Eucalyptus in California
History of Eucalyptus in California
With over 700 species, Eucalyptus is a diverse genus of flowering trees and shrubs part of the Myrtaceae family. Members of the genus dominate the tree flora of Australia, yet they have been planted extensively world-wide for the past 150 years and have been successful as exotics due to their fast growth, capability to adapt and thrive in degraded, acidic and dry soils, prolific seed dispersal abilities, and relatively good resistance to pests (Rockwood et al., 2008; Ritter, 2014). Worldwide among the most commonly grown species are bluegum (Eucalyptus globulus), flooded gum or rose gum (E. grandis), and red gum (E. camaldulensis) (Ritter, 2014).
From the first planting of eucalyptus seeds in 1853, to the eucalyptus "boom" of the 1870s, to the hundreds of thousands of trees decorating the city’s landscape today, eucalyptus trees are an integral, iconic part of San Francisco’s history (Nance, 2014). As the San Francisco-based writer Harold Gilliam stated: “the Eucalyptus seems an indispensable element of this State’s landscapes, as indigenously Californian as the redwoods, the poppy fields, the long white coastal beaches, and the gleaming granite of the High Sierra” (McBride, 2014).
The history of Eucalyptus planting in California is a spectacular story of introduction, naturalization and decline. Once seen as an indispensable component of the landscape, eucalyptus trees quickly became seen as invasive species threatening the native flora and biodiversity of the state (Crawford, 2008; McBride, 2014).
Non-native to California, the main species planted are the blue gum, red gum, sugar gum (Eucalyptus cladocalyx), red ironbark (Eucalyptus tricarpa), silver dollar gum (Eucalyptus polyanthemos), and lemon scented gum (Corymbia citriodora) (McBride, 2014). Initially introduced during the gold rush, the first recorded eucalyptus planting in California was in 1853, in the Golden Gate Nursery (Crawford, 2008; Nance, 2014). Plantings peaked during the “eucalyptus craze” of the 1870’s and the “eucalyptus boom” of 1907 to 1913 in both the Bay Area and in southern California (Farmer, 2014; McBride, 2014).
In the latter half of the 20th century, social engineers and planners promoted widespread eucalyptus planting to achieve a number of different management objectives (Ritter, 2014). Eucalyptus species were preferred since they can adapt and grow in severely dry regions that have been historically unable to maintain vegetation, and therefore, provide shade and shelter in areas that otherwise would have been dominated by dried grasses (Nance, 2014). Eucalyptus trees were used to improve the quality of the farmland, they used as windbreaks, they were used as a source of hardwood timber and firewood supply for coastal communities, and due to their extensive root system they were even used to drain the swamps in the fight against malaria (McBride, 2014; California Invasive Plant Council, 2014). Pioneer planters and state authorities promoted the planting of eucalyptus as a producer of railroad ties, as a preserver of soil and climate, and as a solution to meet future timber needs, supplying lumber for houses, piers, railways and fences, while also providing a source of tannin, fiber and fuel. (Nash, 2013; McBride, 2014; Farmer, 2014). During the early 1900s, planting of eucalyptus was seen as a way of coping with the lack of forest timber. However, after it was discovered that eucalyptus wood is not only unsuitable for rail road ties (since the wood had a tendency to twist while drying), but also is a poor source of timber, these plantation became largely abandoned and left un-managed (Farmer, 2014).
The long history of widespread eucalyptus planting has resulted in several species becoming controversial during the 1980s. During that time, the positive attitude towards eucalyptus shifted swayed by concerns regarding “non-native” and “invasive” species and their impacts on native ecosystems (Nash, 2013; Ritter, 2014). A native species was defined as plants that existed in California prior to 1769, the year when Father Junipero Serra settled the first mission in San Diego (Nash, 2013). Eucalyptus started to be perceived as a "nuisance" due to its ability to quickly spread, its encroachment on habitat deemed critical to native plants, its significant ground water consumptions and high flammability potential (peeling bark and falling eucalyptus leaves that litter the forest floor also represented a significant fire hazard) (Nance, 2014). Shortly after the Oakland hills fire of 1991, numerous parks in the coastal region began removing significant numbers of blue gums, as did other public, and private institutions (Nash, 2013).
Today, eucalyptus groves have become a predominant features of the landscape. They are an integral part of the ecosystem, providing habitat for many native species, such as nesting sites for migratory bird species, and critical habitat for migratory monarch butterflies (Stock et al, 2004; McBride, 2014). Eucalyptus trees grow well at high densities (8 ft average spacing between tress), and subject to few diseases or pests (McBride, 2014).
Eucalyptus trees throughout California are admired for their valuable characteristics (paper pulp, honey production, shade, windbreak functions, aesthetics, and ornamentals functions), while at the same time, they are seen as foreign invaders (California Invasive Plant Council, 2014; Ritter, 2014). Eucalyptus groves have a documented impact on the microclimate (on temperature, relative humidity, wind velocity, precipitation, and fog drip), and trigger changes to light availability by increasing shading, and inhibiting the growth of understory vegetation, (McBride, 2014). Changes in soil characteristics (pH, carbon content, nitrogen, phosphorus, calcium), and stream conditions (increased stream litter), impacts on erosion and sedimentation, and changes in nutrient dynamics have also been recorded.
The next section will further explore the ways in which eucalyptus influences abiotic ecosystem processes and briefly discuss the effects of eucalyptus plantations on the natural ecosystems, biodiversity, wildlife, and ecosystem services.
Modification of site conditions by eucalyptus
Eucalyptus triggers changes to the microclimate, fire regime, and to the hydrologic characteristics of the area, and can potentially have allelopathic effects on other plant species.
Changes to the microclimate: From the edge to the interior of an eucalyptus grove, microclimate conditions change due to shading and leaf litter. This in turn, impacts the species assemblages found in different parts of the forest (McBride, 2014).
Changes to the fire regime: The oily resins characteristic of eucalyptus, are much more ignitable compared to other tree species, and burn at higher fire intensities. Eucalyptus leaves have intermediate flammability in comparison to other species, however, dead leaves are the most energy-rich components with high flammability potential (Dickinson and Kirkpatrick 1985; California Invasive Plant Council, 2014). Drifting burning material has a great potential to ignite spot fires (Boyd, 1997), since ignited leaves and bark can be lofted into the air and distribute embers over long distances (Bossard et al. 2000). However, cool and damp dense eucalyptus forests also have the potential to reduce fire risk (McBride, 2014).
Changes to the hydrology of the area: Eucalyptus species such as eucalyptus globulus, or blue gum, also cause changes to the hydrology of the region. Eucalyptus globulus, for example, is able to withstand prolonged dry summers by tapping into the deep water reservoirs with its far-reaching root systems (California Invasive Plant Council, 2014). It can extract water from the soil at even higher soil moisture tensions than most mesophytic plants (California Invasive Plant Council, 2014). (Soil moisture tension is a negative pressure representing the energy needed for the plant to extract water from the soil. The higher the soil moisture tension, the more energy is needed to extract water from the soil.)
Allelopathic effects: Eucalyptus groves can potentially have allelopatric effects since the aromatic compounds from decaying eucalyptus leaves have been found to interfere with the germination of another native plants (California Invasive Plant Council, 2014; McBride, 2014). Allelopathy is a phenomenon through which a plant releases a chemical compound that interferes with the germination and growth of other plants. If released into the environment, allelopathic materials inside a tree can produce a positive or negative change in the survival, growth, reproduction and behavior of other organisms (Ghnaya et al., 2016).
For more information on Eucalyptus trees please refer to Wolf and DiTomasso, 2016.