Scientists and policy makers have looked at Mediterranean forest biodiversity mainly from a conservation point of view. But biodiversity is increasingly considered an important aspect of ecosystem functioning. In this presentation, we review more specifically diversity-productivity relationships in forests. But it's not an easy task, because the problem is that forest condition and growth are determined by many interacting environmental and management factors. In this context, developing an international network of such experiments, including Mediterranean sites, would be very useful

BUSSOTTI F.

MUYS B.

SCHERER-LORENZEN M.

SELVI F.

VALLADARES F.

I-Revues

385
Tree species diversity
and forest productivity
by Bart MUYS, Fernando VALLADARES, Filippo BUSSOTTI,
Federico SELVI and Michael SCHERER-LORENZEN
Introduction
Primary productivity as a result of photosynthesis and respiration is
an important life support function, contributing largely to the provi-
sioning services of wood and other forest products and to the climate
regulating services of carbon sequestration and storage (MEA 2005).
Sustainable forest management aims at co-optimizing for stock func-
tions (biomass and carbon), rate functions (growth, sequestration) and
stability of stocks and fluxes (PACALA & KINZIG 2002). At least since the
19th century foresters have wondered whether mixed forests would be
more productive than monocultures (PRETZSCH 2005), but so far with-
out conclusive answers. During the last decade strong positive diver-
sity/productivity relationships have been reported from experimental
grassland species assemblages (TILMAN et al. 2001, LOREAU et al. 2001).
They were at least partly explained by species complementarity in
accessing below-ground resources (e.g. VERHEYEN et al. 2008). In this
contribution we further explore if diversity/productivy research in
forests could reveal similar patterns.
Scientists and policy makers have
looked at Mediterranean forest bio-
diversity mainly from a conservation
point of view. But biodiversity is
increasingly considered an impor-
tant aspect of ecosystem functioning.
In this presentation, we review more
specifically diversity-productivity
relationships in forests. But it’s not
an easy task, because the problem is
that forest condition and growth are
determined by many interacting
environmental and management fac-
tors. In this context, developing an
international network of such exper-
iments, including Mediterranean
sites, would be very useful.
Mediterranean Forest Week of Avignon
forêt méditerranéenne t. XXXII, n° 4, décembre 2011
Evidence for biodiversity
/productivity relationships
in forests
Many stand-level observations worldwide
on important timber species like spruce,
Douglas fir, pine or Eucalypt reported higher
productivity in monocultures compared to
mixed stands including the same species
(PRETZSCH 2005). But a few studies with
these species also report positive effects of
species mixture on productivity of up to 10 to
20%. Such positive diversity/productivity
relationships were mainly observed on rich
soils while, on poor soils, mixtures showed
an often negative effect on productivity
(KÖRNER 2005). A synthesis of tropical stand-
level studies showed an overall better diame-
ter growth in mixed forest plantations over
monocultures (PIOTTO 2008). MÖLDER et al.
(2008) found a positive relation between
herb-layer biomass and tree-layer diversity
in deciduous forests in Germany.
So far, very few studies tested
diversity/productivity relationships at larger
than stand scale. A first attempt to test the
biodiversity-productivity hypothesis with
regional forest inventory data was made by
VILA et al. (2007) in Catalonia, see figure 1.
They found increased stand stemwood pro-
duction with tree species richness before
canopy closure in sclerophilous forest, less so
in conifer forests and not in deciduous
forests. This would suggest that in
Mediterranean forests this relationship can
be observed in early successional forests or
on marginal sites. Ever since, some analyses
of national inventory data to deduct effects of
mixed forests are ongoing in France and
Germany (e.g. MORNEAU et al., 2008).
Analysing a global NPP database analysis,
LUYSSAERT et al. (2007) showed that the net
carbon balance of forests is rather similar
over the world because, for average annual
temperatures and precipitation above the
optimum of 10°C and 1500 mm, photosyn-
thesis is increasingly compensated by higher
respiration rates. Global variation between
forest sites is not so much the effect of cli-
matic differences but is rather due to factors
such as forest age, management and history
of disturbance which, among others, influ-
ence the tree species composition.
This short review suggests that results
from observational studies on the diver-
sity/productivity relationship in forests are
not univocal. Observed diversity/productivity
relationships seem context specific and
strongly confounded by environmental fac-
tors and management practices. This finding
underlines the need for well-designed uncon-
founded synthetic community experiments.
In recent years methodological advances
are debated in recent literature (SCHERER-
LORENZEN et al. 2007, LEUSCHNER et al. 2009)
and have led to the establishment of large-
scale long-term tree species assemblages to
explore the functional significance of tree
diversity for forest productivity in Germany,
Finland, Belgium, France, Panama,
Malaysia, China and Canada (see figure 2).
Unfortunately no experiments have been
established in the Mediterranean biome so
far. Next to an experimental platform, the
FP7 project FunDivEurope 1 also includes an
exploratory platform, where plots with dif-
ferent tree species diversity levels were
selected within areas of existing mature for-
est with homogeneous environmental condi-
tions. Six such exploratory sites have been
established in Europe, of which two of them
386
Mediterranean Forest Week of Avignon
Figure 1:
Analysis of the Catalan
forest inventory revealed
positive relationships bet-
ween number of tree spe-
cies and wood produc-
tion, and also between
number of functional
groups and wood produc-
tion in evergreen sclero-
phylous forests, but less
pronounced in broadlea-
ved deciduous and coni-
fer stands (capital letters
above graphs reveal signi-
ficant differences bet-
ween groups). The results
suggest a stronger com-
plementarity effect in
early successional stages
on marginal sites, which
seems contradictory to
findings in cold temperate
climate.
From Vila et al. 2007
1 - www.fundiveurope.eu
387
in the Mediterranean Region (Alto Tajo
Natural Park in Spain and Berignone-Tatti
and Belagaio forests in Italy with sub-
mediterranean climate).
Attempts for mechanistic
explanation
Tree species can show different types of
interactions depending on the abiotic condi-
tions. When two species occupy more or less
the same ecological niche, one being more
productive than the other, then introduction
of the second species would lead to produc-
tivity loss (see figure 3a). Such situation,
where competition occurs between function-
ally similar species is common in forests and
explains the multitude of studies where neg-
ative diversity/productivity relationships are
observed. But, when different species with
clearly distinct niches occur together (fsee
igure 3b), then productivity will depend on
the complex interaction between the two and
can lead to different outcomes, depending on
the species/site interaction (see figure 4).
Scientific Seminar
Figure 2:
The global network of synthetic community experi-
ments evaluating tree species diversity function
consists of more that 2900 plots covering more than
600 hectares and more than 600,000 planted trees
(www.treedivnet.ugent.be). Background map of vas-
cular plant diversity, Barthlott et al. 2005
Figure 3:
Biomass production in relation to site for two tree
species. a) species with similar ecological niche. B)
species with different ecological niche. (Pretzsch
2005). Sections 1-4 refer to species interactions
explained in figure 4.
Figure 4:
Biomass production for two tree species in monocul-
tures and mixed stands under different site conditions
(conditions 1-4 from figure 3b). When two species
with strongly different niche co-occur their producti-
vity will theoretically depend on the complex interac-
tion between 3 types of species interaction (positive
interaction, continuous line; negative interaction, dot-
ted line; neutral interaction, dashed line) at 5 site
types (growing range of species A but outside gro-
wing range of species B (not in figure), growth opti-
mum for species A (fig 4a), same growth potential for
both species (fig 4b), growth optimum for species B
(fig 4c), growing range for species B but outside
range of species A (fig 4d) (Pretzsch 2005).
These possible interactions between
species can be tested in the field by compar-
ing the productivity or any other perform-
ance indicator of monoculture plots/patches
versus mixed plots/patches situated on the
same site. As explained above the interaction
between species can have a negative (compe-
tition) effect, a neutral effect or a positive
effect. But not all positive effects imply
species complementarity, i.e. demonstrated
patterns of niche differentiation and facilita-
tion between the species (HOOPER et al.,
2005). It is also possible that there is simply
a selection/insurance effect. It means that
the higher species diversity leads to a higher
probability of presence for high performing
species (example: admixture of an N-fixing
tree species on poor soil) or to a higher prob-
ability of alternative pathways (example: ash
and alder filling the gaps of elm in a riverine
forest damaged by dutch elm disease). The
simple tests to identify the kind of interac-
tion are given in the following equations.
[1] Net diversity effect (NDE) =
[observed productivity]– [expected productivity]
where the expected productivity is the area
weighted average productivity of the mono-
cultures of species in the mixture.
[2] Transgressive overyielding (Dmax) =
[observed prod.] – [max. single species prod.]
where the maximum single species produc-
tivity is the productivity of the best perform-
ing monoculture of species in the mixture.
The outcome of equations [1] and [2] is com-
pared with the following four equations to
identify the type of species interaction:
NDE<0: antagonistic effect (competition) [3]
NDE=0: no effect [4]
NDE>0; Dmax<0: selection effect [5]
NDE>0; Dmax>0: complementarity effect [6]
In a study of tree growth of Scots pine and
birch stands in the Netherlands, WIJDEVEN et
al. (2000) observed higher productivity in
mixtures than in monocultures of both
species, with an increased effect of the mix-
ture in stands with a higher basal area.
Especially birch was more productive in mix-
ture than in monoculture. The authors
hypothesize resource complementarity, pos-
sibly through rooting in different soil layers,
but do not dispose of measured proof for this
hypothesis. PRETZSCH & SCHÜTZE (2009) were
the first to prove resource complementarity
between tree species in a mixture. While
mixtures of beech and spruce on poor sub-
strates always lead to dominance of spruce,
without complementarity effects, they
observed transgressive overyielding of 14-
29% compared to monocultures on rich sites
in Southern Germany, due to mutual growth
stimulation of both species. This proven com-
plementarity was assumed to be the conse-
quence of more efficient nutrient uptake and
distribution and a higher crown efficiency in
the mixtures.
Conclusion
So far, little evidence for diversity/produc-
tivity has been reported from forests.
Observational studies could not unequivo-
cally find such relationships as they were
possibly masked by confounding factors such
as abiotic resource variability.
Methodological challenges have seriously
retarded the setup of experiments in forests,
as these ecosystems are dominated by tall
long-living organisms. Recently established
experiments are expected to foster a break-
through in understanding the role of biodi-
versity for forest productivity and carbon
sequestration in the future. Meanwhile
advanced statistical analysis of observa-
tional studies and inventory databases
allows further testing the diversity/produc-
tivity hypothesis in forests. Where comple-
mentarity can be demonstrated, more
process-based research is needed to under-
stand the mechanisms behind it. Forest
diversity function is a young science with a
strong need for research results from the
Mediterranean biome. Next to the
exploratory plots of the FunDivEurope proj-
ect in Italy and Spain, it is recommendable
to consider establishing experimental tree
diversity level assemblages with typical
mediterranean species.
Acknowledgements
We would like to thank Greet Willems
(K.U.Leuven) for drawing figures 1, 3 and 4
and Pieter Vangansbeke (Ugent) for draw-
ing figure 2.
388
Mediterranean Forest Week of Avignon
389
Scientific Seminar
Bart MUYS
K.U.Leuven, Belgium
and EFIMED, Spain
Email :
bart.muys@
ees.kuleuven.be
Fernando
VALLADARES
CSIC, Spain
Filippo BUSSOTTI
Federico SELVI
University of
Florence, Italy
Michael SCHERER-
LORENZEN
University
of Freiburg, Germany
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forêt méditerranéenne t. XXXII, n° 4, décembre 2011
390
Mediterranean Forest Week of Avignon
Summary
forêt méditerranéenne t. XXXII, n° 4, décembre 2011
Scientists and policy makers have looked at Mediterranean forest biodiversity mainly from a conserva-
tion point of view due to the status of the Mediterranean biome as a biodiversity hotspot. But biodiver-
sity is increasingly considered an important aspect of ecosystem functioning, underpinning the supply
of other ecosystem services (TEEB 2008). In temperate grasslands for example biodiversity-productivity
relationships have been tested and discussed in detail. In Mediterranean forests, diversity-stability rela-
tionships would be of a higher interest considering the dry climatic conditions, and in a context of
adaptation towards increasing drought. We suspect that many forest managers in the region consider
that mixed forests are more stable against disturbances and even more productive in the long-term,
but there is little scientific evidence for this.
In this presentation we first review some relevant theory of biodiversity function research. We compare
complementarity and selection effects as possible mechanisms for biodiversity function and introduce
the concept of overyielding as a measure of diversity function effect based on comparison of the mixed
forest with the monocultures of its constituents. Then we highlight current and future research activi-
ties in Mediterranean forests. As a matter of fact, demonstrating diversity-productivity or diversity-stabi-
lity relationships in Mediterranean forests is not an easy task. The problem is that forest condition and
growth are determined by many interacting environmental and management factors, some of them
having a much stronger influence than diversity. Controlling for these confounding factors would the-
refore require advanced statistical techniques or a very specific experimental design.
Vilà et al. (2007) made the most successful attempt so far to use national forest inventory data for tes-
ting biodiversity-productivity relationships in the Mediterranean forest. They used the inventory data
from Catalonia, Northern Spain, wich includes high resolution spatial information on many environ-
mental and management related co-variates. Basically, they found higher wood production with increa-
sing tree species mixture in evergreen broadleaved forest, but not in deciduous broadleaved or coni-
fers. This suggests a positive biodiversity-productivity relationship on more extreme or marginal sites.
More controlled observational studies will take place in the framework of the EU FP7 FunDivEUROPE
project, where numerous ecosystem service indicators and associated biodiversity will be measured
along tree species diversity gradients in areas selected for reduced variability of confounding factors.
The two Mediterranean sites included are Alto Tajo Natural Park, in central Spain and the public-owned
forest complexes of Berignone-Tatti and Belagaio in central-southern Tuscany, Italy.
Finally the need for specifically designed tree species diversity experiments that fully control for
confounding factors will be discussed in the context of a developing international network of such
experiments and the appropriateness for some Mediterranean sites to be included in this global effort
will be argued.
Keywords : mixed forest, diversity-productivity relationship, diversity-stability relationship


Tree species diversity and forest productivity -

Scientists and policy makers have looked at Mediterranean forest biodiversity mainly from a conservation point of view. But biodiversity is increasingly considered an important aspect of ecosystem functioning.
In this presentation, we review more specifically diversity-productivity relationships in forests. But it’s not an easy task, because the problem is that forest condition and growth are determined by many interacting environmental and management factors.
In this context, developing an international network of such experiments, including Mediterranean sites, would be very usefulstatus: publishe

Tree species diversity and forest productivity -

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