CUMULATIVE IMPACTS OF VEHICLE TRAFFIC ON HIGH ARCTIC TUNDRA: SOIL TEMPERATURE, PLANT BIOMASS, SPECIES RICHNESS AND MINERAL NUTRITION

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1 CUMULATIVE IMPACTS OF VEHICLE TRAFFIC ON HIGH ARCTIC TUNDRA: SOIL TEMPERATURE, PLANT BIOMASS, SPECIES RICHNESS AND MINERAL NUTRITION Bruce C. Forbes Arctic Centre, University of Lapland Box 122, SF Rovaniemi, Finland Abstract Persistent changes in tundra vegetation and permafrost soils have often been reported to occur within the ruts caused by vehicle tracks. Less well-known are the cumulative effects of these ruts where they traverse gentle slopes which characterize many northern landscapes. Previous research has documented shifts in albedo, soil moisture, ph, and active layer development. Reported here are patterns of near-surface soil temperatures, above-ground vascular plant biomass, total species richness and mineral nutrition of the dominant vascular plants. Measurements were made 21 years after the initial disturbance along transects through hummocky high arctic tundra meadows which included upslope (undisturbed) areas, vehicle ruts, and downslope areas which have been drained by the ruts. It was determined that even a single passage of a tracked vehicle in summer can result in cumulative impacts at the community level which may persist for decades. Introduction Anthropogenic surface disturbance is widespread in the continuous permafrost zone and tundra ecosystems are considered "fragile", although there are few longterm data to justify such a characterization. Off-road vehicle traffic has been and remains one of the most widespread disturbance types active in the circumpolar Arctic. Concerns range from low-amplitude changes in surface thermal regimes, with implications for active layer development and plant community development, to outright exposure and degradation of ice-rich permafrost. This paper reports on some of the cumulative impacts resulting from single- and multi-pass vehicle tracks running perpendicular to a local slope (3-4 ) near the settlement of Clyde River, Baffin Island, Canada (70 26'N, 68 31'W). Methods In June 1989, six points (n=3 each in tracks and upslope undisturbed tundra controls) were established within two vegetation stands (9, 11). The points were randomly chosen, according to the same stratified random grid/transect methods (via a table of random numbers) used for vegetation quadrat placement, to measure soil temperature in vegetation stands with multiple-pass vehicle tracks, and in upslope undisturbed ground (Forbes, 1993a). Fenwall model GB-32 thermistors were taped to wooden stakes and placed at depths of 5, 10 and 15 cm in the mineral soils (below the base of the organic mat) of undisturbed tundra and at the same depths below the soil surface in tracks where no organic mat remained. These were installed as soon as the active layer had developed to a depth of 15 cm. Measurements accurate to ±0.1 C were made at least once (often twice) daily until 14 July and 2-3 times weekly thereafter. Diurnal measurements were made by sampling every few hours for periods of hr. These measurements were made several times during the growing season under a variety of weather conditions. In 1990, temperature in every quadrat (n= 8 each in tracks, upslope undrained controls and downslope drained areas) was measured at the same depths in the same stands as 1989, plus two additional stands (12, 16) using YSI Series 4000 thermistors mounted with epoxy on fiberglass rods inserted into pre-formed holes. Measurements accurate to ±0.1 C were made upon apparent equilibration. Spot readings were taken on several different days chosen to represent variable conditions of radiation and ambient temperature (e.g., completely overcast vs. cloudless, cool vs. warm, and "night" vs. day) during the middle portion of the growing season (25 June-15 July). Only measurements most representative of the conditions in each stand, based on examination of the entire series of measurements for that particular stand during the period of observation, are presented. Bruce C. Forbes 269

2 The majority of biomass sampling was conducted in mid-late July From four to eight 25 x 50 cm quadrats were placed (see above) within disturbed and undisturbed portions of all stands and cover/frequency values recorded as described by Forbes (1992). To provide samples for aboveground standing crop, live and dead material of all vascular species were then clipped at the point where the stems/shoots emerged from the surface (either moss or soil). Air dried samples were later separated by species into living and dead material at McGill University in Montreal and then oven dried at 85 C for 24 hr. and weighed. After drying and weighing for aboveground standing crop, live leaf tissues from species representing the two dominant growth forms (rhizomatous graminoids, deciduous shrubs) were ground in a Wiley mill through a 40-mesh screen and then analyzed for concentrations of the macronutrients N, P, K, Mg and Ca. All digestions and analyses were performed according to the methods of Thomas et al. (1967). The concentrations of N and P in the digestion solutions were measured by a Technicon Auto Analyzer, and Ca, Mg and K by an atomic absorption spectrophotometer. The results are presented as percentages. Given the small number of samples and high variances, tests for statistical differences between means for paired stands from disturbances (vehicle tracks, drained areas) versus controls were made using the Mann-Whitney Test. This was the case for all biomass, tissue nutrient and soil temperature spot measurements. In some instances, indicated below, numerous soil temperature measurements were combined from extended periods and tests were performed on the pooled values by depth. Results SOIL TEMPERATURE Monitoring of soil temperatures among multi-pass tracks and adjacent controls revealed that, over a 24 hr. period under cloud-free skies, temperature change was greatest at a depth of 5 cm in tracks where both vegetation and peat were mostly lacking (Figure 1a) and where vegetation was closed but peat was lacking (Figure 1c). Diurnal change was somewhat reduced at greater depths, particularly in the latter, well-vegetated track. In adjacent undisturbed soils with intact vegetation and organic layers at the surface (Figures 1b and 2b), temperatures at different depths remained mostly distinct from one another throughout the day and "night". This was particularly the case in Stand 11 (Figures 1a and 1b), the wetter of the two. Differences in means between treatments and controls were apparent and statistically significant during both Figure 1. Diurnal soil temperature change (from top to bottom): in Stand 11 - (a) multi-pass vehicle track running perpendicular to an interfluve; (b) upslope control; in Stand 9 - (c) multi-pass vehicle track running parallel to local slope; (d) adjacent control. overcast and cloud-free periods. Depending on the length of each period, differences were usually most pronounced under sunny skies (Figure 2). Taken together, the combined daily and seasonal measurements indicate that the disturbed soils were generally 270 The 7th International Permafrost Conference

3 Figure 2. Mid-day soil temperature in (from top to bottom) (a) single-pass vehicle tracks (Stand 16) running perpendicular to an interfluve; (b) multipass tracks (Stands 12) running perpendicular to a water channel; (c) multipass tracks (Stand 11) running perpendicular to an interfluve. warmer than immediately adjacent natural soils. However, without a closed cover of vegetation and peat, or simply without peat, diurnal changes can be extremely large. Peat and vegetation each provided a measure of insulation which tended to slow heating and cooling of the soil. Wet surfaces tended to heat more quickly than dry ones, due to lower surface albedo and a reduction in the insulating value of the peat layer. On the other hand, exposed mineral soils, even when wet, tended to heat more slowly and to shallower depths than wet organic ones, indicating the relative importance of albedo (see Forbes, 1993a, Figure 1). VASCULAR PLANT BIOMASS Significant changes in peak season aboveground standing crop are evident among the four vascular species with the highest cover/abundance values in this portion of the study site. In the water channel zone (sensu Walker et al., 1989) (Figure 3c), the three graminoid species increased slightly in the tracks while the only deciduous shrub (Salix arctica) was virtually eliminated. In contrast, only Luzula nivalis increased in the tracks through the interfluve zone (Figure 3b). In the drained portions of both water channel zones, the Figure 3. Peak-season live foliar biomass and species richness (including bryophytes and lichens) for upslope controls, vehicle tracks and downslope drained areas in (a - top) Stand 16, (b - center) Stand 12, and (c - bottom) Stand 11, July general responses of the four species were similar and formed a pattern whereby the standing crops of S. arctica and L. nivalis were increased, and both Carex aquatilis var. stans and Eriophorum angustifolium were decreased. However, the relative magnitude of these changes varied between the two zones. The downslope reduction in biomass of E. angustifolium was greater in the water channel zone, while the downslope increases in S. arctica and L. nivalis were greatest in the interfluve zone. The reduction in C. aquatilis var. stans was also greatest in the interfluve zone. In single-pass tracks (Figure 3a), E. angustifolium and L. nivalis increased their biomass. Downslope changes in all four species matched the pattern, though not the magnitude, of change associated with the multi-pass tracks. Vascular biomass in areas drained by single-pass tracks was intermediate between undrained areas and Bruce C. Forbes 271

4 Figure 4. Total foliar biomass for Stands 11 (interfluve), 12 (water channel) and 16 (interfluve), July areas drained by multi-pass tracks, at least in interfluve zones. Whereas E. angustifolium had a standing crop of ca. 37 g m -2 in undrained or control areas, this dropped to ca. 24 g m -2 below single-pass tracks in the interfluve zone and ca. 20 g m -2 below multi-pass tracks in a similar interfluve. Salix, however, increased from ca. 30 g m -2 in the undrained interfluve, to ca. 42 g m -2 below single-pass tracks and to ca. 47 g m -2 below multi-pass tracks. The same pattern was evident for Luzula, which increased from <1 g m -2 in undrained portions of the interfluve, to ca. 7 g m -2 below singlepass tracks and ca. 20 g m -2 below multi-pass tracks. The pattern of decreasing standing crop for Carex was similar to that described for Eriophorum. Total foliar standing crop did not exhibit substantial change in drained areas relative to undrained areas (Figure 4). This was because losses among the aquatic sedges were essentially offset by gains among S. arctica and L. nivalis. Totals remained significantly lower (p=0.01) in all multi-pass tracks, and in many singlepass tracks, especially when woody tissues were included in the analysis (Forbes, 1992). VASCULAR PLANT NUTRIENT STATUS Mid-season concentrations of macronutrients in vascular plant tissue are illustrated for dominant species and growth forms along a disturbance gradient in Figure 5. Levels of phosphorus content among the sedges displayed a pattern similar to that observed for nitrogen (Figures 5a and 5b). Leaf tissue of S. arctica registered a significant reduction in the same set of tracks, but phosphorus levels remained unchanged in drained areas, relative to controls upslope. Potassium levels for Carex increased significantly in the track (p=0.005) (Figure 5c; see also Forbes, 1993b, Appendix 57). Figure 5. Peak season macronutrient (N, P, K) status in leaf tissues of dominant vascular species in Stand 16, including single-pass vehicle tracks and downslope drained areas and upslope controls, July Potassium was significantly lower in drained areas relative to controls in E. angustifolium and S. arctica (p=0.005) (Figure 6c; Forbes 1993b, Appendix 57). In vehicle tracks, magnesium levels (not shown) changed significantly only in Salix, which registered an increase (p=0.05). In drained areas, magnesium decreased significantly in both Carex and Salix (p=0.05) (Forbes, 1993b, Appendix 57). Calcium levels (not shown) decreased in all three species in tracks, significantly so in Carex (p=0.005). Decreases in drained areas were registered in all three species (Carex, Salix and Eriophorum) and were significant for the two sedges (p=0.005). Forbes (1993b, Appendix 55) also details a gradient of drainage for nutrients in Carex. Although all nutrients tended to decrease substantially in areas drained by multi-pass tracks, both nitrogen (p=0.005) and magnesium (p=0.05) were significantly increased in areas drained by single-pass tracks. 272 The 7th International Permafrost Conference

5 Discussion Active layer development is intimately linked to the patterning of surface and near-surface temperatures (Williams and Smith, 1989; Forbes, 1993a). In the case of non-mechanical surface disturbance, such as in the drained peatlands described here, it is clear that environmental factors such as ph, active layer depth and soil temperature are closely correlated with observed variations in vegetation composition (Forbes, 1993a; 1994). Patterns of surface and near-surface temperatures and temperature changes were generally predictable and closely tied to the previously described thaw patterns (Forbes, 1993a). Soil temperatures and, in particular, temperature changes are also of critical importance for vegetation development (Chapin and Shaver 1981). It has been shown that freeze-thaw cycles result in the physical release of nutrients (Jonasson and Skšld, 1983). The present data reveal that mechanically disturbed soils demonstrated significantly greater variation of diurnal and seasonal soil temperature than did immediately adjacent undisturbed soils. An important implication is that if the vegetation cover is slightly damaged and the organic mat even lightly compressed but not entirely destroyed, its nutrients may become available for plant growth through accelerated decomposition in greater quantities than would otherwise be possible at this latitude. Other researchers have also demonstrated that growth and phenology of alpine tundra plants are more strongly controlled by soil temperature than by air temperature (Bliss, 1971; Kudo, 1991). In this study, the dominant graminoids (e.g., Eriophorum angustifolium) exhibited substantial differences in the number of inflorescences between controls (34.6± 19.2 m -2 ), vehicle tracks (69.5±8.7 m -2 ), and drained meadows (7.5±2.7 m -2 ) (Forbes, 1993b). In drained water channels, spot-checks of mid-season soil temperatures revealed areas downslope from multipass tracks to be significantly warmer than controls upslope. However, active layer profiles show that thaw is consistently delayed in these drained areas. These results are in general agreement with findings from northern Alberta peatlands without permafrost, where the substrates of drained areas became warmed to above 0 C slightly later than adjacent undrained areas but maximum summer temperatures were higher in the drained stand (Lieffers and Rothwell, 1987). The same authors reported that the lowering of the water table and removal of excess water changed the physical properties of surface peat and concluded that, after thawing, drained substrates should therefore warm and cool faster, and have greater extreme surface temperatures than undrained substrates (Rothwell and Lieffers, 1987). The biomass of both aquatic graminoids was consistently reduced in drained areas resulting from both single- and multi-pass tracks perpendicular to local slope, although the magnitude of change appeared linked to the original moisture regime within a zone, i.e., interfluve vs. water channel, as well as the extent of the altered hydrology. Sphagnum spp. and associates were eliminated by severe drainage, and species usually associated with more mesic, calcareous substrates have begun to replace them (Forbes, 1993a; 1993b; 1994). It is well documented from research in temperate, boreal and subarctic regions, that peatland drainage can improve tree growth. Increased tree growth after drainage is attributed to improved substrate environment resulting from decreased water content, increased aeration, and higher substrate temperatures (Tamm, 1951; Lieffers and Rothwell, 1987; Rothwell and Lieffers, 1987). After 21 yr., sustained drainage had the effect of significantly increasing both foliar and woody biomass of Salix arctica. This is the dominant shrub present in the control area. These results are consistent with those cited above, although it is not certain whether, in the longer-term, total biomass will decline, as predicted by Gorham (1991). Along with S. arctica, the biomass of the caespitose graminoid Luzula nivalis has increased significantly and is replacing the aquatic rhizomatous sedges (Forbes, 1993a; 1993b). The chemical composition of plants often provides information about the availability of plant nutrients in the soil that in many cases cannot be obtained from soil analyses (Dowding et al., 1981). Different tundra plant species and growth forms have different nutrient demands and different abilities to extract nutrients from the soil (Chapin and Shaver, 1985). In this study, differences among growth forms were often apparent. In single-pass vehicle tracks and drained areas, for example, rhizomatous sedges displayed similar patterns of change in their concentrations of N and P, but both differed markedly from the deciduous Salix arctica with regard to these same nutrients. Controlled fertilization experiments in oligotrophic low and high arctic sedge meadows have resulted in significant increases in above-ground standing crops of foliage and tissue nutrients in Eriophorum angustifolium and Carex spp., in about 2-3 yr. (Chapin and Shaver 1981; Henry et al., 1987). These findings indicate that the changes recorded in this study were probably manifest within the first few years following the initial impacts. The present data are unique in having shown that similar changes can be induced with as little as a single passage of a tracked vehicle and that the effects are persistent for periods of about 20 yr. Furthermore, in drained areas the effects of disturbance have spread far from the point of initial impact, even when the event Bruce C. Forbes 273

6 was limited to a single vehicle pass. These are considered cumulative impacts, and have not previously been documented for the High Arctic. Conclusion In the study area, vegetation and soil changes in vehicle tracks were often similar to those modelled for the Low Arctic by Chapin and Shaver (1981), but the magnitude of change was invariably reduced, and community productivity did not increase as it did in northern Alaska. Responses to wetland drainage were variable depending on local slope hydrology prior to disturbance, but the general effect of eliminating aquatics and stimulating the growth of shrubs and lichens was similar to that reported for subarctic regions. With some exceptions, most of the disturbed patches fit the description of "negative functional recovery" defined by Walker et al. (1987), meaning that disturbed vegetation has recovered, but not to prior levels of diversity and/or biomass. For example, species richness increased over time in drained sedge-moss meadows. However, this was due largely to an increase in lichens at the expense of aquatic vascular species and indicates a net loss of wetland habitat, which is considered "critical" in the High Arctic (Nettleship and Smith, 1975). Soil thermal and plant nutrient cycling regimes have been altered far from the point of the initial vehicle disturbance and represent locally significant cumulative impacts. That these impacts could be induced by as little as a single pass of a tracked vehicle in summer indicate a need for extreme caution during the course of offroad vehicle movement. References Bliss, L.C. (1971). Arctic and alpine plant life cycles. Annual Review of Ecology and Systematics, 2, Chapin, F.S. III and Shaver, G.R. (1981). Changes in soil properties and vegetation following disturbance of Alaskan arctic tundra. Journal of Applied Ecology, 18, Chapin, F.S. III and Shaver, G.R. (1985). Individualistic growth response of tundra plant species to environmental manipulations in the field. Ecology, 66, Dowding, P., Chapin, F.S. III, Wielgolaski, F.E. and Kilfeather, P. (1981). Nutrients in tundra systems. In Bliss, L.C., Heal, O.W. and Moore, J.J. (eds.) Tundra Ecosystems: A Comparative Analysis. Cambridge University Press, New York, pp Forbes, B.C. (1992). Tundra disturbance studies. I. Long-term effects of vehicles on species richness and biomass. Environmental Conservation, 19, Forbes, B.C. (1993a). Aspects of natural recovery of soils, hydrology and vegetation at an abandoned high arctic settlement, Baffin Island, Canada. In Proceedings 6th International Conference on Permafrost, Beijing, China. South China Institute of Technology Press, Vol. 1. pp Forbes, B.C. (1993b). Anthropogenic Tundra Disturbance and Patterns of Response in the Eastern Canadian Arctic. PhD thesis, McGill University, Montreal (475 pp.). Forbes, B.C. (1994). The importance of bryophytes in the classification of human-disturbed high arctic vegetation. Journal of Vegetation Science, 5, Gorham, E. (1991). Northern peatlands: role in the carbon cycle and probable responses to global warming. Ecological Applications, 1, Henry, G.H.R., Freedman, B. and Svoboda, J. (1987). Effects of fertilization on three tundra plant communities of a polar desert oasis. Canadian Journal of Botany, 64, Jonasson, S. and Skšld, S.E. (1983). Influences of frost-heaving on vegetation and nutrient regime of polygon-patterned ground. Vegetatio, 53, Kudo, G. (1991). Effects of snow-free period on the phenology of alpine plants inhabiting snow patches. Arctic and Alpine Research, 23, Lieffers, V.J. and Rothwell, R.L. (1987). Effects of drainage on substrate temperature and phenology of some trees and shrubs in an Alberta peatland. Canadian Journal of Forest Research, 17, Nettleship, D.A. and Smith, P.A. (eds.) (1975). Ecological Sites in Northern Canada. Canadian Committee for the International Biological Programme, Conservation Terrestrial - Panel 9, Ottawa. Rothwell, R.L. and Lieffers, V.J. (1987). Preliminary assessment of air temperatures near the ground on a drained and undrained peatland in central Alberta. In Rubec, C.D.A. and Overend, R.P. (eds.) Proceedings Wetlands/Peatlands Symposium, August 1987, Edmonton Convention Centre, pp Tamm, C.O. (1951). Chemical composition of birch leaves from drained mire, both fertilized with wood ash and unfertilized. Svensk Botanisk Tidskrift, 45, Thomas, R.L., Sheard, R.W. and Moyer, J.R. (1967). Comparison of conventional and automated procedures for N, P and K analysis of plant material using a single digestion. Agronomic Journal, 59, Walker, D.A., Cate, D., Brown, J. and Racine, C. (1987). Disturbance and Recovery of Arctic Alaska Tundra Terrain: a Review of Recent Investigations. CRREL Report, 87-11, Hanover, N.H. Walker, D.A., Binnian, E.F., Evans, B.M., Lederer, N.D., Nordstrand, E. and Webber, P.J. (1989). Terrain, vegetation and landscape evolution of the R4D research site, Brooks Range foothills, Alaska. Holarctic Ecology, 12, Williams, P.J. and Smith, M.W. (1989) The Frozen Earth: Fundamentals of Geocryology. Cambridge University Press, Cambridge (306 pp.). 274 The 7th International Permafrost Conference