The young vegetation on the north side of Upper Muir Inlet in Glacier Bay National Park is developing dramatically differently from older vegetation a few km away. Today, five decades after the retreating Muir Glacier exposed my youngest study site (Site 1) to plant invasion, it supports an open shrubland of willows with most of the ground still carpeted with low-growing Dryas drummondii. At the same age (in 1995), the next older site (Site 2) had no Dryas and was a dense thicket of 6 meter tall Sitka alder shrubs. The distinct successional pathways being followed at these sites have critical ecological differences (e.g., alder is an important nitrogen fixer) and suggest that inferring ecological change from a sequence of different aged sites in this part of Glacier Bay does not work.
Can seed rain explain this?
My favorite hypothesis to explain this is that early seed rain of alder differed between the two areas. Ice margins in Glacier Bay have been retreating to the north for almost three centuries, and invading newly exposed terrain requires that plants can migrate as fast as the ice retreats. Alder was apparently doing a good job of chasing the ice along most of Muir Inlet where my reconstructions of invasion histories at four study sites confirm that alder was an important early invader. Continue reading “When alder can’t keep up”