Coast redwood basal resprouting rose with fire damage after California's 2020 CZU fire, 77-tree study

Researchers Jackson T. Carrasco, Jeffrey M. Kane and Alan J. Tepley examined how coast redwood (Sequoia sempervirens) resprouts from the base after the high-severity 2020 CZU Lightning Complex in the Santa Cruz Mountains, California. The fire burned 34,000 hectares over 37 days in August 2020. According to the study in Fire Ecology, the team measured 77 focal trees in secondary stands in state and county parks. biomedcentral.comFire Ecologybiomedcentral.comOpen source ↗ Both the presence and the abundance of basal resprouting did not detectably decline with increasing fire damage. They increased continuously.

The focal trees were dominant or codominant, with diameter at breast height above 30 cm, heights of 21 to 67 m and diameters of 31 to 170 cm. Trees were kept at least 200 m apart. Crews used a laser rangefinder to measure total height, crown base height, bole char height and crown damage height. Char and crown damage heights were divided by total height to give the bole char ratio and crown damage ratio. Basal shoots were counted, and up to 30 stems per tree were measured for diameter and length.

The authors added data from a companion survey of 65 plots with 718 redwoods. In it, 106 trees had no basal sprouts, and complete genet mortality occurred in 7.1 percent of stems. Climatic water deficit came from the USGS Basin Characterization Model at 270-m resolution. Boosted regression trees, generalized linear models and mixed-effects models were used for the analysis.

Bole char ratio as the main predictor

The bole char ratio, the char height divided by tree height, was the most informative fire-damage metric for both resprout presence and abundance. biomedcentral.comFire Ecologybiomedcentral.comOpen source ↗ In cross-validated models it gave the lowest log loss, 0.33, for resprout presence. The baseline probability of resprouting was 0.49 on trees with damage near zero. Trees with a ratio of 100 percent had the highest probability of sprouting and the greatest basal shoot biomass.

Climate limits and epicormic sprouting

Higher climatic water deficit reduced the probability of basal resprouting and total resprout biomass. Epicormic resprouting of the bole was positively related to the probability of basal resprouting. The authors report reduced resprouting, both basal and epicormic, in warmer and drier locations, which suggests climatic limits on this ability. They note that the long-term trajectory of redwood forests after high-severity fire needs further study. They also say extreme drought events may hinder resprouting in the future.