Once a cohort of weeds has established in a field, its success depends primarily on its survival, discussed in the following section, and its growth, discussed here. Two types of growth rate are relevant to understanding the growth potential of weeds. Absolute growth rate is the addition of biomass per unit time (g week1), whereas relative growth rate (RGR) is the biomass added per unit biomass per unit of time (g g 1 week1). In most species, RGR declines as the plant grows (Grime & Hunt, 1975; Spitters & Kramer, 1985; Ascencio & Lazo, 1997). This occurs because (i) a greater proportion of tissue is nonphotosynthetic in larger plants, (ii) maintenance respiration increases disproportionately with plant size, (iii) self-shading increases as plants grow, and (iv) larger leaves have less favorable source/sink relationships for photosynthesis (Chapin, Groves & Evans, 1989). Since RGR varies with plant size and with environmental conditions, the maximum RGR achieved by young plants in an optimal environment forms a useful basis for comparing species.
Agricultural weeds have the highest maximum RGR of any large category of plants. For example, in Grime & Hunt's (1975) analysis of growth rate of 132 British species, annuals had the highest maximum RGR of the several groups analyzed, and the agricultural annuals were mostly in the higher end of this class. Perennial agricultural weeds also have high RGR. For example, Poa annua and Convolvulus arvensis had the highest and third highest RGR measured. Grime & Hunt (1975) also compared occurrence of species in four RGR categories in 29 British habitats. Plants of manure piles had the greatest proportion of high RGR species, followed by those of enclosed pastures, arable land, and meadows. In short, productive agricultural habitats tend to favor plants with high RGR.
From a management perspective, the most important plants to compare with weeds are crops. Seibert & Pearce (1993) compared growth parameters of four weed and two crop species (Table 2.7). They found that RGR declined as seed size increased, such that Xanthium strumarium, an exceptionally large-seeded weed, behaved more like the crops. High RGR for the small-seeded weed species was primarily due to higher leaf area ratio (LAR, leaf area/plant weight) rather than higher net assimilation rate (NAR, change in plant weight/leaf area). That is, differences in growth rate due to seed size were attributable to morphology rather than physiology. The smaller-seeded species (weeds) put a greater proportion of plant mass into leaves (high LWR, leaf weight ratio) and had thinner leaves (high SLA, specific leaf area) than the large-seeded weed (X. strumarium) and the crops. The proportion of biomass invested in roots was lower in the weeds, but their root diameter was less so that total length of roots increased more quickly than in the crops. To some extent the particular patterns found by Seibert & Pearce (1993) probably depended on the choice of species. Chapin, Groves & Evans (1989) decreased this problem by comparing weed, domestic, and progenitor taxa in a single genus, Hordeum. They too found that seed size explained most of the variation in RGR, and again, the weeds had smaller seeds and higher RGR than the crops. The reason was that large seeds make large seedlings, and larger plants tend to have lower RGR regardless of whether the comparison is within a species or between species.
Because small-seeded weeds have a higher RGR than the larger-seeded crops, they tend to catch up in size eventually. As an extreme example, the initial 500-fold difference in the seed size of maize and redroot pigweed (Table 2.6) may be reduced to a two-fold difference in the size of the mature plants if each species is allowed to grow without competition (Mohler, 1996). Although the large initial size of most crop species gives them a lower RGR than many weeds, the larger size is still competitively advantageous. At emergence, the crop has a greater leaf area and a larger root system than the weed.
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