Above- And Belowground Community Responses to Control Techniques for the Invasive Shrub Amur Honeysuckle (Lonicera maackii [Rupr.] Herder) in Mixed-Hardwood Forests of Indiana
Invasive shrubs in forest understories represent a threat to biodiversity and forest regeneration in the Eastern United States. Lonicera maackii [Rupr.] Herder (Amur honeysuckle, Caprifoliaceae) is a prevalent invasive shrub species in the Midwest, and has been documented in more than 30 states. Once a handful of established L. maackii shrubs reach maturity, prolific fruit production and extensive dispersal by wildlife can lead to explosive population growth and the formation of dense, monotypic thickets across wide areas of forested land. Light limitation, changes to soil nutrient cycling, increased throughfall interception, cascading effects on wildlife (e.g., increased rates of seed predation), and possible allelopathy all contribute to the effects of L. maackii on understory plant communities. While evidence from other invasive shrub taxa indicates that short-term plant community recovery following shrub control may not fully reflect the composition of pre-invasion communities, understory herbaceous cover and native seedling densities can rebound rapidly even after decades of L. maackii invasion. However, the ability of these shrubs to resprout readily from pre-formed buds in the stump and the susceptibility of shrub control sites to reseeding from surrounding invasions provide a challenge for lasting control. Forestry mulching heads are being used with increasing frequency for invasive shrub control in the Eastern U.S. While this technique offers the potential to accelerate the removal of dense shrub thickets over large areas, its ecological effects are understudied in this context. For L. maackii control, the effectiveness of mulching heads for mitigating resprouting and the impacts of this technique on recovery of the understory plant community relative to the widespread cut-stump method are unknown. Despite several lines of evidence that soil processes in forests invaded by L. maackii are modified in ways that are largely associated with accelerated decomposition rates, the effects of honeysuckle removal on soil properties remain poorly understood. Some effects of L. maackii on soil properties can be slow to develop, and invasive shrubs can have lasting legacies on soils, suggesting that the effects of shrub removal may not be evident in the short-term. However, the tendency of these shrubs to resprout when cut may have relatively rapid implications for belowground processes, due to the potential for flushes of root exudates to accelerate decomposition of soil organic matter (SOM), known as a rhizosphere priming effect. I compared the forestry mulching (“Fecon”) head to cut-stump removal in terms of its effectiveness at controlling L. maackii and its impacts on native plant community recovery, while also isolating the effects of mulch deposition from the effects of removal treatments themselves. I also examined soil chemistry and soil microbial community function in bulk soils following shrub removal and in rhizosphere soils directly associated with roots of resprouting (in treatment areas) or undamaged (in reference areas) honeysuckle. Two growing seasons after the initial shrub removal treatments, L. maackii regrowth was higher in Fecon-treated plots than cut-stump plots, but depth of the Fecon head application was an important determinant of stump sprouting in other shrub removal sites. While further data are necessary to determine the effect of Fecon treatment depth on native community recovery, the two removal treatments were no different in the recovery of native seedlings, herbaceous flora, and herbaceous-layer diversity indices following shrub removal. Herbaceous spring perennials were slower to respond to shrub removal than other components of the understory plant community. Soil microbial community function was analyzed using the MicroResp technique for multiple substrate induced respiration (MSIR) responses. MSIR responses in bulk soils were lower in removal areas than reference areas for nearly all substrates the first year, coinciding with relatively elevated responses in the rhizosphere soils of resprouting shrubs. Rhizosphere soils of resprouting shrubs also had higher SOM and organic C than untreated shrubs, particularly in the first year, suggesting a flush of rhizodeposits from cutting and regrowth. The strongest drivers of MSIR profiles were soil organic matter (SOM) and pH. Bulk soil chemistry responses to shrub removal initially reflected a reduction in the effects of honeysuckle on throughfall precipitation—higher ammonium and lower Mg—and did not show any evidence of a rhizosphere priming effect, despite elevated microbial activity and organic C in the rhizosphere of resprouting shrubs. However, changes in bulk soil chemistry between years reflected honeysuckle rhizosphere soil, and this effect was stronger in treatment areas. Honeysuckle mulch was associated with greater soil potassium concentration. Although both shrub removal techniques will require follow-up control methods to kill resprouting shrubs and new seedlings, forestry mulching heads are a promising technique to accelerate the control of invasive shrubs. This study is the first to document the potential for invasive shrub control to affect soil properties through rhizodeposition by the target species, but leaves open questions as to the role of shrub mortality on rhizodeposition, as I did not examine soils associated with the roots of successfully killed L. maackii shrubs. Future work on soil legacy effects of invasive woody species should consider the influence of root inputs from the target species elicited by control efforts.
Jenkins, Purdue University.
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