Responses of wetland graminoids to the relative supply of nitrogen and phosphorus

The biomass production of wetland vegetation can be limited by nitrogen or phosphorus. Some species are most abundant in N-limited vegetation, and others in P-limited vegetation, possibly because growth-related traits of these species respond differently to N versus P supply. Two growth experiments were carried out to examine how various morphological and physiological traits respond to the relative supply of N and P, and whether species from sites with contrasting nutrient availability respond differently. In experiment 1, four Carex species were grown in nutrient solutions at five N:P supply ratios (1.7, 5, 15, 45, 135) combined with two levels of supply (geometric means of N and P supply). In experiment 2, two Carex and two grass species were grown in sand at the same .ve N:P supply ratios combined with three levels of supply and two light intensities (45% or 5%daylight). After 12-13 weeks of growth, plant biomass, allocation, leaf area, tissue nutrient concentrations and rates and nutrient uptake depended signi.cantly on the N:P supply ratio, but the type and strength of the responses differed among these traits. The P concentration and the N:P ratio of shoots and roots as well as the rates of N and P uptake were mainly determined by the N:P supply ratio; they showed little or no dependence on the supply level and relatively small interspeci.c variation. By contrast, the N concentration, root mass ratio, leaf dry matter content and speci.c leaf area were only weakly related to the N:P supply ratio; they mainly depended on plant species and light, and partly on overall nutrient supply. Plant biomass was determined by all factors together. Within a level of light and nutrient supply, biomass was generally maximal (i.e. co-limited by N and P) at a N:P supply ratio of 15 or 45. All species responded in a similar way to the N:P supply ratio. In particular, the grass species Phalaris arundinacea and Molinia caerulea showed no differences in response that could clearly explain why P. arundinacea tends to invade P-rich (N-limited) sites, and M. caerulea P-limited sites. This may be due to the short duration of the experiments, which investigated growth and nutrient acquisition but not nutrient con­servatio

Botanica Helvetica: Entwicklungen 1981-2004 und in der Zukunft

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Litter N:P ratios indicate whether N or P limits the decomposability of graminoid leaf litter

The N:P ratio of leaf litter may determine if decomposability is N-limited (litter with low N:P ratio) or P-limited (litter with high N:P ratio). To test this hypothesis and to determine the threshold between N and P limitation, we studied relationships between litter N and P concentrations, litter mass loss and effects of fertilisation on litter mass loss in laboratory experiments. Leaf litter of 11 graminoid species was collected in Swiss and Dutch wetlands, yielding 84litter samples with a broad range of N and P concentrations (3.2-15.1mg N g−1, 0.04-1.93mg P g−1) and with N:P mass ratios ranging from 5 to 100. On nutrient-free sand, dry mass loss after five or ten weeks (5.5-53% of initial mass) correlated positively with the N and P concentrations of the litter. Within species, mass loss correlated mainly with N for litter with low N:P ratio, and with P for litter with high N:P ratio, in agreement with our hypothesis. Among species, however, these relationships did not exist, and decomposition rather correlated with the specific leaf area. When the litter was incubated on fertilised sand, 35 out of 50litter samples decomposed faster than on nutrient-free sand. Decomposition was generally accelerated by P fertilisation (i.e. P-limited) when the N:P ratio of the litter was above 25 and the P concentration below 0.22mg g−1, supporting our hypothesis. N-limited decomposition was not significantly related to the litter N:P ratio but occurred rarely for litter with N:P ratio greater than 25, and only for litter with N concentration below 11.3mg g−1. We conclude that the N:P ratio of leaf litter indicates whether its decomposability is more likely to be N- or P-limited. The critical N:P ratio (threshold between N and P limitation) appeared to be 25 for graminoid leaf litte

Progressive effects of shading on experimental wetland communities over three years

To investigate how the composition of wetland communities changes over time in response to altered light regimes, experimental communities of five Carex and four grass species were subjected to artificial shading (continuous or seasonal) in a three-year field experiment. Shoot number and size was measured after six weeks, and shoot biomass was harvested five times during the experiment. Communities were initially dominated by three grass species in all treatments, but subsequently, the Carex species increased and reached dominance in the control plots, whereas grasses remained dominant in the shaded plots. Shading had no effect on the biomass of communities or of single species in the first year. In the second year, community biomass was still unaffected, but shading reduced the biomass of three Carex species and also reduced species diversity. In the third year, shading reduced community biomass and all Carex species, but not species diversity. The greater shade tolerance of the grasses could not be explained by differences in morphological plasticity: after six weeks of growth all species had increased shoot height in response to shade by 40-70%. Grasses were hardly more plastic than Carex species. We propose that the long-term success of the Carex species in full light was due to a high allocation of biomass to belowground parts, which may have reduced losses caused by repeated harvesting of shoots (a simulation of management in productive wet meadows). Shading probably caused the Carex plants to change their allocation pattern, and thus prevented their progressive increas

Distribution and functional traits of Solidago gigantea in a Swiss lakeshore wetland

Abstract.: Güsewell S., Zuberbühler N. and Clerc C. 2005. Distribution and functional traits of Solidago gigantea in a Swiss lakeshore wetland. Bot. Helv. 115: 63-75. Alien plant species are of particular ecological concern when they invade native vegetation of high conservation value and alter its functioning.We mapped the area and cover of stands of invasive alien species in the lakeshore wetland "Grande Cariçaie” (CH) and examined whether the main invader, Solidago gigantea Ait., differs functionally from the resident vegetation. Seven alien species considered to be invasive in Switzerland occurred with a total area of 22.2ha. The most frequent invaders were S. gigantea (60 stands) and Robinia pseudoacacia (42 stands, mostly in forest). Less than 1 area was colonised by alien species, mostly S. gigantea. Dense stands of S. gigantea (> 50 vegetation, suggesting that its spread is limited by high water level rather than by low nutrient availability. S. gigantea appeared functionally similar to the invaded wetland vegetation: its shoot size, specific leaf area, and biomass N and P concentrations were similar to or lower than those of co-occurring native species. Total soil N and P concentrations under S. gigantea stands did not differ from those under non-invaded vegetation. Thus, the impact of invasive alien species on the non-forested wetlands of the Grande Cariçaie currently seems rather low but possible changes in soil processes as well as effects on other biota remain to be investigate

Effects of heterogeneous habitat use by cattle on nutrient availability and litter decomposition in soils of an Alpine pasture

Grazing by free-ranging cattle on Alpine pastures in southern Switzerland creates sharp contrasts in plant species composition between small 'camp areas', which are grazed intensely and receive most cattle excreta, and surrounding pasture dominated by Nardus stricta, which is only lightly grazed. We hypothesised that these contrasts are maintained by positive feedbacks related to nutrient availability in soil, in that (a) plant material with rapid decomposition and nutrient release decomposes in camp areas and (b) litter decomposition is further stimulated by enhanced nutrient availability in soil. We compared nutrient availability at three camp areas with that in surrounding Nardus vegetation and investigated how the decomposition of plant material from both vegetation types responds to nutrient availability in soil, both in the field (during 14 weeks) and in the laboratory (during 4, 10, and 16 weeks). At all three field sites P availability was significantly enhanced in camp areas, whereas differences in N availability were inconsistent among the three sites. Laboratory incubations indicated that microbial activity after the addition of labile C (cellulose) was limited by P availability in the Nardus vegetation but not in camp areas. The camp-area plant substrate decomposed much faster (81.5% vs. 27.1% ash-free dry mass loss in the field) and released more N and P than the Nardus substrate, which tended to immobilise soil nutrients. However, the decomposition rate of neither substrate was influenced by nutrient availability in soil, both in the field (comparing camp areas and Nardus vegetation) and in the laboratory (comparing incubations with and without N or P fertilisation). We conclude that the contrasting quality of plant substrates contributes to the greater nutrient availability in camp areas (feedback a) but that the latter does not influence the decomposition of in situ plant material (feedback b) because the latter is not nutrient-limite

Distribution, growth performance and genetic variation of Erigeron annuus in the Swiss Alps

We investigated whether local adaptation has been important in enabling the invasive apomictic species Erigeron annuus to extend its altitudinal range in the Swiss Alps. We first conducted a field survey along several major roads crossing the Swiss Alps to study the distribution and growth performance of E. annuus along an altitudinal gradient. We then used amplified fragment length polymorphism to assess genetic variation within and among populations originating from different altitudes. To complement the molecular analyses, we compared the performance of genotypes with different distributions (i.e. local, occasional, widespread genotypes) in two common gardens at 400m and 1,000m a.s.l. Although E. annuus was seldom found above 1,000m, plant performance in field populations did not decrease with increasing altitude. However, there was a significant decline in genotypic diversity within populations, and highland (711-1,100m) populations were more differentiated (Gst=0.55) than lowland (200-530m) populations (Gst=0.33). In the common garden experiment, local genotypes (i.e. those restricted to a single population) grew less vigorously than widespread genotypes, and were less likely to reproduce. We found no evidence for on-going adaptive changes and conclude that any selection acting on particular genotypes at the altitudinal limit is weak. This leads us to propose that the patterns in the distribution of genotypic diversity in E. annuus are governed by processes of occasional sexual reproduction, dispersal and extinction that are to a large extent independent of altitud

Temporal changes in grazing intensity and herbage quality within a Swiss fen meadow

Güsewell S., Pohl M., Gander A. and Strehler C. 2007. Temporal changes in grazing intensity and herbage quality within a Swiss fen meadow. Bot. Helv. 117: 57 - 73. Grazing is a possible tool for conservation management in wetlands, but a frequent problem is spatial variation in grazing intensity, which may promote the degradation of the vegetation. Temporal changes in grazing patterns may reduce this problem by leading to a more homogeneous overall use of the area. In a lakeshore wetland (2.8 ha) grazed by Scottish Highland cattle from May till September, we studied how the grazing of nine vegetation types changes seasonally and between years, and how this is related to the quantity and quality of the herbage. We observed cattle activity weekly throughout two grazing periods, determined the biomass and nutrient concentrations of the vegetation, analysed dung samples, and carried out clipping experiments to assess shoot regeneration after grazing. The annual grazing intensity varied seven-fold among vegetation types, and this pattern was closely similar in both years. In several vegetation types, however, grazing intensity changed seasonally. Cattle first grazed tall grassdominated vegetation (Phragmition, Phalaridion), which had the most digestible and nutrient-rich herbage, but the poor regeneration of dominant species forced the cattle to then graze small-sedge dominated, nutrient-poor fen vegetation (Caricion davallianae). These temporal changes in grazing patterns slightly reduced the spatial variation in grazing intensity. To take advantage of this effect, relatively long grazing seasons are preferable to shorter ones. Four years of grazing reduced the above-ground biomass production and nutrient concentrations of several vegetation types. Therefore, years without grazing may be needed to prevent a degradation of palatable plant communities with low grazing toleranc

Vegetation patterns maintained by cattle grazing on a degraded mountain pasture

Abstract.: Jewell P.L., Güsewell S., Berry N.R., Käuferle D., Kreuzer M. and Edwards P.J. 2005. Vegetation patterns maintained by cattle grazing on a degraded mountain pasture. Bot. Helv. 115: 109-124. In southern Switzerland the use of mountain pastures for cattle has been in decline since the 19th century, promoting the dominance of unpalatable grasses and shrubs. In an attempt to improve pasture quality, 80 Scottish Highland cattle were introduced to a 73-ha grazing area on acidic soils at 1400-1800 m a.s.l. To assess how this management might affect the vegetation, we surveyed the plant species composition, soil chemistry and spatial patterns of cattle grazing. Large parts of the pasture were low-productive Nardetum grassland or Callunetum heathland. Patches of more productive and nutrient-rich grassland occurred on less acidic soils with higher total P concentration and lower C:N, C:P and N:P ratios. Grazing by cattle focused on these patches, while N. stricta grasslands and heathlands were hardly used. Historical evidence suggests that these patterns of cattle use are similar to those in the past. We conclude that the current grazing regime is unlikely to produce a significant change in vegetation composition and pasture qualit