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Reversion of Forest to Tundra in the Central Yukon

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Reversion of Forest to Tundra in the Central Yukon SUNY Geneseo KnightScholar Biology Faculty/Staff Works Department of Biology 1991 Reversion of forest to tundra in the central Yukon Les C. Cwynar Ray W. Spear SUNY Geneseo Follow this and additional works at: https://knightscholar.geneseo.edu/biology Recommended Citation Cwynar L.C., Spear R.W. (1991) Reversion of forest to tundra in the central Yukon. Ecology 72: 202-212. doi: 10.2307/1938915 This Article is brought to you for free and open access by the Department of Biology at KnightScholar. It has been accepted for inclusion in Biology Faculty/Staff Works by an authorized administrator of KnightScholar. For more information, please contact [email protected]. Ecology, 72(1), 1991, pp. 202-212 ? 1991 by the Ecological Society of Amenica REVERSION OF FOREST TO TUNDRA IN THE CENTRAL YUKON' LES C. CWYNAR Department of Biology, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6E1 RAY W. SPEAR2 Fulton Montgomery Community College, Johnstown, New York 12905 USA Abstract. Pollen and plant macrofossil analyses of sediments from three sites in the central Yukon that are presently in shrub tundra provide a record of former forest estab- lishment. Shrub tundra with groves and gallery forests of balsam poplar occupied the region between 10000 and 8000 BP. At 9400 BP white spruce (Picea glauca) populations ex- panded, and open white spruce woodlands persisted until 6500 BP when black spruce (Picea mariana) and green alder (Alnus viridis) populations increased, resulting in open spruce woodlands with a distribution of species probably similar to that commonly found today in the northern boreal forest: white spruce on drier south-facing slopes and on alluvial sites with balsam poplar, and black spruce on colder, wetter sites on north-facing slopes and valley bottoms. At 5000 BP forest began to revert to shrub tundra, abruptly at first but then more gradually. The modern groves of spruce in the region, which are mostly white spruce, therefore are probably relict populations surviving in favorable microsites. These results support the conclusion from other recent studies that Alaska and northwest Canada experienced warmer summers than today from as early as 10 000 BP to 6000 BP in response to increased summer insolation arising from changes in the earth's orbital parameters as predicted by the Astronomic Theory of climate change. The decline of forest after 5000 BP is not associated with any significant changes in the abundance of shrub or herb pollen types, suggesting that the ground vegetation has behaved independently of the tree populations. Key words: Astronomic Theory; black spruce; central Yukon; forest history; paleoecology; paly- nology; plant macrofossils; tundra; white spruce. INTRODUCTION 1972). Groves of white spruce (Picea glauca), and oc- Ecologists and paleoecologists both attempt to study casionally black spruce (P. mariana), are scattered "the interactions that determine the distribution and throughout this mountainous region on climatically abundance of organisms" (Krebs 1978). In general, the favorable sites on south- to southwest-facing slopes ultimate factor affecting the distribution and abun- (Fig. 2). Previous studies (Brubaker et al. 1983, Ritchie dance of vegetation is climate (Woodward 1981). The et al. 1983, Ritchie 1984a, Edwards et al. 1985) have relationship of vegetation to climate is clearest at veg- shown that areas north of the central Yukon and ad- etation boundaries, a fact that has proved of great value jacent Alaska experienced a period of warmth greater - in reconstructing the past dynamics of climate and veg- than modem climate from 10 500 to 5500 BP, co- etation, as in the study of changes in the prairie-forest incident with higher insolation values predicted by the boundary (McAndrews 1966) and arctic tree line (Rit- Astronomic Theory for high northern latitudes (Berger chie and Hare 1971, Spear 1983, Payette et al. 1985, 1981, Kutzbach and Guetter 1986, COHMAP 1988). Payette and Filion 1985). Vegetation boundaries and We undertook this study to answer a number of re- ecotones are especially sensitive to climate change, lated questions: (1) Was this belt of tundra previously making them ideal for reconstructing past climates and forested? Valley floors in the region are scarcely higher for studying the dynamics of community change. than the altitudinal limit of trees in the boreal forest Yukon Territory is transacted by westward exten- to the south, and as groves of trees occur in favorable sions of the Mackenzie Mountain system, resulting in localities today, the study area is sensitively located to a broad belt of tundra (Fig. 1) between closed boreal respond to past changes of climate. The area would forest to the south and forest-tundra to the north (Rowe likely have been forested during the warm interval cen- tered around 10 000 BP. A somewhat abbreviated pol- len diagram from a bog near Chapman Lake (Fig. 1) I revised 6 Manuscriptreceived 23 October 1989; April analyzed by Terasmae and Hughes (1966) suggests that 1990; accepted 20 April 1990. 2 Present address:Empire State College, State University spruce forests may have grown in the region previously. of New York, 317 WashingtonStreet, Watertown, New York However, Terasmae and Hughes (1966) drew no con- 13601-3744 USA. clusions regarding the past vegetation since they were February 1991 HISTORY OF SPRUCE GROVES 203 chiefly interested in the chronology of glaciation. (2) Whatis the originof these isolatedspruce groves? These robsk 'E li.i.1Bforest a barrens groves may simply represent populations established z~~~~~~~~~~~IEhld boreal1 forest by long-distance dispersal to favorable local habitats in what has alwaysbeen essentiallytundra, or they may Yukon ~ ~ ~ i~ be relict populationsfrom a time when forestwas more extensive. (3) If the areawas previouslyforested, is the presenttundra, then, simply the past vegetationminus the trees or is it fundamentallydifferent from the un- USA derstoryvegetation of past forests?Ritchie (1986) has drawn the important distinction between "tree line" and "tree-linevegetation," noting that changesin tree- .~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.....K. line species are "betterthought of as responsesof par- ticularspecies populationsto climate variations"rath- er than changes in vegetation. He also noted that few species limits coincide with tree line and that in north- ern Quebec Morisset et al. (1983) demonstratedthat Pond most arctic and boreal taxa occur on both sides of tree line. If our study area was forested, it would provide a test of the hypothesis that shrubs and plants com- posing the groundvegetation in forestedareas may act independently of tree populations, and thereforetree line, in the face of climate change. /5> 0 5 o Chapmana^. k SrrE DEscRnoNS The study area lies within the Southern Ogilvie Ranges,which have been glaciatedrepeatedly by mon- tane glaciersthat were independentof the Cordilleran ice sheet (0. L. Hughes, personal communication). Grayday Pond (64057' N, 138015' W, 915 m above sea level (asl);Fig. 2b) lies within the old floodplainof the Blackstone River in a region that was last glaciated more than a million yearsago. Both Honeymoon Pond (64?38'N, 1380241W, 1160 m asl; Fig 2c) and Monks- hood Pond (64?34' N, 138015' W, 1280 m asl; Fig. 2d) 4- - lie just within terminalmoraines of valley glaciersthat spilled into the North Fork Pass duringlate Wisconsin time (Fig. 1). All of the ponds have a surfacearea < 1 ha, and none has an inflowingstream. The names that we have assigned to our sites are informal. The three sites are located in shrub tundra.Isolated stands of predominantly white spruce are scattered throughoutthe area (Fig. 2a, b) and the largestof these, Ioneymoon Pond X which are noted on 1:250 000 scale topographicmaps, i~~~~n sheid are shown in Fig. 1. Black sprucegroves are generally much less common. Balsam poplargroves are restrict- ed to alluvial sites and bluffs along the lower third of the Blackstone River in our study area and appearto Pond be absent in interfluves.We did not, for example, find balsam poplar in any of the white spruce groves that FIG. 1. Map showingthe location of the study area.Shad- is common- ed areas indicate land > 1370 m above sea level, blackened we visited, although such an admixture areas representstands of trees that appearon 1:250000 map place in the boreal forest. Fingersof continuous forest sheets, and heavy lines in lower part of map show the limits of black spruce on valley bottoms and north-facing of Late Wisconsin valley glaciers.The insert shows the veg- slopes and white spruce on drier and warmersites ex- etation regionsaccording to Rowe (1972). tend to within 6 km of Monkshood, the southernmost site, and 3 km of Grayday,the northernmostsite. The main body of the boreal forest lies -22 km south of 204 LES C. CWYNAR AND RAY W. SPEAR Ecology,Vol. 72, No. 1 FiG. 2. Coringsites and environs in the upperBlackstone River. (a) Centralportion of the study area with two stands of white spruce,the farthestindicated by an arrow.The foregroundis dominatedby willows and the valley bottom is covered by shrubbytussock tundra;(b) GraydayPond with dwarf birch shrub tundrain the foregroundand white spruce along the BlackstoneRiver in the background;(c) Honeymoon Pond with willows dominatingthe shrubtundra locally, althoughalder dominates the lower slope of the mountain on the left; (d) Monkshood Pond surroundedby shrub tundra with a solitary white spruceat lower left above the figuredesignation. Monkshood, whereasthe southernlimit of forest-tun- strom (1985). After samplingfor pollen, cores were cut dra occurs -32 km north of Grayday. into 5-cm sections, which were washed througha sieve In our study area, the ground vegetation is domi- with meshes of 0.5 mm in orderto extractplant macro- nated by Eriophorum(cotton grass),Carex (sedge), and fossils. All macrofossilswere identifiedby comparison species of Ericaceae.The dominant shrubs are dwarf with modern reference collections. Bulk samples of birches, Betula glandulosa and B. nana, and willows, sediment for radiocarbondating were submittedto ei- chiefly Salix pulchra.
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