Is higher biodiversity always leading to

better forest ecosystem functioning? Executive Summary by Romain Cottet, Abigail Fellay, Franziska Scheibler

INTRODUCTION As global change threatens ecosystems and forests worldwide, the functions and services they provide are being jeopardized. Following

the Millenium Assessment (MA), the concern that the loss of biodiversity would affect the provisioning of ecosystem service grew. This

led to the development of a new study field whose goal was to assess the relationship between biodiversity and ecosystem functioning

(BEF). In order to dive into this field, one has to first define two

things: biodiversity and ecosystem functioning. Although

biodiversity will inevitably be mentioned a few times, we will

mainly focus here on the definition of ecosystem functioning as it

has been used in the field of forestry.

STATE OF THE ART

BIODIVERSITY AND ECOSYSTEM FUNCTIONS

The overarching conclusion emerging from this field of study

follows the direction of the concerns of the MA in the sense that

biodiversity is found to be a major driving force in ecosystem functions in general, and this for two main reasons: o Biodiversity provides a range of different functions by exploiting complementary spatial and temporal niches (e.g. nitrogen fixing

trees increase growth of other species) (Brockerhoff et al. 2017; Tilman et al. 2001).

o Biodiversity leads to more resistant and resilient forests, in turn ensuring ecosystem functions over a longer period of time,

especially when facing changing conditions (Insurance Hypothesis) (Bengtsson et al. 2000).

However, most studies focus on productivity as a proxy for ecosystem functioning (Brockerhoff et al. 2017). It has been chosen as a

reference point for forestry because it is a good economic indicator and it is linked to carbon sequestration, the most important

ecosystem function regarding climate change. Productivity can be defined as the increasing amount of plant biomass (growth) which is

derived by using the diameter at breast height (DBH) to compute the volume of a tree or of a whole stand (Vilà et al. 2013; Huang et al.

2018). But it can also be defined using NPP measured with Eddy covariance towers.

BIODIVERSITY AND PRODUCTIVITY

The relationship between productivity and species diversity has been explored through experimental and observational studies as well

as simulation models.

EXPERIMENTAL AND OBSERVATIONAL STUDIES

The main findings are the following:

o In experimental studies, positive relationship between tree species richness and stand basal area or stand volume, two indicators

of productivity are often highlighted (e.g. Huang et al. 2018). This positive effect sometimes appears to grow stronger with time.

o Observational studies using worldwide forest inventories show that biodiversity is not the predominant factor influencing plant

biomass production. Indeed, other abiotic and biotic environmental factors (e.g. climate variables, soil fertility or diseases and

herbivory activity) have been shown to be more effective in measuring productivity (Vilà et al. 2013; Poorter et al. 2015).

Difficulties and limitations:

o The slow growth rate of forests implies that long-term experiments are scarce. Most of them have not been carried out for more

than 10 years so far and are therefore not yet representative of natural dynamics. However, a number of experiments have been

set up and are expected to yield results in coming years (Scherer-Lorenzen et al. 2007).

o Observational studies are often based on forest inventories from around the world. But by doing so, it is impossible to control

for the covariates. Therefore, sophisticated statistical analysis needs to be carried out to highlight the effect of biodiversity on

productivity.

SIMULATION MODELS To overcome the limitations of forest experiments and observational studies, scientists have turned to the power of computers. Forest

models allow us to generate thousands of predictions for many different forests in controlled growing conditions over long periods of

time. Those are mechanistic models, in other words a set of complex equations that try to translate into numbers some natural processes

such as light interception in the canopy (Courbaud et al., 2003) or the allometric relationships of trees (Chave et al. 2014). In order to

assess the relationship between biodiversity and productivity, the number of different species in the modelized forests can be

manipulated. In two studies, using simulations models, Morin et al. (2011) and Bohn and Huth (2017) obtained two different results (Figure 1). The

first authors observed a positive linear relationship between the biodiversity and productivity or net biodiversity effect. This second

Ecosystem function are the ecological (biological, chemical, physical)

mechanisms that support the integrity or maintenance of ecosystems.

This concept is ‘‘ecosystem-centered’’ (Brockerhoff et al. 2017).

Ecosystem services retain a connection to the underlying ecosystem

function that generate them. Ecosystem services represent the

contributions that ecosystems make to human well-being. Therefore,

this concept is ‘‘human-centered’’ (Brockerhoff et al. 2017).

Biodiversity is a multifaceted concept that includes the diversity of life

on different levels of organization from genes, to species, to entire

ecosystems. Biodiversity as such is not an ecosystem function but is

important to many ecosystem functions (Dislich et al. 2017).

>>Because of the inherent difficulties, experimental studies confirming

these results are scarce and not yet representative of reality.

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Figure 1: Results from two different simulations assessing the relationship between biodiversity and productivity. With on the x axis the diversity indexes and on the y axis the indicators of productivity.

indicator being the difference between the productivity of a specie in a monoculture with its productivity in a mixed forest. The second

authors did not observe such a relationship between productivity and species number but observed a decline in variability of productivity

with the increase of species number. These differences could be explained by the different assumptions made by the authors when

building the models. Thus, one needs to be aware of these assumptions and understand them to be able to interpret those results.

Limitations of models: o They are only approximations of reality, due to simplifications and choices implied by the technique; the number of processes

included is indeed limited.

o A variety of different pieces of models can be chosen to approximate each of those processes. These will influence the results

but also the type of output which causes comparability problems (e.g. how do BE and AWP compare to each other?).

o The computation time limits the choices.

OTHER PROXIES FOR ECOSYSTEM FUNCTIONING ENERGY FLUXES AS PROXIES FOR ECOSYSTEM FUNCTIONING Study about benthic ocean: Danovaro et al., (2008) used three different ratios of carbon stocks and fluxes as indicators of ecosystem

functioning. In fact, they observed an exponential relationship between functional diversity and ecosystem functioning in all oceans

explored.

Study on land use change: Barnes et al., (2014) explored the impact of land use change on the BEF

relationship. They used the total energy flux between functional feeding guilds from litter material

(predators, omnivores, herbivores and detritivores) as a measure

of multitrophic ecosystem functioning in four different land uses

characterized by strong biodiversity differences (secondary

rainforest, forest rubber, rubber plantation, palm oil plantation).

Their results show a positive power law relationship between

the biodiversity (species richness) and the energy flux between

feeding guilds in different forest systems (Figure 2).

A SET OF ECOSYSTEM SERVICES AS PROXY FOR ECOSYSTEM FUNCTIONING Study on forest ecosystem services: Gamfeldt et al., (2013) identified and quantified six different

ecosystem services provided by northern forests. They used the Swedish National Forest Inventory

to assess the BEF relationship taking the number of tree species as indicator for biodiversity. They

observed a positive linear relationship as well as positively humped shape relationship between biodiversity and ecosystem services.

FINAL WORDS

Assessing the relationship between biodiversity and ecosystem functioning requires before all defining specific proxies for each of these

broad concepts, and further selecting appropriate indicators. Indeed, when measuring BEF, one should first be aware that biodiversity

does not influence all ecosystem functions in the same way. Additionally, as ecosystems are highly complex, a wide range of processes

can be chosen from, as proxies of the overall functionality of the system. Furthermore, selecting a set of processes is likely to lead to

results that are more accurate and help reflect the multifunctionality of ecosystems, although it is more complex to do so. In other words,

the choice of proxies and indicators heavily influences the results of the analysis and should be made considering the type of ecosystem

at hand, the local context as well as the overarching aim of the analysis. Indeed, productivity is a widely used proxy, despite the fact that

numerous studies show that it is not the most suitable one when investigating the role of biodiversity on ecosystems. However, its

economic component is attractive, as it helps attribute a monetary value to biodiversity and thus encourage its conservation. This leads

us to reflect on the overarching aim of investigating BEF and wonder whether biodiversity should not simply be protected for its intrinsic

value.

Figure 2: Results from Barnes et

al. (2014) showing a positive

power law relationship between

species richness and the energy

flux between feeding guilds in

four different forest systems.

>>Indicators such as total energy

flux to assess the BEF relationship

might be a better pick than single

ecosystem properties such as

decomposition or productivity.

>>This selection of different approaches illustrates that results vary, partly because of the field of application, partly because of

the approach itself. However, studies demonstrate that, across a range of ecosystems, the positive relation seems to be

predominant over negative relationships although neutral relations are the most common (van der Plas 2019).

>>This section puts forward productivity as the most commonly used proxy for forest functioning. Although productivity shows to be positively

correlated to biodiversity, it does not seem to be its main driver. Furthermore, regardless of the chosen approach, selecting only one indicator

fails to capture the multifunctionality and trade-offs of ecosystems (Bengtsson et al. 2000; Brockerhoff et al. 2017). Therefore, it would be

positive for this field of study to think out of the box and try to find other ways of measuring BEF by looking at what is done in other fields.

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