Collet Catherine, Ningre François, Barbeito Ignacio, Arnaud Anthony, Piboule Alexandre
INRA, UMR1092, Laboratoire d'Etude des Ressources Forêt Bois (LERFoB), Centre INRA de Nancy, 54280 Champenoux, France.
Ann Bot. 2014 Mar;113(4):711-9. doi: 10.1093/aob/mct285. Epub 2013 Dec 8.
There is considerable evidence for the presence of positive species diversity-productivity relationships in plant populations, but the population parameters determining the type and strength of the relationship are poorly defined. Relationships between species evenness and tree survival or species coexistence are not well established. The objective of this study was to quantify the joint effects of density and species evenness on tree productivity and species coexistence.
A 12-year-old experimental tree plantation mixing two species according to a double gradient of density and species proportion was used. A neighbourhood approach was employed and descriptors of local competition were used to model individual tree growth. Fagus sylvatica and Acer pseudoplatanus were used as model species, as they can be considered as ecologically equivalent in their young stages.
Density and tree size were primary factors determining individual growth and stand productivity. Species identity had a significant, but less pronounced, role. Stand productivity was highest when species evenness was close to 1 and slightly lower in uneven mixtures. The reduction in stand productivity when species evenness decreased was of similar magnitude irrespective of which species became dominant, indicating symmetric effects for the two species. When examining individual tree growth in response to species proportion for each species separately, it was observed for both species that individual trees exhibited greater growth in uneven mixtures in which the other species was more frequent.
The results suggest that mixtures of these two functionally similar species have the highest production at maximum evenness, indicating a complementary effect between them. The presence of a mixture combines both stabilizing mechanisms (individuals from both species show higher growth when surrounded by individuals from the other species) and equalizing mechanisms (the two species have very similar growth curves) that, in turn, determine the species' relative dominance. These processes should act to ensure the long-term coexistence of species.
有大量证据表明植物种群中存在正的物种多样性 - 生产力关系，但决定这种关系类型和强度的种群参数却定义不清。物种均匀度与树木存活率或物种共存之间的关系尚未明确确立。本研究的目的是量化密度和物种均匀度对树木生产力和物种共存的联合影响。
使用了一个种植12年的实验性人工林，该人工林根据密度和物种比例的双重梯度混合了两个物种。采用邻域法，并使用局部竞争描述符来模拟单株树木的生长。欧洲山毛榉和假挪威槭被用作模式物种，因为它们在幼龄阶段可被视为生态等效。
密度和树木大小是决定个体生长和林分生产力的主要因素。物种身份有显著作用，但不太明显。当物种均匀度接近1时，林分生产力最高，不均匀混合时略低。物种均匀度降低时林分生产力的下降幅度与哪个物种成为优势种无关，表明对这两个物种的影响是对称的。分别检查每个物种对物种比例的单株树木生长情况时，发现两个物种的单株树木在另一个物种更常见的不均匀混合中生长得更好。
结果表明，这两个功能相似的物种在最大均匀度时混合具有最高产量，表明它们之间存在互补效应。混合的存在结合了稳定机制（当被另一个物种的个体包围时，两个物种的个体都表现出更高的生长）和均衡机制（两个物种具有非常相似的生长曲线），进而决定了物种的相对优势。这些过程应有助于确保物种的长期共存。
