Sabine Güsewell

RESPONSE OF PLANT PATHOGENS AND HERBIVORES TO A WARMING EXPERIMENT

Pathogens and herbivores can severely reduce host fitness, potentially leading to altered succession rates and changes in plant community composition. Thus, to predict vegetation dynamics under climate change, it is necessary to understand how plant pathogens and herbivores will respond. Pathogens and herbivores are predicted to increase under climate warming because the amount of time available for growth and reproduction will increase. To test this prediction, we used a warming experiment in which heaters were suspended over a natural montane meadow for 12 years. In the summer of 2002, we quantified damage by all the observable (aboveground) pathogens and herbivores on six of the most common plant species (Artemisia tridentata, Helianthella quinquenervis, Erigeron speciosus, Potentilla gracilis, Potentilla hippiana, and Lathyrus leucanthus). We found that plants in the earlier melting plots generally had the most damage and were attacked by a larger number of species, which is consistent with predictions. However, although the overall trend was an increase in damage with warmer temperatures and earlier snowmelt, some pathogens and herbivores performed better in cooler or later melting plots. The idiosyncratic response of each species to environmental conditions suggests that there are likely to be changes in community composition as the planet warms.

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-limited.

Composition of plant species mixtures grown at various N:P ratios and levels of nutrient supply

Abstract Changes in the relative availability of essential nutrients such as nitrogen, phosphorus, and potassium can be associated with changes in the species composition of wetland vegetation, but responsible mechanisms are not always clear. This study examines how the relative supply of N and P influences the composition of mixtures of four herbaceous wetland species (Carex flava, Centaurea angustifolia, Lycopus europaeus and Selinum carvifolia). Mixtures were grown in sand fertilised with nutrient solutions containing N and P in varying proportions (N:P mass ratios of 1.7–135) and total amounts (‘overall supply’, geometric mean of N and P supply). The total above-ground biomass of the mixtures after 11 weeks of growth depended mainly on overall nutrient supply, but also on the N:P supply ratio; it was maximal at a N:P supply ratio of 15 with high overall nutrient supply, 45 with intermediate, and 135 with low nutrient supply. The relative contributions of the four species to total above-ground biomass as well as the root weight ratio differed significantly among nutrient treatments. Treatment effects mostly remained significant when biomass was used as covariate to account for differences in productivity. With increasing N:P supply ratio, the contribution of C. flava increased, while the contribution of L. europaeus and the root weight ratio tended to decrease. These differences were unrelated to interspecific differences in biomass N and P concentrations since L. europaeus had lower concentrations of both N and P than C. flava. Differences in the rates of N and P uptake were apparently most important in this short-term experiment and probably due to differences in biomass allocation to roots. The results also show that the effects of N:P supply ratios should always be investigated at several levels of overall nutrient supply since the two factors interact with each other. Veranderungen der relativen Verfugbarkeit wichtiger Nahrstoffe wie Stickstoff, Phosphor und Kalium in Feuchtgebieten konnen mit Veranderungen der Artenzusammensetzung der Vegetation verbunden sein, doch die verantwortlichen Mechanismen sind nicht immer offensichtlich. Hier wurde untersucht, wie das Verhaltnis von N- zu P-Versorgung die Zusammensetzung (Biomasseanteile) von Mischungen aus vier Feuchtgebietsarten (Carex flava, Centaurea angustifolia, Lycopus europaeus und Selinum carvifolia) beeinflusst. Die Mischungen wurden in Quarzsand kultiviert und mit N und P in unterschiedlichen Verhaltnissen (Massenverhaltnisse von 1.7–135) und Gesamtmengen (geometrisches Mittel von N- und P-Menge) versorgt. Die gesamte oberirdische Biomasse nach elf-wochigem Wachstum hing hauptsachlich von der Gesamtversorgung ab, aber auch vom N:P-Verhaltnis; sie war am hochsten wenn das N:P-Verhaltnis bei hoher Gesamtversorgung 15 war, bei mittlerer Gesamtversorgung 45 und bei niedriger Gesamtversorgung 135. Die Biomasseanteile der vier Arten sowie der Wurzelanteil hingen von den Behandlungen ab. Die Behandlungseffekte blieben zum grosten Teil signifikant wenn die Biomasse als Kovariable verwendet wurde, um den Einfluss von Produktivitatsunterschieden auszuschliesen. Mit zunehmendem N:P-Verhaltnis nahm der Biomasseanteil von C. flava zu, wahrend der Anteil von L. europaeus und der Wurzelanteil abnahmen. Diese Verschiebungen hingen nicht mit unterschiedlichen Nahrstoffkonzentrationen in der Biomasse zusammen, da sowohl die N- als auch die P-Konzentration bei L. europaeus niedriger war als bei C. flava. Demnach waren Unterschiede bei der N- und P-Aufnahme entscheidend, und diese lagen vermutlich an unterschiedlichen Wurzelanteilen. Die Ergebnisse zeigen auch, dass der Einfluss der relativen Verfugbarkeit von N und P stets bei mehreren Gesamtgaben untersucht werden sollte, da die Wirkung der zwei Faktoren voneinander abhangt.

Short‐term and long‐term effects of mowing on the vegetation of two calcareous fens

Short-term field experiments are often used to predict and evaluate long-term management effects. Based on a mowing experiment in two calcareous fens near Lake Neuchatel, Switzerland, we investigated whether shea-term treatment effects (i.e. during the first four years) were confirmed by longterm results (13 - 14 yr). Plots were mown in summer or in winter or left unmown. The main long-term trends in overall species composition (based on percentage cover estimates) were already observable in the first four years: mown and unmown plots diverged, whereas summer-cut and winter-cut plots remained similar. At the individual species level, however, short-term and long-term treatment effects differed considerably: many species whose abundance seemed affected by treatments during the first four years showed no response in the long term, and vice versa. These discrepancies were similar when based on cover estimates or on counts of shoots. Species responses did actually depend on the time scale considered. Short-term and long-term treatment effects on species richness were similar (i.e. a decrease in unmown plots), although only long-term effects were significant. Treatment effects on the above-ground biomass varied considerably, and short-term trends (lower biomass in unmown plots) differed from long-term trends (higher biomass in unmown plots). Our sites showed little overall change in species composition during the period investigated, and treatment effects were low compared with other similar experiments. if study sites are less stable or treatment effects more drastic, a short-term evaluation is expected to be even less reliable. Knowledge on species dynamics at a site may help to choose the adequate spatial and temporal scale of investigation, and thus increase the efficiency of management experiments.

Invasive forbs differ functionally from native graminoids, but are similar to native forbs

• Exotic plant invasions can alter ecosystem processes, particularly if the invasive species are functionally different from native species. We investigated whether such alterations can be explained by differences in functional traits between native and invasive plants of the same functional group or by differences in functional group affiliation. • We compared six invasive forbs in Europe with six native forbs and six native graminoids in leaf and whole-plant traits, plasticity in response to nutrient supply and interspecific competition, litter decomposition rate, effects on soil nutrient availability, and allelopathy. All traits were measured in a series of pot experiments, and leaf traits additionally in the field. • Invasive forbs differed from native forbs for only a few traits; they had less leaf chlorophyll and lower phosphorus (P) uptake from soil, but they tended to have a stronger allelopathic effect. The invasive forbs differed in many traits from the native graminoids, their leaves had lower tissue densities and a shorter life span, their litter decomposed faster and they had a lower nitrogen-use efficiency. • Our results suggest that invasive forbs have the potential to alter ecosystem properties when invading graminoid-dominated and displacing native graminoids but not when displacing native forbs.

Nutrient availability and limitation in traditionally mown and in abandoned limestone grasslands: a bioassay experiment

Management is necessary for the conservation of limestone grasslands. However, the traditional management of hay-making every year in July is no longer profitable for farmers. Hence many species-rich grasslands have become abandoned. The aim of this study is (a) to investigate the consequences of abandonment (as compared with annual mowing) on the availability of soil nutrients, and (b) to determine the type of nutrient limitation. The soil was taken from a long-term experimental site set up 22 years ago in northern Switzerland. The availability of soil N and P was assessed in a bioassay where soil from mown and unmown plots was supplied with different nutrient solutions or deionised water as control. Seedlings of Galium mollugo s.str. L. and Raphanus sativus ssp. oleiferus (DC) Metzg. were used as phytometer plants. Their growth in the control treatment was limited by N and P almost to the same extent, indicating that the availability of both elements was very low in this soil. P limited plant growth slightly more, but was overcome in the case of Raphanus by a high P content in the seeds. The N and P availability responded differently to management. The availability of N was slightly higher in soil from the abandoned plots, whereas the P availability did not differ significantly. Accumulation of nitrogen in the soil after abandonment did presumably not have any decisive effect on the vegetation because the amount was small and because the vegetation seemed primarily P-limited. The direct effect of mowing or abandonment on plants is therefore likely to be much more important for species composition than the minor changes in soil nutrients.

Flood events overrule fertiliser effects on biomass production and species richness in riverine grasslands

Abstract Question: Do severe winter flood events lift the nutrient limitation of biomass production in a river floodplain? How does this affect plant species richness? How long do the effects last? Location: Floodplain grassland on calcareous sandy loam near river Rhine in The Netherlands. Methods: Plots were fertilised with four treatments (control, N, P, N+P) for 21 years; plant species composition, vegetation biomass and tissue nutrient concentrations were determined every year between 1985 and 2005. Results: Fertilisation with N generally increased biomass production and reduced species richness, but these effects varied over time. During the first four years of the experiment, biomass production appeared to be co-limited by N and P, while N fertilisation dramatically reduced plant species richness; these effects became weaker subsequently. Following two extreme winter floods in 1993–94 and 1994–95 and a drought in spring 1996, the effects of fertilisation disappeared between 1998 and 2001 and then appeared again. Flooding caused an overall reduction in species richness (from c. 24 to 15 species m−2) and an increase in biomass production, which were only partly reversed after ten years. Conclusions: Long time series are necessary to understand vegetation dynamics and nutrient limitation in riverfloodplains, since they are influenced by occasional flood and drought events, whose effects may persist for more than ten years. A future increase in flooding frequency might be detrimental to species richness in floodplain grasslands. Nomenclature: van der Meijden (1996).

Management of Phragmites australis in Swiss fen meadows by mowing in early summer

Mowing experiments were carried out from1995 to 2001 in Swiss fen meadows toinvestigate whether the abundance of Phragmites australis is reduced by mowingin early summer in addition to mowing inautumn. Experimental plots of 100 m2were established in three fen meadows thatare mown every year in September; treatedplots received an additional cut in lateJune either every year or every two years.Until 1997, the additional cut had noeffect on the above-ground biomass of Phragmites (monitored every year in lateJune). As from 1998, the biomass of Phragmites was 25–30% lower in the plotswith annual June cut than in the controlplots. However, the pooled biomass of allother plant species decreased similarly, sothat the degree of dominance of Phragmites was not reduced. An additionalJune cut every two years had no effect onthe biomass of Phragmites. In June2001, the shoots of Phragmites weresmaller in annually June-cut plots than incontrol plots, but allometric relationshipsbetween shoot length and diameter, shootgrowth from June to August, and nitrogenand phosphorus concentrations of shoots didnot differ between June-cut and controlplots. The additional June cut increasedthe total export of N with the hay by 18%,and that of P by 50% in 2001. Theseadditional nutrient exports were smallerthan those found in the first years of theexperiment and not larger for Phragmites than for the remainder of thevegetation. Together, the results suggestthat a depletion of below-ground storescaused Phragmites to decrease afterseveral years of additional mowing in June.Eighty further permanent quadrats in fenmeadows with normal management (mownannually in September) were surveyed in1995–96 and in 2001. The above-groundbiomass of Phragmites increasedduring this time in 49 out of 80 plots,with a mean relative difference of +35.5%.Thus, even if additional mowing in earlysummer only slightly reduced theperformance of Phragmites compared toplots mown only in September, thistreatment might help to prevent the speciesfrom spreading under the current conditionsin Swiss fen meadows.

Effects of natural hybrid and non‐hybrid Epichloë endophytes on the response of Hordelymus europaeus to drought stress

Interspecific hybrid endophytes of the genus Epichloë (Ascomycota, Clavicipitaceae) are prevalent in wild grass populations, possibly because of their larger gene variation, resulting in increased fitness benefits for host plants; however, the reasons are not yet known. We tested hypotheses regarding niche expansion mediated by hybrid endophytes, population-dependent interactions and local co-adaptation in the woodland grass Hordelymus europaeus, which naturally hosts both hybrid and non-hybrid endophyte taxa. Seedlings derived from seeds of four grass populations made endophyte free were re-inoculated with hybrid or non-hybrid endophyte strains, or left endophyte free. Plants were grown in the glasshouse with or without drought treatment. Endophyte infection increased plant biomass and tiller production by 10-15% in both treatments. Endophyte types had similar effects on growth, but opposite effects on reproduction: non-hybrid endophytes increased seed production, whereas hybrid endophytes reduced or prevented it completely. The results are consistent with the observation that non-hybrid endophytes in H. europaeus prevail at dry sites, but cannot explain the prevalence of hybrid endophytes. Thus, our results do not support the hypothesis of niche expansion of hybrid-infected plants. Moreover, plants inoculated with native relative to foreign endophytes yielded higher infections, but both showed similar growth and survival, suggesting weak co-adaptation.

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 quality.