Height and clonality traits determine plant community responses to fertilization

Fertilization via agricultural inputs and nutrient deposition is one of the major threats to global terrestrial plant richness, yet we still do not fully understand the mechanisms by which fertilization decreases plant richness. Tall clonal species have recently been proposed to cause declines in plant species richness by increasing in abundance in response to fertilization and competing strongly with other species. We tested this hypothesis in a fertilization experiment in a low productivity grassland by using a novel experimental manipulation of the presence vs. absence of clonal species and by examining the role of height within these treatments. We found that fertilization decreased species richness more in the presence than absence of clonal species. We also found that only tall species increased in biomass in response to fertilization. In the absence of clonal species, fertilization increased biomass of tall nonclonal species. However, in the presence of clonal species, fertilization decreased tall nonclonal biomass and only tall clonal biomass increased. Fertilization caused almost all short species to be lost in the presence, but not the absence, of clonal species and caused greater declines in the mean and variance of light levels in the presence of clonal species. These results show that the traits of species in a community can determine the magnitude of species loss due to fertilization. The strongly negative effect of tall clonals on species richness in fertilized plots is likely a result of their capacity to decrease light levels to a greater extent and more uniformly than nonclonal species, and thereby drive the exclusion of short species. These results help clarify the mechanisms whereby fertilization decreases grassland plant species richness and suggest that efforts to prevent the loss of species under fertilized conditions may be most effective when they focus on controlling the biomass of tall clonal species.

通过农业投入与养分沉降实施的施肥作用，是威胁全球陆生植物物种丰富度的主要因素之一，但目前学界仍未完全阐明施肥导致植物丰富度下降的内在机制。高大克隆物种（clonal species）近来被提出，可通过响应施肥后种群丰度提升、与其他物种激烈竞争的方式，导致植物物种丰富度下降。本研究在一处低生产力草地的施肥实验中，通过创新性设置“有无克隆物种”的实验操控组，并探究不同株高在各处理组中的作用，对上述假说进行了验证。研究结果显示，相较于无克隆物种的组别，施肥处理在有克隆物种存在的样地中对物种丰富度的削弱作用更为显著。同时本研究还发现，仅高大物种的生物量会在施肥后出现提升。在无克隆物种（nonclonal species）的组别中，施肥可提升高大非克隆物种的生物量；但在有克隆物种存在的组别中，施肥反而会降低高大非克隆物种的生物量，仅高大克隆物种的生物量出现增长。在有克隆物种存在的样地中，施肥会导致几乎所有矮小物种消失，而在无克隆物种的组别中则无此现象；同时，有克隆物种存在时，施肥会使光照水平的均值与方差出现更大幅度的下降。上述结果表明，群落内物种的功能性状可决定施肥引发的物种丧失程度。施肥样地中高大克隆物种对物种丰富度的强烈负面影响，大概率源于其相比非克隆物种能够更显著且更均匀地降低光照水平，进而排挤矮小物种。本研究结果有助于阐明施肥导致草地植物物种丰富度下降的内在机制，同时也提示，若要在施肥环境下有效防止物种丧失，将管控重点放在高大克隆物种的生物量上或将取得最佳效果。