The Journal of Ecology editors are delighted to honour David Wardle in our continuing Eminent Ecologist series. David has put together a special Virtual Issue of some of his excellent contributions to the journal and has written a series of blog posts reflecting on his work. Read Part 1, Part 2 and Part 3 on the Journal of Ecology blog.

SWEDISH ISLANDS AND ENVIRONMENTAL GRADIENTS

A significant (and particularly rewarding) component of my research career has focused on a group of 30 forested lake islands in northern Sweden that collectively represent a strong environmental gradient. Here, large islands are often struck by lightning and several have burned in the last 100 years; small islands are a much smaller target and several have not burned for the past 5000 years. As such, these islands collectively represent a lengthy post-fire chronosequence. After Olle Zackrisson introduced me to this study system in 1995, my colleagues and I have continued working on it, with a lot of field work and many adventures, over the following 23 years. Our initial work focused on how increasing time since disturbance caused shifts in the plant community towards species that impair ecosystem processes both above and below ground and that lead to substantial build-up of organic matter through impairment of decomposer activity (Wardle et al. 1997, 2003). My fifth chosen paper (Wardle et al. 2012; one of Journal of Ecology’s ‘Centennial Symposium’ series) provides a synthesis and analysis of the first 15 years of data from that system. It reveals that across contrasting islands differing in exposure to a key extrinsic driver (historical disturbance regime and resulting ecosystem change), there are coordinated responses of soil fertility, vegetation, consumer communities and ecosystem C sequestration, which all feed back to one another.

Swedish islands and environmental gradients. Photo: D. Wardle

The ground layer vegetation on these islands (consisting of ericaceous dwarf shrubs and feather mosses) contributes to over half of their productivity, and is also easily experimentally manipulated. My sixth and seventh chosen papers are both based on ongoing experiments on these islands. The sixth paper (Wardle et al. 2008) is based on an experiment set up in 1996 (now in its 23^rd year) in which ground layer plant functional and species diversity have been continually maintained through manual removals (Wardle & Zackrisson 2005; Fanin et al. 2018; Kardol et al. 2018). In that study, Marie-Charlotte Nilsson, Anna Lagerström and I sowed seeds and planted small seedlings of each of three tree species in all 420 plots of the experiment, as a test of how resident plant species diversity and composition affects community invasibility. The results showed that the effects of loss of components of the resident flora (at either the functional group or species level) on invasibility at the plot scale are not consistent across ecosystems, but are instead highly context dependent and depend greatly on the attributes of the plant species that are lost and the ecosystems that they are lost from.

Swedish islands and environmental gradients. Photo: D. Wardle

My seventh chosen paper (Wardle et al. 2016) emerged from an ongoing factorial nitrogen and phosphorus fertilization experiment set up on each island in 2007 to explore the how the nature of nutrient limitation varies across the gradient. This paper, involving collaboration with my former postdocs Micael Jonsson and Jordan Mayor, as well as Dan Metcalfe, explored the response of a spectrum of aboveground and belowground response variables to these treatments. In contrast to the sixth paper, it did not point to strong differences in treatment responses among ecosystems. Instead we found that there were relatively modest differences among islands in how they responded to nutrient addition despite high variation among them in ecosystem productivity and soil fertility. This is probably because of high species turnover and adaptation of communities to infertile conditions in our system. This finding is in contrast to what has sometimes been shown for ecosystem responses to fertilization across comparable environmental gradients elsewhere (e.g., Vitousek, 2004), highlighting that different study systems often do their own thing in response to the same factors.

Swedish islands and environmental gradients. Photo: M. Jonsson

Any researcher who attempts to publish work from observational or space-for-time studies (such as from chronosequences, see above), or from natural environmental gradients, will soon discover that there is a small number (perhaps 10%) of reviewers (and therefore members of the ecological community) that are inherently negative about such studies and are of the view that they are intractably plagued by confounding factors. Chronosequence studies were getting some bad press late last decade, and my eighth chosen paper (Walker et al. 2010) was an attempt by my colleagues Lawrence (‘Lars’) Walker, Richard Bardgett, Bruce Clarkson and myself to counteract some of that press. In that essay, we outline our view that while we agree that there are certainly cases where the indiscriminate use of chronosequences is problematic, there are also many cases where they are highly appropriate for studying long term environmental change, such as for particular belowground and aboveground community and ecosystem variables, and for convergent (versus divergent) successional pathways. As such, we concluded that when successional trajectories exceed the life span of investigators and the experimental studies that they perform, temporal change can indeed be successfully explored through the judicious use of chronosequences.

David Wardle, Nanyang Technological University, Singapore 

You can read David’s 12 selected papers on the journal website: Eminent Ecologist 2018 Virtual Issue .

Read Part 1, Part 2 and Part 3 on the Journal of Ecology blog.