Anthropogenic Disturbance and Patch Dynamics in Circumpolar Arctic Ecosystems

Review

Conservation Biology, Pages 954–969Volume 15, No. 4, August 2001

BRUCE C. FORBES,* JAMES J. EBERSOLE,† AND BEATE STRANDBERG‡

*Arctic Centre, University of Lapland, Box 122, FIN–96101, Rovaniemi, Finland, email bforbes@urova.fi†Department of Biology, The Colorado College, 14 East Cache La Poudre, Colorado Springs, CO 80903, U.S.A.‡National Environmental Research Institute, Vejlsøvej 25, P.O. Box 314, DK–8600, Silkeborg, Denmark

Abstract:

It has been 30 years since environmental concern was first expressed over the prospect of large-scale resource development in the Arctic. Human effects are more extensive within the tundra biome nowthan at any time in the past. With or without predicted climatic changes, interaction among different naturaland contemporary anthropogenic disturbance regimes are bound to have a significant effect on local and re-gional vegetation patterns and plant migration. We summarize the results of recent studies of patchy anthro-pogenic disturbance. We pay particular attention to the natural regeneration of plant communities, empha-size patch dynamics over the medium term (20–75 years), and discuss the data in the context of popularmodels of vegetation change following disturbance. Disturbance is important because it produces patches ofpartially or totally denuded ground that permit propagule establishment but may also open affected areas toerosion. Even relatively low-intensity, small-scale disturbances have immediate and persistent effects on arc-tic vegetation and soils. On all but the wettest sites, the patches support new, relatively stable vegetationstates. Where slope is minimal, such disturbances are capable of expanding over large areas in as short atime as 4 years. The effects result in an artificial mosaic of patches of highly variable quality and quantitythat comprise feeding and nesting habitats for terrestrial herbivores.

Perturbación Antropogénica y Dinámica de Parches en Ecosistemas del Ártico Circumpolar

Resumen:

Han pasado 30 años desde que se expresó, por primera vez, preoccupación con respecto al desar-rollo de recursos a gran escala en el Ártico. Los efectos humanos dentro del bioma de la tundra son más ex-tensivos ahora que en cualquier otro tiempo del pasado. Con o sin los cambios climáticos previstos, las inter-acciones entre regímenes de perturbación antropogénica naturales y contemporáneos están destinadas atener un efecto significativo en los patrones locales y regionales de vegetación y la migración de las plantas.Resumimos los resultados de estudios recientes sobre la perturbación antropogénica de parches. Pusimos es-pecial atención en la regeneración natural de las comunidades de plantas, enfatizamos en las dinámicas deparches en un periodo de mediano plazo (20-75 años) y discutimos los datos en el contexto de modelos popu-lares de cambios de vegetación posteriores a una perturbación. La perturbación es importante debido a queproduce parches de suelo total o parcialmente desnudos que pemiten el establecimiento de propágulos, peroal mismo tiempo pueden abrir zonas afectadas a la erosión. Aún las perturbaciones de baja intensidad y es-cala pequeña tienen efectos inmediatos y persistentes en la vegetación y suelos del Ártico. Con la excepción delos suelos más húmedos, en todos los sitios, los parches soportaron estados de vegetación nuevos y relativa-mente estables. Donde la pendiente es mínima, estas perturbaciones son capaces de expandirse sobre grandesextensiones en un tiempo tan corto, como cuatro años. Los efectos resultaron en un mosaico artificial deparches con calidad y cantidad altamente variables que comprenden hábitats de alimentación y nidación de

hervíboros terrestres.

Paper submitted July 13, 1999; revised manuscript accepted November 22, 2000.

Conservation BiologyVolume 15, No. 4, August 2001

Forbes et al. Patch Dynamics in Arctic Ecosystems

Introduction

Arctic tundra vegetation has low species diversity, sim-ple structure, and low annual productivity. Nonetheless,tundra ecosystems support large populations of wildand semidomestic animals highly valued by aboriginaland non-native peoples, and they supply critical nestinghabitat for immense numbers of shorebirds, waterfowl,and other birds (Chernov 1995; Forbes et al. 2000). Yetlarge portions of the region are faced with widespreadthreats ranging from petroleum development to eco-tourism (Sippola et al. 1995; Reynolds & Tenhunen1996; Crawford 1997; Komárková & Wielgolaski 1999).Direct human effects on arctic ecosystems may be evenmore important than climatic change over the next fewdecades. These direct effects include disturbance associ-ated with resource exploitation and alterations in grazingregimes due to changing patterns of reindeer husbandry.Recent models acknowledge that land-management poli-cies (e.g., fire suppression, reindeer husbandry) have asmuch or more effect on northern vegetation as ex-pected changes in climate (Starfield & Chapin 1996), buthuman land use is still underrepresented in most models(Cramer 1997).

The challenge is to predict how community composi-tion will respond to these environmental changes andthe consequences for arctic ecosystems (Chapin et al.1997; Callaghan et al. 1998). Even moderate warming islikely to cause a massive increase in thermokarst (subsid-ence of surface due to thawing), particularly in ice-richpermafrost regions such as northern Alaska and north-west Siberia (Nelson & Anisimov 1993; Billings 1997). Ifthis occurs, one can expect the numbers of small andlarge patches with exposed soils to increase. Stimulationof plant reproduction by increased temperatures is likelyto assume the greatest significance in these disturbedareas (Callaghan & Jonasson 1995), and transient re-sponses of vegetation to environmental change wouldbe expected to become more important (Starfield &Chapin 1996). Even without a warming climate, the ex-tent of disturbed surfaces—including thermokarst trig-gered by mechanical effects—is likely to increase asdevelopment continues. It has been suggested that an-thropogenic habitat destruction will prevent many spe-cies from colonizing new habitats when their formerhabitats become unsuitable in the course of climaticwarming (Peters 1990).

Anthropogenic activities in the Arctic up to this timehave generally created small disturbances (

andoften

), and in total they occupy small propor-tions (

10%) of oil and gas fields (Walker & Walker1991). These small-scale disturbances may affect wildlifeout of proportion to their spatial extent by creating mi-croscale heterogeneity with patches that can either at-tract (Truett & Johnson 2000) or repel (Nellemann &Cameron 1996; Volpert & Sapozhnikov 1998) animals.

We consider the soil and vegetation units resulting fromthese small anthropogenic disturbances to be “patches”(sensu Pickett & White 1985). In this sense, a patch isnot limited to any size, discreteness, or internal homoge-neity, but it does imply a relationship to other patchesand to the surrounding, less affected matrix. Anthropo-genic disturbance frequently forms a new patch withinthe existing landscape-level mosaic of “patch ecosys-tems” (sensu Reynolds et al. 1996).

One of the central assumptions of tundra disturbanceecology is that natural regeneration from human distur-bance in the Arctic is slow (Reynolds & Tenhunen1996), and in particular that disturbed High-Arctic tundraregenerates more slowly, but through similar processes,than similar ecosystems in the Low Arctic (Babb & Bliss1974). Until recently, however, there have been insuffi-cient long-term data to test this assumption, to compareresponses of different disturbance types within a givenregion, or to compare responses among different re-gions. We attempt a first circumpolar comparison ofthese responses by summarizing our own research andthat in the relevant literature. We reviewed community-level responses such as changes in floristic composition,species richness, biomass, and nutrient cycling andemphasized medium-term (20- to 75-year) changes. Our

Figure 1. Map of the circumpolar North showing the locations of the study sites and the division between the High and Low Arctic. See Table 3 for latitude and longitude coordinates. Map modified from Bliss and Matveyeva (1992).

Patch Dynamics in Arctic Ecosystems Forbes et al.

specific goals were to (1) compare patterns of responseamong similar disturbances in different regions, (2)place our findings in the context of theories of vegeta-tion change, and (3) discuss implications for the conser-vation of arctic tundra ecosystems.

Patterns of Biota

Biologists define the Arctic as those lands poleward ofthe climatic limit of trees, and they agree that its flora isthat of a single region (Walker et al. 1994). Despite itsoccurrence over several disjunct landmasses, the Arctic

is comprised of a single biome—the tundra. The vascu-lar flora is depauperate and intercontinental similarity isextremely high, particularly in more climatically and edaph-ically severe high-arctic zones (Young 1971). Althoughsimilarity of the nonvascular flora is also high on a cir-cumpolar basis, species richness tends to increase rela-tive to that of temperate and boreal latitudes (Longton1988). The animal biota also contains a high proportionof circumpolar species (Sage 1986).

Summer warmth is considered the dominant macroen-vironmental control delimiting gross distributions of vas-cular plants in the Arctic (Young 1971; Edlund 1990), al-though a mosaic rather than a zonal pattern is usuallyfound at the local level. Within a given bioclimatic zone

Table 1. Physical and biological characteristics of the Low and High Arctic.*

Characteristics Low Arctic High Arctic

Environmentallength of growing season (months) 3–4 1.5–2.5mean July temperature (

C) 4–12 3–8mean January temperature (

35mean annual temperature (

19accumulated degree-days above 0

C 600–1400 150–600mean precipitation June–August (mm) 35–150 25–100mean annual precipitation (mm) 120–800 60–425active layer depth (cm)

fine-textured soils 20–60 20–60coarse-textured soils 100–200+ 70–150soil temperature at

10 cm (

C) 5–12 2–8soil pH mostly 5–6.5 mostly 6–8organic layer (cm)

lowlands 50–300+ 5–50uplands 2–20 0–1

Biologicaltotal plant cover (%)

tundra 80–100 80–100polar semidesert 20–80 20–80polar desert 1–5 1–5

plant height (cm)shrubs 10–500 5–20forbs 5–30 2–10graminoids 10–50 5–20

shoot:root ratios (alive)shrubs 1:1 1:1forbs 1:1–2 1:0.5–1graminoids 1:3–5 1:2–3

Vascular plant flora (species number) 700 380Bryophytes abundant, including abundant, but few

Sphagnum

species

Sphagnum

speciesLichens fruticose and foliose foliose and crustose

growth forms common growth forms commonVascular-plant growth forms woody and graminoid cushion, rosette, and

common graminoid commonLarge land mammals (species number) 4–8 2–4Large ungulates

Rangifer tarandus, Rangifer tarandus pearyi, Ovibos moschatus, R. t. platyrhynchus, Alces alces Ovibos moschatus

Small land mammals (species number) 15–30 5–12Nesting birds (species number) 30–60 2–20Fishes, freshwater (species number) 10–22 1–9

Reprinted from Bliss (1997) with permission from the author and Elsevier Science.

Forbes et al. Patch Dynamics in Arctic Ecosystems

(sensu Edlund 1990), factors such as moisture, nutrientstatus, soil chemistry, and wind become important con-trols, as evidenced by change along local catenas in anumber of landscapes (Bliss & Matveyeva 1992). Grossdistributional patterns among arctic cryptogams are notso easily explained, although there can be pronounceddifferences between both cryptogamic and noncrypto-gamic floras of adjacent areas, where substrates varyeven slightly in age since deglaciation or in chemical sta-tus. This is often the case among bryophytes, which maybe sensitive to subtle spatial and temporal changes infactors such as soil texture, moisture, and pH (Longton1988).

The division into Low and High Arctic has been mappedby Bliss and Matveyeva (1992) (Fig. 1), with phytogeo-graphic subdivisions within each major zone (Walkeret al. 1994). The High Arctic, in general, differs from theLow Arctic in having very low-growing plant communi-ties (polar desert and polar semidesert) with only 10–30% cover of vascular plants (Table 1). Tundra heath–dwarf shrub and sedge-cryptogam communities withcontinuous vascular-plant cover and a variety of growthforms, which are abundant in the Low Arctic, are minorcomponents and usually restricted to coastal lowlands(Bliss 1997). Most of the plots we surveyed (Table 2)occurred within these types of closed tundra communi-ties, except for ancient housing sites in the high arctic.Relatively productive ecosystems such as these, some-times referred to as “polar oases,” comprise only a frac-

tion (2–3%) of the surface area of the High Arctic, yet arecritical to the region’s terrestrial food web (Bliss 1977;Svoboda & Freedman 1994). Cottongrass tussocks (

Erio-phorum vaginatum

), which are important in the struc-ture and function of many low-arctic ecosystems, areabsent in the High Arctic. With regard to response to dis-turbance, we point out differences between High andLow Arctic only where they are substantial.

Disturbance Dynamics

Within a given ecosystem, a disturbance regime has fourbasic dimensions: frequency, intensity, scale (extent), andtiming (season) (Pickett & White 1985; Petraitis et al.1989). Other dimensions can also be important, how-ever, such as spatial pattern. In this review we pay par-ticular attention to the intensity of the initial anthropo-genic effect and to the size and shape of the resultingpatch. A reference state must be defined for each distur-bance regime, but it need not be static. Because of therelative heterogeneity of arctic tundra even at localscales, reference states or “controls” have been situatedimmediately adjacent to each disturbance to clarify whatconstitutes “normal functioning.” Disturbance often im-plies negative changes, such as a reduction in speciesrichness or biomass, but any sudden deviation fromnormal, or reference state, is considered disturbance(Hobbs & Huenneke 1992). In our discussion, we con-

Table 2. Vegetation and soil parameters measured in studies of arctic ecosystems.

Parameter Alaska

Canada Greenland Russia

Vegetationplot size 1

1 to 5

50 cm 100

100 cm 100

100 cmtotal no. of plots 200 466 835 571no. of control plots 127 221 390 245species composition vascular, bryophytes, lichens

vascular, bryophytes, lichens vascular, bryophytes, lichens vascular, bryophytes, lichensspecies richness vascular, bryophytes, lichens

vascular, bryophytes, lichens vascular, bryophytes, lichens vascular, bryophytes, lichenscover vascular, bryophytes, lichens

vascular vascular, bryophytes, lichens vascularfrequency vascular, bryophytes, lichens

vascular, bryophytes, lichens vascular, bryophytes, lichens vascular, bryophytes, lichensheight + + + +biomass vascular, bryophytes, lichens vascular vascular

tissue macronutrients

� � �

N, P, K, Ca, Mg, Na, Fe

Soil, physical

depth of organic horizon + + + +relative moisture + + + +compaction

� �

bulk density

texture +

active layer depth + + +

temperature some plots, at 10 cm at 5, 10, and 15 cm at 1.5 and 5 cm

Soil, chemical

mineral and organic horizon mineral and organic horizon mineral and organic horizonconductivity

mineral and organic horizon mineral and organic horizon mineral and organic horizonmacronutrients available NO

, K, P

total N, C, K, exchangeable P total N, NO

, C, K, exchangeable P total N, C, K, exchangeable Pcation exchange capacity +

decomposition at 25 plots

� � �

Detailed methods are found in references cited in the text. Symbols: +, parameter measured;

, parameter not measured.

Information from 1979–1981. In 1995, 51 plots were resampled for vascular composition, richness, cover, vegetation height and biomass, rel-ative moisture, and active layer depth.

Eighty-seven plots; 113 additional plots have vascular data only.

Soil analyses performed on the 87 plots with data for all plant groups.

Patch Dynamics in Arctic Ecosystems Forbes et al.

5.2–

5.3–

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4–7.

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, P, R

8–7.

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68�1

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4.2–

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, P, R

�54�

69�5

4–6.

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12–3

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Forbes et al. Patch Dynamics in Arctic Ecosystems 959

sider both direct disturbances, mostly mechanical ef-fects, and indirect or cumulative disturbance, meaningaltered resource levels at some distance from the initialimpact (Tables 3 & 4).

In their synthesis, Pickett and White (1985) draw at-tention to physiognomic system structure as a context fordisturbance. Of the four contrasting sorts of communitystructure (Fig. 2 [top]), tundra is considered “root biased.”This distinction is important because the structure of asystem determines (1) what sorts of disturbance have aneffect, (2) the threshold of intensity that is effective, and(3) the dependence of species’ coexistence on distur-bance. Pickett and White contend that disturbance of in-sufficient intensity to open the root mat in root-biasedsystems will have little effect on species coexistence(see also Shaver et al. 1983). We have augmented thePickett and White diagram to illustrate the three levelsof disturbance intensity in root-biased communities con-sidered in this review (Fig. 2 [bottom]).

In addition to intensity, size and shape have provenuseful predictors of the nature of regeneration withinpatches (Forman 1981). This is due primarily to the al-teration of the ratio of edge to interior. In the case of in-direct or cumulative effects, patches may change sub-stantially in size and shape over time, as in the case of a

vehicle track (small strip patch; sensu Forman 1981),which may alter hydrology over an entire slope and/orlead to thermokarst erosion on level ground (large isodi-ametric patches). When we discuss response and the po-tential for recovery, we are referring to the cumulativelyaffected area.

Aspects of Response and Recovery

Walker et al. (1987) emphasize that patch stability is aprerequisite for either assisted or unassisted vegetationregeneration. Vegetation regeneration may, in fact, en-gender site stability. In other words, physical site stabil-ity and environmental stress comprise gradients alongwhich suitable regeneration niches may or may not ex-ist. These concepts are at the core of various theories ofvegetation changes (e.g., safe site theory, facilitation, ini-tial floristics). The distinction between different states ofrecovery proposed by Walker et al. (1987) is useful, butit needs to be expanded to include not only vegetationalcharacteristics but also hydrology (Table 5), edaphic pa-rameters (physical, chemical, and thermal), and patchquality, such as above- and belowground nutrient poolsthat have implications for herbivory.

Table 4. Disturbance ages and sampling dates by disturbance type for studied arctic ecosystems.a

Disturbance Substrateb Year of initial disturbance Year of last disturbance Year measurements made

Organic layer partly removed HIS 1947c (A) 1952 (A) 1979–1983 & 1995 (A)1933 (G) 1933 (G) 1992 (G)

Organic layer totally removed HIP 1947 (A) 1952 (A) 1979–1983 & 1995 (A)1928 & 1933 (G) 1928 & 1933 (G) 1992–1993 (G)1986 (R) 1989 (R) 1993 & 1995 (R)

Mounds of soil/organic matter LIS 1947 (A) 1952 (A) 1979–1983 & 1995 (A)1986 (R) 1989 (R) 1993 & 1995 (R)

Single-pass vehicle tracks LIS/HIS 1947 (A) 1952 (A) 1979–1983 & 1995 (A)1965 (C) 1971 (C) 1989–1990 (C)1982 & 1984 (G) 1982 & 1984 (G) 1983–1986 & 1995 (G)1989 & 1995 (R) 1989 & 1995 (R) 1991 & 1996 (R)

aA, Alaska; C, Canada; G, Greenland; R, Russia.bSee Fig. 2 for explanation of acronyms.cAlaska disturbances took place at oil-drilling wells from 1947 to 1952. Most disturbances took place within 1 field year or during winter over2 calendar years. Dates differed slightly for each well site within this time frame.

960 Patch Dynamics in Arctic Ecosystems Forbes et al.

More recent studies (Forbes 1993a, 1994; Strandberg1996, 1997) have focused on the importance of includingbryophytes and lichens in the evaluation of recovery.We found that evaluation merely on the basis of vascularplants overestimated the degree of recovery in virtuallyevery instance. On the other hand, if a manager simplywants a green, stable surface, then a measure of vascularcover—usually provided by graminoids—may be all thatis feasible under the most severe conditions (Forbes &Jefferies 1999).

Figure 2. Diagram (top) of types of community struc-ture showing distribution of biomass relative to the substrate and attachment of the organisms to the sub-strate: (a) a shoot-biased community with most of the biomass above the substrate, (b) a root-biased com-munity with most of the biomass arrayed within the substrate; and (c) a surface-attached community with all biomass above the substrate and superficial attach-ment to the substrate. Communities of burrowing ani-mals would form a fourth type, (d) substrate con-tained ( from Pickett & White [1985], with permission from Academic Press). Diagram (bottom) of three lev-els of disturbance intensity referred to (see Table 5) as they pertain to root-biased tundra communities, and the relative amount of biomass destroyed. The biologi-cal legacies available to respond to the disturbance range from substantial (LIS, low-intensity secondary substrate) to moderate (HIS, high-intensity secondary substrate) to nonexistent (HIP, high-intensity primary substrate). Intensity refers to the degree of substrate al-teration at the time of initial disturbance by mechani-cal disturbance, the degree of aeolian erosion, or, in the case of ancient dwellings and drained slopes, the degree of organic loading and soil desiccation, respec-tively.

20–7

——

, spec

; D, d

Responses by Disturbance Type

Removal of the Organic Layer

The extent of the disturbance to the organic layer (Fig.2) and the dimensions and moisture regime (Table 5;Fig. 3) of the patch were most important to regenera-tion. If the organic layer was only partly removed, re-sprouting from living rhizomes, fragments, or the seedbank might have been important (Ebersole 1987, 1989;Strandberg 2002b). When the organic layer was re-moved totally, small wet areas (4 m2 to approximately300 m2) had the potential to recover their vegetationphysiognomy (cover, biomass, and height) within 20–75years (Table 5) (Ebersole 1985, 1987; Strandberg 2002c).The dominant vascular taxa, Carex aquatilis, ssp. stans,and Eriophorum angustifolium, were the same as thosein undisturbed wet areas (Fig. 3), apparently becauserhizomatous graminoids are well-adapted to wet distur-bances created by natural thermokarst (Ebersole 1985).

Except for mesic disturbances having less biomass thancontrols, the responses of mesic areas could not be sepa-rated from those of wet areas. Although having the po-tential to recover, mesic sites do not necessarily do sowithin 75 years. Cover, biomass, and species richness werelower on dry substrates with the organic layer totally re-moved. Diversity of functional types was similar to thatof controls, with the exception of deciduous shrubs, whichwere mostly lacking (Fig. 3). Tissue nutrient levels werehigher in plants colonizing rubble or gravel substratesthan in controls, but they were lower in plants coloniz-ing sands (B.C.F. & S. Jonasson, unpublished data).

Mounds of Soil and Organic Matter

Mounds of soil pushed up by bulldozers provide some ofthe most striking examples of vegetation regeneration.In northern Alaska, grasses and erect willows grew vig-orously and tall on these disturbed mesic sites (Table 5;

Figure 3. Dominant plant functional types on various disturbances under different moisture regimes.

Fig. 3). Increased rates of decomposition on these warmer,well-drained soils created a favorable environment forsuch plants (Ebersole & Webber 1983; Ebersole 1985).On mounds with low amounts of organic matter (�10%),grasses strongly dominated species-poor communitiesand willows were virtually absent (Ebersole & Webber1983; Ebersole 1985). Floristic changes in these “early-successional” communities are predicted to be veryslow (see section on alternative vegetation states) be-cause seedlings in small, newly created bare areas werekilled by accumulation of grass litter (Ebersole 1987).On mounds with a higher content of organic matter (gen-erally �25%), a species-rich, physiognomically complexcommunity developed with a high species richness offorbs, bryophytes, and lichens in the understory. Shrubheights on mounds ranged from 1.5 to �3 m after 30years, whereas average height in control areas was 50 cm(Ebersole 1985).

Ancient and Modern Housing

Housing construction resulting from a transient humanpopulation has been a factor in the Arctic for at least1000 years (Fredskild 1961; Forbes 1996; Fedorova1999). Ancient dwellings last used around 1200 AD arecommon on cobble beaches on the coasts of certain is-lands in northern Canada, along the coast of Greenland,and on river bluffs on the Yamal Peninsula of northwestRussia. The plants sampled on these patches had signifi-cantly different (mostly higher) macronutrient contentsin their leaf tissues than the same species occurring onadjoining undisturbed habitats (Forbes 1993b; B.C.F. &S. Jonasson, unpublished data). Phosphorus has beenidentified as the most abundant and persistent nutrientin the soils of northern archaeological sites (Moore &Denton 1988; Strandberg 2002c). Excessive fecal deposi-tion from several herbivores was also evident, indicatingselective grazing within the patches (Forbes 1993b).

Dwellings in all locations were dominated by grami-noids (Fig. 3), along with either dwarf willows (Canadaand Greenland) or various forbs (Russia), and biomasswas consistently greater than on undisturbed ground(see section on alternative vegetation states). Moderndwellings in Canada were constructed directly on thetundra, as in Russia, or on beds of local sand and/orgravel. Where houses were laid directly on the tundra,soils were extremely compacted. In Canada, concentra-tions of macronutrients were significantly higher in theleaves of the dominant rhizomatous grass (Alopecurusalpinus) here than on adjoining trampled ground (Forbes1993b). On sites with added sand or gravel, the samecolonists were present in addition to caespitose grami-noids, forbs (Fig. 3), and ruderal mosses. Evidence of se-lective grazing by lemmings on the ground around mod-ern houses was stronger than that observed on ancienthouse sites (Forbes 1993b). Dwarf shrubs were virtually

absent on all modern dwelling sites in Canada but werestill abundant in Russia (Fig. 3).

Vehicle Tracks

One of the most widespread forms of disturbance through-out the circumpolar arctic is that caused by tracked vehi-cles (Figs. 4–6). Seasonality is critical, and even a singlepass by a heavy vehicle during sensitive periods can re-sult in lasting damage (Forbes 1992a; Rebristaya et al.1993; Kevan et al. 1995; Koroleva 1995; Strandberg2002a) (Fig. 5). In the High Arctic, overall biomass re-ductions more than offset significant and persistent gainsamong rhizomatous graminoids (Forbes 1992a) (Table 5;Fig. 3). In contrast, increases in productivity have beenreported within the Low Arctic (Chapin & Shaver 1981).In High-Arctic tracks, soil nutrients and arthropods weresignificantly different from those of controls, with in-creases and decreases variable among species and growthforms (Forbes 1998). Arthropod populations were con-sistently lower in tracks than in adjacent controls (Kevanet al. 1995). Increases in flowering plants (inflorescencesper unit area) ranged from passing (Dryas octopetala;B.S. & P. Aastrup, unpublished data) to persistent (Erio-phorum and Carex sp., Forbes 1993b).

The shift from scale-of-impact to scale-of-response canbe several orders of magnitude, as was the case in wet-lands drained by vehicle ruts (Forbes 1993c, 1998; Ke-van et al. 1995; B.C.F. & S. Jonasson, unpublished data)(Figs. 5 & 6). Wetland drainage resulted from as little asa single-pass vehicle track in which the organic mat wasretained. Destruction of the organic mat may not be anadequate measure of the intensity of an effect, as recom-mended for the Low Arctic by Shaver et al. (1983).

In the Low Arctic, if damages in mesic areas were slightto moderate and the moisture regime was not changed,most of the original vascular species persisted, andspecies that respond positively to disturbance (rhizoma-tous graminoids, willows, and Equisetum arvense) in-creased (Chapin & Shaver 1981; Ebersole 1985; Strand-berg 2002a). The resulting vegetation often had a verydifferent physiognomy than the original; for example, intusssock tundra Eriophorum vaginatum was reducedin cover and the vegetation became dominated by decid-uous shrubs (Fig. 3). Where tracks churned the uppersoil horizons on ice-rich substrates, thermokarsting cre-ated a wet, species-poor community dominated by rhi-zomatous and caespitose graminoids, and the originalspecies were lost (Fig. 3).

Pedestrian Trampling

In northern Alaska, heavy pedestrian traffic occurred inmesic Dryas-dominated tundra. Many of the originaltaxa remained in reduced amounts, but the physiog-nomy of the community was changed because species

that increased in response to disturbance were primarilyrhizomatous graminoids, and willows, and Equisetumarvense (Ebersole 1985) (Table 5; Fig. 3). In the Cana-dian Arctic, even minimal trampling of mesic tundra re-sulted in significantly decreased species richness (Table5), particularly among lichens and bryophytes (Forbes1992b, 1994, 1996). Trampling also compacted soils,which generally increased bulk density, increased de-composition and mineralization, reduced moisture con-tent, and restricted both horizontal and lateral root pen-etration so that only a few rhizomatous grasses, suchas Alopecurus alpinus and Poa sp., persisted (Forbes1993b). The death of standing vegetation and exposureof the organic layer tends to decrease surface albedo,which increases soil temperatures and thaw depths(Chapin & Shaver 1981; Forbes 1993b). Trampling re-duced or eliminated hummock-hollow microtopogra-phy, an effect that persists indefinitely (Forbes 1992b).

Patterns within and among Ecosystems

Patch Dimensions

Patch size and shape affected the nature and rate of veg-etation regeneration. In general, the inner portions ofdry patches �1 m across remained largely bare after 20years, often due to aeolian and fluvial erosion (Fig. 4),except for occasional clumps of mosses and caespitosegraminoids. Cyperaceae typically invaded from the edges,but did so efficiently only on relatively wet ground andon narrow (�1 m) bared strips (Ebersole 1987; Forbes1992a, 1993b). In northwest Russia, many areas of 1–2km2 have been completely denuded by industrial activ-ity, and aeolian erosion is a problem (Forbes 1995).Within 4 years after abandonment, many taxa, particu-larly caespitose graminoids, various forbs, and decidu-

ous shrubs (Salix sp.) dispersed by propagules to the in-ner portions of such large patches. But establishmentoccurred primarily on microsites with stable substratesand adequate moisture regardless of size or shape (Eber-sole 1985; Forbes 1993b, 1997; Strandberg 1996).

High Arctic versus Low Arctic

In general, similarly created patches returned moreslowly toward the productivity and physiognomy oftheir respective control communities in the High Arcticthan they did in the Low Arctic. The exceptions to thistrend provide important insights into factors controllingthe responses of arctic vegetation to patchy disturbance.In several cases—organic layer totally removed, multi-pass vehicle trails, and heavy trampling—vegetation ondry sites regenerated about equally slowly in High andLow Arctic regions. In areas that experienced nutrientenrichment from deposition of nutrient-rich materials(ancient housing) or accelerated decomposition (mod-ern housing), increased nutrient availability allowedHigh-Arctic sites to develop a vigorous plant cover simi-lar to that of Low-Arctic sites. Species richness was se-verely reduced, however, because of rapidly growingclonal species tolerant of the severely compacted soilsthat were prominent on sites of modern housing and ad-joining heavily trampled ground (Forbes 1996; Strand-berg 1996). In wet areas disturbed by vehicle tracks, theregeneration of vegetation cover was equally vigorous inboth regions, although biomass and species richness re-mained significantly reduced in tracks in the High Arctic(Ebersole 1985; Forbes 1992a, 1998; Strandberg 1997 ).We attribute this to the predominantly belowground al-location of biomass in tundra ecosystems (Callaghan &Emanuelsson 1985). Particularly in communities charac-terized by rhizomatous graminoids, plants can easily re-

Figure 4. Ruts from one pass by a seismic vehicle during the winter of 1984 at Tyskiit Nunaat, Greenland (Fig. 1). The dwarf-shrub heath veg-etation was killed or removed by the vehicle. Since the original dis-turbance, effects to the area have been significantly increased by wind erosion. Photo taken in Au-gust 1996.

sprout from intact tillers and continue to spread vegeta-tively. Recruitment from the seedbank may also occurbut is not common in the High Arctic (Forbes 1993b;Strandberg 1996).

Moisture Regime

There is a strong tendency for surface moisture to limitthe rate of both short- (20-year) and medium-term (20- to75-year) recovery. Vegetation regeneration was gener-ally fastest in wet sites (W), slowest in dry sites (D), andintermediate in mesic (M) sites: W � M � D (Table 5).In wet sites, most parameters in Table 5 had returnedto values approximating those prior to disturbance af-

ter 20–75 years. Only after severe disturbances, such ascomplete removal of the organic soil layer, multipass ve-hicle tracks, and heavy trampling, was recovery slower.The general pattern of W � M � D then changed to W �M � D. In disturbance of even lower intensity—the low-est we studied, light trampling—regeneration was vig-orous in all situations except on dry sites in the HighArctic. In the case of more severe disturbance, such asmultipass tracks, Low-Arctic sites followed the most com-mon pattern of W � M � D (although recovery in mesicsites was extremely variable). In High-Arctic sites, recov-ery was poor in both mesic and dry sites, and the result-ing pattern was W � M � D (Forbes 1993b; Strandberg2002c).

Figure 5 . Deep rutting from a sin-gle passage of a tracked vehicle dur-ing the summer of 1969 at Clyde River, Baffin Island. The tracks run parallel to local slope (2–3�) through a snow-bed community and thus collect runoff during an-nual snowmelt, resulting in serious fluvial erosion and loss of mineral soils. Photo taken in June 1990.

Figure 6. Ruts from a tracked vehi-cle in use during the years 1965–1972 at Clyde River, Baffin Island. View is looking down a 3� slope from the center of a water channel in a sedge-cryptogam mire during the beginning of snowmelt runoff. Water cannot reach the shoreward areas beyond the tracks, which have become extremely desiccated. Photo taken in June 1990.

Growth Forms and Functional Types

One way to simplify the diversity of data on response todisturbance is to group species according to their growthforms or functional types and search for patterns amongthe responses (Komárková & McKendrick 1988; Shaveret al. 1997). For example, the persistence of Carexaquatilis, Eriophorum angustifolium, Alopecurus alpi-nus, Calamagrostis spp., and Poa spp. in many tram-pled and heavily tracked stands indicates that the rhi-zomatous graminoid was the most resistant of all thegrowth forms encountered (Ebersole 1985; Forbes 1992a,1992b; Strandberg 2002c) (Fig. 3). Disturbance is knownto increase shoot turnover in many rhizomatous tundragraminoids (Henry 1987). These species typically pos-sess large systems of hardy, interconnected tillers thatmay represent as many as 27 years of growth (Callaghanet al. 1991). Other researchers working in heavily tram-pled temperate alpine meadows have also reported highlevels of resistance among communities dominated bythis growth form (Cole & Trull 1992).

Dwarf shrubs, both deciduous and evergreen, werethe plants least resistant to mechanical disturbance(Ebersole 1985; Forbes 1992b; Strandberg 2002a). Evena single pass of a vehicle could be lethal for an individ-ual. Seedlings of Salix arctica, the most common decid-uous shrub at high-arctic sites, were rare and restrictedto drier or mesic microsites, evidence of the overall poorresilience of this growth form (Forbes 1992b; Strand-berg 2002a). Viviparous plants such as Poa alpigena sp.colpodea, Polygonum viviparum, Saxifraga rivularis,and S. foliolosa were generally successful in disturbedpatches in the short term (�20 years), regardless of theirfunctional type (Forbes 1993b, 1997; Strandberg 1996).Among lichens, only the foliose Peltigera aphthosa wasable to resist low-intensity disturbance, such as lighttrampling or a single passage of a vehicle. Fruticose andcrustose taxa fared poorly under mechanical disturbanceand generally failed to recolonize disturbed patches.Among bryophytes, acrocarpous mosses tended to colo-nize mesic and dry patches more effectively than pleuro-carpous taxa (Forbes 1992b; Koroleva 1995; Strandberg2002a, 2002c).

Disturbance Theory and Patch Dynamics

Theories and models of patch dynamics are derivedlargely from temperate and tropical systems and need tobe tested within the Arctic. Support has increased formodels of vegetation change that rely on species’ lifehistories (Glenn-Lewin et al. 1992; Hobbs & Huenneke1992). Until now there has been a dearth of analysesfrom patchy anthropogenic disturbance regimes in theArctic, and it has not been certain whether these requiremethodological or theoretical separation from naturaldisturbances. The findings we reviewed are in accord

with several generalizations from naturally disturbedtemperate ecosystems (Pickett & White 1985). Within agiven patch, the level of direct anthropogenic distur-bance ranged from minimal, in the case of light tram-pling, to maximum, in stands where all above- and be-lowground living vegetation was killed or displaced bythe initial impact (e.g., organic layer totally removed). Incontrast to assertions by Pickett and White (1985) andShaver at al. (1983), however, the data we have summa-rized demonstrate that retention of an intact organiclayer after disturbance is not necessarily a guaranteeagainst significant and essentially permanent change toboth vegetation and soils within a patch. The likelihoodof such change increases with the frequency and inten-sity of disturbance.

Many plants are well adapted to the early or pioneer-ing stage following surface disturbance. The plants maybe colonists newly dispersed to the patch, or they mayhave survived the disturbance in situ as adult plants, rhi-zome fragments, or viable propagules within the seedbank. Numerous studies have shown that the role of ini-tial floristics (sensu Egler 1954) within a patch is criticalduring vegetation change in response to disturbance(Glenn-Lewin et al. 1992; Forbes 1993a). A key differ-ence in arctic ecosystems is the reduced role of annualspecies relative to those of temperate regions. Nonethe-less, one of the most important local effects of the dis-turbances we describe may be the development of abank of viable seeds that could respond rapidly to futurepatchy disturbance (Ebersole 1989; Callaghan & Jonas-son 1995).

Alternative Vegetation States

A key difference between temperate-zone ecosystemsand arctic tundra is the ability of moderately disturbedcommunities of temperate systems to recover to theiroriginal state, or nearly so, relatively rapidly once pres-sure (e.g., trampling) is removed (Stohlgren & Parsons1986; Hammitt & Cole 1987). In the Arctic, it appearsthat the early-successional communities that occupy manypatches are self-perpetuating (Forbes 1993b, 1996; Strand-berg 1996; J. J. Ebersole, unpublished data). The reasonfor the persistence of these communities is not clear.One factor may be the higher decomposition rates onsome of these sites (Ebersole & Webber 1983) which fa-vor fast-growing grasses that, in turn, prevent establish-ment from other colonizers ( J. J. Ebersole, unpublisheddata). It is well known from studies in temperate regionsthat invasion into dense, graminoid-dominated swardsby species both within and outside the existing commu-nity is restricted by several factors. Once established,graminoids in northern ecosystems are able to formsimilarly dense swards and can benefit from repeatedgrazing by vertebrate herbivores. This can result in theindefinite persistence of so-called “grazing lawns” on

disturbed patches (McKendrick et al. 1980; Wein et al.1992).

Edaphic parameters can also contribute to communitystability. Physical changes in the soil environment withinpatches, such as soil compaction and increased thawdepths relative to undisturbed ground, can be persistent(Forbes 1993b; Kevan et al. 1995). The soil pH of patchesoften increases (Table 3). This may limit the reinvasionof nonvascular species, which tend to be more sensitivethan vascular plants to subtle changes in surface chemis-try, texture, and moisture (Longton 1988; Forbes 1994).Higher levels of available nutrients are another factorthat may help sward-forming graminoids to occupy apatch indefinitely (Moore & Denton 1988; Ebersole &Webber 1983; Ebersole 1985; Forbes 1993b; Strandberg2002c).

Species Richness

Pronounced variations in species richness have been re-lated to many aspects of communities, including produc-tivity, pH, level of disturbance, and life forms (Huston1994). In our findings, the species richness of both sur-viving and colonizing plants was lower in mechanicallydisturbed areas than in controls. This is consistent withstudies of locally catastrophic anthropogenic disturbancein more temperate regions where low germination ratescombine with reduced habitat heterogeneity to de-crease species richness (Pickett & White 1985). In theArctic, the dearth of efficient colonizers means thatmany of the same species occur in patches of differentorigins and/or ages (Forbes 1992a; 1996).

Summary and Implications for Conservation

We have attempted to draw attention to some of thepatchy, low-intensity human-induced disturbance regimesthat are widespread in most tundra ecosystems in thecircumpolar North. According to Vitousek (1992), “thereis a reasonably strong consensus among terrestrial ecolo-gists that for the next several decades, land use is likelyto be the most certain and the most significant compo-nent of global change, followed by changes in the com-position of the atmosphere.” Human impact is moreextensive within the tundra biome now than at anytime in the past (Reynolds & Tenhunen 1996; Crawford1997). Surface disturbance typically produces patches ofpartially or completely denuded ground. The responsesamong patches described here typically included sig-nificantly altered community composition, temperature,and nutrient fluxes and, at least in the Low Arctic, pro-pagule establishment. Yet patch-level disturbance is of-ten neglected in models of global change employed topredict changes in vegetation zones, whereas the impor-tance of human management, or lack thereof, is increas-

ingly acknowledged (Starfield & Chapin 1996; Cramer1997).

According to Chapin et al. (1996), a major challengefor ecologists is to determine the future distributions oforganisms and their effects on ecosystem processes.Current models of global vegetation distribution assumean equilibrium relationship between present and futureclimate and vegetation and ignore time lags associatedwith migration (Chapin et al. 1996). Even without awarming climate, contemporary anthropogenic distur-bance regimes are already having a significant effect onsedentary and northward-migrating plants (Kuliyev &Morozov 1991; Staniforth & Scott 1991; Wein et al. 1992;Forbes 1997). The dynamics of disturbed patches are ofmuch more than casual or aesthetic concern. The abilityof the fauna to adapt to both increasing disturbance anda changing climate depends in large part on the availabil-ity of suitable forage (Callaghan et al. 1998). Thus, patchquality and persistence are at least as critical as patchquantity. Furthermore, the integrity of the permafrostunderlying and maintaining most tundra ecosystems de-pends on an intact vegetative cover (Billings 1997).

In ecosystems such as these, today’s short-term, low-intensity human effects may be tomorrow’s resourcepatches. These patches may represent alternative stablestates and likely will remain dynamic focal points for bi-otic and abiotic feedbacks for many years to come.These feedbacks, despite their small size, have provedremarkably persistent. As such, our findings can be ap-plied to current issues of Arctic conservation and havespecial relevance for managers seeking to address “small-scale” disturbances such as recreational pedestrian tram-pling, camping, transient housing, and off-road vehicletraffic. In addition to the more obvious and large-scaleeffects associated with petroleum development, mining,and military activities (Crawford 1997), the explosivegrowth of ecotourism is affecting all sectors of the Arc-tic (Anonymous 1990; Hamley 1991; Christensen 1992;Ilyina & Mieczkowski 1992; Kaltenborn & Emmelin 1993;Sippola et al. 1995). We suggest that serious consider-ation should also be given to the less visible effects ofseemingly benign recreational activities that inevitablyaccompany tourism development.

In our findings, none but the smallest and wettestpatches on level ground, with minimal thermokarst orfluvial erosion, recovered unassisted to something ap-proaching their original state in the medium term (20–75 years). The rapid drainage and continuing desiccationof sedge-cryptogam meadows from as little as a singlepassage of a tracked vehicle, as well as erosion of dryareas via deflation, constitute serious “cumulative im-pacts” (sensu Walker 1997) that were not foreseen atthe time the disturbances were initiated. A wide rangeof small disturbances resulted in alternative vegetationstates with reduced species diversity. We must thereforeacknowledge the effect of “nibbling” (sensu Walker 1997),

a slow but essentially permanent change of landscapesaffected by many seemingly insignificant perturbations.Although improvements in mitigation strategies have re-sulted in a significantly smaller “footprint” from petro-leum development in North America (Truett & Johnson2000), this must be balanced with a virtual lack of mean-ingful protocols in northern Russia (Crawford 1997). Interms of conservation, anthropogenic patch dynamicsappear as a force to be reckoned with when plans aremade for even highly circumscribed and ostensibly miti-gative land use in the more productive landscapes of theincreasingly accessible Arctic.

Acknowledgments

We cannot properly thank the huge number of peoplewho contributed to our research and made this sum-mary possible. A few who made important contributionsinclude M. C. Brewer, J. Brown, F. E. Egler (deceased),P. G. Holland, W. G. Howland, O. V. Khitun, V. Komárk-ová, N. V. Matveyeva, C. H. Racine, H. M. Raup (de-ceased), O. I. Sumina, D. A. Walker, P. J. Webber, G. W.Wenzel, and S. B. Young. Agencies and institutes thatsupported our research in various ways include the Arc-tic Institute of North America, the Arctic Long-Term Eco-logical Research Program (U.S.), Arctic Station/Qeqertar-ssuaq/Godhavn, The Colorado College, the Commissionfor Scientific Research in Greenland, the Danish Re-search Academy, the Institute of Arctic and Alpine Re-search, McGill University, the National Academy ofSciences (U.S.), National Science Foundation (U.S.), theNational Environmental Research Institute/Departmentof Arctic Environment (Denmark), the National Geo-graphic Society, the Scientific and Environmental AffairsDivision of the National Aeronautics and Space Adminis-tration, the Science Institute of the Northwest Territo-ries, the Polar Continental Shelf Project (Canada), theRussian Academy of Sciences, the U.S. Army Cold Re-gions Research and Engineering Laboratory, the U.S.Geological Survey, the University of Colorado-Boulder,and the University of Lapland. S. Jonasson kindly re-viewed the manuscript.

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Anthropogenic Disturbance and Patch Dynamics in Circumpolar Arctic Ecosystems

Documents

Object Recognition via Local Patch Labelling

Disturbance Regime and Disturbance Interactions in a Rocky Mountain Subalpine Forest

Dual Frequency Hexagonal Microstrip Patch Antenna

Your guide to tonight's Dan Patch Awards

STUDENT PROJECT PATCH ADAM

Dual-frequency Patch Antennas

patch adams, the wounded healer - DergiPark

Creating a patch and vulnerability management program

Voltage-Clamp and Patch-Clamp Techniques

Searching for Synergies in International Governance Systems Developed in the Circumpolar North

Patch dualities and remarks on nonstandard cosmologies

Ultra-Wideband Shorted Patch Antennas Fed by Folded-Patch With Multi Resonances

Deformable Patch-based Transformer for Visual Recognition

Circularly Polarized Microstrip Patch Antenna

Patch-Based Occlusion Culling for Hardware Tessellation

The baroclinic transport of the Antarctic Circumpolar Current south of Africa

Zackenberg in a circumpolar context

framework for a circumpolar arctic seabird monitoring network

Hypertrophic lagoon management by sediment disturbance

Examining relationships between climate change and mental health in the Circumpolar North