Scotch broom
Cytisus scoparius
Family: Fabaceae
Fire effect on plant
Scotch broom is top-killed by fire in light to moderate severities with high severity fire tending to cause mortality (Zouhar 2005).
Key traits
Scotch broom can resprout after fire from the root crown (Terrega et al. 1992; Zouhar 2005)
This species typically has high seed production (up to 10,000 seeds per year for one plant) and forms large persistent soil seed banks (Allen et al. 1995). Broom has seeds with a hard seed coat allowing survival in soil seeds banks for at least 5 years (Bossard 2005) and as long as 30 years (Zouhar 2005 and references therein). The seed banks are often deeply buried in soil, and seed viability seems to remain >90% (Allen et al. 1995). Seeds are produced in pods that burst and scatter seeds up to 20 feet distances.
Seeds require scarification before germination (Zouhar 2005). Owing to the need for scarification Scotch broom has been noted as having seeds that are stimulated by heat from fire. Evidence suggests that exposure of scotch broom seeds to temperatures ~65oC for 2 mins has been noted to significantly increase germination (Bossard 1993). In laboratory based experiments high germination rates for scotch broom where found following 5 min exposure to 100oC (Terrega et al. 1992).
Plant response to fire
Broom regenerates following fire of low-moderate severity by re-sprouting from root crowns and following higher severities also by seedling establishment.
Owing to seed stimulation by fire broom can increase its abundance post-fire. Broom seeds therefore appear to be considered adapted for postfire germination and is noted as deserving further investigation (Zouhar 2005).
It has been suggested that prescribed burning of broom could increase germination and decrease seed bank density at the soil surface, depending on the duration of heating and temperatures reached during the fire. Despite this adaptation basal resprouting appears to depend on a range of factors including timing of fire/ prescribed fire and controlled burn rotation. Spring burnt broom was able to resprout whilst those burned in autumn were less successful, whilst seedling density was similar between burn seasons (Tveten 1996; and see Zouhar 2005).
Timing of life history
Perennial species. Seeds likely produced from the second or third growing season. Flowering May to June.
Conservation status
None.
References
Allen, R. B., Williams, P. A., Lee, W. G. 1995. Seed bank accumulation of broom (Cytisus scoparius) in South Island. In: Proceedings, 48th New Zealand plant protection conference, Hastings, New Zealand. [Empirical evidence; Academic literature]
Bossard, C.C. 1993. Seed germination in the exotic shrub Cytisus scoparius (Scotch broom) in California. Madrono 40: 47-61. [Empirical evidence; Academic literature]
Bossard, C. 2005. https://www.cal-ipc.org/wp-content/uploads/2017/12/20423.pdf [Expert opinion; Grey literature]
Mallick, A.U. Gimingham, C.H. 1985. Ecological effects of heather burning: II. Effects on seed germination and vegetative regeneration. Journal of Ecology 73: 633-644. [Empirical evidence; Academic literature]
Tarrega, R. Calvo, L., Trabaud, L. 1992. Effect of high temperatures on seed germination of two woody Leguminosae. Vegetatio 102: 139-147. [Empirical evidence; Academic literature]
Tveten, R. K. 1996. Fire and community dynamics on Fort Lewis, Washington. Bellingham, WA: Western Washington University. 58 p. Thesis. [Empirical evidence; Academic literature]
Zouhar, K. 2005. Cytisus scoparius, C. striatus. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.usda.gov/database/feis/plants/shrub/cytspp/all.html [2024, January 22]. [Expert opinion; Grey literature]