Significance: Advanced spring flowering has been described as a fingerprint of climate change—a public, visible display of the detrimental effects of global warming. However, warming experiments fail to account for the full magnitude of observed changes in phenology, suggesting that other factors may play important roles. We show that peak flowering time shifts earlier for most species when we experimentally reduce plant diversity. Additionally, peak flowering times of plant species are more evenly distributed across the season in high-diversity plots. Overall, these results demonstrate the importance of biotic interactions in influencing flowering times and indicate that advancing phenology, one of the most well-described and well-publicized phenomena linking global warming to plant communities, may result equally from biodiversity declines.

The Walker and Syers model of phosphorus (P) transformations during pedogenesis is widely accepted for the development of humid ecosystems, but long‐term P dynamics of more arid ecosystems remain poorly understood. We tested the Walker and Syers model in semiarid piñon–juniper woodlands by measuring soil P fractions under tree canopies and in intercanopy spaces along a well‐constrained, ∼3000 ka (1 ka = 1000 years) volcanic substrate age gradient in northern Arizona, USA. The various pools of soil P behaved largely as predicted; total soil P and primary mineral P declined consistently with substrate age, labile inorganic P increased early in soil development and then declined at later stages, and organic phosphorus increased consistently across the chronosequence. Within each site, soils under tree canopies tended to have higher concentrations of labile and intermediately available P fractions compared to intercanopy soils. However, the degree of spatial heterogeneity conferred by tree islands was moderated by the stage of soil development. In contrast, tree islands had no influence on within‐site distribution of more recalcitrant soil P pools, which appear to be controlled solely by the stage of pedogenesis. Coincident with declines in total P, primary mineral P, and labile inorganic P, we found that phosphatase enzyme activity increased with substrate age; a result consistent with greater ecosystem‐level P demand on older, more highly weathered substrates. Our results suggest that, compared to humid climates, reduced inputs of water, energy, and acidity to semiarid ecosystems slow the rate of change in P fractions during pedogenesis, but the overall pattern remains consistent with the Walker and Syers model. Furthermore, our data imply that pedogenic change may be an important factor controlling the spatial distribution of labile P pools in semiarid ecosystems. Taken together, these data should both broaden and unify terrestrial ecosystem development theory.

Summary: Previous studies in humid forests have shown that the ¹³C and ¹⁵N isotopic composition differs between ectomycorrhizal (ECM) and saprotrophic (SAP) fungi, and that this ECM–SAP ‘divide’ may provide a useful tool for evaluating fungal trophic status. We evaluated whether this method could delineate the trophic status of fungi in two semiarid, temperate forests of the south‐western USA. This technique could be particularly valuable in arid regions where the functional roles of fungi can be difficult to assess because of infrequent sporocarp production. Our data were consistent with the existence of an ECM–SAP divide, although δ¹³C values were more useful than δ¹⁵N values in separating trophic status. Saprotrophic fungi consistently had higher δ¹³C values than their presumed substrates; however, the degree of ¹⁵N enrichment in SAP sporocarps was highly variable. Comparison of 11 sporocarp species common to both sites showed that δ¹⁵N values were higher in one of the forests, even though the δ¹⁵N values of foliage from common understorey and overstorey trees were similar between forests. We conclude that assessment of the isotopic compositions of fungal sporocarps and their substrates is helpful for elucidating ecological relationships in semiarid forests.