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 
	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).