Rhododendron maximum impacts seed bank composition and richness following Tsuga canadensis loss in riparian forests

Tristan M Cofer,Janis K Bush,Katherine J Elliott,Chelcy F Miniat

doi:10.1002/ecs2.2204

Tristan M Cofer, Janis K Bush + Show 2 more

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https://doi.org/10.1002/ecs2.2204

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Abstract

AbstractSouthern Appalachian riparian forests have undergone changes in composition and function from invasive pathogens and pests. Castanea dentata mortality in the 1930s from chestnut blight (Cryphonectria parasitica) and Tsuga canadensis mortality in the 2000s from the hemlock woolly adelgid (Adelges tsugae) have led to the expansion and increased growth of Rhododendron maximum, an evergreen subcanopy shrub. A better understanding of seed bank characteristics and the various abiotic and biotic factors that affect the seed bank may be useful in determining the restoration potential of forest communities following invasion‐related disturbances. We compared the seed bank of two deciduous forest types: hardwood forests with a dense R. maximum subcanopy (hereafter, RR) and hardwood forests without R. maximum (hereafter, HWD). We evaluated numerous microenvironmental variables through principal component analysis (PCA) and correlated the derived PCA axes scores to seed bank density and richness across forest types. We found that seed bank density was comparable between the forests types; however, seed bank richness was much lower in RR than HWD and the species composition was dissimilar between forest types. Twenty‐eight of 64 (44%) species in the seed bank of HWD were not found in the seed bank of RR. Species that were represented in both forest types were often found in contrasting densities. Most notably, seed bank densities of several woody species were considerably higher in RR (85%) than HWD (45%), while herbaceous seed bank density was lower in RR (11%) than HWD (50%). Mineral soil pH, soil nutrient availability, and soil moisture were lower, and organic soil (Oi + Oe + Oa) depth and mass were greater in the RR than HWD forest type. PCA correlations revealed that PCA4 (represented by understory density and Oe + Oa phosphorus and carbon/nitrogen ratio) was negatively correlated with total seed bank density. PCA1 (represented by Oe + Oa cations and phosphorus, understory richness, ground‐layer cover, and mineral soil pH) and PCA4 were positively correlated with total seed bank richness. These results suggest that the soil seed bank will not be the primary mode of recruitment to establish a diverse and herbaceous‐rich community if a RR is present.

Highlights

Human activities over the past century have strongly impacted forest disturbance regimes in many parts of the world
We considered whether the seed bank could contribute to the restoration of plant communities in southern Appalachian riparian forests affected by adelgid-induced T. canadensis mortality
A significant interaction effect (Table 1) revealed that shrub seed density was similar at Coweeta Basin (CWT) between forest types (t1,35.2 = À0.62, P = 0.924); at White Oak Creek Basin (WOC), shrub seed density was much greater in RR than HWD (t1,35.2 = À3.51, P = 0.007; Table 2)

Summary

Introduction

Human activities over the past century have strongly impacted forest disturbance regimes in many parts of the world. Examples of non-native forest insects and pathogens that have resulted in tree mortality include gypsy moth (Lymantria dispar L.), emerald ash borer (Agrilus planipennis Fairmaire), chestnut blight (Cryphonectria parasitica (Murr.) Barr), Dutch elm disease (Ophiostoma novo-ulmi), and hemlock woolly adelgid (Adelges tsugae Annand), and these disturbances have inflicted ecological and economic damage (Lovett et al 2016). Tree mortality in the mid-1930s due to the chestnut blight resulted in expansion of Rhododendron maximum L. shrubs (Elliott and Vose 2012), and the more recent Tsuga canadensis (L.) Carriere tree mortality from infestation by the hemlock woolly adelgid has led to further increased growth of R. maximum (Ford et al 2012). Rhododendron maximum is an ericaceous evergreen shrub that, at high densities, inhibits tree seedling recruitment and limits overstory regeneration (Clinton 1995, Lei et al 2002, 2006)

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