Invasive glossy buckthorn impedes growth of red oak and sugar maple under-planted in a mature hybrid poplar plantation

Caroline Hamelin,Benoit Truax,Daniel Gagnon

doi:10.1007/s11056-016-9551-7

Caroline Hamelin, Benoit Truax + Show 1 more

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https://doi.org/10.1007/s11056-016-9551-7

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Abstract

Native tree seedlings (nursery produced) were planted under control and herbicide treatments in the understory of a mature hybrid poplar plantation, naturally invaded by glossy buckthorn, a major invasive exotic shrub of Eastern North America. The objectives were to (1) test the negative effect of the invasive buckthorn on seedling growth, (2) determine if this effect differed for two tree species with different shade tolerances and edaphic requirements (sugar maple, red oak), and (3) determine if the type of canopy influenced this effect (5 clones). Confounding factors were reduced in this design (canopy composition and structure, age/size of seedlings), and several factors were controlled (transplantation date, deer exclusion). Several factors were measured (canopy openness, soil nutrients, canopy biomass, understory vegetation biomass, buckthorn density and biomass). After two growing seasons, seedlings of both species had reduced diameter and height increments under buckthorn. This difference was statistically significant for diameter increment. Canopy type did not have any effect on environmental variables or seedling growth. Buckthorn reduced light availability, but had no effect on soil moisture or soil nutrient availability. Consistent with sugar maple’s ecological requirements, its diameter growth was explained (multiple regression) firstly by edaphic variables (positive effect: soil humidity and K), and secondly by buckthorn biomass (negative effect). Red oak growth was explained firstly by buckthorn biomass, and secondly by understory vegetation biomass, both negative effects. Seedlings of species with higher light requirements (red oak) may have large growth reductions under buckthorn cover and have difficulty overtopping it. These results indicate that under-planting (plantations, forests) or afforestation should occur rapidly after buckthorn removal, otherwise this introduced invasive shrub may greatly reduce survival and growth of planted trees. Restoration of red oak to areas of former abundance will likely be more difficult because of the competition from glossy buckthorn.

Highlights

Forest managers and conservationists are very concerned by glossy buckthorn (Frangula alnus Mill.) (Fagan and Peart 2004; Webster et al 2007), an exotic invasive shrub that is dominant in several ecosystems in Eastern North America
The objectives of this study are: (1) to test the effect of glossy buckthorn on the growth of tree seedlings planted in the understory of a mature hybrid poplar plantation naturally invaded by buckthorn, (2) to determine if there are differences in this effect in two tree species that differ in their tolerance to shade and in their edaphic requirements, and (3) to determine if the type of hybrid poplar clone forming the canopy (5 clones tested) influences the effect of buckthorn on the growth of the tree seedlings
These results support our first hypothesis about the negative effect of buckthorn on the growth of tree seedlings

Summary

Introduction

Forest managers and conservationists are very concerned by glossy buckthorn (Frangula alnus Mill.) (Fagan and Peart 2004; Webster et al 2007), an exotic invasive shrub that is dominant in several ecosystems in Eastern North America. Its spread is facilitated by openings in the forest canopy, even partial ones, resulting from cutting and thinning operations (Burnham and Lee 2009) It is a somewhat shade-tolerant species that out competes native species in the colonization of canopy gaps, where it tends to form a very dense mono-specific stand (Frappier et al 2004; Nagel et al 2008) limiting light availability for native tree seedlings. When it becomes dominant, glossy buckthorn may possibly modify forest soil conditions through its uptake of resources (nutrients and water) and the addition of its litter to the forest floor (Fagan and Peart 2004). Potential changes in forest species composition may result in a lower value for the future forest (Fagan and Peart 2004)

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