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Changes in the functional traits of the terricolous lichen Peltigera aphthosa with declining soil fertility during ecosystem retrogression were investigated. A well-documented retrogressive chronosequence of 28 forested islands in northern Sweden that differ greatly in fire history and which spans 5000 years was used. The abundance of cephalodia increased, indicative of higher N 2 -fixation rates resulting from lower N availability. Thallus δ 13 C values increased with ageing soils, in line with declining δ 13 C values of the humus substratum along this gradient. However, δ 13 C values were also driven by variation in factors that were at least partly independent of soil ageing. As such, δ 13 C values were mostly related to specific thallus mass (STM), possibly because a higher STM gives a thicker cortical layer and thus greater resistance to CO 2 diffusion, leading to higher δ 13 C values. STM and other measured traits (i.e. thallus N, P, secondary compounds and water-holding capacity) were unresponsive to the gradient, despite these traits being very responsive to the same gradient in epiphytic lichen species.

Soil chronosequences are a powerful tool for understanding how limitation of plant growth by nutrients and light changes throughout ecosystem development, but experimental tests of how availability of these resources interact to influence plant performance as ecosystem development proceeds are rare. We utilise the well‐characterised Franz Josef soil chrononosequence in New Zealand, a sequence of sites caused by a retreating glacier that spans ca 120 000 years and that includes all stages of ecosystem development from primary succession through to retrogression. Soil fertility is relatively low at either end of the sequence due to limitation of biological processes initially by N and ultimately by P whereas light availability is lowest at intermediate stages of the sequence dominated by tall forest. Growth and leaf traits of nine woody plant species, including those that occur widely along the chronosequence and those that are restricted to short portions of it, were quantified in a mesocosm experiment. Phytometers of these species were each grown in each of nine soils collected from throughout the chronosequence at either high (30%) or low (2%) light levels; these soil and light conditions represent the full variation observed along the sequence. Plant growth and biomass were greatest in soils from intermediate stages of the chronosequence and in high light. However, the stimulatory effects of soil fertility largely disappeared under shaded conditions that are characteristic of intermediate stages of ecosystem development. Our results demonstrate that long‐term changes in soil fertility and light availability that occur throughout ecosystem development had direct effects on plant species performance, but that there were stronger interactive effects of soils and light availability. Because light and soil resource availability shift predictably but have different trajectories throughout ecosystem development, our results help to understand variation in plant species performance and community assembly along complex environmental gradients.

We performed a pot experiment to study the effects of varying levels of vegetation shade and humus fertility, simulating natural conditions of the boreal forest, on growth, interspecific competition, and ectomycorrhizal colonization of seedlings of three coexisting tree species: Pinus sylvestris L., Picea abies (L.) Karst., and Betula pendula Roth. In contrast to late-successional Picea abies, early-successional species Pinus sylvestris and particularly B. pendula responded to shade with an increase in stem height and with some changes in biomass, especially in the high-fertility humus. Humus fertility had greater effects on biomass of seedlings than did shade treatments. Interspecific competition among pairwise combinations of the three species was greatest in the high-fertility humus, but was not affected by shade. Betula pendula was the strongest competitor among species; it was not affected by the presence of coexisting species and strongly suppressed coniferous seedlings, especially in the high-fertility humus. Generally, ectomycorrhizal colonization was not affected by shade treatments, but was highest in the low-fertility humus. These results show that effects of light, humus fertility, and presence of neighbours are species specific, and these differences are important for the competitive and recruitment abilities of seedlings of coexisting tree species and ultimately for the species composition of developing forests.

Island area is frequently a major determinant of the species composition of biological communities; community structure, in turn, often has important effects on ecosystem-level properties. Fifty islands of varying area were selected in an archipelago in the northern Swedish boreal forest zone, in which larger islands burn more frequently than smaller ones through wildfire arising from lightning strike, thus inducing a significant relationship between island area and plant species composition. This relationship was found to be a major factor in determining several ecosystem-level properties of these islands, including standing biomass, plant litter decomposition, nitrogen mineralization, terrestrial carbon partitioning, humus accumulation, and plant nitrogen acquisition.

In this article, we evaluate how global environmental change may affect microfood‐webs and trophic interactions in the soil, and the implications of this at the ecosystem level. First we outline how bottom‐up (resource control) and top‐down (predation‐control) forces regulate food‐web components. Food‐web components can respond either positively or negatively to shifts in NPP resulting from global change, thus creating difficulties in developing general principles about the response of soil biota to global change phenomena. We also demonstrate that top‐down effects can be important in soil food‐webs, creating negative feed‐backs which may partially counter bottom‐up effects. Secondly, we determine how soil food‐webs and the processes they regulate respond to various global change phenomena. Enhanced atmospheric CO 2 levels can have two main effects on plants which are relevant for the soil food‐web, i.e. enhanced NPP (often positive) and diminished organic matter quality (with negative effects, at least in the short term). Climate change effects resulting from elevated CO 2 levels may be mainly secondary through alteration of vegetation, as shown by examples. Intensification of land management is usually associated with greater disturbance, which alters soil food‐web composition and key processes; this is particularly apparent in comparisons of conventionally tilled and nontilled agroecosystems. Global change involves shifts in plant species composition and diversity, possibly affecting soil food‐webs; we interpret this in terms of theories relating biodiversity to ecosystem function. We conclude that a more detailed understanding of interactions between NPP, soil organic matter and components of the soil food‐web, as well as their regulation of biogeochemical processes and ultimately ecosystem‐level properties, is essential in better understanding long‐term aspects of global change phenomena.

Following catastrophic disturbances, succession and vegetation development occur, but in the prolonged absence of these disturbances a decline (retrogressive) phase follows in which nutrient availability and tree biomass declines considerably. We measured plant diversity across six long‐term chronosequences that each included retrogressive stages in Australia, New Zealand, Alaska, Hawaii and Sweden. In contrast to theories predicting negative or hump‐shaped responses of tree diversity to biomass or soil fertility, tree species richness often peaked coincidentally with tree basal area (a surrogate of tree biomass), and declined during retrogression. Similar patterns were found regardless of whether or not species richness estimates were rarefraction‐adjusted to correct for variation in stem densities across plots. The Shannon‐Weiner diversity index sometimes showed the same pattern, but in two chronosequences was least when tree basal area peaked; this was driven by the domination of total basal area by single tree species in both cases. The decline in tree diversity during retrogression was often associated with reduced relative amounts of total phosphorus in soil. In contrast, total vascular plant species richness often increased during retrogression. These results demonstrate that forests with high tree diversity and biomass do not persist indefinitely in the long‐term absence of catastrophic disturbance, and that similar patterns occur across the boreal, temperate and subtropical zones.

Studies of the effects of above-ground herbivory on soil organisms and decomposer food webs, as well as the processes that they regulate, have largely concentrated on the effects of non-living inputs into the soil, such as dung, urine, body parts and litter. However, there is an increasing body of information which points to the importance of plant physiological responses to herbivory in regulating soil organisms and therefore, implicitly, key soil processes such as decomposition and nutrient mineralisation. In this review we identify the mechanisms by which foliar herbivory may indirectly affect the soil biota and associated below-ground processes through affecting plants, so as to better understand the nature of interactions which exist between above-ground and below-ground biota. We consider two broad pathways by which above-ground foliar herbivory may affect soil biotic communities. The first of these occurs through herbivore effects on patterns of root exudation and carbon allocation. These effects manifest themselves either as short-term changes in plant C allocation and root exudation or as long-term changes in root biomass and morphology. Evidence suggests that these mechanisms positively influence the size and activity of the soil biotic community and may alter the supply of nutrients in the rhizosphere for plant uptake and regrowth. The second of these involves herbivores influencing soil organisms through altering the quality of input of plant litter. Possible mechanisms by which this occurs are through herbivory enhancing nitrogen contents of root litter, through herbivory affecting production of secondary metabolites and concentrations of nutrients in foliage and thus in leaf litter and through selective foliar feeding causing shifts in plant community structure and thus the nature of litter input to the soil. While the effects of herbivory on soil organisms via plant responses may be extremely important, the directions of these effects are often unpredictable because several mechanisms are often involved and because of the inherently complex nature of soil food-web interactions; this creates obvious difficulties in developing general principles about how herbivory affects soil food-webs. Finally, it is apparent that very little is understood on how responses of soil organisms to herbivory affect those ecosystem-level processes regulated by the soil food-web (e.g. decomposition, nutrient mineralisation) and that such information is essential in developing a balanced understanding about how herbivory affects ecosystem function.

Although island attributes such as size and accessibility to colonizing organisms can influence community structure, the consequences of these for ecosystem functioning are little understood. A study of the suspended soils of spatially discrete epiphytes or treetop "islands" in the canopies of New Zealand rainforest trees revealed that different components of the decomposer community responded either positively or negatively to island size, as well as to the tree species that the islands occurred in. This in turn led to important differences between islands in the rates of ecosystem processes driven by the decomposer biota. This system serves as a model for better understanding how attributes of both real and habitat islands may affect key ecosystem functions through determining the community structure of organisms that drive these functions.

In arctic and boreal ecosystems, ground bryophytes play an important role in regulating carbon (C) exchange between vast belowground C stores and the atmosphere. Climate is changing particularly fast in these high‐latitude regions, but it is unclear how altered precipitation regimes will affect C dynamics in the bryosphere (i.e. the ground moss layer including senesced moss, litter and associated biota) and the closely associated upper humus layer, and how these effects will vary across contrasting environmental conditions. Here, we set up a greenhouse experiment in which mesocosms were assembled containing samples of the bryosphere, dominated by the feather moss Hylocomium splendens , and the upper humus layer, that were collected from across a boreal forest chronosequence in northern Sweden which varies strongly in nutrient availability, productivity and soil biota. We tested the effect of variation in precipitation volume and frequency on CO 2 exchange and dissolved organic carbon (DOC) export, and on moss growth. As expected, reduced precipitation volume and frequency lowered net CO 2 efflux, DOC export and moss growth. However, by regulating moisture, the lower bryosphere and humus layers often mediated how precipitation volume and frequency interacted to drive C dynamics. For example, less frequent precipitation reduced moss growth only when precipitation volume was low. When volume was high, high moisture content of the humus layer helped avoid moss desiccation. Variation in precipitation regime affected C cycling consistently in samples collected across the chronosequence, despite large environmental variation along the sequence. This suggests that the bryosphere exerts a strong buffering effect on environmental variation at the forest floor, which leads to similar responses of C cycling to external perturbations across highly contrasting ecosystems. As such, our study indicates that projected increases in droughts and ground evapotranspiration in high‐latitude regions resulting from climate change will consistently reduce C losses from moss‐dominated ecosystems.

Climate change mitigation initiatives based on biological sequestration of carbon have paid little attention to biodiversity, with important implications both for climate change mitigation and for ecosystem services that depend on biodiversity. Here the chapter reviews the theoretical and empirical evidence for forest biodiversity effects on carbon sequestration. This chapter suggests that protection of primary forests is the most effective option for maximizing carbon sequestration in forest ecosystems, and should be included in future international agreements. Because carbon sequestration is a long term goal, this chapter presents the case that avoidance of losses should be emphasized over short term uptake, and that maintenance of mixtures of dominant and subdominant species and genotypes are the safest option for carbon sequestration in plantations and agroforestry systems. Biodiversity conservation should be included in the development of policy for climate change mitigation initiatives based on carbon sequestration in forested systems, including those related to the Kyoto Protocol.