A multi-site assessment of the effectiveness of Pteridium aquilinum control in Great Britain

Questions: The objectives of this study were to assess the effect of Pteridium aquilinum (bracken) control treatments, at the national scale, and the impact of restoration practices, at the local scale, on P. aquilinum performance. Hypotheses: 1. Geographical location (locally between and within sites) affects the control of P. aquilinum through time. 2. Are the P. aquilinum control treatments successful at all sites, and if so which ones? 3. The treatments applied at the individual site level to restore vegetation influences the performance of P. aquilinum through time. Location: Four geographically distinct acid grassland and heathland sites infested with P. aquilinum across Great Britain. Methods: Six main-plot, bracken control treatments were applied to all sites with site-specific vegetation restoration treatments. Response variables (P. aquilinum cover, frond length and density) were monitored twice yearly, in June and August between 1993 and 2003. Results: Between- and within-site spatial variation was found, although impact is perhaps less than suggested from shorter-term data. Despite local variation all sites responded similarly to bracken control treatments; asulam treatment resulted in a rapid reduction in frond performance followed by a continued recovery taking approximately ten years to return to untreated values. Cutting treatments tended to have a slower impact at the start but an increasing one over time, especially cutting twice per year. Restoration treatments had a limited impact; the only significant effect in August was grass seeding on frond length at Sourhope. In June only, the plots where sheep were fenced out showed a significant reduction in P. aquilinum cover at Peak. Conclusions: Long-term control of Pteridium aquilinum at all sites and on all measures was best achieved using a continuous cutting treatment, preferably twice per year.     

Vegetation re‐establishment on land previously subject to control of Pteridium aquilinum by herbicide

Abstract. Vegetation development after conventional bracken control with herbicide in the uplands is often slow and rarely results in the development of the target community. Two experiments were established on contrasting sites to investigate cost‐effective methods for the re‐establishment of vegetation dominated by Calluna vulgaris (heather), which is the usual target community seen as the desired replacement for Pteridium aquilinum in the uplands by many land managers. At one site, where grazing intensity was low and shelter was sufficient to prevent rapid Pteridium litter loss, the necessary treatments to establish Calluna were to disturb the litter and add a source of Calluna seeds. A difference between continued Pteridium control or a relaxation of control was not observed over the time‐scale of the experiment. At the second site, where grazing intensity was high and shelter minimal, vegetation dominated by Rumex acetosella had developed on a thin but dense litter layer. To enhance Calluna regeneration it was necessary to prevent sheep grazing, disturb the remaining litter layer and add a source of Calluna propagules. A nurse crop hindered Calluna establishment due to its vigorous growth. At neither site was an initial application of fertilizer an aid to Calluna establishment. It is possible to devise methods for the re‐establishment of Calluna at sites where previous attempts at Pteridium control alone have not had the desired effect on vegetation development. However, the precise choice of methods used is dependent on the grazing management at the site. High levels of livestock grazing prevented Calluna establishment, but some trampling may aid litter breakdown. The ability to adjust stocking rates and access to the site by machinery are critical factors to take into account in producing an effective strategy for long‐term Pteridium control and its replacement by target vegetation.     

Lignin signatures show the effects of changes in heather and bracken cover on the composition of organic matter in a moorland soil profile

Bracken (Pteridium aquilinum) is aggressively displacing heather (Calluna vulgaris) on many moorlands in Britain. We investigated the use of lignin derivatives to identify the distribution of soil organic matter (SOM) derived from bracken in moorland soil profiles formed under heather. Phenylpropanoids extracted from recently senesced litters, roots and SOM, using alkaline CuO oxidation, showed distinct signatures for bracken and heather, with vanillyl moieties dominating bracken litter extracts and vanillyl and syringyl dominating heather litter extracts. Ratios of vanillyl and syringyl concentrations characterised the SOM derived from heather and bracken better than the concentrations of the individual moieties. The analysis showed up to a depth of 5 cm under pure bracken cover, and at the interface between heather and bracken, the SOM was largely derived from bracken litter but below that depth SOM was apparently derived from heather. The use of these methods to identify the plant origin of SOM not only enables understanding the effects of changing vegetation cover on organic matter dynamics in moorland soils but could also facilitate management techniques in moorland/heathland restoration which involve the removal of comparatively nutrient-rich SOM derived from bracken. Received: 30 November 1997 / Accepted: 11 April 1998     

The influence of altitude on the distribution of subterranean organs and humus components in Vaccinium myrtillus carpets

Abstract. Humus profiles were sampled along an altitudinal gradient in the Macot‐La‐Plagne Forest (France, northern Alps) to investigate variation occurring under carpets of Vaccinium myrtillus present within Picea abies forests. The vertical distribution of subterranean organs of V. myrtillus was compared with (1) that of P. abies roots and other accompanying vegetation and (2) other components of humus profiles, in particular humified organic matter mainly consisting of animal faeces. It was shown that V. myrtillus roots were mostly concentrated in mineral horizons, while P. abies roots and V. myrtillus rhizomes occupied litter horizons. This was interpreted in terms of competition for nutrient capture between P. abies and V. myrtillus. The effects of altitude were (1) a change in the vegetation accompanying V. myrtillus in dense V. myrtillus carpets, bryophytes at the montane level being replaced by forbs at the sub‐alpine level and (2) a decrease in the thickness of ecto‐organic horizons. This was interpreted as a shift from a moder system characterized by recalcitrant litter (moss) processed by an active faunal community (stabilized in the form of animal faeces) to a mor system characterized by low animal abundance but with litter of better quality which is easily leached in the absence of prominent faunal activity.     

A field assessment of the success of moorland restoration in the rehabilitation of whole plant assemblages

Question: How successful is restoration that is focused on a single dominant plant at enabling the reassembly of the whole vegetation assemblage and what factors affect the relative success of such restoration? Location: Moorlands in England and Scotland, UK. Methods: Vegetation composition was sampled in grassdominated (degraded), restored and long established Calluna vulgaris‐dominated (target) areas within eight moorland restoration sites. Additional soil and biogeographic data were collected. Data were analysed by Canonical Correspondence Analysis, which allowed the impact of moorland management to be examined. Results: All sites showed good restoration success when the dominant managed species (Calluna vulgaris, Molinia caerulea and Nardus stricta) were considered. Restoration success of the remainder of the plant assemblage, disregarding these dominant species, was lower with restored samples at some sites differing little from their respective degraded samples. Moors restored solely by grazing exclusion were more similar to their respective targets than were those restored by more intensive mechanical means. The most important factors in explaining vegetation assemblages were management status (i.e. whether samples represented degraded, restored or target parts of the moor) and latitude. Conclusions: The project demonstrates that, where possible, restoration should be attempted by grazing exclusion alone. Furthermore the importance of applying local restoration targets and of monitoring the whole plant assemblages when assessing the success of moorland restoration is highlighted.     

The influence of position on genet growth: a simulation of a population of bracken (Pteridium aquilinum (L.) Kuhn) genets under grazing

In a spatially explicit simulation model of vegetation dynamics (VegeTate), I labelled the initial mass of Pteridium aquilinum in each of 225 cells as a single, unique genet or clone. The physical environment was homogeneous and all genets shared the same phenotype. The aim was to discover whether and how the success of each genet was affected by its initial position relative to other genets and a competing grass species. In a scenario in which grazing generated a grass-bracken mosaic with complex spatial dynamics, the amount of growth of each genet ranged widely, from frequent extinction to mass increase by over 300 times. The main factor in the impact of position on genet growth was shown to be a benefit from the initial presence or proximity of a large mass of P. aquilinum. This was because a high density of P. aquilinum reduced local grazing intensity, allowing plant mass to accumulate and shifting the balance of competition in favour of P. aquilinum. Thus variations between cells in initial mass of P. aquilinum were greatly amplified. The implications of this amplification of initial differences between sites for population genetics are briefly discussed. Qualitative features of the spatial distribution of genets at the end of simulations matched reported observations on patchily distributed field populations of P. aquilinum. These features included dominance of a large population by a small minority of genets, widespread mixtures of a dominant genet and one or more subordinate genets, and the presence of patches of P. aquilinum formed both by agglomeration from neighbouring foci and by spread of dominant genets. Under less intense grazing, which allowed little or no development of vegetation mosaics, genet growth varied relatively little and initial variations in relative mass between genets were little changed. Based on this study, I hypothesize that any processes that generate non-linear spatial dynamics will also generate complex genet dynamics.     

亚热带不同植被恢复阶段生态系统N、P储量的垂直分配格局

氮(N)、磷(P)是影响生态系统生产力的主要养分因子,为科学评估植被恢复对生态系统N、P积累、分配及其耦合关系的影响,采用时空互代法,以湘中丘陵区地域相邻、环境条件基本一致,且处于不同恢复阶段的4个植物群落(4—5年灌草丛、10—12年灌木林、45—46年马尾松针阔混交林和>90年常绿阔叶林)作为一个恢复序列,设置固定样地,采用收获法和建立主要树种各器官生物量相对生长方程估算群落生物量,采集植被层(叶、枝、干、根)、凋落物层(未分解层、半分解层、已分解层)、土壤层(0—10、10—20、20—30、30—40 cm)样品,测定全N、全P含量,估算生态系统各组分(植被层、凋落物层、土壤层)全N、全P储量。结果表明：植被层全N、全P储量均随植被恢复增加,全N储量增长速率呈先慢后快的特征,而全P储量则呈慢—快—慢增长,地上(叶、枝、干)、地下(根)部分表现为异速增长;随植被恢复,凋落物层全N、全P储量先增加后下降,增长速率为先快后慢,4—5年灌草丛全N、全P储量最低;土壤层全N、全P储量随植被恢复显著增加(P<0.05),全N储量增长速率呈快-慢-快特征,而全P储量呈先慢后快特征...     

Holocene vegetation and climate change from near Lake Pedder, south-west Tasmania, Australia

Aim To use surface pollen and vegetation relationships to aid the interpretation of a Holocene pollen record. Location South‐west Tasmania, Australia. Methods A survey was undertaken of surface‐pollen samples from the major regional vegetation types: alpine, rain forest and moorland. Relationships between vegetation type and surface‐pollen representation were analysed using twinspan classification and ordination. A core was retrieved from moorland vegetation, and interpretation of the fossil pollen sequence was aided using relationships detected in our surface‐pollen analysis. Results Regional vegetation types are reflected in the pollen rain of south‐west Tasmania, despite the over‐representation of important rain forest tree species in samples from non‐forest sites. twinspan classification of the surface‐pollen samples identified the following indicator pollen taxa for each vegetation type: Astelia alpina (alpine); Lagarostrobos franklinii (rain forest); Leptospermum and Melaleuca (moorland). Detrended correspondence analysis of the surface‐pollen samples clearly separates samples from each vegetation type. Correlation of the ordination axes with environmental data identified a dominant temperature/altitudinal gradient in the surface‐pollen data (R = 0.852/0.844). Application of the results of the surface‐pollen analysis to the fossil sequence revealed that fire‐promoted moorland has dominated the local environment around the core site for the entire Holocene. Changes in fossil pollen composition also suggest that temperatures increased through the Late Glacial to peak in the mid‐Holocene and declined thereafter, a trend consistent with other sites in the region. Main conclusions Pollen spectra can successfully be used to predict local vegetation in south‐west Tasmania. At least this part of inland south‐west Tasmania has remained forest‐free throughout the Holocene, conflicting with the dominant palaeoecological paradigm of a mid‐Holocene dominated by rain forest. A comparison with pollen records from moorland vegetation across the region suggests that fire‐promoted moorland has dominated the landscape since the Late Glacial. We suggest that burning by people through the Late Glacial (if not earlier) facilitated the spread of moorland throughout the region, greatly restricting the expansion of rain forest. The continued influence of fire throughout the Holocene in this perennially wet landscape argues for a revision of the dominant human‐occupation model that depicts an abandonment of the interior of south‐west Tasmania in the Late Glacial in response to the expansion of rain forest.     

Effects of the litter layer of Pteridium aquilinum on seed banks under experimental restoration

Questions: Does the litter layer of Pteridium aquilinum (bracken) act as a barrier to certain species in the seed bank? Does bracken control/restoration treatment affect seed transfer through the litter layer? Location: Five experiments at three sites across the UK covering two major vegetation types; acid‐grassland and heath‐land. Methods: At each experiment a range of bracken control and vegetation restoration treatments were applied for about ten years. The seed bank was sampled in both the bracken litter and the soil. The cover (%) of each species in the vegetation and the bracken litter abundance (cover and depth) was also estimated. Results: The bracken litter layer acts as an inert barrier as it contained a large proportion of seeds available in the litter‐soil profile (38%– 67% of the total). Bracken litter depth and cover also influenced significantly the seed bank composition in both the bracken litter and the soil. These effects were site‐specific, and species‐specific. The application of treatments changed significantly the balance between seed inputs and outputs in the bracken litter layer for some species. This was either a positive or negative response relative to the untreated control plots. Conclusion: For heathland and acid‐grassland restoration, the bracken litter layer may be an important seed source, but it must be disturbed particularly before seed addition.     

Control of Pteridium aquilinum: meta-analysis of a multi-site study in the UK

Background and Aims

A great deal of money is spent controlling invasive weeds as part of international and national policies. It is essential that the funded treatments work across the region in which the policies operate. We argue that experiments across multiple sites are required to validate these programs as results from single sites may be misleading. Here, the control of Pteridium aquilinum (bracken) is used as a test example to address the following four questions. (1) Does the effectiveness of P. aquilinum-control treatments vary across sites? (2) Is the best treatment identified in previous research (cutting twice per year) consistent at all sites, and if not why not? (3) Is treatment performance related to P. aquilinum rhizome mass, litter cover or litter depth at the various sites? (4) Does successful P. aquilinum control influence species richness?

Methods

Pteridium aquilinum‐control treatments were monitored for 10 years using six replicated experiments and analysed using meta-analysis. Meta-regressions were used to explore heterogeneity between sites.

Key Results

The effectiveness of treatments varied between sites depending on the measure used to assess P. aquilinum performance. In general, cutting twice per year was the most successful treatment but on some sites other, less expensive treatments were as good. The effectiveness of treatments at different sites was not related to rhizome mass, but the effectiveness of most applied treatments were inversely related to post-control litter. Effective treatment was also associated with high species richness.

Conclusions

It is concluded that successful development of national weed control programs requires multi-site experimental approaches. Here, meta-analyses demonstrate that variation in effectiveness between sites could be explained in part by pre-specified variables. Reliance on data from a single site for policy formulation is therefore clearly dangerous.Key words: Bracken, Pteridium aquilinum, weed, control, herbicide, litter dynamics, species diversity, meta-analysis     

Factors controlling vegetation change in long-term experiments designed to restore heathland in Breckland,UK

Abstract. This paper describes the vegetation change in an 18-year experiment designed to test a range of control of Pteridium aquilinum (Pteridium) and heathland restoration treatments; cutting, asulam application and Calluna vulgar is seeding, in a range of combinations at Cavenham Heath in Breckland, UK between 1978 and 1996. Vegetation change was recorded in two Phases; in Phase I (1978-1986) species biomass was sampled and in Phase II (1986-1996) cover was measured. Initially, Calluna establishment was good in some treatments, especially where Calluna seed was added and Pteridium was controlled. Other plots developed either a grass-heath flora dominated by Agrostis capillaris, Deschampsia flexuosa, Dicranum scoparium, Festuca ovina and Rumex acetosella or were dominated by clonal species such as Calamagrostis epigejos or Carex arenaria. An unconstrained ordination showed significant vegetation change through time and that several treatments influenced the vegetation, especially those involving asulam application. When variation partitioning with constrained ordination was used a different explanation emerged. In Phase I the most important factors were the management treatments applied, elapsed time and spatial factors, with little overlap. In Phase II, elapsed time became irrelevant because the variation time explained overlapped that which could be explained by other variables. The most important of these were management treatments, spatial effects, weather, the amount of bare ground caused by disturbance and Pteridium litter cover (an index of Pteridium recovery). The implications of these results in interpreting vegetation change are discussed.     

Riparian leaf litter decomposition on pond bottom after a retention on floating vegetation

Allochthonous (e.g., riparian) plant litter is among the organic matter resources that are important for wetland ecosystems. A compact canopy of free‐floating vegetation on the water surface may allow for riparian litter to remain on it for a period of time before sinking to the bottom. Thus, we hypothesized that canopy of free‐floating vegetation may slow decomposition processes in wetlands. To test the hypothesis that the retention of riparian leaf litter on the free‐floating vegetation in wetlands affects their subsequent decomposition on the bottom of wetlands, a 50‐day in situ decomposition experiment was performed in a wetland pond in subtropical China, in which litter bags of single species with fine (0.5 mm) or coarse (2.0 mm) mesh sizes were placed on free‐floating vegetation (dominated by Eichhornia crassipes, Lemna minor, and Salvinia molesta) for 25 days and then moved to the pond bottom for another 25 days or remained on the pond bottom for 50 days. The leaf litter was collected from three riparian species, that is, Cinnamomum camphora, Diospyros kaki, and Phyllostachys propinqua. The retention of riparian leaf litter on free‐floating vegetation had significant negative effect on the carbon loss, marginal negative effects on the mass loss, and no effect on the nitrogen loss from leaf litter, partially supporting the hypothesis. Similarly, the mass and carbon losses from leaf litter decomposing on the pond bottom for the first 25 days of the experiment were greater than those from the litter decomposing on free‐floating vegetation. Our results highlight that in wetlands, free‐floating vegetation could play a vital role in litter decomposition, which is linked to the regulation of nutrient cycling in ecosystems.     

亚热带不同植被恢复阶段林地凋落物层现存量和养分特征

为揭示亚热带森林植被自然恢复过程中,凋落物层现存量及其养分元素储存能力的演变,采用空间代替时间的方法,在位于亚热带丘陵区的长沙县选取地域相邻、生境条件基本一致的檵木+南烛+杜鹃灌草丛（Loropetalum chinense+Vaccinium bracteatum +Rhododendron simsii scrub-grass-land,LVR）、檵木+杉木+白栎灌木林（L.chinense+Cunninghamia lanceolata+Quercus fabri shrubbery,LCQ）、马尾松+柯+檵木针阔混交林（Pinus massoniana +Lithocarpus glaber +L.chinense coniferous-broad leaved mixed forest,PLL）、柯+红淡比+青冈常绿阔叶林（L.glaber+Cleyera japonica+Cyclobalanopsis glauca evergreen broad-leaved forest,LAG）作为一个恢复序列,设置固定样地,采集未分解层（U层）、半分解层（S层）、已分解层（D层）凋落物样品,测定凋落物层现存量和主要养分元素含量、储量及其释放率,分析植物多样性指数与凋落物层现存量、养分元素含量的相关性。结果表明：1）凋落物层及各分解层凋落物现存量随着植被恢复而增加;同一恢复阶段D层凋落物现存量最高,占凋落物层现存量的41.59%-51.02%,不同分解层凋落物现存量的差异随着植被恢复而增大;各恢复阶段凋落物分解率为0.44-0.61,周转期为1.65-2.28 a。2）凋落物层及各分解层凋落物主要养分元素含量均表现为：N > Ca > Mg > K > P,随着植被恢复呈现出不同的变化特征,其中N、P含量总体上呈增加趋势,K含量LAG（除U层外）最高,PLL最低,Ca含量LCQ最高,PLL最低,Mg含量LAG（除U层外）最高,LVR最低;同一恢复阶段N、P（除PLL、LAG外）、K、Ca、Mg含量随着凋落物的分解而下降。3）不同恢复阶段凋落物层主要养分元素的储量依次为：N > Ca > Mg > K > P;凋落物层及各分解层凋落物主要养分元素总储量及各种养分元素的储量总体上随着植被恢复而增加;同一恢复阶段随着凋落物的分解,N、P储量增加,而K、Ca、Mg储量变化不大;随着植被恢复,凋落物层养分元素储存能力和转化归还能力提高,特别是N,养分元素总释放率下降,有利于养分的固持。4）乔木层、灌木层、草本层的植物多样性指数对凋落物层现存量和主要养分元素含量的影响不同,其中乔木层的影响最明显。     

Isolation of habitat patches limits colonisation by moorland Hemiptera

There has been a significant loss in the UK, and elsewhere in Europe, of moorland dominated by heather Calluna vulgaris. One response to this has been the initiation of moorland vegetation restoration projects. Heather moorland has distinct assemblages of invertebrates and the ability of these to colonise newly created habitat patches has not previously been experimentally tested. We established a dispersal and colonisation experiment by transplanting C. vulgaris-dominated turfs within grassland at different distances (up to 40 m) from heather moorland vegetation. Hemiptera were cleared from these turfs by the use of an insecticide and were sampled 1 year later to investigate re-colonisation rates. Hemiptera assemblages on transplanted turfs were most dissimilar to those of heather moor at the greatest distances of these turfs from heather moor. Colonisation rates of heathland-indicator Hemiptera declined exponentially with distance. The number of individual heathland-indicator Hemiptera was higher on turfs 5 and 10 m from heather moor than on turfs transplanted back into the heather moor, possibly due to a crowding effect. Our findings indicate that moorland Hemiptera assemblages may be limited by dispersal ability. We recommend that moorland restoration schemes should be prioritised on ground as close as possible to existing heather moors.     

Seed banks of a rare wetland plant community: distribution patterns and effects of human-induced disturbance

Seed banks in rare shoreline communities (Nova Scotia, Canada), were examined in order to determine their role in the persistence of rare plant species. A pristine shoreline which supports globally and nationally significant Atlantic coastal plain species averaged 8500 seeds/m². Seedsweremost abundant high on the shore and where standing crop was greatest. Rare species, which made up 22% of the standing crop and litter of the adult vegetation, comprised only 4% of the seed bank. Common rushes such as Juncus canadensis and J. filiformis, were not well represented in the adult vegetation (5% of the standing crop and litter) but were abundant in the seed bank (52% of all seeds). Seed densities were on average, much lower on an intensely disturbed shoreline (1000 seeds/m²), where heavy all-terrain vehicle traffic had destroyed the adult vegetation. These results suggest that intense disturbances will destroy both existing vegetation and the seed bank and moderate disturbances will alter community composition so that common rushes will replace already endangered Atlantic coastal plain species.     

The effects of cutting and herbicide treatment on Pteridium aquilinum encroachment

Abstract. Pteridium aquilinum (bracken) encroachment is an important factor in the loss of certain habitats in the United Kingdom. However, no information exists as to whether prevention of encroachment is a cost‐effective strategy for Pteridium management. Conventional methods for the control of Pteridium (cutting, asulam application) were tested at one site (Levisham) to quantify their ability to prevent or delay encroachment and to affect the vigour of the Pteridium at the edge of the stand. The effects of encroachment and asulam application on the vegetation present were monitored at a second site (Ramsley), where techniques commonly used for moorland restoration were employed in combination with asulam application. Cutting once per year or a single application of asulam delayed the advance of the Pteridium front. At Levisham, the untreated front advanced 2.7 m in 5 yr, while in the same period the cut front advanced 0.88 m and the sprayed front was 1.5 m behind its initial position. At Ramsley, the untreated front invaded 1.8 m in 5 yr, and the sprayed front was again 1.5 m behind its starting position. Both spraying and cutting reduced frond biomass, frond cover and rhizome biomass. Herbicide spraying prevented the loss of Calluna vulgaris, though the restoration treatments had little effect. The merits of a balanced targeting of control on encroaching fronts or Pteridium at the stand level are discussed for different situations.     

Changes in forest understory associated with Juniperus encroachment in Oklahoma,USA

Question: Does understory vegetation cover and richness decline along a gradient of increasing Juniperus virginiana midstory canopy cover and is that decline best correlated with litter accumulation? Location: Cross Timbers Forest in Payne County, OK, USA. Methods: We measured vegetation in forest gaps as well as forest areas without J. virginiana, at the inner and outer edge of J. virginiana canopies and near J. virginiana trunks (200 plots) and compared vegetation differences among location to light, litter, soil and microclimate variables. Results: Species richness (11 spp m^?2 to 6 spp m^?2) and summer vegetation cover (53.3% to 12.7%) declined with proximity to trunks. Regression indicated that richness declines (R²=0.08) and cover (R²=0.18) were best correlated with J. virginiana litter accumulation. Partial canonical correspondence analysis (pCCA) revealed two strong canonical axes, one related to litter/light and another to cover of Quercus spp. versus J. virginiana. Tree seedlings and woody vines dominated near J. virginiana. Forbs, graminoids and Quercus spp. seedlings were more common in areas without J. virginiana. Conclusions: Increasing J. virginiana and consequent litter additions alter understory biomass and composition and, through inhibiting Quercus spp. recruitment, may lead to changes in overstory composition. Decreases in herbaceous litter, which historically contributed to fuel accumulation, may have positive feedback effects on midstory encroachment by reducing the potential for prescribed burning.     

Nutrient limitation in species-rich lowland fens

Abstract. Nitrogen, phosphorus and potassium were supplied to some Belgian fens of varying nutrient status and productivity. Plant growth in the lowest productive fen with a species-rich Caricion davallianae vegetation was strongly P-limited. N was ineffective when applied alone, but increased the effect of P-addition when applied together. Summer biomass and plant nutrient concentrations were monitored for four years, and showed partial recovery of nutrient limitation. In a more productive fen dominated by Carex lasiocarpa and in a fen meadow, nutrient limitation was less strong. N limited growth in the productive fen, and N and K were co-limiting in the fen meadow. The P-concentration in the productive fen vegetation showed a marked increase after P-fertilization, but it did not result in higher standing crop. The significance of P-limitation for the conservation of species rich low productive fens is discussed. P-limitation may be an essential feature in the conservation of low productive rich fens: because it is less mobile in the landscape than N and/or because it is an intrinsic property of this vegetation type. Plant nutrient concentrations and N:P-ratios may be used as an indication for the presence and type of nutrient limitation in the vegetation. We found N:P-ratios of 23 to 31 for a P-limited site and 8 to 15 in N-limited sites. This was in agreement with critical values from the literature: N:P > ca. 20 for P-limitation and N:P < 14 for N-limitation. Thus, this technique appears valid in the vegetation types that were studied here.     

Patch colonisation in a savanna grassland

Patches (1 m diameter) were cleared in a heavily grazed and a lightly grazed savanna in South Africa; half of them were covered once with grass litter. The greatest colonisation over three years occurred on heavily grazed patches with litter, the least on open, lightly grazed patches. Annual rainfall affected colonisation rate. Basal cover was lower on patches than in the surrounding vegetation. Patch composition was weakly affected by treatment, and was partly related to the composition of adjacent vegetation, but the proportional representation of species on patches differed from the surrounding vegetation. The stoloniferous Digitaria eriantha and the obligate seed reproducers Aristida bipartita and Heteropogon contortus were major components of patch communities but were uncommon in the surrounding vegetation and in the seed bank. Setaria incrassata and Themeda triandra were the predominant components of the surrounding vegetation and of the seed bank, but Setaria established very poorly and Themeda established well only on heavily grazed patches with litter. The pattern of seedling establishment was the same as that of colonisation. Most seedlings emerged atthe beginning of the wet season, with ca. 50% mortality soon thereafter. The colonising species can be partly predicted from the availability of propagules (vegetative or seed), emergence and establishment success, and subsequent growth.     

Influences of neighbouring plants on seedling establishment in limestone grassland

Abstract. Influences of neighbouring plants on seedling establishment of six dicotyledonous species was investigated in a nutrient-poor limestone grassland (Mesobrometum) in northern Switzerland. Microsites with different vegetational structure were created and seeds sown in them. The fate of emerging seedlings was followed for one or two years. A plant surviving its first growing season was regarded as established. Shelter by neighbouring plants appeared to be necessary for the establishment of Arabis hirsuta and Primula veris. Because of drought and frost heave, hardly any seedlings of these species established in gaps (4 - 21 %). In microsites with vegetation, their survival was significantly higher (40–57 %). Neighbouring plants slightly reduced the survival of Plantago lanceolata and Sanguisorba minor, but these species established well both in gaps (74 - 81 %) and in vegetated microsites (54 - 67 %). Medicago lupulina established well in all microsites in one year (71 - 79 %), but poorly in the next year (18–32 %). Linum catharticum emerged poorly in one year and was completely extinguished by a fungal pathogen the following year. The overall conclusion is, that gaps are of minor importance for recruitment of these species in this grassland. Physical hazards and pathogens control seedling establishment to a greater extent than competition by neighbouring plants. Some species are hardly able to establish without shelter of vegetation. Seed size is an important factor for success of establishment, especially in gaps. It is suggested, that the relatively low productivity and the absence of litter accumulation (due to mowing and biomass removal) are important conditions for the observed behaviour of the seedlings and juvenile plants in this community.