Plant–plant interactions change in response to environmental conditions, and riparian species are commonly influenced by flooding. This study tested whether flooding affects the intraspecific and interspecific competition of two riparian species and whether such effects depend on the topographic positions where plants have established. Seeds of the riparian species Polygonum hydropiper were collected from both low and high positions within the shoreline of the Three Gorges Reservoir. Groups of P. hydropiper seedlings from each position were either grown alone (i.e., without competition), with another group of P. hydropiper seedlings (i.e., intraspecific competition), or with a group of seedlings of the companion species Xanthium sibiricum (i.e., interspecific competition). Each group comprised six replicates. In total, 288 plants of P. hydropiper and 84 plants of X. sibiricum were selected for the experiment. Seedlings were subjected to control and flooding treatments for 60 days. Irrespective of competition type (i.e., intra- or interspecific), both flooding and competition negatively affected the growth and/or photosynthetic capacities of P. hydropiper. Flooding only interacted with competition to explain total biomass. Flooding reduced total biomass in a larger proportion in the absence of competition, and, to a lesser extent, with intraspecific competition, compared to interspecific competition. However, such interaction effects were independent of the positions where the seeds that originated from the plants were collected from. Interspecific competition significantly decreased the chlorophyll content and photosynthetic efficiency of plants, while intraspecific competition did not. In general, plants from lower positions had higher total chlorophyll content than plants from higher positions. These results suggest that flooding may regulate the population dynamics of P. hydropiper by altering its competitive interactions.

Rubus sachalinensis, a Mongolian medicinal plant, serves as a valuable raspberry breeding resource due to its high value and stress resistance. Global climate change and human logging have contributed to its gradual decline in population. In this study, we utilized MaxEnt modeling to forecast the potential distribution of suitable habitat for R. sachalinensis under changing climatic conditions, aiming to facilitate controlled cultivation to lessen dependence on wild populations and support conservation efforts. In this study, by integrating 573 global distribution records of R. sachalinensis with 22 environmental variables related to climate and terrain, we developed the MaxEnt model and selected 11 variables based on the correlation (correlation threshold r < 0.8) for analysis. Through the jackknife method, we found that the distribution of R. sachalinensis is primarily influenced by precipitation during the warmest quarter (bio18, 43%), mean temperature of the coldest quarter (bio11, 20.5%), temperature seasonality (bio4, 17.1%), and slope (12.2%). The habitat of R. sachalinensis reached its maximum extent during the Last Interglacial period, subsequently decreased, and then expanded to 4726.25 × 10³ km² during the Mid-Holocene period, and currently stands at 5293.45 × 10³ km² in northeastern China, eastern Mongolia, and southeastern Russia, with a smaller distribution in North America, particularly in western Canada. Future projections indicated a declining trend in SSP1_2.6 and SSP5_8.5, with the centroid expected to shift eastward. Consequently, due to environmental changes, the suitable habitat distribution of R. sachalinensis will shift eastward, leading to a reduction in habitat area, and human logging will further exacerbate the reduction in wild resources. The resources within the area that remain constant from the ancient climate period to the future suitable habitat should be strengthened for protection, as well as the resources in the shrinking areas. These findings provide potential distribution information and dynamic changes in R. sachalinensis, which will provide vital data for identifying important conservation areas and sustainable harvesting.