长的影响,以及克隆整合在空心莲子草适应不利环境过程中所起的作用。酸雨设3种浓度梯度：pH值3.5 、pH值4.5和 pH值6.5（对照）;采食设3种水平：不去叶、去叶50%和去叶90%;整合水平：匍匐茎切断和连接。结果表明：无论保持或切断匍匐茎的连接,酸雨处理都不影响空心莲子草生物量。当保持匍匐茎连接时,pH值4.5酸雨处理增加了空心莲子草匍匐茎长度和分株数目,因此,低度酸雨可能对空心莲子草生长有一定的促进作用。同样,无论匍匐茎是否被切断,采食处理都显著降低了空心莲子草克隆片段生物量,而显著增加了叶片数目。当切断匍匐茎连接时,采食处理使空心莲子草分株数目显著增加。本文得出的结论是：空心莲子草能较好地适应酸雨和采食的环境压力,当空心莲子草全部克隆分株遭受均质环境胁迫时,克隆整合并不能显著改善它的生长。     

Intersexual differences in phenology and damage by herbivores and pathogens in dioecious Rubus chamaemorus L.

Summary Shoot development and damage by herbivores and pathogens to male, female and non-floral ramets of the dioecious, perennial herb Rubus chamaemorus were studied in the field during three consecutive years. Leaves on male ramets were usually consumed more by herbivores and attacked more by fungi than were leaves on female ramets. Male ramets unfolded their leaves later than did female ramets. In 1983, when the level of herbivory was comparatively high, ramets that were fully developed in early June were more damaged by herbivores than were ramets that unfolded their leaves later. Non-floral ramets usually showed intermediate levels of damage compared to male and female flowering ramets.Defoliation caused a greater increase in ramet mortality among females than among males in a field experiment. It is suggested that the different reproductive roles of males and females differently constrain the evolution of defense against herbivores and pathogens in dioecious plants. This may result in intersexual differences both in palatability to herbivores and susceptibility to pathogens.     

Effects of the loss of clonal integration on four sedges that differ in ramet aggregation

Although clonal growth is a dominant mode of plant growth in wetlands, the importance of clonal integration, resource sharing among ramets, to individual ramet generations (mother and daughter) and entire clones of coexisting species has not been well investigated. This study evaluated the significance of clonal integration in four sedge species of varying ramet aggregations, from clump-forming species (Clumpers –Carex sterilis, Eleocharis rostellata), with tightly aggregated ramets (rhizomes<1cm), to runner species (Runners –Schoenoplectus acutus, Cladium mariscoides), with loosely aggregated ramets. We manipulated clonal integration by either severing connections between target mother and daughter ramets or leaving connections intact, and then planted them in an intact neighborhood of a fen in Michigan, USA. We measured growth parameters of original and newly produced ramets over two growing seasons and conducted a final biomass harvest, to address four hypotheses. First, we expected integrated clones to accumulate more biomass than severed clones. However, final clone-level biomass and ramet production were the same for both treatments in all species although severing initially stimulated ramet production by Schoenoplectus and produced a more compact ramet aggregation in Cladium. Second, we hypothesized that mother ramets would experience a cost of integration, through reduced ramet or biomass production, while daughters would experience a benefit, through increased resource availability from mothers. Mother ramets of Cladium suffered a cost from integration, while Schoenoplectus mothers suffered a slight cost and Carex daughters saw a slight benefit. Finally, we hypothesized that integration would be more active in runner species than in clumper species. Indeed, we documented more active integration in runners than clumpers, but none of the study species were dependent upon integration for growth or survival once daughter ramets were established with their own roots and shoots. This study demonstrates that integration between established ramets may not be the most important advantage to clonal growth in this wetland field site. The loss of integration elicited varied responses among coexisting species in their natural habitat, somewhat but not completely related to their growth form, suggesting that a combination of plant life history traits contributes to the dependence upon clonal integration among established ramets of clonal species.     

Responses of Hedysarum Laeve , a guerrilla clonal semi-shrub in the Mu Us sandland, to local sand burial

In arid and semi-arid inland deserts, one of the environmental stresses for plants is recurrent sand burial, which can influence the physical and biotic microenvironments of the plants and soil. Previous studies have shown that different levels of sand burial have different effects on plants. Slight sand burial could increase the height increment, leaf biomass and the number of new ramets of the plants while heavy sand burial could impair the growth of the plants and even decrease their chances of survival. In other words, below a certain threshold level of burial, the growth of plants is stimulated probably because of multiple factors. However, as the level of burial increases, the positive response starts to decline until it becomes a negative value. Arid and semi-arid inland deserts are frequently colonized and stabilized by many rhizomatous clonal plants. Clonal physiological integration often helps clonal plants buffer local environmental stress encountered by ramets. A rhizomatous clonal semishrub, Hedysarum laeve (H. laeve), is the dominant plant species and important for vegetation restoration in the Mu Us sandland. To investigate whether clonal integration can increase the threshold of sand burial and help rhizomatous H. laeve tolerate heavy sand burial, we conducted a field experiment. The results showed that slight sand burial could accelerate ramet growth and enhance leaf biomass, stem biomass and shoot biomass, while heavy sand burial reducesed the biomass of the plant and impairs survival and growth of the ramets. Clonal integration increased the threshold of sand burial. Under heavy sand burial, ramets connected to other ramets not buried in sand were more in terms of height increment, stem biomass, leaf biomass and shoot biomass compared to the ramets encountering sand burial but disconnected from other ramets. It suggested that clonal physiological integration could help H. laeve ramets tolerate relatively heavy sand burial. We also discussed that clonal integration plays a role in H. laeve presence in the Mu Us sandland. __________ Translated from Journal of Plant Ecology (formerly Acta Phytoecologica Sinica), 2006, 30(2): 278–285 [译自: 植物生态学报]     

Clonal growth of sagebrush (Artemisia tridentata) (Asteraceae) and its relationship to volatile communication

To increase systemic resistance to herbivory, some clonal plants transmit internal cues among clonal ramets to prevent widespread damage to genets. Sagebrush (Artemisia tridentata) (Asteraceae) is known to use volatile cues to induce resistance within and between plants (so‐called volatile communication). In the present study, we observed the extent and frequency of clonal growth in a natural sagebrush population in western North America to understand the influence of clonal growth on volatile communication. We used genetic analysis involving microsatellite markers and excavation of the root systems. In addition, we characterized the volatile profiles from the headspace of sagebrush ramets. Excavation of the root system of sagebrush plants revealed that sagebrush propagates clonally below ground and that daughter ramets grow near the mother stem. Volatiles were variable among genetically different ramets, although clonal ramets (genetically identical ramets) released similar volatiles, suggesting a genetic basis for volatile similarity. Sagebrush has been shown to be most responsive to volatiles released from artificially produced clones and suffers less herbivore damage as a result. Therefore, these results, taken into consideration together, imply that volatile communication may occur among genetically identical ramets under natural conditions, and that volatile similarity between the releaser and receiver may be recognized by the receiver and increase resistance against herbivory.     

Internal heterogeneity of ramet and flower densities of <Emphasis Type="Italic">Typha angustifolia</Emphasis> near the boundary of the stand

Fifteen relatively narrow stands of Typha angustifolia, including both stands bounded to open water and ridged boundary, were investigated in terms of the distributions of ramet density as well as the fraction of flowering ramets. The ramet density, fraction of flowering ramet and morphological characteristics of ramets were measured continuously from one side of the stand to the other, together with depth and the nutrient concentration of pore water. The rhizome biomass was also measured at the periphery and at the center of the stand. The results indicated that the shoot density declined gradually from the edge to the center, however, the fraction of the flowering ramets were significantly higher at the edge while it rapidly declined away from the edge. Statistical analyses indicate that the ramet density depends on either depth or nutrient level of the pore water; however, the distance from the edge was the most probable factor to reduce the fraction of the flowering ramets. The rhizome biomass was mostly larger at the periphery than at the center, likely because the accumulation of the horizontal rhizomes as extension was blocked by the boundary. High flowering fraction near the periphery was probably induced by the hormonal change caused by the blockage. However, a large fraction of ramets flowered simultaneously with large rhizome biomass at the periphery, likely because of the preferable light climates compared with inside the stand.     

Effect of reservoir drawdown on biomass of three species of aquatic macrophytes in a large sub-tropical reservoir (Itaipu, Brazil)

The biomass and maximum depth of colonization (Z _max) of Egeria najas Planchon (submerged), and the biomass and area covered by a stand of Eichhornia crassipes (Mart.) Solms and Salvinia herzogii Raddi (floating species) were assessed to measure the effects of a 5 m drawdown in water level of the Itaipu Reservoir (Brazil–Paraguay), which lasted about 3 months. The frequency of occurrence of the two first species, and of Salvinia spp was also assessed in the main arms of the reservoir. A conspicuous decrease of E. najas biomass was observed and this attribute did not reach the previous values even 10 months after the water level had returned to normal. Rapid growth of free-floating species (surface t _doub = 2.9 and 3.2 days for total biomass of S. herzogii and E. crassipes, respectively, and 2.3 days for surface area covered by both species) was recorded immediately after the water level returned to normal. This fast growth was related to phosphorus increases in water. A clear succession was observed over a period of 103 days, during which S. herzogii was slowly substituted by E. crassipes. The effects of water level drawdown were also observed on E. najas frequency, given that this species occurred in 38% of the stands investigated before the water drawdown, but in only 9 and 6% of stands following 1 and 10 months of water level recovery, respectively. Water drawdown did not affect the frequency of occurrence of the floating species, which remained approximately constant, and lower than 15% in several of the reservoir arms investigated.     

Evolution of an invasive phenotype: shift to belowground dominance and enhanced competitive ability in the introduced range

In response to novel selection pressures in an introduced range, non-native species may evolve more competitive phenotypes unique from those of their native range. We examined the existence of an invasive phenotype in the herbaceous perennial Artemisia vulgaris, a frequent invader of the Northeast and Mid-Atlantic US. Populations from both the native (European) and the introduced (North American) ranges were grown in intra-specific competition (same population), inter-specific competition with the native perennial herb Solidago canadensis, and alone in a common garden to quantify shifts in resource allocation and neighbor effects on performance and competitive ability. Without competition, introduced A. vulgaris populations were much shorter than native populations, but germinated earlier, produced more ramets, more belowground and total biomass, and maintained higher root-to-shoot ratios. Under inter- and intra-specific competitions, introduced A. vulgaris populations were shorter, but produced more ramets, belowground, and total biomass than native populations. S. canadensis belowground and total biomass were more highly suppressed by introduced than native A. vulgaris. Our data suggest that since the introduction to North America, A. vulgaris has evolved a more competitive invasive phenotype characterized by many short ramets with more extensive root/rhizome networks. This rapid evolutionary shift likely benefits A. vulgaris in its introduced range by allowing establishment and subsequent dominance in dense stands of existing vegetation.     

Root damage and aboveground herbivory change concentration and composition of pyrrolizidine alkaloids of Senecio jacobaea

Thus far not many studies focussed on how herbivory in one plant part affects plant defence in the other. The effects of root damage and a leaf-feeding herbivore (Mamestra brassicae) on pyrrolizidine alkaloid (PA) levels of Senecio jacobaea were investigated in a controlled environment. Three cloned S. jacobaea genotypes, which differed in PA concentrations, received four treatments: (1) no damage, (2) root damage (removing half of the root system), (3) shoot herbivory by M. brassicae larvae, (4) root damage and shoot herbivory.Shoot herbivory did not significantly affect shoot biomass, while root damage decreased both root and shoot biomass. Shoot herbivory decreased PA concentrations in the roots. Conversely, root damage increased PA concentrations in the roots. Alkaloid concentrations in the shoot showed a weak response to root damage, shoot herbivory had no effect on PA levels in the shoot. The effect of damage on the allocation of PAs to shoot and roots depended on genotype. One genotype allocated more PAs to the damaged site, another genotype did not change allocation and the third genotype allocated more PAs to the shoot if the roots were damaged. Changes in PA composition were observed in one genotype. Shoot herbivory increased erucifoline concentrations in the shoot and decreased concentrations of senecionine in the roots. In conclusion, we have shown that even in an alleged constitutively defended plant, damage of one compartment affects secondary metabolite level in the other.     

Carbon pools in a boreal mixedwood logging chronosequence

Mixedwood forests are an ecologically and economically important forest type in central Canada, but the ecology of these forests is not as well studied as that of single-species dominated stands in the boreal forest. Northern boreal mixedwood forests have only recently been harvested and the effects of harvesting on carbon content in these stands are unknown. We quantified the carbon content and aboveground net primary production (NPP) for four different-aged mixedwood boreal forest stands in northern Manitoba, Canada. The stands included 11-, 18-, and 30-year-old stands that originated from harvesting and a 65-year-old fire-originated stand that typifies the origin of all northern boreal mixed-wood forests that are coming under management. Trees included black spruce (Picea mariana (Mill.) B.S.P.), jack pine (Pinus banksiana Lamb.), balsam poplar (Populus balsamifera L.), and quaking aspen (Populus tremuloides Michx.). Overstory biomass was estimated using species-specific allometric models that generally explained greater than 95% of the observed variation in biomass. Carbon content of the overstory vegetation was greatest in the 65-year-old stand and was 74% larger than the 11-year-old stand and showed a positive relationship with stand age (F_{1, 2}=122.62, P=0.0081 R²=0.99). The slope of mineral soil carbon did not differ significantly among stands (F_{1, 2}=0.39, P=0.5956, R²=0.16). Coarse woody debris carbon content followed a U-shaped pattern among stands. Aboveground NPP differed by 24% between the youngest and oldest stand. Mean annual carbon accumulation and aboveground NPP rates of the mixedwood forests were on average two times greater than nearby relatively pure stands studied during the BOREAS (BOReal Ecosystem Atmospheric Study) project. The trends in the results, along with other field studies, suggest that harvesting does not significantly affect the total soil carbon content. The results of this study suggest that scientists should be cautious about extrapolating results from BOREAS stands to a broader region until more data on other forest types and regions are available.     

扎龙湿地保护区异质生境芦苇种群分株构件的数量特征

采用单位面积取样,计数和测量的调查与统计方法,对扎龙湿地保护区4个生境单优群落芦苇种群分株构件数量特征进行比较分析。结果表明,4个生境芦苇种群从5月10日左右返青后进入营养生长期,分株高度和分株密度均持续增加到生殖生长初期的8月份,其中6-8月份差异均达到显著水平(P0.05),8-10月份差异均未达到显著水平(P0.05),芦苇进入生殖生长期后,分株便停止高度生长,地下芽的输出也不再形成分株补充现实种群;分株生物量和种群生物量均持续增加到生殖生长旺盛期的9月份,至休眠期的10月份均有所降低,各月份间的差异均达到显著水平,芦苇种群在生长季末期,具有将生产的物质分配转移到地下储藏与营养繁殖器官的形成与生长上的特性。芦苇种群分株数量特征与返青后实际生长时间之间均较好地符合对数函数关系,R~2在0.818-0.994之间,拟合方程均达到了P0.05的显著水平。4个生境芦苇种群分株密度以湿生生境最大,依次为水生生境、旱生生境,盐碱生境最小,分株高度、分株生物量和种群生物量均以水生生境最大,依次为湿生生境、旱生生境,盐碱生境最小。因此,4个生境芦苇种群分株构件数量特征均表现出基本一致的生育期节律性,同时,芦苇种群的个体生长和种群动态存在明显的环境效应,土壤含水量、pH是影响该地区芦苇分株数量特征的主要因子。     

扎龙湿地芦苇分株生态可塑性及其对土壤因子的响应

扎龙湿地的芦苇既可形成大面积的单优群落,也可形成不同群落斑块。采用大样本抽样调查与统计分析方法,对湿地内水生生境、湿生生境、旱生生境和盐碱生境芦苇种群分株高度和生物量进行比较。结果表明,6—10月份,4个生境芦苇种群分株高度及生物量均以水生生境最高,盐碱生境最低,水生生境株高为盐碱生境的1.5—2.3倍,分株生物量为2.0—5.1倍,生境间的差异性以及差异序位均相对稳定。4个生境株高生境间变异系数(19.45%—31.56%)均高于生境内变异系数(8.07%—17.61%),分株高度在生境间的可塑性更大;分株生物量中水生生境、湿生生境和盐碱生境3个生境间的变异系数(33.43%—55.61%)均低于生境内变异系数(44.85%—79.82%),分株生物量在生境内的可塑性更大。不同生境条件下芦苇种群分株,在生长和生产上均存在较大的生态可塑性,表现出明显的环境效应,其中土壤含水量是该地区芦苇分株生态可塑性变异的主要驱动因子(R0.80),为正向驱动。     

Little strokes fell great oaks: minor but chronic herbivory substantially reduces birch growth

Modern concepts of plant tolerance to herbivory are primarily based on studies of short‐term severe damage, whereas the effects of minor chronic damage to long‐lived woody plants, corresponding to background herbivory (2–15% annual loss of foliar biomass in boreal and temperate forests), remain poorly understood. In our experiment, the annual removal of 2, 4, 8 and 16% of the leaf area from naturally growing mountain birch Betula pubescens subsp. czerepanovii saplings during a seven‐year period resulted in a pronounced reduction of plant vertical growth (–30, –34, –45 and –78%, respectively). Leaf size decreased first (already after one year of the 16% treatment), resulting in the reduction of the total leaf area. This effect was followed by a considerable decrease in the length of long shoots in all treatments. Leaf number on the plant was maintained for a longer time, being reduced by the end of the experiment in 16% treatment only; no changes in specific leaf area or chlorophyll fluorescence were observed in either of the treatments. This pattern may indicate that the plant reallocates resources from the growth of the woody parts to the maintenance of the photosynthetic area, and can be seen as a strategy of tolerance to minor herbivory, whereas compensatory responses typical of severe herbivory (increased photosynthesis rates and shoot regrowth) have not been detected. The predicted 2–5% increase in background herbivory due to climate warming can potentially produce previously unrecognised negative impacts on tree growth. We conclude that in the long term, background herbivory is likely to impose stronger effects on the growth of woody plants than short‐term devastating outbreaks of defoliators, thus contributing more to the development of plant evolutionary adaptations to herbivory than severe but episodic bouts of damage.