DIVERSITY, REPRODUCTION, and POTENTIAL for INVASIVENESS of EUCALYPTUS in CALIFORNIA Biological Sciences Department, Cal Poly, 1
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MADRON˜ O, Vol. 56, No. 3, pp. 155–167, 2009 DIVERSITY, REPRODUCTION, AND POTENTIAL FOR INVASIVENESS OF EUCALYPTUS IN CALIFORNIA MATT RITTER AND JENN YOST Biological Sciences Department, Cal Poly, 1 Grand Ave., San Luis Obispo, CA 93407 [email protected] ABSTRACT In the 150 yr since their introduction to the state, species in the genus Eucalyptus have become the most common non-native trees in California. A clearer understanding of the ability of different species to reproduce in the state is important for how we monitor the ecological impact of these abundant non-native trees and for predicting possible future invasions. Here we present current data on the diversity of Eucalyptus in California, which species are spontaneously reproducing, or have the potential to do so, where they can be found, how they can be identified, and our analysis, based on herbarium and field observations, of the potential ecological impacts of various species in the locations where they have been introduced. We also present a new dichotomous identification key, and botanical drawings of all naturalized species. We discuss the degree to which factors such as life history traits, commonness of planting, and native range influence reproductive behaviors of different species. Key Words: Australia, California, Eucalyptus, invasive, key, naturalization, weed. The genus Eucalyptus L’ He´r. (Myrtaceae) species, mostly E. globulus, have become contro- includes some of the most important solid timber versial in the regions where they are now and paper pulp forestry trees in the world conspicuous features of the landscape. There are (Doughty and Places 2000). They have also many popular articles containing the most become the most abundant, ecologically success- emotive writing, much of which is based loosely ful, and controversial exotic trees in California. at best on scientific observation, regarding the Species in the genus were first imported into the various impacts of eucalypts on California’s state as early as the mid 1850s, to be grown landscapes and wildlife. They are admired as initially as horticultural oddities for the nursery erosion control, wildlife habitat, and aesthetically trade, then later as a promising fiber source and valuable landscape and heritage trees or demon- possible savior of a forecasted timber drought ized as America’s largest, most fire prone, most (Butterfield 1935; Santos 2006). By 1880, a dangerous, bird killing, weeds (Bulman 1988; number of species, but primarily E. globulus Williams 2002). Labill. (Fig. 1A), were being extensively planted The genus, which contains more than 700 for lumber, pilings and posts, fuel wood, medic- species, according to the most recent formal inal products, tannin, oil, windbreaks, and as classification (Brooker 2000), is almost entirely street and park trees (Groenendaal 1983). As the endemic to the Australian continent with a small California forestry and fuel economy evolved number of species occurring natively in the many of the thousands of hectares of Eucalyptus southern Philippines, New Guinea and parts of plantations remained uncut, and parts of the state Indonesia (Williams and Woinarski 1997). Euca- are now bearing the ecological legacy of this vast lypts exhibit a great range of adaptation to unharvested crop. different moisture conditions (Boland and Hall Since the time of its initial introduction into 1984), and they rival other large tree genera (i.e., California, the genus was promoted by various Ficus, Pinus, Quercus) in having great diversity in agents including private landholders, commercial mature tree size. Species range from small multi- firms, and state and federal agencies. The stemmed shrubs (mallees), to some of the tallest planting craze that took place around the turn and largest forest trees on earth. In fact, the of the last century, the enthusiasm of certain putative tallest angiosperm in North and South individuals in high-profile state public office, and America is a 75.05 m (246.2 ft) E. globulus off the biomass fuel experimentation by the U.S. De- coast of California, on Santa Cruz Island (Steve partment of Energy and the California Depart- Sillette personal communication, Humboldt State ment of Forestry following the 1973 Arab oil University). embargo have all significantly helped propagate A great number of species, representing the the genus in the state. The prevalence of eucalypts gamut of diversity in the genus, have been in California is more the result of large scale introduced into cultivation in California over intentional plantings than it is the result of the last 150 yr. The correct identification of extensive naturalization. Needless to say, these Eucalyptus species in cultivation is often difficult, Madron˜ o madr-56-03-03.3d 18/1/10 14:48:18 155 156 MADRON˜ O [Vol. 56 FIG. 1. Illustrations of fruits and buds of: A. Eucalyptus globulus subsp. globulus,(Howell 32582,CAS).B. Eucalyptus camaldulensis,(Sanders 21982, CAS). C. Eucalyptus fastigata,(Ritter 345,OBI).D.Eucalyptus citriodora,(Eastwood s.n., CAS 45201). All size bars 5 1 cm. but is essential for studying the potential of leaves in phyllotaxis, shape, petiolation, and different species to become invasive. The bark, glaucousness (Jacobs 1955). Juvenile leaves are leaves, and reproductive structures are greatly commonly sessile, decussate, glaucous, oriented varied and at times all need to be examined for horizontally, discolorous (dorsiventral) and often accurate identification (Pryor 1976; Brooker and cordate, orbicular, or ovate in shape, whereas Kleinig 1996). Many species retain the dead bark adult leaves tend to be petiolate, alternate, year after year, giving rise to a trunk covered in a glabrous, pendulous, lanceolate, and concolorous hard, weathered, outer layer (e.g., E. sideroxylon (isobilateral) (Coppen 2002). A. Cunn. ex Woolls), while others annually As in most genera, the defining aspects of decorticate, resulting in a completely smooth Eucalyptus are in the reproductive structures. The trunk (e.g., E. citriodora Hook. 5 Corymbia flowers of only a small number of species develop citriodora (Hook.)K.D.Hill&L.A.S. singly in leaf axils (e.g., E. globulus), while more Johnson). Eucalypts are definitively heterophyl- commonly they develop in 3-, 7-, 9-, 11-, etc. lous, with juvenile leaves that differ from adult flowered umbels (intact inflorescences always Madron˜ o madr-56-03-03.3d 18/1/10 14:48:19 156 2009] RITTER AND YOST: EUCALYPTUS IN CALIFORNIA 157 FIG. 2. Illustrations of fruits and buds of: A. Eucalyptus polyanthemos,. (Twisselmann 18559, CAS). B. Eucalyptus kitsoniana,(Ritter 263,OBI).C.Eucalyptus conferruminata,(McClintock s.n., CAS 994288). D. Eucalyptus pulchella,(Kawahara 800, CAS). All size bars 5 1cm. have odd numbers of flowers) or heads. Individ- capsule with valves dehiscing at the top, allowing ual umbels may develop singly or paired in leaf tiny, wind-dispersed seeds to be shed (Slee et al. axils (e.g., E. camaldulensis Dehnh., Fig. 1B and 2006). E. fastigata H. Deane & Maiden, Fig. 1C) or in There have been a number of past treatments branched axillary or terminal panicles (e.g., E. of cultivated and naturalized eucalypts in Cali- citriodora,Fig.1DandE. polyanthemos Schauer, fornia. In Eric Walther’s 1928 key to the species Fig. 2A). Individual flowers, which are often grown in California, he included 99 distinct small, white, and inconspicuous in the tree crown, species, known to be growing in the state at the have either one or two bud caps (opercula) time (not necessarily naturalized however), and derived from the fused petals and/or sepals. In a made mention of over 100 others (Walther 1928). number of species (mostly subgenus Symphyo- Three species, E. polyanthemos, E. globulus,and myrtus (Schauer) Brooker ) the outer bud cap, E. tereticornis Sm., were treated in Munz’s 1959 derived from united sepals, sheds early in the flora of California, and a number of other development of the flower, leaving a diagnostic commonly planted species including E. siderox- ring-like scar around the middle of the inner bud ylon, E. viminalis Labill., and E. camaldulensis cap. The inner bud cap is shed at anthesis, have been listed as naturalized in the floras of exposing numerous spreading stamens. The various regions and counties (Howell 1958; Munz inferior ovary is sunken in and fused to the and Keck 1959; Howell 1970; Beauchamp 1986; hypanthium (invaginated pedicle) wall. After Thomas 1991; Smith and Wheeler 1992; Junak et fertilization, when the stamens and the style fall al. 1995; Moe et al. 1995; Matthews 1997; Best et from the flower, the ovary develops into a woody al. 2000). The two most commonly planted Madron˜ o madr-56-03-03.3d 18/1/10 14:48:44 157 158 MADRON˜ O [Vol. 56 eucalypts in California, E. globulus and E. on field observations of living specimens and camaldulensis, are treated in the California construction of the key was accomplished by Invasive Plant Inventory Database (California conventional means. Invasive Plant Council 2006–2009) with invasive For all sites observed, the level of reproduction ratings of moderate and limited respectively. In was gauged based on the number, if any, of the most recently published state flora, The spontaneously occurring new individual trees Jepson Manual: Higher Plants of California, nine (from seed, not stump sprouting). In order to species were included as naturalized (McClintock determine naturalization the number of young 1993). Since this 1993 volume, new observations trees (those that were clearly not planted) in the of eucalypt naturalization have been made and area of adult trees were counted, distances from those discoveries are reported here. original introduction sites were approximated, In order to further elucidate the status of the sapling juvenile leaves were visually inspected for genus in California, we have generated a database proper identification, and saplings were pulled of all, or nearly all, past and current collections of up, with the root included, for herbarium Eucalyptus in the state’s many arboreta, botanical vouchers.