Tussock Tundra Research Articles

Physical short‐term changes after a tussock tundra fire, Seward Peninsula, Alaska

The Kougarok area, situated on the central Seward Peninsula, Alaska, experienced a severe fire in August 2002. This may be the only tundra fire where high‐quality prefire (1999–2002) and postfire (2003–2006) active layer and meteorology measurements have been collected in the same locations. After fire, near‐surface soil showed increased moisture at the burned tussock site, remaining close to saturation throughout the thawed season 2003–2006. Despite wetter soil after the fire, freezing occurred earlier at the burned tussock site than at the control, indicating the importance of a reduced organic layer. Severe combustion of lichen and moss left 15–25 cm high tussocks, resulting in a doubling of the surface roughness coefficient. Average September temperature at the tussock site increased 2.3 ± 0.7°C throughout the 1 m soil profile, doubling the active layer depth, although this is due partly to favorable meteorological conditions. The shrubby control station experienced a mean annual temperature increase of 1.1 ± 0.3°C in the upper 0.5 m. A similar annual change was found at the burned tussock site. Cooler weather conditions in 2006 stagnated the soil‐warming trend, which occurred after 2002. How the thermal and moisture regimes in tundra will be affected after fire is highly influenced by weather, fire severity, vegetation regrowth, prefire vegetation, and ground ice conditions.

Climate and species affect fine root production with long-term fertilization in acidic tussock tundra near Toolik Lake, Alaska

Long-term fertilization of acidic tussock tundra has led to changes in plant species composition, increases in aboveground production and biomass and substantial losses of soil organic carbon (SOC). Root litter is an important input to SOC pools, although little is known about fine root demography in tussock tundra. In this study, we examined the response of fine root production and live standing fine root biomass to short- and long-term fertilization, as changes in fine root demography may contribute to observed declines in SOC. Live standing fine root biomass increased with long-term fertilization, while fine root production declined, reflecting replacement of the annual fine root system of Eriophorum vaginatum, with the long-lived fine roots of Betula nana. Fine root production increased in fertilized plots during an unusually warm growing season, but remained unchanged in control plots, consistent with observations that B. nana shows a positive response to climate warming. Calculations based on a few simple assumptions suggest changes in fine root demography with long-term fertilization and species replacement could account for between 20 and 39% of the observed declines in SOC stocks.

Quality and potential biodegradability of soil organic matter preserved in permafrost of Siberian tussock tundra

Soil organic matter (SOM) quality and biodegradability the permafrost underlying Siberian wet tussock tundra (Kolyma river basin, northeast Siberia) were analyzed and compared to the characteristics of the contemporary active layer. For this purpose, three permafrost affected soil cores (down to 3 m depth) and seven active layer soil cores (down to 0.3 m depth) were sampled. The samples were divided into particular layers, which were analyzed separately. SOM stability was assessed using a simple chemical fractionation (sequential extraction by cold and hot water, and hot acid). SOM biodegradability and soil mineralization potentials were tested in short-term laboratory incubations. The active layer contained 24 kg C m −2 and 70 kg C m −2 was preserved in 3 m of permafrost. The chemical quality and biodegradability of permafrost SOM were very similar to that of the active layer mineral horizon, and independent from depth. The only exceptions were (1) higher solubility of permafrost SOM in water, indicating its higher mobility and potential leakage after permafrost thawing and (2) higher nutrient (N, P) concentrations available to a dense permafrost microbial community, which could support decomposition of more complex substrates under suitable temperature conditions after thawing. The mineralization potential of the upper 1 m deep permafrost, which could melt by 2100 according to permafrost degradation models, was 6.7 g C m −2 d −1 (optimum conditions of 20 °C, field water capacity), which is comparable to that of the contemporary active layer of 0.5 m depth (7.5 g C m −2 d −1). Under field conditions, SOM mineralization rate would reasonably be significantly lower due to prevailing anoxia (high water table) and diffusion constraints in the deep and flooded soil profile. We conclude from our results that the permafrost (1) cryopreserves a high SOM amount, which is distributed to considerable depths, being of similar chemical quality and biodegradability to that of the active layer mineral horizon SOM, and (2) contains a dense living microbial community, which is able to decompose the present SOM rapidly without any obvious chemical limitation under suitable conditions.

Carbon in tundra soils in the Lake Labaz region of arctic Siberia

SummaryLarge amounts of carbon are stored in permafrost‐affected soils of the Arctic tundra. The quantity, distribution and composition of this carbon are important, because much of the carbon is likely to be released as a result of global warming. We have studied soils of the central Siberian Arctic to determine the carbon content and the nature of the organic matter by density fractionation, and 13C‐NMR‐ and 13C‐stable‐isotope analyses. There are pronounced differences in the profile and variations from place to place in the quantity and nature of soil organic matter. We estimated that the mean stock of carbon was 14.5 kg m–2 within the active layer. We found a total of about 30.7 kg C m–3 in the entire upper metre of the soils. Carbon of the tussock tundra showed strong vertical differentiation, with a large proportion comprising decomposed, recalcitrant compounds. We identified within the soil several zones of aerobe and anaerobe decomposition. Mobile carbon fractions have precipitated under the influence of low temperatures.

Climate, fire, farming and the recent vegetation history of subantarctic Campbell Island

ABSTRACTCampbell Island is a small, uninhabited peat-covered island lying in the cool southern ocean 600 km south of the New Zealand mainland. Dracophyllum scrub is the main cover from sea level to 200 m, above which tussock grassland, macrophyllous forbs and tundra dominate. Seven peat profiles from sea level to the tundra zone provide an elevational transect for pollen and charcoal records spanning the last 500 years. Scrub density was relatively low between 200 and 400 cal yrs BP, possibly due to Little Ice Age cooling, but had recovered by the time Europeans discovered the island in AD 1810. Burning and grazing during a brief farming episode (AD 1895–1931) severely reduced scrub and palatable grasses and forbs. Vegetation recovery is now well advanced following cessation of farming and the later elimination of all feral grazing animals, cats and rats. Climates were cool in the southwest Pacific during the farming period, and since AD 1970 the island has warmed by c. 0·5°C. However, there has been no upwards movement of the scrubline despite vigorous regeneration of scrub at lower altitudes. The island's cloudy, highly oceanic climate appears to offset increasing summer warmth, and scrubline is likely to rise only if clearer and less windy, as well as warmer, summers eventuate.

Effects of climate variability on carbon sequestration among adjacent wet sedge tundra and moist tussock tundra ecosystems

Temporal and spatial variability in the Arctic introduces considerable uncertainty in the estimation of the current carbon budget and Arctic ecosystem response to climate change. Few representative measurements are available for land‐surface parameterization of the Arctic tundra in regional and global climate models. In this study, the eddy covariance technique was used to measure net ecosystem CO2 exchange (NEE) of Alaskan wet sedge tundra and moist tussock tundra ecosystems during the summer (i.e., 1 June to 31 August) from 1999 to 2003 in order to quantify the seasonal and spatial variability in NEE and to determine controlling factors on NEE in these tundra ecosystems. Warmer and drier conditions prevailed for the moist tussock tundra compared with that of the wet sedge tundra. Over the 5‐year period, the wet sedge tundra was a sink for carbon of 46.4 to 70.0 gC m−2 season−1, while the moist tussock tundra either lost carbon of up to 60.8 gC m−2 season−1 or was in balance. The contrasting patterns of carbon balance at the two sites demonstrate that ecosystem difference can be more important in determining landscape NEE than intraseasonal and interseasonal variability due to environmental factors with respect to NEE. The wet sedge tundra showed an acclimation (e.g., over days) to temperature, while the moist tussock tundra illustrated a strong temperature dependence. Warming and drying accentuated ecosystem respiration in the moist tussock tundra, causing a net loss of carbon. Better characterization of spatial variability in NEE and associated environmental controls is required to improve current and future estimates of the Arctic terrestrial carbon balance.

A simple model for analyzing climatic effects on terrestrial carbon and nitrogen dynamics: An arctic case study

We developed a simplified plant‐soil model (PSM) composed of four coupled differential equations that simulate the effects of climate change on major stocks and fluxes of carbon (C) and nitrogen (N) in terrestrial ecosystems. Here we use the model to examine past, present, and future changes in C storage in arctic Alaska, a region undergoing rapid climate change. Model parameters were initialized to simulate the buildup of C and N stocks from the beginning of the current postglacial period (∼10,000 years BP) to present‐day levels for tussock tundra at Toolik Lake, Alaska. For projected rates of warming during the next century, the model predicts an increase in aboveground plant biomass and a net loss of soil carbon, resulting in almost no net change in total ecosystem C. The simplified model structure serves to clarify several important issues that have not been adequately addressed in previous studies. These issues include altered residence times of C and N in soils and plants, decreased synchrony of above and belowground processes, and the relationship between a model's initial conditions and the ecosystem's trajectory at the point of initialization.

MICROBIAL COMMUNITY COMPOSITION AND FUNCTION ACROSS AN ARCTIC TUNDRA LANDSCAPE

Arctic landscapes are characterized by a diversity of ecosystems, which differ in plant species composition, litter biochemistry, and biogeochemical cycling rates. Tundra ecosystems differing in plant composition should contain compositionally and functionally distinct microbial communities that differentially transform dissolved organic matter as it moves downslope from dry, upland to wet, lowland tundra. To test this idea, we studied soil microbial communities in upland tussock, stream-side birch-willow, and lakeside wet sedge tundra in arctic Alaska, USA. These are a series of ecosystems that differ in topographic position, plant composition, and soil drainage. Phospholipid fatty acid (PLFA) analyses, coupled with compound-specific 13C isotope tracing, were used to quantify microbial community composition and function; we also assayed the activity of extracellular enzymes involved in cellulose, chitin, and lignin degradation. Surface soil from each tundra ecosystem was labeled with 13C-cellobiose,13C-N-acetylglucosamine, or 13C-vanillin. After a five-day incubation, we followed the movement of 13C into bacterial and fungal PLFAs, microbial respiration, dissolved organic carbon, and soil organic matter. Microbial community composition and function were distinct among tundra ecosystems, with tussock tundra containing a significantly greater abundance and activity of soil fungi. Although the majority of 13C-labeled substrates rapidly moved into soil organic matter in all tundra soils (i.e., 50-90% of applied 13C), microbial respiration of labeled substrates in wet sedge tundra soil was lower than in tussock and birch-willow tundra; approximately 8% of 13C-cellobiose and approximately 5% of 13C-vanillin was respired in wet sedge soil vs. 26-38% of 13C-cellobiose and 18-21% of 13C-vanillin in the other tundra ecosystems. Despite these differences, wet sedge tundra exhibited the greatest extracellular enzyme activity. Topographic variation in plant litter biochemistry and soil drainage shape the metabolic capability of soil microbial communities, which, in turn, influence the chemical composition of dissolved organic matter across the arctic tundra landscape.

External mycorrhizal mycelia – the importance of quantification in natural ecosystems

External mycorrhizal mycelia – the importance of quantification in natural ecosystems

Effect of tree line advance on carbon storage in NW Alaska

We investigated the size, distribution, and temporal dynamics of ecosystem carbon (C) pools in an area of recent tree line advance, northwest Alaska. Repeat aerial photographs show forest cover increased ∼10% in our study area since 1949. We sampled C pools of four principal ecosystem types, tussock tundra, shrub tundra, woodland, and forest, all located on a 600–800 year old river terrace. Significant differences between ecosystem C pools, both above ground and below ground existed. Tundra sites store >22.2 kg C/m2, shrub tundra sites and woodland sites store 9.7 kg C/m2 and 14.3 kg C/m2, respectively, and forest sites store 14.4 kg C/m2. Landscape variation of total ecosystem C was primarily due to organic soil C and was secondarily due to C stored in trees. Soil C/N profiles of shrub tundra sites and woodland sites showed similarities with forest site soils at surface and tundra site soils at depth. We hypothesize that tundra systems transformed to forest systems in this area under a progression of permafrost degradation and enhanced drainage. On the basis of C pool estimates for the different ecosystem types, conversion of tundra sites to forest may have resulted in a net loss of >7.8 kg C/m2, since aboveground C gains were more than offset by belowground C losses to decomposition in the tundra sites. Tree line advance therefore might not increase C storage in high‐latitude ecosystems and thus might not, as previously suggested, act as a negative feedback to warming. Key to this hypothesis and to its projection to future climate response is the fate of soil carbon upon warming and permafrost drainage.

Cold-season Production of CO2 in Arctic Soils: Can Laboratory and Field Estimates Be Reconciled through a Simple Modeling Approach?

Microbial activity in arctic tundra soils has been evaluated through both lab incubations and field flux measurements. To determine whether these different measurement approaches can be directly linked to each other, we developed a simple model of soil microbial CO2 production during the cold season in tussock tundra, moss tundra, and wet meadow tundra in the Alaskan Arctic. The model incorporated laboratory-based estimates of microbial temperature responses at sub-zero temperatures with field measurements of C stocks through the soil profile and daily temperature measurements at the sites. Estimates of total CO2 production overestimated in situ cold season CO2 fluxes for the studied sites by as much as two- to threefold, suggesting that either CO2 produced in situ does not efflux during the cold season or that microbial respiration potentials are constrained by some other factor in situ. Average estimated winter CO2 production was near 120 g C m−2 in moist tundra and 60 g C m−2 in wet meadow tundra. Production was strongly seasonal, with most of the winter CO2 production happening early in the winter, before soils froze completely through. Roughly two-thirds of the total estimated CO2 production was from deep soils, largely mineral soils, in contrast to growing season CO2 dynamics.

A Bayesian Random Effects Discrete-Choice Model for Resource Selection: Population-Level Selection Inference

Modeling the probability of use of land units characterized by discrete and continuous measures, we present a Bayesian random-effects model to assess resource selection. This model provides simultaneous estimation of both individual- and population-level selection. Deviance information criterion (DIC), a Bayesian alternative to AIC that is sample-size specific, is used for model selection. Aerial radiolocation data from 76 adult female caribou (Rangifer tarandus) and calf pairs during 1 year on an Arctic coastal plain calving ground were used to illustrate models and assess population-level selection of landscape attributes, as well as individual heterogeneity of selection. Landscape attributes included elevation, NDVI (a measure of forage greenness), and land cover-type classification. Results from the first of a 2-stage model-selection procedure indicated that there is substantial heterogeneity among cow–calf pairs with respect to selection of the landscape attributes. In the second stage, selection of models with heterogeneity included indicated that at the population-level, NDVI and land cover class were significant attributes for selection of different landscapes by pairs on the calving ground. Population-level selection coefficients indicate that the pairs generally select landscapes with higher levels of NDVI, but the relationship is quadratic. The highest rate of selection occurs at values of NDVI less than the maximum observed. Results for land cover-class selections coefficients indicate that wet sedge, moist sedge, herbaceous tussock tundra, and shrub tussock tundra are selected at approximately the same rate, while alpine and sparsely vegetated landscapes are selected at a lower rate. Furthermore, the variability in selection by individual caribou for moist sedge and sparsely vegetated landscapes is large relative to the variability in selection of other land cover types. The example analysis illustrates that, while sometimes computationally intense, a Bayesian hierarchical discrete-choice model for resource selection can provide managers with 2 components of population-level inference: average population selection and variability of selection. Both components are necessary to make sound management decisions based on animal selection.

Identifying Differences in Carbon Exchange among Arctic Ecosystem Types

Our objective was to determine how varied is the response of C cycling to temperature and irradiance in tundra vegetation. We used a large chamber to measure C exchange at 23 locations within a small arctic catchment in Alaska during summer 2003 and 2004. At each location, we determined light response curves of C exchange using shade cloths, twice during a growing season. We used data to fit a simple photosynthesis-irradiance, respiration-temperature model, with four parameters. We used a maximum likelihood technique to determine the acceptable parameter space for each light curve, given measurement uncertainty. We then explored which sites and time periods had parameter sets in common—an indication of functional similarity. We found that seven distinct parameter sets were required to explain observed C flux responses to temperature and light variation at all sites and time periods. The variation in estimated maximum photosynthetic rate (Pmax) was strongly correlated with measurements of site leaf area index (LAI). The behavior of tussock tundra sites, the dominant vegetation of arctic tundra, could largely be described with a single parameter set, with a Pmax of 9.7 μmol m−2 s−1. Tussock tundra sites had, correspondingly, similar LAI (mean = 0.66). Non-tussock sites (for example, sedge and shrub tundras) had larger spatial and temporal variations in both C dynamic parameters (Pmax varying from 9.7–25.7 μmol m−2 s−1) and LAI (0.6–2.0). There were no clear relationships between dominant non-tussock vegetation types and a particular parameter set. Our results suggest that C dynamics of the acidic tussock tundra slopes and hilltops in northern Alaska are relatively simply described during the peak growing season. However, the foot-slopes and water tracks have more variable patterns of LAI and C exchange, not simply related to the dominant vegetation type.

Carbon dioxide and methane exchange of a north‐east Siberian tussock tundra

AbstractCarbon dioxide, energy flux measurements and methane chamber measurements were carried out in an arctic wet tussock grassland located on a flood plane of the Kolyma river in NE Siberia over a summer period of 155 days in 2002 and early 2003. Respiration was also measured in April 2004. The study region is characterized by late thaw of the top soil (mid of June) and periodic spring floods. A stagnant water table below the grass canopy is fed by thawing of the active layer of permafrost and by flood water. The climate is continental with average daily temperature in the warmest months of 13°C (maximum temperature at midday: 28°C by the end of July), dry air (maximum vapour pressure deficit at midday: 28 hPa) and low rainfall of 50 mm during summer (July–September). Summer evaporation (July–September: 103 mm) exceeded rainfall by a factor of 2. The daily average Bowen ratio (H/LE) was 0.62 during the growing season. Net ecosystem CO2 uptake reached 10 μmol m−2 s−1 and was related to photon flux density (PFD) and vapour pressure deficit (VPD). The cumulative annual net carbon flux from the atmosphere to the terrestrial surface was estimated to be about −38 g C m−2 yr−1 (negative flux depicts net carbon sink). Winter respiration was extrapolated using the Lloyd and Taylor function. The net carbon balance is composed of a high rate of assimilation in a short summer and a fairly large but uncertain respiration mainly during autumn and spring. Methane flux (about 12 g C m−2 measured over 60 days) was 25% of C uptake during the same period of time (end of July to end of September). Assuming that CH4 was emitted only in summer, and taking the greenhouse gas warming potential of CH4 vs. CO2 into account (factor 23), the study site was a greenhouse gas source (at least 200 g Cequivalent m−2 yr−1). Comparing different studies in wetlands and tundra ecosystems as related to latitude, we expect that global warming would rather increase than decrease the CO2‐C sink.

Neighbor effects on germination, survival, and growth in two arctic tundra plant communities

In relatively harsh environments such as arctic tundra, abiotic factors have traditionally been considered the primary determinants of community structure, overwhelming any effects of biotic interactions such as competition. Two common low arctic tundra types that differ in soil properties, moist acidic and moist non‐acidic tussock tundra (MAT and MNT, respectively), occur in close proximity in northern Alaska. Several plant species occur in both communities with different relative abundance, while others are restricted to one. This study experimentally examined how neighboring vegetation affects germination, survival, and growth of species in these two communities that differ in soil pH, cation availability, and other characteristics. Germination of sown seeds was greater than background levels suggesting seed limitation may restrict recruitment of these clonal, perennial species. Germination of sown seeds was greater at both sites when both mosses and vascular plants had been removed compared to plots with intact vegetation. However, neighbors had almost no effect on survival and growth of adult transplants. Patterns of germination, survival and growth of several species differed depending on the community of origin and the community of destination of the seeds or transplanted adults. For example, transplants of the sedge Eriophorum vaginatum grew better if they were from MAT, and this species germinated better when sown at MNT. Although of relatively short duration (three growing seasons), this study suggests that biotic interactions may affect local species composition by restricting germination and establishment in these two communities, but have less of an effect on adult plants. Not surprisingly, site‐specific abiotic conditions also exhibit control over species occurrence and relative abundance. Without disturbance to clear bare ground for recruitment of new individuals, these populations for the most part must rely on clonal growth to persist.

The seasonal dynamics of amino acids and other nutrients in Alaskan Arctic tundra soils

Past research strongly indicates the importance of amino acids in the N economy of the Arctic tundra, but little is known about the seasonal dynamics of amino acids in tundra soils. We repeatedly sampled soils from tussock, shrub, and wet sedge tundra communities in the summers of 2000 and 2001 and extracted them with water (H2O) and potassium sulfate (K2SO4) to determine the seasonal dynamics of soil amino acids, ammonium (NH4+), nitrate (NO3−), dissolved organic nitrogen (DON), dissolved organic carbon (DOC), and phosphate (PO42−). In the H2O extractions mean concentrations of total free amino acids (TFAA) were higher than NH4+ in all soils but shrub. TFAA and NH4+ were highest in wet sedge and tussock soils and lowest in shrub soil. The most predominant amino acids were alanine, arginine, glycine, serine, and threonine. None of the highest amino acids were significantly different than NH4+ in any soil but shrub, in which NH4+ was significantly higher than all of the highest individual amino acids. Mean NO3− concentrations were not significantly different from mean TFAA and NH4+ concentrations in any soil but tussock, where NO3− was significantly higher than NH4+. In all soils amino acid and NH4+ concentrations dropped to barely detectable levels in the middle of July, suggesting intense competition for N at the height of the growing season. In all soils but tussock, amino acid and NH4+ concentrations rebounded in August as the end of the Arctic growing season approached and plant N demand decreased. This pattern suggests that low N concentrations in tundra soils at the height of the growing season are likely the result of an increase in soil N uptake associated with the peak in plant growth, either directly by roots or indirectly by microbes fueled by increased root C inputs in mid-July. As N availability decreased in July, PO42− concentrations in the K2SO4 extractions increased dramatically in all soils but shrub, where there was a comparable increase in PO42− later in the growing season. Previous research suggests that these increases in PO42− concentrations are due to the mineralization of organic phosphorus by phosphatase enzymes associated with soil microbes and plant roots, and that they may have been caused by an increase in organic P availability.

Inter-annual variability of NDVI in response to long-term warming and fertilization in wet sedge and tussock tundra

This study explores the relationship between the normalized difference vegetation index (NDVI) and aboveground plant biomass for tussock tundra vegetation and compares it to a previously established NDVI-biomass relationship for wet sedge tundra vegetation. In addition, we explore inter-annual variation in NDVI in both these contrasting vegetation communities. All measurements were taken across long-term experimental treatments in wet sedge and tussock tundra communities at the Toolik Lake Long Term Ecological Research (LTER) site, in northern Alaska. Over 15 years (for wet sedge tundra) and 14 years (for tussock tundra), N and P were applied in factorial experiments (N, P and N+P), air temperature was increased using greenhouses with and without N+P fertilizer, and light intensity was reduced by 50% using shade cloth. during the peak growing seasons of 2001, 2002, and 2003, NDVI measurements were made in both the wet sedge and tussock tundra experimental treatment plots, creating a 3-year time series of inter-annual variation in NDVI. We found that: (1) across all tussock experimental tundra treatments, NDVI is correlated with aboveground plant biomass (r2 = 0.59); (2) NDVI-biomass relationships for tussock and wet sedge tundra communities are community specific, and; (3) NDVI values for tussock tundra communities are typically, but not always, greater than for wet sedge tundra communities across all experimental treatments. We suggest that differences between the response of wet sedge and tussock tundra communities in the same experimental treatments result from the contrasting degree of heterogeneity in species and functional types that characterize each of these Arctic tundra vegetation communities.

Leaf mineral nutrition of Arctic plants in response to warming and deeper snow in northern Alaska

Articulating the consequences of global climate change on terrestrial ecosystem biogeochemistry is a critical component of Arctic system studies. Leaf mineral nutrition responses of tundra plants is an important measure of changes in organismic and ecosystem attributes because leaf nitrogen and carbon contents effect photosynthesis, primary production, carbon budgets, leaf litter, and soil organic matter decomposition as well as herbivore forage quality. In this study, we used a longterm experiment where snow depth and summer temperatures were increased independently and together to articulate how a series of climate change scenarios would affect leaf N, leaf C, and leaf C:N for vegetation in dry and moist tussock tundra in northern Alaska, USA. Our findings were: 1) moist tundra vegetation is much more responsive to this suite of climate change scenarios than dry tundra with up to a 25% increase in leaf N; 2) life forms exhibit divergence in leaf C, N, and C:N with deciduous shrubs and graminoids having almost identical leaf N contents; 3) for some species, leaf mineral nutrition responses to these climate change scenarios are tundra type dependent (Betula), but for others (Vaccinium vitis‐idaea), strong responses are exhibited regardless of tundra type; and 4) the seasonal patterns and magnitudes of leaf C and leaf N in deciduous and evergreen shrubs were responsive to conditions of deeper snow in winter. Leaf N is was generally higher immediately after emergence from the deep snow experimental treatments and leaf N was higher during the subsequent summer and fall, and the leaf C:N were lower, especially in deciduous shrubs. These findings indicate that coupled increases in snow depth and warmer summer temperatures will alter the magnitudes and patterns of leaf mineral nutrition and that the long term consequences of these changes may feed‐forward and affect ecosystem processes.

Seasonal protein dynamics in Alaskan arctic tundra soils

In the arctic tundra of Alaska, plant growth is limited by N supply, especially in tussock tundra. Because proteins are the predominant form of soil organic N, proteolysis is considered to be the rate-limiting step in both the release of amino acids and in N mineralization. To help understand the controls on N availability in tundra soils, and to determine whether proteolysis is controlled by enzyme activity or by substrate availability, we measured soil protein concentrations, and proteolysis rates with and without added protein, every 1–2 weeks through the summer of 2000 and twice in the summer of 2001. Protease activity with added protein (‘potential’) was higher than without added protein (‘actual’). However, differences between the two tended to be driven by relatively brief peaks in potential protease activity. In fact, actual and potential rates were usually similar, suggesting that much of the time proteolysis was not limited by substrate availability, but rather by enzyme activity. Our data suggest that protease activity was actually only substrate limited at the times when it was highest. Potential rates peaked at the same times that soluble proteins were also high. These increases in protease activity and soluble protein concentrations occurred when soil amino acid and NH 4 + concentrations were at their lowest, drawn down by the seasonal peaks in root growth. The fact that the peaks in protease activity coincided with the peak in soil amino acid and NH 4 + demand strongly suggests that proteolysis was stimulated by high soil amino acid demand, and resulted in increases in soluble protein concentrations caused by the solubilization of larger proteins.

Spatial and Temporal Heterogeneity of Vegetation Properties among Four Tundra Plant Communities at Ivotuk, Alaska, U.S.A.

Intraseasonal patterns of normalized difference vegetation index (NDVI), leaf area index (LAI), and phytomass were compared for four tundra vegetation types at Ivotuk, Alaska, during summer 1999. The vegetation types included moist acidic tundra (MAT), moist nonacidic tundra (MNT), mossy tussock tundra, and shrub tundra. The seasonal curves of NDVI were similar among the vegetation types but with varying magnitudes of the peak values. Peak NDVI in the shrub tundra (0.83) was significantly greater than in MAT (0.76), which was significantly greater than in MNT (0.71) and mossy tussock tundra (0.70). LAI and phytomass exhibited high temporal variability with distinct seasonality only in shrub tundra. Seasonal LAI and NDVI patterns were therefore correlated only in shrub tundra, which was attributed to the high quantity of deciduous shrub foliage present in this community and absent in the other vegetation types. Shrub tundra peak live above-ground phytomass (1256 ± 123 g m−2) was significantly greater than peak live above-ground phytomass for MAT, MNT, and mossy tussock tundra (722 ± 71, 773 ± 53, 703 ± 39 g m−2 respectively, P < 0.05). Relative abundances of deciduous shrubs, mosses, and graminoids were revealed as key components controlling differences in NDVI, LAI, and phytomass among tundra vegetation types.