Dr. Elly Morriën

VENI-NWO programme - Secondary succession belowground: top-down or bottom-up controlled?

Secondary succession is a key process in the restoration of semi-natural grasslands on former agricultural land. Current practice shows that when former agricultural land is assigned to nature restoration, the secondary succession appears to be very slow and often does not result in the targeted plant community. Secondary succession involves myriad transitions within a complex network of plants and belowground bacterial and fungal decomposers, microbial consumers, and predators. Changes in plant and soil communities during secondary succession have been well described, however, fundamental understanding of mechanisms that drive these transitions is lacking. My research aims to unravel the mechanisms that drive secondary succession in plant and soil communities. I will test the hypothesis that changes in root exudation and organic matter caused by changes in plant species composition favors soil fungi which in turn will benefit the establishment of later successional plants. In particular, I expect that a combination of a decrease in nutrient availability and an increase in the activity of soil fungi and their higher trophic levels will stimulate later successional plant species. I will use mathematical modelling to capture the network of ecological interaction and identify which interactions may potentially drive succession, and how. These interactions are further analyzed using empirical methods such as soil transplants, faunal gut content analysis and the analysis of root exudates and organic matter during succession. Knowledge gained from the empirical part will then be used to further parameterize the model to get more accurate predictions on the crucial drivers of successional change. I will disentangle the plant effect from the soil community effect by testing root exudate and organic matter influences on soil with and without a living soil community. This will yield potential tools to speed up transitional shifts in plant and soil community composition during secondary succession.

Traditionally, it is assumed that during secondary succession nutrient availability decreases, which enables slow growing plant species with recalcitrant tissues to take over from early successional fast growing plant species with easily decomposable tissues. However, until recently little attention has been given to the role of the soil food web in plant succession. Later successional plant species stimulate fungal communities that have the ability to break down complex substrates. Fungi recycle organic material at a relatively slow rate. Therefore, nutrients become increasingly immobilized, which favors slow growing plants. However, the question is to what extent the soil food web already initiates the transition from fast to slow growing plant species.

In my recent work using stable isotope labelling and sequencing of the soil biodiversity, I provided novel evidence that fungal communities increase their activity by >20% during succession. Especially wood rot fungi, which can break down recalcitrant organic matter and arbuscular mycorrhizal fungi (AMF), which can provide scarce nutrients to plants become more dominant in later successional stages (Morriën et al. in review.). However, these results do not reveal whether soil communities drive changes in the plant community or the other way around. To be able to manipulate the rate of succession successfully it will be crucial to enhance understanding of who drives who, when and where.

The aim of my study is to unravel the mechanisms that drive secondary succession in plant and soil communities. I focus on decomposer- and mutualistic fungi, because they play an important role in later successional stages. I will examine if these fungi may drive or follow secondary succession in vegetation. I will test the hypothesis that changes in root exudation and organic matter caused by plants favors soil fungi, which will enhance later successional plants. I expect that a combination of a decrease in nutrient availability and an increase in the activity of soil fungi and their higher trophic levels will stimulate later successional plant species.

Ecofinders – Functional Soil Biodiversity (post-doc project)

The next step in future research would be to study the process of range-expansion itself. Since range-expanding exotics gradually shift range within a continent, they provide a perfect opportunity for detailed studies on changes that may take place along the range expansion gradient. Since climate change is one of the major global factors that drive changes in ecosystems, it is important to be able to make predictions on the future consequences of range expansions for biodiversity and the functioning of ecosystems.

In the next study I examined how the soil nematode community from the new range responds to exotic plant species compared to related native plants species. In line with the enemy release hypothesis, I found fewer root-feeding nematodes per unit root biomass on range expanders than on related native plant species. This would point at range expanders being resistant rather than tolerant against root-feeding nematodes from the new range. However, lower densities of root-feeding nematodes were partially caused by higher amounts of root biomass in the range-expanding exotic plants compared to the root biomass produced by the related native plant species. But when I correlated root biomass with the number of root-feeding nematodes I did not find a significant correlation. This implies that although range-expanding exotic plants had higher root biomass and fewer root-feeding nematodes, root biomass only cannot explain the root-feeding nematode numbers. As differences between range expanding and native plant species were quite consistent across the various types of root-feeding nematodes, our results suggest that the exotic plants did not promote numbers of root-feeding nematodes as much as related natives. On the other hand, the overall taxonomic and functional nematode community composition was influenced by plant species rather than by plant origin. Indeed, the plant identity effects declined in nematodes with higher trophic positions, as predicted. This was due to plant feeders being influenced more by species identity than higher trophic level nematodes, like bacterial and fungal feeders.
 

As a follow up on this study, I determined the rhizosphere community composition of bacteria, fungi, arbuscular mycorrhizal fungi (AMF) and fusaria. All groups of microbes were analyzed qualitatively and the non-mycorrhizal fungal biomass and fusaria were also analyzed quantitatively. I tested the hypothesis that range-expanding plant species have a different rhizosphere microbial structure than natives. I found different bacterial rhizosphere communities in native and range-expanding exotic plants, whereas I found no effects in the case of fungi, arbuscular mycorrhizal fungi (AMF) and fusaria. Quantitatively, I found less non-mycorrhizal fungal biomass and fewer fusaria genome copies in the rhizosphere of exotic range-expanding plant species than in related native plants. Overall, range-expanding plants appear to have a soil community different from natives, which supports my previous observations that the range expanders had the least negative soil feedback. However, my results did not reveal a causal link between rhizosphere microbial community composition and plant-soil feedback.

Finally, I compared the early establishment of range-expanding exotics and phylogenetically related plant species that are native in the invaded habitats. In a greenhouse I grew five range-expanding plant species and five related natives in sterilized and non-sterile soils from the new range, both alone and with a background community of plant species that are also present in the invaded habitat. In the field, I grew the same plants species in artificially created sparse and dense plant communities. I tested whether range-expanding exotic plant species establish better under competition with native vegetation than phylogenetically related natives, because exotics may benefit from less negative interactions with the soil community compared to natives. In the greenhouse, there was a strong negative effect of competition from the background community on the shoots and the roots of both range expanders and native plant species when compared to their performance in pots without background community. The shoot and root biomass of the range-expanding plants did not differ from the native congeners, neither alone nor in competition with the background community. Soil biota effects were detectable only when the exotics and natives were grown in monoculture. In the field experiment the shoot and root biomass of range expanders and native focal plants species were also not different. Since the results of the field experiment are in line with those of the greenhouse, I conclude that in this experiment range-expanding plant species do not benefit from their tolerance or resistance to soil-borne pathogens from the new range.