On the indices of plant–plant competition and their pitfalls

Abstract

Oksanen, L., Sammul, M. and Mägi, M. 2006. On the indices of plant Á/plant competition and their pitfalls. Á/ Oikos 112: 149 Á/155. The index of relative competition intensity (RCI) has serious built-in biases, due to its asymptotic behavior when competition intensity is high and its tendency to obtain very low values when plants with neighbors intact perform better than neighbor removal plants. These biases have been partially corrected in the index of relative neighbor effect (RNE), but the existence of fixed upper and lower bounds ((/15/RNE5/'/1) still creates problems and biases in communities where the average intensity of competition or facilitation is high and plant performance pronouncedly varies in space. The third commonly used index, the logarithm of response ratio (lnRR), is mathematically and statistically sound, but when computed from pair-wise comparisons between neighbor removal and control plants, this index reflects the geometric mean of the treatment effect. Moreover, linear patterns in lnRR reflect exponential patterns in the intensity of competition. As the interest of ecologists usually focuses on arithmetic means, we propose a corrected index of relative competition intensity, CRCI0/arc sin (RNE). This index is fairly linear within the observed ranges of competition and facilitation, and for the range of competition intensities where RNE behaves reasonably, the two indices obtain almost identical values. We compared the performance of the four indices, using both imagined and real data, the latter from systems where the responses of plants to neighbor removal ranged from weak to moderate, so that RNE and CRCI were expected to behave similarly. The indices were computed both from pooled data for each community and as averages of pair-wise comparisons. lnRR and CRCI were found to behave in a consistent and bias-free manner, yielding similar results regardless of method of computation. This was, by and large, the case with RNE, too, but as the values of indices grew, the values from pair-wise comparisons became increasingly smaller than values computed from pooled data. RCI yielded grossly aberrant results in computations based on pair-wise comparisons. Therefore, the further use of RCI is unadvisable and studies where RCI has been derived from pair-wise comparisons should be excluded from meta-analyses.

Competitive interactions between native and invasive exotic plant species are altered under elevated carbon dioxide

“…The corrected index of relative competition intensity (CRCI; Oksanen et al 2006) of native species on average was significantly increased under elevated compared with ambient CO 2 , indicating that the competitive response of natives under elevated CO 2 was reduced. The CRCI is based on the competitive response of the target plant grown in competition with neighbors but can also be interpreted as a measure of the competitive effect of the neighbor plants.…”

“…For each species-pair, the competitive response of the target native and invasive exotic species was determined. There are a number of methods that have been developed to measure competitive ability, including the relative competition index (Grace 1995), relative neighbor effect (Markham and Chanway 1996) and logarithm of response ratio (Goldberg et al 1999), that have varying advantages and disadvantages (Goldberg et al 1999;Oksanen et al 2006). We chose to measure the corrected index of relative competition intensity (CRCI) following the method of Oksanen et al (2006).…”

“…Subsequently, using the RNE value, the corrected index of relative competition intensity (CRCI) was calculated (Oksanen et al 2006).…”

Variation in weed infestation of sugar beet (Beta vulgaris L. subsp. vulgaris) depending on the intensity of chemical protection of plantations

“…Secondly, the chemical agents used were not fully effective in relation to the above-mentioned species and they could occupy a part of the released ecological niche. Such phenomena of mutual competition and interspecific interactions between the components of segetal communities have been the object of research and have been confirmed in the studies of W e i n e r [17], W e i g e l t and J o l l i f f e [18], V i l et al [19], and O k s a n e n a et al [20]. Moreover, according to some authors, a single species occurring in high abundance exerts a greater negative effect on a cultivated plant than a multi-species community without clearly dominant species [21][22][23].…”

The Relatioship between Plant Density and Microclimate and Nutmeg (Myristica fragrans Houtt) Production in Nutmeg and Coconut Mixed-Planting System in Wakatobi District in Indonesia

“…i) To explore the interaction among individual plants in the planting system, Relative Neighbor Effect (RNE) analysis was used based on equation from Markham and Chanway (1996) in Bachmann et al (2005), Oksanen et al (2006). RNE was calculated based on the sum of branches per plant and weight of kernel per plant per year, as follows:…”