UC Agriculture & Natural Resources Agriculture

Title Management of blue gum 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 () is a large native to that was wild areas) in some parts of California widely planted throughout California for , 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, 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 even more? Or should these “weeds” (i.e., out of place) be removed? While an often contentious or many Californians, 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 within least 23 counties (UC Regents 2014). the , 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 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 , blue gum is pres- Reproductive traits and dispersal tributing to spread, (2) describe biotic and ent in (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 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, , 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 is not a systematic review of the However, after planting millions of Richardson 2011). scientific literature. Limiting our search to trees, production intended for In California, blue gum produces peer-reviewed scientific literature could railroad ties was abandoned because during the wet season, generally result in the omission of critical informa- blue gum 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 , 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 . As a result, blue average weight of just over 2 milligrams scapes containing blue gum (including gum 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 . As Blue gum, a large tree in the Myrtle have been poorly documented in the sci- such, it does not often encroach into tree- () 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 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 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, layer and allelochemicals (natural sub- wide range of climatic adaptability, with or a species that is widespread, but stances produced by plants that can sup- the most successful introductions in causes significant ecological impacts in press growth), which may inhibit not only mild, temperate climates, or cool, high el- specific regions or areas of the state, yet its own germination, but also that of other evations in tropical areas. Ideal climates minimal or no impact in other areas. plants (Molina et al. 1991; Watson 2000). for establishment and growth have no Cal-IPC found that blue gum’s limited Despite this, establishment of blue gum severe dry season, mean annual rain- status corresponds to the latter category, in undisturbed and scrub has been fall of 35 inches (90 centimeters) and a where significant negative ecological observed repeatedly in coastal areas of

40 CALIFORNIA AGRICULTURE • VOLUME 70, NUMBER 1 California (Cal-IPC 2015), and young trees can produce seeds within 2 to 5 years of germination, although not in great quan- tities (Burns and Honkala 1990; Metcalf 1924). Seeds in the soil under natural con- ditions probably remain viable less than a year (Rejmánek and Richardson 2011). Vegetative reproduction can also contribute to invasive potential, making control or removal difficult. Blue gum sprouts readily from stumps of all sizes and ages, as well as from the (woody swelling of the crown at or below ground level) and . Blue gum can survive for many years in the soil after stems die back (Esser 1993; Skolmen 1983). If a tree is cut down, ligno- tubers become active and each bud may produce many new shoots, commonly known as “sucker growth” or coppice and Kim Starr Forest shoots (Bean and Russo 2014; Davidson Blue gum eucalyptus trees can produce a large quantity of seeds in woody capsules, but germination 1993), which may be even more vigorous rates are generally low. and difficult to control than the original growth (Farmer 2013). Resprouting is grasslands, although some plantings oc- difficult, blue gum scored 1 to 2 (very common after fire or cutting, but is not a curred in, or later escaped into, native tree high ignition potential). For comparison, primary mode of spread. Although repro- stands (Griffiths and Villablanca 2013). /bay received a score of 6 to duction can also occur when new shoots Because of the dramatic shift in plant 8, redwood 8, scrub vegetation 4 to 8, and arise from roots/rhizomes (Esser 1993), communities (e.g., from grasslands to tree annual grassland vegetation 1 to 3 (LSA this has rarely been noted in the litera- plantations), it is not unexpected that the Associates 2009). ture, even in repeatedly harvested stands historic fire intensity and frequency (i.e., In the 2013 environmental impact of blue gum (Skolmen 1983). the natural fire regime) were also dramat- statement for the FEMA Hazardous Fire Dispersal. Most blue gum seeds are ically altered (Bossard et al. 2000; FEMA Reduction grant in the East Bay hills not dispersed long distances and are 2013; LSA Associates 2009; Russell and of California, blue gum flame lengths generally distributed by and grav- McBride 2002). As a consequence, vegeta- (used as a proxy for flammability) were ity. In one study, the fruit capsules were tion management plans in blue gum– estimated at 6 to 21 feet (1.8 to 6.4 m); in calculated to disperse only 66 feet (20 m) dominated communities should consider comparison, flame length for oak/bay from a 131-foot (40 m) tree height with historic fire regimes, goals for fire risk woodland was 1 to 34 feet (0.3 to 10.4 m), of 6 miles/hour (10 kilometers/ management and potential hazards to ad- Monterey ( D. Don) 1 hour) (Burns and Honkala 1990). On oc- jacent businesses and homes. to 6 feet (0.6 to 4.9 m), coastal redwood casion, blue gum seeds can be dispersed In Australia, Dickinson and ( (Lamb. ex D. Don) long distances by water when growing Kirkpatrick (1985) found that live blue Endl.) 7 to 31 feet (2.1 to 9.4 m), northern near streams or rivers. The lack of a long- gum leaves were resistant to combustion, coastal scrub 14 to 32 feet (4.3 to 9.8 m) distance dispersal mechanism would ac- but dead leaves were highly flammable and nonnative grasslands 2 to 10 feet (0.6 count for the relatively slow, if any, rate of and the most energy-rich component of to 3.0 m). Flame lengths in young blue spread (Rejmánek and Richardson 2011), the tree. Juvenile and adult leaves of blue gum plantations range from 7 to 31 feet although rate of spread across the state gum had intermediate flammability in (2.1 to 9.4 m), depending on fuel volumes, has not been rigorously documented in comparison to other species evaluated. stand structure, treatment history and experimental or observational evidence. They concluded that living blue gum trees slope (FEMA 2013). and fuel components within litter had the In addition to being generally more Potential impacts of blue gum greatest tendency to propagate fires rela- ignitable and highly flammable in com- Potential risks and impacts can be tive to species from wet sclerophyll (high parison with some species, blue gum grouped into two categories — abiotic and moisture, low light, dense eucalyptus for- accumulates more fuel for than biotic — that may work independently or ests) and gully habitats or dry grasslands and native tree species. Blue interact to produce blue gum impacts on forest communities. gum can accumulate 68,000 pounds per ecosystem processes. Species that produce oily resins, such acre (lb/ac) of dropped limbs, bark and as blue gum, are far more ignitable than leaves (76,000 kilograms/hectare [kg/ha]), Abiotic impacts. those that do not. On a scale of 1 to 10 for compared to 42,000 lb/ac (47,000 kg/ha) Fire regime changes and fire hazards. Blue ignition potential, with 1 representing for California bay (Umbellularia californica gum was most frequently planted in species most easy to ignite and 10 most (Hook. and Arn.) Nutt.) and 26,000 lb/ac

http://californiaagriculture.ucanr.edu • JANUARY–MARCH 2016 41 Blue gum bark sheds in long strips that accumulate high fuel loads, contribute to high flammability and propagate fire.

(29,000 kg/ha) for coast live oak ( Née; also called “California live oak”) (NPS 2006). As a result, blue gum stands are particularly susceptible to fire during the dry season in California. The flammability of blue gum litter may be exacerbated by rare deep freezes, which cause die-back of the trees and contribute to fuel loads (Rejmánek and Richardson 2011). Blue gum also has a tendency to propa- gate fires via open tree crowns and long swaying branches that encourage maxi- mum updraft (Esser 1993; LSA Associates 2009). Multiple stems originating from a single trunk create a basket structure that catches dead materials, which burn easily and intensely (Burns and Honkala 1990; Landrum 2013). When ignited, leaves and bark of blue gum are lofted into the air, sending firebrands (fragments of burning wood) “kilometers” from the fire front to ignite new spot fires. Because leaves and bark firebrands are large, embers are generally still burning when they land, which can rapidly increase fire spread and Kim Starr Forest (Rejmánek and Richardson 2011). Overall, blue gum has a high fire water delivered to the base of trees via the untreated control. Given these mixed hazard rating in comparison with native stemflow. Thick litter layers may also results, Watson (2000) concluded that res- grass and tree species, which have low to interact with allelopathic chemicals to toration projects designed to replace blue moderate ratings (LSA Associates 2009). further suppress germination (May and gum with native plants should at least In summary, blue gum is highly ignit- Ash 1990). consider the potential effects of persistent able and flammable, accumulates high Watson (2000) found that the germina- allelochemicals in the soil. fuel loads, propagates fire quickly, and tion of two California natives, yarrow Concentrated extracts or leachates can increase rate of fire spread to adjacent (Achillea millefolium L., a perennial forb) may not have the same effect as uncon- areas. In fact, the and blue wild rye (Elymus glaucus Buckley, centrated water flowing off plant materi- (2006) estimated that 70% of the energy re- a perennial grass), was significantly als, however. For example, a separate leased through combustion of vegetation reduced following water extract treat- experiment with soil from under blue was due to blue gum in the deadly 1991 ments with blue gum compared with gum showed no significant inhibition of Oakland hills fire. an untreated control or water extract germination for any of the tested species Potential allelopathic effects. Del Moral treatments from coast live oak (Q. agri- relative to the control (although germina- and Muller (1969) reported that natural folia Née). Germination of yarrow and tion was reduced, albeit not significantly). unconcentrated fog drip from blue gum blue wild rye was 89% and 33% lower, The author hypothesized that winter inhibited growth of annual grass seed- respectively, in blue gum treatments rela- rains may have leached allelochemicals lings, and unconcentrated stemflow from tive to the untreated control, and 92% deeper into the soil profile and suggested blue gum inhibited germination of some and 33% lower, respectively, in blue gum that in future studies samples be taken in . The volume of water channeled treatments relative to the coast live oak multiple seasons (Watson 2000). down the stem is about eight times more treatment. In addition, average time for A similar hypothesis was proposed than that of falling rain, such that soil yarrow germination was delayed by 4.5 by Lange and Reynolds (1981), who indi- at the base of trunks could receive large days in the oak treatment and 6.2 days cated that may be exacerbated quantities of water containing potentially in the blue gum treatment. In contrast, in areas with low rainfall because al- allelopathic compounds. The associated germination and root length in the na- lelopathic chemicals would concentrate suppression of plant growth at the base tive perennial grass California brome in the upper soil surface rather than of trees is more likely due to allelopathy (Bromus carinatus Hook. & Arn.) were not leach through the soil profile with heavy than water shortage given the amount of affected by blue gum treatment relative to rainfall. Molina et al. (1991) also found

42 CALIFORNIA AGRICULTURE • VOLUME 70, NUMBER 1 allelopathic properties of blue gum in can alter nutrient dynamics and germina- somewhere in between. Sampled areas the Mediterranean (). Additional tion, emergence and growth of seedlings. where naturalization was documented in- research is needed to clarify the naturally Consistent with this, Aggangan et al. cluded riparian corridors and sites along occurring allelopathic potential of blue (1999) reported a reduction in nitrogen the coast from Monterey Bay north, where gum. mineralization rates in soil below blue summer fog provides sufficient water in Changes to hydrology. Despite its abil- gum litter. However, in a riparian area, an otherwise long, dry summer season. ity to withstand prolonged dry summers Lacan et al. (2010) found no difference in Blue gum plantings in grasslands rep- (Florence 1996; Pryor 1976), blue gum is litter breakdown between blue gum and resent a dramatic change to community not particularly drought tolerant (USDA native vegetation. composition from open grassland to for- PLANTS 2015), and is only able to survive Blue gum shading reduces light avail- est. In grasslands supporting livestock by tapping into deep water reservoirs and ability and might create conditions that and native ungulates, blue gum has a transpiring freely (DiTomaso and Healy inhibit the growth of seedlings and most considerable competitive advantage as 2007), which can alter water availability other plants in the understory (DiTomaso compared with many other tree species, to depths of 45 feet (14 m) and distances and Healy 2007). While native trees grow- as its juvenile foliage is seldom browsed of 100 feet (30 m) from the trunk. While ing in crowded conditions also shade by cattle or sheep (its unpalatability may blue gum does not economize in the use understory plants, the combination of a be related to its moderate toxicity rating of water, it has deep and extensive root thick litter layer and potential allelopathy [CPCS 2009]). This condition not only systems and can extract water from the may exacerbate the effects of blue gum contributed to its popularity for planting soil at higher soil moisture tensions than shading (Bossard et al. 2000). Moreover, in open grasslands years ago, but also most mesophytic (terrestrial plants that blue gum alters light availability in other- permits newly recruited seedlings outside are not adapted to very wet or very dry wise open grasslands and within invaded planted stands to survive in the presence conditions) plants (Florence 1996; Pryor native forests, which could interfere with of grazing animals (Burns and Honkala 1976). This allows blue gum to compete the germination and growth of some 1990). strongly with other vegetation for water plant species (Peter Warner, Botanical and Reports on the impact of blue gum (HEAR 2007). Ecological Consulting, personal observa- stands on plant diversity are variable, Compared with , blue gum tion as listed on original Cal-IPC assess- with some observations noting the pres- uses less water per unit of , but ment form, 2004). ence of several native species supported blue gum’s high biomass production, In general, while some evidence is in the understory (LSA Associates 2009). even under low rainfall conditions, may observational, or additional research is While native plants may be found be- reduce nearby streamflows more rapidly needed to clarify the abiotic environmen- neath blue gum trees at some locations in than other slower-growing tree species tal impacts, blue gum appears to alter his- California (CAPRC 2011), evidence regard- (Davidson 1993). Thus, in dry areas the torical abiotic conditions and ecosystem ing the relative amounts of native cover benefits of lower water use per unit bio- processes not only where it is planted, and trends in native vegetation is mini- mass of blue gum may be outweighed by but also in natural areas to which it has mal. However, most reports indicate that impacts on soil moisture content due to spread. In some cases, these impacts are vegetation is “sparse” under blue gum high total water consumption (Rejmánek severe, hazardous or ecologically irrevers- stands (Bean and Russo 2014; DiTomaso and Richardson 2011). ible (Cal-IPC 2003). and Healy 2007; Esser 1993), which may be In support of this finding, Williams due to the combined effects of competi- Biotic impacts. (2002) noted that streambeds became tion for water, tree density and shading, eroded and dewatered near blue gum Changes to plant community dynamics. allelopathy, non-wettability of soils and a plantations. Potential allelopathic effects While blue gum stands are often mono- thick inhibitory litter layer. For example, on the germination and cover of under- cultures (in terms of tree cover), this is at Mount Davidson in San Francisco, story species in combination with the not necessarily due to competitive exclu- only 36% of the blue gum understory is high water use capacity of blue gum may sion of native trees — it could be that native, with 29 of 50 species recorded as also result in a greater risk of erosion in they were purposefully planted at high nonnative (SFRP 2006). Even other rapidly hillsides covered by blue gum (HEAR densities. Some sources indicate that blue growing nonnative trees may have dif- 2007). Davidson (1993) also implicated gum outcompetes native vegetation as it ficulty persisting in blue gum plantations: non-wettability of soils (water repellence naturalizes in mesic areas (see review by Metcalf (1924) reported that Lombardy and hydrophobicity due to blue gum oils Griffiths and Villablanca 2013), but experi- poplar ( nigra L.) planted among deposited on soil particles) as a contribut- ments have not been conducted to con- blue gum only persisted for 15 to 20 years ing factor to low understory cover, which firm this. However, in both planted and before it was overtopped by blue gum. may alter erosion rates. However, research invaded areas, blue gum stands can form Without removal of blue gum, plant that more clearly elucidates the impacts of near monotypic canopy covers, particu- community composition is not likely to blue gum plantations on hydrology and larly in coastal ecosystems (Cal-IPC 2015). support historic community composi- erosive processes is necessary. Ritter and Yost (2012) sampled 52 unique tion. Even with removal, treatments Changes to nutrient cycles and light avail- stands of blue gum in coastal regions must be repeated multiple times due ability. Leaves and branches of blue gum of California. Of these stands, 21 had to resprouting or new flushes of blue have been noted to decompose very not naturalized at all, 11 had extensive gum seedlings (LSA Associates 2009), slowly (DiTomaso and Healy 2007), which naturalization, and the remainder were resulting in continued disturbance and

http://californiaagriculture.ucanr.edu • JANUARY–MARCH 2016 43 potentially detrimental impacts on com- 2011). Sax (2002) found that species rich- riparian vegetation for many other spe- munity composition for several years. For ness was nearly identical for invertebrates, cies. Some avifauna (e.g., cavity-nesting, example, Davidson (1993) points out that amphibians and birds in native forests foliage-gleaning and ground-nesting following blue gum removal, soils may be and blue gum plantations, although birds) are comparatively less abundant in persistently non-wettable due to the oils rodents had significantly more species blue gum stands than in native habitats, deposited by blue gum. This could further in native forests; moreover, species com- and many breeding species historically affect remaining vegetation through det- position was different between the two represented in the oak and riparian habi- rimental impacts on water infiltration and forest types for all groups. Many birds, tats that blue gum replaced make little or runoff. Therefore, the potentially negative and invertebrates utilize blue no use of blue gum in the Monterey Bay impacts of blue gum removal and long- gum plantations at some point, although region. term management should be carefully there is no consistent trend across all spe- In Santa Clara County, Rottenborn considered when developing a manage- cies for relative use of blue gum as com- (2000) found that red-shouldered hawks ment plan. pared with native forests (LSA Associates nesting in blue gum and other nonnative Impacts on higher trophic levels. Impacts 2009; Rejmánek and Richardson 2011). tree species had higher fitness due to non- of blue gum on terrestrial vertebrates Macroinvertebrate species diversity did natives’ better stability and cover than are mixed, with some reports indicating not differ between blue gum and native that of native trees. In contrast, Williams significantly lower species diversity of vegetation in riparian areas in California (2002) stated that while native birds do arthropods, small mammals and birds in (Lacan et al. 2010). use blue gum groves, species diversity blue gum stands (Cal-IPC 2015; Rejmánek Impacts on avifauna are mixed, al- drops by at least 70%. Moreover, 50% of and Richardson 2011). For example, in though there is little experimental or the Anna’s hummingbird nests in blue Angel Island State Park (San Francisco observational information available. gum were shaken out by the wind, while Bay), 41 species of birds were observed in At an Elkhorn Slough Coastal Training only 10% of nests were destroyed by wind native vegetation, but only 30 species in Program Eucalyptus workshop, Suddjian in native trees. The presence of nonnative blue gum stands. However, approximately (2004) stated that habitat quality of blue blue gum may also alter migratory bird three times more California slender sala- gum depends on many factors, including patterns, as rare wintering species are manders (Batrachoseps attenuatus) were tree size, stand density, canopy closure, attracted to the blue gum sources found in blue gum vegetation than in understory development and of sur- (Suddjian 2004). To our knowledge, no native forests (Rejmánek and Richardson rounding habitat. Compared with dense, studies have been conducted to determine homogeneous blue gum stands of only whether this had a positive or negative Oils and resins in leaves may increase one age, blue gum stands of low to moder- impact on rare bird populations. flammability of blue gum and contribute ate density with mixed age structure in Suddjian (2004) reported that when to non-wettability of soils. proximity to native woodland habitat and birds feed among flowers of blue gum, water provide the highest habitat value. the feathers on their faces become cov- In the Monterey Bay region, over 90 birds ered with black pitch-like residue (often make regular use of blue gum habitat, incorrectly called “gum”) from although many species that nest in blue , resulting in feather loss and gum appear to do so at lower densities plugging of nostrils or bills that theoreti- than in native habitats. More specifically, cally may prevent breathing or feeding. the decay-resistant wood of blue gum of- Articles published in the Point Reyes Bird fers limited opportunities for nesting to Observatory newsletter (Stallcup 1997) woodpeckers and birds that excavate their and Audubon magazine (Williams 2002) own holes. Birds that glean from also implicated blue gum in mortality foliage are also present at notably lower of North American bird species feed- densities in blue gum stands (Suddjian ing amid blue gum flowers. However, 2004). Suddjian’s (2004) observations revealed lit- Some bird species nest preferentially tle evidence of deaths among birds due to in blue gum compared with native trees, plugged beaks or nostrils. Nevertheless, possibly due to blue gum’s tall growth Williams (2002) noted experienced bird patterns and large limbs. In Santa Cruz watchers had reported finding hundreds County, great blue , great egrets of moribund warblers with blue gum and double-crested cormorants only nest nectar covering their faces, as well as in blue gum, while 85% of red-shouldered Townsend’s warblers, yellow-rumped and red-tailed hawk nests were found warblers, ruby-crowned kinglets, Anna’s in blue gum (Suddjian 2004). However, and Allen’s hummingbirds, and Bullock’s Suddjian also noted that while some orioles. Suddjian (2004) suggested that birds use blue gum stands more than more research is needed in this area due native stands, blue gum plantations do to ambiguities in observations and a lack not provide equivalent habitat compared of rigorously documented evidence of Forest and Kim Starr Forest to native oak woodland and deciduous deaths due to blue gum flower nectar.

44 CALIFORNIA AGRICULTURE • VOLUME 70, NUMBER 1 Impacts on arthropods are also mixed: A Cal-HIP PlantRight survey of California (2012) noted that blue gum of the same Fork (2004) showed that abundance of nurseries indicated that few nurseries genotype can be invasive in some areas, Diptera (flies) was higher in blue gum continue to sell blue gum (< 1%). Thus, while rarely reproducing in others. Thus, plantations as compared with oak retailers, growers and landscaping profes- invasiveness does not appear to be related , but that Coleoptera () sionals have largely phased blue gum out to genotypic variability, but rather envi- abundance was higher in oak woodlands of California’s garden center supply chain, ronmental conditions, particularly reli- than in blue gum groves. Overall, order and replaced the species with noninvasive able access to water. In the Central Valley, richness, total abundance and diversity of alternatives (Cal-HIP 2011). where blue gums were cultivated as a arthropods were not significantly differ- CalWeedMapper (2014) allows land- source of fuel, timber and windbreaks, ent between oak woodland and blue gum owners and managers to report on the they do not receive enough moisture to habitats (i.e., they were either equally rare status (spreading; spreading or decreas- propagate from seed (HEAR 2007) and, as in both habitats, or equally abundant in ing with management; eradicated) and such, spread into wildlands is generally both habitats [Fork 2004; Sax 2002]). occurrence of blue gum in the state, and rare. Under ideal conditions where mois- Blue gum is a major source of nec- these are reported at a resolution of USGS ture is not limited, once a tree matures it tar and for honeybees, as well 7.5-minute quadrangles, or approximately can produce a large number of progeny in as an important overwintering site for 8-mile × 6-mile areas. This may not ac- a few years, doubling stand area within 10 monarch butterflies (Rejmánek and count for duplicate reports or naturally years, or spreading at a rate of 10 to 20 feet Richardson 2011). Monarch butterflies decreasing populations. At the time of (3 to 6 m) in diameter per year (Boyd 1997; may utilize nonnative habitat preferen- this review, of the approximately 250 Esser 1993). tially over native habitats (Meade 1999; reported occurrences, about 100 were Coastal California — and in particular, Oberhauser et al. 2001; Pleasants and spreading (all of which occurred along Sonoma, Monterey and Humboldt coun- Oberhauser 2012); however, historical the coast and coastal ranges). Of these, ties — is most at risk for the continued records suggest that monarch butterflies eight were documented as spreading with spread of blue gum. Observations by land clustered on native trees prior to the management, 18 were decreasing with managers and agency personnel (Tim introduction of blue gum (Riley and management, 52 required verification of Hyland and Vince Cicero, California State Bush 1882; Shepardson 1914). Moreover, occurrence, and 30 required verification of Parks, personal communication, 2014) an observational study in mixed stands the species identification. Including those indicate blue gum has invaded disturbed (native trees mixed with blue gum) requiring occurrence or species verifica- coastal prairie and riparian corri- showed that monarch butterflies did not tion, approximately 150 did not indicate a dors at Natural Bridges State Park in Santa consistently cluster preferentially on trend in spread. Cruz, and Montaña de Oro State Park in blue gum, and at times, preferred native While total cover may be increasing, Los Osos, respectively. Aerial photos from trees in some seasons and locations. For decreasing or remaining stable at differ- Humboldt State University (Bicknell 1990) example, during mid-season overwinter- ent sites, it is possible there may be no from 1949 (original established ing (~ December 31 in California) when overall change in cover statewide (Cal-IPC in 1907 to 1908) showed seven species of habitat conditions are generally the least 2015) even without management. eucalypts covering 119 acres (48 ha). By favorable for monarch butterflies, they Climate change may also influence 1989, the grove had expanded to 181 acres are likely to express a preference for the blue gum cover. In California, climate- (73 ha), of which blue gum covered 108 most favorable microclimate. It was dur- matching models estimate that blue acres (43 ha). Overall, the area covered by ing this sensitive time that monarchs gum already exists in the regions with all Eucalyptus species increased by 52% clustered disproportionately on native the most suitable climate (Calflora 2014). between 1908 and 1989. Van Dyke (2004) trees. Planting of additional native co- CalWeedMapper (2014) climate-matching reported a 50% to 400% increase in blue nifers, rather than removal of blue gum reports predict a considerable increase gum stand size between 1930 and 2001 (which could reduce total habitat), may in the suitable range for blue glum along across six sites in coastal California, al- provide additional beneficial microhabi- the northern coast of California by 2050. though one location experienced an initial tat conditions for monarch butterflies However, the climate-matching program increase in the first 25 years and remained (Griffiths and Villablanca 2013). does not have the capability to predict stable thereafter. Overall, blue gum impacts on plant acres of potential invasion. In addition, Research needs and animal abundance and diversity climate-matching, in itself, does not are mixed, and target species should be mean that blue gum will occupy all these Much research has been conducted on managed accordingly, depending on the suitable climatic areas, as other plant the commercial production of blue gum potential positive or negative impacts that characteristics, including viable seed, in California and international produc- blue gum presence or removal may have germination rates, seedling competition tion for timber and other consumptive on their populations and behaviors. and survival all play a role in its potential purposes (e.g., see Standiford and Ledig spread. 1983). However, to guide future manage- Blue gum spread and removal In most cases, establishment of new ment, observational and experimental re- Blue gum is no longer widely sold populations in California wildlands search is still needed on some of the basic or planted in California, and only one is dependent on proximity to previ- impacts of blue gum (or other Eucalyptus California nursery was identified that re- ously planted or otherwise established, species) on abiotic conditions or other portedly sells seeds (Dave’s Garden 2014). seed-producing stands. Ritter and Yost trophic levels.

http://californiaagriculture.ucanr.edu • JANUARY–MARCH 2016 45 While Davidson (1993) pointed out that in California across a variety of different Management implications controlled, replicated and realistic experi- habitats needs to be clarified. Further Management of blue gum must be site ments testing allelopathy are few and con- research should elucidate whether or not and context specific and goal oriented, clusions remain tenuous, he also indicated blue gum reliably produces seed crops requiring sufficient time be spent on that trees should probably not be grown each year, and under what climatic condi- clarifying the desired outcomes of vegeta- in low rainfall areas (15.75 inches/year, or tions seeds are produced, so that regional tion management, compiling information < 400 millimeters) due to the risk for ad- weed risk assessments are more accurate regarding climate and native plant and verse effects from allelopathy and compe- in predicting potential invasiveness. animal communities, and considering so- tition for water, especially when soils are In addition, it is not known exactly cial factors. For example, while blue gum coarse textured and nutrient poor. On the where and to what extent populations is a nonnative plant that in some cases other hand, plantings in areas with higher of blue gum are naturally decreasing, can be particularly invasive or hazard- rainfall or reliable summer fog are also increasing or remaining stable. While ous, eradication of blue gum populations more likely to exhibit invasive tendencies CalWeedMapper (2014) provides reports is not always appropriate. Where current than those in drier areas and are therefore of blue gum occurrence, data regarding plantings may be desirable for alternative a concern, even if allelopathy is not an is- the status of these populations (rate of habitat, for instance, sue in these regions. Continued research spread or decrease, if populations are land managers should carefully consider under local conditions is needed to test the impact of natural levels of potentially While blue gum is a nonnative plant that in some cases can be allelopathic compounds in the soil and litter layer on growth of other nonnative particularly invasive or hazardous, eradication of blue gum naturalized and native plants at different moisture levels. populations is not always appropriate. Suddjian (2004) indicated that research is needed to evaluate the impacts on birds naturally spreading or decreasing) is in- potential outcomes on monarch popula- that feed among blue gum flowers. At sufficient to determine the actual rate of tions. However, caution should be simul- the very least, a systematic observational spread or area covered locally, regionally taneously exercised, because monarch study should be conducted to investigate or statewide. butterflies overwinter in coastal regions the occurrence and extent of nectar on Finally, while weed risk assessments (Marriot 1997) where blue gum is more feathers and beaks and determine if this have been conducted on blue gum for the likely to spread naturally and become has a detrimental effect on fitness and state of California, these assessments are invasive (Ritter and Yost 2012). In many survival. Further information is also not regional or context specific, do not ac- areas, blue gum is considered an aestheti- needed regarding the avian use of blue count for the great variety of ecoregions cally desirable landscape component, and gum forests relative to native vegetation. within the state, and do not incorporate these cultural considerations should be As many bird populations and behaviors management goals, safety considerations accounted for when determining best are at risk, the implications of blue gum or species-specific concerns or benefits. methods for ensuring community safety spread or removal in areas of concern Local climate is particularly influential (e.g., risks associated with blue gum in- would be particularly helpful to land in determining whether or not blue gum clude fire hazards and falling limbs). c managers. is likely to spread, or will be difficult to Some reproductive characteristics of either eradicate or maintain. Thus, assess- blue gum are unclear as well. The num- ments that allow for area-specific climatic ber of seeds produced per square meter information would be useful in guid- K.M. Wolf is Graduate Student Researcher and J.M. DiTomaso is UC ANR Cooperative Extension Specialist each year has not been clearly assessed. ing management efforts by state parks, and Professor in the Department of Plant Sciences at How quickly and under what circum- conservation-based institutions and city UC Davis. stances vegetative reproduction occurs planning organizations.

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