Introduction
Positive
biodiversity-productivity
relationships are a predominant pattern in global forests that are based
on both observational and experimental studies
(Zhang et al. 2012;
Liang et al. 2016). Species richness, as
a surrogate of biodiversity, has been demonstrated to increase ecosystem
productivity (Diaz & Cabido 2001;
Hooper et al. 2012).
Traditionally, numerous
biodiversity-ecosystem functioning experiments have been implemented
through the manipulation of species richness to understand the
mechanisms that drive this relationship
(Tilman et al. 1996;
Grossman et al. 2017;
Huang et al. 2018). However, there has
been controversy in regard to the relationship between
species richness and productivity,
being either positive (Grossman et al.
2017; Huang et al. 2018), insignificant,
or even negative (Vila et al. 2003;
Tobner et al. 2016) in forest ecosystems.
Still, a deeper mechanistic elucidation of which ecological processes
drive variant species richness-productivity relationships remains
incomplete.
Experimental plant communities involve random assemblages of species
from a species pool (Tilman et al. 1996;
Grossman et al. 2017;
Huang et al. 2018), which typically have
distinct sets of functional traits (Leps
2004). The functional traits of individual species and their
interactions can lead to different species mixtures outcomes
(Diaz & Cabido 2001;
Loreau et al. 2001;
Tobner et al. 2016). Species with various
functional traits lead to increased niche differentiation and positive
interspecific interactions, more comprehensive resource use, and
improved community-level biomass production, i.e., the so called
complementarity effects (Tilman et al.
1997; Cardinale et al. 2011). Meanwhile,
higher productivity in species mixtures can result from a selection
effect, that is, species mixtures may be more productive in contrast to
monocultures, due to the increased probability of particular productive
species that dominate in mixtures (Grime
1997; Loreau & Hector 2001). However,
the outcomes of various species mixtures might be influenced through the
extent of trait variations, even though the species richness is constant
(Tobner et al. 2016). Thus far, the
functional significance of tree diversity on ecosystem productivity has
been rarely tested, both across and within species richness levels.
Recent studies have confirmed that effects of biodiversity on ecosystem
functioning may be predicted by the degree of functional differences
between constituent species in mixtures
(Heemsbergen et al. 2004;
Chen et al. 2019). Functional differences
might result in variable interactions between species
(Heemsbergen et al. 2004). For instance,
greater interspecific functional dissimilarities increase niche
differentiation and facilitative interactions to enhance the usage of
resources; thereby, increasing ecosystem productivity
(Loreau et al. 2001;
Wright et al. 2017). Functional trait
dispersion (FDis) is theoretically associated with niche differentiation
(Laliberte & Legendre 2010).
Accordingly, we hypothesized that
FDis in species mixtures would be
positively associated with positive diversity effects on forest
productivity, both across and within species richness levels. We
expected that the positive effects of species richness on productivity
were attributable to the positive association between species richness
and functional dispersion. Furthermore, the effects of diversity on
productivity increased with higher levels of functional dispersion, when
the species richness was constant (Fig. 1).
The effects of plant mixtures on productivity are also driven by the
functional identities of species mixtures, which represent the
characterized functional strategies for resource acquisition of species
assemblages (Mokany et al. 2008).
Acquisitive species with significant production investments in their
stems and leaves have higher efficiencies in terms of resource
acquisition and utilization, than conservative species
(Reich 2014;
Diaz et al. 2016). High productivity
associated with acquisitive traits in mixed communities have been
matched with high productivity in corresponding monocultures
(Mokany et al. 2008). Moreover, the
functional characteristics of species determine the intensity of the
interactions between constituent species in plant communities along
abiotic stress gradients (Maestre et al.
2009). The varied intensities of species interactions, therefore, can
lead to different outcomes in terms of community-level productivity
(Lusk et al. 2008;
Fichtner et al. 2017). Additional
available resources allow for the improved realization of niche
differentiation in communities dominated by acquisitive traits
(Sterck et al. 2011;
Baez & Homeier 2018). Alternatively, in
stressed or resource-limited environments, conservative traits dominate
and interspecific facilitation tends to be stronger, as predicted by a
stress-gradient hypothesis (Prado-Junior
et al. 2016). Therefore, it may be anticipated that the positive
effects of species mixtures on productivity are contingent on the
community-weighted means (CWM) of acquisitive traits (Fig. 1). Further,
the trajectory of the influence of the CWM indirectly reflects the
relative strength of its effects on niche differentiation versus
interspecific facilitation.
Here, we aimed to
investigate how functional
differences and identity determine the various outcomes of tree mixture
effects on ecosystem productivity, both across and within species
richness levels. We conducted a global meta-analysis based on 210 paired
observations of tree mixtures and corresponding monocultures from 59
tree diversity experiment studies (Fig. 1). We collected data on
specific leaf area, leaf nitrogen content, and wood density of the
selected species for each observation to determine whether FDis and the
CWM of acquisitive traits of species mixtures might be positively
associated with positive diversity effects on forest productivity, both
across and within species richness levels.