 |
| Bigleaf maple leaf (Acer macrophyllum). Source |
Digger Spade, MG Plant Detective
In mid August, not just one but three clients came in with concerns about the health and/or death of bigleaf maples (Acer macrophyllum) on their properties. Their concerns ranged from what had killed the dead ones to could anything be done for those that look likely to follow suit.A search of the DNR website revealed that the bigleaf maple problems have been of concern for several years and that quite a lot of effort has gone into trying to answer our client’s questions. These trees have been known for years to be susceptible to Verticillium wilt and to Armillaria root rot but field surveys made since 2011 didn’t find the former in the dying trees and the latter in only 11% of those studied. Similar inconclusive findings have been made for other possible biotic causes. A long-term study is underway that may find a biological agent that can be held responsible and that may be controllable. But as a recent DNR report indicated “the truth is, we don’t know exactly what’s going on.”
Our first thoughts about the problem had been along the lines of “here’s another result of our several years of ever longer droughts.” At the time of the inquiry we had had no significant rain for four months and last year we had none for more than ten weeks. This spring we had many dead hemlocks in the county and many other trees showing signs of severe drought stress.
To maintain healthy leaves a plant has to draw sufficient water from the ground to maintain turgor pressure and keep up with the transpiration rate that is determined by the environment of the leaves. That rate is very sensitive to temperature, humidity, air movement etc. An increase in temperature of 20 degrees F may increase transpiration demand threefold. And when the ground in the root zone is moisture-free, roots die, so when water becomes available the tree cannot use it. So when there has been no rain for weeks and temperatures are setting records, a bigleaf maple probably wishes it had been born a conifer, though not a hemlock.
After talking these things over with the clients. We advised the one with a small tree that still looked as though it might survive to water it outside the drip line with a soaker hose, to do it slowly to give the dry soil time to absorb the water, then do it again every week or so 'til nature takes over. We also gave the clients the DNR link so they could get reports on the ongoing research.
There is no doubt that drought is a challenge for all trees and in Whatcom many are too close together, making the competition for water even more severe. Nature is making the case for xeriscape landscapes in the Pacific Northwest.
Digger’s “solution” in his junior forest is to cut about a third or more of them down. A solution with little appeal to most.
Resources
Bigleaf Maple Decline (DNR)
https://dnrtreelink.wordpress.com/2016/08/10/bigleaf-maple-decline-update-and-next-steps/
Water Cycle Transpiration (USGS)
http://water.usgs.gov/edu/watercycletranspiration.html
Hardy Plants for Waterwise Landscapes (WSU)
http://public.wsu.edu/~lohr/wcl/
