No abstract is provided for this article.
. Understanding the impacts of plant community characteristics on soil carbon dioxide efflux (R) is a key prerequisite for accurate prediction of the future carbon balance of terrestrial ecosystems under climate change. In this review, we synthesize relevant information from a wide spectrum of sources to evaluate the current state of knowledge about plant community effects on R, examine how this information is incorporated into global climate models, and highlight priorities for future research. Plant species consistently exhibit cohesive suites of traits, linked to contrasting life history strategies, which exert a variety of impacts on R. As such, we propose that plant community shifts towards dominance by fast growing plants with nutrient rich litter could provide a major, though often neglected, positive feedback to climate change. Within vegetation types, belowground carbon flux will mainly be controlled by photosynthesis, while amongst vegetation types this flux will be more dependent upon the specific characteristics of the plant life form. We also make the case that community composition, rather than diversity, is usually the dominant control on ecosystem processes in natural systems. Individual species impacts on R may be largest where the species accounts for most of the biomass in the ecosystem, has very distinct traits to the rest of the community, or modulates the occurrence of major natural disturbances. We show that climate-vegetation models incorporate a number of pathways whereby plants can affect R, but that simplifications regarding allocation schemes and drivers of litter decomposition may limit model accuracy. This situation could, however, be relatively easily improved with targeted experimental and field studies. Finally, we identify key gaps in knowledge and recommend them as priorities for future work. These include the patterns of photosynthate partitioning amongst belowground components, ecosystem level effects of individual plant traits, and the importance of trophic interactions and species invasions or extinctions for ecosystem processes. A final, overarching challenge is how to link these observations and drivers across spatio-temporal scales to predict regional or global changes in R over long time periods. A more unified approach to understanding R, which integrates information about plant traits and community dynamics, will be essential for better understanding, simulating and predicting feedbacks to R across terrestrial ecosystems and the earth-climate system.
Five weed-management strategies (sawdust mulching, repeated spring-summer cultivation, hand-hoeing, two herbicide treatments) were applied to asparagus and maize cropping systems near Hamilton, New Zealand. Assessments of 27 nematode populations on four sampling occasions over an entire cropping cycle are related to published microbial, arthropod and environmental data. Under asparagus cropping abundance of 11 nematode populations (at genus or family level) in 0–5 cm soil showed significant treatment effects on at least two sampling occasions; under maize 6 populations showed treatment effects. Overall, the most obvious trends were for some taxa of bacterial feeding nematodes to have their greatest abundances under different treatments. The ratio of bacterial feeding to fungal feeding nematodes varied significantly with time and treatment, and indicates shifts in trophic structure of the nematode fauna. Canonical correspondence analysis demonstrated that nematode populations were more strongly related to environmental factors at the prior sampling than those at the contemporary sampling time. Under asparagus cropping the sawdust mulch was the predominant factor affecting ordinations; bacterial and fungal feeding nematodes were most abundant or showed greatest treatment responses, but the increase in populations of predacious nematodes (Nygolaimus, Mononchidae, Aporcelaimidae) may be responsible for absence of marked increases in these other groups. Under maize, effects were similar but less significant. Helicotylenchus and Pratylenchus were present under the maize crop but not under the asparagus crop. The responses of nematode taxa to weed management practices were very variable but, given the range of life history strategies within trophic groups, responses follow a predictable pattern. Detailed correlation of management-induced changes in nematode populations and biological environmental factors is confounded by the effect of nematode feeding activity on the microbial populations. Overall, the results confirm the important influence of microfaunal grazing on microfloral populations and the cycling of plant nutrients in the soil.
Predicting how plants will respond to global warming necessitates understanding of local plant adaptation to temperature. Temperature may exert selective effects on plants directly, and also indirectly through environmental factors that covary with temperature, notably soil properties. However, studies on the interactive effects of temperature and soil properties on plant adaptation are rare, and the role of abiotic versus biotic soil properties in plant adaptation to temperature remains untested. We performed two growth chamber experiments using soils and Bistorta vivipara bulbil ecotypes from a subarctic elevational gradient (temperature range: ±3 ° C) in northern Sweden to disentangle effects of local ecotype, temperature, and biotic and abiotic properties of soil origin on plant growth. We found partial evidence for local adaption to temperature. Although soil origin affected plant growth, we did not find support for local adaptation to either abiotic or biotic soil properties, and there were no interactive effects of soil origin with ecotype or temperature. Our results indicate that ecotypic variation can be an important driver of plant responses to the direct effects of increasing temperature, while responses to covariation in soil properties are of a phenotypic, rather than adaptive, nature.
The technical feasibility of operating commercial aircraft on low concentration biodiesel in kerosene blends is reviewed. Although the analysis is preliminary, it seems plausible that a biodiesel component could be introduced without significant modification to aircraft, airport infrastructure, and flight operations. The use of a biodiesel component, even for only a subset of flight operations, would open the possibility of giving all passengers, the world over, regardless of route, the option to pay a premium to make their journey on “green” fuel (actually biodiesel). In this way, the airline industry could recover the additional cost of biodiesel in comparison to kerosene. The costs associated with such a scheme are estimated, as is consumer demand. Although the analysis is preliminary, the scheme appears commercially viable. From a humanitarian and/or environmental perspective, marketing flight on biodiesel as “green air travel” is problematic. On the one hand, the use of biodiesel in aviation would reduce addition of carbon dioxide to the atmosphere and foster development of sustainable technology. On the other hand, it would require that agricultural resources be dedicated to air travel, nominally a luxury, in a world where agricultural resources appear destined to come under increasing strain merely to satisfy humanity’s basic food and energy needs. A preliminary discussion of these issues is presented. It is hoped that this can serve as the starting point for further discussion, at an international level, to reach consensus on whether marketing of flight on biodiesel as “green air travel” should be allowed to proceed, or whether it should be declared unethical.
In this Citation Classics commentary, we reflect upon a paper that we published in Soil Biology & Biochemistry in 1995 and that provided a critical assessment of the use of the microbial metabolic quotient (qCO 2 or the ratio of microbial respiration to biomass) as a bioindicator of soil quality. This quotient is based on Eugene Odum's ‘Theory of Ecosystem Succession’ published in 1969 and has been widely used in soil ecology for over 30 years; a higher qCO 2 value is reflective of greater energetic inefficiency of the soil microbial biomass. At the time of our study a strong demand was emerging for easily measured soil quality bioindicators and qCO 2 was an obvious candidate. In our paper we drew on the plant ecological strategy theory of J. Philip Grime by highlighting that, in addition to the two major types of ecosystem identified in Odum's theory (‘disturbed’ and ‘developed’), there is a third type, ‘stressed’ that is characterized by persistently harsh conditions (e.g., nutrient poor, cold, dry). We measured qCO 2 for soils collected from along the Franz Josef Glacier chronosequence in New Zealand that spans 120,000 years and which includes all three types of ecosystems, going from disturbed to developed to stressed. We found that qCO 2 was least for the developed stage and was similarly high for the disturbed and stressed stages; it was not possible to discriminate between early successional stages characterized by high disturbance and high quality resource inputs from late successional stages characterized by substantial nutrient limitation and poor quality resource inputs. We further reanalyzed data from several publications from which qCO 2 could be calculated and these reinforced our empirical findings. Upon reflecting on our earlier work, we conclude that highly contrasting types of ecosystems that vary in their disturbance regime and stress cannot be arranged to any satisfaction along a unidimensional axis, and that no simple, unidimensional bioindicator (including qCO 2 ) is capable of adequately summarizing variation of soil quality among ecosystems. However, we also emphasize that despite its limitations as a bioindicator of soil quality, the qCO 2 concept has proven to be highly useful for advancing conceptual understanding of the functioning of the soil microbial community and its contribution to ecosystem processes.
In the prolonged absence of catastrophic disturbance, ecosystem retrogression occurs, and this involves increased nutrient limitation, and reduced aboveground and belowground ecosystem processes rates. Little is known about how the nitrogen and carbon stable isotope ratios (delta(15)N and delta(13)C) of plants, soils and consumer organisms respond to retrogression in boreal forests. We investigated a 5000 year chronosequence of forested islands in the boreal zone of northern Sweden, for which the time since lightning-induced wildfire increases with decreasing island size, leading to ecosystem retrogression. For this system, tissue delta(15)N of three abundant plant species (Betula pubescens, Vaccinium myrtillus and Pleurozium schreberi) and humus all increased as retrogression proceeded. This is probably due to enhanced ecosystem inputs of N by biological fixation, and greater dependency of the plants on organic N during retrogression. The delta(13)C of B. pubescens and plant-derived humus also increased during retrogression, probably through nutrient limitation increasing plant physiological stress. Unlike the plants, delta(15)N of invertebrates (lycosid spiders and ants) did not increase during retrogression, probably because of their partial dependence on aquatic-derived prey that had a variable delta(15)N signature. The delta(13)C of the invertebrates increased as retrogression proceeded and converged towards that of an aquatic prey source (chironomid flies), suggesting increased dependence on aquatic-derived prey during retrogression. These results show that measurement of delta(15)N and delta(13)C of plants, soils, and consumers across the same environmental gradient can provide insights into environmental factors that drive both the aboveground and belowground subsystems, as well as the linkages between them.
Forest ecosystems world-wide are being subjected to invasion by organisms representing all domains of life. Here we use a combined aboveground-belowground approach to provide a conceptual framework for assessing how forests respond to biological invasions. We first address mechanisms by which invasive plants and aboveground and belowground consumers impact on forests, and highlight that although we have a growing understanding of the determinants of the effects of invasive plants, for invasive consumers we have yet to move from a series of iconic case studies to the development of general principles. We also address the effects of invasive biota in the context of the drivers of invasion, co-invasion and invasional meltdown, the issue of simultaneous species gains and losses, and forest restoration and recovery post-invasion. We then highlight areas that would benefit from further work, particularly regarding underlying mechanisms, determinants of context-dependency of invader effects, and linkages between causes and consequences of invasion. In concluding, we emphasize that biological invaders have the potential for large-scale and long-term impacts on forest processes, and consideration of these impacts in an aboveground-belowground context will enable better prediction of future responses of forests to invaders and their management as well as of restoration efforts.
This study explores soil nutrient cycling processes and microbial properties for two contrasting vegetation types along an elevational gradient in subarcti
Immediately upon graduating from the University of Durham in 1961 with a BA (Hons) in Classical and General Literature, John Atkinson set off for the former Rhodesia, where he served for two years as assistant lecturer in the then University College of Rhodesia and Nyasaland. There T.F. Carney's ideas and interests exercised a powerful, long-lasting influence on him. After a short stay of 18 months as a lecturer at the University of South Africa (Unisa), the honorand took up another lectureship at the University of Cape Town in January 1965. In the following forty years he progressed through the ranks to full professor and was the last Dean of the Faculty of Arts, before its amalgamation into the Faculty of Humanities.
Questions We investigated some commonly held assumptions of community assembly theory needed to provide accurate predictions of changes in plant species assemblages across environmental gradients or following environmental change. Do (1) dominant and subordinate species respond in the same way to changes in environmental variables; (2) plant species assemblages show higher interspecific than intraspecific trait responses; and (3) co‐existing dominant species differ in their responses to the same environmental variables? Location Islands in L akes U ddjaure and H ornavan, northern S weden. Methods We explored the responses of forest understorey vegetation assemblages to variation in environmental resources across a chronosequence of 30 lake islands that differ in fire history, above‐ground and below‐ground resource availability and species diversity. For one plot on each island, we measured specific leaf area, leaf dry matter content and foliar N and P of all dominant and subordinate understorey plant species to assess species‐specific and weighted and non‐weighted community‐level trait responses to variation across islands in all major local environmental drivers. Results Consistent with our expectations, we found that species responses to environmental conditions were not homogenous within assemblages, and that responses of dominant and subordinate species differed. Further, intraspecific variation was often an important component of local‐scale plant community‐level responses. Responses were often relatively consistent across species, but dominant species sometimes showed contrasting responses of the same trait to the same environmental factor. Finally, environmental factors that influenced community average trait values also affected functional diversity. Conclusions This study has shown that several common assumptions that underpin community assembly theory do not necessarily hold, and this can cause inaccuracies in predicting plant functional composition responses to changes in environmental variables. Because these assumptions are central to current models that predict vegetation responses to environmental change, it is crucial to further test in which particular environmental context and to what extent these assumptions are critical for model accuracy.
