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 ****************************************************************************** Shoreline Vegetation The predicted 1 m rise in sea level will have significant effect on shoreline habitats and flora. Higher water will drown low-lying coastal plant communities and substantially modify shoreline morphology, resulting in the redistribution of species and possibly the loss of some. The greatest impact will be on estuaries. All estuaries will be inundated to some degree. The inundation may be gradual, but also there will be catastrophic inundation and erosion as winter storm surges roar across the deltaic platform. Changes in distributary channels will be sudden through a process called channel avulsion, where at a high-water phase a channel suddenly finds a less resistant path to the ocean. In major estuarine systems, where dikes have been constructed, the impact behind the dikes will be reduced, but this will concentrate the effect in the undiked portion of the estuary with strong impact on, if not destruction of, plant communities. In some cases, species may be able to disperse up the floodplain and re-establish populations. In other cases, such as the Fraser Delta, there is evidence (Williams and Hebda 1991) that the estuarine part of the flood plain may very well build up (aggrade) quickly enough to maintain the estuary surface at sea level. In smaller estuaries, with low sediment supply, this may not occur. Another problem arises because many estuaries and indeed coastal wetlands just above sea level are bounded on the landward side by urban or rural land. If sea level rises there is no physiographic continuity for the wetland to re-establish further up the gradient. Dikes serve as such an artifical environmental boundary too. In such cases the estuarine systems or wetlands will simply be squeezed out. Some species may persist along the steep gradient of the dike face or natural banks. It would seem, therefore, that the greatest impact would be in small constrained estuaries or highly developed estuaries. Any rare or endangered species in such settings would face extirpation. Henderson's checker-mallow (Sidalcea hendersonii Wats) would be under considerable risk under these conditions. One way in which such extirpations can be mitigated is through the provision for natural estuarine expanses or embayments excluded from diked land. These embayments must contain sufficient area and elevational gradient to accommodate upslope dispersal and re- establishment of estuarine communities. In other shoreline environments, the danger is that a rapidly rising sea level or more specifically a catastropic storm surge will destroy a population of rare species. Almost all of the populations of California bayberry (Myrica californica Cham.) in B.C. could be eliminated this way. Two taxa of sand verbena (Abronia latifolia Eschsch., Abronia umbellata Lam. subsp. acutalata (Standl.) Hitchc. could similarly be affected. More common species would very likely simply re-occupy newly created shore zone habitats. Historically speaking, sea-level change has been a normal process with rapid submergence and emergence during the late Pleistocene and early Holocene in B.C. (Clague et al. 1982). There was a relatively major sea-level rise of about 10 m along the southern B.C. coast between 7 000 - 5 000 years ago. Paleoecological studies reveal that some shoreline taxa (e.g Salicornia) survived, but we do not know how many were lost or severely decimated. The last 5 000 years has seen relatively stable shorelines and a well developed habitat zones. Notably, the predicted rise will occur about twice the rate (1 m in 100 years) experienced between 7 000 and 5 000 years ago (1 m in 200 years).