History and future of mid-high elevation steppe/grasslands in
the southern interior of British Columbia

Introduction

With the arrival of Europeans into the southern interior of the province, the written documentation of the extent and quality . of the valley grasslands and forests began. Mather (1996) describes some of the earliest use of grasslands during the 19th and 20th centuries, with many cattle operations occurring on the lower elevation slopes and valley bottoms. The remote, high elevation grasslands were probably spared sustained use, not only because they were difficult to get to, but there was an abundance of available land further north into the Cariboo district. High elevation steppe grasslands have recently been subject to cattle grazing, and timber harvesting occurs in adjacent forest stands. As logging has moved further upslope into the Engelmann Spruce Subalpine Fir (ESSF) biogeoclimatic zone, the open steppe meadow habitats have been impacted and indeed anthropogenically "enlarged". Thus, recent attention is being directed towards these high elevation grasslands, to answer questions about their history and past importance, their sensitivity and disturbance regimes, special insect and arthropod fauna, and to identify issues pertaining to the preservation of biodiversity. In the account that follows, we first describe how we uncover the history of past steppe/grasslands. We follow this with a description of the history of these ecosystems since the retreat of the last ice sheet. Our account concludes with some thoughts about the future of these important ecosystems, especially in the context of climate change.

How we study ancient ecosystems

Pollen, spores and other plant remains are most widely used to reconstruct past terrestrial ecosystems (Moore et al., 1991). The study of pollen and spores (called palynology) has been particularly well developed during the past century. Different plant taxa (genera and species) produce distinctive pollen and spore types. For example, the pollen grain of grasses has the form of a sphere with a single distinct pore surrounded by a ridge (Figure 1a). No other group of plants produces such a pollen grain. The pollen grains of sagebrush or sage species (Artemisia spp.) are also relatively distinctive (Figure 1b).

Pollen grains and spores are dispersed widely by various agents, especially wind and water, often blanketing the surface of nearby lakes and wetlands (Figure 2). Constructed of a remarkably durable waxy substance called "sporopollenin", many pollen and spores become incorporated in sediments where they withstand the onslaught of time. Only a few cubic centimetres of suitable sediment harbours thousands, if not millions, of grains and spores. These plant microfossils reflect in a direct way, the composition of the vegetation that covered the region near the study site, at the time that the sediment under investigation was deposited.

Palynologists (those who study pollen) extract the pollen and spores through a series of physical and chemical treatments involving the use of strong acids (such as concentrated sulphuric and acetic acid) and mount the residues on glass microscope slides. The glass slide from each sediment level is examined using a microscope under magnifications of 400-1000 times, and the pollen and spores are identified and counted.

The results of this counting and identification are usually presented as percentage data, arranged by sample level, from oldest to youngest in a pollen diagram (Figures 3a and 3b). The pollen diagram is then divided into zones of neighbouring samples with similar pollen and spore assemblages. Each pollen zone is taken to represent vegetation or a collection of nearby plant communities that has persisted in the vicinity of the study site during the interval of time represented by the zone.

Key to the interpretation of pollen zones is an understanding of each species' ecological characteristics. Though species behave individualistically in the natural environment, they also have relationships with other species and these establish a basis for reconstructing past vegetation. For example, a sample with relatively abundant grass pollen by itself could be hard to interpret because grasses occur in a wide range of environments including wetlands. However grass pollen associated with notable amounts of sage pollen almost certainly implies grassland steppe vegetation.

Information from other components of the sediment record helps verify, fill in, and add new information when reconstructing an ancient environment. Insect and fungal remains as well as sediment chemistry and structure are widely used. More and more, charcoal is receiving attention particularly because it helps reveal the role of disturbance in shaping the landscape.

Charcoal counts are obtained by sieving sediment through 180 µm mesh (holes about the thickness of a pin), and counting the black, shiny fragments, often showing cellular structure, under a low-magnification microscope. Charcoal of this size reflects fire activity from the local to watershed region. Due to mixing of charcoal within the upper sediments, periods of time with increased fire activity are usually identified, rather than specific events. Determining the severity of these fire-dominated periods from charcoal counts alone is difficult, as a grass fire obviously has a different amount of biomass burnt than a forest fire. However, when a conifer forest stand is burned and replaced with a deciduous stand, the pollen change is reflected immediately in the sediment, thus allowing a correct interpretation of the charcoal record. Thus, it's best to make fire and vegetation interpretations together.

Vegetation History

The history of the high elevation steppe of the arid southern interior of British Columbia begins with the recession of glacial ice after 15000 years ago. The earliest plant communities, as interpreted from lake sediments, consist largely of sage (Artemisia) and grasses adapted to a dry and cold climate. These types of communities also flourished at the southern margins of the glacial ice sheet in what is today the northwest United States (Thompson et al., 1993). The extent to which this high elevation steppe occurred across southern BC depended on the amount of land free of ice at a given time during the last glaciation. Many high peaks near the margins of the glacial sheet and network likely stood as "islands" above the ice, called nunataks. We may not have any record yet of these glacial landscapes because we have no lake sediments of this earliest age. High peaks rarely hold lakes and even basins with lakes today may have been too dry to hold water under cold dry full-glacial climates. Such nunataks may well have been the sources for many of the high-elevation steppe species of southern interior BC today.

As it stands, the earliest certain-dated lake sediment is about 11,000 radiocarbon years old. Radiocarbon years back to this time actually represent more time than real years, so we are talking about 12,000-13,000 calendar years ago. We recently studied two separate sites near modern high-elevation steppe vegetation, both located just within the upper reaches of the ESSF biogeoclimatic zone (Meidinger and Pojar, 1991). These sites, one on Mt. Kobau and the other on Crater Mountain, provide insight into steppe history, and show that Artemisia played a much greater role than it does today.

McLean (1970) describes Mt. Kobau's vegetation as a unique combination labelled the Artemisia-Abies lasiocarpa zone and identified several other peaks in the southern interior with this vegetation. The earliest vegetation (Zone 1, about 11,000 year ago) at Mt. Kobau consisted of steppe dominated by sagebrush and grasses. Open forests of spruce and fir followed and lasted to approximately 9000 years ago. Then grass-dominated steppe with some sagebrush replaced the parkland (Zone 2). Forests probably disappeared from the mountain slopes during the warm and dry interval, having been replaced by grasslands. These persisted until about 6000 years ago. This three thousand year interval of widespread high elevation steppe corresponds with the time of maximum extent of valley bottom grasslands (Hebda, 1995 1996). At high elevations, the importance of sagebrush increased through the interval whereas at low elevations it seems that grasses became relatively more abundant. After 6000 years ago, modern ESSF forests of spruce and fir developed reducing the extent of steppe (Zones 3 and 4). Pine was, and remains a successional component of the forest stands on Mt. Kobau.

At Crater Mountain, the current, open grassland vegetation is similar to that of Mt. Kobau, however sagebrush is much less common. What is fascinating about this site is the nearly continuous grassland vegetation from the valley bottoms, typically Bunch Grass-Ponderosa Pine (BG-PP), reaching up the steep valley walls to the high-elevation grasslands. The common pattern of forested zones, from Interior Douglas Fir (IDF) to Montane Spruce (MS) to ESSF is absent. Late-glacial vegetation around Crater Mountain consisted of sagebrush-grass steppe, likely with scattered clumps of krummholz spruce and subalpine fir (Zone 1). Early Holocene vegetation consisted of pine parkland, likely interspersed with scattered birch and alder (Zone 2). Douglas fir was likely present from 8000 to 6500 yr BP. Pine-dominated forests, with successional alder and birch patches, occurred from approximately 6500 to 4000 yr BP (Zone 3). Increased moisture and cooling since 4000 yr BP saw the return of subalpine fir and spruce resulting in a mosaic of successional pine forests (Zone 4). Increased proportions of spruce and fir have occurred since 1600 yr BP, although large open areas of grassland remain prevalent (Zone 5).

Past and Present Disturbance

Vegetation histories are largely unique from site to site, and so likely is the history of natural disturbance regimes. In the ESSF, disturbance is mainly under the influence of fire. Fires were more active in the southern interior during the warm dry early Holocene, from about 9000 to 6500 years ago. During this interval, fire in combination with dry climate ensured that forests did not persist. At Crater Lake, repeated burning gave rise to pine parkland (Figure 4). The recent record at Mt. Kobau suggests that fires occurring under human influence happen often and destroy forest stands. High elevation Artemisia tridentata steppe replaces forest stands and requires 50 years or more for lodgepole pine to displace the sagebrush. The return of spruce and subalpine fir takes much longer, likely in the order of centuries. Nearby steppe communities obviously return more quickly, thus repeated burning might ensure that high-elevation steppe could remain to the exclusion of forest stands. Fires at Crater Mountain, seemingly subject to less fire activity under the current climate, likely lead to the replacement of spruce-fir forests with successional pine stands rather than extensive and persistent steppe communities.

This differing sensitivity and response to fire is especially relevant when additional disturbances such as logging and overgrazing are considered. Mt. Kobau and similar sites might appear to be more sensitive to conversion to steppe and hence less tolerant of any disturbance in addition to fire.

The Future: Climate change and unsustainable use

Mt. Kobau has been identified as an area with unique flora (Miller, unpublished) and fauna (Blades and Maier, 1996). An increasingly warm climate favours expansion of high-elevation steppe of the southern Okanagan in two ways. First, drier summers climate will undoubtedly lead to expansion of sites too dry to support trees. Second, the warmer climate will likely lead to increased fire activity favouring sagebrush communities. Human activities in the mountains may hasten the conversion of forest and parkland to steppe. Clear-cut logging, overgrazing, and disruptive recreational activities may not only lead to the progressive loss of tree cover but may also introduce and facilitate the spread of weedy noxious species. Clearly the biodiversity of high-elevation forest, parkland and steppe ecosystems cannot sustain intensive use.

Conclusions

Our studies of the long-term history of the Okanagan's high-elevation landscape reveals that its origins are complex and that seemingly similar sites have different histories. One point is clear, our 20 years of pollen studies now show that at all elevations grassland steppe ecosystems have played a predominant role in the evolution of the region's landscape. Further studies of past ecosystems and sensitivity to climate change at all elevations are now needed to understand what the future may hold for the region as climate warms and summers become increasingly dry.

REFERENCES

Blades, D.C.A. & Maier, C.W. (1996). A survey of grassland and montaine arthropods collected in the southern Okanagan region of British Columbia. Journal of the Entomological Society of British Columbia 93, 49- 61.

Hebda, R.J. (1995). British Columbia vegetation and climate history with focus on 6 ka BP. Géographie physique et Quaternaire 49, 55- 79

Hebda, R.J. (1996). Interior grasslands past and future. Cordillera, 344- 346.

Mather, K. (1996). Bunchgrass ecosystems and the early cattle industry in the Thompson-Okanagan [On-line]. Kelowna, BC: Living Landscapes.

McLean, A. (1970). Plant communities of the Similkameen Valley, British Columbia, and their relationships to soils. Ecological Monographs 40, 403- 423.

Meidinger, D. & Pojar, J. (1991). Ecosystems of British Columbia. Victoria, BC: Research Branch, Ministry of Forests.

Thompson, R.S., Whitlock, C., Bartlein, P.J., Harrison, S.P., & Spaulding, W.G. (1993) . Climatic changes in the western United States since 18,000 yr B.P. In H.E. Wright, Jr., J.E. Kutzbach, T. Webb III, W.F. Ruddiman, F.A. Street-Perrott, & P.J. Bartlein (Eds.), Global climates since the last glacial maximum (pp. 468-513). Minneapolis, MN: University of Minnesota Press.