Human societies depend on an Earth System that operates within a constrained range of nutrient 68 availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. 69 Examining patterns of foliar N concentrations ([N]) and isotope ratios (δ 15 N) from more than 42,000 samples acquired over years, here we show that foliar [N] declined by 8% and foliar δ 15 N declined by 0.8 -1.9 ‰. Examining patterns across different climate spaces, foliar δ 15 N declined across the entire range of MAT and MAP tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ 15 N of tree rings and leaves from herbarium samples over the past 75-150 years. 76These patterns are consistent with the proposed consequences of elevated atmospheric CO 2 and longer growing seasons. These declines will limit future terrestrial C uptake and increase nutritional stress for herbivores. 235 much. Preventing these declines in N availability further emphasizes the need to reduce 236 anthropogenic CO 2 emissions.Data and code availability. The datasets generated during and/or analysed during the current study are available in the Dryad repository [link to be generated upon acceptance]. All code used for statistical analyses and figure generation are available on Dryad (XXX).

The stability of ecological communities is critical for the stable provisioning of ecosystem services, such as food and forage production, carbon sequestration, and soil fertility. Greater biodiversity is expected to enhance stability across years by decreasing synchrony among species, but the drivers of stability in nature remain poorly resolved. Our analysis of time series from 79 datasets across the world showed that stability was associated more strongly with the degree of synchrony among dominant species than with species richness. The relatively weak influence of species richness is consistent with theory predicting that the effect of richness on stability weakens when synchrony is higher than expected under random fluctuations, which was the case in most communities. Land management, nutrient addition, and climate change treatments had relatively weak and varying effects on stability, modifying how species richness, synchrony, and stability interact. Our results demonstrate the prevalence of biotic drivers on ecosystem stability, with the potential for environmental drivers to alter the intricate relationship among richness, synchrony, and stability.

Plant phenology is expected to be sensitive to climate warming. In boreal trees, spring flush is primarily temperature driven, whereas height growth cessation and autumn leaf senescence are predominantly controlled by photoperiod. Cuttings of 525 genotypes from the full range of balsam poplar were planted into two common gardens (Vancouver and Indian Head, Canada) at similar latitudes, but with differing winter temperatures and growing seasons. There was clinal variation in spring and, particularly, summer and fall phenology. Bud flush and, despite milder climate, bud set and leaf drop were earlier at Vancouver than at Indian Head by 44, 28 and 7 d, respectively. Although newly flushed growth is insensitive to photoperiod, many genotypes at both sites became competent before the summer solstice. At Vancouver, high-latitude genotypes set dormant terminal buds in mid-spring. Most other genotypes grew until midsummer or set bud temporarily and then experienced a second flush. In both gardens and in a growth chamber experiment, earlier bud set was associated with reduced height growth and higher root/ shoot ratios. Shoots attained competency~5 weeks after flushing, which would normally prevent dormancy induction before the solstice, but may be insufficient if spring advances by more than a few weeks.