Effects of ‘Target’ Plant Species Body Size on Neighbourhood Species Richness and Composition in Old-Field Vegetation

<div><p>Competition is generally regarded as an important force in organizing the structure of vegetation, and evidence from several experimental studies of species mixtures suggests that larger mature plant size elicits a competitive advantage. However, these findings are at odds with the fact that large and small plant species generally coexist, and relatively smaller species are more common in virtually all plant communities. Here, we use replicates of ten relatively large old-field plant species to explore the competitive impact of target individual size on their surrounding neighbourhoods compared to nearby neighbourhoods of the same size that are not centred by a large target individual. While target individuals of the largest of our test species, <i>Centaurea jacea L.</i>, had a strong impact on neighbouring species, in general, target species size was a weak predictor of the number of other resident species growing within its immediate neighbourhood, as well as the number of resident species that were reproductive. Thus, the presence of a large competitor did not restrict the ability of neighbouring species to reproduce. Lastly, target species size did not have any impact on the species size structure of neighbouring species; i.e. they did not restrict smaller, supposedly poorer competitors, from growing and reproducing close by. Taken together, these results provide no support for a size-advantage in competition restricting local species richness or the ability of small species to coexist and successfully reproduce in the immediate neighbourhood of a large species.</p></div

Size of target species.

<p> = 3 replicates) dry mass, lateral extent, plant height, and size index for the ten target species, listed in order of increasing size index. Nomenclature follows Gleason and Cronquist (1991). Mean (N</p

Effects of target individual height on mean neighbour plant species height.

<p>Relationship between target species size index and (mean height of species in target neighbourhood-mean height of species in random neighbourhood) (species height is the typical maximum from published data) for: (a) inner target neighbourhoods only; (b) outer target neighbourhoods only; and (c) total target neighbourhoods. The same relationship is also depicted for only reproductive species in target and random neighbourhoods (d-f). The 3 replicates of each of the 10 target species are coded by numbers 1–10 in order of increasing average species size index: 1- <i>Rudbeckia hirta</i>, 2<i>- Solidago juncea</i>, 3- <i>Erigeron philadelphicus</i>, 4- <i>Asclepias syriaca</i>, 5- <i>Aster umbellatus</i>, 6- <i>Daucus carota,</i> 7- <i>Verbascum thapsus</i>, 8- <i>Cirsium vulgare</i>, 9- <i>Solidago canadensi</i>s, 10- <i>Centaurea jacea.</i> Dashed line indicates where species richness in target neighbourhood = species richness in random neighbourhood. r_s and associated P-values are from Spearman Rank correlation analyses.</p

Effects of target individual size on neighbour plant species reproduction.

<p>Relationship between target species size index and [(proportion of reproductive species in target neighbourhood) (proportion of reproductive species in random neighbourhood)] for (a) inner neighbourhoods only; (b) outer neighbourhoods only; and (c) total neighbourhoods. The 3 replicates of each of the 10 target species are coded by numbers 1–10 in order of increasing average species size index: 1- <i>Rudbeckia hirta</i>, 2<i>- Solidago juncea</i>, 3- <i>Erigeron philadelphicus</i>, 4- <i>Asclepias syriaca</i>, 5- <i>Aster umbellatus</i>, 6- <i>Daucus carota,</i> 7- <i>Verbascum thapsus</i>, 8- <i>Cirsium vulgare</i>, 9- <i>Solidago canadensi</i>s, 10- <i>Centaurea jacea</i>. Dashed line indicates where species richness in target neighbourhood = species richness in random neighbourhood. r_s and associated P-values are from Spearman Rank correlation analyses.</p

Effects of large target individuals on neighbour plant species diversity.

<p>Relationship between target species size index and [(species richness in target neighbourhood) (species richness in random neighbourhood)] for: (a) inner neighbourhoods only; (b) outer neighbourhoods only; and (c) total neighbourhoods. The 3 replicates of each of the 10 target species are coded by numbers 1–10 in order of increasing average species size index: 1- <i>Rudbeckia hirta</i>, 2- <i>Solidago juncea</i>, 3- <i>Erigeron philadelphicus</i>, 4- <i>Asclepias syriaca</i>, 5- <i>Aster umbellatus</i>, 6- <i>Daucus carota</i>, 7- <i>Verbascum thapsus</i>, 8- <i>Cirsium vulgare</i>, 9- <i>Solidago canadensis</i>, 10- <i>Centaurea jacea</i>. Dashed line indicates where species richness in target neighbourhood = species richness in random neighbourhood. rs and associated P-values are from Spearman Rank correlation analyses. With <i>Centaurea jacea</i> (species 10) omitted,(a) rs =  −0.366, P = 0.062, n = 27; (b) rs = −0.376, P = 0.0533, n = 27; (c) rs = −0.143, P = 0.472, n = 27.</p

Diagram illustrating the sampling methods used.

<p>For the target plot (left), TA represents the inner neighbourhood of radius A (rA), determined as the extreme lateral extent of the target species. TB is extended 0.5 times rA, extending beyond the area in TA. An identically sized random plot, separated by 2 metres from the target neighbourhood was located randomly and had the same dimensions as the associated target neighbourhood.</p