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Biophysical variables have both direct and indirect effects on the uptake and release of carbon dioxide (CO2) within tundra ecosystems. Arctic landscape shows high levels of spatial heterogeneity. High spatial-resolution remote sensing data has the ability to capture the fine-grain spectral response of various biophysical variables at the landscape scale. To accurately model CO2 flux patterns using remote sensing data we first need to model the relationships between biophysical variables and their spectral response. In this study we model percent vegetation cover (PVC), aboveground biomass (AGB), and soil moisture using high spatial-resolution (IKONOS 4 m) normalized difference vegetation index (NDVI) values. At two non-overlapping Arctic landscape sites statistically robust landscape-scale sampling procedures were used to characterize the biophysical variables. NDVI values were extracted from IKONOS data, and linear bivariate regression models were calibrated and validated using a k-fold cross-validation technique. PVC and percent soil moisture produced the strongest and most consistent results (r2≥ .84 and .73, respectively). Analysis of covariance tested the use of common models for each site. The models were not coincidental—combining data from various sites should be done with caution—but illustrated parallelism in that NDVI responds to each biophysical variable equally, regardless of site.
Environmental trends and ecosystems' ranges of variability are little known in tropical very high elevation Andean ecosystems (above 4400 m a.s.l.). We combined satellite image analyses and dendrochronological methods with instrumental records at lower elevation to assess changes in lake size and indices of plant productivity of subtropical high-elevation ecosystems in northern Argentina and southern Bolivia. Between 1985 and 2009, interannual lake fluctuations assessed with Landsat images were positively correlated with interannual variations in regional precipitation and de Martonne's aridity index, showing a decreasing trend in moisture availability during the period. Changes in lake size were positively correlated with radial growth of Polylepis tarapacana, and with MODIS-derived phenological parameters of enhanced vegetation index (EVI; an index of vegetation “greenness”) between 2001 and 2010. This indicates that water balance has a significant effect on ecosystem functioning, which is related to regional scale atmospheric circulation. A long-term tree ring chronology (starting in 1750) showed that tree growth during recent decades was lower than the last 180 years, and were comparable to growth patterns that occurred between 1775 and 1825. These results suggest that if recent climatic trends continue, long-term ranges of variability in ecosystem functioning could be exceeded.
Antarctic studies have indicated that during summer, ice-free areas experience greater temperatures than those along the glacial boundaries. This allows for the availability of liquid water and thus influences the biogeochemical processes of Antarctic terrestrial ecosystems. In this study we explore whether the patterns of soil phosphorus cycling differ between the glacial boundary and ice-free areas. To do so, we chose two sites on the Fildes Peninsula in Antarctica, one at the boundary of the Collins Glacier and another within an ice-free area close to Lake Uruguay. In each location, we determined soil phosphorus distribution, iron and aluminum fractions, soil mineralogy, alkaline phosphatase activity, and fluorescein diacetate hydrolysis. The results showed that soils of the ice-free area had a greater content of phosphorus sorbed on iron/aluminum oxyhydroxides and occluded forms. An opposite pattern was obtained for the calcium-bound phosphorus pool. Accordingly, soil samples from the ice-free area showed the greatest levels of iron/ aluminum oxyhydroxides and the presence of secondary minerals such as hematite and the intergrade chlorite-vermiculite-montmorillonite. Alkaline phosphatase activity and the fluorescein diacetate hydrolysis were also favored at the ice-free area. The overall results suggest that the less extreme microclimatic conditions and the presence of liquid water in the ice-free area promote the biogeochemical cycling of soil phosphorus. In a context of climate warming this study may contribute to the comprehension of how the expansion of the ice-free areas due to glacial retreat influences biogeochemical cycling of essential nutrients in the Antarctic Peninsula.
The quantification of the relationship between accumulation of snow and vegetation is crucial for understanding the influence of vegetation dynamics. We here present an analysis of the thickness of the snow and hydrological availability in relation to the seven main vegetation types in the High Arctic in Northeast Greenland. We used ground penetrating radar (GPR) for snow thickness measurements across the Zackenberg valley. Measurements were integrated to the physical conditions that support the vegetation distribution. Descriptive statistics and correlations of the distribution of each vegetation type to snow thickness, as well as to external factors that influence the redistribution of snow were performed. We found that although there is wide variability in the snow packing, there is strong correlation between snow thickness and the distribution of certain plant communities in the area. The accumulation of snow and occurrence of vegetation types such as Dryas octopetala heath and Salix arctica snowbed showed more influence by the microtopography than by other vegetation types that showed independence of the terrain conditions.
Recent observations of divergence between tree growth at high-latitude sites and temperature, as well as the decline of yellow cedar [Callitropsis nootkatensis (D. Don) Oerst. ex D.P. Little] in southeast Alaska due to warming, emphasize a need to investigate nonstationary climate response of Alaskan coastal forests to warming in other tree species. Comparison of annual tree growth in mountain hemlock [Tsuga mertensiana (Bong.) Carrière] to mean monthly temperature and precipitation data from Sitka, Alaska, from A.D. 1830s to 1990s along an elevational transect reveals nonstationarity in tree growth response to climate that suggests an ongoing biome shift. We observe a marked weakening in the positive relationship between annual growth and warmer growth season temperatures at low-elevation hemlock sites, and a concurrent increase in growth and sensitivity at higher elevations coupled with increased correlation between growth and April precipitation at all sites along our transects. As previously observed with yellow cedar, the mechanism of the hemlock biome shift may be due to an increased susceptibility of roots to damage from late frosts resulting from earlier seasonal loss of protective snowpack.
Winter desiccation and mortality of coniferous foliage are important determinants of carbon balance in trees and thus can influence the location of the subarctic treeline ecotone. The purpose of this study was (i) to assess variation in winter desiccation and viability of first-year conifer needles at several heights and orientations along tree boles across the forest-tundra ecotone near Churchill, Manitoba, from 2008 to 2010, and (ii) determine if there is a noticeable influence of needle health on ongoing treeline advance in the area. Growing season air temperatures around Churchill were significantly cooler in 2009, resulting in the development of significantly shorter needles during 2010. Minimum epidermal conductance (gmin) varied little with height on the tree or orientation to the prevailing wind direction. The highest values of gmin occurred in 2010, when temperatures during the previous June (t — 1) were 2.9 °C cooler than normal, and the lowest gmin occurred during 2009 when June (t — 1) was 1.2 °C warmer than normal. There were few correlations between needle viability and water content, and little consistency among years. However, significant correlations occurred during all 3 yr in northwest-facing needles at treeline, which suggests that treeline trees could be the most susceptible to water loss and dieback, relative to forest and tundra stems. Despite the occurrence of some winter desiccation, death of coniferous foliage (<10%) and sapling mortality (4–17%) was low, and rapid height growth of live saplings suggests passage through the wind-blown snow abrasion zone does not significantly impede wood production. Ostensibly winter desiccation and foliage mortality does not significantly influence sapling height growth and treeline dynamics around Churchill.
Wind exposure is known to have stressful effects on plant growth, particularly at high altitudes. We studied how environmental factors affected carbon assimilation in Pinus pumila needles at a wind-exposed site. Needle gas exchange rates were determined for detached shoots in the laboratory where the needles were free from field environmental stresses, and also determined for attached shoots in the field under in situ environment. There was no difference in gas exchange characteristics determined in the laboratory between shoots from the wind-exposed and the wind-protected sites, suggesting that wind exposure did not affect the photosynthetic potential. In the field, however, the photosynthetic rate of one-year-old needles (in situ Aarea) was significantly lower at the windexposed site than that at the wind-protected site. There was a positive correlation between the in situ Aareaand the xylem pressure potential, suggesting that water deficit caused photosynthetic suppression at the wind-exposed site. The in situ Aarea was lower at the wind-exposed site, even with the same electron transport rate and the same stomatal conductance. These results suggest that CO2 assimilation is suppressed by lower mesophyll CO2 conductance at the wind-exposed site. We conclude that the carbon gain is limited by water stress in wind-exposed regions.
Chloroplast pigments and chlorophyll fluorescence were characterized in needles of Korean fir (Abies koreana) in summer, winter, and spring at three altitudes on Jeju Island, Korea. High light-harvesting efficiency (intrinsic photosystem II efficiency) and indirect evidence for high photosynthetic rates (high levels ofβ-carotene and chlorophyll b) during the growing season contrasted with mid-winter downregulation of light-harvesting efficiency involving retention of high zeaxanthin levels and locked-in photoprotective thermal dissipation (from low chlorophyll fluorescence emission). Neoxanthin levels were inversely correlated with sustained photoprotection in the winter, and lutein to xanthophyll cycle carotenoid levels decreased from summer to winter, suggesting that zeaxanthin plays the prominent role in winter photoprotection of Korean fir needles. Summer was apparently most conducive to photosynthesis, consistent with high levels of summer precipitation on Jeju Island, and in contrast to fir and other conifers in a climate with dry summers at high altitudes in Colorado, U.S.A., where studies have shown that the wet spring is the season most favorable for photosynthesis. Lastly, despite there being only 300 m difference in altitude among the three sites, there were discernible differences in (i) accumulation of zeaxanthin in winter (as an indicator for the severity of conditions, with highest levels at the highest altitude), (ii) apparent photosynthesis rates in summer (from β-carotene levels, with highest levels at the highest altitude), and (iii) transition to increased photosynthesis in spring (from fluorescence emission levels, slowest at the highest altitude).
NDVI (Normalized Difference Vegetation Index) calculated from coarse-resolution sensors has shown strong increases since the 1980s on Alaska's North Slope. Finer-resolution satellite data and ground studies are needed to understand the changes in the vegetation that are causing these increases. Analysis of an 823 km2 area using a Landsat NDVI time series showed that the homogeneous greening at coarser scales was very heterogeneous at 30-m pixel resolution, with a strong influence due to glacial history. Small scattered patches of pixels with significant increases in NDVI occurred throughout the younger, late Pleistocene glacial deposits. On older, mid-Pleistocene deposits, increases occurred in few, larger patches of mostly tussock-sedge, dwarf-shrub, moss tundra, possibly a result of release of nutrients from thawing of ice-rich permafrost. Five percent of pixels had significant linear increases in NDVI from 1985 to 2007 (n = 6, p < 0.05), while 0.4% showed significant decreases, in small patches whose causes were evident when sampled on the ground. Trends in NDVI varied by glacial history, elevation, slope, and the resulting vegetation conditions. This heterogeneity in response to climate change can be expected throughout much of the Arctic, where complex glacial histories determine existing soil and vegetation characteristics.
Vegetation-banked terraces with alternating bands of vegetation and gravel are spectacularly developed on the plateau of subantarctic Macquarie Island. Previous work has described two distinct, apparently unrelated types of vegetation-banked terraces on the island, “windward” and “leeward,” depending on their exposure to the prevailing westerly winds. Here we have documented aspects of terrace morphology and vegetation. The terraces are dynamic, with mobile gravel, substrate erosion, and vegetation growth all in evidence. By focusing on the processes occurring, we conclude that the “windward” and “leeward” types are actually two related forms that grade from one into the other, depending on hillside aspect. In addition, we describe stone-banked terraces, not previously reported from Macquarie Island. Recent changes on the island resulting in widespread death of the cushion plant Azorella macquariensis may impact on the maintenance of terrace stability in the long term.
Extrapolating biosphere-atmosphere CO2 flux observations to larger scales in space, part of the so-called “upscaling” problem, is a central challenge for surface-atmosphere exchange research. Upscaling CO2 flux in tundra is complicated by the pronounced spatial variability of vegetation cover. We demonstrate that a simple model based on chamber observations with a pan-Arctic parameterization accurately describes up to 75% of the observed temporal variability of eddy covariance—measured net ecosystem exchange (NEE) during the growing season in an Abisko, Sweden, subarctic tundra ecosystem, and differed from NEE observations by less than 4% for the month of June. These results contrast with previous studies that found a 60% discrepancy between upscaled chamber and eddy covariance NEE sums. Sampling an aircraft-measured normalized difference vegetation index (NDVI) map for leaf area index (L) estimates using a dynamic flux footprint model explained less of the variability of NEE across the late June to mid-September period, but resulted in a lower root mean squared error and better replicated large flux events. Findings suggest that ecosystem structure via L is a critical input for modeling CO2 flux in tundra during the growing season. Future research should focus on quantifying microclimate, namely photosynthetically active radiation and air temperature, as well as ecosystem structure via L, to accurately model growing season tundra CO2 flux at chamber and plot scales.
We compared in three shallow Alaskan Arctic lakes physical properties of bulk sediment and the fine-scale (1 mm increments to 20 mm sediment depth) vertical distribution of dissolved O2, scalar irradiance, chlorophyll a, and radiocarbon assimilation by microphytobenthos to better understand the structural and functional significance of this community. Sediments showed water contents of 86–98%, and dry bulk densities of 0.012–0.146 g cm-3, depending on depth. Chlorophyll a displayed no clear vertical pattern, suggesting mixing of surface layers and showed lakewise averages of 5.1–23.7 µg cm-3. Sediments were oxic to 1.5–5.5 mm and showed attenuation coefficients of 1.17–2.07 mm-1 for photon scalar irradiance. Photosynthetic activity was localized near the sediment surface, as 57–81% of H14CO3- added to intact cores was recovered in the 0–2 mm zone. Only 26–44% of the chlorophyll a in vertical profiles was sited in the euphoric zone, but microphytobenthos in underlying, aphotic sediments immediately photosynthesized at or near rates for the surface sediment when artificially irradiated. Area-based chlorophyll a in the euphotic plus photosynthetically capable aphotic microphytobenthos was 62–105 times higher than that of the phytoplankton, pointing to the potential importance of benthic autotrophs to Arctic lake food webs.