Above- and below-ground biodiversity jointly regulate temperate forest multifunctionality along a local-scale environmental gradient

1. Tree diversity has been shown to promote a broad range of ecosystem
functions in forests. However, how important these effects are in driving
ecosystem multifunctionality in natural forests, relative to other
drivers, such as below-ground biodiversity (e.g., soil microbial
diversity), community-level functional traits and environmental
conditions, remains poorly understood. Here, we hypothesise that tree
species or phylogenetic diversity, stand structure, functional traits and
soil microbial diversity jointly regulate temperate forest
multifunctionality along a local-scale environmental gradient. 2. Using
repeated census data from a 25-ha old-growth temperate forest, we first
quantified eight ecosystem functions and properties related to above- and
below-ground nutrient cycling. We then used these to estimate ecosystem
multifunctionality using both an averaging and multiple thresholds (50,
75, and 95%) approaches. Finally, we used structural equation models to
explore how different facets of tree (tree species, functional and
phylogenetic diversity) and soil (bacteria, fungi, and nematode diversity)
biodiversity influence ecosystem multifunctionality, as well as how these
relationships are modulated by stand structural attributes and
environmental conditions (topography and soil nutrients). 3. Forest
multifunctionality was positively related to stand structural complexity
but negatively related to acquisitive traits (i.e. community-weighted mean
of specific leaf area). Plant phylogenetic diversity had no significant
direct effect on forest multifunctionality, but it had a significant
indirect effect via increased stand structural complexity. The effect of
soil microbial diversity on forest multifunctionality increased with
increasing threshold levels of forest multifunctionality and outperformed
tree diversity and environmental conditions at the highest threshold level
(i.e. 95%). Forests on steep slopes had lower levels of ecosystem
multifunctionality due to decreased stand structural complexity. Soil
nutrients were responsible for regulating forest multifunctionality via
plant trait composition and, to a lesser extent, via tree diversity, stand
structure and soil microbial diversity. 4. Synthesis: Plant phylogenetic
diversity, stand structure and soil microbial diversity jointly regulated
forest multifunctionality, and these effects were influenced by
local-scale changes in environmental conditions. Soil microbial diversity
was a key driver of highly multifunctional forests, whereas conservation
of complex stand structure and conservative trait dominance could enhance
mean values of multiple functions. 1. Tree diversity has been shown to
promote a broad range of ecosystem functions in forests. However, how
important these effects are in driving ecosystem multifunctionality in
natural forests, relative to other drivers, such as below-ground
biodiversity (e.g., soil microbial diversity), community-level functional
traits and environmental conditions, remains poorly understood. Here, we
hypothesise that tree species or phylogenetic diversity, stand structure,
functional traits and soil microbial diversity jointly regulate temperate
forest multifunctionality along a local-scale environmental gradient. 2.
Using repeated census data from a 25-ha old-growth temperate forest, we
first quantified eight ecosystem functions and properties related to
above- and below-ground nutrient cycling. We then used these to estimate
ecosystem multifunctionality using both an averaging and multiple
thresholds (50, 75, and 95%) approaches. Finally, we used structural
equation models to explore how different facets of tree (tree species,
functional and phylogenetic diversity) and soil (bacteria, fungi, and
nematode diversity) biodiversity influence ecosystem multifunctionality,
as well as how these relationships are modulated by stand structural
attributes and environmental conditions (topography and soil nutrients).
3. Forest multifunctionality was positively related to stand structural
complexity but negatively related to acquisitive traits (i.e.
community-weighted mean of specific leaf area). Plant phylogenetic
diversity had no significant direct effect on forest multifunctionality,
but it had a significant indirect effect via increased stand structural
complexity. The effect of soil microbial diversity on forest
multifunctionality increased with increasing threshold levels of forest
multifunctionality and outperformed tree diversity and environmental
conditions at the highest threshold level (i.e. 95%). Forests on steep
slopes had lower levels of ecosystem multifunctionality due to decreased
stand structural complexity. Soil nutrients were responsible for
regulating forest multifunctionality via plant trait composition and, to a
lesser extent, via tree diversity, stand structure and soil microbial
diversity. 4. Synthesis: Plant phylogenetic diversity, stand structure and
soil microbial diversity jointly regulated forest multifunctionality, and
these effects were influenced by local-scale changes in environmental
conditions. Soil microbial diversity was a key driver of highly
multifunctional forests, whereas conservation of complex stand structure
and conservative trait dominance could enhance mean values of multiple
functions. 1. Tree diversity has been shown to promote a broad range of
ecosystem functions in forests. However, how important these effects are
in driving ecosystem multifunctionality in natural forests, relative to
other drivers, such as below-ground biodiversity (e.g., soil microbial
diversity), community-level functional traits and environmental
conditions, remains poorly understood. Here, we hypothesise that tree
species or phylogenetic diversity, stand structure, functional traits and
soil microbial diversity jointly regulate temperate forest
multifunctionality along a local-scale environmental gradient. 2. Using
repeated census data from a 25-ha old-growth temperate forest, we first
quantified eight ecosystem functions and properties related to above- and
below-ground nutrient cycling. We then used these to estimate ecosystem
multifunctionality using both an averaging and multiple thresholds (50,
75, and 95%) approaches. Finally, we used structural equation models to
explore how different facets of tree (tree species, functional and
phylogenetic diversity) and soil (bacteria, fungi, and nematode diversity)
biodiversity influence ecosystem multifunctionality, as well as how these
relationships are modulated by stand structural attributes and
environmental conditions (topography and soil nutrients). 3. Forest
multifunctionality was positively related to stand structural complexity
but negatively related to acquisitive traits (i.e. community-weighted mean
of specific leaf area). Plant phylogenetic diversity had no significant
direct effect on forest multifunctionality, but it had a significant
indirect effect via increased stand structural complexity. The effect of
soil microbial diversity on forest multifunctionality increased with
increasing threshold levels of forest multifunctionality and outperformed
tree diversity and environmental conditions at the highest threshold level
(i.e. 95%). Forests on steep slopes had lower levels of ecosystem
multifunctionality due to decreased stand structural complexity. Soil
nutrients were responsible for regulating forest multifunctionality via
plant trait composition and, to a lesser extent, via tree diversity, stand
structure and soil microbial diversity. 4. Synthesis: Plant phylogenetic
diversity, stand structure and soil microbial diversity jointly regulated
forest multifunctionality, and these effects were influenced by
local-scale changes in environmental conditions. Soil microbial diversity
was a key driver of highly multifunctional forests, whereas conservation
of complex stand structure and conservative trait dominance could enhance
mean values of multiple functions.