Fig. 3. Plant species richness (above) and plant functional group richness (below) effects on the phenological synchrony above and belowground dynamics. The first column (A, D) refers to the spring (pre-mowing) period, the middle column (B, E) refers to the summer (post-mowing) period, and the third column (C, F) refers to the winter period. Only significant (p <0.05) relationships are shown (solid lines). Each point refers to one plot. For the full results, please see Table S2.

Seasonal effects of plant diversity and abiotic factors on aboveground and belowground responses

We tested the direct and indirect effects (via soil microclimate) of plant diversity (i.e. species richness and functional group richness) on plants and soils. Across the seasonal cycle, the timing of events above and below the ground was markedly influenced by the interplay of plant diversity and abiotic dynamics. However, across the seasons, these influences shifted in strength and direction.
During spring (Fig. 5a), richer communities showed higher plants, with increased root growth and enhanced activity of detritivores, but with less plant greenness. Instead, plant greenness increased with soil temperature, increasing root production and detritivore feeding activity. Plant functional group richness and soil moisture did not explain significant variation in response variables during spring. Plant community height was associated with all other activities - positively with greenness and root production, and negatively with detritivore feeding activity. Moving into summer (Fig. 5b), higher plant species richness corresponded to enhanced root production and detritivore activity. However, increased plant functional group richness was observed to decrease detritivore activity. Notably, the impacts of abiotic factors on all studied ecosystem processes were predominantly negative. Root growth declined with rising soil temperature and moisture, while detritivore activity decreased with increasing soil moisture. Communities with taller plants showed cooler soil temperatures and higher soil moisture content. Interestingly, in contrast to spring observations, taller plant communities favored detritivore activity during summer while reducing plant greenness. Moreover, greenness was found to affect detritivore activity positively. As winter approached (Fig. 5c), richer plant communities showed enhanced root growth and soil moisture retention. Conversely, increased root growth and detritivore activity were observed in colder soils. Surprisingly, root production decreased detritivore activity during this season.
All models fitted the data well (Fisher’s C and p -values in Fig. 5). Plant height was only explained by a fixed term in the spring model (marginal R2 = 0.42), but its variation was also explained by the random terms (conditional R2 = 0.45). Greenness was explained only by the fixed terms (marginal and conditional R2 = 0.39 in the spring and R2 = 0.05 in the summer model). The variation of root production was greatly explained by fixed terms in all models (marginal R2 = 0.24, 0.32, and 0.20 for spring, summer, and winter, respectively). Still, the random terms increased explanatory power greatly (conditional R2 = 0.90, 0.75, and 0.64 for spring, summer, and winter, respectively). The variation in detritivore feeding activity explained by random terms decreased along the year (marginal R2 = 0.06 and 0.30 for spring and summer; conditional R2 = 0.23 and 0.37 for spring and summer, respectively), to the point that random terms increased explanatory power only slightly in winter (marginal and conditional R2 = 0.14 and 0.15, respectively).