Vegetation development on pumice at Mount St. Helens, USA

This study explores early vegetation development on pumice at Mount St.Helens. We monitored species annually in a grid of 200 contiguous100-m² plots between 1989 and 1999. Of interestwere how vegetation changed and if it became more homogeneous over time.Speciesrichness and cover increased annually. Diversity(H) stabilized by 1996 and began to decline aslong-livedstress-tolerant species such as Agrostis pallens, Carexmertensii and Penstemon cardwellii began todominate. Protected sites had more species and higher cover than did exposedones. Plots next to relict vegetation had more species and cover than diddistant plots. The vegetation initially was dominated by species with gooddispersal, but subsequently those with poor dispersal became dominant. Wecompared species expansion patterns to a model based on random colonization.Theresults implied that populations with poor dispersal derived from a fewcolonists that then produced seeds for local expansion. Detrendedcorrespondenceanalysis showed a pronounced shift in species composition. This analysis alsoshowed that species composition was becoming more homogeneous over time.However, significant heterogeneity remained and some plots are diverging fromothers. As yet, this vegetation is not developing towards a regional vegetationtype. Rather, an unusual community with Agrostis spp.,Carex spp., Penstemon cardwellii, Lupinuslepidus, Anaphalis margaritacea and Salixcommutata has developed. The accumulation phase of primarysuccessionis nearly complete. The next phase, in which vertical structure develops asSalix and conifers mature, has scarcely begun. It shouldbemarked by the invasion of forest understory species and loss of subalpinemeadowspecies. Assembly rules based on biotic interactions may then become evident.     

The Fundamentals of Vegetation Change - Complexity Rules

Long-term vegetation dynamics based on paleo-pollen data display transient behaviour, often alternating in phase between predominant determinism and predominant 'turbulence', when viewed as a trajectory in a multivariate phase space. Given this, the metaphor of vegetation dynamics as a 'flowing stream', first introduced by Cooper in his classic 1926 paper entitled "The fundamentals of vegetation change", is re-examined and revealed to be not only useful, but strikingly realistic. Vegetation dynamic theory is reviewed and classic theories are found to reflect reality poorly. It is suggested that vegetation dynamics is a far from equilibrium system, and that the application of nonequilibrium thermodynamic theory is appropriate.     

Scale,succession and complexity in island biogeography: are we asking the right questions?

• 1 This paper offers a commentary on the development of island ecological theory since the publication of MacArthur & Wilson’s equilibrium theory in the 1960s. I distinguish the simple model at the core of their Equilibrium Theory of Island Biogeography (ETIB) and the broader body of their theory, which embraces evolutionary as well as ecological patterns — all, however, within the overarching framework or assumption of equilibrium.
• 2 The basic problems with the ETIB have long been known, and its status as a ruling paradigm has been the subject of concern for more than two decades. With the development of nonequilibrium ideas in ecology, island biogeographers arguably now have viable theoretical frameworks to set alongside or around the ETIB. Four conditions are highlighted as extremes: i) dynamic equilibrium; ii) dynamic nonequilibrium; iii) ‘static’ equilibrium; and iv) ‘static’ nonequilibrium: together providing a conceptual framework for island ecological analyses.
• 3 The importance of scale is stressed and attention is drawn to Haila’s spatial‐temporal continuum as an organizational device. It is argued that the processes represented within the ETIB (and by extension, other island theories) may be prominent within only a limited portion of this continuum, while elsewhere they are generally subsumed by other dominant processes.
• 4 Colonization and ecosystem development of near‐shore islands constitute just a special case of ecological succession, and thus the development of theories of island assembly may benefit accordingly from efforts to incorporate ideas from the ecological succession literature.
• 5 The desirability of specifying answerable questions is stressed, as is the need to build a greater degree of complexity into the development of island ecological models. Notwithstanding which, it is also recognized that key advances are often brought about by simple, but bold models, of the form exemplified elsewhere in this issue.

     

Does seed dispersal limit initiation of primary succession in desert playas?

To investigate the initiation of primary succession in a cold-desert playa-dune complex, we studied the large-scale (2000 m) seed (diaspore) dispersal patterns at Mono Lake, California. Seeds of seven of the ten species reaching the barren playa had wind-dispersal adaptations. Rates of dispersal (numbers of seeds per square metre per day) were as much as three orders of magnitude lower on the playa than in the diverse dune vegetation. However, seed input appeared sufficient to reach potential safe sites on the playa, with a peak input of 66 ± 8 total seeds·m·d. The smooth playa surface, the virtual absence of aboveground barriers, and the high windspeed environment promote the long-distance dispersal of seeds (at least 1300 m for Chrysothamnus spp. and at least 700 m for Sarcobatus vermiculatus). The large spatial scale of sampling revealed a relatively high seed input onto the playa by the dominant pioneer species S. vermiculatus, despite the low abundance of parent vegetation in this region. All of these results implicate low rates of seed entrapment as an obstacle to establishment on this desert playa, rather than a lack of seed input.     

Early succession on lahars spawned by Mount St. Helens

The effects of isolation on primary succession are poorly documented. I monitored vegetation recovery on two Mount St. Helens lahars (mud flows) with different degrees of isolation using contiguous plots. Seventeen years after the eruption, species richness was stable, but cover continued to increase. That isolation affects community structure was confirmed in several ways. The dominance hierarchies of the lahars differed sharply. Detrended correspondence analysis on Lahar I showed a trend related to distance from an adjacent woodland, whereas vegetation on Lahar II was relatively homogeneous. Spectra of growth forms and dispersal types also differed. Lahar I was dominated by species with modest dispersal ability, while Lahar II was dominated by species with better dispersal. Variation between plots should decline through time, a prediction confirmed on Lahar II. Lahar I remained heterogeneous despite having developed significantly higher cover. Here, the increasing distance from the forest has prevented plots from becoming more homogeneous. At this stage of early primary succession, neither lahar is converging towards the species composition of adjacent vegetation. This study shows that isolation and differential dispersal ability combine to determine initial vegetation structure. Stochastic effects resulting from dispersal limitations may resist the more deterministic effects of competition that could lead to floristic convergence.     

Seasonal succession of Cladocerans in a ricefield in Italy

A biological ricefield in northern Italy, without periodic dry spells in its growing cycle and therefore more familiar to naturally humid zones, was studied for its heleoplankton community. The biocoenosis reached a greater level of complexity than reported in literature. In particular, the seasonal succession of Cladocerans, the dominant group throughout the study period, was analyzed. Wlassicsia pannonica (Daday, 1904; Anomopoda Macrothricidae), is new to Italy; its morphology is compared to that of other populations of the same species and its biological cycle is compared with that of other dominant Cladocerans.     

中国北方化石昆虫群的建立及其演化序列(英文)

作者几十年来已积累了众多的中国北方古生代晚期、中生代和第三纪的昆虫化石标本,对其长期的系统研究结果表明：中国北方昆虫化石演化序列可以划分为三个大的演化阶段,即古生代晚期、中生代和第三纪演化阶段,其中包含14个昆虫群虫和29个昆虫组合。这些昆虫群及其组会既反映了北方昆虫在地质历史上兴衰的发展规律,也为地层划分、对比与地质时代的鉴定提供了依据。