Crown Scorch

Crown scorch is the yellowing or browning of needles following damage from direct flames and/or convectional heat produced by prescribed burns or wildfires. In cases where crown scorch is a major cause of death in pines following fire (Cooper and Altobellis 1969, Van Wagner 1970), crown scorch can be a good indicator of crown damage and subsequent mortality in pines (Allen 1960, McNab 1977, Chambers and Villarrubia 1980, Waldrop and Van Lear 1984, Cain 1984, Johansen and Wade 1987, Lilieholm and Hu 1987). It is often used to assess the effectiveness of prescribed burns or plan salvage operations following wildfires.

Methods for Measuring Crown Scorch

There are several ways of measuring and expressing crown scorch.

• Scorch height is the vertical height from ground level to the highest point in the crown delineated by yellowing or browning needles (m or ft).
• Crown scorch volume is the volume of the crown that is yellowed or browned (m³ or ft³). Crown scorch volume can be visually estimated or calculated based on crown scorch height and tree dimensions, such as crown length and width.
• Percent crown scorch is the percent of the total crown that is yellowed or browned. Percent crown scorch is usually estimated visually rather than measured, and crown scorch classes are often used (Johansen and Wade 1987).
• Needle drop has been used as an indicator of the degree of crown scorch (Waldrop and Van Lear 1984) but is rather labor intensive and does not work if needles are consumed by fire. Needle drop is collected in litter traps and expressed on a (weight/area) basis. Needle drop from unburned areas are also collected as controls.

Of these methods, percent crown scorch is the most commonly used, however no studies have compared their accuracy when assessing southern pines. Studies using western conifer species have reported that percent crown scorch is often a better predictor of mortality and decreased growth than scorch height (Saveland and Neunenschwander 1990). Also, observed crown scorch volume more accurately estimates crown damage than crown scorch volume calculated from measured dimensions (Peterson 1985). The popularity of using scorch height is due to its relationship with fire intensity- it can be predicted prior to burning using expected fireline intensity, ambient temperature, and windspeed (see Estimating Scorch Height).

Relationships between Crown Scorch and Decreased Growth or Mortality

There are several general patterns that relate crown scorch with decreased growth and mortality in southern pines.

• The effects of crown scorch vary by season of burn. Southern pines are likely to survive crown scorch if stands are burned during the dormant season, even if tree crowns are completely scorched (slash pine and longleaf pine: Story and Merkel 1960, slash pine: Johansen 1975). However, death can result from needle consumption, even during winter burns. For example, only 13% of slash and longleaf pines with over half of their needles consumed survived a winter burn in Florida (Story and Merkel 1960).
• Mortality increases with increasing crown scorch. Pines have a low chance of surviving to the next growing season if their entire crown is consumed by fire (Johansen and Wade 1987). However, the effects of total crown scorch is more variable. Villarrubia and Chambers (1978) found that 75% of non-suppressed loblolly pines trees with total crown scorch died. However, Waldrop and Van Lear (1984) found only 20-30% of non-suppressed loblolly pine died, and Johansen and Wade (1987) found that no mortality among slash pine suffering total crown scorch.
• Effects of crown scorch vary by crown class. Small-diameter suppressed pines are more likely to die from crown scorch than unsuppressed pines (Allen 1960, McNab 1977, Chambers and Villarrubia 1980, Waldrop and Van Lear 1984, Cain 1984). Suppressed pines can even die from partial crown scorch (Villarrubia and Chambers 1978). Mortality from crown scorch also increases with increasing stand density, partly because of the increase in the number of suppressed trees.
• Crown scorch can decrease growth of surviving trees. In general, crown scorch above a certain threshold will decrease growth of pines during the first one or two seasons following burns. However, the level of this threshold appears to vary. For example, Johansen and Wade (1987) found growth loss among even slightly scorched following a moderately intense burn of 25-yr-old slash pine. Villarrubia and Chambers (1978) found decreasing growth only among loblolly pine with more than 30% crown scorch. Mann and Whitaker (1955) found reduced height growth among slash pine when crown scorch was greater than 50%. However, Waldrop and Van Lear (1984) found that diameter growth of loblolly pines with even 100% crown scorch was not affected. Some studies have found that very low levels of crown scorch can even increase growth due to a pruning effect (Johansen 1975, Villarrubia and Chambers 1978). The USDA Forest Service provides estimates of potential growth loss in loblolly and slash pines over 3 inches dbh (see Table: Damage estimates from crown scorch).
• Effects of crown scorch are short-lived. Decreased growth rates due to crown scorch usually last no more than 2 growing seasons (Johansen and Wade 1987, Lilieholm and Hu 1987) and most mortality occurs within 2 or 3 growing season after fire (Lilieholm and Hu 1987).
• Wind helps to minimize the effects of crown scorch. Backing fires should be used with relatively high wind speeds (3 mph at eye-level) in order to dissipate the heat, particularly during the summer when ambient temperatures are high (USDA Forest Service 1988). This is particularly true when burning in closed canopies, where heat can be trapped under low wind conditions.