Effects of tree sapling diversity and nutrient addition on herb-layer invasibility in communities of subtropical species

Sabine Both,Helge Bruelheide,Teng Fang,Bernhard Schmid,Martin Baruffol,Alexandra Erfmeier

doi:10.4236/oje.2012.21001

Abstract

Exotic species are assumed to alter ecosystem functioning. However, little is known of the relationships within vertically structured plant communities such as forests, where tree saplings interact with herbaceous species, especially in the early phases of succession. This relationship was tested in a common garden experiment which assessed the impacts on tree saplings and herbaceous species following nutrient addition and the introduction of exotic herb species. The experiment was established in South- East China using four broad-leaved tree species (Elaeocarpus decipiens, Schima superba, Castanea henryi and Quercus serrata) to study the relationships between tree sapling diversity, herb-layer productivity and invasibility. Tree saplings were planted in monoculture and in mixtures of two and four species. A full factorial design was applied, within which species composition was crossed with nutrient and exotic seed-addition treatments. The seed-addition treatment included mixtures of seeds from eight exotic herb species, and herb community attributes were assessed after a four month growing season. Results indicate that certain tree species negatively affect native as well as exotic herbs; however, the high productivity of native herbs had a stronger negative impact on exotic species than tree saplings. Nutrient addition increased the productivity of exotic herbs but had no effect on native herbs. Remarkably, exotic species introduction had a negative feedback effect on the growth of tree saplings, which highlights the potential of exotic herbs to diminish tree recruitment. Although tree saplings reduced invasive effects on the herb-layer during the earliest phase of forest succession, nutrient addition had a more profound and opposite effect on these invaders.

Highlights

Given recent and predicted rates of global biodiversity loss, ecological research has shifted toward exploring the relationships between biodiversity and ecosystem functioning as a matter of priority [1,2,3,4]
Native herbaceous species recolonised the plots immediately after weeding and exotic species were not detected in plots without seed-addition, which confirms that exotics were absent in the local seed bank
Within our early-successional communities of tree saplings and herb species, we found evidence for mutual effects in the very first growing season

Summary

Introduction

Given recent and predicted rates of global biodiversity loss, ecological research has shifted toward exploring the relationships between biodiversity and ecosystem functioning as a matter of priority [1,2,3,4]. Studies from various ecosystem types have revealed the importance of species richness to a range of ecosystem functions and identified that diverse communities are both more productive [5,6] and more stable (e.g. increased invasion resistance [7,8]). Investigations into current threats to biodiversity have revealed that in addition to habitat loss and exotic species invasion, increased nitrogen deposition negatively affects plant diversity [9,10,11]. Subtropical regions of East and South-East Asia host high levels of vascular plant species richness [12], but countries of the regions, and in particular China with its high level of economic and industrial growth and vast population, still have nature conservation systems that lag behind their increasing environmental problems [15, 16]. Negative impacts of human activities on natural ecosystems have led to an increasing number of naturalized exotic species [17,18,19], increasing the risk of OPEN ACCESS

Methods

Results

Conclusion