Recent woody invasion of wetlands on the Kenai Peninsula Lowlands, south-central Alaska: a major regime shift after 18 000 years of wet Sphagnum–sedge peat recruitment

Abstract

We document accelerating invasion of woody vegetation into wetlands on the western Kenai Peninsula lowlands. Historical aerial photography for 11 wetland sites showed that herbaceous area shrank 6.2%/decade from 1951 to 1968, and 11.1%/decade from 1968 to 1996. Corresponding rates for converting herbaceous area to shrubland were 11.5% and 13.7%/decade, respectively, and, for converting nonforest to forest, were 7.8% and 8.3%/decade, respectively. Black spruce (Picea mariana (Mill.) BSP) forests on three wetland perimeters established since the Little Ice Age concluded in the 1850s. Dwarf birch shrubs at three wetland sites showed median apparent tree-ring age of 13 years, indicating recent shrub colonization at these sites. Peat cores at 24 wetland sites (basal peat ages 1840 -18 740 calibrated years before present) indicated that these peatlands originated as wet Sphagnum-sedge fens with very little woody vegetation. Local meteorological records show a 55% decline in available water since 1968, of which one-third is due to higher summer temperatures and increased evapotranspiration and two-thirds is due to lower annual precipitation. These results suggest that wet Sphagnum-sedge fens initiating since the end of the Wisconsin glaciation began to dry in the 1850s and that this drying has greatly accelerated since the 1970s.Résumé : Nous documentons l'invasion accélérée de la végétation ligneuse dans les zones humides de l'ouest des basses terres de la péninsule de Kenai. D'anciennes photographies aériennes de 11 stations humides montrent que la superficie occupée par les plantes herbacées a diminué de 6,2 % par décennie entre 1951 et 1968 et de 11,1 % par décennie entre 1968 et 1996. Les taux correspondants de conversion de la superficie occupée par les plantes herbacées en prairie boisée étaient de 11,5 et 13,7 % alors que les taux de conversion des aires non forestières en aires forestières étaient de 7,8 et 8,3 % par décennie. Des forêts d'épinette noire se sont établies en périphérie de trois zones humides depuis la fin du Petit  ge glaciaire dans les années 1850. Des bouleaux nains établis dans trois stations humides avaient un âge médian apparent de 13 ans d'après le décompte des cernes annuels; ce qui indique que la colonisation de ces stations par les arbustes est ré-cente. Des carottes de tourbe provenant de 24 stations humides (âge de la tourbe basale allant de 1840 à 18 740 années calibrées BP) indiquent qu'elles étaient initialement des tourbières humides dominées par des sphaignes et des graminées avec très peu de végétation ligneuse. Des archives météorologiques locales montrent une diminution de 55 % de l'eau disponible depuis 1968, due pour un tiers à l'augmentation des températures estivales et de l'évapotranspiration et pour les deux tiers à la diminution des précipitations annuelles. Ces résultats indiquent que les tourbières dominées par les sphaignes et les graminées formées depuis la fin de la glaciation du Wisconsin ont commencé à s'assécher durant les années 1850 et que cet assèchement s'e...

Decline of an endangered amphibian during an extreme climatic event

“…3C). In wetland environments tree invasion occurs as a result of increased dryness (Bren 1992, Berg et al 2009), indicating that extinction sites have experienced prolonged periods of desiccation. At the pool and nest level there were no significant relationships between habitat variables measured at randomly selected pool and randomly selected nest locations at the three different site types, with the exception of presence of water (Fig.…”

Presence of rapidly degrading permafrost plateaus in south-central Alaska

“…While previous reports of permafrost on the Kenai Peninsula exist (Hopkins et al, 1955;Jorgenson et al, 2008b), they were restricted to the wetland complex (Mystery Creek) north of Sterling (Berg et al, 2009). Information on its dynamics here and elsewhere was lacking.…”

“…Berg and Anderson (2006) caution that overall drier conditions on the western Kenai Peninsula, combined with standing dead spruce stands, may alter the future fire regime of this region. Wetland drying (Klein et al, 2005) and establishment of woody vegetation in wetlands (Berg et al, 2009) may be attributed to warmer air temperatures and decreases in precipitation. Furthermore, tectonic activity associated with the Great Alaska Earthquake of 1964 caused the western Kenai Peninsula to lower in elevation by 0.7-2.3 m (Plafker, 1969), while the northern portion of the peninsula subsequently uplifted 0.8-0.9 m (Cohen and Freymueller, 1997), potentially altering groundwater flow paths (Gracz, 2011).…”

“…However, ecosystem-protected permafrost persists in southcentral Alaska in regions with present-day climatic conditions that are no longer conducive to its formation (Shur and Jorgenson, 2007). Isolated permafrost patches in southcentral Alaska exist in the western Kenai Peninsula lowlands (Hopkins et al, 1955;Berg et al, 2009;Jorgenson et al, 2008b) and in the vicinity of Anchorage (Jorgenson et al, 2003;Riddle and Rooney, 2012;Kanevskiy et al, 2013). Enhanced insight into the resilience and vulnerability of ecosystem-protected permafrost is important due to its utility as a climate indicator and a forecaster of the environmental consequences expected to arise from permafrost thaw elsewhere in the boreal forest, where MAAT is expected to warm beyond 0 • C in the coming decades (Beilman et al, 2001).…”

Distribution and abundance of exotic earthworms within a boreal forest system in southcentral Alaska

“…Wetlands in Alaska are warming and drying (Klein et al 2005, Riordan et al 2006, Berg et al 2009) and average winter temperatures have warmed 3.5 °C in the last 50 years (Karl et al 2009). Drying wetlands and warmer winters may provide increasingly suitable habitat for exotic earthworms.…”