Response to soil biota by native, introduced non-pest, and pest grass species: is responsiveness a mechanism for invasion?

Comparative assessment of soil quality parameters after implementing afforestation programme in forest areas of Hong Kong

The objective of this research is to investigate and compare the alteration of soil parameters with and without afforestation programme in a Hong Kong forest, analysed by Principal Component Analysis (PCA). One hundred soil samples were collected from the following sites: Pak Ngau Shek (PNS), Shing Mun (SM), Tai Po Kau (TPK), Tai Tong (TT) (forest with afforestation programme) and Lantau Peak (LP) (control site). A significant difference was found in only two out of 16 parameters: pH (8.34-8.87) and PAHs (4.35-6.32 μg/kg) by comparing the soils taken in the forest with and without an afforestation programme implemented. Three principle components are responsible for soil quality variations in the studied sites. The first, second and third components included pH (0.167) and EC (0.176), PAHs (0.331) and PAHs (0.207), respectively. This framework provides information on the least disturbance of soil properties for the afforestation programme. To conclude, a rigorous monitoring of soil quality is necessary to assess forest health after an afforestation programme. Besides, in the long term, an appropriate forest preservation programme should be implemented to achieve rural area sustainability.

Biogeographic differences in soil biota promote invasive grass response to nutrient addition relative to co-occurring species despite lack of belowground enemy release

Multiple plant species invasions and increases in nutrient availability are pervasive drivers of global environmental change that often co-occur. Many plant invasion studies, however, focus on single-species or single-mechanism invasions, risking an oversimplification of a multifaceted process. Here, we test how biogeographic differences in soil biota, such as belowground enemy release, interact with increases in nutrient availability to influence invasive plant growth. We conducted a greenhouse experiment using three co-occurring invasive grasses and one native grass. We grew species in live and sterilized soil from the invader's native (United Kingdom) and introduced (New Zealand) ranges with a nutrient addition treatment. We found no evidence for belowground enemy release. However, species' responses to nutrients varied, and this depended on soil origin and sterilization. In live soil from the introduced range, the invasive species Lolium perenne L. responded more positively to nutrient addition than co-occurring invasive and native species. In contrast, in live soil from the native range and in sterilized soils, there were no differences in species' responses to nutrients. This suggests that the presence of soil biota from the introduced range allowed L. perenne to capture additional nutrients better than co-occurring species. Considering the globally widespread nature of anthropogenic nutrient additions to ecosystems, this effect could be contributing to a global homogenization of flora and the associated losses in native species diversity.

Putative linkages between below- and aboveground mutualisms during alien plant invasions

Evidence of the fundamental role of below-aboveground links in controlling ecosystem processes is mostly based on studies done with soil herbivores or mutualists and aboveground herbivores. Much less is known about the links between belowground and aboveground mutualisms, which have been studied separately for decades. It has not been until recently that these mutualisms-mycorrhizas and legume-rhizobia on one hand, and pollinators and seed dispersers on the other hand-have been found to influence each other, with potential ecological and evolutionary consequences. Here we review the mechanisms that may link these two-level mutualisms, mostly reported for native plant species, and make predictions about their relevance during alien plant invasions. We propose that alien plants establishing effective mutualisms with belowground microbes might improve their reproductive success through positive interactions between those mutualists and pollinators and seed dispersers. On the other hand, changes in the abundance and diversity of soil mutualists induced by invasion can also interfere with below-aboveground links for native plant species. We conclude that further research on this topic is needed in the field of invasion ecology as it can provide interesting clues on synergistic interactions and invasional meltdowns during alien plant invasions.

Mutualistic Interactions and Biological Invasions

Mutualisms structure ecosystems and mediate their functioning. They also enhance invasions of many alien species. Invasions disrupt native mutualisms, often leading to population declines, reduced biodiversity, and altered ecosystem functioning. Focusing on three main types of mutualisms (pollination, seed dispersal, and plant-microbial symbioses) and drawing on examples from different ecosystems and from species- and community-level studies, we review the key mechanisms whereby such positive interactions mediate invasions and are in turn influenced by invasions. High interaction generalization is “the norm” in most systems, allowing alien species to infiltrate recipient communities. We identify traits that influence invasiveness (e.g., selfing capacity in plants, animal behavioral traits) or invasibility (e.g., partner choice in mycorrhizas/rhizobia) through mutualistic interactions. Mutualistic disruptions due to invasions are pervasive, and subsequent cascading effects are also widespread. Ecological networks provide a useful framework for predicting tipping points for community collapse in response to invasions and other synergistic drivers of global change.

Transgenerational soil-mediated differences between plants experienced or naïve to a grass invasion

Invasive species may undergo rapid change as they invade. Native species persisting in invaded areas may also experience rapid change over this short timescale relative to native populations in uninvaded areas. We investigated the response of the native Achillea millefolium to soil from Holcus lanatus-invaded and uninvaded areas, and we sought to determine whether differential responses between A. millefolium from invaded (invader experienced) and uninvaded (invader naïve) areas were mediated by soil community changes. Plants grown from seed from experienced and naïve areas responded differently to invaded and uninvaded soil with respect to germination time, biomass, and height. Overall, experienced plants grew faster and taller than their naïve counterparts. Naïve native plants showed negative feedbacks with their home soil and positive feedbacks with invaded soil; experienced plants were less responsive to soil differences. Our results suggest that native plants naïve to invasion may be more sensitive to soil communities than experienced plants, consistent with recent studies. While differences between naïve and experienced plants are transgenerational, our design cannot differentiate between differences that are genetically based, plastic, or both. Regardless, our results highlight the importance of seed source and population history in restoration, emphasizing the restoration potential of experienced seed sources.