Ten days after planting (16 October, 2003), 12·5 % of the bulbs had emerged. Emergence rapidly increased to 72 % after a further week, reaching a maximum of 95 % one month after planting. Emergence tended to be delayed in the smallest bulbs, with 5 % of bulbs (all less than 0·09 g) failing to emerge. While failure to emerge tended to occur in smaller bulbs, the three smallest bulbs (less than 0·033 g) did emerge. Oxalis attained maximum biomass and flowering 3·5 months earlier (mid-January) than Lolium (mid- to late-April), with complete die-back of Oxalis above-ground biomass at the time of Lolium maximum biomass.
Oxalis responded to high nutrients with a significant increase in above-ground biomass (F1,36 = 106·2, P < 0·0001; Fig. ), and a significant decrease in root biomass (F1,18 = 12·6, P = 0·003; Fig. A) and root mass ratio (RMR; F1,18 = 47·8, P < 0·0001; Fig. B). High nutrients also caused a significant increase in leaf N (F1,36 = 28·0, P < 0·0001; Fig. A) and, to a lesser degree, P concentration (F1,36 = 11·5, P = 0·002; Fig. B), but did not significantly change N/P ratios (F1,36 = 3·7, P = 0·064; Fig. C). High nutrients increased the total number of Oxalis daughter bulbs (F1,38 = 73·2, P < 0·0001; Fig. A), but did not have an overall effect on total daughter bulb biomass (Fig. B). However, the increase in the number of daughter bulbs was due to much higher values under no competition compared with all other treatments, while under competition the total number and biomass of bulbs was as low as under low nutrients (FN × C; 1,38 = 46·2, P < 0·0001 and FN × C;1,38 = 13·4, P < 0·001, respectively; Fig. A, B). Daughter bulbs were smaller (lower weight) at high than at low nutrient levels (F1,38 = 21·3, P < 0·0001; Fig. C).
Fig. 1. Above-ground biomass of Oxalis and Lolium grown under high (H, no hatching) and low nutrients (L, hatched) and either alone or in competition with the reciprocal species. Data are for the first harvest at the time of Oxalis maximum growth (see text). (more ...)
Fig. 2. (A) Below-ground biomass and (B) root mass ratio (g roots g−1 total biomass) of Oxalis and Lolium grown under high and low nutrients. Data are for the first harvest at the time of Oxalis maximum growth (see text). Error bars are s.e. of the mean. (more ...)
Fig. 3. (A) Leaf nitrogen concentration, (B) phosphorous concentration and (C) N/P ratios of Oxalis and Lolium grown under high (H, no hatching) and low nutrients (L, hatched) and either alone or in competition with the reciprocal species. Data are for the first (more ...)
Fig. 4. (A) Total number of daughter bulbs, (B) total daughter bulb biomass and (C) mean mass of individual daughter bulbs of Oxalis grown under high (H, no hatching) and low (L, hatched) nutrients and either alone or in competition with Lolium. Error bars are (more ...)
Competition by Lolium caused a significant decrease in above-ground biomass (F1,36 = 7·0, P = 0·012; Fig.1) and of leaf N concentration in Oxalis (F1,36 = 5·5, P = 0·025; Fig. A) but it did not affect leaf P concentration and N/P ratios (F1,36 = 0·39, P = 0·54, and F1,36 = 0·99, P = 0·33, respectively; Fig. B, C). The effect on leaf N concentration tended to be stronger under low nutrients (FN × C;1,36 = 3·8, P = 0·06; Fig. A). Competition by Lolium strongly reduced the total number and total biomass of Oxalis daughter bulbs (F1,38 = 215·4 and 60·2, respectively; P < 0·0001 for both), with a much stronger effect at high nutrients (see above; Fig. A, B); however, it did not affect the mass of individual daughter bulbs (Fig. C).
Larger initial bulbs in Oxalis resulted in higher above- and below-ground biomass (F1,36 = 40·0, P < 0·0001, and F1,18 = 21·2, P < 0·0001, respectively; Fig. A, B). However, this effect was mostly restricted to smaller bulbs up to approx. 0·2 g, after which there was no change in the response variables (Fig. ). An exception was when Oxalis was grown alone under low nutrients, where below-ground biomass continued to increase (although not linearly) with initial bulb mass (Fig. B). Larger mother bulbs also produced larger daughter bulbs (i.e. higher mass per bulb; F1,38 = 10·2, P < 0·003; Fig. C) and tended to result in an increase in the total biomass of daughter bulbs (i.e. a nearly significant effect of the initial bulb mass covariate in the ANOVA; F1,38 = 3·9, P = 0·055). When treatments were considered separately, a positive linear correlation between initial bulb mass and total daughter bulb biomass was only significant when Oxalis was grown alone under low nutrients (r2 = 0·4, P = 0·04, F1,8 = 6·4; data not shown).
Fig. 5. Relationship between (A) above-ground biomass and (B) below-ground biomass and the mass of the mother bulb of Oxalis plants grown under high or low nutrientsand either alone or in competition with Lolium. For each treatment data were fitted to an exponential (more ...)
In Lolium, high nutrients increased tiller biomass in the first (F1,38 = 94·8, P < 0·0001) and final (F1,40 = 89·9, P < 0·0001) harvests, and the total spike biomass in the final harvest (F1,40 = 41·8, P < 0·0001; Figs and ). While Lolium root biomass in the first harvest was not affected by the nutrient treatment (Fig. A), RMR decreased significantly under high nutrients (F1,20 = 6·3, P = 0·02; Fig. B). Nutrients resulted in a significant increase of Lolium leaf N concentration in the first harvest (F1,38 = 28·8, P < 0·0001; Fig. A), but a significant decrease in leaf P (F1,38 = 11·4, P = 0·002; Fig. B). Consequently, Lolium leaf N/P ratios for the first harvest decreased under low nutrients (F1,38 = 50·5, P < 0·0001; Fig. C).
Fig. 6. Total biomass of tillers and spikes of Lolium at its peak biomass when grown under high (H, no hatching) and low (L, hatched) nutrients and either alone or in competition with Oxalis. Error bars are s.e. of the mean. P-values for the effects of competition (more ...)
Competition by Oxalis significantly reduced the first-harvest above-ground biomass of Lolium regardless of nutrient treatment (F1,38 = 12·0, P = 0·001; Fig. ). However, Oxalis competition did not affect final Lolium tiller and spike biomass (F1,40 = 0·5, P = 0·13 and F1,40 = 2·3, P = 0·14, respectively; Fig. ). Leaf N concentration and N/P ratios of Lolium were significantly reduced under competition with Oxalis (F1,38 = 5·2, P = 0·03 and F1,38 = 5·1, P = 0·03, respectively; Fig. A, C), while leaf P concentration was not affected by competition (F1,38 = 0·05, P = 0·55; Fig. B).
Overall, biomass responses to nutrients and to competition were similar for both species (FN × SP 1,75 = 0·61, P = 0·08; FC × SP 1,75 = 0·3, P = 0·21; three-way ANOVA with species, nutrients and competition; Figs and ). Decreases in leaf N concentration under low nutrients were greater for Oxalis than for Lolium (FN × SP 1,75 = 5·2, P = 0·03; Fig. A). In contrast, under low nutrients leaf P concentration decreased in Oxalis while it increased in Lolium (FN × SP 1,75 = 22·8, P = 0·0001; Fig. B). Consequently, nutrient effects on the N/P ratio were stronger for Lolium than for Oxalis (FN × SP 1,75 = 4·6, P = 0·04; Fig. C).
Oxalis-invaded soils sampled in Menorca during 2003 had significantly higher P concentrations than paired non-invaded soils (one-tail paired t-test; t22 = 1·7, P = 0·02; Fig. ).
Fig. 7. Average soil available P from field sites either invaded or not invaded by Oxalis. Error bars are s.e. of the mean. Although overall averages are shown, field sampling and statistics were conducted with a paired design (Oxalis-invaded, non-invaded) for (more ...)