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：大小不对称竞争（size-asymmetric competition）指体型更大的植物在与小型植物的竞争中占据不成比例的竞争优势。既往研究提出假说认为，养分异质性可能会促进该竞争模式。但目前验证该假说的相关实验结果尚无定论，且多数研究均单独评估养分异质性的效应，未结合其他环境因素开展实验。本研究以黑麦草（Lolium perenne）、长叶车前（Plantago lanceolata）和绒毛草（Holcus lanatus）种群为研究对象，旨在验证两个假说：其一，养分异质性可促进大小不对称竞争；其二，养分异质性会与大气二氧化碳分压（PCO2）及养分有效性共同作用，决定该竞争响应的强度。
研究方法：本研究采用析因实验设计，构建包含各供试物种单一种植群落的微宇宙培养系统（microcosms），并设置以下处理因素开展为期90天的培养实验：大气二氧化碳分压（PCO2：37 5 Pa与70 Pa）、养分有效性（NA：以有机质形式添加的氮含量为40 mg与120 mg），以及有机质的不同空间分布模式（NH：均匀分布与异质分布）。通过种群内单株植物的体型差异（体型不平等性），以单株地上生物量的变异系数及每个微宇宙中两株最大个体的总生物量进行量化。体型不平等性的提升与大小不对称竞争直接相关。
主要研究结果：当养分以异质性方式供应时，3个供试物种的体型不平等性均显著升高。对于长叶车前与绒毛草，养分异质性（NH）对体型不平等性的增强效应在高养分供应条件下更为显著（存在显著的NA×NH交互作用）；而对于长叶车前，该效应在升高的PCO2条件下更为明显（存在显著的PCO2×NA×NH三阶交互作用）。黑麦草未检测到显著的二阶及三阶交互作用。
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3个供试物种的大小不对称竞争。养分有效性与PCO2会改变长叶车前与绒毛草的该竞争效应强度，但对黑麦草无显著影响。因此，本研究的第二个假说得到了部分验证。