Biomass allocation in response to salinity and competition in native and invasive species

Biomass allocation to different plant parts affects the subsequent capture rate of resources and reproduction. Thus, many studies have been conducted on the biomass allocation to learn growth, reproduction and competitive ability of plants. However, few researches have explored how biomass allocation responses to non-resource factor and interspecific competition over time. We experimentally investigated the effects of soil salinity and competition on root:shoot ratio (RS), reproductive effort (RE; seed biomass:total biomass ratio), the relationship between belowground and aboveground biomass, the relationship between reproductive and vegetative biomass, growth, and reproduction of invasive Spartina alterniflora and native Phragmites australis. The biomass of P. australis decreased with increasing salinity, whereas that of S. alterniflora did not significantly change. The reproduction of P. australis decreased with increasing salinity under competitive conditions, and that of S. alterniflora increased. Therefore, P. australis was less-tolerant, and S. alterniflora was more-tolerant. The RS of P. australis increased over time under competitive conditions and with increasing salinity, and that of S. alterniflora did not significantly change. The RE of P. australis decreased to zero with decreasing total biomass, and that of S. alterniflora did not significantly correlate with total biomass. Both species exhibited a linear relationship between belowground and aboveground biomass. The relationship between reproductive and vegetative biomass in P. australis was linear, and the reproductive biomass of S. alterniflora did not significantly correlate with the vegetative biomass. Competitive dominance shifted from P. australis to S. alterniflora with increasing salinity. The findings demonstrated that the plastic biomass allocation of less-tolerant species facilitates performance of less-tolerant species in favorable environments, while the fixed biomass allocation of more-tolerant species facilitates performance of more-tolerant species in stressful environments, suggesting that S. alterniflora invasion driven by competitive exclusion probably occur in high salinity zones, and reproductive ability of invasive species should be relatively stronger during exclusion. More broadly, linking level of environmental stresses with tolerance of plants is crucial to understanding and predicting the biomass allocation and its effects on plant performance, which is an expansion of predictions from optimal theory and allometric theory and therefore illustrates the conditionality of these predictions.

植物向不同器官分配生物量，会影响后续的资源获取效率与繁殖能力。因此，学界已开展诸多围绕生物量分配的研究，以解析植物的生长、繁殖及竞争能力。然而，鲜有研究探讨生物量分配如何随时间推移响应非生物因子与种间竞争。本研究通过实验探究了土壤盐度与种间竞争对入侵物种互花米草（*Spartina alterniflora*）和本土物种芦苇（*Phragmites australis*）的根冠比（root:shoot ratio, RS）、繁殖投入（reproductive effort, RE；即种子生物量与总生物量之比）、地下生物量与地上生物量的关系、繁殖生物量与营养生物量的关系，以及二者的生长与繁殖情况的影响。芦苇的生物量随盐度升高而降低，而互花米草的生物量无显著变化。竞争条件下，芦苇的繁殖能力随盐度升高而下降，而互花米草的繁殖能力则随之提升。据此可认为，芦苇的耐逆性较弱，互花米草的耐逆性较强。竞争条件下，芦苇的根冠比随时间推移与盐度升高均呈上升趋势，而互花米草的根冠比无显著变化。芦苇的繁殖投入随总生物量下降而降至零，而互花米草的繁殖投入与总生物量无显著相关性。两种植物的地下生物量与地上生物量均呈线性关系。芦苇的繁殖生物量与营养生物量呈线性相关，而互花米草的繁殖生物量与营养生物量无显著相关性。随着盐度升高，种间竞争优势从芦苇转向互花米草。本研究结果表明，耐逆性较弱的物种通过可塑性生物量分配，可在适宜环境中提升自身表现；而耐逆性较强的物种通过固定的生物量分配模式，可在胁迫环境中维持自身表现。这提示，由竞争排斥驱动的互花米草入侵大概率发生在高盐度区域，且入侵物种在竞争排斥过程中需具备相对更强的繁殖能力。从更广泛的视角来看，将环境胁迫水平与植物耐逆性相结合，对于理解和预测生物量分配模式及其对植物表现的影响至关重要。这一研究拓展了最优分配理论与异速生长理论的预测范畴，同时也阐明了这些理论预测的条件依赖性。