The role of succession in the evolution of flammability

Fire-prone ecosystems contain plants that are both fire-adapted and flammable. It has been hypothesized that these plants were under selection to become more flammable, but it is unclear whether this could be adaptive for an individual plant. We propose arrested succession as a robust mechanism that supports the evolution of flammability in surface fire ecosystems without the need to invoke group selection or additional fitness benefits. We used the natural history of lodgepole pine (Pinus ponderosa) forests, longleaf pine (Pinus palustris) forests, and tall grass prairies to create a general mathematical model of surface fire ecosystems and solved for the evolutionarily stable strategy (ESS) level of flammability. In our model, fires always kill understory plants and only sometimes kill overstory plants. Thus, more flammable plants suffer increased mortality due to fires, but also more frequently arrest succession by clearing their understory of late successional competitors. Increased flammability was selected for when the probability of an overstory plant dying from an individual fire was below a maximum threshold and the rate of succession relative to fires was above a minimum threshold. Future studies can test our model predictions and help resolve whether or not plants have been selected to be more flammable.     

Rate of succession in restored wetlands and the role of site context

Question: Are changes in plant species composition, functional group composition and rates of species turnover consistent among early successional wetlands, and what is the role of landscape context in determining the rate of succession? Location: Twenty‐four restored wetlands in Illinois, USA. Methods: We use 4 years of vegetation sampling data from each site to describe successional trends and rates of species turnover in wetlands. We quantify: (1) the rate at which composition changes from early‐successional to late‐successional species and functional groups, as indicated by site movement in ordination space over time, and (2) the rate of change in the colonization and local extinction of individual species. We correlate the pace of succession to site area, isolation and surrounding land cover. Results: Some commonalities in successional trends were evident among sites. Annual species were replaced by clonal perennials, and colonization rates declined over time. However, differences among sites outweighed site age in determining species composition, and the pace of succession was influenced by a site's landscape setting. Rates of species turnover were higher in smaller wetlands. In addition, wetlands in agricultural landscapes underwent succession more rapidly, as indicated by a rapid increase in dominance by late‐successional plants. Conclusions: Although the outcome of plant community succession in restored wetlands was somewhat predictable, species composition and the pace of succession varied among sites. The ability of restoration practitioners to accelerate succession through active manipulation may be contingent upon landscape context.     

Changes in life history trait composition during undisturbed old-field succession

This paper describes the patterns of vegetation change by spontaneous succession on former agricultural land. In 1968, initiated by Heinz Ellenberg, an experimental study site was set up on an arable field in the new botanical garden of Göttingen University (Germany). The undisturbed successions on four plots of a long sere (since 1968/69) and on four additional plots of a shorter sere (since 1982) were evaluated. The seres were classified into stages by cluster analysis, which yield to four subsequent stages for the long sere and to three subsequent stages for the short sere. The early succession is characterized by a high proportion of species invasion, whereas with developing time species extinction increased. On all study plots undisturbed succession directed to the development of pioneer forests. Altogether 247 vascular plant species were recorded. The total species richness shows a fluctuating course during the successional seres, with significantly increased numbers shortly after succession has started and a decline in the pioneer-forest stage, when stands age and close down. The primary aim was to compare the characteristics (particular traits) of species occurring at different stages of succession. The majority of the traits concerned exhibited some clear trends in the course of succession. Exclusive reproduction by seeds decreased and the ability for additional vegetative reproduction increased. Anemochorous dispersal significantly decreased, whereas the importance of dispersal by animals, especially endochorous, dominates in the pioneer-forest stage. The mean seed weight significantly increased. During early succession, plant species staying green over winter dominated, but are of none relevance in the pioneer-forest stage, where summergreen plant species dominate. The change in strategy type features a significant increase of competitive species. The mean indicator value for light in the herb layer decreased significantly in the pioneer-forest stage.     

The role of frugivorous bats in tropical forest succession

Discussion of successional change has traditionally focused on plants. The role of animals in producing and responding to successional change has received far less attention. Dispersal of plant propagules by animals is a fundamental part of successional change in the tropics. Here we review the role played by frugivorous bats in successional change in tropical forests. We explore the similarities and differences of this ecological service provided by New and Old World seed-dispersing bats and conclude with a discussion of their current economic and conservation implications. Our review suggests that frugivorous New World phyllostomid bats play a more important role in early plant succession than their Old World pteropodid counterparts. We propose that phyllostomid bats have shared a long evolutionary history with small-seeded early successional shrubs and treelets while pteropodid bats are principally dispersers of the seeds of later successional canopy fruits. When species of figs (Ficus) are involved in the early stages of primary succession (e.g. in the river meander system in Amazonia and on Krakatau, Indonesia), both groups of bats are important contributors of propagules. Because they disperse and sometimes pollinate canopy trees, pteropodid bats have a considerable impact on the economic value of Old World tropical forests; phyllostomid bats appear to make a more modest direct contribution to the economic value of New World tropical forests. Nonetheless, because they critically influence forest regeneration, phyllostomid bats make an important indirect contribution to the economic value of these forests. Overall, fruit-eating bats play important roles in forest regeneration throughout the tropics, making their conservation highly desirable.     

Ecological conditions on the post-solifluction areas and their role in the forming of primary succession

We studied the ecological conditions forming on the solifluction areas often occurring in cryolithic zone of Central Evenkia. A significant change of microenvironment was registered on some solifluction “microecosystems” that differ sharply form the control ones in plantation. The ecological conditions formed in these areas result in the change of species composition of vegetation and its diversity. However, we revealed no significant differences of separate ecological groups depending on hydrothermal and trophic conditions.     

The role of remnant forest trees in tropical secondary succession

This paper emphasizes the contribution of remnant trees to the establishment of woody species during succession on abandoned fields and pastures in the Mexican rain forest area, Los Tuxtlas, Veracruz. Remnant trees, original large forest trees left in the clearings by traditional farmers will become natural perching sites for both passing and resident birds. Frugivorous birds drop or regurgitate seeds and fruits which fall under the canopies of remnant trees during their stay, thus contributing to an accumulation of species, which make these remnant trees into ‘regeneration nuclei’. The species transported into these sites belong chiefly to older stages of successional development and reach these otherwise isolated areas, counteracting the depauperization of tropical land, brought about by both intensive and extensive clearing. In a study of seven remnant trees, 29 woody species and two climbers were found, 86% of which are bird dispersed. The total number of species per tree varied from 6 to 15 and was higher under remnant trees with fruits attractive to birds. Floristic variations of the understorey as detected by detrended correspondence analysis was correlated with the relative amount of shade-tolerant primary and late secondary trees versus light dependent pioneers and early successional trees.     

Maastrichtian facies succession and sea-level history of the Hossein-Abad,Neyriz area,Zagros Basin

The Maastrichtian shallow-water carbonate platform (Tarbur Formation) is described from outcrop in southwest Iran. It is characterised by eight microfacies types, which are dominated by larger foraminifera, rudist debris and dasycladacean algae. They are grouped into four distinct depositional settings: tidal flat, lagoon, barrier and open marine. The depositional settings include stromatolitic boundstone of tidal flat, peloidal dasycladacean miliolids wackestone and peloid bioclastic imperforate foraminifera wackestone of restricted lagoon, Omphalocyclus miliolids bioclast packstone–grainstone and miliolids intraclast bioclast packstone–grainstone of open lagoon, rudist bioclast grainstone of inner-platform shoals and rudist bioclast floatstone–rudstone and bioclastic wackestone of open-marine environments.

The facies and faunal characters are typical of a ramp-like open shelf. The lack of reef-constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. On the basis of facies analysis, three depositional sequences (third order) are defined.     

Co-dominance and succession in forest dynamics: the role of interspecific differences in crown transmissivity

Forests that are composed of two or more tree species with similar ecological strategies appear to contradict the competitive exclusion principle. Beech-maple communities are a well-known example of such a system. On a local scale, a number of mechanisms have been proposed to explain the coexistence of these two species. These are reciprocal replacement, external factors that favour alternatively one or the other species and demographic stochasticity. This paper presents and analyses a simple mathematical model that shows that external factors are not an essential requirement for coexistence. Rather, coexistence requires interspecific differences in light transmissivity through the crowns of adult trees. However, all the three mechanisms mentioned above can be interpreted within the framework of the model. Furthermore, many models of forest dynamics make use of shade tolerance as a key feature in describing successional dynamics. Despite its importance, however, shade tolerance does not have a commonly accepted quantitative definition. Here, a simple scheme is proposed where the relationship between shade tolerance, individual traits (growth and survival) and successional status is defined. This might have important implications in understanding the overall dynamics. Theoretical results have been compared with a number of studies carried out in North American forests. In particular, coexistence in beech-maple communities and the relation between shade tolerance and successional status in a beech-hemlock-birch community have been discussed.     

遗传学史在遗传学教学中的作用

科学史的研究和发展状况能反映一个国家的科学技术水平,遗传学史是生命科学发展史的一个重要分支,21世纪是生命科学的世纪,在遗传学教学中加强遗传学发展史的介绍,不仅具有教育功能,使学生了解遗传学的产生和发展,而且可以培养学生的思维能力和科学素质。本文就遗传学史的教育功能及在教学中的作用进行论述。     

A role of Ia-associated invariant chains in antigen processing and presentation

Most native antigens require processing in a cellular compartment for efficient presentation to T helper cells. The cellular elements that permit processing are not known. We investigated a possible role of the class II MHC-associated invariant chains in antigen processing. Fibroblast cells that were transfected with class II genes were compared with fibroblasts supertransfected with the invariant chain gene for their capacity to present the fifth component of complement (C5) to C5-specific class II restricted T cell clones or influenza virus protein to a virus-specific T cell clone. Only fibroblasts supertransfected with the invariant chain gene were able to present native antigen, even at very low antigen concentration, whereas both fibroblast types could present cyanogen bromide-fragmented C5 or the virus peptide. Presentation of intact antigen but not of fragmented antigen was totally abrogated by treatment of fibroblasts with chloroquine. The invariant chain gene encodes two polypeptides, li31 and li41. Expression of either li31 or li41 was sufficient to render class II-expressing fibroblasts capable of presenting intact antigen.     

Studies on the actomyosin ATPase and the role of the alkali light chains

Myosin isoenzymes, highly enriched in either alkali 1 or alkali 2 light chains have been prepared by light chain exchange in 4.7 M ammonium chloride, under conditions where there is minimal loss of ATPase activity. While the actin-activated ATPase measurements were complicated by a biphasic dependence on actin concentration, the two myosin isoenzymes behaved in a similar manner; at a variety of ionic strength conditions their maximum rates of ATP hydrolysis were nearly identical. Furthermore, under conditions where their Km values could be reliably determined, their apparent affinities for actin in the presence of ATP did not differ greatly. These results suggest that the presence of a particular alkali light chain does not influence the maximum rate of ATP turnover by actomyosin under ionic strength conditions approximating physiological.     

On the role of aromatic side chains in the photoactivation of BLUF domains

BLUF (blue-light sensing using FAD) domain proteins are a novel group of blue-light sensing receptors found in many microorganisms. The role of the aromatic side chains Y21 and W104, which are in close vicinity to the FAD cofactor in the AppA BLUF domain from Rhodobacter sphaeroides, is investigated through the introduction of several amino acid substitutions at these positions. NMR spectroscopy indicated that in the W104F mutant, the local structure of the FAD binding pocket was not significantly perturbed as compared to that of the wild type. Time-resolved fluorescence and absorption spectroscopy was applied to explore the role of Y21 and W104 in AppA BLUF photochemistry. In the Y21 mutants, FADH*-W* radical pairs are transiently formed on a ps time scale and recombine to the ground state on a ns time scale. The W104F mutant shows a spectral evolution similar to that of wild type AppA but with an increased yield of signaling state formation. In the Y21F/W104F double mutant, all light-driven electron-transfer processes are abolished, and the FAD singlet excited-state evolves by intersystem crossing to the triplet state. Our results indicate that two competing light-driven electron-transfer pathways are available in BLUF domains: one productive pathway that involves electron transfer from the tyrosine, which leads to signaling state formation, and one nonproductive electron-transfer pathway from the tryptophan, which leads to deactivation and the effective lowering of the quantum yield of the signaling state formation. Our results are consistent with a photoactivation mechanism for BLUF domains where signaling state formation proceeds via light-driven electron and proton transfer from the conserved tyrosine to FAD, followed by a hydrogen-bond rearrangement and radical-pair recombination.     

The role of spontaneous vegetation succession in ecosystem restoration: A perspective

Abstract. The paper summarizes ideas which were discussed during the ‘Spontaneous Succession in Ecosystem Restoration’ conference and elaborated through further discussion among the authors. It seeks to promote the integration of scientific knowledge on spontaneous vegetation succession into restoration programs. A scheme illustrating how knowledge of spontaneous succession may be applied to restoration is presented, and perspectives and possible future research on using spontaneous vegetation succession in ecosystem restoration are proposed. It is concluded that when implementing spontaneous succession for ecological restoration the following points must be considered: setting clear aims; evaluation of environmental site conditions; deciding whether spontaneous succession is an appropriate way to achieve the aims; prediction of successional development; monitoring of the results. The need for interdisciplinary approaches and communication between scientists, engineers and decision‐makers is emphasized.     

Species-area curve,life history strategies,and succession: a field test of relationships

Changes of species richness along temporal and environmental gradients were investigated. Two data sets were used: a successional sere of old-field plant communities in the Bohemian Karst, and a set of plant communities under various intensities of disturbance in the Krkonoe (Giant) Mts, both in Czechoslovakia. The species richness of a plant community is a spatial phenomenon, and should be described by the species-area relationship (using e.g. the power function S=c\A ^z) rather than by a single number. In the old-field succession, the number of species in very small plots (0.1×0.1 m) tends to increase with successional age while the number of species in larger plots (4×4 m) decreases from the third year of succession. The plant community under the highest rate of disturbance of the Krkonoe Mts data set shows the lowest number of species on small plots and the highest number of species on large plots. The results may be explained using the distinction between founder-and dominance-controlled communities (Yodzis 1978, 1984). In accordance with this theory, the species-area relationship within a community is shaped mainly by the type of competitive interaction and may be predicted on the basis of life-history strategies of constituent species. Disturbance causes a shift from dominance to founder control. On the landscape scale, the species-area relationship is shaped by other factors, and so it is unjustified to extrapolate the relationship outside the range in which it was originally assessed.