西天目山黄山松阔叶林的冠层干扰与动态推测

根据年龄结构和直径生长图分析研究了浙江省西天目山黄山松阔叶混交林的冠层干扰年表和发展动态。结果表明黄山松在该林地的定居发生在１８６１－１８８０年之间,黄山松定居３０－４０年后,其他阔叶树开妈在林下萌发生长,形成目前的黄山阔叶混交林,该林浆被阔叶叶树取代。     

The impact of grazing management on Orthoptera abundance varies over the season in Mediterranean steppe-like grassland

As semi-natural grassland has a high level of biological diversity, understanding the effects of grazing and its variation over time is important in order to identify sustainable grazing practices. We measured temporal variation in Orthoptera abundance and spatial vegetation structure during seasonal grazing in an extensive sheep-farming system. We studied five grazed pasture areas (pre-grazing and post-grazing) and two adjacent ungrazed grasslands. We recorded the total abundance of Orthoptera and described the vegetation structure of 175 replicate plots (25 per pasture/grassland) during six field sampling sessions. We demonstrated that the impact of grazing on Orthoptera abundance is species-specific and greatly varies over the grazing season. The decrease of phytovolume is significant after 4–7 weeks of sheep grazing. Total Orthoptera abundance was higher in pre-grazed plots than in ungrazed plots, and higher in ungrazed plots than in post-grazed plots. These differences were particularly high during the peak of adult abundance. No difference in species richness was observed between grazing intensities. Total Orthoptera abundance positively correlated to phytovolume only when grazing pressure was high. However, the relationship between abundance and phytovolume differed between species. Extensive grazing by sheep tends to homogenize spatial vegetation structure and to temporarily reduce total Orthoptera abundance at pasture scale. However, rotational grazing allows spatial and temporal heterogeneity in vegetation structure to be maintained at farm scale, heterogeneity that is beneficial for Orthoptera. In contrast, absence of grazing has a negative impact on Orthoptera abundance as it favours the accumulation of litter, which is detrimental for a high proportion of xerothermophilic Orthoptera associated with bare ground and short vegetation.     

Multivariate analysis of plant communities in the Narok district, Kenya: The influence of environmental factors and human disturbance

Species type in 42 samples was examined using ordination analysis to establish whether the vegetation pattern displays gradation due to environmental factors, and whether the pattern was being altered by human disturbance. The analysis displayed two patterns. First, the ordination on axis 1 demonstrated vegetation gradation from xeric communities on the plains to humid forests on the highland. This pattern is explained on the basis of moisture and nutrient variations. Vegetation vectors based on species presence-absence were correlated with average rainfall, carbon, nitrogen and zinc.The second was separation of disturbed and undisturbed forest and bushland site categories on axis 2 of the ordination space. This pattern suggests that disturbance response is interactive with moisture so that only in moist communities does a significant vegetation response to disturbance occur.     

Conservation implications of grazing practices on the plant and dipteran communities of a turlough in Co. Mayo,Ireland

Turloughs, which are classified as priority habitats under the European Habitats Directive, are seasonally flooded depressions found almost exclusively in Ireland. In 2001, three adjacent fields with different stocking densities were selected and plant/dipteran communities within the same vegetation zone of each field (site) were investigated using quadrats and sweep netting, respectively. There was a significant positive relationship between Diptera morphospecies richness/Diptera abundance and mean vegetation height (P < 0.001). However, no significant relationship between Diptera morphospecies richness and plant species richness was found. Median Diptera morphospecies richness per sweep was lower at the site with the highest stocking density (17) than at the other two sites (22 and 31, respectively). Total species richness of Sciomyzidae was greater at the least grazed site (7) than at the more heavily grazed sites (2 and 1, respectively). The results suggest that an evaluation of turlough management practices based on plant communities alone is not sufficient and that at least some areas within the turlough basin remain ungrazed on a rotational basis to ensure maximum diversity of Diptera.     

Effects of introduction and exclusion of large herbivores on small rodent communities

In this study we analysed the effects of large herbivores on smallrodent communities in different habitats using large herbivore exclosures. Westudied the effects of three year grazing introduction by red deer(Cervus elaphus L.) in previously ungrazed pine and oakwoodland and the exclusion of grazing by red deer, roe deer(Capreoluscapreolus L.) and mouflon (Ovis ammon musiminL.) in formerly, heavily grazed pine woodland and heathland. At eight exclosuresites within each habitat type, small rodents were captured with live trapsusing trapping grids. At each trapping grid, seed plots of beechnuts(Fagus sylvatica L.) and acorns (Quercusrobur L.) were placed to measure seed predation by rodents.Exclusion of grazing by large herbivores in formerly, heavily grazedhabitats had a significant effect on small rodent communities. Insideexclosureshigher densities of mainly wood mice (Apodemus sylvaticusL.) and field voles (Microtus agrestis L.) were captured.Introduction of grazing by red deer appeared to have no significant negativeeffects on small rodent communities. The seed predation intensity of beechnutsand acorns by small rodents was significantly higher in ungrazed situations,particularly in habitats that were excluded from grazing. The differencesbetween grazing introduction and exclusion effects on small rodent communitiescan be explained by differences in vegetation structure development. Therecovery of heavily browsed understory vegetation after large herbivore grazingexclusion proceeded faster than the understory degradation due to grazingintroduction. Small rodents depend on structural rich vegetations mainly forshelter. We conclude that large herbivores can have significant effects onvegetation dynamics not only via direct plant consumption but also throughindirect effects by reducing the habitat quality of small rodent habitats.     

The dynamics and regulation of small rodent populations in the woodland ecosystems of Killarney, Ireland

Populations of Wood mice Apodemus sylvaticus and Bank voles Clelhrionomys glareolus were studied by mark-recapture techniques in five different woodlands but concentrated on an oak Quercus petraea wood and a yew Taxus baccata wood, where food supply was also investigated. Much of the yew wood was on limestone pavement which was greatly creviced thus providing static and quantifiable cover. Indices of the intensity with which food was being searched for were obtained on all areas by seed-removal experiments.
Trappability varied within the populations and seasonally, being highest in summer. Unusually high densities of mice were recorded in the yew wood but not elsewhere; vole densities were always low. There was an inverse relationship between range length and density: mean range length in the yew wood was exceptionally low. Males generally ranged further than females, voles further than mice. Females outnumbered males in the oak wood, where cover was minimal, when the population was low. In the two areas with most cover, males outnumbered females. In a further area, where numbers were initially exceedingly low, the population consisted entirely of males for almost a year. Sexes differed in habitat preference but numbers, especially of males, were significantly correlated with cover. Voles usually bred for shorter periods than mice. Overwinter breeding and increased overwinter survival followed good seed years. Individuals recruited when food was plentiful lived longest. In seed-removal experiments rate of removal increased with shortage of natural seed.
Food and intraspecific aggression were the probable major factors in regulating population size, of mice at least. Cover probably modifies intrinsic mechanisms by reducing numbers of encounters between individuals.     

The interaction between nutrient availability and disturbance frequency on the diversity of benthic marine communities on the north-east coast of England

1. Several theoretical models predict under what conditions maximum species diversity can be maintained, and they are often used to develop effective ecosystem management plans. 2. Two models that are currently used to predict patterns of species diversity were empirically tested in marine subtidal benthic communities of different successional stages. 3. The two models were: the interactive effects of nutrient availability and disturbance frequency proposed by Kondoh (2001; Proceedings of the Royal Society London B, 268, 269-271), and the intermediate disturbance hypothesis (IDH) proposed by Connell (1978; Science, 199, 1302-1310). 4. Interactive effects were found to be transient and only occurred in the older communities, while the unimodal pattern suggested by the IDH was not supported in either successional stage. 5. It is concluded that these models are very general and thus lack sufficient explanatory power. Both models require a number of specific prerequisites for maximum diversity to be found, and though applicable in many different ecosystems they need to be refined as tools in order that they can be effectively used in habitat management plans.     

The impact of diffusion and stirring on the dynamics of interacting populations

We investigate the combined effects of diffusion and stirring on the dynamics of interacting populations which have spatial structure. Specifically we consider the marine phytoplankton and zooplankton populations, and model them as an excitable medium. The results are applicable to other biological and chemical systems. Under certain conditions the combination of diffusion and stirring is found to enhance the excitability, and hence population growth of the system. Diffusion is found to play an important role: too much and initial perturbations are smoothed away, too little and insufficient mixing takes place before the reaction is over. A key time-scale is the mix-down time, the time it takes for the spatial scale of a population to be reduced to that of a diffusively controlled filament. If the mix-down time is short compared to the reaction time-scale, then excitation of the system is suppressed. For intermediate values of the mix-down time the peak population can attain values many times that of a population without spatial structure. We highlight the importance of the spatial scale of the initial disturbance to the system.     

The role of gap phase processes in the biomass dynamics of tropical forests

The responses of tropical forests to global anthropogenic disturbances remain poorly understood. Above-ground woody biomass in some tropical forest plots has increased over the past several decades, potentially reflecting a widespread response to increased resource availability, for example, due to elevated atmospheric CO2 and/or nutrient deposition. However, previous studies of biomass dynamics have not accounted for natural patterns of disturbance and gap phase regeneration, making it difficult to quantify the importance of environmental changes. Using spatially explicit census data from large (50 ha) inventory plots, we investigated the influence of gap phase processes on the biomass dynamics of four 'old-growth' tropical forests (Barro Colorado Island (BCI), Panama; Pasoh and Lambir, Malaysia; and Huai Kha Khaeng (HKK), Thailand). We show that biomass increases were gradual and concentrated in earlier-phase forest patches, while biomass losses were generally of greater magnitude but concentrated in rarer later-phase patches. We then estimate the rate of biomass change at each site independent of gap phase dynamics using reduced major axis regressions and ANCOVA tests. Above-ground woody biomass increased significantly at Pasoh (+0.72% yr(-1)) and decreased at HKK (-0.56% yr(-1)) independent of changes in gap phase but remained stable at both BCI and Lambir. We conclude that gap phase processes play an important role in the biomass dynamics of tropical forests, and that quantifying the role of gap phase processes will help improve our understanding of the factors driving changes in forest biomass as well as their place in the global carbon budget.     

Nutrient dynamics in milpa agriculture and the role of weeds in initial stages of secondary succession in Belize,C.A.

Nutrient levels, soil moisture and soil organic matter were monitored monthly during a three year period in a site cleared for shifting agriculture (milpa) and in adjacement undisturbed high bush forest soils in Belize. Throughout the study there was no evidence of a decline in available nutrient levels in the cleared site. Sharp changes in P and Mn were considered indicative of wetting and drying periods. Corn yields for the rainy season crop in the first two years were average for the region ca 2500 kg ha⁻¹ dry wt, but declined dramatically in the third year. Total weed biomass for the rainy season crop conversely increased every year and reduced grain yield probably through competition for nutrients (especially N), light and water. Substantial amounts of nutrients were also immobilized in the weed biomass, which served the agroecological purpose of preventing nutrient loss by leaching.     

Variability in flood disturbance and the impact on riparian tree recruitment in two contrasting river systems

The vegetation within the riparian zone performs animportant ecological function for in-stream processes.In Australia, riparian zones are regarded as the mostdegraded natural resource zone due to disturbancessuch as river regulation and livestock grazing. Thisstudy looks at factors influencing vegetation dynamicsof riparian tree species on two contrasting riversystems in Western Australia. The Blackwood River insouth-western Australia is influenced by aMediterranean type climate with regular seasonalwinter flows. The Ord River in north-western Australiais characterized by low winter base flows andepisodic, extreme flows influenced by monsoon rains inthe summer. For both rivers, reproductive phenology ofstudied overstory species is timed to coincide withseasonal river hydrology and rainfall. An evendistribution of size classes of trees on the BlackwoodRiver indicated recruitment into the population iscontinual and related to the regular predictableseasonal river flows and rainfall. In contrast, on theOrd River tree size class distribution was clustered,indicating episodic recruitment. On both rivers treeestablishment is also influenced by elevation abovethe river, microtopography, moisture status and soiltype. In terms of vegetation dynamics riparianvegetation on the Ord River consists of long periodsof transition with short lived stable states incontrast to the Blackwood river where tree populationstructure is characterized by long periods of stablestates with short transitions.     

Relationship of butterfly diversity with nectar plant species richness in and around the Aokigahara primary woodland of Mount Fuji,central Japan

We examined the relationships between the diversities of vegetation, adult nectar plants, and butterflies in and around the Aokigahara primary woodland on the northwestern footslopes of Mount Fuji, central Japan. The results showed that the nectar resource utilization by adult butterflies was significantly biased to herbaceous plants, especially to perennials, compared to woody species, although most of the study area was in and near a primary woodland. There were greater nectar plant species in sites with greater plant species richness. Among the butterfly community indices analyzed, the strongest correlation was detected between butterfly species richness and nectar plant species richness at each site. Another close correlation was detected between the species richness of nectar plants and herbaceous plants at each site. These results suggest that herbaceous plant species richness in a habitat plays a central role in its nectar plant species richness, and the nectar plant richness is a highly important factor supporting its adult butterfly species richness. Consequently, we propose that the maintenance and management of herbaceous plant species richness in a butterfly habitat, which lead to those of its nectar plant species richness, are very important for conservation of butterfly diversity even in and around woodland landscapes of temperate regions.     

Prehistoric Pinus woodland dynamics in an upland landscape in northern Scotland: the roles of climate change and human impact

Pollen, microscopic charcoal, palaeohydrological and dendrochronological analyses are applied to a radiocarbon and tephrochronologically dated mid Holocene (ca. 8500–3000 cal b.p.) peat sequence with abundant fossil Pinus (pine) wood. The Pinus populations on peat fluctuated considerably over the period in question. Colonisation by Pinus from ca. 7900–7600 cal b.p. appears to have had no specific environmental trigger; it was probably determined by the rate of migration from particular populations. The second phase, at ca. 5000–4400 cal b.p., was facilitated by anthropogenic interference that reduced competition from other trees. The pollen record shows two Pinus declines. The first at ca. 6200–5500 cal b.p. was caused by a series of rapid and frequent climatic shifts. The second, the so-called pine decline, was very gradual (ca. 4200–3300 cal b.p.) at Loch Farlary and may not have been related to climate change as is often supposed. Low intensity but sustained grazing pressures were more important. Throughout the mid Holocene, the frequency and intensity of burning in these open Pinus–Calluna woods were probably highly sensitive to hydrological (climatic) change. Axe marks on several trees are related to the mid to late Bronze Age, i.e., long after the trees had died.