Tree-shrub associations in grazed woodlands: first rodents,then cattle? |
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Authors: | Christian Smit Mart Verwijmeren |
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Institution: | (1) Environmental Sciences Group, Utrecht University, P.O. Box 80115, 3508 TC Utrecht, The Netherlands;(2) Community Ecology and Conservation Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, P.O. Box 14, NL-9750 AA Haren, The Netherlands |
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Abstract: | Spatial associations of tree saplings with spiny or toxic plants in grazed woodlands are generally explained by associational
resistance, i.e., protection against grazing via a well-defended neighbor. In this study, we tested whether directed seed
dispersal and post-dispersal seed removal by wood mice are additional explanations for the observed spatial association between
thorny shrubs and trees, hence before associational resistance. We performed three studies in grazed woodlands in the Netherlands
to test this idea. Our first seed dispersal experiment with tagged acorns showed indeed that wood mice disperse acorns directed
towards shrubs. The majority of these dispersed acorns were, however, consumed. Our second experiment revealed that post-dispersal
removal of cached acorns was higher under shrubs than in grassland and under trees, but also indicated the importance of within-shrub
position: shrub centers were faster depleted than shrub edges. Also the number of freshly emerged seedlings and older saplings
differed between micro-habitats, being higher under trees and at shrub inner-edges than at shrub outer-edges, shrub centers,
and in grassland. The spatial associations with shrubs got stronger with the age of saplings, which probably reflect accumulated
grazing effects over time, being higher in the open than in shrubs. We conclude that directed acorn dispersal and post-dispersal
acorn removal by wood mice are two additional explaining mechanisms for the spatial associations between thorny shrubs and
trees in grazed woodlands, before associational resistance. Our findings change the view that large herbivores are the sole
main drivers behind tree recruitment patterns in grazed woodlands. |
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