LB 10-125 - Stop-and-go primary succession: Episodic vegetation development on a recent lava flow in northwestern British Columbia

Background/Question/Methods:

The 26 km² lava plain left by the Tseax Volcano is found in the Nass Valley of northwestern British Columbia, a transition zone between coastal and interior temperate rain forests. The volcanic eruption that took place sometime in the 1700s and its impacts are central to the history of the Nisga’a First Nation. The current lava landscape supports a wide variety of plant cover, ranging from bare basalt and thick blankets of lichens and mosses to closed forest dominated by mature trees. Such contrasts prompted the question, “Why are there such pronounced differences in ecosystem development more than two centuries after a single historical disturbance?” In our first year of field research, visual estimates of plant cover by species and substrate descriptions were made in 66 4m x 4m quadrats distributed randomly across 13 strata defined by differences apparent in aerial photographs. The following year, another 108 quadrats were sampled (primarily arrayed in transects away from lava flow edges and from roads), increment cores were taken from apparent cohorts of trees, nearby streamflow records were examined, surface fuels were monitored for moisture content, and dataloggers recording air temperature and humidity were installed in selected vegetation patches.

Results/Conclusions:

Vegetation analysis revealed significant effects of substrate differences as expected, but also a pronounced legacy of recent disturbance events. A’a surfaces often support continuous lichen cover dominated by the cyanolichen Stereocaulon paschale, which can be replaced by the moss Racomitrium lanuginosum if left undisturbed. Pahoehoe surfaces are less vegetated, except in cracks and collapsed lava tubes, which support several fern, shrub and tree species. Vascular plant cover is negatively related to distance from lava flow edges, and exotic plant cover is primarily associated with roadsides. Evidence of recurrent wildfires was observed on the well-travelled lava plain, which have repeatedly reset the pattern of succession there. The distinctive climate of the basaltic plain means that surface fuels (especially moss blankets) dry out more rapidly than expected, making them susceptible to ignitions. Conversely, other disturbances such as localized flooding and dust from road use lead to silt deposition that promotes the establishment of trees and other vascular plants. As extreme weather resulting in wildfires and floods increases, vegetation development on the lava beds will be set back in some places and accelerated elsewhere. This complexity challenges textbook descriptions of linear ecological succession and foretells an uncertain future in a changing climate.