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.