Allelopathy as a mechanism for resisting invasion: the case of Polygonella myriophylla - Invasive Plants: Ecological and Agricultural Aspects - page 147

bare zone, and within the bahiagrass stand. Root mass of P. myriophylla at the
edge of the patch was only 37% compared to P. myriophylla root mass in the
center of the patch, and only 3% in the bare zone (Fig. 2). Given the low
growth habit of P. myriophylla, which precludes significant shading, these
results suggest a noncompetitive interaction between the two species [9].
Animal activity, which has been shown to contribute to the development of
bare zones around shrubs in the California chaparral [10], has been ruled out
as a cause of bare zones in the scrub [9].
Bioassays were conducted of soils collected biweekly for one year from
beneath P. myriophylla , the bare zone, and adjacent grassed areas. Effects on
both germination and growth of bahiagrass were pronounced. Relative to adja-
cent grassed area soil controls, average germination of bahiagrass was reduced
29% in P. myriophylla soil, and 19% in bare zone soil. Shoot dry weights were
reduced 52% in P. myriophylla soil, and 19% in bare zone soil (Fig. 3).
Nutrient analyses showed that bare zone soil was only slightly lower in P, K,
Mg, organic matter, and total N than the P. myriophylla soil. The reduced
growth in P. myriophylla soil relative to the bare zone is, thus, attributed to the
presence of higher concentrations of allelochemicals. The reduced growth in
P. myriophylla soil relative to grassed area controls was attributed to the com-
bined effect of phytotoxins and slightly lower levels of P, K, and N in the P.
myriophylla soil [9].
Figure 2. The distribution of root mass around a mature shrub shows that P. myriophylla root mass is
concentrated beneath the shrub and does not extend into the bare zone, making resource competition
an unlikely explanation for the bare zones around the plant. Data originally appeared in tabular form
in [9].