Influence of river channelization and the invasive shrub, Ligustrum sinense, on oak (Quercus spp.) growth rates in bottomland hardwood forests


Question: Do river channelization and a common invasive shrub, Ligustrum sinense Lour. (Chinese privet), affect tree growth rates in forested riverine wetlands?. Location: Three sites along the Wolf River, in and near Memphis, Tennessee, USA. Methods: We cored and analysed 83 oak trees within three study sites. Correlations of river channelization and climatic variables with basal area increments (BAI) and detrended BAI (SFS) were analysed to investigate channelization effects. Significant correlations were removed from BAI chronologies (BAI-HR) to isolate the effect of L. sinense. We investigated effects of L. sinense on tree growth using a before–after/control–impact (BACI) experimental design with one location impacted by invasion (INV), two control locations that were not invaded (NON and REF) and covariates of tree age and size. We conducted a two-factor mixed effects ANCOVA to determine if there was a significant interaction (BACI) effect. Results: Channelization promoted faster canopy oak growth in all sites, spring drought significantly slowed growth in NON, while summer precipitation promoted wood production in NON. Investigation of L. sinense impact on tree growth revealed a significant BACI effect, such that trees in INV grew slower after invasion than trees in the non-invaded control sites (NON and REF). Conclusions: We suggest that faster growth after channelization may be a consequence of increased aerobic conditions, by enabling higher oxygen availability to tree roots. However, many of the obligate bottomland tree species struggle to regenerate in drier, post-channelized conditions. Thus, native drought-tolerant species may have an advantage in the sub-canopy and regeneration layers of these altered forests. Instead, L. sinense, an invasive and drought-tolerant species, has invaded most locations adjacent to the channelized portion of the Wolf River. L. sinense presence reduces the channelization advantage by slowing growth of canopy oaks and possibly contributing to accelerated mortality. These results suggest an emerging shift to a commonly held paradigm with respect to invasive shrub species. Not only do invasive shrubs crowd out other small woody species and herbaceous plants, but they also have the ability to impact the structure of the forest canopy.

Publication Title

Applied Vegetation Science