The instrumental Arctic climate record is both temporally and spatially limited. Therefore, there is a need for reliable climate proxies to increase knowledge of past and future Arctic climate ...change. Annual shoot length increase of the circumarctic dwarf shrub species
Cassiope tetragona represents such a new climate proxy. We measured annual shoot length increase of 32 plant samples of the circumarctic dwarf shrub species
C. tetragona using the presence of wintermarksepta within the stems, resulting in a 169 year growth chronology (1840–2008) for a High Arctic site on Svalbard. This is the longest growth chronology for dwarf shrub species produced up to now.
Relationships between climate and
Cassiope growth were investigated through correlative, response function and forward stepwise multiple regression analysis. July average air temperature was found to be the most important factor determining growth, by itself capable of explaining 41% of the variance in shoot growth between 1912 and 2008. The second best predictors were previous year September precipitation sums and average air temperatures, along with several previous growth parameters. A multiple regression model explaining growth with current July and previous year September temperature, combined with previous growth of lag 1, 2 and 5 years as predictors explains 70% of the observed variance in growth. July temperatures and previous year September precipitation sums alone explain 59% of the variance in standardized growth.
Mean July air temperature was reconstructed for the period between 1876 and 2007 by a growth-temperature transfer model, using current and following year’s growth. The estimated temperatures correlated well with measured temperatures over the calibration (1912–1959) and verification (1960–2007) period:
R
2 = 0.34 and
R
2 = 0.47, respectively. The instrumental record (1912–2008) extended with these reliable mean July temperature estimates (1876–1911) reveals a significant warming trend on Svalbard since 1876 of 0.07 °C decade
−1 on average.
This study shows that the climate–growth relationships in
C. tetragona, its longevity, its annual resolution, the availability of (sub)fossil fragments in tundra soil cores and its circumartic distribution make it a very valuable tool for climate reconstructions beyond the instrumental record and in areas lacking meteorological data, throughout the Arctic.
1. Climate change in the subarctic is expected to influence vegetation composition, specifically bryophyte and lichen communities, thereby modifying litter decomposition rates and carbon (C) dynamics ...of these systems with possible feedbacks to climate. 2. In a 2-year experiment, we investigated decomposition rates and chemical traits of 27 bryophytes, 17 lichens and 5 vascular plants in litter beds in subarctic Sweden. The majority of the sampled cryptogam species are widespread at higher northern latitudes. 3. Average 2-year litter decomposition rates (exponential mass loss constant k) of lichen (0.44 ± 0.01) and vascular plant (0.56 ± 0.03) species were higher than that of bryophytes (0.11 ± 0.01), while within main cryptogam taxa, species identity was an important determinant of mass loss rates. At cryptogam group level, 2-year litter mass loss of Sphagnum was significantly lower than for non-Sphagnum mosses and liverworts. Within lichens, N₂-fixing versus non-N₂-fixing lichens showed no variation in decomposability. 4. In a subset of the large species set, mass loss differed both among incubation environments (reflecting nutrient-rich and poor birch forest and Sphagnum peatlands, respectively) and species. The pattern of mass loss across incubation environments was not consistent among cryptogam species. N₂-fixing, in contrast to non-N₂-fixing lichens with lower nitrogen (N) levels displayed similar decomposition rates across incubation environments. Mass loss of non-Sphagnum mosses was correlated with initial N irrespective of incubation environment. 5. Litter mass loss of cryptogam taxa could be predicted very well from infrared spectra of the initial chemical composition of the species, by application of Fourier transform infrared using an attenuated total reflectance probe. The initial macronutrient concentrations (N, phosphorus, C and cations) and initial litter pH correlated less well. 6. Synthesis. We showed comprehensively that decomposition rates of bryophytes are generally lower than those of lichens and vascular plants. Among bryophyte or lichen species there is also great variation in litter decomposability which depends strongly on species-specific chemistry. Our data will help predict changing land surface feedback to C cycles and climate in cold biomes by understanding long-term climate effects on litter decomposability through shifting vegetation composition.
Annual shoot length of the circumarctic dwarf shrub Cassiope tetragona has proved to be a reliable proxy for past and ongoing climate change in the Arctic. This is based on its strong linear ...relationship with monthly climate parameters. Monthly means are, however, coarse units for prediction of growth in marginal regions with short growing seasons. An alternative to monthly averages are parameters that quantify the growing season length (GSL) and its intensity (growing degree-days; GDD5). GDD5 is defined as the cumulative daily mean temperature above 5°C. GSL is defined as the number of days on which the average temperature exceeds 5°C. The aims of this study were to test whether these parameters are a better predictor of growth than monthly means and to reconstruct past High Arctic growing season climate. Correlative analysis shows that GDD5 is a better predictor of annual shoot length growth than mean monthly temperatures and GSL, both at C. tetragona’s European northern and southern distribution limit, as well as at its assumed climatic optimum. Svalbard Airport GDD5 was reconstructed back to 1857. The reconstruction shares 61% of variance with the instrumental record. This opens the possibility to obtain an Arctic network of climate reconstructions with high temporal and spatial resolution through construction of C. tetragona shoot length chronologies.
1. Interspecific variation in plant functional traits is fast becoming popular as a tool for understanding and predicting ecosystem biogeochemistry as dependent on vegetation composition. Leaf pH has ...recently been shown to be a promising new candidate trait for this purpose. But how robust is leaf pH as a species trait in the face of environmental variation? We hypothesized that inherent interspecific variation in leaf pH should be greater than phenotypic variation of given species in response to soil environments. 2. We tested this hypothesis in a temperate herbaceous flora by growing 23 species experimentally in three soils of contrasting pH (ranging by almost three pH units) and related chemistry. 3. As predicted, there was large and consistent variation in leaf pH among these species, which was robust to the differences between soil types. Indeed both the species rankings and the absolute species values for leaf pH were remarkably constant in comparisons between soil types. 4. The fact that a given species can maintain a leaf pH very different from that of their soil environment, combined with the great interspecific variation in leaf pH, indicates that leaf pH really is largely a species-specific trait. Linked with recent field evidence we suggest that interspecific variation in leaf pH, while easy and cheap to assess, has important predictive power of biogeochemical properties and processes in ecosystems.
Annual growth of the polar evergreen shrub Cassiope tetragona on Svalbard was evaluated as a proxy for Arctic summer temperatures. Transfer functions were derived from temperature-growth correlations ...of shoots and from a temperature-growth response, obtained from experimental warming using open top chambers (OTC) in high Arctic tundra vegetation at Isdammen approximately 1.5 km southeast of Longyearbyen, Svalbard (78°N, 15 E) and in Longyeardalen, 3 km west of Isdammen from 2004 to 2006. Air temperatures, monitored throughout the summer months, were 1.3 °C higher inside the OTCs than in the control plots. Annual stem growth was measured by tagging stems and leaves, and in the lab with shoots harvested from OTCs and control plots. Annual growth parameters assessed were leaf production, sum of length and weight of individual leaves, and stem length increment derived from leaf scar distances and the distances between wintermarksepta in the stem. Wintermarksepta are formed at the end of the summer growth period when the pith is narrowing and consist of dense and dark tissue ( Fig. 1b ). The variation of annual growth in a 34-year site chronology (based on Cassiope shoots from the surroundings of the OTCs and control plots) correlated strongly with the mean summer temperature on Svalbard. The number of leaf pairs, leaf length and stem length also increased in the OTC warmed plots in the second and third year of warming. Transfer functions were derived from the temperature-annual growth correlations from a single shoot from Longyeardalen, from the cross-dated Isdammen site chronology and from the growth response to experimental warming. Based on leaf scar distances and distances between wintermarksepta of well-preserved subfossil shoots in arctic tundra soil, annual stem length increase was assessed for the layers of a soil core collected at the Isdammen site. Based on the derived transfer functions summer temperature of the period relating to the 15 cm deep tundra soil core layer, radiocarbon dated at 4230±40 bp, may have been 3.0 °C lower than the present-day 6.2 °C value. These results indicate that the transfer functions can be used to reconstruct past temperatures, beyond the time range of instrumental temperature and ice core records of Svalbard.
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► Salicornia dolichostachya had its growth optimum at 300mM NaCl in the root medium. ► The highest values of succulence and stem diameter coincide with the growth optimum. ► Plants ...maintained a lower leaf osmotic potential than that of the medium. ► Carbon isotope discrimination decreased linear with increasing external salinity. ► Leaf and root glycine betaine concentrations did not change with external salinity.
Salinization of agricultural land is an increasing problem. Because of their high tolerance to salinity, Salicornia spp. could become models to study salt tolerance; they also represent promising saline crops. The salinity-growth response curve for Salicornia dolichostachya Moss was evaluated at 12 salt concentrations in a hydroponic study in a greenhouse and at 5 different seawater dilutions in an outside setting. Salt concentrations ranged between 0mM and 500mM NaCl (≈seawater salinity). Plants were grown for six weeks and morphological and physiological adaptations in different tissues were evaluated.
S. dolichostachya had its growth optimum at 300mM NaCl in the root medium, independent of the basis on which growth was expressed. The relative growth rate (RGR) in the greenhouse experiment was comparable with RGR-values in the outdoor growth experiment. Leaf succulence and stem diameter had the highest values at the growth optimum (300mM NaCl). Carbon isotope discrimination (δ13C) decreased upon salinity. S. dolichostachya maintained a lower leaf sap osmotic potential relative to the external solution over the entire salinity range, this was mainly accomplished by accumulation of Na+ and Cl−. Glycine betaine concentrations did not significantly differ between the treatments. Na+:K+-ratio and K+-selectivity in the shoots increased with increasing salinity, both showed variation between expanding and expanded shoot tissue. We conclude that S. dolichostachya was highly salt tolerant and showed salt requirement for optimal growth. Future growth experiments should be done under standardized conditions and more work at the tissue and cellular level needs to be done to identify the underlying mechanisms of salt tolerance.
A method was developed for the analysis of the UV-absorbing sporopollenin monomers p-coumaric acid and ferulic acid in very low numbers of pollen. This enables the analysis of pollen or spores from ...cultured plants, from herbarium collections, and from sediment, soil, and peat cores. The method involves thermally assisted hydrolysis and methylation using tetramethylammonium hydroxide combined with gas chromatography and mass spectrometry. Pyrolysis, gas chromatographic, and mass spectrometric conditions were optimized for the analysis of minimal amounts of pollen. The method has a detection limit of ∼60 fresh pollen of Alnus glutinosa and a relative standard deviation of ∼10% between 100 and 600 pollen.
UV-B absorbance and UV-B absorbing compounds (UACs) of the pollen of
Vicia faba, Betula pendula,
Helleborus foetidus and
Pinus sylvestris were studied. Sequential extraction demonstrated considerable ...UV-B absorbance both in the soluble (acid methanol) and insoluble sporopollenin (acetolysis resistant residue) fractions of UACs, while the wall-bound fraction of UACs was small. The UV-B absorbance of the soluble and sporopollenin fraction of pollen of
Vicia faba plants exposed to enhanced UV-B (10 kJ m
−2 day
−1 UV-B
BE) was higher than that of plants that received 0 kJ m
−2 day
−1 UV-B
BE. Pyrolysis gas chromatography–mass spectrometry (py-GC–MS) analysis of pollen demonstrated that
p-coumaric acid and ferulic acid formed part of the sporopollenin fraction of the pollen. The amount of these aromatic monomers in the sporopollenin of
Vicia faba appeared to increase in response to enhanced UV-B (10 kJ m
−2 day
−1 UV-B
BE). The detection limit of pyGC–MS was sufficiently low to quantify these phenolic acids in ten pollen grains of
Betula and
Pinus.The experimental data presented provide evidence for the possibility that polyphenolic compounds in pollen of plants are indicators of solar UV-B and may be applied as a new proxy for the reconstruction of historic variation in solar UV-B levels.
Soil organic matter decomposition is limited at waterlogged conditions by the low activity of extracellular enzymes like phenol oxidases. In this paper, we show that ferrous iron ($\text{Fe}^{2+}$), ...which is abundant in waterlogged soils, significantly stimulates phenol oxidase activity both in pure enzyme assays and in waterlogged soil slurries from nutrient-poor dune slacks. However, the effects in soil slurries were less strong than in enzyme assays. Both the addition of$\text{Fe}^{2+}$and the initial presence of$\text{Fe}^{2+}$stimulated phenol oxidase activity at the microaerophilic conditions tested. This stimulation is attributed to the catalysis of additional OH radical production, promoting the oxidation of phenolics. Subsequently, the presence of$\text{Fe}^{2+}$strongly increased total decomposition rates of soil organic matter, measured as CO₂ production and Cotton strip Tensile Strength Loss. There is circumstantial evidence that this stimulation by$\text{Fe}^{2+}$could be important for decomposition in wetlands at field conditions, but its relevance compared to the effects of other compounds still needs to be elucidated. These results emphasise the crucial role of water quality in determining extracellular enzyme activity and decomposition in water-logged wetlands.
•Fuel ladders are potentially important determinants of future boreal fire regimes.•Tree species identity determines the ignition probability of organic soils.•Flames from smoldering soils ignite ...conifer branches more easily than birch branches.•Chemical branch traits contribute to fire ladder behavior in both directions.
Peat fires in boreal and tundra regions can potentially cause a high CO2 release, because of their large soil carbon stocks. Under current and future climate warming the frequency and intensity of droughts are increasing and will cause the plant community and organic soil to become more susceptible to fire. The organic soil consumption by fire is commonly used as a proxy for fire severity and is a large source of carbon release. However, the role of organic soils in both above- and belowground fire behavior has only rarely been studied. In this study we collected soil and branches from Betula pubescens, Pinus sylvestris and Picea abies/obovata from the taiga/tundra ecotone across a large spatial scale. In laboratory fire experiments we burned different fuel type combinations to examine the fire spread through fuel ladders both from branches to soil and vice versa. We found that the tree species identity influences the fire spread from branches to soil and vice versa. The combination of chemical and structural plant traits could explain the stronger interaction between soil and coniferous spruce and pine fuels in a fire ladder compared to the deciduous birch. Therefore, total carbon emission from a boreal forest fire may not only depend on burned plant fuel, but also on the species-specific potential of the trees to ignite the soil. Carbon emission models and forest management could be improved if not only the aboveground plant fuel consumption is considered, but also the interaction between fuels in a fuel ladder and the probability of soil ignition by a forest crown fire and vice versa.
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