How Wildfire Affects Tree Physiology

• Wildfire effects on trees depend on fireline intensity and residence time and can occur from heat transfer or smoke. To understand how this process unfolds, we dive deep into the study of how wildfire affects tree physiology.
• Wildfire effects can be first-order or second-order and are evaluated based on carbon and water transport and are determined by gas exchange rate change and crown, stem, and root damage.
• Trees either succumb to first-order effects immediately or survive with physiological impairment depending on fire intensity and duration.
• Second-order effects can gradually enhance growth, functionality loss, or mortality.
• Tree physiological responses to fires weaken trees, making them susceptible to disturbances like drought, pathogens, and insects.

Wildfires are common global natural disturbances whose frequency and severity increase due to climate change. Fire causes varying degrees of harm to trees and triggers a cascade of responses. Knowing specific tree physiological responses and traits that help resist damage is crucial for anticipating the postfire ecosystem and developing informed risk assessment and management tools. Therefore, tree physiological responses to fire are the focus of intense research. Find out what we know so far.

Wild Fires

Anthropogenic-driven climate change has increased wildfires. For example, burnt areas have increased by 320% in California from 1996 to 2021. The extent of land affected in the USA has grown yearly, with the most extensive regions burnt in fires since 2004. According to the EPA, the USA spends $1 billion annually to fight wildfires, reaching $2.3 billion in 2020. Wildfires have killed over 1000 firefighters and caused significant health problems to people and society, besides destroying biodiversity, wildlife, and property.

Due to climate change, wildfires start early, with the worst damage coinciding with drought and hot years.

Figure 1: “Overview of fire effects in trees. Heat transfer into the crown, stem, and functional traits mediate root tissues and can immediately lead to first-order injuries, which can potentially induce second-order effects. Both first- and second-order effects can lead to physiological impairments in tree carbon (C) and water relations, consequently limiting functioning and growth. Depending on postfire environmental conditions and species-specific traits (e.g., abilities to balance C and/or water restrictions), affected trees may either recover from postfire limitations or succumb to fire legacy effects.” Bär et al. 2019. (Image credits: https://doi.org/10.1111/nph.15871)

Degree of Wildfire Impact

The degree of wildfire impact on vegetation depends on heat transfer and smoke. Fireline intensity and fire residence time determine the heat flux on tree parts. Meanwhile, wildfire smoke is determined by fuel loads, fire intensity, and burning duration and can persist for weeks in the air.

• High-intensity fires burn live and dead canopy, foliage, and meristems in the crown, causing immediate tree mortality in most cases. Sometimes, a tree resprouts from heat-resistant organs.
• Low and moderate-intensity fires are not lethal to mature trees but can cause injuries that impact tree functioning.

Wildfire impacts are classified into two types: first-order and second-order effects.

Cambium and phloem necrosis, xylem, root damage, and gas exchange that alter carbon and water transport are some of the main physiological alterations to fire-affected trees. Burnt plants are more susceptible to subsequent drought.

First Order Effects

First-order effects are the immediate result of heat transfer to plant tissues. They occur due to heat transfer from wildfires to crown, stem, and root tissues. Convection and radiation transfer heat to tissue and soil. Heat is transferred within plant parts and soil via conduction.

Roots: Heat transfers from wildfire causes the following damage to roots:

• Root heat-induced injuries occur from conduction through the soil and into root interiors. It can cause root mortality, especially fine roots and phloem, and cambium necrosis. Tissue mortality is complete at 60°C because of protein denaturation.
• Cell necrosis increases with temperature and can occur if exposed to more extended periods to lower temperatures.
• Phloem and cambium necrosis occur in roots in the topsoil and near root crowns due to ground fires, which lead to significant soil and root heating over hours and days.

Stem: Heat transfer rates into tissues are affected by plant functional traits. The bark is thermal insulation in the stems since heat must pass through it before it can affect other internal tissue. Bark thickness, moisture content, and density are crucial in controlling heat fluxes and potential damage to underlying tissues, like phloem, xylem, and vascular cambium. If the bark insulation is insufficient, fires can be lethal for these tissues.

Crown: Heat transfer to the crown results in foliage, bud necrosis, and damage to phloem and cambium in branches.

The injuries depend on functional traits, crown base height, fire intensity, and residence times. Traits like crown width, shape, surface area, orientation, and degree of shielding by foliage control heat fluxes. Features like mass, water content, and specific heat capacity determine the energy required to raise temperatures. Larger buds in species, like longleaf pine and ponderosa pine, suffer less necrosis than smaller sugar maple buds.

Condition and internal temperature gradients are crucial for more significant crown parts- fruits, cones, and branches. The size or diameter of features insulating inner tissues becomes critical as conduction decreases with radial depth. The bark is the insulator in branches; cones and fruits can also protect embryos from heat necrosis.

In cases where first-order effects do not cause mortality, heat injuries can lead to second-order impacts and mortality, see Figure 2.

Figure 2: “Example of delayed postfire mortality in a Norway spruce (Picea abies) specimen. The tree survived a forest fire in March 2014 (Absam, Austria) with a scorched stem and leaf and bud necroses in the lower crown portion, but eventually died in the following year. Although mortality occurred from interactions of potential injuries to cambial, vascular, and resource acquisition tissues, it is not known whether carbon starvation or hydraulic dysfunction was the ultimate cause. Pictures of the same tree were taken shortly before (a, June 2015) and after (b, September 2015) its death” Bär et al. 2019. (Image credits: https://doi.org/10.1111/nph.15871).

Second-order Fire Effects

Second-order wildfire effects are complex, and their mechanisms are less understood than first-order effects. Two hypotheses have been proposed to explain second-order functionality and mortality of fire injuries on trees.

1. Cambium Necrosis Hypothesis (Hunger): This hypothesis suggests that fire-induced cambium and phloem necrosis occur and limit carbohydrate translocation, leading to carbon starvation.
2. Hydraulic Dysfunction Hypothesis (Thirst): The hypothesis posits that the heat from forest fires can damage the xylem of trees, leading to hydraulic failure and disrupting the plant’s ability to transport water.

Both effects can trigger physiological cascades that impact the overall function of the plant and may contribute to tree mortality.

Crown Damage

The response of plants to fire injuries varies widely and can be positive or negative, depending on the extent of crown damage.

Positive Effects

Crown injuries lead to reduced leaf area in affected trees. Loss of foliage after a fire decreases the leaf area, improving water availability for the remaining vegetation. This improved water availability can enhance stomatal conductance and photosynthesis rates in undamaged leaves.

Trees with crown injuries and decreased competition due to the absence of herb and shrub layers may exhibit unaffected or enhanced postfire production and growth. Reduced competition and improved water availability can contribute to these positive effects. So, injured trees may benefit in the short- and mid-term from reduced competition due to fire.

Negative Effects

However, high levels of leaf necrosis can limit the beneficial effects. Extensive leaf necrosis may prevent the remaining foliage from sustaining the tree’s carbohydrate requirements, leading to decreased growth. Stem cambial and vascular tissue injuries can compromise functionality, affecting stomatal behavior and tree growth. So, surviving trees could suffer from compromised physiological functionality, reduced growth, and an increased likelihood of delayed death.

Besides cambium and phloem necrosis and xylem damage, other physiological features can also lead to hunger or thirst, like root injuries or alteration to gas exchange.

Root Injuries

Knowledge about how root injuries affect postfire tree viability and productivity is limited.

• Fires can reduce fine root biomass, potentially compromising water uptake. Root injuries may mediate tree decline and mortality through hydraulic limitation, even when root cell damage is not lethal.
• Fine root degradation may reduce water uptake.
• Denaturation of aquaporins, essential for water uptake, can disable radial water transport in roots, even if root cell damage is not lethal.
• Also, reducing microbial biomass can change carbon storage and cycling and affect the whole ecosystem.

Understanding below-ground nutrient cycling post-fire will be crucial to developing management practices.

Gas Exchange in Fire-Affected Trees

Wildfires can also affect physiological processes like gas exchange. These processes can be affected by heat transfer and smoke from wildfires.

The effect of fire and smoke can differ in perennial and deciduous trees.

Deciduous Forests

Wildfires harm deciduous forests.

• Photosynthesis: Burned trees with high defoliation rates exhibited reduced photosynthesis and carbon availability for growth. Twenty minutes of smoke exposure can lead to a 50% reduction in photosynthetic capacity. Photosynthesis reduction was due to less stomatal conductance and biochemical limitations.Smoke has nearly 100 compounds, many of which are harmful to plants, including nitrogen dioxide (NO2), sulfur dioxide (SO2), and ozone (O3). Combining NO2 and SO2 has an additive inhibitory effect on photosynthesis. Ozone is linked to chlorophyll destruction and inhibition of stomatal regulation. SO2 also reduces stomatal conductance and disrupts photosynthesis.

• Transpiration: Reduced stomatal conductance also reduces transpiration. Even if the fire doesn’t affect the xylem hydraulics properties, severe crown damage can cause growth reduction and transpiration Also, there will be limited development of thicker xylem conduit walls in burned trees compared to unburned trees.

However, conifer forests can be warmer and drier after a fire, and smoke makes the area darker. These conditions lead to enhanced stomatal conductance, photosynthesis, and transpiration.

Long-term exposure to NO2 and SO2 in smoke reduces essential antioxidant enzymes in plants, potentially leading to oxidative stress and weakening the tree, especially when combined with high ozone levels during extended smoke exposure.

Second-order effects, like physiological carbon and water restrictions, can increase tree susceptibility to pests and insects. Altered physiology and a weak defense impact whole-plant functioning and can result in postfire tree mortality.

Measuring Tree Responses to Wildfires

Scientists who study urgent hereto unknown tree physiological responses to wildfires need all the help they can get. CID Bio-Science Inc. produces several scientific devices suitable for non-destructive measurements in the field or laboratory, which are trusted by researchers globally and can help in above- and below-ground studies of tree responses to wildfires:

• CI-110 Plant Canopy Imager with a 150^o fisheye measurement for canopy cover and Leaf Area Index (LAI) estimation.
• CI-340 Handheld Photosynthesis System can simultaneously measure photosynthesis, transpiration, and stomatal conductance.
• CI-202 Portable Laser Leaf Area Meter and CI-203 Handheld Laser Leaf Area Meter are suitable for non-destructive measurements of several leaf types, sizes, and thicknesses.
• CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager for minirhizotron systems are excellent tools for estimation of live, dead, growth, and turnover of fine and coarse roots, mycorrhizal hyphae, and pathogen infestation. Root tubes installed in the field will allow long-term monitoring of belowground dynamics.

Precision scientific tools that estimate complex internal physiological processes like photosynthesis, transpiration, and stomatal conductance that determine carbon and water transport or morphological traits like leaf area or canopy cover can speed up research in this crucial area to inform risk assessment and postfire management plans.

Sources

Bär, A., Michaletz, S. T., & Mayr, S. (2019). Fire effects on Tree physiology. New Phytologist, 223(4), 1728–1741. https://doi.org/10.1111/nph.15871

Calder, W. J., Lifferth, G., Moritz, M. A., & Clair, S. B. (2010). Physiological effects of smoke exposure on deciduous and conifer tree species. International Journal of Forestry Research, 2010, 1–7. https://doi.org/10.1155/2010/438930

EPA. (2021, April). Technical Documentation: Wildfires. Retrieved from https://www.epa.gov/sites/default/files/2021-04/documents/wildfires_td.pdf

Keane, R. E., Ryan, K. C., & Running, S. W. (1996). Simulating effects of fire on northern Rocky Mountain Landscapes with the ecological process model fire-BGC. Tree Physiology, 16(3), 319–331. https://doi.org/10.1093/treephys/16.3.319

Light, B., Bryant, K., Mathias, J. M., Stenzel, J., Lynch, L., & Hudiburg, T. W. (2022, December). How does belowground functioning vary among trees with different levels of fire damage? In AGU Fall Meeting Abstracts (Vol. 2022, pp. B55G-1046).

82. I. Daws, J. Davies, H. W. Pritchard, N. A. C. Brown, and J. Van Staden. (2007). Butenolide from plant-derived smoke enhances germination and seedling growth of arable weed species. Plant Growth Regulation, 51(1), 73–82.

O’Brien, J. J., Kevin Hiers, J., Mitchell, R. J., Varner, J. M., & Mordecai, K. (2010). Acute physiological stress and mortality following fire in a long-unburned longleaf pine ecosystem. Fire Ecology, 6(2), 1–12. https://doi.org/10.4996/fireecology.0602001

Partelli‐Feltrin, R., Smith, A. M., Adams, H. D., Thompson, R. A., Kolden, C. A., Yedinak, K. M., & Johnson, D. M. (2022). Death from hunger or thirst? phloem death, rather than xylem hydraulic failure, as a driver of fire‐induced conifer mortality. New Phytologist, 237(4), 1154–1163. https://doi.org/10.1111/nph.18454