The Northern Inland Temperate Rainforest of British Columbia: Old Forests With a Young History?

The inland temperate rainforest (ITR) in east-central British Columbia is marked by superlative examples of old-growth cedarhemlock forest. How long has this old-growth forest structure been a major component of this landscape? What is the biological and conservation significance of the history of this forest type? Here we present paleoecological evidence from a study in the Robson Valley of the Upper Fraser River. Sediment cores from Gerry Lake and Redmountain Lake show that western hemlock and western redcedar increased in abundance only within the last 2000 years. Thus, the old-growth forests of the northern ITR may have been present for only a few generations of trees. It is even possible, based on our preliminary evidence, that the oldest western redcedar in these stands may be the first colonizing individuals at these sites. Further paleoecological studies, including a combination of stand-age structure and pollen analysis from small lakes and forest hollows, are needed in order to understand the historical significance of these stands. A recent establishment of the ITR has implications for understanding the assembly of the modern diverse biota of the region as well as how the biota will respond to future climate change. 1 Author to whom correspondence should be addressed. E-mail: [email protected] Introduction The occurrence of disjunct populations of mesicadapted biota in the Columbia Mountains of British Columbia, ranging from 160 to 320 km from their main coastal distributions, is a pattern that has long been of interest to biogeographers (Daubenmire 1952, 1975; Detling 1968). This region of mesic disjunctions typically is defined by the range of Tsuga heterophylla (western hemlock) and Thuja plicata (western redcedar). Especially towards their northern limit, these forests display ecologically significant old-growth structure and a highly diverse flora of epiphytic macrolichens and bryophytes, supporting their characterization as an “inland temperate rainforest” (Newmaster et al. 2003, Goward and Spribille 2005). Only recently have rigorous approaches been applied to unravel the history of these disjunctions (Brunsfeld et al. 2001, Gavin et al. 2006). Pollen records from the central and southern end of interior Thuja-Tsuga forests show that these stands have been present for less than 4000 years (Mehringer 1996, Rosenberg et al. 2003, Chase et al. 2008). However, no pollen records exist from the northern limit of the ITR, and thus their antiquity on the landscape is poorly understood (Sanborn et al. 2006). The establishment of the disjunct biota that comprises the ITR is an intriguing problem when considering that the region was glaciated until ca. 12,000 years ago. Species that comprise the northern ITR must have migrated there from either ice-free refugia south of the ice-sheet in Idaho, or dispersed across dry rainshadow climates of eastern Oregon, Washington, or British Columbia. How important were these historical events in the formation of the current ITR biota? For example, if the mesic-adapted species had to disperse from the coastal region over a barrier where warm and dry growing conditions would not support these species, then the rare probability of these dispersal events may have delayed the establishment of the ITR biota for millennia after suitable climatic conditions were present. Such a scenario would stand in contrast with a general view that vegetation is in equilibrium with climate (e.g., Shuman et al. 2004). In this paper we present a summary of the pollen records from two lakes in the Robson Valley in order to assess the timing of Thuja-Tsuga Northwest Science, Vol. 83, No. 1, 2009 © 2009 by the Northwest Scientific Association. All rights reserved. 71 BC Inland Temperate Rainforest establishment. The first lake was chosen for its potential for climate reconstruction using carbonate-rich sediments, and also because it is ca. 2 km from Thuja-Tsuga forest. The second lake is located in subalpine forest just above the Thuja-Tsuga forest. This site was selected to serve the dual purpose of providing a paleoclimate reconstruction from fossil midges (requiring the subalpine location) as well as a vegetation reconstruction from regionally dispersed pollen. The midge-based climate reconstruction proved to be unreliable, probably due to the unusual geomorphic (inorganic sediments) and microclimatic (persistent snow banks) setting, as described in Chase et al. (2008). Here, we focus on the pollen records and additional evidence of past climate inferred from the sediment stratigraphy.