Dealing With Forest Impacts After a Hurricane
This article was submitted by Robert P. Trickel Forest Health Branch Head NC Registered Forester
After the winds settled and the floods receded following Hurricane Floyd in
1999, there were questions from landowners and land managers about the
impacts of flooding on our forests. If you work in an area recently
flooded by Hurricane Matthew, you will likely hear some of these same
- · What’s going to happen to my trees?
- · Are my trees going to die due to being flooded?
- · Should I harvest my forest that was just flooded?
- · Some guy just came by and said I should cut my trees down because the flood will cause [pick a tragedy].
- What should I do?
The answer to the first three questions is easy. For all three, you can
confidently answer, “I don’t know” or “It depends.” There are way too many
unknowns to give a more definite answer. Impacts to trees from flooding
are dependent upon a complex mix of :
Tree species — some trees (bottomland) are more tolerant of flooding
and are thus not as likely to be impacted, but that does not mean that
upland trees will automatically die if water covers their roots for a
Length of flooding occurrence – the longer tree roots are deprived of
atmospheric oxygen, the more likely a tree will suffer,
Depth of inundation – what is flooded, just the roots? Branches?
Leaves? Small trees and shrubs that are totally submerged can have
trouble with silt clogging stomatas,
What is in the water – Including silt? Pollutants? Nutrients?
Time of year – impacts may be greater during the growing
season–especially early in the growing season (active photosynthesis
So, how do you handle the fourth question above? Unless you see obvious
signs of a problem, the only defensible recommendation for landowners is,
“wait and see.” Impacts of flood exposure are neither as evident nor as
desperate as with wind damage (wind throw and/or breakage). Tree health
and value will not likely take a nose dive overnight and recommendations to
manage or salvage immediately are seldom necessary.
In response to these types of questions, and the lack of available research
related to flooding of upland forests, the then Pest Control (now Forest
Health) Branch teamed up with the Southern Research Station to do a flood
impact study after Hurricane Fran. Below is a summary of this study.
Hurricane Floyd reached North Carolina on September 16, 1999, and dumped
nearly 20 inches of rain in parts of the coastal plain that had been
saturated by Hurricane Dennis a few weeks earlier. This created an inland
flooding disaster that caused rivers to crest well above flood stage and
inundated urban areas, agricultural land, and both bottomland and upland
forests. Two years after Floyd, 95 Forest Inventory and Analysis plots
(with historic data) in the coastal plain were intentionally re-measured to
document immediate impacts of flooding to upland forests. The plot sites
contained upland forest types including oak-hickory, oak-pine and loblolly
pine forests that contained pulpwood or sawtimber sized stands. Just over
half of the plots had been flooded during the hurricane and the rest were
spared from flooding. Comparisons of the plots showed there were no
immediate or dramatic effects of flooding related to tree mortality,
leaning angle of living trees, or abundance of saplings after almost two
years. There were, however, indications that crown conditions were
somewhat poorer in the flooded areas. This was expressed as slightly
higher crown dieback and lower crown density of flooded trees. While these
crown observations were statistically significant, it was unknown whether
they were biologically significant.
Since every stand and every flood event is different, only time will tell
if there will be any long-term impacts related to flooding. For landowners
and forest managers, this means that post-flood management decisions can be
justifiably delayed and follow-up visits are recommended to determine
long-term impacts to forest health and management.
If you have any questions, please do not hesitate to contact me.
A similar blog article will be posted to
week—this post is written for more general audiences.
Robert P. Trickel
Forest Health Branch Head
NC Registered Forester No. 1071
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