What impact did 2018’s wildfires have on orchards?

Kat Jarvis-Shean, UCCE Orchard Systems Advisor, Sacramento, Solano & Yolo Counties

Luke Milliron, UCCE Farm Advisor for Butte, Tehama, and Glenn Counties

Rick Buchner, UCCE Farm Advisor Emeritus, Tehama County

Originally published in West Coast Nut

As has widely been reported, 2018 was the most destructive wildfire season on record in California. Quite rightly, primary attention has been paid to the direct loss of lives, homes and livelihoods that resulted from these fires. Less discussed are the potential impact downwind of these fires. Many growers, managers and advisors have been asking what impact weeks of wildfire smoke may have had to their fruit and nut orchards.

We’ve gone in circles recently trying to get a good answer to that question. We can’t point to a solid number or a conclusive experiment that will answer the question of how wildfire smoke impacts orchard production. Most of the research on impacts of smoke has focused on human health. Research into air pollution effects on trees has largely focused on urban air pollution (which is a slightly different mix of contaminants). Additional research reports describe air quality impacts to forest productivity, and a few studies have been done on impacts to wine grape berries and wine quality.

The best we can do is piece together what we know from other systems to estimate, theorize and speculate at what might be happening to fruit and nut orchards when they experience prolonged periods of smoke. So what do we know?

Unusual, late season leaf discoloration in an otherwise healthy walnut orchard in Tehama County. Photo: Allan Fulton

How Wildfire Smoke Can Hurt Plants

Ozone (O3) is a common pollutant in wildfire smoke. Surface level (“tropospheric”) ozone enters tree leaves through the same stomata that let in carbon dioxide for photosynthesis and let out water. Ozone inside the leaf is toxic. It splits into oxygen and peroxide, oxidizing proteins and lipids of cell membranes (chemically combining with them), interfering with enzyme processes. This is bad news for trees. Trees need carbon dioxide to make sugars during photosynthesis, but they don’t want ozone coming in and throwing a wrench in the works.

Research with peach trees in China has found elevated ozone decreases photosynthetic rate and the content of pigments (like green, photosynthesizing chlorophyll), while increasing antioxidant enzyme activity (Dai et al, 2017). In other words, the plant is dedicating resources to fighting and repairing the oxidizing effects of ozone, while also having its sugar production decreased by ozone damage. Illinois forestry researchers have found that increasing ozone from ambient levels of 40 parts per billion (ppb) to 64 ppb decreased tree biomass by 11% (Wittig et al, 2009). Elevating it to 97 ppb decreased biomass by 17%.

Examining the ozone records kept by the California Air Resources Board (https://www.arb.ca.gov/aqmis2/aqdselect.php), there were days in early August 2018, in the midst of the Carr Fire, when daily average ozone in Tehama County was higher than 70 ppb. This is squarely in the range of ozone that decreased biomass in deciduous trees in the Illinois study. UCCE Advisors Allan Fulton and Rick Buchner shared photos from Tehama County in October that are consistent with cellular leaf damage caused by high ozone exposure.

On the other hand, one can find ozone levels recorded that high for the same period in 2016, when corresponding symptoms were not recorded. Additionally, symptoms of leaf damage caused by high ozone exposure can be difficult to differentiate from other causes, such as over-irrigation. The story gets more complex when we take into account that chronic exposure to elevated ozone decreases stomatal conductance. The Illinois forestry study, for example, found reduced transpiration rates in trees exposed to smoke. It may be that smoke-exposed orchards were being over-irrigated because there were not transpiring as much as they usually would mid-summer. Given this variety of factors that can cause the same symptoms, controlled studies and purposeful measurements are needed to better grapple with what effects smoke has on tree functions.

Sulfur dioxide (SO2) and nitrogen dioxide (NO2) are two other components of wildfire smoke. Both can interfere with important steps in the energy-making process of photosynthesis. However, neither are as widely researched as the impacts of ozone.

How Wildfire Smoke Can Help Plants

In orchard production, one of the potential benefits of smoke exposure is the increase in aerosols. Fine mode aerosols (particulate matter less than 2.5 μm in diameter, also known as PM2.5) diffuse solar radiation. We’ve heard anecdotally from many walnut growers and processors that smoky years are the best years for nut quality, possibly because there are fewer sunburned nuts and less discoloration-producing heat stress. We don’t know of any experiments that have tested this, but the logic makes some sense if the smoke exposure is not so prolonged as to lead to other problems.

Another impact of increased aerosols can be increased photosynthesis. You might think plants want all the solar radiation they can get for photosynthesis, but actually, on a clear day in the height of a Central Valley summer, that solar radiation is often more than the plant can utilize. Aerosols promote photosynthesis by enhancing diffuse eradiation. In essence, these aerosols can soften the intense radiation of peak summer. But there’s a balance, since too much particulate matter interferes with photosynthesis. A recent study on how aerosols impact plant activity cites maximum plant photosynthesis as occurring when the diffuse fraction is between 0.4 to 0.6, whether that diffusion results from cloud or aerosols (Yue 2018). Modeling work in this same paper found that photosynthesis enhancement by aerosols in smoke can partially offset the decrease in photosynthesis that results from increased ozone in smoke. However, aerosols only weakened the ozone-caused photosynthesis decrease by about 8-30%. In other words, the ozone in the smoke decreases photosynthesis much more than aerosol light diffusion increases it, resulting in a significant net decrease in photosynthesis from smoke exposure.

So where does that leave us?

On the whole, smoke and its byproducts can have positive and negative impacts to plant health and production. Acute (short but severe) and chronic (steady, prolonged) exposure can have different impacts. This summer’s fires may have benefited walnut quality but decreased energy production, which may have impacted tree growth or carbohydrate reserves for the spring. There are still more questions than answers. Whether we like it or not, we’re likely to have plenty more opportunities in the future to collect data and observations on what smoke does to our trees and their production.

References:

Dai, L., et al. (2017). "Differential responses of peach (Prunus persica) seedlings to elevated ozone are related with leaf mass per area, antioxidant enzymes activity rather than stomatal conductance." Environmental Pollution 227: 380-388.

Wittig, V. E., et al. (2009). "Quantifying the impact of current and future tropospheric ozone on tree biomass, growth, physiology and biochemistry: a quantitative meta-analysis." Global Change Biology 15(2): 396-424.

Yue, X. and N. Unger (2018). "Fire air pollution reduces global terrestrial productivity." Nature Communications 9: 5413.