For the three invader species, we examined if there was a relationship between percentage germination or germination time (days after seed insertion in the gaps), and species richness, percentage light transmittance or neighbour biomass. Percentage germination decreased significantly with species richness in F. arundinacea (linear regression, y = 70·608 −2·243x, P = 0·048, r2 = 0·08), while in L. perenne a negative relationship was found between germination percentage and biomass of neighbour plants (linear regression, y = 106·355 − 50·984x, P = 0·016, r2 = 0·12). The influence of light on germination percentage was not significant, nor were there significant regressions for germination time (P > 0·05 for all species).
Seed mass (averaged per invader species) could not explain the observed variation in germination percentage (all invaded gaps; linear regression, P > 0·05), but had a significant effect on germination time, with longer germination times in larger seeds (linear regression, y = 5·043 + 0·316x, P = 0·038, r2 = 0·03).
Because several invaders were smaller than the harvesting height, total leaf length per plant rather than biomass was used as measure for invader growth. Invader leaf length at the end of the first growing season did not depend on the species richness of its neighbours, but increased significantly with increasing percentage photosynthetically active radiation (PAR) transmittance and was negatively related to the biomass of its neighbour plants at the end of the first growing season (linear regressions, Fig. 2). This pattern was observed for the three invader species (except for biomass in L. perenne), with percentage PAR transmittance O (measured when the plants around the gaps were recently mown, and a better predictor for leaf length in year 1 than percentage PAR transmittance C) being the best predictor in F. arundinacea and L. perenne (r2 = 0·21 and r2 = 0·23, respectively), and neighbour plant biomass in P. trivialis (r2 = 0·18). In the second growing season, in contrast, invader leaf length declined significantly towards the higher richness levels (linear regression, Fig. 3). As in year 1, leaf length was positively associated with percentage PAR transmittance (in year 2 percentage PAR transmittance C, measured in more closed gaps, was a better predictor for leaf length than percentage PAR transmittance O), and negatively with biomass of the neighbour plants (measured in year 2), but this time the regression slopes were steeper (except for neighbour biomass in P. trivialis). In F. arundinacea and L. perenne the percentage of PAR penetrating in the closed gaps was the best predictor of leaf length in the second season (r2 = 0·24 and r2 = 0·45, respectively), while in P. trivialis leaf length was associated most strongly with species richness (r2 = 0·30). For P. trivialis, the relative importance of biomass and light in affecting leaf length changed over time, with light becoming more important in year 2. Neighbour plant biomass and percentage PAR transmittance were correlated, with the strongest correlations between percentage PAR transmittance O and neighbour biomass year 1 (Pearson correlation: n = 143, r = −0·458, P < 0·001) and between percentage PAR transmittance C and neighbour biomass year 2 (Pearson correlation: n = 143, r = −0·217, P = 0·009).
Figure 2. Effect on invader performance of: species richness; percentage transmittance of photosynthetically active radiation (PAR transmittance O, measured when vegetation around gaps was recently mown and invaders were younger); and biomass of neighbour plants in year 1, expressed as total leaf length per plant at the end of the first growing season, for the three different invader species.
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Figure 3. Effect on invader performance of: species richness; percentage transmittance of photosynthetically active radiation (PAR transmittance C, measured after prolonged regrowth of neighbour plants); and biomass of neighbour plants in year 2, expressed as total leaf length per plant in the second growing season, for the three different invader species.
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To explain differences in richness effect on invader by year, we determined whether and how species richness modified percentage PAR transmittance, biomass of the neighbour plants or Imax for both times of measurement. As community traits were not affected by invader identity (anova with factors invader species and richness, invader effect for PAR O: F2,131 = 0·236, P = 0·790; for PAR C: F2,131 = 0·406, P = 0·667; for biomass year 1: F2,131 = 0·786, P = 0·458; for biomass year 2: F2,131 = 1·202, P = 0·304), correlations were calculated with all invaders combined. Species richness correlated negatively with percentage PAR transmittance and positively with the biomass of the plants surrounding the gaps in both harvesting years (Table 1). For Imax, however, a different result was found between the two years. When Imax was calculated with the biomass data of the first growing season, no relationship with species richness was observed, whereas Imax increased with richness in year 2. Regressions of invader leaf length on Imax show that Imax negatively affected invader growth in the second growing season, while leaf length in the first year was not associated with Imax (Fig. 4). Values of Imax were also higher in year 2 than in year 1 (anova, factor = year, F1,283 = 23·568; P < 0·001), with transgressive over-yielding (Imax > 1) occurring in 80% of the mixtures in year 2 vs. only in 55% of the mixtures in year 1. To summarize, in year 1 species richness did not affect the level of complementarity, and differences in complementarity did not affect invader growth. In contrast, in year 2 species richness did increase complementarity which, in turn, suppressed the invaders.
Table 1. Spearman's rank correlations between species richness of communities and percentage light transmittance in gaps, biomass of neighbour plants surrounding gaps, and Imax (index for assessing degree of transgressive over-yielding)
|Species richness||% PAR transmittance||Biomass neighbours||Imax|
|O||C||Year 1||Year 2||Year 1||Year 2|
|Correlation coefficient||−0·273||−0·373|| 0·438|| 0·367||−0·124|| 0·514|
|P|| 0·001||<0·001||<0·001||<0·001|| 0·140||<0·001|
Figure 4. Relationship between invader leaf length and Imax (index indicating complementary resource use if Imax > 1) in the first and second growing seasons with all richness levels combined.
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Invaders had a significantly higher leaf length in year 2 compared with year 1 (Figs 2 and 3; anova with factors invader and year, for year: F1,216 = 15·302, P < 0·001). Especially in the monocultures and two-species mixtures, invader leaf length had increased. Community age also affected neighbour plant productivity, but not in the same way at all richness levels (Fig. 5). At the end of the first growing season, a wide range of monoculture productivity was observed, while the two-species mixtures all had very similar biomass. Also, the range was small in the four- and eight-species mixtures (Fig. 5a). However, in year 2 the monoculture biomasses converged (especially through elevated productivity of the mixtures that were least productive in year 1), while a wide range in productivity had developed in the mixtures (Fig. 5b). Whereas monoculture biomass increased only little in year 2, most of the two-, four- and eight-species mixtures became much more productive.
Figure 5. Neighbour plant biomass in (a) first and (b) second growing season for different species compositions. □, Monocultures; •, bicultures; ▵, four-species mixtures; ◆, eight-species mixtures. Ae, Arrhenatherum elatius; At, Agrostis tenuis; Cc, Cynosurus cristatus; Dg, Dactylis glomerata; Fp, Festuca pratensis; Fr, Festuca rubra; Hl, Holcus lanatus; Pp, Phleum pratense. Symbols represent means of cumulative biomass of the eight neighbours for each composition ± 1 SE (n = 6).
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Further, a positive relationship between invader leaf length in all the gaps and invader seed mass (averaged per invader species) was found in both year 1 (linear regression, y = 0·990 + 0·750x, P < 0·001, r2 = 0·41) and year 2 (linear regression, y = 1·718 + 0·724x, P = 0·005, r2 = 0·09).
Survival of the invaders was highest in F. arundinacea (78%), while in L. perenne 65% and in P. trivialis only 53% of the plants survived until the second year. A logistic regression was used to examine whether invader survival until the second growing season was influenced by species richness, percentage PAR transmittance, neighbour biomass or invader leaf length. Neighbour biomass was a significant predictor of survival for the invader F. arundinacea, with more surviving individuals as neighbour biomass decreased (d.f. = 1, Wald = 5·350, P = 0·021). Survival of L. perenne could be predicted by means of its leaf length at the end of the first growing season, with a higher probability of survival when leaf length was higher (d.f. = 1, Wald = 6·487, P = 0·011). For P. trivialis no significant relationship was found.