Data from: Nutrient availability and atmospheric CO2 partial pressure modulate the effects of nutrient heterogeneity on the size structure of populations in grassland species

Background and Aims: Size-asymmetric competition occurs when larger plants have a disproportionate advantage in competition with smaller plants. It has been hypothesized that nutrient heterogeneity may promote it. Experiments testing this hypothesis are inconclusive, and in most cases have evaluated the effects of nutrient heterogeneity separately from other environmental factors. The aim of this study was to test, using populations of Lolium perenne, Plantago lanceolata and Holcus lanatus, two hypotheses: (a) nutrient heterogeneity promotes size-asymmetric competition; and (b) nutrient heterogeneity interacts with both atmospheric CO2 partial pressure (PCO2) and nutrient availability to determine the magnitude of this response. Methods: Microcosms consisting of monocultures of the three species were grown for 90 d in a factorial experiment with the following treatments: PCO2 (37 5 and 70 Pa) and nutrient availability (NA; 40 and 120 mg of N added as organic material) combined with different spatial distribution of the organic material (NH; homogeneous and heterogeneous). Differences in the size of individual plants within populations (size inequality) were quantified using the coefficient of variation of individual above-ground biomass and the combined biomass of the two largest individuals in each microcosm. Increases in size inequality were associated with size-asymmetric competition. Key Results: Size inequality increased when the nutrients were heterogeneously supplied in the three species. The effects of NH on this response were more pronounced under high nutrient supply in both Plantago and Holcus (significant NA x NH interactions) and under elevated PCO2 in Plantago (significant PCO2 x NA x NH interaction). No significant two- and three-way interactions were found for Lolium. Conclusions: Our first hypothesis was supported by our results, as nutrient heterogeneity promoted size-asymmetric competition in the three species evaluated. Nutrient supply and PCO2 modified the magnitude of this effect in Plantago and Holcus, but not in Lolium. Thus, our second hypothesis was partially supported.

研究背景与目的：当大型植物在与小型植物的竞争中占据不成比例的竞争优势时，便会产生体型不对称竞争（size-asymmetric competition）。已有假说提出，养分异质性可促进该类竞争的发生。目前验证这一假说的实验结果尚无定论，且多数实验均单独评估养分异质性的影响，未结合其他环境因素开展研究。本研究以多年生黑麦草（Lolium perenne）、长叶车前（Plantago lanceolata）和绒毛茅（Holcus lanatus）种群为研究对象，旨在验证两个假说：（a）养分异质性可促进体型不对称竞争；（b）养分异质性与大气CO₂分压（PCO₂）及养分有效性（NA）共同作用，决定该响应的强度。
研究方法：本研究采用析因实验设计，构建包含三个物种单种栽培的微宇宙（microcosms），设置如下处理组合：大气CO₂分压（37.5 Pa与70 Pa）、养分有效性（以有机质形式添加40 mg与120 mg氮），以及有机质的不同空间分布模式（NH；均匀分布与异质分布），培养周期为90天。采用个体地上生物量的变异系数，以及每个微宇宙中最大两个个体的总生物量，量化种群内个体间的体型差异（体型不平等性）。体型不平等性的提升与体型不对称竞争直接相关。
主要研究结果：三个物种在养分异质供应条件下，其体型不平等性均显著提升。对于长叶车前与绒毛茅，NH对该响应的影响在高养分供应条件下更为显著（存在显著的NA×NH交互作用）；而长叶车前的该效应在升高的CO₂分压条件下更为明显（存在显著的PCO₂×NA×NH三重交互作用）。多年生黑麦草未观测到显著的两因素与三因素交互作用。
研究结论：本研究结果验证了第一个假说，即养分异质性可促进三个受试物种的体型不对称竞争。养分供应水平与CO₂分压可改变长叶车前与绒毛茅的该效应强度，但对多年生黑麦草无显著影响。因此，本研究的第二个假说得到部分验证。