Less obvious, but worse by far, is the toxic effect salt has on roots. Salt may get directly deposited onto a tree s root zone by plowing, etc., or runoff may wash salt into the soil. In high enough concentrations, salt will kill a root system. At lower concentrations salt makes water unavailable to tree roots, producing drought stress even in moist soil. Roots need to get oxygen from soil pores, and healthy soil forms tiny natural clumps which allow pore space.
Salt breaks the chemical bonds holding the clumps together and the pore spaces close.
Somewhat confusing is the fact that salt damage may not become evident until late summer, when it often causes brown, scorched- looking leaf margins. Deicing salt may be the last thing on people s minds at that point. Salt damage is cumulative- each year s injury causes further stress to a tree, until eventually symptoms become evident and physical decline begins. Some tree species are very sensitive to salt and are easily injured, for example sugar maple. Other species, like honeylocust, are more salt tolerant.