Discussion
This meta-analysis explicitly revealed that both functional trait
variability and identity influenced the effects of tree mixtures on
forest productivity, both across and within species richness levels of
tree mixtures in experimental tree communities at a global scale.
Specifically, we found that the functional dispersion of tree mixtures
increased the extent of the positive mixture effects on productivity
overall, and within the two- and four-species mixtures. Moreover, the
CWM of acquisitive traits of species mixtures enhanced the positive
effects of mixtures on forest productivity. Our findings offer novel
insights into the importance of plant functional traits in determining
the magnitude (and even directionality) of the biodiversity-productivity
relationships that have been under debate for more than two decades.
To date, a few experimental
studies have segregated functional aspects from species richness to test
the effects of functional diversity or individual traits involved in
tree productivity
(Tobner
et al. 2016; Grossman et al. 2017). The previous studies
emphasized that a particular combination of functional attributes (e.g.,
deciduous and shade-intolerant species, high leaf-nitrogen, and calcium)
(Tobner et al. 2016;
Grossman et al. 2017;
Huang et al. 2018), or shade tolerance
heterogeneity between constituent species
(Zhang et al. 2012), caused the observed
species diversity effect. However, these researches also demonstrated
that functional diversity per se could not explain the additional
variation in ecosystem productivity across communities at a given
species richness level. Beyond such conventional wisdom, our
meta-analysis revealed that both functional dispersion and identity in
tree mixtures determined the extent of diversity effects on productivity
across and within the species richness levels. Our results, therefore,
provided evidence that increased functional diversity should enhance
ecosystem functioning through the coincidental dominance of influential
species, or through niche partitioning
(Tilman et al. 1997;
Diaz & Cabido 2001;
Loreau et al. 2001).
The degree of functional differences between species drive the effects
of plant mixtures on ecosystem productivity, due to niche partitioning
and positive interactions between constituent species at the community
level (Tilman et al. 1997;
Diaz & Cabido 2001;
Loreau & Hector 2001). Plant leaf and
wood economics traits are associated with plant resource acquisition,
shade tolerance, hydraulic transport, mechanical support, and carbon
storage (Reich 2014). Communities
consisting of species with contrasting leaf and wood economics traits
caused niche differentiation with respect to the utilization of light
and water and facilitative interactions
(Fichtner et al. 2017;
Baez & Homeier 2018), which might
increase the community-level acquisition and efficient use of light and
water (Anderegg et al. 2018;
Huang et al. 2018). Species-diverse
mixtures with higher FDis, in turn, enhances the efficcacy of resource
use in mixtures due to recourse niche differentiation
(Tilman et al. 1997;
Cardinale et al. 2011), thereby improving
ecosystem productivity (Flynn et al.
2011). Moreover, we found that increasing FDis resulted in higher
productivity, even at a given species richness level. These outcomes
suggested that trees in the communities with the same number of species
that occupied various positions in the leaf and wood economics spectrum,
increased the efficacy of resource utilization, and tended to promote
forest productivity.
Consistent with our hypothesis, we found that the higher CWM of LNC
increased the positive effects of species mixtures on productivity,
which indicated the important role of the N-acquisitive strategies
involved in diversity effects for improving forest productivity (Fig.
4). The functional characteristics of plant species determined the
interactions between constituent species in plant communities along
abiotic stress gradients (Maestre et al.
2009). In this study, mixtures dominated by acquisitive species were
found at experimental sites with warmer climates and higher
precipitation (Fig. S3). Additional available resources allowed for
intense species interaction caused by effective light acquisition of
fast-growing species, and hence niche differentiation in communities
(Bertness & Callaway 1994;
Callaway et al. 2002). These interactive
processes appeared to be more intense in mixtures that included
acquisitive plants compared with conservative mixtures, which
consequently improved the effects of mixtures on productivity
(Tobner et al. 2016;
Fichtner et al. 2017). It is noted that
the impacts of species mixtures on productivity were enhanced with the
CWM of leaf nitrogen but independent of wood density
(Sakschewski et al. 2015). Wood density
correlates to a large number of structural characteristics of wood
plants (Chave et al. 2009), and species
with high wood density generally represent the conservative-end of the
fast-slow plant economics spectrum (Reich
2014). Communities being characterized by great CWM of wood density
reflect the coincidental dominance of slow growing species for
maintaining ecosystem productivity. In such case, the interactive
processes should be weak in mixtures that dominated by slow growing
plants, which consequently cannot enhance the effects of mixtures on
productivity.
In conclusion, our meta-analysis integrated the functional differences
of species between global scale tree diversity experiments and
investigated how functional trait variability and identity determined
the outcomes of tree mixture effects on ecosystem productivity. Our
results revealed that the effects of tree mixtures on productivity
increased with the functional dissimilarity of the leaf and wood
economics traits, and the community-weighted mean of leaf nitrogen
content overall and within the two- and four- species mixtures. These
results revealed the key role of the functional dispersion and
composition of species mixtures toward explaining the variations in the
effects of plant mixtures on ecosystem productivity, both across and
within the species richness levels. We anticipate that our analysis will
stimulate future inquiries into the role of functional traits in the
diversity-productivity relationships.