ENVIRONMENTAL HISTORY

ECOSYSTEMS

Ecosystem history varies from south to north in the zone and from valley bottom to mountain top. The history is best known for the southern interior where studies have focused on valley bottom grasslands (Hebda 1982b, 1995,1996) (see Figure 1 for summary) and on high elevation forests and the subalpine environment (Pellatt 1996).

Today the southern part of the Montane Cordillera supports Bunchgrass and Ponderosa Pine Biogeoclimatic zones in the valley bottoms and on lower slopes (Meidinger and Pojar 1991). Interior Douglas-fir (IDF) forests occupy warm dry mid slopes and extensive tracts of similar climate on the southern interior plateau. Montane spruce forests occur on slopes above these Douglas-fir forests, but below the subalpine coniferous forests and parklands of the Engelmann Spruce (Picea engelmannii)-Subalpine Fir (Abies lasiocarpa) zone. A mixed coniferous forest, the Interior Cedar Hemlock (ICH) biogeoclimatic zone covers moist lower and mid slopes of the west-facing windward sector of the mountains of the eastern Montane Cordillera. Patches of alpine tundra crown high peaks, becoming more widespread in the north.

The earliest ecosystems after deglaciation are not well described yet. Based on observations at Finney Lake (west of Cache Creek) and Nazko Cone west of Quesnel low to mid elevations supported open herbaceous vegetation perhaps with scattered shrubs (Hebda 1982a, Souther et al. 1987). Marshes and aquatic vegetation seems to have been well established and was dominated perhaps by cattail (Typha latifolia) (Hebda 1982a). A species of Populus was the first tree to arrive and form stands at Finney Lake 11,000 - 12,000 years ago. Pine, assumed to be lodgepole pine (Pinus contorta,) arrived by 10,000 years ago and spread at higher elevations and possibly in northern sites. Its sources are not established, though certainly it moved northward from refugia south of the Cordilleran ice sheet. However, early occurrence of abundant pine pollen in the Heckmann Pass of west central British Columbia (Hebda and Whitlock 1997) suggests a coastal source too.

Though forests or forest-tundra developed between 12 000 and 10 000 years ago, open ecosystems containing sage species (Artemisia spp.) and grasses persisted on valley bottoms (Hebda 1995). During the hot, dry early Holocene (10 000 - 8000 years ago) grasslands and sagelands were widespread, ranging up to at least 1300 m above sea level, and on southern south-facing slopes connecting valley-bottom to mountain-top non-arboreal communities. An interesting feature of montane sites of the southern interior at this time was the occurrence of shrubby vegetation probably including Rocky Mountain juniper (Juniperus scopulorum) soapberry (Shepherdia canadensis) and willows (Salix spp) (Hebda 1995).

From 8000 to 4500 years ago the grassland area shrank, a decline well under way by 6000 BP. Nevertheless these steppe ecosystems were still more extensive than today. At Kilpoola Lake (Figure ), just west of Osoyoos BC, a species of sage, likely big sagebrush (Artemisia tridentata), dominated the mid elevation valley floor and slopes (Heinrichs et al. unpublished). Under the moister climate of this interval, forest species, especially Douglas-fir, expanded and modern day forested and savannah ecosystems like the Ponderosa Pine, Interior Douglas-fir and Montane Spruce zones began to take shape. The forest steppe ecotone descended the slopes toward valley bottoms.

Between 4500 and 3000 years ago grasslands reached their minimum extent, being restricted to valley bottoms. Modern forests finally developed under progressively modern climate which had become relatively cool and moist. Pollen diagrams from several sites suggest that grasslands have expanded slightly during the last 3000 BP, even before major disturbance and clearing by European settlers in the 1800's (Hebda 1982b).

The relatively moist western redcedar and western hemlock mixed conifer forest was a late development in the history of the cordillera. Though these two diagnostic species may have been present in the middle Holocene, they only rose to dominance between 2000 and 4000 years ago (Hebda 1995).

The study of high elevation sites has focused largely on tree-line changes as a measure of climate fluctuations. In southern British Columbia and adjacent Alberta the upper tree line stood higher than present throughout most of the first half of the Holocene until about 5000 years ago (Hebda 1995). In some regions tree line declined to its present position as late as 2000 - 3000 years ago. In Alberta timber lines have been similar to present or lower since 4500 BP (Luckman and Kearney 1986).

Northern interior plateau sites have been little studied. Today ecosystems transitional from those typical of the Cordillera to those related to the great Boreal Forest occupy lower to mid elevations and Engelmann spruce and subalpine fir forests and alpine tundra occur at higher elevations. Much of the plateau and lower slopes are cover by mixed lodgepole pine and spruce forests of the Sub-Boreal-Pine-Spruce biogeoclimatic zone and by spruce stands of the Sub Boreal Spruce zone. This area's environmental changes seem, at this point of our knowledge, seem to be less extreme than those further south. Lodgepole pine appears to have been a major element of the vegetation from the beginning. Though there is a hint that the central plateau may have supported open forest and even extensive patches of steppe in the early Holocene (Hebda 1995). The only other site from the area, is from higher elevation and does not show this feature. Spruce and true fir (presumably subalpine fir) became more abundant in the mid Holocene suggesting that modern zonal ecosystems have developed since that time.

Disturbances have been important in shaping the ecosystems of the Montane Cordillera Ecozone. Fire intensity, extent and frequency as measured by charcoal production has certainly changed with time with fires likely more frequent during the warm dry climate of the early Holocene. Ongoing research will shed more light on the nature of these fires and their impact. Several eruptions from volcanic centres in the United Sates and British Columbia have spread ash over the southern interior of British Columbia and adjacent Alberta. Preliminary studies suggest that these eruptions had dramatic effects on the landscape, leading to increased erosion, possibly, establishment of successional plant communities and changes in the characteristics of wetland and aquatic ecosystems (Heinrichs et al. submitted). The impact of single or combined disturbances may have lasted for several centuries before the return of pre-disturbance ecosystems.