Impact of the 2012 drought on woody vegetation invading alvar grasslands in the Burnt Lands Alvar, eastern Ontario

Abstract

Counts of dead and living trees in open alvar indicated that the 2012 drought had a significant impact on the Burnt Lands Alvar in eastern Ontario, with mortality of woody species ranging from 10% to 100% at open alvar sites. The maximum age of dead trees suggested that the most recent drought of similar impact occurred more than 35 years ago, possibly in 1974. Some of the killed junipers were 87–90 years old. All dry periods indicated on a summer rainfall diagram may not be severe enough to kill woody vegetation, but a drought causing an average 50% kill of woody vegetation in open alvar may have occurred at least once every 30 years over the past century. Consequently the encroachment of woody species into alvar grassland appears not to be a threat to this habitat. Open alvar could have existed on the landscape as isolated occurrences since early postglacial times, if the climate fluctuated in the distant past as it has recently. With evidence for drought in a fluctuating climate to maintain the disjunct occurrences of midwestern plant and animal species, these occurrences may be considered as reliable indicators of a more continuous distribution of such species more than 10 000 years ago.