🏆Eminent Ecologist 2022: Hans Cornelissen (part 1)

The Journal of Ecology Editors are delighted to announce that Hans Cornelissen is our Eminent Ecologist award winner for 2022!

In recognition of his work, we asked Hans to put together a virtual issue of some of his favourite contributions to the journal. Hans has also written this blog series, and was interviewed by Richard Bardgett about his motivations and career to date, how he sees the field of functional traits developing in the future, and what advice he’d give to ECRs interested in plant ecology. Han’s full blog series can be found here: Part 1👇 | Part 2 | Part 3

When I got the message that I had been elected by Journal of Ecology to be Eminent Ecologist 2022, my first response, after staring at the screen in disbelief and re-reading the message, was to look at the list of previous laureates… and to feel humble in the company of seven ecologists I greatly admire. I then did a web search of my papers to verify that Journal of Ecology really has been my favorite journal by a wide margin for two and a half decades. But not only a favorite as a frequent author; I have also been a happy reader and associate editor of the journal for many years. Talking of associate editorship; I may just as well use this opportunity to apologize to quite a few other authors of Journal of Ecology manuscripts who have had to wait for my feedback for longer than hoped for. All too often good intentions get pushed aside by other commitments in research and, especially,  teaching; time management has never been my forte. I also use this blog intro to say a big thank you to all the colleagues and friends who have provided a lot of hard work, insight, companionship and support over the years, directly or indirectly contributing to my (co-)authorships in this journal and to the fact that I am now in the happy position to write this blog. It’s a cliché, but all my scientific output has been the result of teamwork one way or the other. So, if you are one of my many coauthors or collaborators related to Journal of Ecology publications: “Thank you!” Having said this; three of my many dear colleagues at Vrije Universiteit over the past two decades have been so central to many of my Journal of Ecology contributions, and other papers, that I must thank them by name: Rien Aerts, Richard van Logtestijn and Matty Berg. Finally, before moving on to the science, I’d like to mention here that, over many years, I have greatly enjoyed the professionalism, generosity (with time and encouragement) and friendliness of the people running Journal of Ecology, be it as an editor, administrator or policy person anywhere in the workflow. Together with the great team of associate editors and reviewers, you have enabled Journal of Ecology to grow from strength to strength steadily, to reach the high impact and visibility that it enjoys today.

When sifting through my Journal of Ecology papers, I had a hard time selecting which ones I would like most to get new exposure via the virtual issue. Many more than those included are precious to me, but hey, tough choices are part of life. I decided to go mostly for those where my own contribution was particularly large and which together represent and link four of my main research themes, all of which are somehow linked to ecological aspects of variation in functional traits among species: (1) Comparative ecology of living plants; (2) Comparative ecology of plant and tissue afterlife effects on carbon and nutrient cycling; (3) Trait evolution and carbon and nutrient cycling; (4) Cryptogam ecology.

(1) COMPARATIVE ECOLOGY OF LIVING PLANTS

This research line started, in a way, during my PhD with Professor Marinus Werger at Utrecht University in the Netherlands. This work took me to China from 1989 to 1991. In the subtropical broad-leaved evergreen forest belt of SW China I did nursery experiments on the growth performance of different tree species in different light environments, trying to mimic the light conditions typical for different  forest succession stages after gap formation. This was China when it was only at the very start of its incredible subsequent development in science and technology, so working conditions were physically tough. I had to find my way to the local street market to buy bamboo sticks for constructing frames and mosquito netting to tie over the frames to create different light regimes (photo 1a) below which I grew my little trees (photo 1b). With the generous help of colleagues and field assistants, under the kind wings of nationally renowned Professor Zhong Zhangcheng (photo 1c) at SW China University near Chongqing, I managed to germinate and grow a range of tree species from the various succession stages and measured their growth, leaf and root traits and biomass allocation in response to different light environments. In spite of practical and language constraints, and some disasters, I managed to complete my PhD thesis and published four papers from it, albeit not in journals of the impact that most readers of this blog would aim for nowadays. Journal of Ecology was still a dream, well beyond my level… However, this whole PhD period did set me up as a resourceful researcher not easily beaten by adversity. And it did leave me with a passion for China and its people that was permanent and would again become a big factor later on in my career.

 Following on from my PhD study, back in 1993, one of the defining moments for me as a scientist was the move to the NERC Unit of Comparative Plant Ecology (UCPE) at Sheffield University, UK, initially as a postdoc. Its director, the late Philip Grime (who now fittingly has a section in this journal named after him!), but certainly also Ken Thompson, John Hodgson, Roderick Hunt and (regular visitor) Sandra Diaz, were my new colleagues and also my heroes; they were among the real pioneers of trait-based comparative ecology before it became globally hot in the current millennium. My main task was to run an “Integrated Screening Programme” (ISP) for woody species, parallel to the then already well established ISP focusing on herbaceous species (Grime et al. 1997). Central to this work was to screen dozens and dozens of temperate trees and shrubs in the Sheffield region, and dozens more from Spain (with Pilar Castro Diez and Jean-Philippe Puyravaud), for inherent relative growth rate (RGR) and the traits underpinning RGR, as these traits were already known to be critical assets and predictors for the ecological strategies of plant species. In practice, this work entailed trying to first germinate seeds, many of which had complex dormancies to be broken first. For instance, seeds from berry species that normally travel through a bird’s gut to lose their dormancy often had to be washed thoroughly first, then put into rich compost thus going through two lengthy cycles of warm and cold treatment. Once germinated, we grew seedlings for three weeks from unfolding of the first true leaf in standard greenhouse conditions with plenty water and nutrients (see photo 2). We showed that, across taxonomically wide-ranging species, shrubs generally grew faster than trees and deciduous species faster than evergreens; and that these differences were underpinned by variation in specific leaf area (leaf area per mass), proportional allocation of biomass to leaves at the whole-plant level (Cornelissen et al. 1996a, virtual issue) and (somewhat) by stem tissue density (Castro-Diez et al 1998).

With Bruno Cerabolini, a great contributor to comparative plant ecology, I then went on to measure leaf traits of adult plants of broadly the same shrub and tree species, which showed broadly corresponding patterns to those of the seedlings (Cornelissen et al. 2003). This dataset, and a somewhat similar one collected from the subarctic tundra area in N Sweden, turned out not only to be useful for our own regional studies, but also for global ones. The publication of the “leaf economic spectrum”  (LES, Wright et al. 2004) seemed to be a catalyst that made plant trait study hot all over the world, perhaps partly because people realized that comparative plant ecological studies could be published in the very top journals. And this turned out to be the case with multiple papers extending the LES or placing it into a different environmental context (e.g. Diaz et al. 2016; Björkman et al. 2018; Bruelheide et al 2018; Joswig et al. 2021), undoubtedly greatly helped by massive efforts to assemble trait data for building global databases (Kattge et al. 2020) and to standardize methodology for trait measurement (Pérez-Harguindeguy et al. 2013). Where the plants on planet Earth were once classified by taxonomy, growth form or functional type, they were now organized along continuous trait-based strategy axes of variation; and rightly so.

It had to happen sooner or later, but I was happy that we, and this journal, presented the “plant economics spectrum” (PES), supported by convincing empirical evidence from a regional flora in N Sweden (Freschet et al. 2010, virtual issue). We showed that a wide range of functional traits related to the carbon and nutrient economy of plants (e.g. dry matter content, lignin and nutrient concentrations, pH), were coordinated among different plant organs, i.e. among leaves, roots and (fine) stems. This paper helped to rank species according to their whole-plant ecological strategy in terms of fast tissue turnover supporting rapid growth versus slow tissue turnover promoting resource conservation (Reich 1994). I had the honour of being the day-to-day PhD supervisor of Grégoire Freschet, who is one of many young scientists (see also below and next blogs) who deserve much credit for me getting this award. There is no greater reward for a supervisor than when PhD students or postdocs equal or exceed their “master” in academic quality; especially when they subsequently pursue a successful career in academia.

The PES is also part of a development from a very dominant focus on leaf traits to a stronger focus on the traits of belowground parts. It is nice to see new papers coming out about the different strategy axes that root traits follow worldwide, with a special axis attributed to the outsourcing of root functions to mutualists such as mycorrhizal fungi (Bergmann et al. 2020). This new focus has led to some amazing discoveries, such as specialised snow roots in the Cauacasus Mountains (Onipchenko et al. 2009).

(2) COMPARATIVE ECOLOGY OF PLANT AND TISSUE AFTERLIFE EFFECTS ON CARBON AND NUTRIENT CYCLING

Litterbed expriments: the “common garden”

If I had to choose one paper that I am the most proud of, it would have to be the one in which I linked the decomposition rates (“decomposability”) of leaf litter of different species to their functional traits – even though “trait” was not really in my vocabulary back then (Cornelissen 1996, virtual issue). While collecting seeds for my RGR studies in autumn 1993, roaming the beautiful countryside in the Peak District near Sheffield, I got intrigued by the pretty autumn leaf colours. At UCPE we’d been discussing about litter decomposition, a topic I knew virtually nothing about. I started to wonder whether the autumn colours had anything to do with the (pre-death) functioning of leaves  – yes, they do! While collecting pretty leaf litter, gradually the idea took hold to compare many plant species for their litter decomposition rates and somehow relate these to functional aspects of the species. And once you start to pick up leaf litter it’s pretty much impossible to stop; greed got the better of me – and still does when let loose in the field in autumn. I must say that Ken Thompson and John Hodgson were super helpful to collect litter of a substantial number of herbaceous species for me. The tally came to 125 species by the end of autumn. By then I still had no clear idea how to go about the next step other than pre-weighing litter samples and putting them in litterbags (with mesh to allow decomposers in)…. until I noticed a huge pile of leaf mould at Tapton Garden in Sheffield. This leaf mould contained a great diversity of common plant species swept up in the local park; and presumably a broad decomposer community living inside it. The penny suddenly dropped: why not bury all 125 litter species (as litterbag samples) in this leaf mould, so that they would all have similar environmental conditions for decay? In hindsight the idea seems ridiculously simple, but this Sheffield-based study became the first of many multispecies “litterbed” studies worldwide (e.g. Cornwell et al. 2008) that would help to link decomposability to functional groups or traits of plants. Because the litterbags were hidden below the leaf mould surface (see photos 3), they were invisible to the naked eye. On an “open day” at Tapton Garden I had been asked to be one of the scientists to show their experiment to the broader public, which was a bit of a challenge given the invisibility of what was happening to my precious litter samples. So when I got interviewed live by Sheffield Radio, I jokingly said that the earthworms, who were doing some of the decomposition, were so active that you could actually hear them. I made some very loud chomping noises in the microphone to back up this statement. A relative, who’d been listening, told me afterwards how fascinating it was that earthworms could be so noisy 😊. Anyway, what came out of this study (after reweighing the litter samples for % mass loss) were things like: “deciduous species generally decompose faster than evergreens”; “shrub litter decomposes faster than tree litter”; “among deciduous woody species, green litter (e.g. chlorophyll wasters like alder – Alnus glutinosa and ash – Fraxinus excelsior) decomposes faster than brown litter (showing their nasty tannins, e.g. beech – Fagus sylvatica, sweet chestnut – Castanea sativa)”; and “species with large SLA decompose faster than species with small SLA” (although the latter not after phylogenetic correction; see the previous eminent ecologist blogs by Mark Westoby and Michelle Leishman). These things are useful to know when you want to predict whether ecosystem-level decomposition rates will accelerate or slow down with changing plant species (trait) composition; or if you want to use remote sensing to link autumn leaf colours of deciduous trees and shrubs to decomposition rates. However, it is important to take into account, as commonly known, that the decomposition rates of given litter types also greatly depend on the soil environment that they are in, both in terms of abiotic and abiotic drivers. There has been an extensive literature by now on the “home-field” advantage (e.g. Ayres et al. 2006, with involvement of Richard Bardgett; talking of heroes in ecology), which essentially means that litter of certain species does relatively better (than “expected” when compared with other species’ litters) in litter layers dominated by their own litter. The theory is that this local litter layer has promoted a decomposer community that is adapted to consuming litter of a particular quality. We extended this theory to include the whole range of positive to negative effects of the overall litter layer quality on the relative ranking of species’ litter types, showing that the decomposition of a litter particle of a given species is relatively promoted (or inhibited) if the similarity (or dissimilarity) with the overall litter matrix is greater: the substrate matrix hypothesis (SMI, Freschet et al. 2012a in this journal). On the other hand, we showed in this journal that relatively rare litter of species deviating in quality from the general community litter matrix in subtropical forests may change the overall litter decomposition compared to the expected rates based on the abundance-weighted individual rates of the component species (Guo et al. 2020 in this journal); possibly because of non-additive litter mixture effects (Guo et al. 2018).

Of course, plants are more than leaves and a lot of the litter in and on the soil is derived from roots and stems. So, to understand the role of vegetation composition in the turnover of dead plant material, we need to compare the litter decomposabilities of the different plant organs across tree species. This is exactly what we tried to do, again with Grégoire as the driving force (photos 4). We showed that the PES for living plant organs can be extended to a PES for decomposability in a subarctic flora (Freschet et al. 2012), while fine root and leaf decomposability across species tends to be coordinated also across various ecosystems worldwide (Freschet et al. 2013, virtual issue). This means that the whole-plant species ranking from slow to fast tissue turnover of living plants (see above) can be extended to the tissue turnover of their dead parts. Again, these findings are important for our understanding of how plant species composition in ecosystems affects their rates of carbon and nutrient cycling.

Wood decomposition: tree cemeteries

It should be said that our evidence for the PES of decomposability is based on evidence from very wide-ranging species, from small herbs to trees, and not taking into account the big trunks of trees. In fact, studies that have focussed on trees only found weaker or poorer coordination of the decomposability between plant organs across species, especially when  focussing on coarse dead wood and leaves (Pietsch et al. 2014; Zuo et al. 2018). Which brings me to one of my pet topics in this millennium: deadwood decomposition. This interest started in 2005 during a workshop in Sydney, organised by Mark Westoby (see above), one of my other heroes in comparative plant ecology. Mark had brought together a few dozen plant trait people and global vegetation-climate modellers to see how continuous plant trait variation could help to improve current models or build new models to better predict the consequences of climate and vegetation change worldwide; and how to standardize and assemble trait datasets for this. I presented some of my work on leaf litter decomposability, which (I think…) drew some interest because of its link to nutrient (and somewhat also carbon) cycling globally. But it also led to discussions in the corridors with the climate modellers, who told me that for global carbon dynamics and climate the turnover (or not) of coarse deadwood is of much greater importance than that of leaves. And that they had so far focussed on deadwood quantity without much regard for its quality. This gradually set us thinking about the importance of interspecific variation in wood traits as drivers of coarse wood decomposition. It led to Will Cornwell, Christian Wirth and I organising two follow-up workshops on deadwood decomposition in Sydney, and one in New Zealand, also as part of Mark’s highly productive ARC Network of Vegetation Function. In those workshops we brought together current knowledge and data on relationships between wood traits and decomposition rates worldwide (Cornwell et al. 2009; Weedon et al. 2009; Pietsch et al. 2014). These papers made the point that variation in wood traits in given regions or ecosystems are very important for deadwood decomposition rates, probably as important as macroclimate or local environmental variation. They also made it clear that multi-species empirical data specifically collected with trait-decomposition relationships in mind were sadly missing in the literature. This is how the idea was born for a tree cemetery experiment, akin to the forensic “body bag experiment” you see in movies. After gradually assembling a growing team of enthusiasts from Wageningen University, Utrecht University and “my” Vrije Universiteit Amsterdam, and after much discussion about theory and the huge logistic hurdles to overcome, we decided to simply get the diaries out and book two weeks of field days in the winter of 2012, i.e. to act rather than talk even more. Deadlines really focus the mind when research designs and protocols need to be devised… To me, the whole process of setting up and running “Loglife” (Cornelissen et al. 2012), in my own temperate lowland country, has been one of the most heart-warming and exciting research experiences ever (and still is now). This experiment, including its extensions in 2013 and 2015, has involved well over 1 km length of 25 cm diameter logs (of 1 m length each) of a total of 25 tree species of wide-ranging phylogeny and functional types, all decomposing in two contrasting sites (photos 5). One vivid memory from the ambitious setting up fieldwork is that my arms failed one night after carrying log after log all day and I had to be spoon-fed my dinner like a baby. It is amazing how much can be done with a huge amount of enthusiasm, team spirit and complementary expertise. And I am proud to say that 10 years and many papers later, with the experiment still running successfully now, we have only ever talked about the science, practicalities and manuscripts around Loglife, having hardly ever talked about money. All groups have always volunteered some funds, tools and precious time if and when needed and the collaboration has been even more harmonious and pleasant than I could have hoped for. Bottom up enthusiasm, that’s what science needs. Amongst many interesting findings from Loglife on decomposition rates and (invertebrate and microbial) diversity (literally Log Life!), I here like to highlight our recent paper on  how the (resource-rich, decomposable) inner bark and (protective, decay-recalcitrant) outer bark play very different roles in deadwood decomposition (Lin et al. 2022). This work complements other recent papers about how important bark traits in general are for decomposition rates of both bark itself and the wood inside it (Dossa et al. 2018 in this journal; Tuo et al. 2021; Chang et al. 2018); and for invertebrate communities (Zuo et al. 2016; Andringa et al. 2019).

As I mentioned earlier, since my PhD time I have been very fond of China and, since 2006, have enjoyed ever increasing intensity and geographical breadth of collaboration with Chinese colleagues, without exception supported by wonderful hospitality when out there. I really like the way in which especially young Chinese scholars are so super dedicated and eager to learn and improve. My main job with my numerous own Chinese PhD students and postdocs in Amsterdam over the years, besides discussion about research ideas, designs and manuscripts, has always been to emphasize the importance of taking a break in the weekends and relaxing more to stay healthy. In this blog (see also chapter 3) I will only highlight a few China collaborations directly related to some of the Journal of Ecology papers based on them. With regard to deadwood decomposition, Guofang Liu (one of several bright young collaborators at the Institute of Botany, Beijing) and co-workers (advised by long-term research friends Ming Dong, Zhenying Huang, Kunfang Cao and Will Cornwell) carried out a large deadwood experiment comparing deadwood decomposability of (monocot) bamboos with that of (dicot) broadleaf trees in the beautiful tropical botanical garden of Xishuangbanna, in the deep south of China (photos 6). I have come to really appreciate the great additional benefit of botanical gardens (besides the obvious ones they are meant for) of serving as a “common garden” where all species grow in a similar climate and mostly also in rather similar soil; and generally the plants have labels with the correct species name. This is ideal for comparative studies on traits and carbon cycling including decomposition (which I will revisit in the next blog). It was easy for us to get permission for this work, as our massive sampling operation basically meant we were “cleaning up” the garden by removing its dead wood (of all our target species). Our team collected deadwood samples of various standard diameters, measured some of their key traits and incubated all samples, in litterbags, in a mixed forest leaf mould layer, i.e. a common garden litterbed again (see above). Our litter turned out to be also accessible to termites. The big story from this work (Liu et al. 2015, virtual issue) was not simply that, as hypothesized, bamboo wood decomposed generally more slowly than dicot wood because of its very tough dry matter. Excitingly, we found that the overall negative relationships between initial  dry matter content or wood density and decomposition rates, across many bamboo and dicot species, were explained for about half by microbial respiration without termite interference, and for the other half by the preferences of termites for softer wood. This positive feedback, whereby termites amplify the relationship between wood quality and microbial decomposition rate, is very important to carbon cycling in warm-climate forests, given the great abundance of termites there. I may come back to this work in a subsequent blog, hereby announcing a cliff hanger about “celebrating disasters”.

A few years later Enrong Yan, at East China Normal University in Shanghai, kindly invited me to help design an experiment building on Loglife, but with very different context (subtropical monsoon forest, termites). I am so grateful to this day that I got involved. FUNLOG (where “fun” stands for functional ecology as well as fun) is an enormous tree cemetery experiment involving deadwood of 43 subtropical species in two sites and a gold mine for testing hypotheses about, for instance, dynamic relationships among species’ wood (and bark) traits, invertebrates and decomposition rates (photos 7). I have to mention Chao Guo in particular here; her work on  these dynamic relationships (e.g. Guo et al. 2021, virtual issue) is of great originality and quality. I am very lucky to count her, and subsequently other prodigies of Enrong (Tuo et al. 2021 on the bark economics spectrum, termites and wood decomposition; Ci et al. 2022 in this journal on foliar nutrient homeostasis in trees ) among the young scholars I guide with him. My next blog will feature various other key young Chinese scientists and their supervisors at other institutes…

I’d like to share a piece of good (old) news to round off this wood trait – decomposition section. If you are not in a position to cut a kilometer of tree trunks into logs, and to carry them around and follow their fate in a tree cemetery for a decade or more, you might consider the short-cut method published here (Freschet et al. 2012c, virtual issue). What started as an embryonic idea and basic scribble on the back of an envelope was turned into a promising model and methodology by a couple of bright young people. It involves collecting dead wood pieces of given diameter of multiple decay stages of a range of tree species, incubating them in a “common garden” (see above) for one or two years and then putting all the mass (or density) loss vectors of the wood samples of each species on a time axis following an initial basic linear model of mass remaining over time; and then replotting them on this graph multiple times following sigmoid, exponential or linear models until the data fit with the model shows its lowest variance. This way we can compare different species for their long-term (decadal-scale) mass loss curves and, for instance, time until 50 % mass loss, by doing only shorter-term experiments. This method for comparing decomposition rates of different tree species can be used in other common garden studies but is now also being used in natural forest settings. A research suggestion for young scientists: let us not forget about shrubs; they are also important for carbon cycling worldwide but poorly represented in the trait and decomposition literature!

Litter and ecosystem services

Finally a few words about how species’ traits and litter fates and (turnover) rates matter to you and me in daily life, now and in the future. In other words, what ecosystem services does dead plant material of different species provide to people? The field linking biodiversity (including trait diversity) to ecosystem functions and services is superhot (Van der Plas et al. 2018; IPBES 2019) but has so far focussed strongly on the life rather than the afterlife of organisms or their parts. I had been toying with various ideas about the broader ecological role of leaf size and shape for a few years starting from their key importance for the flammability and fire behaviour of surface litter layers (Cornwell et al. 2015; Grootemaat et al. 2017). Then, based on a most pleasant picnic-and-beer brainstorm that André Dias, Matty Berg and I had in the rainforest outside Rio de Janeiro (photo 8), we wrote about how litter matters to people in various ways, and how these various services that litter provides depend on the traits related to the leaf or plant economics spectrum (see chapter 1 above) and the “size and shape spectrum” (SSS, Dias et al. 2017, virtual issue; see also Cornelissen et al. 2017). I can highly recommend observing nature’s beauty in situ for inspiration while talking about ecological concepts! It certainly works for me. Since that time I have focused more and more on the importance of traits related to the SSS in relation to ecosystem services, which has led to off-shoots into invertebrate diversity with long-term research friends Saori Fujii and Matty (Fujii et al. 2020) and into woody organs with, again, Chao and Enrong (Guo et al. 2022).

Wrapping up (for now)

It’s so nice to have been able for once, in a blog like this, to write about science without worrying continuously about word limits and maximum efficiency of information transfer. Still, while writing all this, so many interesting memories and research highlights come to the surface, that already now I find myself in serious danger of being too long-winded. So I’d better press the pause button and keep the promised other two topics on (3) TRAIT EVOLUTION AND CARBON AND NUTRIENT CYCLING and (4) CRYPTOGAM ECOLOGY for blog part 2 and part 3. So please watch this space! And thanks, Journal of Ecology, for giving me the great opportunity and enjoyment to revisit so many precious research experiences.

Hans was interviewed by Executive Editor Richard Bardgett, about his motivations and career to date, how he sees the field of functional traits developing in the future, and what advice he’d give to ECRs interested in plant ecology:

REFERENCES

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