We conducted the phylogenetic comparison to minimize phylogenetic bias as trait differences among species strongly depend on phylogenetic relationships, growth forms and life forms [7, 19, 38]. R. typhina has high total biomass than C. coggygia in the mixture treatment. This indicates the strong competitive effects of R. typhina on C. coggygria and that R. typhina is the superior competitor. The lnRR further validated this observation as the lnRR of R. typhina was always lower than that of C. coggygria, indicating the competitive effects on R. typhina was less than that on C. coggygria. However, we cannot conclude that mixed culture with C. coggygria facilitated the growth of R. typhina in well-watered conditions even though the lnRR of R. typhina was below zero under W3 condition. This is because the total biomass of R. typhina remained unchanged across different water regimes. Rhus typhina has also been reported to outcompete native Quercus acutissima  and Vitex negundo .
Interaction between R. typhina and C. coggygria under mixed culture condition
The shifts in biomass allocation pattern of the introduced species can create a advantage for a species over their neighbors in response to different environmental conditions [14, 39]. Both R. typhina and C. coggygria had high biomass in the W3 treatment but the total biomass of C. coggygria rapidly decreased with the soil moisture decreasing (Fig. 3). It indicated that R. typhina have stronger ability of adapting the drought environment than C. coggygria. Previous study found that R. typhina have higher total biomass facing both interspecific and intraspecific pressure than native plant under both drought and wet conditions . Drought may be the disturbance for plant growth and invasive plant always can adapt to disturbance condition, based on the disturbance hypothesis . Drought decreased the resource allocation of C. coggygria to stem but had no influence on the SBR of R. typhina (Fig. 3), and the R. typhina had lower height than C. coggygria (Fig. 1). R. typhina may have stronger the wood density and increase drought tolerance. Previous study found invasive tree Acacia mearnsii have higher wood density and resistive capacity to drought-induced cavitation than native trees in the introduced habitat . Drought changes the species composition of native plant community and make more susceptible to invasion .
Shoot height and allocation to growth have been shown to promote invasiveness . Higher shoot height, crown area, and leaf biomass in the present study together conferred R. typhina a competitive advantage over C. coggygria to obtain more light in mixture pots. According to the productivity-dependent scaling hypothesis, high aboveground productivity of the neighboring plant will impose strong asymmetric competition on the target plant [44, 45]. This asymmetric competition for light has the strongest effect on the growth of small trees , i.e., C. coggygria seedlings in the present study, as the largest share of a vital resource can be gained by a plant through a slight height advantage over its neighbors . The growth rate is probably a key functional trait linked to the invasiveness of a tree species, and it is highly associated with the invasion success of tree species [7, 38, 47]. In the mixture pots, R. typhina had slightly higher RGR-H and thus taller shoot to cover C. coggygria rapidly, suppressing the growth of the native species by shading it [11, 21]. Besides, R. typhina enabled to inhibit the growth of shoot adjacent native plant Vitex negundo var. heterophylla in the interspecific competition although native plant had higher height than R. typhina facing intraspecific competition .
For C. coggygria, the RBR increased but the root biomass decreased in mixed culture, indicating that its ability to obtain water and potential nutrients was decreased by the strong competition from R. typhina. Larger root biomass of R. typhina than that of native species were found previously [26, 27]. Furthermore, the RBR, root biomass, and WUE of R. typhina were higher than those of C. coggygria in mixed pots, which conferred R. typhina a conspicuous advantage to absorb more water and nutrients, and utilize the water more efficiently, hindering the growth of C. coggygria by decreasing its resource supply . In this situation, C. coggygria invests more on biomass production to the root and leaf to cope with the intrusion of R. typhina, but this alteration in biomass allocation strategy still cannot offset the strong competitive effects from R. typhina. Considering the aboveground advantages mentioned above, we conclude that invasive alien species possess stronger light-capturing and water/nutrients-absorbing abilities, and water use efficiency than the native species by exhibiting more acquisitive functional traits. Previous found that facing the pressure of intraspecific competition, the R. typhina have higher total biomass and higher root biomass ratio to increasing the absorption nutrient than native plant Quercus acutissima Carr. and this advantage is also maintained during interspecies competition .
This conferred R. typhina an assimilatory advantage over C. coggygria; thus, contributing to higher performance than that of C. coggygria in the mixture pot. Higher photosynthetic traits are considered typical features for the success of invasive species [7, 11, 27, 38] and may be the reason for the higher growth rate of shoot height of R. typhina in the present study. Cotinus coggygria seedlings tend to grow upward (higher shoot height, RGR-H, and SBR than those of R. typhina) to reach the upper space with higher A than that of R. typhina. Rhus typhina seedlings tend to grow horizontally (higher crown area than that of C. coggygria) to capture more light and deeply to obtain more water and nutrient (higher root biomass and RBR than those of C. coggygria). Rhus typhina maintained larger crown area, leaf biomass, and root biomass than those of C. coggygria. The upward growth of C. coggygria seedlings was strongly inhibited (indicated by distinctly lower shoot height than that of C. coggygria in monoculture) and its A was decreased most likely due to the shade environment caused by the exceedingly larger canopy of R. typhina seedlings in the upper layer in the mixture. Eventually the total biomass of C. coggygria seedlings was extremely lower than that of R. typhina seedlings in the mixture pots. Therefore, the invasive R. typhina and the native C. coggygria displayed differences in most crucial functional parameters related to growth when they were cultivated separately or together, indicating that our results support the phenotypic divergence hypothesis  rather than the phenotypic convergence hypothesis [12, 13].
Invasive alien R. typhina possesses three advantages over the native C. coggygria in the mixed culture. First, with the stronger photosynthetic capacity than C. coggygria, R. typhina was able to achieve a higher growth rate and larger amount of carbohydrate production. Secondly, in the mixture pots, higher shoot height, crown area, RGR-H, and leaf biomass enabled R. typhina to exhibit dominance earlier by obtaining more light resource and shading the native C. coggygria. This asymmetric competition caused by the higher aboveground productivity of R. typhina over C. coggygria results in positive-feedback  to R. typhina, leading to even higher competitive advantages of R. typhina. Finally, the higher root biomass and WUE of R. typhina than C. coggygria conferred R. typhina the ability to obtain water and nutrients, and utilize water more efficiently.
Drought relieves the competitive effects of R. typhina on C. coggygria
Rhus typhina maintained its dominance under drought condition despite the fact that the competitive ability of R. typhina over C. coggygria decreased and concomitantly the competitive ability of C. coggygria over R. typhina increased to some extent. This observation generally supports the fluctuating resource availability theory , which holds that a plant community’s invasibility decreases under limited resource conditions. The decrease in the competitive ability of R. typhina over C. coggygria under drought conditions was probably related to the growth inhibition of R. typhina (i.e., decrease in the shoot height, crown area, RGR-H, and leaf biomass) under drought conditions. However, the shoot height, crown area, RGR-H, and leaf biomass of R. typhina were still substantially higher than those of C. coggygria under drought condition, consistently conferring R. typhina an aboveground competitive advantage. Therefore, the asymmetric competition of R. typhina over C. coggygria was alleviated to some extent, because the light-capturing ability of R. typhina decreased to some extent, although this ability was still conspicuously stronger than that of C. coggygria. Rhus typhina has been shown to outperform native Vitex negundo under variable levels of water supply frequency but with a constant level of total supplied water .
Drought decreased all the biomass parameters of C. coggygria in the monoculture pots, whereas, those in the mixture pots was unaffected by drought, suggesting that the mixed culture alleviated the negative effects of drought on C. coggygria. This might be because R. typhina has a larger crown area than that of C. coggygria, which would intercept more light, reduce the temperature of the topsoil and evaporation of soil water, and eventually alleviate the negative effects of drought on C. coggygria when it is grown close to R. typhina. It has been reported that shade can alleviate the negative effects of drought on Acer buergerianum , which is consistent with our results. The decreased chlorophyll a to chlorophyll b ratio of C. coggygria under drought conditions provided further evidence to this view, because decreased chlorophyll a to chlorophyll b ratio enables the plant to obtain more light in shade . Therefore, the increase in the competitive ability of C. coggygria over R. typhina under drought condition was likely caused by weaker drought effects on C. coggygria in the mixture pots compared with that in the monoculture pots. In mixture pots, the root biomass and RBR of both species remained unchanged with the increasing drought stress, and R. typhina had higher root biomass, RBR, and WUE than those of C. coggygria, indicating that the belowground advantage of R. typhina to obtain water and nutrients, and utilize water more effectively than C. coggygria was independent of soil water content.
The A and Gs of C. coggygria simultaneously decreased by both drought and the competition with R. typhina, indicating the stomatal closure of C. coggygria in response to drought and competition. The responses of C. coggygria in the present study support the carbon-starvation hypothesis [49, 50], in which a plant reduces Gs as the soil water potential decreases, decreasing photosynthetic carbon uptake; thus, resulting in lower biomass compared with that under other water treatment conditions in monoculture and mixed culture. Carbohydrate reserves deplete due to continued demand for carbohydrates in order to maintain metabolism, and the plant might die due to its inability to resist attack from biotic agents or starvation, whichever occurs first . The stomatal conductance and net photosynthesis rate of Acer platanoides and Fagus sylvatica were found to decrease simultaneously under drought plus competition conditions , which is consistent with that observed in C. coggygria in the present study. In contrast to C. coggygria, R. typhina maintained constant A, and Gs in the mixture pots, regardless of the water conditions, indicating that R. typhina might have adopted another mechanism to cope with drought. Some species maintain positive carbon gain under drought conditions, allowing the midday leaf water potential to decline [49, 50] by keeping the stoma open. In this case, R. typhina continued to accumulate carbohydrate under drought condition, maintaining the biomass constant compared with that under well-watered condition , and they can withstand prolonged drought condition before carbon starvation .
In conclusion, the invasive R. typhina and the native C. coggygria displayed differences in many growth-related parameters when they were cultivated either separately or together. This is consistent with the phenotypic divergence hypothesis  holding that the functional traits differences contribute to the invasion success of exotic species. Rhus typhina maintained its absolute dominance against C. coggygria in both well-watered and drought environments by possessing higher photosynthetic capacity, larger crown area, taller shoot height, and higher RGR-H, leaf and root biomass, and WUE, outcompeting C. coggygria vertically and horizontally. The growth of C. coggygria was strongly limited by the competition from R. typhina; however, this competition alleviated the negative effects of drought on C. coggygria mainly by providing shade environment. Drought alleviated the asymmetric competition of R. typhina over C. coggygria to some extent because drought had stronger negative effects on than on C. coggygria in the mixture pots and R. typhina is better capable of exploiting excess water resource. Our observation provides further evidence for the fluctuating resource availability theory . Under drought conditions, C. coggygria suffered from carbon starvation, whereas, R. typhina retained the normal carbohydrate synthesis, contributing to the dominance of R. typhina. However, both the species would experience the risk of hydraulic failure under severe drought conditions. Our observation confirmed the robustness of the comparative trait differences between invasive and non-invasive species across environmental gradients .