DISCUSSION
Plant diversity effects of aboveground-belowground processes
are present throughout the
year
The magnitude and/or direction of the plant diversity effects on plant
and soil processes changed throughout the year. This pinpoints phenology
as a key, yet often overlooked, component of relationships between
biodiversity and ecosystem functioning. On the one hand, plant diversity
effects on overall plant height, root production, and detritivore
feeding activity are consistent with previous works (Liang et al., 2016;
Ma & Chen 2016; Birkhofer et al., 2011). On the other hand, the lack of
plant diversity effects on overall greenness was unexpected, which may
be explained by several reasons. One possible explanation is that the
competitive environment in species-rich plant communities may be
translated into enhanced flower production (Ebeling et al., 2008),
decreasing greenness and suggesting the need to include more
flower-related metrics to understand diversity effects on plant
community production fully (Motohka et al., 2010; Schiefer et al.,
2021). Another plausible explanation may be that greenness is in this
case a better proxy of community structure aspects (Guimarães-Steinicke
et al., 2019), such as density and volume, not representing
photosynthetic activity well in our study (but see Muraoka et al.,
2013). Furthermore, plant diversity effects on plant shoot production
were shown in the first phase of the growing season, when diverse
communities started growing earlier than previously observed
(Guimarães-Steinicke et al., 2019). Given that other experiments are
younger (as in the case of Guimarães-Steinicke et al., 2019), our
results are consistent with the suggestion that temporal niche
partitioning and legacy effects strengthen BEF relationships over time
(Dietrich et al., 2021; Guerrero-Ramírez et al., 2017; Reich et al.,
2012; Vogel et al., 2019).
Plant diversity effects on plant processes disappeared in summer when
the positive effects of soil temperature were also absent. Instead, the
plant community showed several effects on soil temperature and moisture
during this period, including a buffer effect by plant height on soil
temperature, as shown before (Huang et al., 2023). The direction of
biotic-abiotic relationships is a classical discussion in Ecology (Mori
et al., 2017). Several authors have debated whether biotic factors (e.g.
plant structure) are driven by abiotic factors (e.g. soil temperature
and moisture) or whether abiotic factors regulate diversity (Mori et
al., 2017). Our results support both relationships and suggest that the
strength and causal direction may change over the year even in the same
community.
Belowground activity is high during
winter
We found surprisingly high belowground activity in winter, revealing the
influence of plant species richness and plant functional group richness
on soil functioning. This is particularly important in grasslands, where
roots account for about 70% of plant biomass (Jackson et al., 1996;
Poorter et al., 2012). Root production in winter suggests that niche
differences among species allow diverse communities to initiate growth
at low temperatures, possibly resulting from a shift in carbon
allocation to roots when temperature decreases (Poorter et al., 2012;
Reich et al., 2014). Early onset of root production in a plant can
convey a competitive advantage and diversify carbon allocation
strategies (Harris 1977), which may, in turn, contribute to enhanced
productivity of diverse plant communities during the aboveground growing
season. The winter peak in detritivore feeding activity was unexpected,
given that other works found low activities during winter (Sünnemann et
al., 2021; Siebert et al., 2019, but see Gottschall et al., 2022). Plant
diversity continuously enhanced detritivore feeding activity during
spring and summer, but only functional group effects were observed in
winter. Altogether, these results demonstrate that an accurate depiction
of winter activity is required for a mechanistic description of temporal
niche dispersion and biodiversity effects on ecosystem functioning
(Gottschall et al., 2022).
Aboveground-belowground (a)synchrony along the seasons is
mediated by plant
diversity
Plant diversity effects on aboveground-belowground phenological
synchrony shifted throughout the year, with independent impacts of plant
species richness and functional groups shown. During spring, plant
diversity generally increased phenological synchrony. Plant species
richness enhanced height-greenness synchrony, indicating an optimal
growth strategy, with the highest photosynthetic capacity reaching the
timing of the highest physical structure (Zhao et al., 2022). During
summer (post-mowing), height-greenness dynamics were asynchronous on
average but showed a tendency towards positive synchrony with increasing
species richness. This indicates that species-rich plant communities
regrow faster after mowing, then plant height matches the high
greenness. Plant diversity strongly enhanced height-root synchrony
during spring, probably due to earlier shoot growth and extended root
production in species-rich plant communities. Due to the high synchrony
of height-greenness, roots were also synchronous with greenness in the
same period. Even though plant functional richness was not a significant
driver of above- and belowground activities in spring, it affected
aboveground-belowground phenological synchrony. Specifically,
greenness-feeding and height-feeding activity shifted from asynchrony in
lower diversity to a non-correlation in higher diversity. This was the
opposite in summer, when communities with higher plant functional group
richness showed not synchronous greenness-feeding activity dynamics.
This may reflect the negative effect of functional group richness on
detritivore feeding activity alone in the same period, causing a
mismatch with greenness that was still high. Even though detritivore
feeding activity and root production were high in winter, their
within-winter dynamics differed, resulting in non-correlated root
growth-feeding activity dynamics.