Holocene biogeography of Tsuga mertensiana and other conifers in the Kenai Mountains and Prince William Sound, south-central Alaska

Abstract

Several important North American coastal conifers – having immigrated during the Holocene from the southeast – reach their northern and upper elevation limits in south-central Alaska. However, our understanding of the specific timing of migration has been incomplete. Here, we use two new pollen profiles from a coastal and a high-elevation site in the Eastern Kenai Peninsula–Prince William Sound region, along with other published pollen records, to investigate the Holocene biogeography and development history of the modern coastal Picea (spruce)– Tsuga (hemlock) forest. Tsuga mertensiana became established at Mica Lake (100 m elevation, near Prince William Sound) by 6000 cal. BP and at Goat Lake (550 m elevation in the Kenai Mountains) sometime after 3000 years ago. Tsuga heterophylla was the last major conifer to arrive in the region. Although driven partially by climate change, major vegetation changes during much of the Holocene are difficult to interpret exclusively in terms of climate, with periods of slow migration alternating with more rapid movement. T. mertensiana expanded slowly northeastward in the early Holocene, compared with Picea sitchensis or T. heterophylla. Difficulty of invading an already established conifer forest may account for this. We suggest that during the early Holocene, non-climatic factors as well as proximity to refugia, influenced rates of migration. Climate may have been more important after ~2600 cal. BP. Continued expansion of T. mertensiana at Goat Lake at the Medieval Climate Anomaly (MCA)–‘Little Ice Age’ (‘LIA’) transition suggests warm and wet winters. But expansion of T. mertensiana at both sites was arrested during the colder climate of the ‘LIA’. The decline was more extensive at Goat Lake, where climatic conditions may have been severe enough to reduce or eliminate the T. mertensiana population. T. mertensiana continued its expansion around Goat Lake after the ‘LIA’.

Postglacial vegetation community change over an elevational gradient on the western Kenai Peninsula, Alaska: pollen records from Sunken Island and Choquette Lakes

“…() suggested that Picea may have arrived even earlier (11.0–10.0 ka) at Swanson Fen. The tree moved slowly south, so that it occurred throughout the Kenai Mountains by ~6.5–6.0 ka, ~6.0 ka in the southern Caribou Hills (Circle Lake), and reaching its southernmost Kenai Peninsula site (Homer Spit) by ~5000 years ago (Ager, ; summarized in Anderson et al ., ).…”

“…The record from the coastal Mica Lake stands alone in contrast to this pattern, with two different conifers – Tsuga mertensiana and Picea sitchensis –establishing there by~6.0 and ~5.0 ka, respectively (Fig. ; Anderson et al ., ). These ages represent the earliest establishment of these two conifers on the Kenai Peninsula, and this site could well have served as a node for subsequent expansion both to the west and to the east.…”

“…Comparing the latest Holocene conifer record at the Mica Lake and Goat Lake sites, Anderson et al . () noted a significant impact of the cold Little Ice Age (LIA; maximum cold ~1660–1820 ad ) on pollen deposition of both Picea glauca / sitchensis and T. mertensiana at the higher Goat Lake site. Both trees were in decline by ~0.65 ka, being replaced by a denser Alnus woodland.…”

“…More recently, Anderson et al . () compared pollen records from two sites at different elevations in the eastern Kenai Peninsula – western Prince William Sound area, where major conifer species currently reach their northern and upper elevational limits in south‐central Alaska. Comparison of pollen data from Mica Lake (~100 m a.s.l.)…”

“…In this paper we continue our research on postglacial paleovegetation change over an elevational range by presenting two new sites from the western Kenai Peninsula: Sunken Island Lake from the Kenai Lowlands, and Choquette Lake from the Caribou Hills upland. As with our previous study (Anderson et al ., ), we use pollen and plant macrofossil data from well‐dated cores, but also include new data on Holocene fire occurrence from the higher elevation Choquette Lake that we compare with a low‐elevation fire record from previously published Paradox Lake (Anderson et al ., ). We also provide additional perspective on the timing of postglacial expansion of major conifers – Picea glauca , P. mariana (black spruce) and Tsuga mertensiana – for the western Kenai Peninsula.…”