Palaeogeography, Palaeoclimatology, Palaeoecology 383–384 (2013) 59–71 Contents lists available at SciVerse ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Phytoliths of Southeastern Vancouver Island, Canada, and their potential use to reconstruct shifting boundaries between Douglas-fir forest and oak savannah Jenny L. McCune a,⁎, Marlow G. Pellatt b a Department of Botany and Biodiversity Research Centre, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada b Parks Canada, Natural Resource Conservation, Vancouver, BC, V6B 6B4, Canada article info abstract Article history: Phytolith assemblages in terrestrial soils have the potential to reveal local shifts between open savannah and Received 24 January 2013 conifer forests in the past. On southeastern Vancouver Island, this could help quantify the range of variability Received in revised form 26 April 2013 of Garry oak savannahs prior to European colonization, and the degree of influence of indigenous people in Accepted 5 May 2013 maintaining open savannahs. We catalogued the phytolith morphotypes found in 72 of the most common Available online 12 May 2013 plant species in this system, and extracted phytoliths from two soil cores on either side of the boundary be- tween a conifer forest and an oak savannah. Grasses are the most prolific producers of phytoliths in savan- Keywords: fi Phytoliths nahs. In forests, Douglas- r(Pseudotsuga menziesii) produces large, distinctive asterosclereid phytoliths. Douglas-fir Phytolith assemblages from soil cores showed that phytoliths from grasses dominated, even in soils from Grassland the forest, where grasses are rare. However, the presence of asterosclereid phytoliths reliably distinguished Savannah between Douglas-fir forest and Garry oak savannah habitats within 20 m of the vegetation boundary. This Asterosclereid finding may be useful for reconstructing past vegetation not only on Vancouver Island, but also across a Canada large geographic area in North America where grasslands tend to be invaded by Douglas-fir following fire suppression. The use of the phytolith record from terrestrial soils at local sites where the history of human occupation is well-known could help establish the degree of influence of indigenous peoples in maintaining open savannahs on southeastern Vancouver Island. © 2013 Elsevier B.V. All rights reserved. 1. Introduction have shown that the extent of these savannahs was much greater just prior to European settlement, and that the remnants of this vegetation In many places across the Americas indigenous peoples had a long type now exist mainly in steep, rocky areas not suitable for agriculture and substantial influence on vegetation long before colonization by (Lea, 2006; Vellend et al., 2008; Bjorkman and Vellend, 2010). Oak Europeans (Denevan, 1992; Delcourt and Delcourt, 2004). However, savannahs on deep soil sites are highly susceptible to invasion by the there are serious debates concerning the degree and extent of this conifer Douglas-fir(Pseudotsuga menziesii; Fuchs, 2001). Palynological influence in time and space for particular regions (e.g. Vale, 2000; studies have shown that oak savannahs were established on southeast- Whitlock and Knox, 2002). It is very challenging to tease apart the ern Vancouver Island by approximately 8000 years ago, and persisted interacting effects of climatic and other non-human agents from cul- despite a cooling, moistening trend beginning about 6000 years ago tural impacts on vegetation over long time periods. Even with strong (Pellatt et al., 2001). Ethnographic and historical evidence suggests evidence from ethnography, dendroecology and palynology that the that on deep soil sites in particular, management by indigenous people activities of indigenous peoples were an important influence on veg- in the form of frequent, low-intensity prescribed burns maintained the etation structure, mysteries often remain about the details. open character of Garry oak savannahs (Turner, 1999; MacDougall et al., This is definitely true for the Garry oak savannahs of Vancouver 2004). However, the spatial and temporal extent of this influence, and Island, Canada. These savannahs represent the northernmost edge of the timing of transitions from open savannah to Douglas-fir forest or the range of the Garry oak tree (Quercus garryana), which stretches as vice versa prior to European settlement, is not clear. far south as northern California (Fig. 1). Historical ecological studies Phytolith analysis has the potential to increase our understanding of the history of vegetation change in this system. Phytoliths are plant microfossils formed when hydrated silicon dioxide (SiO2·nH20) is de- ⁎ Corresponding author at: 3529-6270 University Blvd., Vancouver, BC V6T 1Z4, Canada. posited within and between the cells of a living plant (Pearsall, 2000; Tel.: +1 604 822 2133; fax: +1 604 822 6089. E-mail addresses: [email protected] (J.L. McCune), Prychid et al., 2003). The resulting cell-shaped silica casts are released [email protected] (M.G. Pellatt). upon decomposition of the plant, and can survive in sediments for 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.05.003 60 J.L. McCune, M.G. Pellatt / Palaeogeography, Palaeoclimatology, Palaeoecology 383–384 (2013) 59–71 Fig. 1. Main map: the West Coast of North America showing Vancouver Island, with the range of Garry oak indicated in grey. Inset: Southeastern Vancouver Island and the Gulf Islands, showing Tumbo Island, the location of the soil samples. Lower right: photo of the boundary between oak savannah and Douglas-fir forest at the soil sampling location. Garry oak range map based on map by Erickson (2008). thousands of years (e.g. Blinnikov et al., 2002). Phytoliths may be 2. Materials and methods transported by wind or water, but phytolith assemblages are derived from more local vegetation sources than pollen, which can travel for 2.1. Reference collection hundreds of kilometres (Jacobson and Bradshaw, 1981; Pearsall, 2000; Blinnikov et al., 2013). Therefore, phytoliths have the potential We collected tissue samples from common plant species of the to reveal vegetation changes on a local spatial scale. Phytoliths are not three main non-riparian vegetation types in the region: Douglas-fir produced by all plant taxa, but are particularly abundant in certain forests, Garry oak savannahs, and Douglas-fir/Arbutus forests, which plant families, and in some cases are specific to the level of genus are dominated by Douglas-fir but include a substantial component (Pearsall, 2000). In North America, phytoliths extracted from buried of arbutus (Arbutus menziesii) trees in the canopy. We selected spe- palaeosols and/or modern soil profiles have been used to infer past cies based on a vegetation survey of 184 20 × 20 m relevé plots sur- changes in vegetation in diverse regions including the Great Plains, veyed in 2009 (McCune and Vellend, in prep). For each of the three the Columbia Basin, Arizona, Utah, Minnesota, California, and Manitoba vegetation types, we collected samples of all species that had an aver- (Bartolome et al., 1986; Fisher et al., 1995; McClaran and Umlauf, 2000; age cover of at least 1% in all relevés in which they were present. We Kerns et al., 2001; Blinnikov et al., 2002; Boyd, 2002; Morris et al., supplemented our collection with the following: as many grass spe- 2010). cies as we could collect, including two native bunchgrasses that are The first steps in utilizing phytoliths to study long-term vegetation now very rare and one very rare exotic grass from the Panicoideae change are to establish a reference collection of the phytolith types sub-family; one species from the Juncaceae family; and a few recently produced by plants common to the region, and to determine whether introduced exotic non-grass species whose phytoliths could poten- the proportions of different phytolith types in surface soils can reli- tially be used as stratigraphical markers. In total we collected samples ably distinguish between current vegetation types (Fredlund, 2005). from 72 species in 28 plant families, including 13 herbs, 3 ferns, 15 Therefore, our study was designed to answer the following questions: shrubs, 10 trees, 1 sedge, and 30 grasses (Tables 1, 2). Samples were collected in the late spring or summer of 2010 or 2011 from mature (1) What are the phytolith types and type frequencies in common specimens, and air-dried until they could be processed. plants of Douglas-fir forests and Garry oak savannahs of We extracted phytoliths from plant tissue samples using the southeastern Vancouver Island? dry-ashing technique (Pearsall, 2000). We cut plant samples into (2) Do phytolith assemblages from terrestrial soils reliably differ- small pieces and separated inflorescences, fruit, and vegetative por- entiate between the two vegetation types? tions (stems and leaves). Shrubs and tree samples were separated (3) How sensitive are phytolith assemblages to vegetation into leaves (needles), twigs, fruit, and in some cases bark. We did boundaries? not collect below-ground plant parts except for the bulbs of camas J.L. McCune, M.G. Pellatt / Palaeogeography, Palaeoclimatology, Palaeoecology 383–384 (2013) 59–71 61 (Camassia quamash and C. leichtlinii), which were the most important heavy liquid flotation procedure modified from Pearsall (2000) to ex- starch source for indigenous peoples prior to European colonization tract phytoliths. First we air-dried the soil sample overnight, and then (Turner and Bell, 1971). We rinsed herb and grass samples thoroughly filtered it through a 500 μm sieve. We weighed out approximately 5 g in distilled water and placed them in a drying oven at approximately of the filtered soil to process for phytoliths. We removed carbonates 70 °C overnight. We washed twigs and bark samples in a sonic bath by heating in 10% hydrochloric acid for 1 h. Following rinsing to re- for 30 min, and then placed them in a drying oven at approximately move the acid, we removed organic material with a five minute 70 °C overnight.