An Ounce of Prevention

Thanks to Grant Kirker for writing this article spotlighting how homeowner’s can better maintain their wood decks. Kirker is a Research Forest Products Technologist at FPL in the Durability and Wood Protection Research unit.

The experimental test block setup used to examine the role of accumulated leaf litter on material performance and wood durability in aboveground exposure. The test block is surrounded by untreated pine and the channel between the block and frame is filled with leaf litter. The two black fittings on the top are able to take repeated moisture measurement using a pin-type moisture probe. USDA Forest Service Forest Products Laboratory

Benjamin Franklin is credited with the saying “an ounce of prevention is worth a pound of cure”, which suggests that taking steps to avert a problem before it starts is far better than taking corrective steps after the problem arises. Forest Products Laboratory (FPL) Wood Durability and Protection researchers, in collaboration with research partners at Oregon State University, attempted to apply this concept to a situation close to home for many homeowners—the wooden deck.

The global wooden decking market in 2020 was valued at $15 billion USD, of which the North American markets made up about 35%, or $5.25 billion USD1. In a 2019 National Association of Homebuilders (NAHB) survey2, 20.3% of all new houses included decks. Although this estimate is lower than historical averages, the global pandemic has led to an increasing interest in outdoor living spaces, which will likely cause this market to increase. Wood is an excellent building material for outdoor decking because of its  reasonable cost and low maintenance requirements; if properly installed and maintained, a wood deck can provide a long-lasting benefit to the homeowner.

This is the complete array of test samples. This box was mounted on top of an above ground rack ~3 feet off the ground and exposed to full weathering at our field site outside of Madison.

However, some periodic maintenance is an important component of owning a wood deck. Over time, organic detritus, most commonly leaf litter, accumulates and settles into cracks and crevices and can cause situations that closely resemble ground contact due to excess moisture, nutrients, and fungal material. This research, published in the journal Microorganisms, examined the role of accumulated leaf litter on material performance and wood durability in aboveground exposure.

Simulated deck microcosms were constructed to mimic worst-case situations for wood rot in aboveground exposure and were installed outdoors at two locations in South central Wisconsin. Untreated pine was exposed to both fresh and aged leaf litter to determine if decay capacity of leaf litter increases as it ages. Moisture readings of the wood were collected over time along with visual decay ratings at 4, 13, 24, and 41 months. Wood sawdust and leaf litter samples were taken at 25 and 41 months of exposure, and DNA was extracted and analyzed using amplicon-based sequencing to identify fungal inhabitants.    

A partial shade cloth was also placed over the samples to reduce drying and facilitate faster development of decay. USDA Forest Service Forest Products Laboratory

Results showed that moisture and resultant decay were accelerated in the presence of leaf litter and that onset of wood decay happened most rapidly where aged leaf litter was present. Analysis of the fungal sequencing data showed that leaf litter contains a significant number of fungal species that are available to inhabit and decay the wood and that the species composition shifts over time to include more basidiomycete fungi, which are primarily responsible for wood decay.

The treated wood in this study showed signs of soft rot in later samplings. The sequencing data from the treated wood was not presented in this paper, but analysis is underway. It’s important to note that untreated pine is not recommended for above ground decking especially exposed to the elements as it will readily decay in only a few years.The reasoning for using it in this study was to highlight worst case scenarios and to eventually compare to pressure treated materials. For more information on pressure treatments that are approved for use above ground and in close to ground contact, consult the most recent AWPA book of standards or check out GTR 275. USDA Forest Service Forest Products Laboratory

Some overlap was seen between those species found in the leaf litter and wood, but wood with leaf litter absent showed dissimilar fungal communities. These results highlight the importance of incorporating a maintenance schedule that minimizes accumulation of leaf litter to prolong the service life of the wood.  The published results from this study are for untreated wood only, but analyses of samples from preservative-treated wood are currently underway.

An ounce of prevention goes a long way for wood to have a more robust service life. For more on leaf litter and its role in wood decay, read the entire journal article: “Role of Leaf Litter in Above-Ground Wood Decay.”

This shows a great example of what the addition of leaf litter can do. The moss growing on the litter indicates extremely moist conditions and fertile substrate. Even windblown seeds are beginning to sprout (middle right-hand side) and the abundance of fungal fruiting bodies (Dacrymyces spp.) shows that the wood is heavily decayed to the point where the fungus is moving on. USDA Forest Service Forest Products Laboratory
Carpenter ants can often be found in rotted wood and this test was no exception. The ants don’t actually eat the wood instead they excavate galleries in the wood to lay eggs. For this reason carpenter ants are considered a secondary pest of wood because you typically have an underlying moisture or fungus problem, unlike termites which directly feed on the wood.  USDA Forest Service Forest Products Laboratory
Here is a close up of what untreated wood looks like at the end of the test. The wood is water soaked and you can see evidence of fungal mycelia inside the wood, mycelia are the threadlike appendages of fungi and how they move, eat and defend themselves. This wood rot is indicative of white rot fungi and the fruiting bodies on the surface are most likely Dacrymyces spp. which was detected in the sequencing data. USDA Forest Service Forest Products Laboratory
This is the fruiting body of a wood decay fungus on the surface of an untreated sample. Seeing fruiting bodies on wood, especially in construction, is usually a bad sign because that indicates the fungus has already used up most of the resources in the wood and its ready to move on. The gills or pores on the underside contain copious amounts of fungal spores that are spread by wind or splashing water. Once the spores land on a damp surface, they germinate and the cycle repeats. USDA Forest Service Forest Products Laboratory


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