UC Agriculture & Natural Resources California Agriculture Title Management of blue gum eucalyptus in California requires region-specific consideration Permalink https://escholarship.org/uc/item/9db933pg Journal California Agriculture, 70(1) ISSN 0008-0845 Authors Wolf, Kristina M DiTomaso, Joseph Publication Date 2016 DOI 10.3733/ca.v070n01p39 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Review Article Management of blue gum eucalyptus in California requires region-specific consideration by Kristina M. Wolf and Joseph M. DiTomaso Blue gum eucalyptus (Eucalyptus globulus) is a large tree native to Australia that was wild areas) in some parts of California widely planted throughout California for reforestation, building and timber, but in (Esser 1993; Ritter and Yost 2009). And, in some areas, invasive populations — some areas has spread beyond its planted borders and substantially altered wildlands. those that have naturalized and cause Due to its fast growth, large size and reproductive potential, blue gum’s impacts on na- economic or ecological harm — have so tive vegetation, wildlife and ecosystem processes are of concern, particularly in areas altered landscapes and ecosystem pro- with reliable year-round rainfall or fog, where it is most likely to spread. Depending on cesses that the impacts raise many eco- logical, social and cultural questions. For levels of invasion and rate of spread, blue gum may have negative, positive or neutral example, should blue gum be retained as impacts on fire regimes, water and nutrient availability, understory vegetation and overwintering habitat for monarch but- higher trophic levels. Additional research on the abiotic and biotic impacts of blue gum, terflies Danaus( plexippus L.), whose popu- quantitative estimates of area covered by blue gum, and weed risk assessments that al- lations have dropped by an estimated 90% due to declines in suitable habitat low for region-specific climatic information and management goals to be incorporated (Griffiths and Villablanca 2013)? Should are needed to guide management of blue gum populations. we perhaps plant even more? Or should these “weeds” (i.e., plants out of place) be removed? While an often contentious or many Californians, eucalypts of blue gum were planted in California subject with proponents on both sides (Eucalyptus spp.) are a valued part between 1856 and the 1930s (Butterfield (Jones 2009), it is important to consider Fof the natural landscape, while for 1935), extending from Humboldt County the pros and cons when making decisions others, they are a nightmare that fueled in the north to San Diego County in the regarding management of blue gum (LSA the disastrous 1991 Oakland hills fire that south, with best individual growth and Associates 2009). claimed 25 lives (NPS 2006). Introduced to survival occurring in the coastal fog belt California from Australia circa 1856 (Esser in the vicinity of San Francisco (Burns Online: http://californiaagriculture.ucanr.edu/ 1993), Eucalyptus globulus Labill. (blue gum and Honkala 1990). Herbarium collections landingpage.cfm?article=ca.v070n01p39&fulltext=yes eucalyptus, hereafter “blue gum”) was today show blue gum occurrences in at doi: 10.3733/ca.v070n01p39 the most widely planted species within least 23 counties (UC Regents 2014). the genus, and mainly occurs in grass- However, blue gum has naturalized lands and some previously forested areas. (escaped from its original plantings into About 40,000 acres (> 16,000 hectares) Naturalized blue gum trees in Tilden Regional Park, Contra Costa County. http://californiaagriculture.ucanr.edu • JANUARY–MARCH 2016 39 Will Suckow Will Our objectives are to (1) summarize minimum temperature above 20°F (−7°C) impacts occur in limited areas along the the traits of blue gum that may contribute at all times. California coast. to invasiveness and identify factors con- In the United States, blue gum is pres- Reproductive traits and dispersal tributing to spread, (2) describe biotic and ent in Hawaii (National Tropical Botanical abiotic impacts of nonnative blue gum Garden 2015) and California, where it Reproductive traits. Depending on the in California, (3) describe current trends has naturalized (Baldwin 2012). Its fast region and climatic conditions, certain in the spread, removal and introduction growth, large size and ability to thrive in reproductive traits can be significant con- of blue gum in California, and (4) clarify California’s Mediterranean climate made tributors to the invasiveness of a plant, research needs and management implica- it an attractive choice for building, furni- such as asexual reproduction and the pro- tions regarding blue gum presence and ture, firewood, medicinal uses, cleaning duction of a high number of propagules invasiveness. products and, originally, reforestation and (Radosevich et al. 2007). Reproductive afforestation efforts. Many naturalists, sci- traits that could contribute to blue gum’s Literature review methods entists and government agencies extoled ability to spread include yearly seed Because much information regarding its merits, recommending the species for production (in many areas), seed produc- the biotic and abiotic impacts of blue gum large-scale planting, even offering awards tion for more than 3 months per year exists in the non-peer-reviewed literature, for individuals who planted the largest (November to April, in California) and including agency reports, blogs and per- number of trees (Farmer 2013; Santos a tendency to resprout prolifically after sonal observations by land managers, this 1997). damage (e.g., cutting, fire) (Rejmánek and paper is not a systematic review of the However, after planting millions of Richardson 2011). scientific literature. Limiting our search to trees, lumber production intended for In California, blue gum produces peer-reviewed scientific literature could railroad ties was abandoned because flowers during the wet season, generally result in the omission of critical informa- blue gum wood often split, twisted and from November to April, and the fruit (a tion on the general status of blue gum and cracked. Further, the wood could not be distinctive top-shaped woody capsule, research needs. Rather, we obtained in- treated properly for lumber or furniture 15 millimeters long and 2 centimeters formation via scientific database searches (Groenendaal 1983). However, this did wide) ripens between October and March. (including Web of Science, Google not prevent ardent supporters from rec- Although many sources indicate prolific Scholar and AGRICOLA), general Internet ommending it for other uses, including seed production at 3- to 5-year intervals, searches, and solicitation of information ornamental plantings, windbreaks, shade, these “heavy seed crops” have not been regarding invasions (or lack thereof) medicinal purposes, firewood and an- verified in the scientific literature. The and management via emails and phone ticipated environmental benefits such as seeds of blue gum are very small, with an calls to professionals working in land- reductions in soil erosion. As a result, blue average weight of just over 2 milligrams scapes containing blue gum (including gum plantations continued to persist in per seed (460 seeds/gram) (Burns and California State Parks, California Invasive California (Farmer 2013). Honkala 1990). However, little is known Plant Council, California Polytechnic Some plantings exhibit invasive char- about what fraction of blue gum seeds are State University San Luis Obispo, UC acteristics and environmental impacts viable. Berkeley and UC Davis). that contributed to an initial “moderate” While blue gum might produce abun- invasive status by the California Invasive dant seed, it does not generally find ap- Origin and characteristics Plant Council (Cal-IPC), although these propriate conditions for germination. As Blue gum, a large tree in the Myrtle have been poorly documented in the sci- such, it does not often encroach into tree- family (Myrtaceae) that is native to entific literature. Opposing views of blue less areas without purposeful cultivation. southeastern Australia (UC Regents gum’s invasive potential have sparked Germination rates are typically very low 2014), is the most extensively planted heated debate in recent decades, and in under natural conditions, ranging from a Eucalyptus species in the world (Burns 2015, its status was reexamined in re- high success rate of 1% to the more typical and Honkala 1990). Trees can grow to sponse to a request from stakeholders for low of 0.1% (Bean and Russo 2014). Seed 180 feet (55 meters [m]) tall with bark another review. The reassessment of the germination is highest on bare mineral that sheds in long strips, leaving smooth available ecological evidence resulted in soil (Bean and Russo 1989), particularly surfaces of contrasting colors (Farmer Cal-IPC downgrading blue gum’s invasive under high light conditions, such as after 2013; Skolmen 1983). Mature leaves are a status to “limited” (Cal-IPC 2015) logging or fire (Burns and Honkala 1990). waxy grey-blue-green and sickle shaped, According to the Cal-IPC criteria, a Germination within dense plantations while young leaves are oval shaped and limited invasive status is either due to is even less common (Bean and Russo bluish green, with distinctive square a species that is widespread, but does 1989). Blue gum produces a thick litter stems (Brooker 2000). The species has a not cause significant negative impact,