GLOBAL CHANGE AND BRITISH COLUMBIA NATIVE FLORA Richard Hebda Botany Unit Royal British Columbia Museum Victoria, B.C. V8V 1X4 Presented at the "British Columbia Native Plants, their current Status and Future Colloquium at Botany Dept., University of British Columbia, May 12, 1990 ****************************************************************************** Future of British Columbia's Flora and Vegetation WETLANDS The greatest effects of global warming will likely be on wetland taxa and and wetland ecosystems including lakes, ponds, swamps, bogs, fens, marshes and related entities. There are at least four reasons for this. First, any change in moisture availability results in a change in wetland hydrology and hence character of the wetland. Wetlands, unlike the alpine habitat, cannot "migrate" along the climatic gradient, - they are physiographically limited. A change in hydrologic regime often results in wholesale changes in plant community structure and complete elimination of certain habitats and species. In some cases, a sufficient hydrologic change leads to disappearance of the wetland. Second, even a small decline in moisture supply at the margins results in proportionately large loss or change in wetland habitat. For example, in a circular wetland with a radius of 2 km, a decline of moisture sufficient to maintain a wetland in a zone of only 0.4 km from today's margin, results in a loss of 64% (ır2) of the original wetland surface and hence habitat. Third, many if not most, wetland species have very narrow, physical requirements with respect to moisture (duration of inundation), nutrient flux and water quality (nutrient concentration indirectly measurable by pH). We observe this in the wild by the concentric pattern of species distribution around wet zones. These species exist on an environmental tightrope, unable to compete in the terrestrial realm, squeezed against an inhospitable aquatic environment. Changes in any of the elements of the wetland gradient and indeed changes in the steepness of the gradient could easily eliminate their niche along the gradient. Further, the species cannot easily escape because habitat opportunities are not continuous. Fourth, of all the habitat systems, wetlands may be the most directly affected by human activity because there will likely be increased demands for water. Water will be the critical natural resource in the coming decades. At the local/rural level, people will demand more from their small neighbouring wetlands, lowering their water table or digging them out. Also, drying wetlands will be seen as excellent sites to expand agriculture under droughty conditions. On a community or regional scale, water consumption may drastically reduce water levels in large wetland reservoir sources or reduce the ground water table. The latter effect has already been noticed in places such as Grand Forks in the southern interior, where, according to local residents, water levels in wells are lower than ever in living memory. A further concern is that many water sources are already polluted beyond health standards. Hence, people and municipalities must look for alternate sources of clean water: Look out wetlands!! What kinds of changes can we expect in wetland systems as a result of warming? First, the area covered by wetland and aquatic environments will shrink. My work (Hebda 1982) and that of Rolf Mathewes and students (Mathewes and King 1989) at Simon Fraser University reveals that, especially in southern and central interior B.C., lakes, ponds and wetland systems shrunk or dried-up completely during the warmest part of the early Holocene about 10 000 - 8 000 years ago. Finney Lake in the Hat Creek Valley provides an excellent example. Today the lake covers about 15 hectares, and, has a mean depth of 3 m and a maximum depth of 5 m. During the early Holocene there was little or no water in the lake - a decline of 10 m (Holocene sediments included) in lake level and a decrease of 95-100% of lake volume. This aquatic body obviously could not support the aquatic flora it does today. At best it might have harbored alkaline or salt-tolerant taxa in the bottom of the basin, like much smaller and shallower basins do today. Even a small climatic change resulting in a net moisture loss can create dramatic changes in a lake and in dependent plant species and habitats. It is my experience that many interior lakes have a wide, shallow, littoral platform. At the lakeward edge of the platform the bottom drops off. Finney Lake exhibits such morphology. During the winter season the lake is full to the limits of the landward edge of the platform. As the lake level drops during the summer, ephemeral species grow along a marginal community gradient forming typical concentric vegetation zones. Many uncommon species thrive in this setting. Under slightly drier conditions this zone may revert largely to a terrestrial setting and the peculiar ephemeral habitat disappear or be greatly reduced. Furthermore, it is just in such a marginal zone that cattle in the interior trod the soil into mush and weedy species thrive. With changing climate the natural native species of this zone may be decimated or extirpated and weeds such as Taraxacum officinale Weber and Tragopogon spp will thrive. Species such as the rare fern Marsilea vestita Hook. and Grev. would possibly disappear. Ammania coccinea Rottb. and Rotalla ramosior (L.) Kuehne could likewise be extirpated from British Columbia. Organic wetlands will experience great changes. The types of changes possible are recorded in wetland deposits along the coast of B.C. Banner et al. (1988) illustrate organic sediment records from several wetlands in the Pacific Temperate Wetland Region and the Pacific Oceanic Wetland Region. These sequences reveal major changes in organic sediment type, hence environment of deposition and hence plant habitat and community composition since deglaciation. Notable is the occurrence of a slimy, humic horizon, usually in the mid to lower part of the sequence which at "Bear Cove Bog" near Port Hardy occurs during the early Holocene Xerothermic Interval (Hebda 1983). It is also notable that the development of Sphagnum- dominated wetland is a mid to late Holocene phenomenon. I believe that many, if not most, of the changes in organic wetland sequences on the coast are primarily the result of climatic change and related hydrological and hence community changes. Hydrosere succession is an ongoing and important element too. But, the last 5000 years has seen paludification (wetland expansion into non-wetland areas) rather than loss of wetlands due to forest encroachment on the coast of British Columbia (Quickfall 1987). These observations imply that we may well experience the decline of the paludification process with the predicted climatic change. Certainly the conditions favourable to Sphagnum growth and peat accumulation will be reduced. In the long run, this may result in a major reduction of acidic bog and bog forest presently so typical of the Pacific Oceanic Wetland Region. With this will come a major reduction in appropriate habitat for acid-requiring species. In general, I would expect a shift from acidic, rain-water fed systems to less acidic and even alkaline ground-water fed wetland systems, resulting in a shift from bogs and bog woodlands to fens, swamps and marshes. The changes would be most profound in regions with greatest summer moisture deficits such as the Coastal Douglas-fir Biogeoclimatic Zone of southwest British Columbia. In the interior, fens and boggy fens may shift to alklaline marshes especially at low to mid elevations. Species most seriously affected would be those of hyperacid bog environments or boreal species surviving in southern bog enclaves, such as cloudberry (Rubus chamaemorus) L.) in Burns Bog, Fraser Delta. Species of hypermaritime acidic habitats would be most at risk. However, even with major net reduction in moisture availability, the hyperoceanic settings may remain moist and cool enough for these species to survive. Studies on the Brooks Peninsula (Hebda in press) reveal that moist acidic subalpine-alpine communities survived during the Xerothermic interval. Within those were preserved several of our rarest species such as Ligusticum calderi Math. and Const., Geum schofieldii Calder and Taylor. If temperature increases and moisture decreases more than that during the Xerothermic, then we may loose these botanical treasures forever. What do we need to do to help plan for and mitigate possible changes in our wetland flora? First, we must develop a functional classification system and a model for wetlands that accommodates principles of succession and predictability. I briefly mentioned such an approach in Banner et al. (1988) where wetlands could be viewed within three interdependent aspects of the associated water supply: 1) water source; 2) water flow rate; and 3) water table depth. I called this the "hydrologic gradient" approach. Water source determines the nutrient levels of the water supply to the wetland. Ground-water dominated wetlands, such as springs, have a rich supply of nutrients, especially bases, because the water has flowed through the ground of the watershed, dissolved nutrients, and brought them to the wetland plants. At the other extreme of the spectrum is nutrient-poor rain water such as feeds raised bogs. A simple, although perhaps not ideal way to measure nutrient availability could be along a ground water ph scale. Water flow, where combined with nutrient measurement in soil water, provides a measure of nutrient flux. Thus stagnant water, even if rich in nutrients, supplies less nutrients for plant consumption per unit time than moderately flowing low nutrient water. Basin bogs show the results of this effect clearly. The centre of the bog will be occupied by species and communities typical of nutrient poor circumstances. The lagg or margin, where water flows relatively quickly, is occupied by nutrient-requiring species such as willows and red osier dogwood (Cornus sericea L.) Water table depth establishes the degree of inundation of a wetland. This parameter influences how much organic matter accumulates or breaks down above the water table. One way of measuring it is by an estimate of the number of centimeter-days per year above or below the ground surface. Water table depth or some measure of inundation would likely be the parameter most affected by global change. Today's wetland communities and indeed species distributions could be plotted on a multi-dimensional coordinate axis system according to the measure of quantitative parameters which reflect the three factors of the hydrologic gradient. By inspecting the arrangement of communities or species on the gradient it should be possible to predict in which direction a wetland would change given a change in one of the hydrologic gradient variables. Of greatest interest with respect to global warming would be the water table axis. Another step we must take to develop an understanding of wetlands is to study and classify interior wetlands. One of the great omissions of National Wetlands Working Group (1988) The Wetlands of Canada was the absence of a description of Montane Wetlands. These wetlands are most important in southern British Columbia. We know very little about these habitats yet it is these that are most likely to suffer the greatest change. These wetlands need to be described, classified and the occurrence of species noted.