Data from: Direct litter interference and indirect soil competitive effects of two contrasting phenotypes of a spiny legume shrub drive the forb composition of an oromediterranean community

Contrasting phenotypes of foundation species are known to differentially affect understorey plants. However, there is little knowledge on both the mechanisms of competition (resource competition versus interference) of stress-tolerant phenotypes and the importance of indirect interactions. In an oromediterranean community from Mount Lebanon we assessed the effects on understorey forbs of two contrasting phenotypes, a tight competitive from stressful habitat and a loose facilitative from more benign habitat. In a dry south and a wet north exposure we assessed short-term resource effects removing shrub canopy and long-term soil effects (including litter interference) with the comparison of forb performances in adjacent naturally open areas vs no shrub. Indirect effects were quantified through the removal of grasses. Abundance, richness and biomass of forbs were measured in all treatments after one year of experiment, together with litter depth and soil moisture. We found strong direct negative soil effects of the tight phenotype on all forb performances and in particular in south exposure. These effects were due to litter interference on water availability, but not to resource competition. They were likely explained by the high hydrophobicity of organic matter accumulating in the stressful habitat of the tight phenotype. We also found an indirect competition of the loose phenotype for forb richness, due to its direct positive soil effect on competitive grasses, and in particular in south exposure. Our results improve our knowledge on the importance of litter interference in dry nutrient-poor habitats and the role of indirect interactions in phenotypic effects on understorey species.

已有研究表明，建群种 (foundation species) 的不同表型会对下层植被 (understorey plants) 产生差异化影响。然而，目前我们对于耐胁迫表型 (stress-tolerant phenotypes) 的竞争机制（资源竞争 (resource competition) 与干扰竞争 (interference competition)）以及间接相互作用的重要性仍知之甚少。本研究以黎巴嫩山的山地地中海群落 (oromediterranean community) 为研究对象，探究两种对立表型对下层杂类草 (forbs) 的影响：一种是来自胁迫生境的紧凑竞争型表型，另一种是来自较温和生境的松散促进型表型。在干燥的南向坡与湿润的北向坡生境中，我们通过移除灌丛冠层 (shrub canopy) 以评估短期资源效应；同时通过对比相邻自然裸地与无灌丛样地中杂类草的生长表现，来探究长期土壤效应（包括枯落物干扰 (litter interference)）。我们通过移除禾本科植物来量化间接相互作用效应。本实验持续一年后，我们测定了所有处理样地中杂类草的多度、物种丰富度与生物量，同时测定了枯落物厚度与土壤含水量 (soil moisture)。研究发现，紧凑竞争型表型对所有杂类草的生长表现均产生了显著的直接负面土壤效应，且该效应在南向坡生境中尤为突出。该效应源于枯落物对水分可利用性的干扰，而非资源竞争，这大概率是因为紧凑竞争型表型所在的胁迫生境中积累的有机质具有较高的疏水性 (hydrophobicity)。此外，我们还发现松散促进型表型会通过间接竞争降低杂类草的物种丰富度：该表型可对具有竞争能力的禾本科植物产生直接的正面土壤效应，且该效应在南向坡生境中更为显著。本研究结果加深了我们对干燥贫营养生境中枯落物干扰的重要性，以及间接相互作用在表型对下层植被物种效应中所扮演角色的认知。