Tundra Systems Research Articles

Size-selective predation on pelagic microorganisms in Arctic freshwaters

Herbivorous zooplankton may have a pronounced influence on pelagic microorganisms in Arctic freshwaters. We quantified experimentally the size-selective feeding of several zooplankton groups on pelagic microorganisms in high Arctic tundra systems. Our experiments and field study focused on dominant herbivores in Arctic freshwaters, including the cladoceran Daphnia, the copepod Diaptomus and the anostracan Branchinecta, and their effects on prey ranging in size from bacteria to large phytoplankton. Grazing experiments showed that Daphnia were effective predators on all types of prey, whereas Diaptomus grazed preferentially on larger phytoplankton with low clearance rates for bacterial cells. Further analysis by flow cytometry indicated that Diaptomus grazed selectively on the largest bacteria. In contrast to the results obtained in the controlled experiments, Arctic lakes and ponds with a zooplankton community dominated by Daphnia had a higher bacterial production and abundance than systems not dominated by this grazer. This may indicate that the stimulatory effect of grazers on bacterial growth is more pronounced in natural systems, or that factors other than zooplankton grazing are more important in regulating bacterial abundance and production in natural systems. Although Arctic waters differ considerably from temperate systems with respect to temperature and light regime, herbivore–prey dynamics as well as the bacterial response to temperature appear to be similar between the climatic regions.

Phase and amplitude of ecosystem carbon release and uptake potentials as derived from FLUXNET measurements

As length and timing of the growing season are major factors explaining differences in carbon exchange of ecosystems, we analyzed seasonal patterns of net ecosystem carbon exchange ( F NEE) using eddy covariance data of the FLUXNET data base ( http://www-eosdis.ornl.gov/FLUXNET). The study included boreal and temperate, deciduous and coniferous forests, Mediterranean evergreen systems, rainforest, native and managed temperate grasslands, tundra, and C 3 and C 4 crops. Generalization of seasonal patterns are useful for identifying functional vegetation types for global dynamic vegetation models, as well as for global inversion studies, and can help improve phenological modules in SVAT or biogeochemical models. The results of this study have important validation potential for global carbon cycle modeling. The phasing of respiratory and assimilatory capacity differed within forest types: for temperate coniferous forests seasonal uptake and release capacities are in phase, for temperate deciduous and boreal coniferous forests, release was delayed compared to uptake. According to seasonal pattern of maximum nighttime release (evaluated over 15-day periods, F max) the study sites can be grouped in four classes: (1) boreal and high altitude conifers and grasslands; (2) temperate deciduous and temperate conifers; (3) tundra and crops; (4) evergreen Mediterranean and tropical forests. Similar results are found for maximum daytime uptake ( F min) and the integral net carbon flux, but temperate deciduous forests fall into class 1. For forests, seasonal amplitudes of F max and F min increased in the order tropical<Mediterranean and temperate coniferous<temperate deciduous and boreal forests, and the pattern seems relatively stable for these groups. The seasonal amplitudes of F max and F min are largest for managed grasslands and crops. Largest observed values of F min varied between −48 and −2 μmol m −2 s −1, decreasing in the order C 4-crops>C 3-crops>temperate deciduous forests>temperate conifers>boreal conifers>tundra ecosystems. Due to data restrictions, our analysis centered mainly on Northern Hemisphere temperate and boreal forest ecosystems. Grasslands, crops, Mediterranean ecosystems, and rainforests are under-represented, as are savanna systems, wooded grassland, shrubland, or year-round measurements in tundra systems. For regional or global estimates of carbon sequestration potentials, future investigations of eddy covariance should expand in these systems.

Early Spring Nitrogen Uptake by Snow-Covered Plants: A Comparison of Arctic and Alpine Plant Function under the Snowpack

We compared nitrogen (N) acquisition by alpine and arctic tundra plants during snowmelt, and assessed its significance relative to season-long N demand. We then related plant responses in alpine and arctic tundra to soil and air temperature. Parallel experiments were conducted in an alpine moist meadow at Niwot Ridge, Colorado, and in arctic dry heath and moist tussock tundra near Toolik Lake, Alaska. We added 99%-enriched 15NH415NO3 (0.5 g N m−2) to the snowpack, and traced the fate of the 15N into plants and soil by harvesting samples before snowmelt ended. All alpine and arctic plants acquired N during snowmelt. However, alpine plants acquired 100 times more N than arctic plants. Using data from published studies, we estimated season-long N demand for the vascular plant species. In the alpine tundra, N uptake during snowmelt constituted over 12% of season-long uptake for a graminoid species, and averaged 7.4% for perennial forbs. In contrast, N uptake during snowmelt by arctic plants averaged less than 0.1% of season-long N uptake. Soil temperatures were similar in these systems during snowmelt, averaging −0.4°C in the arctic and −0.5°C in the alpine tundra. However, arctic plants experienced lower winter soil temperatures than alpine plants. Winter soil surface temperatures averaged −7.75°C in arctic and −2.43°C in alpine tundra systems, and the lowest weekly mean temperature was −14.5°C in the arctic and −6.14°C in the alpine systems. Thus, differences in plant N uptake are likely due to winter conditions, rather than conditions that occurred during uptake. In conclusion, plant acquisition of N resources during snowmelt is important in alpine systems.

Early Spring Nitrogen Uptake by Snow-Covered Plants: A Comparison of Arctic and Alpine Plant Function under the Snowpack

We compared nitrogen (N) acquisition by alpine and arctic tundra plants during snowmelt, and assessed its significance relative to season-long N demand. We then related plant responses in alpine and arctic tundra to soil and air temperature. Parallel experiments were conducted in an alpine moist meadow at Niwot Ridge, Colorado, and in arctic dry heath and moist tussock tundra near Toolik Lake, Alaska. We added 99%-enriched 15NH415NO3 (0.5 g N m−2) to the snowpack, and traced the fate of the 15N into plants and soil by harvesting samples before snowmelt ended. All alpine and arctic plants acquired N during snowmelt. However, alpine plants acquired 100 times more N than arctic plants. Using data from published studies, we estimated season-long N demand for the vascular plant species. In the alpine tundra, N uptake during snowmelt constituted over 12% of season-long uptake for a graminoid species, and averaged 7.4% for perennial forbs. In contrast, N uptake during snowmelt by arctic plants averaged less than 0.1% of season-long N uptake. Soil temperatures were similar in these systems during snowmelt, averaging −0.4°C in the arctic and −0.5°C in the alpine tundra. However, arctic plants experienced lower winter soil temperatures than alpine plants. Winter soil surface temperatures averaged −7.75°C in arctic and −2.43°C in alpine tundra systems, and the lowest weekly mean temperature was −14.5°C in the arctic and −6.14°C in the alpine systems. Thus, differences in plant N uptake are likely due to winter conditions, rather than conditions that occurred during uptake. In conclusion, plant acquisition of N resources during snowmelt is important in alpine systems.

Carbon dioxide fluxes of soils and mosses in wet tundra of Taimyr Peninsula, Siberia: controlling factors and contribution to net system fluxes

Knowledge of the environmental controls of carbon dioxide fluxes is essential for understanding the dynamics of carbon exchange between ecosystems and atmosphere. In this study we investigated soil respiration and moss photosynthesis as well as their contribution to the net carbon dioxide flux of two different wet tundra systems. During two summers, in situ carbon dioxide fluxes were measured in a tussock tundra and in a low-centre polygonal tundra on Taimyr Peninsula, central Siberia. Measurements were carried out by means of a multichannel gas exchange system. Results show pronounced differences in soil respiration rates as related to microscale topography, mainly due to differences of soil water table and soil temperatures. Modelling of soil respiration for individual microsites revealed differences of process performance with respect to both factors. The wet microsites showed the highest potential regarding an increase of soil respiration rates in warmer and drier climate change scenarios. Another important process compensating the CO2 release from the soil was the photosynthesis of the moss layer, assimilating as much as 51% to 98% of the daily amount of carbon dioxide released from wet tundra soils. This result demonstrates the importance of mosses in the context of tundra ecosystem processes. The magnitude of net system fluxes of the whole system at the depression of the polygonal tundra was strongly influenced by changes in soil water table. Consequently, any changes of the hydrology, as anticipated in the context of global change, would effectively alter the carbon balance of wet tundra systems.

Hydrology, water availability and tundra ecosystem function in a changing climate: the need for a closer integration of ideas?

AbstractHydrologists and ecologists studying tundra ecosystems have worked largely independently, with little cross‐fertilization between disciplines. Their disciplines are, however, inextricably linked by a need to understand the dynamics and significance of the common substance water, in its liquid, solid and gaseous state within tundra environments. The impacts of predicted long‐term changes in climate have particularly important consequences for the functioning of tundra systems and there is a pressing need to initiate studies that integrate hydrological and ecological methodologies and concepts. Our paper attempts to summarize existing information on the role of water within tundra ecosystems, to emphasize the fundamental links between the biotic and the physico/chemical environments and to suggest how a closer integration of ideas might be achieved. Given the breadth of the subject matter the paper is intended to be illustrative rather than comprehensive. The paper examines the physical impacts of water in its various states on the tundra environment, emphasizing in particular the causes of spatial variation in water availability to living organisms. The significance of water is discussed for a range of organism groups, including plants, invertebrates and microorganisms and its pivotal role in ecosystem function and disturbance stressed. The need to develop integrated hydological/ecological models for tundra systems on different spatial scales is emphasized.

Carbon dioxide fluxes of soils and mosses in wet tundra of Taimyr Peninsula, Siberia: controlling factors and contribution to net system fluxes

Knowledge of the environmental controls of carbon dioxide fluxes is essential for understanding the dynamics of carbon exchange between ecosystems and atmosphere. In this study we investigated soil respiration and moss photosynthesis as well as their contribution to the net carbon dioxide flux of two different wet tundra systems. During two summers, in situ carbon dioxide fluxes were measured in a tussock tundra and in a low-centre polygonal tundra on Taimyr Peninsula, central Siberia. Measurements were carried out by means of a multichannel gas exchange system. Results show pronounced differences in soil respiration rates as related to microscale topography, mainly due to differences of soil water table and soil temperatures. Modelling of soil respiration for individual microsites revealed differences of process performance with respect to both factors. The wet microsites showed the highest potential regarding an increase of soil respiration rates in warmer and drier climate change scenarios. Another important process compensating the CO2 release from the soil was the photosynthesis of the moss layer, assimilating as much as 51% to 98% of the daily amount of carbon dioxide released from wet tundra soils. This result demonstrates the importance of mosses in the context of tundra ecosystem processes. The magnitude of net system fluxes of the whole system at the depression of the polygonal tundra was strongly influenced by changes in soil water table. Consequently, any changes of the hydrology, as anticipated in the context of global change, would effectively alter the carbon balance of wet tundra systems.

Carbon cycling in the Kuparuk basin: Plant production, carbon storage, and sensitivity to future changes

The Marine Biological Laboratory General Ecosystem Model was calibrated for an arctic tussock tundra system using data from long‐term observations and experiments at Toolik Lake, Alaska. These experiments include the effects of changes in temperature, light, CO2, and nutrients, so the model could be applied to five regions comprising the entire Kuparuk River basin. Net primary production, averaged for the entire basin, was 92 g C m−2 yr−1. A 150 year simulation of carbon storage under a doubling of CO2 (slow ramp‐up) and a temperature increase of 3.5°C gave an estimate of +400 g C m−2 when soil moisture increased and +500 g C m−2 when soil moisture decreased. Drier soils stimulated decomposition producing an increase in nitrogen availability; the increased N led to increased net primary production. If this result is applicable to other arctic ecosystems, then it is unlikely that warming will enhance carbon loss to the atmosphere to further enhance warming.

Effects of drainage and temperature on carbon balance of tussock tundra micrososms

We examined the importance of temperature (7°C or 15°C) and soil moisture regime (saturated or field capacity) on the carbon (C) balance of arctic tussock tundra microcosms (intact blocks of soil and vegetation) in growth chambers over an 81-day simulated growing season. We measured gaseous CO2 exchanges, methane (CH4) emissions, and dissolved C losses on intact blocks of tussock (Eriophorum vaginatum) and intertussock (moss-dominated). We hypothesized that under increased temperature and/or enhanced drainage, C losses from ecosystem respiration (CO2 respired by plants and heterotrophs) would exceed gains from gross photosynthesis causing tussock tundra to become a net source of C to the atmosphere. The field capacity moisture regime caused a decrease in net CO2 storage (NEP) in tussock tundra micrososms. This resulted from a stimulation of ecosystem respiration (probably mostly microbial) with enhanced drainage, rather than a decrease in gross photosynthesis. Elevated temperature alone had no effect on NEP because CO2 losses from increased ecosystem respiration at elevated temperature were compensated by increased CO2 uptake (gross photosynthesis). Although CO2 losses from ecosystem respiration were primarily limited by drainage, CH4 emissions, in contrast, were dependent on temperature. Furthermore, substantial dissolved C losses, especially organic C, and important microhabitat differences must be considered in estimating C balance for the tussock tundra system. As much as ∼ 20% of total C fixed in photosynthesis was lost as dissolved organic C. Tussocks stored ∼ 2x more C and emitted 5x more methane than intertussocks. In spite of the limitations of this microcosm experiment, this study has further elucidated the critical role of soil moisture regime and dissolved C losses in regulating net C balance of arctic tussock tundra.

Soil aeration in relation to soil physical properties, nitrogen availability, and root characteristics within an arctic watershed

The seasonal change in soil oxygen availability was determined in several habitats along a topographic moisture gradient in an arctic watershed. The effect of changes in soil aeration on soil chemical and plant properties was examined by comparison of the driest (tussocks) and wettest (wet sedge tundra) sites along this gradient. Spatial variability and seasonal change in soil oxygen availability was closely linked to the hydrologic regime and the thickness of the organic soil horizon. The greatest extension of the aerobic soil layer was found beneath well-drained tussocks, while less than 10% of the unfrozen soil layer is aerated in flooded wet sedge tundra. Intertussock areas and watertracks (channels of water drainage) have intermediate levels of aeration. In tussock tundra, soil oxygen diffusion is restricted in the mineral soil layer below the organic horizon due to reduced pore space. Organic matter constituents and their change with depth were similar beneath tussocks and in wet sedge tundra, indicating that factors other than soil aeration (e.g. low soil temperatures, short growing season) are the primary controls on decomposition in these two arctic tundra systems. NH4+, the dominant form of inorganic N, was more available in wet sedge tundra than in tussock tundra. At both sites, extractable and soil solution NO3- concentrations increased 4 to 8 fold in the second part of the growing season, indicating increased nitrifier activity with improved soil oxygen availability. Although soils thawed as deep as 60 cm, approx. 90% of the root biomass was concentrated within 20 cm of the surface. Despite the anaerobic soil environment in wet sedge tundra, the dominant species there, Eriophorum angustifolium, reached slightly greater rooting depths than E. vaginatum, whose roots grow in the elevated, aerobic portion of tussocks. E. angustifolium had a root porosity of 31%, within the range found for wetland species, while roots of E. vaginatum had a porosity close to 12%. Rhizome porosity were low in both species (11%).

Amino Acid Absorption by Arctic Plants: Implications for Plant Nutrition and Nitrogen Cycling

Recent studies of nitrogen (N) cycling in arctic tundra have indicated that inorganic N supplied to plants by mineralization is not sufficient to meet the annual requirement of N by many tundra species. Whereas N mineralization is slow in tundra soils and concentrations of inorganic N are low, these soils have large stocks of both structural and soluble organic N. In light of these observations, kinetics of absorption of three amino acids (glycine, aspartic acid, and glutamic acid) were measured in dominant vascular plant species of the four major ecosystems types in arctic Alaska and compared with concentrations of free amino acids in soils. Absorption rates were measured on roots using 14C—labeled substrates. Concentrations of free amino acids in soil were measured on water—extracted samples by high pressure liquid chromatography. All species had higher capacity (Vmax) for ammonium uptake (measured using methylamine as an ammonium analogue) than for any amino acid. However, at concentrations observed in the field, uptake rates estimated for amino acids were similar to (glycine) or less than (aspartic and glutamic acids) that for ammonium. On the basis of these comparisons, uptake rates of the three amino acids together may account for between 10 and 82% of the total N uptake in the field, depending on species and community. Deciduous shrubs had higher uptake rates than the more slowly growing evergreen shrubs, suggesting that new growth created a sink that strongly influenced capacity for amino acid uptake. In general, ectomycorrhizal species had higher amino acid uptake than did non—mycorrhizal species. In species that were sampled from more than one community, amino acid uptake rates were highest in the community where a given amino acid was most abundant in the soil. The results indicate that, in arctic tundra, plants short—circuit the mineralization step of decomposition by directly absorbing amino acids. This implies that in the organic soils of these tundra systems (1) inorganic nitrogen is an inadequate measure of plant—available soil nitrogen, (2) mineralization rates underestimate nitrogen supply rates to plants, (3) the large differences among species in capacities to absorb different forms of N provide ample basis for niche differentiation of what was previously considered a single resource, and (4) by short—circuiting N mineralization, plants accelerate N turnover and effectively exert greater control over N cycling than has been previously recognized.

Response of alpine tundra to a changing climate: A hierarchical simulation model

The present paper describes an alpine tundra simulation model that has been developed within a hierarchical framework based primarily on data from Niwot Ridge and adjacent areas of the Colorado Front Range. Our objective is to simulate the response of alpine tundra to climatic change. The model consists of three hierarchically nested submodels. The level i submodel represents the alpine zone of the Front Range of Colorado (16.6 km 2) and operates on a time scale of centuries. The Level ii submodel represents Niwot Ridge alpine (6.8 km 2) and operates on a time scale of decades. The Level iii submodel, which is itself divided into six submodels, represents each of the six major soil-plant communities that occur on Niwot Ridge (several hectares each) and operates on a time scale of years. Simulations were run representing tundra system dynamics under (a) constant environmental temperatures and (b) a cooling trend of 0.07°C per year. Simulation results show shifts in the elevation of treeline (Level i), shifts in the proportion of tundra occupied by each of six different soil-plant communities (Level ii), and changes in rates of primary production and decomposition in each community type (Level iii).

Holism and Reductionism in Microbial Ecology

Microbial ecology is often regarded as the epitome of ecological reductionism. Although many microbial ecologists adopt reductionist approaches to achieve causal explanation, I feel this is due to the small size of the organisms and systems they work with rather than to any necessary preoccupation with small scale entities or approaches. Further, I believe that the relative proportion of holists and reductionists in microbial ecology does not differ from other disciplines. Microbial ecologists, however, must brave a nebulous episteme when laboratory studies of processes are to be related to field measures of the results of these processes. Two very different aspects of my research, one clearly reductionist (in the sense of small vs large) and the other more holistic, eventually brought me to the same point in understanding how tundra ecosystems function. Here I will discuss how observations and measurements of climate variables and respiration in organic residues, on the one hand, and data on microbial taxa, physiology and biomass on the other, gave me the same insight as to how and to what extent microbes contribute to or relate to system functions such as primary productivity, organic matter turnover and nutrient cycling. These ecosystem perspectives emerged fortuitously from studying some of the parts of the system. My basic questions were: 1) How are the processes which result in CO2 emission from soils influenced by climate? 2) To what degree and how efficiently do microbes use energy and mineral resources in the field? To answer the first question we measured the seasonal climate variables of temperate and moisture and the respiration response of various litters, organic soils and litterbag weight losses (Flanagan and Veum 1974). Using climatic data we attempted to predict microbial activities in decaying tundra substrates and relate them to the turnover of organic matter and plant productivity. This could be considered a more holistic approach because it concentrated on microbial activities (respiration) and ignored microbial taxa. As a result of this work, three simulation models were constructed relating microbial activities quantitatively and unambiguously to the environmental phenomena that govern them. One model (GRESP, Flanagan and Bunnell 1976, Bunnell et al. 1977a) relates the respiratory response of different kinds of litter to changing climatic conditions. The second (DECOMP, Flanagan and Bunnell 1976, Bunnell et al. 1977b) expresses the respiration rate as a function of the quality of the litter. The third (ABISKO II, Bunnell and Scoullar 1975) integrates the effects of changing meteorological condition and litter quality within the ecosystem. These models document relationships between weight loss and microbial activities, as they are influenced by abiotic variables and litter quality and the relationship of the processes of microbial populations to primary production and turnover of organic matter. Although the development of these models are based on research in tundra (Flanagan and Bunnell 1976), their predictive abilities have also been tested for conditions found in taiga forest and moors (Bunnell et al. 1977a, Bunnell and Scoullar 1981). Thus the models represent the inference of causation, involving both smaller and larger scales of organization than that of the measurements and observations that went into the formation of the models. The second question (energy and mineral processing by microbes) was approached in my laboratory from what is commonly perceived as a reductionist approach. I isolated the major species components of the microflora and subjected them to experiments on their growth and respiration, on field substrate (cellulose, hemicellulose, sugars, etc.) and evaluation of their yield and maintenance demands on different substrates at varying temperatures (consistent with field conditions) (Flanagan and Scarborough 1974, Flanagan 1978. Having established yield coefficients and maintenance demands in the laboratory (Flanagan 1978) these numbers were used, together with estimates of the total fungal biomass of the study sites, to compute the amount of organic matter necessary to maintain an average standing crop of microbes. The equation used was that of Marr et al. (1963). The remainder of the annual input from plants or the measure of decay in g m-2 yr-' was used as a numerator to be divided by an average microbial standing crop (denominator) to calculate potential annual microbial turnover (productivity). Using the measured yield coefficient (Y) with the calculated productivity gave an estimate of average CO2 emission from decaying organic matter. The potential number of microbial generations was estimated using the equation of Gray and Williams (1971). This equation includes a term allowing for recycling of dead microbial tissues. The microbial biomass (per square meter) in the tundra system of Alaska was 18.1 g and the yield efficiency (g microbial tissue per g of substrate) was 0.35. Based on an average (litter)

Plant‐Soil Processes in Eriophorum Vaginatum Tussock Tundra in Alaska: A Systems Modeling Approach

The Arctic Tundra Simulator (ARTUS) is a computer—based simulation model of Eriophorum vaginatum tussock tundra ecosystems found in north central Alaska. ARTUS simulates the annual patterns of heat and water balance, carbon fixation, plant growth, and nitrogen and phosphorus cycling. ARTUS runs in 1—d time steps for a growing season from 1 May to 17 September and is intended to run for several years. The abiotic section of ARTUS encodes the seasonal input of the environmental driving variables and calculates the resultant thermal and water regimes to define the heat and water environments for the tussock tundra system. The primary driving variables are daily total solar radiation, air temperature, precipitation, surface albedo, wind, and sky conditions. The soil compartment contains three organic horizons, which are recognized by their state of physical and chemical decomposition, and one mineral horizon. Six vascular plant species and four moss species are simulated. The model has seven compartments for each vascular plant species: total nonstructural carbohydrates, total nitrogen, total phosphorus, leaves grown in the current season, leaves grown in previous years, conducting and storage stems plus roots, and absorbing roots. In ARTUS the functional unit of the plant is the shoot system or ramet. Each shoot system consists of leaves, stems, fine roots (which do not have secondary growth and have a limited life—span), and larger roots, which have secondary growth and an extended life—span. Although plant processes are based on individual shoots, the ARTUS model as a whole is based on a square metre of ground. Values per square metre are calculated from the values per shoot by multiplying by the shoot density of each species. The model was validated by comparing calculated and measured peak season biomasses and nutrient contents, and the seasonal progression of environmental processes, biomass, carbohydrate contents, and nutrient contents. ARTUS successfully simulated the seasonality of the physical environment, but simulated thaw depths were deeper than those measured at all sites. The simulated value for total vascular plant production was 77% of the measured value. The simulated values for ecosystem respiration for Eagle Creek were within the range of measured values. Simulations with ARTUS indicated different patterns of growth and different storage—carbohydrate levels in deciduous shrubs, evergreen shrubs, and graminoids. The simulated seasonal course of net primary production of vascular plants and mosses was similar to the pattern measured at Eagle Creek. Sensitivity analysis using ARTUS indicated that the tussock tundra is more sensitive to external environmental factors, such as increased temperature, than to internal ecosystem variables. The development of ARTUS was limited by the unavailability of data on whole—plant carbon balance including root and stem respiration. More data are also needed on decomposition processes and nitrogen and phosphorus cycling. Adequate climatological data for northern Alaska are needed for extensive validations of the model. While caution should be used in basing managerial decisions on model simulations, ARTUS can be used to identify and quantify the magnitude and direction of plant responses to changes in state variables in the model.

Lichens of Anaktuvuk Pass, Alaska, with Emphasis on the Impact of Caribou Grazing

Anaktuvuk Pass in the central Brooks Range of northern Alaska provides a major migration route for the western arctic caribou herd. A por- tion of the herd overwintered in the Anaktuvuk Pass area 8 years prior to sampling. Caribou grazing resulted in the almost complete removal of mac- rolichens in grazed areas and this has been followed by the invasion of crus- tose and squamulose species. An analysis by species biomass composition for grazed and nongrazed areas on both north and south slope exposures revealed a lichen composition of 31 species, with Cladonia stellaris, C. rangiferina, C. arbuscula and C. mitis being the most common and present in almost equal abundance on both exposures. In addition, 288 species in 71 genera are re- ported for the Anaktuvuk Pass area. Aspicilia plicigera (Zahlbr.) Thoms. is reported as a new combination and new to North America. Tundra occupies approximately 15% of the earth's land surface, principally con- fined to the northern portion of the Northern Hemisphere (Bliss, 1971). These vast treeless expanses are dominated by prostrate shrubs, short-stemmed herbaceous pe- rennials, lichens and mosses (Billings, 1973). The lichens are particularly important in upland portions of tundra systems where they form extensive mats (Thomson, 1972) that are utilized by caribou and reindeer as a winter food source (Hanson, 1966a; White et al., 1975). Although lichens are more diverse than higher plants in the arctic, most ecological studies have been conducted on higher plants. Lichens are capable of existing in this heat deficient ecosystem, in part because they are highly resistant to frost injury and can survive long periods of inactivity while in a frozen state (Kap- pen, 1973). Consequently, lichens are opportunistic species which are capable of efficiently using favorable conditions present during the brief summer period for pro-

Vehicle Perturbation Effects upon a Tundra Soil‐Plant System: I. Effects on Morphological and Physical Environmental Properties of the Soils

AbstractChanges obtaining in the soil abiotic physical environment due to tracked‐vehicle disturbance were determined in Tundra soils near Barrow, Alaska. The study was a contribution of the U.S.I.B.P., Tundra Biome Program.Six years following the scarring of the tundra surface by several years of infrequent passage by tracked‐vehicles (Weasel), the soils within the track had higher bulk densities and temperatures, accelerated and deeper thaw, and lower moisture percentages than undisturbed soil. Soil morphology was little altered except for direct physical alteration of surface organic horizons and a slight increase in mottling of mineral horizons in all but the wetter portions of the track. The intensity of alteration and associated environmental changes were related to the degree of evident disturbance (the amount of visible damage to the vegetative cover and soil surface) which, in turn, were related to differences in the natural drainage of the soils along the tracks; disturbance being greater, the poorer the natural drainage.The study indicated that a slight amount of disturbance to the tundra surface may be beneficial in terms of increased productivity and nutrient content of vegetation (see Part II of this paper). On the other hand, heavy disturbance can result in severe damage to the tundra system due to subsidence and erosion preventing the revegetation of the disturbed areas.