Linking aboveground and belowground diversity

Aboveground and belowground species interactions drive ecosystem properties at the local scale, but it is unclear how these relationships scale-up to regional and global scales. Here, we discuss our current knowledge of aboveground and belowground diversity links from a global to a local scale. Global diversity peaks towards the Equator for large, aboveground organisms, but not for small (mainly belowground) organisms, suggesting that there are size-related biodiversity gradients in global aboveground-belowground linkages. The generalization of aboveground-belowground diversity relationships, and their role in ecosystem functioning, requires surveys at scales that are relevant to the organisms and ecosystem properties. Habitat sizes and diversity gradients can differ significantly between aboveground and belowground organisms and between ecosystems. These gradients in biodiversity and plant community trait perception need to be acknowledged when studying aboveground-belowground biodiversity linkages.     

Earthworm and belowground competition effects on plant productivity in a plant diversity gradient

Diversity is one major factor driving plant productivity in temperate grasslands. Although decomposers like earthworms are known to affect plant productivity, interacting effects of plant diversity and earthworms on plant productivity have been neglected in field studies. We investigated in the field the effects of earthworms on plant productivity, their interaction with plant species and functional group richness, and their effects on belowground plant competition. In the framework of the Jena Experiment we determined plant community productivity (in 2004 and 2007) and performance of two phytometer plant species [Centaurea jacea (herb) and Lolium perenne (grass); in 2007 and 2008] in a plant species (from one to 16) and functional group richness gradient (from one to four). We sampled earthworm subplots and subplots with decreased earthworm density and reduced aboveground competition of phytometer plants by removing the shoot biomass of the resident plant community. Earthworms increased total plant community productivity (+11%), legume shoot biomass (+35%) and shoot biomass of the phytometer C. jacea (+21%). Further, phytometer performance decreased, i.e. belowground competition increased, with increasing plant species and functional group richness. Although single plant functional groups benefited from higher earthworm numbers, the effects did not vary with plant species and functional group richness. The present study indicates that earthworms indeed affect the productivity of semi-natural grasslands irrespective of the diversity of the plant community. Belowground competition increased with increasing plant species diversity. However, belowground competition was modified by earthworms as reflected by increased productivity of the phytometer C. jacea. Moreover, particularly legumes benefited from earthworm presence. Considering also previous studies, we suggest that earthworms and legumes form a loose mutualistic relationship affecting essential ecosystem functions in temperate grasslands, in particular decomposition and plant productivity. Further, earthworms likely alter competitive interactions among plants and the structure of plant communities by beneficially affecting certain plant functional groups.     

Linking aboveground and belowground interactions via induced plant defenses

Plants have a variety of chemical defenses that often increase in concentration following attack by herbivores. Such induced plant responses can occur aboveground, in the leaves, and also belowground in the roots. We show here that belowground organisms can also induce defense responses aboveground and vice versa. Indirect defenses are particularly sensitive to interference by induced feeding activities in the other compartment, and this can disrupt multitrophic interactions. Unravelling the involvement of induced plant responses in the interactions between aboveground and belowground communities associated with plants is likely to benefit from comprehensive metabolomic analyses. Such analyses are likely to contribute to a better understanding of the costs and benefits involved in the selection for induced responses in plants.     

Effects of plant evolution on nutrient cycling couple aboveground and belowground processes

Plant strategies for nutrient acquisition and recycling are key components of ecosystem functioning. How the evolution of such strategies modifies ecosystem functioning and services is still not well understood. In the present work, we aim at understanding how the evolution of different phenotypic traits link aboveground and belowground processes, thereby affecting the functioning of the ecosystem at different scales and in different realms. Using a simple model, we follow the dynamics of a limiting nutrient inside an ecosystem. Considering trade-offs between aboveground and belowground functional traits, we study the effects of the evolution of such strategies on ecosystem properties (amount of mineral nutrient, total plant biomass, dead organic matter, and primary productivity) and whether such properties are maximized. Our results show that when evolution leads to a stable outcome, it minimizes the quantity of nutrient available (following Tilman’s R* rule). We also show that considering the evolution of aboveground and belowground functional traits simultaneously, total plant biomass and primary productivity are not necessarily maximized through evolution. The coupling of aboveground and belowground processes through evolution may largely diminish predicted standing biomass and productivity (extinction may even occur) and impact the evolutionary resilience (i.e., the return time to previous phenotypic states) of the ecosystem in the face of external disturbances. We show that changes in plant biomass and their effects on evolutionary change can be understood by accounting for the links between nutrient uptake and mineralization, and for indirect effects of nutrient uptake on the amount of detritus in the system.     

Linking aboveground and belowground communities: the indirect influence of aphid species identity and diversity on a three trophic level soil food web

There has been a growing recent interest in how foliar herbivory may indirectly affect the belowground sub-system, but little is known about the belowground consequences of the identity, species composition or diversity of foliar herbivores. We performed an experiment, utilising model grassland communities containing three plant species, in which treatments consisted of addition of each of eight aphid species in single and in two- four- and eight-species combinations, as well as an aphid-free treatment. While aphid species treatments did not affect total plant biomass or productivity, aphid species identity had important effects on the relative abundance of the three plant species. This in turn affected the abundances of each of three groups of secondary consumers in the soil food web (bacterial- and fungal-feeding nematodes, and enchytraeids) but not primary consumers (microbes, herbivorous nematodes) or tertiary consumers (predatory nematodes). The fact that some trophic levels responded to treatments while others did not is consistent with trophic dynamic theory. Aphid species treatments also affected the community composition within each of the herbivorous, microbe-feeding and top predatory nematode groups, as well as diversity within the first two of these groups. However, aphid species diversity per se had few effects. There were specific instances in which specific aboveground and belowground response variables in two aphid species combinations differed significantly from those in both of the corresponding single aphid species treatments (apparently as a consequence of resource use complementarity between coexisting aphid species), but no instance in which increasing aphid diversity beyond two species had any effect. Our results provide evidence that the identity of aboveground consumers can have effects that propagate through multiple trophic levels in soil food webs in terms of consumer abundance, and composition and diversity within trophic levels.     

Sequential effects of root and foliar herbivory on aboveground and belowground induced plant defense responses and insect performance

Plants are often simultaneously or sequentially attacked by multiple herbivores and changes in host plants induced by one herbivore can influence the performance of other herbivores. We examined how sequential feeding on the plant Plantago lanceolata by the aboveground herbivore Spodoptera exigua and the belowground herbivore Agriotes lineatus influences plant defense and the performance of both insects. Belowground herbivory caused a reduction in the food consumption by the aboveground herbivore independent of whether it was initiated before, at the same time, or after that of the aboveground herbivore. By contrast, aboveground herbivory did not significantly affect belowground herbivore performance, but significantly reduced the performance of later arriving aboveground conspecifics. Interestingly, belowground herbivores negated negative effects of aboveground herbivores on consumption efficiency of their later arriving conspecifics, but only if the belowground herbivores were introduced simultaneously with the early arriving aboveground herbivores. Aboveground–belowground interactions could only partly be explained by induced changes in an important class of defense compounds, iridoid glycosides (IGs). Belowground herbivory caused a reduction in IGs in roots without affecting shoot levels, while aboveground herbivory increased IG levels in roots in the short term (4 days) but only in the shoots in the longer term (17 days). We conclude that the sequence of aboveground and belowground herbivory is important in interactions between aboveground and belowground herbivores and that knowledge on the timing of exposure is essential to predict outcomes of aboveground–belowground interactions.     

Pine invasion impacts on plant diversity in Patagonia: invader size and invaded habitat matter

Conifers, which are widely planted as fast growing tree crops, are invading forested and treeless environments across the globe, causing important changes in biodiversity. However, how small-scale impacts on plant diversity differ according to pine size and habitat context remains unclear. We assessed the effects of different stages of pine invasion on plant communities in forest and steppe sites located in southern Chile. In each site, we sampled plant diversity under and outside the canopy of Pinus contorta individuals, using paired plots. We assessed the relative impact of pine invasion on plant species richness and cover. In both sites, richness and cover beneath pine canopy decreased with increasing pine size (i.e. height and canopy area). A significant negative impact of pines on species richness and plant cover was detected for pines over 4 m in height. The impact of pines on plant richness and cover depended on pine size (i.e. canopy area) and habitat type. Larger pines had more negative impacts than smaller pines in both sites, with a greater impact for a given pine size in the Patagonian steppe compared to the A. araucaria forest. Species composition changed between under and outside canopy plots when pines were 4 m or taller. Pine presence reduced cover of most species. The impacts of pine invasions are becoming evident in forested and treeless ecosystems of southern Chile. Our results suggest that the magnitude of pine invasion impacts could be related to how adapted the invaded community is to tree cover, with the treeless environment more impacted by the invasion.     

Effects of plant diversity on invasion of weed species in experimental pasture communities

Studies have shown that weed invasion into grasslands may be suppressed if the resident plant community is sufficiently diverse. The objective of this study was to determine whether increased forage plant diversity in grazed pasture communities might be associated with reduced weed abundance both in the aboveground vegetation and soil seed bank. Data were collected from a pasture experiment established in 1994 in Missouri, USA. The experiment consisted of 15 m×15 m plots sown with Festuca arundinacea Schreb. or Bromus inermis Leysser as a base species in mixtures of 1, 2, 3, 6, or 8 forage species. The plots were grazed by cattle during each growing season from 1998 to 2002. Aboveground plant species composition in each plot was measured using a point step method. Soil cores were collected in 1999 and 2002, and the species composition of germinable weed seeds in plots were evaluated by identifying seedlings as they germinated over an 8-week period. Species diversity was measured using several indices: species richness (S), Shannon–Wiener diversity index (H^′), and forage species evenness (J). Aboveground weed abundance in plots was unrelated to forage species richness (S), but weed abundance declined as the evenness (J) of resident forage species increased in mixtures. The species composition of mixtures may have affected weed abundance. Weeds both in the soil seed bank and aboveground vegetation were less abundant in mixtures that contained F. arundinacea compared with mixtures that contained B. inermis. Although variables like forage plant productivity may also suppress weed abundance in pastures, our results suggest that maintaining an evenly distributed mixture of forage species may help suppress weeds as well.

Zusammenfassung

Untersuchungen haben gezeigt, dass die Unkrautinvasion in Grünländer unterdrückt sein kann, wenn die ansässige Pflanzengemeinschaft ausreichend divers ist. Die Zielsetzung dieser Untersuchung war es zu bestimmen, ob eine erhöhte Futterpflanzendiversität in beweideten Grünlandgemeinschaften mit einer verringerten Unkrautabundanz sowohl bei der oberirdischen Vegetation als auch in der Bodensamenbank verbunden sein kann. Die Daten wurden in einem Weidelandexperiment gesammelt, das 1994 in Missouri, USA, etabliert wurde. Das Experiment bestand aus 15 m×15 m Probeflächen, die mit Festuca arundinacea Schreb. oder Bromus inermis Leysser als Basisarten in Mischungen von 1, 2, 3, 6 oder 8 Futterarten eingesät waren. Die Probeflächen wurden während jeder Wachstumssaison von 1998 bis 2002 stark mit Vieh beweidet. Die oberirdische Pflanzenartenzusammensetzung wurde in jeder Fläche mit einer Punktstopmethode gemessen. Bodenproben wurden 1999 und 2002 gesammelt und die Artenzusammensetzung der keimfähigen Unkrautsamen wurde in den Probeflächen bewertet, indem die Keimlinge identifiziert wurden, die in einer 8-wöchigen Periode keimten. Die Artendiversität wurde unter Verwendung verschiedener Indizes gemessen: Artenreichtum (S), Shannon–Wiener-Diversitätsindex (H^′) und Futterarten-Äquitabilität (J). Die oberirdische Unkrautartenabundanz in den Probeflächen stand in keiner Beziehung zum Futterartenreichtum (S), aber die Unkrautabundanz nahm ab, wenn die Äquitabilität (J) der ansässigen Futterarten in den Mischungen zunahm. Die Artenzusammensetzung der Mischungen könnte die Unkrautabundanz beeinflusst haben. Sowohl die Unkräuter in der Bodensamenbank, als auch in der oberirdischen Vegetation waren weniger abundant in Mischungen, die F. arundinacea enthielten, im Vergleich zu denen, die B. inermis enthielten. Obgleich Variablen wie die Futterpflanzenproduktivität möglicherweise ebenfalls die Unkrautabundanz im Weideland unterdrücken, lassen unsere Ergebnisse vermuten, dass die Aufrechterhaltung einer gleichmäßigen Mischung von Futterarten ebenfalls helfen kann, die Unkräuter zu unterdrücken.     

The variable effects of soil nitrogen availability and insect herbivory on aboveground and belowground plant biomass in an old-field ecosystem

Nutrient availability and herbivory can regulate primary production in ecosystems, but little is known about how, or whether, they may interact with one another. Here, we investigate how nitrogen availability and insect herbivory interact to alter aboveground and belowground plant community biomass in an old-field ecosystem. In 2004, we established 36 experimental plots in which we manipulated soil nitrogen (N) availability and insect abundance in a completely randomized plot design. In 2009, after 6 years of treatments, we measured aboveground biomass and assessed root production at peak growth. Overall, we found a significant effect of reduced soil N availability on aboveground biomass and belowground plant biomass production. Specifically, responses of aboveground and belowground community biomass to nutrients were driven by reductions in soil N, but not additions, indicating that soil N may not be limiting primary production in this ecosystem. Insects reduced the aboveground biomass of subdominant plant species and decreased coarse root production. We found no statistical interactions between N availability and insect herbivory for any response variable. Overall, the results of 6 years of nutrient manipulations and insect removals suggest strong bottom-up influences on total plant community productivity but more subtle effects of insect herbivores on aspects of aboveground and belowground production.     

Effect of aboveground litter on belowground plant interactions in a native Rough Fescue grassland

Chemical compounds from plants may exhibit stimulatory and/or inhibitory effects on surrounding organisms. However, research on belowground biochemical interactions among plants has focused more effort on elucidating negative effects. Moreover, the effect of shoot litter on belowground plant–plant interactions has remained relatively unexplored. In a field experiment with four target plant species (Artemisia frigida Willd., Solidago missouriensis Nutt.), Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths and Poa pratensis L.) interacting with intact grassland neighbours, we manipulated root competition using PVC tubes and shoot litter, and belowground chemical interaction by adding activated carbon (AC) to the soil. In A. frigida, shoot litter significantly interacted with root competition and root chemicals. Plants grown plus AC were larger than those minus AC when shoot litter was left intact suggesting inhibitory effects from neighbours and/or decomposing products. However, when shoot litter was removed, plants grown minus AC were larger suggesting stimulatory effects of root exudates. B. gracilis showed a similar trend but results were non-significant. Results demonstrate that the effects of neighbours can be inhibitory or facilitative depending on the presence or absence of shoot litter and mediation through AC.     

Diversity- and density-mediated allelopathic effects of resident plant communities on invasion by an exotic plant

Plant and Soil - Resident plants can exert allelopathic effects on introduced exotic plants, and resistance to exotic plant invasions usually increases with diversity and density of the resident...     

Larger aboveground neighbourhood scales maximise similarity but do not eliminate discrepancies with belowground plant diversity in a Mediterranean shrubland

Plant and Soil - An unresolved question in plant ecology is whether diversity of the aboveground and belowground compartments of a plant community is similar at different neighbourhood scales. We...     

Negative relationship between diversity and productivity under plant invasion

The relationship between diversity and productivity of plant community under plant invasion has been not well known up to now. Here, we investigated the relationship between diversity and productivity under plant invasion and studied the response of species level plant mass to species richness in native and invaded communities. A field experiment from 2008 to 2013 and a pot experiment in 2014 were conducted to study the effects of plant invasion on the relationship between diversity and productivity and the response of species level plant mass to species richness in native and invaded communities. The community level biomass was negatively correlated to plant species richness in invaded communities while the same relationship was positive in native communities. The species level plant mass of individual species responded differently to overall plant species richness in the native and invaded communities, namely, most of the species’ plant mass increased in native communities, but decreased in invaded communities with increasing species richness. The complementarity or selection effects might dominate in native communities while competition effects might dominate in invaded communities. Accordingly, the negative relationship between diversity and productivity under plant invasion is highlighted in our experiments.     

Influence of plant diversity and elevated atmospheric carbon dioxide levels on belowground bacterial diversity

Background  

Changes in aboveground plant species diversity as well as variations of environmental conditions such as exposure of ecosystems to elevated concentrations of atmospheric carbon dioxide may lead to changes in metabolic activity, composition and diversity of belowground microbial communities, both bacterial and fungal.     

Non-native and native shrubs have differing impacts on species diversity and composition of associated plant communities

The spread of non-native plants has been depicted as a serious threat to biodiversity. However, it remains unclear whether the indigenousness of the invading plant plays a marked role for the ecological consequences of an invasion as few studies have compared the ecological impacts of non-native shrubs with structurally or functionally comparable native shrubs. We studied patches of introduced and native shrubs to assess whether there are general differences in plant species composition or biomass between patches formed by non-native versus native shrubs. The indigenousness of the shrub (non-native vs. native) did not explain the variation in soil nutrients, neither the production of shoot biomass or allocation of growth to different parts of the shoot. The amount of light reaching ground level did not differ between patches of a non-native and a native shrub. However, species richness and biomass of herbaceous plants were lower in patches of non-native than native shrubs and the amount of litter was higher below non-native than native shrubs. Our results suggest that the indigenousness of the patch-forming plant may be an important factor for the diversity and composition of associated herbaceous vegetation. Based on our results, resource availability (light and nutrients) is not a sufficient explanation for the negative effects of non-native shrubs on plant communities. Further research is needed to investigate whether alternative explanations, such as the novelty of the toxic compounds produced by non-native plants, can explain the differences we observed.     

Disparate effects of plant genotypic diversity on foliage and litter arthropod communities

Intraspecific diversity can influence the structure of associated communities, though whether litter-based and foliage-based arthropod communities respond to intraspecific diversity in similar ways remains unclear. In this study, we compared the effects of host-plant genotype and genotypic diversity of the perennial plant, Solidago altissima, on the arthropod community associated with living plant tissue (foliage-based community) and microarthropods associated with leaf litter (litter-based community). We found that variation among host-plant genotypes had strong effects on the diversity and composition of foliage-based arthropods, but only weak effects on litter-based microarthropods. Furthermore, host-plant genotypic diversity was positively related to the abundance and diversity of foliage-based arthropods, and within the herbivore and predator trophic levels. In contrast, there were minimal effects of plant genotypic diversity on litter-based microarthropods in any trophic level. Our study illustrates that incorporating communities associated with living foliage and senesced litter into studies of community genetics can lead to very different conclusions about the importance of intraspecific diversity than when only foliage-based community responses are considered in isolation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Host plant species effects on arbuscular mycorrhizal fungal communities in tallgrass prairie

Symbiotic associations between plants and arbuscular mycorrhizal (AM) fungi are ubiquitous in many herbaceous plant communities and can have large effects on these communities and ecosystem processes. The extent of species-specificity between these plant and fungal symbionts in nature is poorly known, yet reciprocal effects of the composition of plant and soil microbe communities is an important assumption of recent theoretical models of plant community structure. In grassland ecosystems, host plant species may have an important role in determining development and sporulation of AM fungi and patterns of fungal species composition and diversity. In this study, the effects of five different host plant species [Poa pratensis L., Sporobolus heterolepis (A. Gray) A. Gray, Panicum virgatum L., Baptisia bracteata Muhl. ex Ell., Solidago missouriensis Nutt.] on spore communities of AM fungi in tallgrass prairie were examined. Spore abundances and species composition of fungal communities of soil samples collected from patches within tallgrass prairie were significantly influenced by the host plant species that dominated the patch. The AM fungal spore community associated with B. bracteata showed the highest species diversity and the fungi associated with Pa. virgatum showed the lowest diversity. Results from sorghum trap cultures using soil collected from under different host plant species showed differential sporulations of AM fungal species. In addition, a greenhouse study was conducted in which different host plant species were grown in similar tallgrass prairie soil. After 4 months of growth, AM fungal species composition was significantly different beneath each host species. These results strongly suggest that AM fungi show some degree of host-specificity and are not randomly distributed in tallgrass prairie. The demonstration that host plant species composition influences AM fungal species composition provides support for current feedback models predicting strong regulatory effects of soil communities on plant community structure. Differential responses of AM fungi to host plant species may also play an important role in the regulation of species composition and diversity in AM fungal communities. Received: 29 January 1999 / Accepted: 20 October 1999