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Associations between canopy and understory species increase along a rainshadow gradient in the Alps: habitat heterogeneity or facilitation?

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Abstract

Spatialassociations among overstory and understory species tend to increase ongradients from wet to dry climates. This shift in the strength of spatialassociations has usually been attributed to shared abiotic requirements betweencanopy species and understory assemblages within communities and/or to anincrease in habitat heterogeneity in dry climates and therefore higher betadiversity. On another hand, more important positive effects of tree canopies onunderstory species in drier climates may also explain stronger associations andhigher beta diversity. We examined these three hypotheses along a strongrainshadow gradient that occurs from the wet external Alps to the dry innerAlpsby analyzing with correspondence analysis and canonical correspondence analysisthe species composition of 290 relevés of forests dominated to differentdegrees by Abies alba and Piceaabies.We found important differences in climatic requirements forAbies and Picea, withAbies occurring in warmer and drier habitats thanPicea. The understory species associated with these twospecies showed similar correlations with temperature but not with moisture,withunderstory species of Picea-communities having strongerxeric affinities than understory species ofAbies-communities. We found no significant associationsbetween canopy species and understory composition in the external Alps despitethe fact that Abies and Piceaoccurredin substantially different environments. In contrast,Abiesand Picea occurred in more similar environments in theinner Alps, but the understory assemblages associated with eitherAbies or Picea were significantlydifferent. This increase in canopy-understory associations was in partdetermined by strong differences in moisture between southern and northernaspects in the inner Alps, which affected both canopy and understory speciesdistributions. However, differences between the canopy effects ofPicea and Abies also appeared tocontribute to stronger associations between canopy and understory species, andconsequently to increase beta diversity. This pattern only occurred on southernaspects of the inner Alps but was highly significant. Our results suggest thatspecies distributions may be continuous on the wet ends of moisture gradientsbut discrete on dry ends. Relatively discrete communities at stressful ends ofgradients appear to develop as a result of both habitat differentiation and thepositive effects of overstory species.

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  1. Laboratoire Ecosystèmes et Changements Environnementaux, Centre de Biologie Alpine, Université Joseph Fourier, BP 53 X, F- 38 041, Grenoble, France

    R. Michalet, Ch. Rolland & D. Joud

  2. Department of Plant Biology, Babes-Bolyai University, 42 Republic Street, 3400, Cluj-Napoca, Romania

    D. Gafta

  3. Division of Biological Sciences, University of Montana, Missoula, 59812, USA

    R.M. Callaway

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  1. R. Michalet
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Michalet, R., Rolland, C., Joud, D. et al. Associations between canopy and understory species increase along a rainshadow gradient in the Alps: habitat heterogeneity or facilitation?. Plant Ecology 165, 145–160 (2003). https://doi.org/10.1023/A:1022297624381

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