van de Weg
Martine J.
van de Weg
Martine J.
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PreprintVegetation shifts observed in arctic tundra 1.5 decades after fire( 2012-01-27) Barrett, Kirsten ; Rocha, Adrian V. ; van de Weg, Martine J. ; Shaver, Gaius R.With anticipated climate change, tundra fires are expected to occur more frequently in the future, but data on the longer term effects of fire on tundra vegetation composition are scarce. This study therefore addresses changes in vegetation structure that have persisted for 17 years after a tundra fire on the North Slope of Alaska. Fire-related shifts in vegetation composition were assessed from remote sensing imagery and ground observations of the burn scar and an adjacent control site. Early-season remotely sensed imagery from the burn scar exhibits a low vegetation index compared to the control site, while the late-season signal is slightly higher. The range and maximum vegetation index is greater in the burn scar, although the mean annual values do not differ among the sites. Ground observations revealed a greater abundance of graminoid species and an absence of Betula nana in the post-fire tundra sites, which is a likely explanation for the spectral differences observed in the remotely sensed imagery. Additional differences in vegetation composition in the burn scar include less moss cover and a greater cover of herbaceous species. The partial replacement of tundra by graminoid-dominated ecosystems has been predicted by the ALFRESCO model of disturbance, climate, and vegetation succession.
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PreprintContrasting effects of long term versus short-term nitrogen addition on photosynthesis and respiration in the Arctic( 2013-07) van de Weg, Martine J. ; Shaver, Gaius R. ; Salmon, Verity G.We examined the effects of short (<1 to 4 years) and long-term (22 years) nitrogen (N) and/or phosphorus (P) addition on the foliar CO2 exchange parameters of the arctic species Betula nana and Eriophorum vaginatum in northern Alaska. Measured variables included: the carboxylation efficiency of Rubisco (Vcmax), electron transport capacity (Jmax), dark respiration (Rd), chlorophyll a and b content (Chl), and total foliar N (N). For both B. nana and E. vaginatum, foliar N increased by 20-50% as a consequence of 1 to 22 years of fertilisation, respectively, and for B. nana foliar Nincrease was consistent throughout the whole canopy. However, despite this large increase in foliar N, no significant changes in Vcmax and Jmax were observed. In contrast, Rd was significantly higher (>25%) in both species after 22 years of N addition, but not in the shorter-term treatments. Surprisingly, Chl only increased in both species the first year of fertilisation (i.e. the first season of nutrients applied), but not in the longer-term treatments. These results imply that: 1) Under current (low) N availability, these Arctic species either already optimize their photosynthetic capacity per leaf area, or are limited by other nutrients; 2) Observed increases in Arctic NEE and GPP with increased nutrient availability are caused by structural changes like increased leaf area index, rather than increased foliar photosynthetic capacity and 3) Short-term effects (1-4 years) of nutrient addition cannot always be extrapolated to a larger time scale, which emphasizes the importance of long-term ecological experiments.
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PreprintResponse of dark respiration to temperature in Eriophorum vaginatum from a 30-year-old transplant experiment in Alaska( 2012-09) van de Weg, Martine J. ; Fetcher, Ned ; Shaver, Gaius R.Background: In the Arctic region, temperature increases are expected to be greater under anticipated climate change than the global average. Understanding how dark respiration (Rd) of common Arctic plant species acclimates to changes in the environment is therefore important for predicting changes to the Arctic carbon balance. Aims: The aim of this study is to investigate the influence of genotype and growing environment on Rd, the temperature response (Q10) of Rd, and foliar N (Nleaf) of the Arctic sedge Eriophorum vaginatum. Methods: We measured Rd, and determined its Q10 and Nleaf of E. vaginatum populations that were reciprocally transplanted 30 years previously along a latitudinal transect of 370 km in northern Alaska. Results: Rd and Q10 did not differ among populations (ecotypes) of E. vaginatum, but the local environment had a significant effect on both variables. Rd as well as Nleaf was higher in northern, colder sites, while Q10 was lower there. Conclusions: Rd in the different populations of E. vaginatum is a very plastic trait and controlled by growing environment, as is Nleaf. The lower Q10 values in the northern sites were most likely a consequence of substrate inhibition of Rd at higher temperatures.
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ArticleShrub encroachment in Arctic tundra : Betula nana effects on above- and belowground litter decomposition(John Wiley & Sons, 2017-04-07) McLaren, Jennie ; Buckeridge, Kate M. ; van de Weg, Martine J. ; Shaver, Gaius R. ; Schimel, Joshua P. ; Gough, LauraRapid arctic vegetation change as a result of global warming includes an increase in the cover and biomass of deciduous shrubs. Increases in shrub abundance will result in a proportional increase of shrub litter in the litter community, potentially affecting carbon turnover rates in arctic ecosystems. We investigated the effects of leaf and root litter of a deciduous shrub, Betula nana, on decomposition, by examining species-specific decomposition patterns, as well as effects of Betula litter on the decomposition of other species. We conducted a 2-yr decomposition experiment in moist acidic tundra in northern Alaska, where we decomposed three tundra species (Vaccinium vitis-idaea, Rhododendron palustre, and Eriophorum vaginatum) alone and in combination with Betula litter. Decomposition patterns for leaf and root litter were determined using three different measures of decomposition (mass loss, respiration, extracellular enzyme activity). We report faster decomposition of Betula leaf litter compared to other species, with support for species differences coming from all three measures of decomposition. Mixing effects were less consistent among the measures, with negative mixing effects shown only for mass loss. In contrast, there were few species differences or mixing effects for root decomposition. Overall, we attribute longer-term litter mass loss patterns to patterns created by early decomposition processes in the first winter. We note numerous differences for species patterns between leaf and root decomposition, indicating that conclusions from leaf litter experiments should not be extrapolated to below-ground decomposition. The high decomposition rates of Betula leaf litter aboveground, and relatively similar decomposition rates of multiple species below, suggest a potential for increases in turnover in the fast-decomposing carbon pool of leaves and fine roots as the dominance of deciduous shrubs in the Arctic increases, but this outcome may be tempered by negative litter mixing effects during the early stages of encroachment.