Here is another issue arising in the aftermath of the wildfires. This question came from someone who lost his home. Some of the trees were destroyed, some damaged and some relatively unscathed:
We need your advice on site cleanup. The demolition contractor is coming to bulldoze the house next week and has asked if we need to scrape the landscape areas of the property away from the house where the ground cover was charred. Are there any considerations for or against removal of ash in those areas? Are there any risks to the tree root systems if we scrape off the top layer?
First, regarding ash, I have looked at some articles on the subject and found a few points of interest:
— Ash from wood and other plant parts has a high pH. That is, it increases alkalinity. High alkalinity can be harmful to plants and trees.
— Ash varies depending on the intensity of the fire. Lower heat intensity produces darker wood ash, which tends to have some valuable plant nutrients. Ash from higher intensity heat tends to be lighter in color.
— Ash from high-intensity fire forms a crust after wetting. This can interfere with subsequent water infiltration.
— High-intensity fire can make soil water-repellant (“hydrophobic”)
A scientific paper “Wildfire ash: Production, composition and eco-hydro-geomorphic effects” at this link: http://cesonoma.ucanr.edu/files/272305.pdf offers a close look at the subject. The introduction notes:
“As a new material present after a wildland fire, ash can have profound effects on ecosystems. It affects biogeochemical cycles, including the Carbon cycle, not only within the burned area, but also globally. Ash incorporated into the soil temporarily increases soil pH and nutrient pools and changes physical properties such as albedo (light absorption/reflection), soil texture and hydraulic properties including water repellency. Ash modifies soil hydrologic behavior by creating a two-layer system: the soil and the ash layer, which can function in different ways depending on (1) ash depth and type, (2) soil type and (3) rainfall characteristics. Key parameters are the ash’s water holding capacity, hydraulic conductivity and its potential to clog soil pores. Runoff from burned areas carries soluble nutrients contained in ash, which can lead to problems for potable water supplies. Ash deposition also stimulates soil microbial activity and vegetation growth.”
What does this mean with respect to site cleanup?
At the risk of over-simplifying, I’ll say: Do not leave a thick layer of ash over tree root zones. Check for hydrophobic conditions by pouring a little water on a sample area and watching to see if it soaks in. If it does not, you can help correct the condition by lightly cultivating the top 1 to 2 inches and applying a thin layer of straw or compost. If the current rainfall does not soak the ground, irrigate lightly to moisten the tree root zones to a depth of about 6 to 12 inches.
As always, maintaining a 3 to 4-inch thick layer of wood chip mulch is an excellent treatment for tree root zones.
As to the second question regarding the risk from scraping off the soil surface, normal caution applies. The scraping is a grade cut, a common cause of harm to trees in construction projects.
Grade cuts must be planned and limited, depending on the tree species, age, health, and proximity to the trunk.
Again, at the risk of over-simplifying, I would limit “scraping” to remove only thick layers of ash, melted irrigation pipe and other unsightly or potentially toxic debris.