13C isotopic signature and C concentration of soil density fractions illustrate reduced C allocation to subalpine grassland soil under high atmospheric N deposition.

We followed soil C fluxes in a subalpine grassland system exposed to experimentally increased atmospheric N deposition for 7 years. Earlier we found that, different from the plant productivity response, the bulk soil C stock increase was highest at the medium, not the high N input as hypothesized. This implies that a smaller N-deposition rate has a greater potential to favor the biological greenhouse gas-sink. To help elucidate the mechanisms controlling those changes in SOC in response to N deposition, we produced four soil density fractions and analyzed soil organic C concentration [SOC], as well as δ13C signatures (δ13CSOC) of SOC components. Soil respired CO2 (δ13CCO2) was analyzed to better distinguish seasonal short term dynamics from N-deposition effects and to identify the predominant substrate of soil respiration. Both at the start of the experiment and after 7 years we found a strong, negative correlation between [SOC] and δ13CSOC of the soil density fractions in the control treatment, consistent with an advanced stage of microbial processing of SOC in fractions of higher density. During the experiment the [SOC] increased in the two lighter density fractions, but decreased in the two heavier fractions, suggesting a possible priming effect that accelerated decomposition of formerly recalcitrant (heavy) organic matter pools. The seasonal pattern of soil δ13CCO2 was affected by weather and canopy development, and δ13CCO2 values for the different N treatment levels indicated that soil respiration originated primarily from the lightest density fractions. Surprisingly, [SOC] increases were significantly higher under medium N deposition in the <1.8 fraction and in bulk soil, compared to the high N treatment. Analogously, the depletion of δ13CSOC was significantly higher in the medium compared to the high N treatment in the three lighter fractions. Thus, medium N deposition favored the highest C sequestration potential, compared to the low N control and the high N treatment. Clearly, our results show that it is inappropriate to use plant productivity N response as an indicator for shifts in SOC content in grassland ecosystems. Here, isotopic techniques illustrated why atmospheric N deposition of 14 kg N ha-1 yr-1 is below, and 54 kg N ha-1 yr-1 is above a threshold that tips the balance between new, assimilative gains and respiratory losses towards a net loss of [SOC] for certain soil fractions in the subalpine grassland.

Simulated heat waves affected alpine grassland only in combination with drought.

The Alpine region is warming fast, and concurrently, the frequency and intensity of climate extremes are increasing. It is currently unclear whether alpine ecosystems are sensitive or resistant to such extremes. We subjected Swiss alpine grassland communities to heat waves with varying intensity by transplanting monoliths to four different elevations (2440-660 m above sea level) for 17 d. Half of these were regularly irrigated while the other half were deprived of irrigation to additionally induce a drought at each site. Heat waves had no significant impacts on fluorescence (Fv /Fm , a stress indicator), senescence and aboveground productivity if irrigation was provided. However, when heat waves coincided with drought, the plants showed clear signs of stress, resulting in vegetation browning and reduced phytomass production. This likely resulted from direct drought effects, but also, as measurements of stomatal conductance and canopy temperatures suggest, from increased high-temperature stress as water scarcity decreased heat mitigation through transpiration. The immediate responses to heat waves (with or without droughts) recorded in these alpine grasslands were similar to those observed in the more extensively studied grasslands from temperate climates. Responses following climate extremes may differ in alpine environments, however, because the short growing season likely constrains recovery.

Elevated ozone affects the genetic composition of Plantago lanceolata L. populations.

The genetic composition and diversity of Plantago lanceolata L. populations were analysed using amplified fragment length polymorphism (AFLP) as well as simple sequence repeat (SSR) markers to test for differences in an old semi-natural grassland after five years of treatment with ambient or elevated ozone (O3) using a free-air fumigation system. Genetic diversity in populations exposed to elevated O3 was slightly higher than in populations sampled from control plots. This effect was significant for AFLP-based measures of diversity and for SSR markers based on observed heterozygosity. Also, a small but significant difference in genetic composition between O3 treatments was detected by analysis of molecular variance and redundancy analysis. The results show that micro-evolutionary processes could take place in response to long-term elevated O3 exposure in highly diverse populations of outbreeding plant species.

Legacy Effects of Climate Extremes in Alpine Grassland.

Climate change is particularly apparent in many mountainous regions, with warming rates of more than twice the global average being reported for the European Alps. As a result, the probability of climate extremes has increased and is expected to rise further. In an earlier study, we looked into immediate impacts of experimentally imposed heat waves in alpine grassland, and found that these systems were able to cope with heat as long as enough water was available. However, concomitant drought led to increased stress, and reduced aboveground biomass production and green plant cover. Here, we studied the legacy effects (lag-effects) of the imposed climate extreme to see whether delayed responses occurred and how fast the alpine grassland could rebound from the initial changes. Green cover continued to be suppressed the two following years in communities that had been exposed to the most intense hot drought, while aboveground biomass production had returned to control levels by year 2. The initial lower resistance of the forb fraction in the communities was not compensated by faster recovery later on. This resulted in alpine communities that became (and remained) relatively enriched with graminoids, which resisted the original extreme better. The responses of alpine grassland to heat extremes with or without drought observed in this study resemble those typically found in lowland grassland in the short term. However, alpine grassland exhibited longer legacy effects from an annual perspective, with delayed recovery of aboveground production and persistent changes in community composition. This suggests that once initial resistance thresholds are exceeded, impacts may be longer-lasting in alpine grassland, where recovery is constrained by both the short growing season and difficult seedling establishment.

High-resolution modelling of AOT40 and stomatal ozone uptake in wheat and grassland: a comparison between 2000 and the hot summer of 2003 in Switzerland.

The aim was to compare the ozone risk for agricultural crops in Switzerland during the hot and dry year 2003 with the more 'normal' situation in 2000. An improved version of the Ozone DEposition Model ODEM was used at a 2 x 2 km resolution. The distribution of the index AOT40 was compared with the accumulated stomatal ozone flux, AF(st). Averaged AOT40 at 2 m and at canopy height was much higher in 2003 than in 2000, but inter-annual differences in AF(st) for wheat and grasslands were small due to the limiting effect of low soil water contents in 2003. AOT40 suggested larger potential yield losses in wheat in 2003, while using AF(st) with a threshold of 6 nmol m(-2) s(-1) (AF(st)6) yielded similar estimates for both years. The data show that modelling of AF(st) can be used to differentiate ozone risks between regions and years at a national scale.

Elevated ozone and nitrogen deposition affect nitrogen pools of subalpine grassland.

In a free-air fumigation experiment with subalpine grassland, we studied long-term effects of elevated ozone (O3) and nitrogen (N) deposition on ecosystem N pools and on the fate of anthropogenic N. At three times during the seventh year of exposure, N pools and recovery of a stable isotope tracer ((15)N) were determined in above- and belowground plant parts, and in the soil. Plants were much better competitors for (15)N than soil microorganisms. Plant N pools increased by 30-40% after N addition, while soil pools remained unaffected, suggesting that most of the extra N was taken up and stored in plant biomass, thus preventing the ecosystem from acquiring characteristics of eutrophication. Elevated O3 caused an increase of N in microbial biomass and in stabilized soil N, probably resulting from increased litter input and lower litter quality. Different from individual effects, the interaction between the pollutants remained partly unexplained.

Intra-specific variability of ozone sensitivity in Centaurea jacea L., a potential bioindicator for elevated ozone concentrations.

Brown knapweed (Centaurea jacea L.) has been suggested as a potential bioindicator for tropospheric ozone (O3), but little is known about the intra-specific variation in O3 sensitivity in this wild species. The aim of this study was to quantify the differences in O3 sensitivity among and within five populations, and to relate the differences to morphological, phenological, and genetic characteristics. These parameters were periodically recorded in two consecutive experiments on a total of 357 plants from five different European countries (Norway, Hungary, Switzerland, Italy, Slovenia). They were grown from seed in natural soil under ambient conditions at a site with seasonally elevated O3 concentrations (Cadenazzo, southern Switzerland). The populations differed significantly both in frequency and extent of O3 injury, as well as in phenological development. The observed degree of O3 injury was highest in the Slovenian and the Swiss populations, while only few Hungarian and Norwegian plants showed slight symptoms of injury. Plants were generally most sensitive to O3 when reaching the reproductive stage, and insensitive at the rosette stage. Amplified fragment length polymorphism analysis (AFLP) demonstrated genetic distinctiveness of the five C. jacea populations. All individuals of four of the five populations were correctly assigned to the respective populations based on principal component analysis. Cluster analysis quite accurately reflected the geographic origin of each population. Overall, the analysis revealed a high degree of intra-specific variability in O3 sensitivity in C. jacea, and underlined the important influence of the climate-dependent population-specific plant development on O3 sensitivity. These observations may constrain the development of a standardized biomonitoring system.

High tolerance of subalpine grassland to long-term ozone exposure is independent of N input and climatic drivers.

In a seven-year study, we tested effects of increased N and O3 deposition and climatic conditions on biomass of subalpine grassland. Ozone risk was assessed as exposure (AOT40) and as stomatal flux (POD0,1). We hypothesized that productivity is higher under N- and lower under O3 deposition, with interactions with climatic conditions. Aboveground biomass was best correlated with growing-degree days for May (GDDMay). Nitrogen deposition increased biomass up to 60% in the highest treatment, and 30% in the lowest addition. Also belowground biomass showed a positive N-response. Ozone enrichment had no effect on biomass, and no interaction between O3 and N was observed. Growth response to N deposition was not correlated to GDDMay or precipitation, but indicated a cumulative effect over time. Productivity of subalpine grassland is tolerant to increasing ozone exposure, independent of N input and climatic drivers. N deposition rates at current critical loads, strongly increase the grassland yield.

Ozone visible symptoms and reduced root biomass in the subalpine species Pinus uncinata after two years of free-air ozone fumigation.

Concentrations of ozone often exceed the thresholds of forest protection in the Pyrenees, but the effect of ozone on Pinus uncinata, the dominant species in subalpine forests in this mountainous range, has not yet been studied. We conducted an experiment of free-air ozone fumigation with saplings of P. uncinata fumigated with ambient O(3) (AOT40 May-Oct: 9.2 ppm h), 1.5 × O(3amb) (AOT40 May-Oct: 19.2 ppm h), and 1.8 × O(3amb) (AOT40 May-Oct: 32.5 ppm h) during two growing seasons. We measured chlorophyll content and fluorescence, visible injury, gas exchange, and above- and below-ground biomass. Increased exposures to ozone led to a higher occurrence and intensity of visible injury from O(3) and a 24-29% reduction of root biomass, which may render trees more susceptible to other stresses such as drought. P. uncinata is thus a species sensitive to O(3), concentrations of which in the Pyrenees are already likely affecting this species.

Effects of combined ozone and nitrogen deposition on the in situ properties of eleven key plant species of a subalpine pasture.

Tropospheric O(3) and deposition of reactive N threaten the composition and function of natural and semi-natural vegetation even in remote regions. However, little is known about effects of these pollutants individually or in combination on plant species in alpine habitats. We analyzed 11 frequent plant species of a subalpine Geo-Montani-Nardetum pasture exposed at 2,000 m a.s.l. in the Swiss Alps during 3 years using a factorial free-air exposure system with three concentrations of O(3) and five rates of N application. The aim was to detect subtle effects on leaf chlorophyll and N concentrations, leaf weight, specific leaf area (SLA), and delta(18)O and delta(13)C as proxies for gas exchange. We expected that the species' responsiveness to O(3) and N would be related to their functional traits and that N-induced changes in these traits would modify the species' response to O(3) via increased growth and higher leaf conductance (g (s)). Most species reacted to N supply with the accumulation of N and chlorophyll, but with no change in SLA, g (s), and growth, except Carex sempervirens which showed increased water use efficiency and leaf weight. Elevated O(3) reduced g ( s ) in most species, but this was not related to a reduction in leaf weight, which was recorded in half of the species. Contrary to our expectation, the magnitude of the response to both O(3) and N was not related to species-specific traits such as SLA or g (s). No pronounced O(3) x N interactions were observed. In conclusion, since for most species neither N nor gas exchange limited growth, their short-term response to O(3) and N and to their combination was small. O(3) x N interactive effects are expected to be more pronounced in habitats where species are more responsive to N due to favorable growth conditions in terms of nutrient availability and temperature.

Nitrogen deposition but not ozone affects productivity and community composition of subalpine grassland after 3 yr of treatment.

A field experiment was established at 2000 m above sea level (asl) in the central Swiss Alps with the aim of investigating the effects of elevated ozone (O(3)) and nitrogen deposition (N), and of their combination, on above-ground productivity and species composition of subalpine grassland. One hundred and eighty monoliths were extracted from a species-rich Geo-Montani-Nardetum pasture and exposed in a free-air O(3)-fumigation system to one of three concentrations of O(3) (ambient, 1.2 x ambient, 1.6 x ambient) and five concentrations of additional N. Above-ground biomass, proportion of functional groups and normalized difference vegetation index (NDVI) were measured annually. After 3 yr of treatment, the vegetation responded to the N input with an increase in above-ground productivity and altered species composition, but without changes resulting from elevated O(3). N input > 10 kg N ha(-1) yr(-1) was sufficient to affect the composition of functional groups, with sedges benefiting over-proportionally. No interaction of O(3) x N was observed, except for NDVI; positive effects of N addition on canopy greenness were counteracted by accelerated leaf senescence in the highest O(3) treatment. The results suggest that effects of elevated O(3) on the productivity and floristic composition of subalpine grassland may develop slowly, regardless of the sensitive response to increasing N.