Biodiversity effects and rates of spread of nonnative eucalypt woodlands in central California

Abstract

Woodlands comprised of planted, nonnative trees are increasing in extent globally, while native woodlands continue to decline due to human activities. The ecological impacts of planted woodlands may include changes to the communities of understory plants and animals found among these nonnative trees relative to native woodlands, as well as invasion of adjacent habitat areas through spread beyond the originally planted areas. Eucalypts (Eucalyptus spp.) are among the most widely planted trees worldwide, and are very common in California, USA. The goals of our investigation were to compare the biological communities of nonnative eucalypt woodlands to native oak woodlands in coastal central California, and to examine whether planted eucalypt groves have increased in size over the past decades. We assessed site and habitat attributes and characterized biological communities using understory plant, ground-dwelling arthropod, amphibian, and bird communities as indicators. Degree of difference between native and nonnative woodlands depended on the indicator used. Eucalypts had significantly greater canopy height and cover, and significantly lower cover by perennial plants and species richness of arthropods than oaks. Community composition of arthropods also differed significantly between eucalypts and oaks. Eucalypts had marginally significantly deeper litter depth, lower abundance of native plants with ranges limited to western North America, and lower abundance of amphibians. In contrast to these differences, eucalypt and oak groves had very similar bird community composition, species richness, and abundance. We found no evidence of "invasional meltdown," documenting similar abundance and richness of nonnatives in eucalypt vs. oak woodlands. Our time-series analysis revealed that planted eucalypt groves increased 271% in size, on average, over six decades, invading adjacent areas. Our results inform science-based management of California woodlands, revealing that while bird communities would probably not be affected by restoration of eucalypt to oak woodlands, such a restoration project would not only stop the spread of eucalypts into adjacent habitats but would also enhance cover by western North American native plants and perennials, enhance amphibian abundance, and increase arthropod richness.

Invasive vegetation affects amphibian skin microbiota and body condition

“…We found that the body condition index of salamanders in Eucalyptus forest was significantly lower than those found in Quercus woodlands. The effect on body condition is consistent with prior work demonstrating a negative effect of introduced Eucalyptus on amphibian diversity (Fork et al, 2015;Russell & Downs, 2012). Importantly, our results also suggest that although Eucalyptus may not always decrease amphibian abundance or diversity (Keane & Morrison, 1990;Sax, 2002), it can have more cryptic negative effects.…”

“…Although we cannot determine what proximate factor is driving body condition decline in Eucalyptus habitat, there are multiple possible explanations including decreased prey availability. For example, Fork et al (2015) found lower arthropod richness and lower abundance of some arthropod classes in Eucalyptus relative to oak woodland habitat-although an earlier study did not find differences in leaf litter arthropod richness in Eucalyptus vs. Quercus woodlands; Sax, 2002. In addition, Eucalyptus leaf extract has been found to compromise chemical communication in salamanders, and this could contribute to stress in Batrachoseps residing in invasive vegetation dominated habitat (Iglesias-Carrasco et al, 2017).…”

“…In California one of the invasive plants of greatest concern are the Eucalyptus sp. (Fork et al, 2015;Wolf & DiTomaso, 2016). Eucalyptus were introduced into the state in the 1850's as a timber species (Butterfield, 1935), and multiple members of this genus are now abundant and ecologically successful throughout the state (Ritter & Yost, 2009).…”

Going nowhere fast: a review of seed dispersal in eucalypts

“…Greatest dispersal distances for individual stands, estimated from the report's maps for this review, ranged from zero for roadside plantings (Site 14) to 133 m (Site 9). Similarly, Fork et al (2015) analysed the spread of six eucalypt stands in the Elkholm Slough watershed (California). They found an average increase of 271% over 70 years , which was almost identical to that reported for the stands at Angel Island (i.e.…”

Testing an invasion mechanism for Eucalyptus globulus: Is there evidence of allelopathy?

“…At the turn of the 19 th century, E. globulus was introduced to California to provide a new supply of timber (Santos, 1997; Farmer, 2013), and, despite its limited viability as a timber species, continues to occupy ~16,000 hectares of the state (Ritter and Yost, 2009). In these plantations, plant understory diversity is low (Westman, 1990; Fork et al, 2015), and allelopathy is credited, both anecdotally (DiTomaso et al, 2013; Nash, 2013; McBride, 2014; Nance, 2014) and in the literature (Baker, 1966; del Moral and Muller, 1969; May and Ash, 1990; Becerra et al, 2018), as the mechanism by which native understory vegetation is inhibited.…”