The CIs of the treatment in which the total amount of nutrient was 4 g per pot were the largest between amounts of nutrient supply. In two of the three 4 g per pot treatments, CIs differed significantly from zero (Table 4-3). On the contrary, in the three 16 g per pot treatments, CIs did not differ significantly from zero (Table 4-3). However, there were no significant differences in means of CIs between the amounts of nutrients (F2, 71 = 1.802, P = 0.17) probably because variances of CIs within each treatment were not necessarily small (Table 4-3). Means of CIs in heterogeneous treatments were not always larger than those in homogeneous treatments. A result of ANOVA also showed that there were no differences in means of CIs between the patterns of nutrients (F2, 71 = 0.175, P = 0.84).
Discussion
Means of the total biomass per plant did not significantly differ between the heterogeneous and homogeneous conditions although selective root placement in a rich patch was clearly observed in the heterogeneous conditions. No significant difference in total biomass between patterns of nutrient contrasts with Prediction 1 that plant with selective root placement in a heterogeneous condition would result in an increase in total biomass. There are a few reasons. First, construction and maintenance of roots in a nutrient-poor patch would not pay in comparison with absorbed amounts of nutrients. Reasonable amounts of roots in a nutrient-poor patch may contribute to the growth of a plant in a heterogeneous environment. The statistically significant difference in root biomass in a poor patch between patterns of nutrients suggests that plants responded to heterogeneity, although the responses did not increase the total biomass in the heterogeneous conditions. Second, if roots were selectively placed in a rich patch and only if there had been no increases in rates of nutrient uptake per unit of roots in a rich patch, the obtained results could occur. Though we do not know the rates of nutrient uptake of I.tricolor in a nutrient-rich patch, values of the root proportion significantly differed between the patterns of nutrients, which means that I.tricolor can respond to a heterogeneous environment somehow. This point is inconsistent with the observation of Blair (2001).
Moreover, the extent of the root proportion of a rich patch in our study (for example, 0.64 for the 16 g treatment) is more or less the same as mean (0.67) of the root proportions of many species. The root proportions were calculated as a proportion of root biomass in nutrient-rich patches to the total of root biomass of nutrient-rich patches and nutrient-poor patches from the appendix of Robinson (1994).
The competition intensities in the heterogeneous conditions were not always severer than those in the homogeneous condition despite the selective root placement to rich patches occurred in the plants growing with neighbours. These results contrast with Prediction 2 that selective root placement to a nutrient-rich patch would cause intense competition in a heterogeneous environment.
This inconsistency could be caused by a lack of the dominant use of a rich patch by a few plants in a heterogeneous condition because of large proportion of the substrate volume of a rich patch. The size
of a rich patch of our study was much larger than that of other studies. The large-sized patch including large amounts of nutrients would allow all plants in a pot to access to the rich patch, which is less likely to cause the dominant use of the rich patch by a few plants. Actually, a percentage of the substrate volume of a rich patch was 1.3% (Robinson et al. 1999; Hodge et al. 1999) and 6.5%
(Day et al. 2003c) although it was 50% in our study. The lack of the dominant use of a rich patch by a few plants could also be caused by relatively large amounts of nutrient supply per pot for plants of I. tricolor, but this was not the case of our study. In this study, the total amount of nutrient supply per pot was not large enough to make saturate all plants in a pot because there was significant reduction of growth of the plants with neighbours at least in the 4 g treatments. Therefore, competition would not be intensified if a nutrient-rich patch occupies a relatively large area in the total area in a heterogeneous environment.
The CIs in the 4 g treatment were larger than that in the 8 g or 16 g treatment, and they differed significantly from zero irrespective of patterns of nutrients. These results were consistent with Prediction 3 that competition would be intense in a nutrient-poor environment regardless of distribution of nutrients. In this study, the patterns of nutrient supply did not change absolute amounts of available nutrients, but they did spatial distributions of nutrients within a range in which a plant would respond. That is, even if extent of heterogeneity (i.e., contrast, sensu Kotliar and Wiens 1990) varies, the amount of nutrients potentially available to plants should not vary between the heterogeneous environments. This means that reduction of nutrients would inevitably affect growth of plants independent of heterogeneity. Thus, the intensified competition in a nutrient-poor environment was independent of spatial patterns of nutrient supply. Especially in a nutrient-poor environment, heterogeneity has been underappreciated although the competition is thought to be intensified (Schenk 2006). Indeed, results of our study exhibited that limited nutrient supply would intensify competition in a heterogeneous environment as well as in a homogeneous environment.
Competition and amount of nutrients were more pronounced on growth of I. tricolor than spatial pattern of nutrients. In this study, heterogeneity only changes distribution of nutrients and the
total amount of nutrients is constant between a heterogeneous condition and a homogeneous condition. On the other hand, competition and amount of nutrients directly restrict the amount of available resources to plants. Thus, they would negatively affect the resource acquisition of plants.
Therefore, our results suggest that growth of plants might be more affected by competition and/or amounts of nutrients than by heterogeneity.
In conclusion, amount of nutrients has more profound effects on growth of a plant than pattern of nutrients, which can be enhanced by competition. This tendency would be conspicuous if the amount of nutrients is very limited.
Table 4-1 Multiple analysis of variance of root biomass in a rich patch and a poor patch. Multivariate test statistics (a) and the corresponding univariate test statistics (b) are given for pattern of nutrients, amount of nutrients, competition and the interactions between them. The data were log transformed prior to analysis. A significant difference is showed in bold fonts. df denotes degrees of freedom; F, F-statistics.
(a)
Sources Wilk’s Lambda df F P
Pattern of nutrients (P) 0.668 4,194 10.855 < 0.001
Amount of nutrients (A) 0.918 4,194 2.123 0.079
Competition (C) 0.966 2,97 1.724 NS
P × A 0.926 8,194 0.951 NS
P × C 0.979 4,194 0.530 NS
A × C 0.923 4,194 1.984 0.098
P × A × C 0.986 8,194 0.166 NS
(b)
Sources df Rich patch Poor patch
F P F P
P 2 1.408 NS 3.275 < 0.05
A 2 0.901 NS 3.076 0.051
C 1 1.321 NS 0.011 NS
P × A 4 0.933 NS 0.230 NS
P × C 2 0.044 NS 0.519 NS
A × C 2 3.353 < 0.05 2.973 0.056
P × A × C 4 0.179 NS 0.160 NS
Error 98
Table 4-2 Results of three-way multivariate ANOVAs of shoot, root and total biomass (a). Pattern of nutrients, amount of nutrients and competition are independent variables. The corresponding univariate ANOVAs is shown as (b). The data were log-transformed prior to analysis. A significant difference is showed in bold fonts. df denotes degrees of freedom; F, F-statistics.
(a)
Sources Wilk’s lambda df F P
Pattern of nutrients (P) 0.921 6,282 1.965 0.071
Amount of nutrients (A) 0.583 6,282 14.567 < 0.001
Competition (C) 0.615 3,141 29.422 < 0.001
P × A 0.946 12,373 0.657 NS
P × C 0.956 6,282 1.060 NS
A × C 0.905 6,282 2.419 < 0.05
P × A × C 0.960 12,373 0.483 NS
(b)
Sources df Shoot Root Total
F P F P F P
P 2 2.38 0.096 0.40 NS 1.93 NS
A 2 15.62 < 0.001 1.57 NS 8.66 < 0.001
C 1 43.91 < 0.001 1.59 NS 33.40 < 0.001
P × A 4 0.21 NS 0.16 NS 0.09 NS
P × C 2 0.14 NS 0.005 NS 0.16 NS
A × C 2 2.29 NS 4.90 < 0.01 2.93 0.057
P × A × C 4 0.16 NS 0.44 NS 0.22 NS
Error 143
Table 4-3 Means of competitive intensity in treatments. Treatments are the combination of amount and pattern of nutrients. Results of one-sample t-tests. A significant difference is showed in bold fonts. The significance at P < 0.05 was assessed using α-levels calculated according to the sequential Bonferroni technique (Rice 1989). N denotes number of replicates; Mean CI (SE), mean competitive intensities (standard errors); SD, standard deviations; T, T-statistics.
Treatments
Amount of nutrients
Pattern of nutrients
N Mean CI (SE) SD T P
4 g/pot 100:0 9 0.686 (.20) 0.60 3.427 0.0090
75:25 9 1.098 (.28) 0.83 3.951 < 0.05
50:50 9 0.963 (.23) 0.69 4.207 < 0.05
8 g/pot 100:0 9 0.632 (.33) 0.98 1.929 0.0899
75:25 9 0.659 (.35) 1.03 1.912 0.0922
50:50 8 0.783 (.24) 0.68 3.270 0.0137
16 g/pot 100:0 9 0.429 (.30) 0.90 1.430 0.1906
75:25 9 0.447 (.54) 1.61 0.831 0.4301
50:50 9 0.350 (.37) 1.10 0.958 0.3661
Figure 4-1 Schematic diagram of a pot of a plant without neighbours (a) and a pot of a plant with neighbours (b). Diagrams on left-hand side show even fertilization conditions and those on right-hand side uneven fertilization conditions. Shaded regions represent fertilized areas.
Concentration of fertilizer in a black area is twice as high as that in a grey area. White dots denote positions of plants.
Figure 4-2 Means (± SE) of proportion of root biomass in a rich patch to the total root biomass per pot were plotted against patterns of nutrients: left a plant in a pot was grown without neighbours (a);
right a plant in a pot was grown with neighbours (b). Circles indicate the treatments in which a pot contained 4 g of fertilizer; squares, 8 g; triangles, 16 g. The same letter indicates no significant difference between treatments at P < 0.05.
Figure 4-3 Mean (± SE) of total biomass of Ipomea tricolor: left a plant was grown without neighbours (a); right a plant was grown with neighbours (b). Circles indicate the treatments in which a pot contained 4 g of fertilizer; squares, 8 g; triangles, 16 g. See Table 2 for the associated analyses.
The same letter indicates no significant difference between treatments at P < 0.05.
Appendix 4-1 Mean of root biomass in a rich patch and a poor patch in each treatment. Treatments are the combinations between competition, amounts of nutrients and patterns of nutrients. Mean denotes mean root biomass and SE, standard error.
Treatments Rich patch Poor patch
Competition
Amount of nutrients
Pattern of
nutrients Mean (g) SE Mean (g) SE
Without 4 g/pot 100:0 0.310 0.041 0.204 0.019
neighbours 75:25 0.378 0.049 0.262 0.033
50:50 0.305 0.032 0.291 0.029
8 g/pot 100:0 0.299 0.051 0.228 0.045
75:25 0.283 0.029 0.206 0.033
50:50 0.269 0.028 0.254 0.026
16 g/pot 100:0 0.289 0.034 0.167 0.028
75:25 0.280 0.061 0.165 0.036
50:50 0.226 0.029 0.213 0.032
With 4 g/pot 100:0 0.213 0.039 0.163 0.025
neighbours 75:25 0.237 0.011 0.195 0.033
50:50 0.222 0.009 0.213 0.015
8 g/pot 100:0 0.270 0.034 0.192 0.032
75:25 0.260 0.024 0.207 0.027
50:50 0.252 0.038 0.224 0.046
16 g/pot 100:0 0.333 0.053 0.205 0.040
75:25 0.325 0.061 0.240 0.050
50:50 0.226 0.054 0.224 0.044