Invasion-induced trait plasticity differences explain plant competitive responses under altered rainfall amounts in arid regions

Traditional functional trait studies explain plant competition using trait means measured under a single environmental or competitive state. However, under altered rainfall amounts and increasing invasion pressure, competitive responses and trait expression can change dynamically along competition gradients. We conducted a controlled experiment in the arid region of Xinjiang, China, crossing three rainfall-amount treatments with five invasive-plant proportions (0, 0.25, 0.50, 0.75, and 1.00). Using two invasive Xanthium species and four native species, we measured 14 functional traits. We compared static functional trait differences between invasive and native plants at a single invasion proportion with invasion-induced trait plasticity differences along the invasion-pressure gradient, and assessed their relationships with competitive responses. Variation in rainfall amount altered competitive responses along the invasion-pressure gradient: native plants generally showed negative responses, whereas invasive plants showed positive responses only under the increased-rainfall treatment. Compared with static functional trait differences, invasion-induced trait plasticity differences were more closely associated with the competitive-response slopes of native plants. The multitrait principal component axis based on plasticity differences was significantly associated with the competitive responses of native plants, whereas the axis based on static functional trait differences was not. These findings suggest that, under altered rainfall amounts in arid regions, competitive responses to plant invasion are closely associated with asymmetric functional trait adjustments between invasive and native plants along the increasing invasion-pressure gradient. This result offers a process-oriented perspective for understanding plant invasion dynamics in arid regions under climate change.