Fluctuations in resource availability shape the competitive balance among non-native plant species

Fluctuating resource availability plays a critical role in determining non-native plant invasions by mediating the competitive balance between non-native and native species. However, the impact of fluctuating resource availability on interactions among non-native species remains largely unknown. This represents a barrier to understanding invasion mechanisms, particularly in habitats that harbor multiple non-native species with different responses to fluctuating resource availability. To examine the responses of non-native plant species to nutrient fluctuations, we compared the growth of each of 12 non-native species found to be common in local natural areas to nutrients supplied at a constant rate or supplied as a single large pulse in a pot experiment. We found that seven species produced more biomass with pulsed nutrients compared to constant nutrients (hereafter 'benefitting species'), while the other five species did not differ between nutrient enrichment treatments (hereafter 'non-benefitting species'). To investigate how nutrient fluctuations influence the interactions among non-native plant species, we established experimental non-native communities in the field with two benefitting and two non-benefitting non-native species. Compared with constant nutrient supply, the single large pulse of nutrients did not influence community biomass, but strongly increased the biomass and cover of the benefitting species and decreased those of the non-benefitting species. Furthermore, the benefitting species had higher leaf N content and greater plant height when nutrients were supplied as a single large pulse than at a constant rate, whereas the non-benefitting species showed no differences in leaf N content and were shorter when nutrients were supplied as a single large pulse than at a constant rate. Our results add to the growing evidence that the individual responses of non-native species to nutrient fluctuation are species-specific. More importantly, benefitting species were favored by nutrients coming in a pulse, while non-benefitting ones were favored by nutrients coming constantly when they grew together. This suggests that nutrient fluctuations can mediate the competitive balance among non-native plants and may thus determine their invasion success in a community harbouring multiple non-native plant species.

资源可获得性波动通过调控外来物种（non-native species）与本土物种（native species）间的竞争平衡，在外来植物入侵过程中发挥关键作用。然而，资源可获得性波动对外来物种种间相互作用的影响仍鲜为人知，这成为理解入侵机制的一大障碍，尤其是在栖息有多种对资源可获得性波动响应各异的外来物种的生境中。为探究外来植物物种对养分波动的响应，本研究通过盆栽实验（pot experiment），对比了12种在本地自然区域常见的外来植物物种分别在恒量养分供应与单次大量脉冲式养分供应下的生长情况。研究发现，相较于恒量养分供应，7个物种在脉冲式养分供应下的生物量（biomass）更高（以下简称"获益物种"），其余5个物种在两类养分添加处理间的生物量无显著差异（以下简称"非获益物种"）。为探究养分波动如何影响外来植物物种间的相互作用，我们在野外构建了由2种获益物种与2种非获益物种组成的外来植物实验群落。结果显示，与恒量养分供应相比，单次大量脉冲式养分供应并未改变群落总生物量，但显著提升了获益物种的生物量与盖度（cover），同时降低了非获益物种的对应指标。此外，相较于恒量养分供应，脉冲式养分供应下的获益物种叶片氮含量更高、植株更高大；而非获益物种的叶片氮含量无显著变化，且植株高度较恒量供应时更低。本研究结果进一步佐证了日益增多的研究证据，即外来物种对养分波动的响应具有物种特异性。更重要的是，当多种外来物种共同生长时，获益物种更适应脉冲式养分供应，而非获益物种则更偏好恒量养分供应。这表明养分波动可调控外来植物间的竞争平衡，进而决定其在多外来物种群落中的入侵成功率。