Precommercial thinning maintains the abundance but reduces spatial aggregation of fruit abundance but reduces spatial aggregation of fruit

shrubs in a boreal forest

2 _{Ce chapitre sera soumis pour publication dans la Revue canadienne de recherche forestière. Mélanie Major}

RÉSUMÉ

Les petits fruits sont une ressource clé pour de nombreux oiseaux forestiers en période post- reproductrice. En sapinière boréale, on trouve les arbustes fruitiers surtout dans les jeunes peuplements au stade gaulis. Or, c’est à ce stade que s’applique l’éclaircie précommerciale (ÉPC), un traitement sylvicole communément utilisé pour augmenter la croissance en diamètre des tiges éclaircies et orienter la composition en espèces des jeunes peuplements. Ce traitement soulève d’importantes préoccupations quant aux répercussions sur la faune associée aux milieux denses et à l’élimination possible des arbustes fruitiers dans les peuplements traités en ÉPC. Ces préoccupations ont mené à l’application expérimentale d’ÉPC à valeur faunique (ÉPCvf) à la Forêt Montmorency, où l’évaluation des tiges compétitrices est moins sévère que dans l’ÉPC conventionnelle. J'ai examiné l’impact de cette ÉPC sur l’abondance et la répartition des arbustes fruitiers en effectuant un inventaire d’arbustes fruitiers par transects dans des peuplements traités en ÉPCvf et non traités (témoins). J'ai modélisé l’abondance d’arbustes fruitiers en fonction du traitement et de variables de sites (pente, altitude et exposition). De façon générale, les résultats indiquent que le nombre d’arbustes fruitiers semble être plus élevé dans les peuplements traités en ÉPCvf par rapport aux peuplements témoins, mais la différence n’est pas significative. L’abondance d’arbustes fruitiers était très variable dans les coupes et la réponse à l’ÉPCvf variait selon les espèces. Néanmoins, l’ÉPC à valeur faunique ne semble pas avoir d’impact négatif sur l’abondance d’arbustes fruitiers. J’attribue le maintien des arbustes fruitiers au fait qu’une bonne partie ne sont pas coupés lors de l’éclaircie et, dans le cas où les tiges sont coupées, à l’augmentation de lumière et de nutriments suivant l’éclaircie qui favorise les rejets de souche. Finalement, la répartition des arbustes fruitiers était fortement agrégée (contagieuse), mais n’était pas liée à la distance aux chemins ni aux lisières de forêt mature. Cependant, l’ÉPC a significativement réduit le taux d’agrégation des arbustes fruitiers.

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, fruit is an abundant resource and many forest birds become chiefly frugivores. 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.

INTRODUCTION

Small berries are a key resource for forest birds as they undergo molt and pre-migratory fat deposition during the post-reproduction period (Vega Rivera et al. 1998a, 1999, White et al. 2005, Vitz and Rodewald 2007). Dense fruit shrubs may also provide cover to postbreeding birds as their flight ability is impaired by molt (Vega Rivera et al. 1998b, Vitz and Rodewald 2007). Fruit shrubs are mainly pioneer species, favored by increased light, daily temperatures and precipitation reaching the ground in more open areas. Consequently, they are mainly found in early-successional forests in cutover or disturbed sites (New Jersey temperate forests: Baird 1980, Costa Rica second growth: Levey 1988a, European temperate forests: Herrera 1995, South Carolina temperate forests: Kwit et al. 2004, southern New England temperate forests: DeGraaf and Yamasaki 2003, oak dominated temperate forests and shrublands in southern Spain: Telleria and Perez-Tris 2007), and in forest gaps (isolated hardwood forest in Illinois: Blake and Hoppes 1986, Costa Rica tropical wet and premontane wet forest: Levey 1988b, Pennsylvania mixed forest: Stutchbury et al. 2005). Fruit shrubs may also be abundant near forest edges and roadsides (eg. Haeussler and Coates 1986) where light is more available than in denser mature forest. Although fruit shrubs are occasionally found under forest canopy, shrubs growing in more open habitat generally produce larger crops and fruit over a longer period of time (Levey 1988b, Wender et al. 2004).

In the boreal forests of Europe and North America, the main silvicultural practice used to manage early-successional forest stands is precommercial thinning (PCT). PCT is typically used to increase diameter growth of residual trees by reducing density of overstocked stands and thus minimising competition (Brissette et al. 1999, Pothier 2002,

Zhang et al. 2006). PCT also controls stand species composition and selection of crop trees (Brissette et al. 1999). Coniferous stems are generally retained, while the number of deciduous stems is reduced (Homyack 2003). Crown volume of residual crop trees increases rapidly with reduced competition (Brissette et al. 1999, Lindgren et al. 2006, Zhang et al. 2006) and treated stands bypass the self-thinning stage of forest succession.

The consequences of the widespread application of PCT on biodiversity is a cause for concern, as the effects of PCT on wildlife, both immediately and several years post-

young, dense stands for finding food or escaping predators. Consequently, manipulation of stem densities in regenerating clearcut stands may have negative impacts on species associated with dense early-successional habitat, particularly the snowshoe hare (Lepus

americana Erxleben 1977; Sullivan and Sullivan 1988, de Bellefeuille et al. 2001,

Homyack 2003, Bujold 2004, Etcheverry et al. 2005) Bicknell’s thrush (Catharus Bicknellii Ridgway 1882; Connolly et al. 2002, Chisholm and Leonard 2008) or other songbirds. For example, the breeding densities of alder flycatcher (Empidonax alnorum), chestnut-sided warbler (Dendroica pensylvanica), mourning warbler (Oporornis philadelphia), red-eyed vireo (Vireo olivaceus), and veery (Catharus fuscescens) decreased following thinning of young spruce plantations in northwestern Ontario because of the removal of foraging and nesting substrates (Woodcock et al. 1997). Removal of deciduous trees following thinning may also reduce potential nesting sites for forest song-birds (Easton and Martin 2002). As a result, excluding some stands from PCT has been proposed to maintain ecosystem diversity at the landscape scale (Etcheverry et al. 2005). Maintaining untreated strips or patches throughout treated stands has also been suggested to maintain cover for wildlife (de Bellefeuille et al. 2001, Ausband and Baty 2005, Woodcock et al. 1997). Finally, modifying the application of PCT to maintain a higher number of competing poles after treatment has been tested, resulting in higher lateral cover than in conventional PCT (Sansregret 2000). This last treatment, called wildlife-enhanced PCT, is only applied experimentally at the Forêt Montmorency, Québec.

Few studies have concentrated on how PCT affects food availability and to my knowledge, no study has directly addressed the effect of PCT or wePCT on fruit shrub abundance and distribution. Thompson et al. (2003) suggests that PCT eliminates fruit- bearing species. However, one could argue that thinning increases the number of fruit- bearing shrubs due to canopy opening (Doerr and Sandburg 1986) and the resulting increase in light reaching the ground (Pothier and Margolis 1991). Furthermore, PCT may also increase the availability of mineral nutrients and water which are limited by intra- and inter-specific competition in dense stands (Pothier and Margolis 1991, Thibodeau et al. 2000). Thus, growth of shrubs may be favoured by increased resources such as light and

Sullivan 2001, Thomas et al. 1999, Sullivan et al. 2001, Doerr and Sandburg 1986).

We tested the influence of wildlife-enhanced PCT (wePCT) on abundance and spatial distribution of fruit shrubs over time. We hypothesized that wePCT would increase the abundance of fruit shrubs and reduce their aggregation by providing uniformly favourable conditions for fruit shrub establishment and growth. We also tested whether proximity to forest edges and roads influenced fruit shrub distribution, because of associated lower light exposure and higher soil disturbance respectively.

METHODS

Study area

Field work was conducted at the Forêt Montmorency, Quebec (47º20’N, - 71º10’W), a 66 km2 forest managed for timber production and recreational purposes. The forest landscape is composed of regenerating forest stands of various ages, harvested primarily by the clearcut method, as well as premature and mature stands. Mature (> 40 y), premature (21 – 40 y) and regenerating (< 20 y) forest stands comprise 49 %, 23 % and 24 % of the study area, respectively (for more details see Darveau et al. 1997). Mature forest in the study area is mostly second-growth dominated by balsam fir (Abies balsamea L.), with black spruce (Picea mariana Miller), white spruce (P. glauca Moench), and white birch (Betula papyrifera Marshall) as companion species. Regenerating forest stands are dominated by balsam fir and white birch, as well as planted white spruce and several fruit shrubs. Fruit shrub and fruit tree species at our study site are red elderberry (Sambucus

racemosa), pin cherry (Prunus pensylvanica), mountain ash (Sorbus Americana), shadbush

(Amelanchier sp.), squashberry (Viburnum edule) and raspberry (Rubus idaeus). These will subsequently be refered to as Sambucus, Prunus, Sorbus, Amelanchier, Viburnum and

Rubus respectively. 3

All stands used for this study were clearcut, i.e., all trees with diameter at breast height > 9.1cm were felled. Sixty-eight stands cut between 1988 – 2001, were selected for study (Figure 9), 15 of which were treated with wePCT 7 to 20 years post-clearcut

Only approximately 30 % of stands eligible to PCT are treated each year. Most untreated stands in the study area met criteria for PCT and therefore, we considered treated stands as a random sample of all 7-20 y old stands. All clearcuts were mapped and delimited by GPS or orthoimages obtained by fixed-wing aircraft. Elevation and slope of selected clearcuts ranged from 612 to 902 m (mean ± SE: 750 ± 58) and 3 to 34% (mean ± SE: 18 ± 8 %) respectively. Aspect of all clearcuts was also measured. Surficial deposit of clearcuts consist predominantly of till, with moderate drainage.