The more (carbon in forest), the better (for biodiversity)?

Credits: S. Burrascano, F.M. Sabatini, Pixabay.com

 Forests host a huge range of animals and plants, and provide a wealth of services to us, including the provision of timber and other forest products (mushrooms!), protection from landslides and avalanches in mountain areas, clean water and clean air. In addition, forests capture and store large amounts of carbon, mostly in wood and soils, thus contributing to mitigating climate change. When managing forests, we should keep all these services in mind, and since not all of them can be maximised at the same time, make choices and set priorities. In our paper ‘Trade‐offs between carbon stocks and biodiversity in European temperate forests, recently published in Global Change Biology, we focus on one of the possible trade-offs. Can we manage forests to both support biodiversity and maximize the amount of carbon they store? In short, can we fill two needs with one deed?

Ours aren’t easy times to live in harmony with nature. A largely unsustainable economic model, high population densities, the increasing economic inequalities and the irresponsibly high pro-capita consumption of a significant share of the population translated into high pressures for natural resources, the climate system and biodiversity. Forests are not exempt from such pressures, also given their important economic value and the significant role they play in the carbon cycle. It is therefore vital to learn whether and how we can fulfil multiple objectives when managing forests.

Global and regional environmental policies, for instance, assume that increasing the carbon stored in a forest (= wood!) benefits biodiversity. Indeed, recent research including a paper of my former colleague Moreno Di Marco, show that at global scale, forest storing the highest amounts of carbon, are also the most rich in vertebrates. Still, it is unclear whether this correlation holds at fine scales relevant for management. When looking at an individual forest stand, is it true that the more carbon it stores, the better for biodiversity? And for which elements of biodiversity, specifically?

I stressed more than once that ‘biodiversity’ is an umbrella concept encompassing a wide variety of metrics, biological levels and organism groups, from ants to zelkovas. Biodiversity is also pretty difficult to measure comprehensively, so that often biologists have to rely on well-known species groups (e.g. trees, or vertebrates), to infer patterns in overall biodiversity. We discussed the limitations of such an approach in a previous post, as well as how the correlation between species groups varies with varying spatial scale. In the work we have just published, instead, we focused on understanding which components of biodiversity, individual or group of species, benefit from increasing levels of carbon stored in a forest, if any.

As often, this is no work for lone wolves. Six research groups joined forces under the leadership of Sabina Burrascano, and brought data and expertise together to answer these questions. Tens of biologists spent hundreds of person-hours measuring forest carbon, and sampling and identifying vascular plants, lichens, bryophytes, saproxylic beetles, fungi and birds in more than 352 plots in 22 beech and deciduous oak forest sites across Hungary, France and Italy. To them goes my sincerest thanks. We used these data to model statistically the relationship between biodiversity and forest carbon, not only considering the traditional aggregated measures of biodiversity (i.e., species richness = the number of species), but also the individual response of each individual species.

Indeed, when considering biodiversity in an aggregated manner, we found little evidence that above‐ground live carbon and species richness in temperate forests are congruent at the extent of individual forest stands (Fig. 1).

Fig1_SB.png
Figure 1 – We did not find a strong and univocal relationship between species richness of different organism groups and forest carbon (=above‐ground live carbon) using Boosted Regression Trees. Scaled richness represents the fraction of species of the species pool size estimated for a given plot. Ticks on the x‐axis represent forest carbon (above‐ground live carbon) data distribution. For each taxonomic group, we report in parenthesis the relative importance of aboveground live carbon in the respective boosted regression tree model. Credits: freedownloads.com; openclipart.org.

 

Does it mean that forest carbon just doesn’t matter? Not at all.

Simply, considering species richness does not return the whole picture. Our analyses clearly indicate that, for increasing carbon levels in a forest, there is a turnover between species better adapted to carbon-dense forests (we called them ‘win-win’ species), which slowly replace open-habitat species (‘trade-off’ species). Even if species richness does not differ between the two ends of the carbon gradient, what differs is the identity of the species (Fig 2).

 

Fig4_WinWin.barplot_20180621
Figure 2 – Proportion of win‐win versus trade‐off species across taxonomic groups and forest types (sorted for increasing number of win‐win species). (a) Oak‐dominated forests. (b) Beech‐dominated forests (c) comparison of the two forest types across all taxonomic groups. Win‐win*(dark blue, left) and trade‐off* (dark red, right) species are pure and reliable species. Win‐win (light blue, left) and trade‐off (orange, right) species are pure but not reliable indicators.

We also checked whether a specific threshold in forest carbon exist, where most of this turnover is concentrated. Finding evidence for such a threshold would provide indication on how forest managers could manipulate forest carbon without having to fear negative repercussions on biodiversity. Still, species and community‐level change points were neither congruent nor abrupt, and differed between win‐win and trade‐off species, as well as across taxa and forest types, suggesting that a clear ecological threshold along the carbon‐stock gradient may not exist in temperate forests.

Overall, our results suggest that, at the extent of an individual forest stand, biodiversity and carbon cannot be jointly maximized. Rather, forest planners and managers should carefully evaluate whether to give priority to biodiversity conservation or other carbon‐related goals, since maximizing forest carbon stock (e.g., by altering the number and arrangement of trees via thinning or planting) may only benefit some species, while being detrimental to others. Reconciling biodiversity and carbon objectives requires therefore planning across multiple scales, so to find the optimal arrangement of management types which delivers highest co‐benefits, while integrating stand‐level structural and biodiversity features. For instance, retaining blocks of undisturbed forest within managed forests may represent an effective option for incorporating multiple environmental goals in forest management, and reconcile carbon‐storage and conservation goals, while allowing for the persistence of the full range of both win-win and trade‐off species.

Originally published in https://forestsandco.wordpress.com/ by F.M. Sabatini

Full Reference

Sabatini FM, de Andrade RB, Paillet Y, Ódor P, Bouget C, Campagnaro T, Gosselin F, Janssen P, Mattioli W, Nascimbene J, Sitzia T, Kuemmerle T, Burrascano S, 2018. Trade‐offs between carbon stocks and biodiversity in European temperate forests. GLOBAL CHANGE BIOLOGY. n\a, 1-13. DOI: 10.1111/gcb.14503

 

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