Title: Growth and Competition Between Seedlings of Protea repens (L.) L. and the Alien Invasive, Acacia saligna (Labill.) Wendl. in Relation to Nutrient Availability
Abstract: Seedlings of Acacia saligna (Labill.) Wendl., an alien invasive shrub of coastal fynbos, and Protea repens (L.) L., an indigenous shrub, were grown in pots, as single seedlings and as a mixture. These were amended with one of a range of treatments representing a broad gradient in nutrient availability. When grown in isolation, both species displayed increases in dry mass, and leaf nitrogen and phosphorus contents in response to increasing levels of nutrient availability. A. saligna however, exhibited a greater response than P. repens, particularly at high levels of nutrient availability. No significant differences in root/ shoot ratio were found in response to variation in nutrient availability, but A. saligna consistently allocated a greater proportion of its total dry mass to below-ground growth. When grown as a mixture, P. repens exhibited reduced growth at high levels of nutrient availability in response to competition with A. saligna, which totally overtopped the indigenous species. In root observation chambers, the depth penetration of the soil by the tap root of seedlings of A. saligna after germination was approximately twice as rapid as that of P. repens. Seed nitrogen and phosphorus contents were significantly linearly related to seed size, except the phosphorus content of A. saligna. The relatively high absolute growth rate of A. saligna seedlings, at practically all levels of nutrient availability found in the fynbos, appears to be one of the key factors which enables them to compete successfully with P. repens seedlings. Key-words: Alien invasion, competitive interaction, phenotypic plasticity, root penetration, root/shoot ratio, seed quality, seed size * Present address: Dr E.T.F. Witkowski, School of Biology, Curtin University of Technology, GPO Box U 1987, Perth 6001, Western Australia. Introduction Coastal fynbos of the south-western Cape has been extensively invaded by Acacia saligna (Labill.) Wendl., a shrub or small trees, native to southwestern Australia, which was introduced during the mid-nineteenth century to stabilize shifting sand-dunes (Shaughnessy, 1980). It has invaded both disturbed and pristine fynbos and allied shrublands, on a wide variety of substrata, and often forms dense monospecific stands (Milton & Hall, 1981). The success of A. saligna has been largely attributed to the large number of viable seeds it produces (Milton & Hall, 1981; Macdonald, 1984; Holmes, Macdonald & Juritz, 1987), although little is know of its resource requirements, particularly in terms of nutrients and soil moisture. However, A. saligna grows to a height of from 3 to lOm and overtops the indigenous vegetation which generally attains a height of less than 2 m. The soils of the south-western Cape, like those of south-western Australia, have a low nutrient status (Witkowski & Mitchell, 1987). The availability of phosphorus and nitrogen in coastal fynbos is raised after a fire for a short period of time (Brown & Mitchell, 1986; Stock & Lewis, 1986), and may influence the establishment of seedlings. Seed germination of both indigenous and alien species in the fire-prone fynbos vegetation occurs largely after fire, during the moist winter period of this mediterranean-climate region. Fire frequencies in fynbos vegetation usually occur at intervals between 6 and 40 years (Kruger & Bigalke, 1984). Seedling establishment may critically depend on the ability of these plants to produce a rooting system which penetrates to the deeper layers of the soil profile, where sufficient supplies of moisture for survival and growth during the period of summer drought are available. In addition, the ability to produce a deeply penetrating root system may in part require seeds with high contents of limiting nutrients. In this study, the effects of variation in nutrient This content downloaded from 157.55.39.59 on Mon, 17 Oct 2016 04:52:55 UTC All use subject to http://about.jstor.org/terms 102 availability on the growth of seedlings of A. E. T. F. saligna are compared with those of an indigenous Witkowski species, Protea repens (L.) L. (Proteaceae), as single seedlings and as a mixture. Many of the nutrient-poor fynbos areas invaded by A. saligna were previously dominated by shrubs of the Proteaceae, and field observations at a sand-plain lowland fynbos site at Pella, strongly suggest that P. repens is excluded by dense stands of A. saligna. P. repens is a sclerophyllous shrub or small tree, which attains dominance in the mature post-fire period in coastal fynbos (Hoffman, Moll & Boucher, 1987), although it has a relatively wide distribution within the fynbos biome. In contrast, in its native habitat A. saligna dominates during the early post-fire period when competition from other species is reduced, and it is also often found in disturbed sites (Milton & Hall, 1981). The effects of variation in nutrient availability on the uptake of nitrogen and phosphorus were also determined, these nutrients having been shown experimentally to be the most limiting in sand-plain lowland fynbos (Witkowski, 1989ab, 1990; Witkowski, Mitchell & Stock, 1990). In addition, the penetration of the soil by the tap root of seedlings of these species were compared in root observation chambers. Variations in seed size and quality (nitrogen and phosphorus contents) were also determined. It was hypothesized that A. saligna would display a greater absolute growth rate than P. repens, in terms of both shoot and root growth, at practically all levels of nutrient availability found in the fynbos (see Witkowski & Mitchell [1987] for variations in the availability of soil phosphorus). Materials and methods
Publication Year: 1991
Publication Date: 1991-01-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 101
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