Postglacial evolution and spatial differentiation of seasonal temperate rainforest in western Canada

Abstract

Surface samples from Vancouver Island, Canada, were used to assess the relationship between discrete seasonal temperate rainforest (STR) plant communities and their corresponding pollen signatures. Pollen from 10 sediment cores was further used to evaluate the post-glacial development of these communities. Principal components analysis (PCA) of the surface data revealed the distinctiveness of the modern pollen rain, with samples from the Coastal Douglas Fir (CDF) zone, the dry Coastal Western Hemlock (CWH) zone, the wet CWH sub-zones and the Mountain Hemlock (MH) zone clustering distinctly. PCA of the fossil data revealed early-seral open canopy, wet rainforest, subalpine rainforest and late-glacial plant associations and showed that the STR has changed markedly through time. Pinus woodlands with low palynological richness prevailed in the early late-glacial period, only to be supplanted by mixed conifer forest with increased pollen richness. In the early Holocene, STR vegetation differentiated spatially as early-seral open canopy forests expanded, though a non-analog Picea-dominated forest persisted on the moist outer coast. Generally high pollen richness is attributed to the expansion of dryland habitat coupled with the development of a firemaintained vegetation mosaic. In the mid and late Holocene intervals, open canopy communities persisted in eastern areas, eventually developing into modern CDF and dry CWH forest. In contrast, moist and oceanic CWH rainforest developed on central and western Vancouver Island, whereas subalpine forest established at high elevation. Pollen richness declined in the mid Holocene concomitant with increased precipitation and a general reduction in the incidence of fire, though this trend was offset somewhat in the late Holocene by paludification.

Pollen assemblage richness does not reflect regional plant species richness: a cautionary tale

“…The low taxonomic resolution in pollen assemblages, the large number of entomophilous taxa and the taphonomic processes that occur between pollen liberation and its eventual preservation in depositional basins result in the lack of a meaningful statistical relationship between measures of plant and pollen richness. This calls into question the use of fossil pollen assemblages as a measure of species richness through time and has implications for understanding relationship between changes in climate and richness over time (Birks & Birks 2008;Brown et al 2008). Further research to investigate relationships between pollen and plant richness at broad spatial scales is needed in other regions in order to assess the extent to which our results indicate a universal shortcoming in the use of palynological richness as a proxy for plant richness.…”

“…Zea and Cereale in Poaceae). Nonetheless, palynological richness is often used as an indicator of past species richness, either explicitly or implicitly (Birks & Line 1992;Jaramillo, Rueda & Mora 2006;Berglund et al 2008;Birks & Birks 2008;Brown et al 2008;van der Knaap 2009; Morales-Molino, Garc õa Ant on & Morla 2011; Colombaroli et al 2012), despite the fact that the relationship between plant richness and pollen richness has not been well studied. While pollen richness at a site may vary over time, temporal changes in richness may not be directly correlated with regional plant species richness.…”

Long-term climate, vegetation and fire regime change in a managed municipal water supply area, British Columbia, Canada

“…Silt and clay deposits characterized by markedly high magnetic susceptibility values at the bottom of the Begbie Lake core (BL-1) reflect this open, erodible landscape. The preponderance of diploxylon Pinus pollen (Figures 3 and 4), suggests that pioneering pine trees, likely P. contorta (Brown and Hebda, 2003; Brown et al, 2008), quickly colonized the deglaciated landscape. Given that Pinus is overrepresented in the modern pollen rain (Allen et al, 1999; Hebda and Allen, 1993), it is hypothesized that open-canopy pine-dominated woodlands prevailed at this time.…”

Multi-millennial streamflow dynamics in two forested watersheds on Vancouver Island, Canada

“…As per the marine environment, vegetation likewise changed in response to climate forcing. In the area on southern Vancouver Island near the San Juan and Koksilah watersheds, Douglas-fir forests with alder (Alnus) and bracken fern (Pteridium) expanded westward during the warm dry early Holocene, with spruce (likely Picea sitchensis) and western hemlock forests prevailing on the outer western coast (Hebda, 1995;Brown et al, 2008). The incidence of fire disturbance was generally greater at this time (Brown and Hebda, 2002a).…”