Don’t Neglect the Value of Crown Closure to Improve Wildfire Resilience and Wood Quality

Managing post harvest fuel loads is the best thing we can do to protect our next generation of trees but there is more we can do.

In Lodge-pole Pine (Pl) stands, we should be establishing earlier crown closure through increased density of planted and natural trees. This will more closely mimic fire origin forests which will improve the stands resilience to wildfire, improve log quality and shorten the rotation time to get the stand to merchantable size. It appears to me that 6000 to 10,000 sph seems to be a good intial density. Post harvest fuel reduction treatments may acheive this without increasing required planting.

I am just back from working with heavy equipment groups on a few local wildfires. We stopped a Rank 4/5 wildfire by constructing a control line in a +/- 100 ha. stand of lodge-pole pine (Pl) that was clearcut in 1972, broadcast burned and then planted and juvenile spaced (thinned). There was crown closure so grass growth was limited with no understory. This was a stand of merchantable sawlogs after only 50 years. We figured we could get two 20 ft logs out of each tree if we needed to salvage log.

The BC Wildfire Fire Behavior folks made a good video on the fire and control line. It is great to see them considering the interactions of land management treatments with wildfire and looking for win/win opportunities.

Over the years I have seen a substantial amount of younger planted stands in the Southern Interior burn severely. I have walked them to try and figure out why.

The value of post harvest fuel reduction in protecting the next generation of trees is well known and accepted. However, even on sites with low post harvest fuel loads I often see high losses and damage to regenerated trees. On a fire in July this year I saw a patch of 5 year old regen Pl destroyed on an area that burned very intensely in 2018.

As discussed in the video I feel grass is often a main contributing factor. Grass dries quickly, ignites easily and allows fire to spread rapidly. Even though the fire intensity associated with grass fuels is relatively low it is enough to kill most young trees.

The current common inter-tree planting distance of +/- 1.5 to 2.5 meters between trees allows a lot of moisture and sunlight to hit the ground for the first 20 years of a planted trees life. This contributes to grass growth and / or raises the surface fuel temperature and lowers the humidity more than in a denser stand. Grazing may help keep the grass down but cattle damage and wide spacing may lower the eventual log quality.

Pl planted at low density. Heavy grass, large branches to ground, big rings, heavy taper. Very susceptible to wildfire and very low lumber quality

I believe the stocking standards for wildfire risk reduction projects are even lower which may make the situation worse.

We are always going to suffer losses during extreme fire weather but I think we could reduce the frequency and severity of the damage by increasing stand density to reduce grass ingress.

Background Info on Lodge-pole Pine

1. We are planting trees to produce lumber at some point in the future. Pl is the most widely planted species. We can achieve a lot of other objectives but lumber, and associated products, are what pay the bills. Woodlots, Community Forests, First Nations Licensee’s and large corporations all need to sell logs to produce lumber to make money and continue operating.
2. The best quality and highest value logs for lumber have tight grain, no (or few) knots, and little taper.
3. Increasing the height of a tree increases the volume of the tree far more than increasing the diameter of the tree.
4. Pl is a fire origin species. It needs fire to regenerate. After a fire it is very common to see 100,000 stems per ha. of natural regeneration.
5. When the branches of young Pl touch the branches of an adjacent tree something happens and that tree will start putting growth on it’s leader (top) and start to shed its lower limbs that are no longer gathering sunlight.
6. Fire origin Pl is tall, skinny, has very tight grain, very few branches or knots and very little taper.
7. Hence, a stand of dense, tall, small diameter Pl generally has far more volume and value than low density stands.
8. Clear cut harvest is the method that most closely resembles the effects of wildfires on a site.
9. Juvenile spacing (thinning) of 20 year old stands was very common treatment to try and encourage diameter growth. In fact it generally devalued the next stand substantially by delaying crown closure and allowing prolonged branch growth.

+/- 200,000 sph Pl germinants the spring following a wildfire

+/- 250,000 sph 10 years old fire origin Pl stand. Crown closure has occurred. Lowest limbs starting to shed.

No grass under the trees. Reduced chance of re-burn except in extreme fire weather.

Note: +/- 10,000 sph expressing dominance over neighbours (blue arrows)

Fire Origin Pl stand at 30-40 years – Tall skinny stems, no branches, little taper. Self spacing and pruning.

Fire origin Pl stand at +/- 60 years. Substantial surface fuel but crown closure lowers temp and raises RH. This also accelerates decompositon of surface fuels.

Reduced potential fire behavior in all but the hottest, driest and windiest conditions.

Fire origin Pl stand at 60-80 years. Crown closure has accelerated decomposition of surface fuels. Few ladder fuels. Lower chance of crown fire.

Fire origin Pl stand at 80 -100 years. Reaching the end of its life span. Understory of Sx and Bl ingressing. Highly susceptible to pests, disease and fire.

Pl 100 plus years – tick tock tick tock

These pictures are from various locations but I am sad to say I have observed most of this progression first hand in a stand of trees next to our family property.

I have laid out and harvested two moderate to high volume stands of fire origin Pl that were younger than me (<65 years). The stands were dense, the trees were tall, had little taper, had no branches for 20 meters, only the top 20% had live limbs, the grain was tight with only a few small knots. The lumber produced from these stands was the highest quality and value with almost all going to engineered wood products.

Our industry has ever increasing demands on how we operate and an ever diminishing area to work on. Based on my walk throughs I feel that by establishing and maintaining crown closure very early we will improve a stands wildfire resiliency, increase the eventual lumber volume and quality while shortening the time required to get a stand to marketable size.

These suggestions are obviously generalizations and are all based on my walkthroughs and initial ponderings. They will need some real science to solidfy my thoughts.

This may apply to Firesmart and fuel mod projects as well. The current standard of 3 m. intercrown spacing has me concerned. Increasing the value of logs from fuel modified areas may make treatments profitable.

Give me a call if you want some more information or drink some coffee and go look around.

Doug