3.1 Effects of P level, light intensity, and competition on plant height
Under full light and with the increase in P content, the height of PP and PC decreased, but the other plants changed not obvious. The plant height of PP was significantly higher with low and medium P than with high P (p<0.05) (Fig. 2d). With 10% light intensity, the height of C. odorata plants increased with the increase of P in all polyculture patterns (CC, CP, and CS). The height of CS plants was significantly higher with high P than with low or medium P (p<0.05) (Figs. 2a, 2c). However, the growth of PP plants was inhibited but that of PC plants tended to be increased with the increase in P level (Fig. 2d), but no significance was detected (Fig. 2b).
Light intensity significantly affected the heights of C. odorata, S. sericea, and P. grandis plants (p<0.05). The height of P. grandis plants was also significantly affected by competition (p<0.05). The height of CS was significantly affected by P level, and the interaction between light intensity and competition affected C. odorata growth in different polyculture patterns. (p<0.05) (Table 1).
3.2 Effects of phosphorus level, light intensity, and competition on leaf area
With full light intensity, C. odorata leaf area was largest with medium P regardless of the polyculture pattern; C. odorata leaf area in CC and CP pots was smallest with high P; and C. odorata leaf area in CS pots was smallest with low P (Figs. 3a, 3c). As the P content of the soil increased, the leaf area increased for S. taccada and decreased for P. grandis in spite of monoculture or interplanting. The leaf area of P. grandis was significantly smaller with high P than with low or medium P (p < 0.05) (Figs. 3b, 3d). However, the leaf area of all plants was reduced by the low light intensity. Interestingly, the leaf area of C. odorata tended to increase with the increase in P level and was significantly larger with high P than with low P (p < 0.05) (Figs. 3a, 3c). The leaf area of PC was significantly greater with high P than with low P (p < 0.05) (Fig. 3d). Under the low light intensity, the leaf area of S. taccada did not significantly differ among P levels (Fig. 3b).
Leaf surface area was significantly affected by light intensity and also by competition. The P content of the soil significantly affected the leaf area of C. odorata in CC, CP, and CS pots (Table 1). The leaf area of CC, CP, and CS were also significantly influenced by the interaction between P level and light intensity (p < 0.05). The interaction between light intensity and competition significantly affected the leaf area of CS and SC (p < 0.05) (Table 1).
3.3 Effects of phosphorus level, light intensity, and competition on photosynthesis
To examine the effects of P level, light intensity, competition, and their interactions on photosynthesis, we measured the maximum net photosynthetic rate (MNPR) of mature leaves of the experimental plants. The MNPR of CS was highest among all plants of C. odorata with full light and low P. Interestingly, the MNPR of CS decreased as the P content increased, and was significantly lower with high P than with low or medium P (p < 0.05) (Figs. 4a, 4c).
In comparison, the MNPR of SS with low light were significantly lower than those of S. taccada with full light regardless of P level, but the MNPR of SC with low P was significantly higher than that of SC with medium or high P under full light intensity (p < 0.05) (Fig. 4b). The MNPR of PP was the highest with low P and with full light intensity and decreased with the increase in P level. The MNPR of PC was significantly higher with high P than with low or medium P (p < 0.05) (Fig. 4d).
In general, the MNPR was lower for C. odorata than for S. taccada, but was greater for C. odorata than for P. grandis under the same conditions. Light intensity, P level, and their interactions significantly affected the MNPR of CS and tended to affect the maximum photosynthetic rate of CP although the effect was not statistically significant in the latter case. The competition had slight effects on the maximum photosynthetic rate of C. odorata but had significant effects on S. taccada and P. grandis (Table 1).
3.4 Effects of phosphorus level, light intensity, and competition on relative growth rates
Among the treatments, the light intensity had the largest effect on the relative growth rates (RGR). The RGR of C. odorata and S. taccada were significantly greater with full light intensity than with low light intensity, (p < 0.05), but were unaffected by P level or polyculture pattern (Figs. 5a, 5b, 5c). The RGR of PP and PC with low or medium P level under full light was also significantly greater than those under shading light (p < 0.05). Interestingly, the RGR of CP was significantly increased by high P in the full light intensity treatment (p < 0.05) (Fig. 5d).
However, the RGR was significantly lower for SC than for SS with high P (p < 0.05) (Fig. 5b). PP grew faster than PC, and the RGR of both was significantly greater under full light than those under low light with low or medium P (p < 0.05). However, the RGR of PC was significantly lower with low and medium P than with high P and with low light intensity (p < 0.05) (Fig. 5d).
Comparing the same treatments, the RGR of C. odorata was significantly higher than that of P. grandis and S. taccada, indicating that C. odorata had better adaptability to the environment. Light and the interaction of P and light significantly affected the RGR of C. odorata, while P had a significant effect on CS. The RGR of S. taccada was significantly affected by light and the interaction of light and competition, and P. grandis growth was significantly affected by light, competition, and P and their interactions (p < 0.05) (Table 1).
3.5 Effects of phosphorus level, light intensity, and competition on relative yield and aggressivity coefficient
The relative competitiveness of P. grandis, S. taccada, and C. odorata was compared using relative yield (RY) and the aggressivity coefficient (AG). Light intensity significantly affected the RY of C. odorata and S. taccada, and P level only significantly affected the RY of C. odorata when competing with S. taccada (p < 0.05). The AG of C. odorata was significantly affected by light intensity and the interaction between light intensity and P level (p < 0.05) (Table 2). The RY of P. grandis was significantly affected by light intensity, P level, and their interaction, while the RY of C. odorata was significantly affected only by light intensity (p < 0.05). The AG of C. odorata was significantly affected by light intensity, P level, and competition with P. grandis (p < 0.05) (Table 3).
With full light, the RY of C. odorata increased as the P level increased, and was significantly greater with medium and high P than with low P (p < 0.05). However, the RY of S. taccada decreased with the increase in P level and was significantly greater with low P than with high P (p < 0.05). The AG of C. odorata increased significantly as the P level increased, and was significantly greater with high P than with medium or low P (p < 0.05). Interestingly, under low light, the AG values < 0 indicated that S. taccada was more competitive than C. odorata (Table 4).
The RY of P. grandis was smallest with low P and full light and was significantly greater with high P than with medium or low P under low light (p < 0.05). However, the RY of C. odorata was the smallest with a high P. The AG of C. odorata was significantly higher with high P than with low or medium P under full light (p < 0.05). With the increase in P level, the AG of C. odorata declined and that indicated the competitiveness was reducing with full or low light (Table 5).
Overall, competition between C. odorata and P. grandis or S. taccada was affected by both light and P level. Especially with low light, increases in the P content of the soil reduced the ability of C. odorata to compete with the native species.