Assessing the spatial variability in peak season CO2exchange characteristics across the Arctic tundra using a light response curve parameterization
Mbufong, H.N. ; Lund, M. ; Aurela, M. ; Molen, M.K. van der - \ 2014
Biogeosciences 11 (2014)17. - ISSN 1726-4170 - p. 4897 - 4912.
carbon-dioxide exchange - net ecosystem exchange - photosynthetically active radiation - growing-season - thermal-acclimation - vascular plants - tussock tundra - climate-change - energy flux - alaska
This paper aims to assess the spatial variability in the response of CO2exchange to irradiance across the Arctic tundra during peak season using light response curve (LRC) parameters. This investigation allows us to better understand the future response of Arctic tundra under climatic change. Peak season data were collected during different years (between 1998 and 2010) using the micrometeorological eddy covariance technique from 12 circumpolar Arctic tundra sites, in the range of 64-74° N. The LRCs were generated for 14 days with peak net ecosystem exchange (NEE) using an NEE-irradiance model. Parameters from LRCs represent site-specific traits and characteristics describing the following: (a) NEE at light saturation (Fcsat), (b) dark respiration (Rd), (c) light use efficiency (a), (d) NEE when light is at 1000 µmol m-2s-1(Fc1000), (e) potential photosynthesis at light saturation (Psat) and (f) the light compensation point (LCP). Parameterization of LRCs was successful in predicting CO2flux dynamics across the Arctic tundra. We did not find any trends in LRC parameters across the whole Arctic tundra but there were indications for temperature and latitudinal differences within sub-regions like Russia and Greenland. Together, leaf area index (LAI) and July temperature had a high explanatory power of the variance in assimilation parameters (Fcsat, Fc1000and Psat, thus illustrating the potential for upscaling CO2exchange for the whole Arctic tundra. Dark respiration was more variable and less correlated to environmental drivers than were assimilation parameters. This indicates the inherent need to include other parameters such as nutrient availability, substrate quantity and quality in flux monitoring activities.
Topographic controls on the leaf area index and plant functional type of a tundra ecosystem
Spadavecchia, L. ; Williams, M. ; Bell, R. ; Stoy, P.C. ; Huntley, B. ; Wijk, M.T. van - \ 2008
Journal of Ecology 96 (2008)6. - ISSN 0022-0477 - p. 1238 - 1251.
arctic ecosystems - statistical variables - principal components - soil properties - tussock tundra - global change - co2 flux - vegetation - alaska - biomass
Leaf area index (LAI) is an emergent property of vascular plants closely linked to primary production and surface energy balance. LAI can vary by an order of magnitude among Arctic tundra communities and is closely associated with plant functional type. We examined topographic controls on vegetation type and LAI distribution at two different scales in an Arctic tundra ecosystem in northern Sweden. `Micro-scale' measurements were made at 0.2-m resolution over a 40 m × 40 m domain, while `macro-scale' data were collected at approximately 10-m resolution over a 500 m × 500 m domain. Tundra LAI varied from 0.1-3.6 at the micro-scale resolution, and from 0.1-1.6 at the macro-scale resolution. The correlation between dominant vascular species and LAI at the micro-scale (r2 = 0.40) was greater than the correlation between dominant vegetation and LAI at the macro-scale (r2 = 0.14). At the macro-scale, LAI was better explained by topographic parameters and spatial auto-correlation (pseudo r2 = 0.32) than it was at the micro-scale (r2 = 0.16). Exposure and elevation were significantly but weakly correlated with LAI at the micro-scale, while on the macro-scale the most significant explanatory topographic variable was elevation (r2 = 0.12). The distribution of plant communities at both scales was significantly associated with topography. Shrub communities, dominated by Betula nana, were associated with low elevation sites at both scales, while more exposed and/or high elevation sites were dominated by cryptogams. Synthesis. Dominant vegetation, topography and LAI were linked at both scales of investigation but, for explaining LAI, topography became more important and dominant vegetation less important at the coarser scale. The explanatory power of dominant species/functional type for LAI variation was weaker at coarser scales, because communities often contained more than one functional type at 10 m resolution. The data suggest that remotely sensed topography can be combined with remotely sensed optical measurements to generate a useful tool for LAI mapping in Arctic environments.
The effect of temperature on growth and competition between Sphagnum species
Breeuwer, A.J.G. ; Heijmans, M.M.P.D. ; Robroek, B.J.M. ; Berendse, F. - \ 2008
Oecologia 156 (2008)1. - ISSN 0029-8549 - p. 155 - 167.
interspecific competition - litter quality - tussock tundra - climate-change - water-level - mosses - bog - decomposition - mire - photosynthesis
Peat bogs play a large role in the global sequestration of C, and are often dominated by different Sphagnum species. Therefore, it is crucial to understand how Sphagnum vegetation in peat bogs will respond to global warming. We performed a greenhouse experiment to study the effect of four temperature treatments (11.2, 14.7, 18.0 and 21.4°C) on the growth of four Sphagnum species: S. fuscum and S. balticum from a site in northern Sweden and S. magellanicum and S. cuspidatum from a site in southern Sweden. In addition, three combinations of these species were made to study the effect of temperature on competition. We found that all species increased their height increment and biomass production with an increase in temperature, while bulk densities were lower at higher temperatures. The hollow species S. cuspidatum was the least responsive species, whereas the hummock species S. fuscum increased biomass production 13-fold from the lowest to the highest temperature treatment in monocultures. Nutrient concentrations were higher at higher temperatures, especially N concentrations of S. fuscum and S. balticum increased compared to field values. Competition between S. cuspidatum and S. magellanicum was not influenced by temperature. The mixtures of S. balticum with S. fuscum and S. balticum with S. magellanicum showed that S. balticum was the stronger competitor, but it lost competitive advantage in the highest temperature treatment. These findings suggest that species abundances will shift in response to global warming, particularly at northern sites where hollow species will lose competitive strength relative to hummock species and southern species.
What is the relationship between changes in canopy leaf area and changes in photosynthetic CO² flux in artic ecosystems?
Street, L.E. ; Shaver, G.R. ; Williams, M. ; Wijk, M.T. van - \ 2007
Journal of Ecology 95 (2007)1. - ISSN 0022-0477 - p. 139 - 150.
long-term nutrient - wet sedge tundra - tussock tundra - plant-communities - carbon storage - alaskan wet - responses - biomass - vegetation - fertilization
1 The arctic environment is highly heterogeneous in terms of plant distribution and productivity. If we are to make regional scale predictions of carbon exchange it is necessary to find robust relationships that can simplify this variability. One such potential relationship is that of leaf area to photosynthetic CO2 flux at the canopy scale. 2 In this paper we assess the effectiveness of canopy leaf area in explaining variation in gross primary productivity (GPP): (i) across different vegetation types; (ii) at various stages of leaf development; and (iii) under enhanced nutrient availability. To do this we measure net CO2 flux light response curves with a 1 × 1 m chamber, and calculate GPP at a photosynthetic photon flux density (PPFD) of 600 µmol m2 s1. 3 At a subarctic site in Sweden, we report 10-fold variation in GPP among natural vegetation types with leaf area index (LAI) values of 0.05¿2.31 m2 m2. At a site of similar latitude in Alaska we document substantially elevated rates of GPP in fertilized vegetation. 4 We can explain 80% of the observed variation in GPP in natural vegetation (including vegetation measured before deciduous leaf bud burst) by leaf area alone, when leaf area is predicted from measurements of normalized difference vegetation index (NDVI). 5 In fertilized vegetation the relative increase in leaf area between control and fertilized treatments exceeds the relative increase in GPP. This suggests that higher leaf area causes increased self-shading, or that lower leaf nitrogen per unit leaf area causes a reduction in the rate of photosynthesis. 6 The results of this study indicate that canopy leaf area is an excellent predictor of GPP in diverse low arctic tundra, across a wide range of plant functional types.
Optical instruments for measuring leaf area index in low vegetation: application in arctic ecosystems
Wijk, M.T. van; Williams, M. - \ 2005
Ecological Applications 15 (2005)4. - ISSN 1051-0761 - p. 1462 - 1470.
tussock tundra - carbon-dioxide - exchange - productivity - responses - light - ndvi - lai
Leaf area index (LAI) is a powerful diagnostic of plant productivity. Despite the fact that many methods have been developed to quantify LAI, both directly and indirectly, leaf area index remains difficult to quantify accurately, owing to large spatial and temporal variability. The gap-fraction technique is widely used to estimate the LAI indirectly. However, for low-stature vegetation, the gap-fraction sensor either cannot get totally underneath the plant canopy, thereby missing part of the leaf area present, or is too close to the individual leaves of the canopy, which leads to a large distortion of the LAI estimate. We set out to develop a methodology for easy and accurate nondestructive assessment of the variability of LAI in low-stature vegetation. We developed and tested the methodology in an arctic landscape close to Abisko, Sweden. The LAI of arctic vegetation could be estimated accurately and rapidly by combining field measurements of canopy reflectance (NDVI) and light penetration through the canopy (gap-fraction analysis using a LI-COR LAI-2000). By combining the two methodologies, the limitations of each could be circumvented, and a significantly increased accuracy of the LAI estimates was obtained. The combination of an NDVI sensor for sparser vegetation and a LAI-2000 for denser vegetation could explain 81% of the variance of LAI measured by destructive harvest. We used the method to quantify the spatial variability and the associated uncertainty of leaf area index in a small catchment area
Tight coupling between leaf area index and foliage N content in arctic plant communities
Wijk, M.T. van; Williams, M. ; Shaver, G.R. - \ 2005
Oecologia 142 (2005)3. - ISSN 0029-8549 - p. 421 - 427.
brooks-range-foothills - tundra ecosystems - northern alaska - tussock tundra - responses - biomass - productivity - toposequence - vegetation - landscape
The large spatial heterogeneity of arctic landscapes complicates efforts to quantify key processes of these ecosystems, for example productivity, at the landscape level. Robust relationships that help to simplify and explain observed patterns, are thus powerful tools for understanding and predicting vegetation distribution and dynamics. Here we present the same linear relationship between Leaf area index (LAI) and Total foliar nitrogen (TFN), the two factors determining the photosynthetic capacity of vegetation, across a wide range of tundra vegetation types in both northern Sweden and Alaska between leaf area indices of 0 and 1 m(2) m(-2), which is essentially the entire range of leaf area index values for the Arctic as a whole. Surprisingly, this simple relationship arises as an emergent property at the plant community level, whereas at the species level a large variability in leaf traits exists. As the relationship between LAI and TFN exists among such varied ecosystems, the arctic environment must impose tight constraints on vegetation canopy development. This relationship simplifies the quantification of vegetation productivity of arctic vegetation types as the two most important drivers of productivity can be estimated reliably from remotely sensed NDVI images.
Long-term ecosystem level experiments at Toolik Lake, Alaska, and at Abisko, Northern Sweden: generalizations and differences in ecosystem and plant type responses to global change
Wijk, M.T. van; Clemmensen, K.E. ; Shaver, G.R. ; Williams, M. ; Callaghans, T.V. ; Chapin, F.S. ; Cornelissen, J.H.C. ; Gough, L. ; Hobbie, S.E. ; Jonasson, S. ; Lees, J.A. ; Michelsen, A. ; Press, M.C. ; Richardsons, S.J. ; Rueth, H. - \ 2004
Global Change Biology 10 (2004)1. - ISSN 1354-1013 - p. 105 - 123.
simulated environmental-change - arctic polar semidesert - uv-b radiation - climate-change - tussock tundra - growth-responses - betula-nana - reproductive development - terrestrial ecosystems - eriophorum-vaginatum
Long-term ecosystem-level experiments, in which the environment is manipulated in a controlled manner, are important tools to predict the responses of ecosystem functioning and composition to future global change. We present the results of a meta-analysis performed on the results of long-term ecosystem-level experiments near Toolik Lake, Alaska, and Abisko, Sweden. We quantified aboveground biomass responses of different arctic and subarctic ecosystems to experimental fertilization, warming and shading. We not only analysed the general patterns but also the differences in responsiveness between sites and regions. Aboveground plant biomass showed a broad similarity of responses in both locations, and also showed some important differences. In both locations, aboveground plant biomass, particularly the biomass of deciduous and graminoid plants, responded most strongly to nutrient addition. The biomass of mosses and lichens decreased in both locations as the biomass of vascular plants increased. An important difference between the two regions was the smaller positive aboveground biomass response of deciduous shrubs in Abisko as compared with Toolik Lake. Whereas in Toolik Lake Betula nana increased its dominance and replaced many of the other plant types, in Abisko all vascular plant types increased in abundance without major shifts in relative abundance. The differences between the responses of the dominant vegetation types of the Toolik Lake region, i.e. tussock tundra systems, and that of the Abisko region, i.e. heath systems, may have important implications for ecosystem development under expected patterns of global change. However, there were also large site-specific differences within each region. Several potential mechanistic explanations for the differences between sites and regions are discussed. The response patterns show the need for analyses of joint data sets from many regions and sites, in order to uncover common responses to changes in climate across large arctic regions from regional or local responses.
Luxury consumption of soil nutrients: a possible competitive strategy in above-ground and below-ground biomass allocation and root morphology for slow-growing arctic vegetation?
Wijk, M.T. van; Williams, M. ; Gough, L. ; Hobbie, S.E. ; Shaver, G.R. - \ 2003
Journal of Ecology 91 (2003). - ISSN 0022-0477 - p. 664 - 676.
nitrogen-fertilization - mineral-nutrition - plant-communities - tussock tundra - carbon storage - wild plants - growth - diversity - alaska - availability
1 A field-experiment was used to determine how plant species might retain dominance in an arctic ecosystem receiving added nutrients. We both measured and modelled the above-ground and below-ground biomass allocation and root morphology of non-acidic tussock tundra near Toolik Lake, Alaska, after 4 years of fertilization with nitrogen and phosphorus. 2 Compared with control plots, the fertilized plots showed significant increases in overall root weight ratio, and root biomass, root length and root nitrogen concentration in the upper soil layers. There was a strong trend towards relatively more biomass below ground. 3 We constructed an individual teleonomic (i.e. optimality) plant allocation and growth model, and a competition model in which two plants grow and compete for the limiting resources. 4 The individual plant model predicted a strong decrease in root weight ratio with increased nutrient availability, contrary to the results obtained in the field. 5 The increased investment in roots in the fertilized plots found in the field could be explained in the competition model in terms of luxury consumption of nutrients (i.e. the absorbance of nutrients in excess of the immediate plant growth requirements). For slow-growing species with relatively low phenological and physiological plasticity it can be advantageous to increase relative investment into root growth and root activity. This increased investment can limit nutrient availability to other fast-growing species and, thereby, preclude the successful invasion of these species. 6 These results have implications for the transient response of communities and ecosystems to global change.