Forest Ecology and Management 309 (2013) 1–3
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Tree species effects on soils in temperate and boreal forests: Emerging themes and research needs
1. Introduction The manners in which the tree species growing on a site alter the underlying soil have long been of interest (Müller, 1887; Ovington, 1954, 1956; Stone 1975), and inform several topics of current interest in ecology, including of plant functional trait effects (Cornelissen and Thompson, 1997; Cornwell et al., 2008), above- and belowground interactions (Wardle et al., 2004; Sylvain and Wall, 2011), functional implications of increasing species diversity (Scherer-Lorenzen et al., 2007; Cesarz et al., 2013), context-dependency in ecological interactions and processes (Eviner and Hawkes, 2008; Hoeksema et al., 2010), and species-induced changes in ecosystem processes (Chapin et al., 1997; Clarholm and Skyllberg, 2013). Understanding inﬂuences of tree species on soils also informs decision-making around environmental service provisioning and environmental issues such as effects of species shifts in response to climate change, C sequestration in soil, ecosystem responses to invasive species, restoration of degraded sites, afforestation, and short-rotation forestry for biomass production. The challenge of distinguishing effects of tree species have been alleviated to some extent in recent studies that employ common garden experiments or retrospective designs that minimize the inﬂuence of site-related factors (Reich et al., 2005; Vesterdal et al., 2008; 2012; Hansson et al., 2011). Heightened interest in tree species effects and recent developments in research on this topic prompted us to suggest a session on ‘‘Inﬂuences of tree species on forest soils’’ at the 4th EUROSOIL Congress in Bari, Italy, 2–6 July, 2012. The papers included in this special issue include reviews of tree species effects on soil C (Vesterdal et al., 2013) and soil microbial communities (Prescott and Grayston, 2013), syntheses of large studies (Hansson et al., 2013; Aponte et al., 2013), and individual investigations that explore the mechanisms underlying tree species effects and application of this knowledge to soil reclamation, C sequestration, soil water seepage and nitrate leaching from forests. 2. Emerging themes The evidence presented in these papers and the associated literature suggest that progress has been made in our collective ability to predict effects of tree species on soils, at least in temperate and boreal forests. The key themes that emerge from these papers are: We can expect forest ﬂoor mass and C stock to be greater under coniferous species such as spruce and pine than under deciduous 0378-1127/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2013.06.042
species such as lime, ash and maple (Vesterdal et al., 2013; Hansson et al., 2013). However, several common species, including beech, oak, larch and Douglas-ﬁr, tend to produce forest ﬂoors that are intermediate to these extremes. Rather than a deciduous– coniferous dichotomy, species appear to occur along a spectrum with regard to the amount and proportion of soil C stock represented by the forest ﬂoor. Differences in C stocks in mineral soils among species are smaller than differences in forest ﬂoors, and less consistent. There is a strong tendency for species with small forest ﬂoors to have greater C accumulation in mineral soil, due to the activities of soil fauna, particularly earthworms which redistribute litter into the soil proﬁle (Frouz et al., 2013; Vesterdal et al., 2013; Rajapaksha et al., 2013). This pattern mirrors the tri-class or spectrum pattern of forest ﬂoor development described above, and can be largely predicted by characteristics of the litter of each species. There is a clear potential for increasing the soil organic carbon stock by a change in tree species. Forest ﬂoor C stocks are most sensitive and can be increased 2–5-fold, but more stable mineral soil C stocks can also be increased by 40–50% by targeted selection of tree species (Vesterdal et al., 2013). As mentioned above, these proportional differences within forest ﬂoor and mineral soil are not always additive: in some cases C distribution between forest ﬂoor and mineral soil (rather than total C stock) differs among tree species. Thus some tree species may increase sequestration of C in stable forms in mineral soil, through greater root litter input or by stimulating macrofaunal activity. The mediating effect of soil macrofauna is a key factor for long-term sequestration of C originating from high-quality, labile litter (Frouz et al., 2013; Cotrufo et al., 2013). This association can be used to predict tree species that will be particularly useful for soil reclamation, such as rowan, ash, lime and eucalypts, which will stimulate development of soil organic matter. However, the association between particular tree species (or species with certain functional traits) and particular soil fauna exists only when site factors such as soil texture, pH or moisture do not preclude fauna, as demonstrated across a soil-quality gradient in Denmark (Gurmesa et al., 2013). The inﬂuence of tree species can be confounded or overwhelmed by inﬂuences of site factors such as soil ‘quality’ (deﬁned as a composite of texture, fertility and acidity) and climate. However, the context-dependent nature of the tree-species inﬂuence can be accommodated by understanding the mechanisms that underlie the tree species inﬂuence. For example, the lack of inﬂuence of the deciduous species on mineral soil C on
Preface / Forest Ecology and Management 309 (2013) 1–3
some sites in Denmark was attributed to the lack of earthworms in coarse, acidic and nutrient-poor soils (Gurmesa et al., 2013). Thus, along a gradient of site quality we could expect to ﬁnd no faunal-mediated soil C sequestration at the poorest sites (regardless of tree species); as soil quality improved it would occur only under species with the highest litter quality, then under intermediate species and ﬁnally, at the richest sites, it might be evident under all species. In this scenario, the greatest apparent differences in soils under different tree species would occur at sites of intermediate quality. Differences among tree species in soil C stocks seem to be more closely related to rates of litter decomposition than litter input (Vesterdal et al., 2013). However, species differences in litter production rate are important for soil rehabilitation, as species such as rowan, which produce more leaf litter than other species (also of higher nutrient content), can quickly boost soil organic matter quality on reclaimed sites (Carnol and Bazgir, 2013). The growth rates of different tree species on a given site also inﬂuence the development of soil C and N stocks and leaching losses of nutrients from soil (Hansson et al., 2013; Carnol and Bazgir, 2013). Forest ﬂoors under different tree species harbor distinct microbial and faunal communities, and this appears to be most closely related to the pH and calcium content of the litter (Frouz et al., 2013; Diaz-Aguilar et al., 2013). There is less discrimination in mineral soils except in the rhizosphere or mycorrhizosphere, where microbes are inﬂuenced both by the tree species and the mycorrhizal fungal communities (Prescott and Grayston, 2013). Even on the same site, roots under different tree species differ in mass (related to tree growth rate), distribution with respect to soil depth, degree of mycorrhizal colonization, mycorrhizal species composition, distribution of root orders, and characteristics such as density and speciﬁc root length (Hansson et al., 2013). Each of these factors has implications for longevity, turnover and decay of roots and C sequestration in soil. Tree species also inﬂuence moisture in soil through interception of precipitation and transpiration (Sprenger et al., 2013; Carnol and Bazgir, 2013) and by inﬂuencing the organic matter content of soil, which may feedback to site suitability for certain species (Aponte et al., 2013). In regions with elevated atmospheric N deposition, forest ecosystems retain N which might otherwise lead to eutrophication of surface and drinking waters. Certain conifers retain N more efﬁciently than broadleaves during the ﬁrst 4–5 decades of stand development in spite of higher deposition inputs (Gurmesa et al., 2013; Hansson et al., 2013). Species traits related to high N retention and soil C sequestration were consistently coupled in these studies, even along an extensive soil quality gradient, indicating that both processes could be encouraged through selection of appropriate tree species. Functional trait analysis provides a new lens through which to examine effects of plant species on soils, and consequent feedbacks and interactions between above- and belowground components of ecosystems. Aponte et al. (2013) provide an example in their comparison of soils beneath two co-existing oak species in southern Spain. The evergreen Quercus suber is dominant on nutrient-poor soils on ridges, while the deciduous Quercus canariensis dominates fertile valley bottoms. Q. canariensis leaves had lower mass per unit area and higher nutrient concentrations in foliage and in litter, and its foliar litter decomposed faster and returned more nutrients each year. Soils under Q. canariensis had greater microbial biomass, N and P concentrations and higher abundance of mycorrhizae with saprotrophic abilities relative to Q. suber. This study provides an example of how the functional traits of species growing in rich or poor environments create positive feedbacks which exacerbate dif-
ferences in nutrient supply and thereby enhance the species’ suitability for the site. Interestingly, despite faster initial decomposition of its leaf litter, Q. canariensis produced more soil organic matter, another indication that species with higher-quality litter generate more soil organic matter than those with low-quality litter (Cotrufo et al., 2013). The functional signiﬁcance of species diversity per se is difﬁcult to distinguish from the additive effects of individual species in mixed-species forests. Sprenger et al. (2013) found that water recharge in six-species mixtures was lower than expected based on corresponding pure species plots (indicating an overyielding effect), but this was not the case in three-species mixtures. There are also indications that soil organic carbon stocks are greater in stands of high diversity, but the role of diversity per se remains to be unraveled through new rigorous experimental designs (Vesterdal et al., 2013).
3. Research needs The articles in this special issue demonstrate the multiple mechanisms through which tree species inﬂuence soils: aboveground litterfall, microclimate (temperature, precipitation and soil moisture), ground vegetation, throughfall nutrient ﬂuxes, nutrient uptake, root turnover and exudation, and root-associated and freeliving soil organisms. Most studies to date have examined only one or a few of these processes and even when correlations are found, one cannot be certain that these are the causal means of inﬂuence. Concerted studies of multiple pathways of inﬂuence in well-replicated common garden experiments (e.g. Vesterdal et al., 2013; Hansson et al., 2013; Gurmesa et al., 2013) will allow better elucidation of the processes and mechanisms underlying tree species inﬂuences on soils. Conﬁrmation of the apparent differences between tree species and the communities of organisms in decomposing litter requires that organisms be studied in litter at the same stage of decomposition, rather than the same time since placement (Prescott and Grayston, 2013). Relative to aboveground litter, little is known about the roots of different tree species, such as their distribution in the soil proﬁle, functional traits, lifespan, decomposition and turnover rate (Hansson et al., 2013). Given their contribution to stable soil C pools (Rasse et al., 2005; Prescott, 2010), much more should be known. Better identiﬁcation of soil C pools, their relative stabilities and the material from which they originate are needed to establish which types of organic matter and which processes are most important in forming stable soil C. It is known that mycorrhizal fungal associates vary among tree species (Aponte et al., 2013; Prescott and Grayston, 2013), but there is not yet enough information to know if such relationships are consistent or context-dependent. Indications that mycorrhizal fungi inﬂuence the communities of other organisms and the release of C in exudates (Prescott and Grayston, 2013), and may be a signiﬁcant source for stable C in some ecosystems (Godbold et al., 2006; Clemmensen et al., 2013), speak to the need to better understand the relationships between tree species and mycorrhizal fungi. Finally, recognition that tree species inﬂuences on soils are context-dependent (see above) obliges us to reﬁne our research questions – for example, rather than asking ‘do broadleaves stimulate soil fauna?’ we should instead ask ‘under which conditions would broadleaves ameliorate soil by stimulating soil fauna and under which conditions would they not?’ Thus reframing our research questions might enable us to understand why species effects do not consistently occur and to better predict when and where a tree species will have characteristic effects on soil.
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Cindy E. Prescott a, Lars Vesterdal b a Faculty of Forestry, University of British Columbia, Vancouver, BC, Canada b
Department of Geosciences and Natural Resource Management, University of Copenhagen, Frederiksberg, Denmark ⇑ Corresponding author. Tel.: +1 604 822 4701. E-mail address: [email protected]
(C.E. Prescott) Available online 6 August 2013