Competition along productivity gradients: news from heathlands
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The importance of competition in low productive habitats is still debated. Studies which simultaneously evaluate preemption of resources and consequences for population dynamics are needed for a comprehensive view of competitive outcomes. We cultivated two emblematic species of European heathlands (Calluna vulgaris and Molinia caerulea) in a nursery for 2 years at two fertility levels, reproducing the productivity gradient found in phosphorus (P)-depleted heathlands in southwest France. The second year, we planted Ulex europaeus seedlings, a ubiquitous heathland species, under the cover of the two species to evaluate its ability to regenerate. Half of the seedlings were placed in tubes for exclusion of competitor roots. We measured the development of the competitors aboveground and belowground and their interception of resources (light, water, inorganic P). Ulex seedlings’ growth and survival were also measured. Our results on resources interception were consistent with species distribution in heathlands. Molinia, which dominates rich heathlands, was the strongest competitor for light and water in the rich soil. Calluna, which dominates poor heathlands, increased its root allocation in the poor soil, decreasing water and inorganic P availability. However, the impact of total competition and root competition on Ulex seedlings decreased in the poor soil. Other mechanisms, especially decrease of water stress under neighbouring plant cover, appeared to have more influence on the seedlings’ response. We found no formal contradiction between Tilman and Grime’s theories. Root competition has a primary role in acquisition of soil resources in poor habitats. However, the importance of competition decreases with decreasing fertility.
KeywordsGrime–Tilman debate Competition importance Resource-ratio hypothesis Resource supply pre-emption Plant–plant interactions
This study was funded by the Bordeaux Sciences Agro—Institute of Agricultural Sciences and the French National Institute of Agricultural Research (INRA). We thank Patrick Pastuszka, Frederic Bernier, and all the INRA-UE570 Forêt Pierroton team for making the tree nursery available and for experimental support. We are grateful to Helene Budzinsky and Karine Lemenach from the CNRS UMR 5805 EPOC for N isotope measurements and %Ndfa determination. We thank Catherine Lambrot, Sylvie Milin and Nathalie Gallegos for measurements of nutrient concentrations, Christian Morel and Anne Gallet-Budynek for help regarding 32P labelling and isotopic dilution method. We thank Christophe Chipeaux for help with the soil water content probes. We thank Sylvie Niollet for field support. We also thank Daphne Goodfellow and Fabien Delerue for revising the English.
Author contribution statement
FD, MG, LP and LA conceived and designed the experiments. FD and LP coordinated fieldwork. DLA took in charge phosphorus availability estimation. FD analysed the data. FD, MG, DLA and LA wrote the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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