L'influence des arbustes fruitiers sur la répartition des oiseaux en sapinière boréale
|Abstract:||The post-reproductive period is critical for many forest birds, especially for juveniles who must learn to forage on their own before the fall migration. At this period, many forest birds become mainly frugivores and songbirds of mature boreal forests often shift to early-successional stands. There are at least three explanations for this late-summer habitat shift: (1) songbirds may seek abundant fruit resources or cover against predators, (2) restrict their use of clearcuts to mature-forest edges or (3) use clearcuts mostly for transit between different patches of mature forest via early-seral stands. We tested frugivory, edge and transit hypotheses at the Forêt Montmorency, Quebec during summers 2007 and 2008. We tested the frugivory hypothesis by conducting a fruit removal experiment testing the prediction that bird capture rates in mist nets would be lower in fruit removal plots than in control plots. Additionally, we evaluated the ripening and consumption of available red elderberry (Sambucus racemosa) berries in control plots following each mist-netting session, and we modeled capture rates as a function of fruit shrub abundance at varying spatial scales. We tested edge and transit hypotheses by comparing capture rates in clearcuts near vs. away from adjoining mature forest edges, as well as in mist nets placed parallel vs. perpendicular to forest edges. Sixty-four percent of individuals, representing 15 of the 33 species captured, were seasonal frugivores. In both years of this study, fruit consumption of red elderberry approached 100 % by the end of the sampling periods, suggesting that fruit could be a limiting resource to boreal forest birds. Experimental fruit removal reduced capture rates of frugivores by 45 %, but did not affect capture rates of other species. There was no relationship between bird and fruit abundance beyond a few meters from mist nets. Capture rates were independent of distance from mature forest edges, but they were greater in nets parallel to forest edges than in those perpendicular to edges in the case of species nesting in mature forests. In the boreal forest, fruit shrubs are mainly found in dense, early successional stands. In managed forests, it is within these stands that precommercial thinning (PCT) is applied, a treatment designed to reduce stand density so as to increase diameter growth of residual poles and orient stand species composition. The consequences of PCT on wildlife associated to dense habitat as well as the possible elimination of fruit shrubs in treated stands is a cause for concern. These preoccupations have lead to the experimental application of wildlife-enhanced PCT at the Forêt Montmorency, where the evaluation of competing stems is less severe as in conventional PCT. I examined the impact of wildlife-enhanced PCT (wePCT) on fruit shrub abundance and distribution by conducting a fruit shrub inventory in thinned and control stands, and modeling the abundance of fruit shrubs as a function of treatment and site variables (slope, elevation and aspect). Results indicate that the abundance of fruit shrubs generally seemed higher in thinned stands, but differences were not significant. Fruit shrub abundance was highly variable in young stands and the response to thinning was species specific. Nonetheless, wePCT does not seem to have a negative impact on fruit shrub abundance. I suggest two possible explanations for the maintenance of fruit shrub following wePCT: either fruit shrubs were left uncut during thinning or, when cut, increased light and available nutrients rapidly restored stem abundance by favouring stump sprouting. Finally, fruit shrub distribution was highly aggregated, but was not related to distance to roads or to mature forest edges. However, PCT significantly reduced fruit shrub aggregation. Given the support for frugivory and transit hypotheses, silvicultural treatments should be closely monitored to maintain fruit shrubs in small-scale patchworks of different successional stages. PCT in our study area appeared unharmful to birds, as fruit shrub abundance in PCT sites was similar to that of control stands. However, reduced fruit shrub aggregation could negatively affect foraging efficiency of frugivorous forest birds. Fruit shrub abundance and distribution results cannot be directly extrapolated to conventional PCT, but they provide insight on the fruit shrub dynamics of the boreal forest. Better understanding the effects of PCT and other sylvicultural treatments on fruits shrubs and trees is important, as fruit is possibly a limiting resource for post-breeding birds.|
|Document Type:||Mémoire de maîtrise|
|Open Access Date:||17 April 2018|
|Collection:||Thèses et mémoires|
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