Previous studies on competition between two or more plants used to focus on the differences in above- and belowground biomass between target plant with and without neighbors, to express the performance of the target plant under competition. In this study, we measured hydraulic conductance in the entire root system (kR) with high pressure flow meter, to compare the ability of two co-occurring plants to compete for water. Root-specific hydraulic conductance (kS) was calculated by dividing kR by the biomass of entire root system. Loblolly pine (Pinus taeda L.) and cherrybark oak (Quercus pagoda Raf.) seedlings were used in this study. Two seedlings were planted in one 2-liter pot, which could be pine and pine, oak and oak, or pine and oak. There was a control group in each of the three cultivations, in which a vertical board was installed across the middle of the pot to avoid root competition. Meanwhile, in order to minimize aboveground competition, a vertical board covered by polished aluminum foil was installed across the middle of each pot. High and low rates of fertilization were applied to each cultivation pattern. There were eight replications in each treatment group composed of different cultivations and fertilization rates.
Results/Conclusions
The kR of pine seedlings was 2-3 times higher than that of oak seedlings. Pine and oak seedlings also had different hydraulic responses to the different rates of fertilization. Under low fertility, kR of oak seedlings growing with pine was increased by the development of larger root and shoot systems, indicating a size-symmetric competition mode, while pine seedlings did not show any significant differences in kR due to the presence of competitors. However, under high fertility, kR in pine seedlings growing in mixed culture with oak was increased through increased kS, instead of larger root or shoot systems, indicating a size-asymmetric root competition mode, but no significant differences were observed in kR or kS in oak seedlings due to the presence of competitors. Loblolly pine and cherrybark oak are early and late successional species, respectively. Our results suggest that they have different ways to adjust root hydraulic conductance. More information is needed before we can determine of these changes in hydraulic conductance are species or genera specific, or are general characteristics of early and late successional tree species. We conclude that root hydraulic conductance is dependent on species, soil nutrient availability and the presence of competitors.