Much has been made of the lethal effects of eucalyptus when living plants and their residues are near other desirable plants. Blue gum eucalyptus probably is the most widely grown euc in California and it particularly carries the onus of being a bad neighbor. The use of the tree as a mulch material has also been widely viewed as a problem. A recent master's thesis by Kristen Nelson may help allay fears about using this wonderful woody material, which when correctly used as a mulch can have beneficial effects (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=28917; https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=26911).
ABSTRACT
Evaluating the Myth of Allelopathy in California Blue Gum Plantations
Kristen Marie Nelson
It is widely accepted that allelopathy is not only significant, but more or less singular, in the inhibition of understory vegetation in California Eucalyptus globulus (blue gum) plantations. However, there is no published documentation of allelopathy by blue gums against California native species. Here, we present evidence that germination and early seedling growth of five California native species are not inhibited by chemical extracts of blue gum foliage, either at naturally-occurring or artificially concentrated levels. In the greenhouse, seeds were germinated in field-collected soil from mature blue gum plantations and the adjacent native, coastal scrub communities. In petri plates, seeds of native species were germinated in the presence of concentrated volatile and water-soluble compounds from fresh foliage of blue gum, coast live oak (Quercus agrifolia)as a negative control, or white sage (Salvia apiana)as a positive control, or in a water control. In the greenhouse, blue gum soil supported germination and early seedling growth of native species equal to or better than coastal scrub soil. In the lab, germination of native species was not inhibited when grown in the presence of volatile compounds from blue gum foliage, compared to the native control (coast live oak) or the neutral water control. Germination of three out of five native species tested was not inhibited in the presence of water-soluble compounds from blue gum foliage, compared to coast live oak or the water control. Our results contradict the long-standing paradigm that blue gums are toxic to California natives, which may have significant implications for management and restoration of land historically occupied by blue gum plantations.
Nelson, K.M. 2016. Evaluating the myth of allelopathy in California blue gum plantations. M.S. Thesis, Cal Poly SLO. https://pdfs.semanticscholar.org/f32b/82fa387fad9fe5a42dcd35ff8900539f7f2e.pdf
Photo: 1930 Hogg, Auto Club CA
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