The Florida scrub

Over the past 15 years, several investigators have studied the role of allelopathic interference in the Florida scrub community. This work has recently been reviewed (Williamson et al. 1992b Fischer et al. 1994 Weidenhamer 1996). 

The scrub forms distinct boundaries with roadsides consisting primarily of ruderal species, with a characteristic bare zone 1 m or more in extent between the scrub and roadsides. Similar sharp ecotones occur between the scrub and Florida sandhill community, which is dominated by longleaf pine (Pinus palustris Mill.), slash pine (P. elliotti var. densa Little Dorman), and oaks (Quercus laevis Walt, and Q. geminata Small). Unlike the scrub, the sandhill has a dense herbaceous understory. The scrub vegetation is fire-sensitive, and Richardson and Williamson (1988) proposed that chemical interference by fire-sensitive scrub perennials prevents the invasion of the scrub by herbs and grasses, which would provide fuel for fires. 

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A number of phenylpropanoid-derived compounds are capable of inhibition of seed germination and seedling growth of other plants. Dihydrocinnamic acid (79), from Ceratiola ericoides (Empetraceae), a common plant in the Florida scrub, was shown to inhibit both germination and growth of Schizachyrium scoparium (little blue stem, Poaceae), Leptochloa dubia (Poaceae), Rudbeckia hirta (As-... 

Fischer, N. H., N. Tanrisever, and G. B. Williamson, Allelopathy in the Florida scrub community as a model for natural herbicide actions, in Biologically Active Natural Products Potential in Agriculture (H. G. Cutler, ed.), ACS Symposium Series 380, 233-249, American Chemical Society, Washington, DC, 1988. 

Obviously, much more quantitative work remains to establish the hypothesis of allelopathy in the Florida scrub. It is likely that sophisticated bioassays will also be required, in which low doses of allelochemicals are applied for long periods of time under environmental conditions that mimic the harsh environment of the scrub. Environmental stress factors such as temperature and nutrient limitation are known to increase the toxicity of allelochemicals (Einhellig Eckrich 1984 Einhellig 1987, 1989). Soil surface temperatures in the scrub are high... 

In the Florida scrub, strong evidence from field and greenhouse bioassays that have ruled out other possible factors and implicated allelopathy has been supported by chemical studies that have identified phytotoxic inhibitors in the leaf dip of five scrub perennials. In several cases, the breakdown products of these compounds are more phytotoxic than the original compound. However, measured concentrations of these compounds in the environment are low, and long-term bioassays and field studies that measure the effects of phytotoxins on plant growth under harsh environmental conditions comparable to those of the scrub will be required to confirm the hypothesis of allelopathy. The studies of allelopathy in the Florida scrub illustrate the crucial role that both bioassays and chemical studies must play in the examination of allelopathy. 

Burger J, Woolfenden GE, Gochfeld M. 1999. Metal concentrations in the eggs of endangered Florida scrub-jays from central Florida. Arch Environ Contam Toxicol 37 385-388. 

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