Trait value and phenotypic integration contribute to the response of exotic Rhus typhina to heterogeneous nitrogen deposition: A comparison with native Rhus chinensis.

The temporal heterogeneity of nitrogen availability in soils is increasing due to agricultural deposition. We here compared the effects of gradually increasing nitrogen deposition rate and its increasing temporal heterogeneity patterns on the functional traits of seedlings of exotic species Rhus typhina and the native species Rhus chinensis. Nitrogen deposition rates of 0, 8, 20 g N m-2 year-1 and constant, single-peak, and double-peak nitrogen were added to simulate deposition rate and temporal heterogeneity. After 60 days of treatment, R. typhina seedlings had several advantageous growth trait values, such as higher total biomass production, but lower phenotypic plasticity than R. chinensis seedlings. R. typhina seedlings also had higher phenotypic integration, measured as the correlation among functional traits. The increased nitrogen deposition rate affected several traits of the two species differently. Thus, while R. chinensis seedlings allocated more biomass to leaves and less to roots with increasing N deposition, R. typhina seedlings had stable biomass allocation among all N treatments. Chlorophyll content, leaf phosphorus concentration, and water use efficiency increased, but the maximum net photosynthetic rate decreased, with N availability in R. chinensis, but not in R. typhina. Temporal heterogeneity had no significant effect on the total biomass of R. typhina and R. chinensis seedlings. Overall, the performance of R. typhina is better than that of R. chinensis seedlings under different nitrogen deposition treatments, which is due to the significantly advantageous trait values and greater phenotypic integration of R. typhina seedlings, whereas R. chinensis seedlings have higher phenotypic plasticity.

Increased soil moisture aggravated the competitive effects of the invasive tree Rhus typhina on the native tree Cotinus coggygria.

BACKGROUND: Invasive exotic species have caused significant problems, and the effects of extreme precipitation and drought, which might occur more frequently under the global climate change scenarios, on interspecific relationship between invasive and native species remain unclear. RESULTS: We conducted a greenhouse experiment with three soil water levels (30-40%, 50-60%, and 70-80% of field capacity) and two cultivation treatments (monoculture pots, one seedling of either species and mixture pots, one seedling of each species) to investigate soil water content effects on the relationship between invasive Rhus typhina and native Cotinus coggygria. Rhus typhina had lower height but bigger crown area than C. coggygria in the monoculture treatment. Rhus typhina had higher height, bigger crown area and total biomass than C. coggygria in the mixture treatment. Drought decreased the growth parameters, total chlorophyll concentration, and leaf biomass, but did not change gas exchange and other biomass parameters in R. typhina. The growth parameters, leaf area index, biomass parameters, total chlorophyll concentration, and net photosynthetic rate of C. coggygria decreased under drought conditions. The log response ratio (lnRR), calculated as ln (total biomass of a target plant grown in monoculture/total biomass of a target plant grown in mixed culture), of R. typhina was lower than that of C. coggygria. The lnRR of R. typhina and C. coggygria decreased and increased with increase in soil water content, respectively. CONCLUSIONS: Rhus typhina has greater capacity to relatively stable growth to the drought condition than C. coggygria and has strong competition advantages in the mixture with C. coggygria, especially in the drought condition. Our study will help understand the causes of invasiveness and wide distribution of R. typhina under various moisture conditions and predict its expansion under climate change scenarios.