2 nd International workshop on deer-forest relationships :

Transcription

1 Deer browsing creates cascading effects on herbaceous plant diversity through changes in dominant plant-plant interactions by Julien Beguin, David Pothier, Steeve D. Côté 2 nd International workshop on deer-forest relationships : Impacts of the overabundance of large herbivores on forest ecosystem management

2 Background: Developing sustainable practices in forest management requires a good understanding of fonctional units in forest ecosystems (e.g. plant communities). ) What does literature say about the ecological processes driving plant communities: Drivers Biotic Abiotic Historic - Herbivory - Competition (-) - Facilitation (+) - Dispersal (e.g. animals) - Recruitment - Resource availability - Productivity - Physical disturbance - Dispersal (e.g. wind) - Legacies - Pool of species ) What does literature not say : What about the importance of each of these factors in a context of deer overabundance? - Does one factor predominate over the others? Which one, herbivory? - Do they interrelate with each other? If yes, how? at which spatial scale?

3 Objective of the study: Quantify how multiple biotic and abiotic processes interact in shaping small-scale richness of herbaceous plant species after clearcutting. Drivers Biotic - Herbivory - Competition (-) - Facilitation (+) Abiotic -Physical disturbance (soil) How to disentangle these processes?

4 Methods: Multifactorial randomised experiment with paired fenced/unfenced vegetation quadrats (4 m 2 ) located within three levels of soil disturbance intensity. Sampling date: 7 years after treatments We sampled the horizontal cover of each vascular plant taxon in each quadrat (n = 104) To account for the effect (- and +) of dominant plants on herb richness, we selected dominant taxa (one browsing-tolerant and one browsing-intolerant) based on their frequency and abundance within the entire survey. paper birch (browsing-intolerant) graminoids/thistle (browsing-tolerant) Source: Beguin et al Ecological Applications, 21:

5 Hypotheses : Directed acylcic graph or DAG Hypothesis 1 Hypothesis 2 Statistical Analyses: One way of testing if these hypothetical graphs mirror the data is to use multilevel path analyses with directed-separation tests (see Shipley. B Ecology 90: ). I will not describe all the methodology here but essentially this analysis allows testing all the claims of dependence and independence among variables that are proposed in a DAG. If the data disagree with these claims (tests), the graph is rejected.

6 Results: R 2 = 0.23; P < Cover of graminoïds + thistle (%)

7 (following) 1. The relationship between disturbances (deer and soil) and herb richness is not direct. it involves: 1) negative interactions between dominant browsing-tolerant and browsingintolerant taxa, and 2) a positive relationship between browsing-intolerant taxa and herb richness. 2. Vegetation dominated by grasses and thistle contained more diversity as previously thought. Do they act as ecological refuges for neighbouring herbaceous species?

8 Conclusion: Deer browsing and soil disturbance have direct impacts on abundance of paper birch but indirect effects on the richness of herbaceous plants. At small scale, plant-plant interactions (+ and -) play an important role in structuring plant communities, even in presence of overabundant deer populations. As these interactions are influenced by the abundance of dominant plants, we found the following cascade: Deer browsing dominant plants plant-plant interactions herb richness Hence, deer browsing initiate cascading effects on herbaceous plant communities. Interesting questions remain: - do beneficiary plants share common traits? - do plant-plant interactions and their effects on species richness change over time? - what about other dominant plants: fireweed, raspberry, etc

9 Thank you for your attention! Acknowledgment / Remerciements: - Vanessa Saint-Pierre - Sonia de Bellefeuille - Denis Duteau - François Lebel - Marcel Prévost (MRNF) - J.-P. Lapointe (MRNF) - M. Gagnon (MRNF) - Bill Shipley (U. Sherbrooke) - Gaétan Daigle (U. Laval)