Ecologist USDA Forest Service, Rocky Mountain Research Station
The proximate mechanisms of delayed tree mortality from wildland fire are poorly understood. Trees that initially survive fire may sustain injuries to the crown that impair carbon acquisition, eventually leading to death. We investigated the temporal effects of burning on non-structural carbohydrates (NSC) in ponderosa pine (Pinus ponderosa). Ponderosa pine has large buds and thick terminal branches such that fire may kill foliage but not buds, allowing for the possibility of crown recovery and long-term tree survival. We followed 21 trees that burned in a low-intensity surface fire and 9 trees in an adjacent unburned unit. Trees were 2.0-16.8 cm diameter at 3.37 m. We collected 1-year-old foliage, branch phloem, and stem phloem at seven timesteps from 4/17/2021-9/2/2022: T0: immediately prior to burning, T1:4 days, T2:23 days, T3:61 days, and T4:128 days, T5:14 months, and T6:16 months postfire. We estimated crown volume needle scorch and bud kill at T2. Crown scorch ranged from 0-100%, and bud kill from 0-80%. All burned trees initially survived, but two trees died by T4 and six more by T5. At T0, NSCs were similar between burned and unburned trees. Differences in burned and unburned NSC concentrations were most consistent and enduring in main stem phloem. By T2, during bud elongation, soluble sugars, starch, and total NSC were lower in stem phloem of burned trees compared to unburned trees, with differences peaking in T3 during active needle growth, and largely persisting through T4. Stem phloem NSCs in burned trees that later died steadily declined starting at T2, while NSCs in surviving burned trees began increasing at T2 but remained lower than unburned trees through T4. NSCs levels in burned, surviving trees largely recovered to unburned levels by the next year. Crown scorch was strongly negatively correlated with T4 total NSC stem phloem; NSCs declined precipitously as scorch increased above 60%. We show that crown scorch decreases the ability of trees to acquire carbon, causing a depletion of NSCs in stem phloem at higher crown injury levels. However, ponderosa pine recovered if bud kill was low, such that by the end of the first season after a spring burn, total NSC in main stem phloem was the same as unburned trees. Quick recovery of NSCs postfire may minimize the time which fire-injured trees are susceptible to insect attack and other stressors, although this may not be true of less fire-tolerant conifers.
