Temperate forest ¹⁵N isotope trace experiments find nitrogen (N) addition‐driven carbon (C) uptake is modest as little additional N is acquired by trees; however, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked whether N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply ¹⁵N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional ¹⁵N additions typically trace mineral ¹⁵N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutrition, we conducted a mesocosm experiment, applying 54 g N/¹⁵N ha⁻¹ yr⁻¹ to Sitka spruce saplings. We compared tree and soil ¹⁵N recovery among treatments where enrichment was due to either (1) a ¹⁵N‐enriched litter layer, or mineral ¹⁵N additions to (2) the soil or (3) the canopy. We found that 60% of ¹⁵N applied to the canopy was recovered above ground (in needles, stem and branches) while only 21% of ¹⁵N applied to the soil was found in these pools. ¹⁵N recovery from litter was low and highly variable. ¹⁵N partitioning among biomass pools and age classes also differed among treatments, with twice as much ¹⁵N found in woody biomass when deposited on the canopy than soil. Stoichiometrically calculated N effect on C uptake from ¹⁵N applied to the soil, scaled to real‐world conditions, was 43 kg C kg N⁻¹, similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N⁻¹. Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.

Summary: A minuscule fraction of the Earth's paleobiological diversity is preserved in the geological record as fossils. What plant remnants have withstood taphonomic filtering, fragmentation, and alteration in their journey to become part of the fossil record provide unique information on how plants functioned in paleo‐ecosystems through their traits. Plant traits are measurable morphological, anatomical, physiological, biochemical, or phenological characteristics that potentially affect their environment and fitness. Here, we review the rich literature of paleobotany, through the lens of contemporary trait‐based ecology, to evaluate which well‐established extant plant traits hold the greatest promise for application to fossils. In particular, we focus on fossil plant functional traits, those measurable properties of leaf, stem, reproductive, or whole plant fossils that offer insights into the functioning of the plant when alive. The limitations of a trait‐based approach in paleobotany are considerable. However, in our critical assessment of over 30 extant traits we present an initial, semi‐quantitative ranking of 26 paleo‐functional traits based on taphonomic and methodological criteria on the potential of those traits to impact Earth system processes, and for that impact to be quantifiable. We demonstrate how valuable inferences on paleo‐ecosystem processes (pollination biology, herbivory), past nutrient cycles, paleobiogeography, paleo‐demography (life history), and Earth system history can be derived through the application of paleo‐functional traits to fossil plants.

Mediterranean grasslands are highly seasonal and co-limited by water and nutrients. In such systems, little is known about root dynamics which may depend on individual plant properties and environment as well as seasonal water shortages and site fertility. Patterns of root biomass and activity are affected by the presence of scattered trees, grazing, site management, and chronic nitrogen deposition, all of which can affect nutrient ratios and potentially cause development of nitrogen : phosphorus (N : P) imbalances in ecosystem stoichiometry. In this study we combined observations from minirhizotrons with root measurements from direct soil cores and ingrowth cores, along with measures of above-ground biomass to investigate seasonal root dynamics and root : shoot ratios in a Mediterranean tree–grass “savanna”. We investigated responses to soil fertility, using nutrient manipulation (N∕NP addition) and spatial microhabitat treatments between open-pasture and microhabitats under the tree canopy. Root dynamics over time were also compared with indices of above-ground growth drawn from proximal remote sensing. Results show distinct differences in root dynamics and biomass between treatments and microhabitats. Root biomass was higher with N additions, but did not differ from the control with NP additions in early spring. By the end of the growing season root biomass had increased with NP in open pastures but not higher than N added alone. In contrast, root length density (RLD) in pastures responded stronger to the NP than N-only addition, while beneath trees root biomass tended to be higher with only N. Even though root biomass increased, the root : shoot ratio decreased under nutrient treatments. Timing of root and shoot growth was reasonably well paired, although in autumn root growth appeared to be substantially slower than “regreening” of the system. We interpret these differences as a shift in community structure and/or root traits under changing stoichiometry induced by the fertilization. We also consider seasonal (phenology) differences in the strength and direction of effects observed.

Fundamental axes of variation in plant traits result from trade-offs between costs and benefits of resource-use strategies at the leaf scale. However, it is unclear whether similar trade-offs propagate to the ecosystem level. Here, we test whether trait correlation patterns predicted by three well-known leaf- and plant-level coordination theories – the leaf economics spectrum, the global spectrum of plant form and function, and the least-cost hypothesis – are also observed between community mean traits and ecosystem processes. We combined ecosystem functional properties from FLUXNET sites, vegetation properties, and community mean plant traits into three corresponding principal component analyses. We find that the leaf economics spectrum (90 sites), the global spectrum of plant form and function (89 sites), and the least-cost hypothesis (82 sites) all propagate at the ecosystem level. However, we also find evidence of additional scale-emergent properties. Evaluating the coordination of ecosystem functional properties may aid the development of more realistic global dynamic vegetation models with critical empirical data, reducing the uncertainty of climate change projections.

The responses of forest carbon dynamics to fluctuations in environmental conditions at a global scale remain elusive. Despite the understanding that favourable environmental conditions promote forest growth, these responses have been challenging to observe across different ecosystems and climate gradients. Based on a global annual time series of aboveground biomass (AGB) estimated from radar satellites between 1992 and 2018, we present forest carbon changes and provide insights on their sensitivities to environmental conditions across scales. Our findings indicate differences in forest carbon changes across AGB classes, with regions with carbon stocks of 50–125 MgC ha⁻¹ depict the highest forest carbon gains and losses, while regions with 125–150 MgC ha⁻¹ have the lowest forest carbon gains and losses in absolute terms. Net forest carbon change estimates show that the arc‐of‐deforestation and the Congo Basin were the main hotspots of forest carbon loss, while a substantial part of European forest gained carbon during the last three decades. Furthermore, we observe that changes in forest carbon stocks were systematically positively correlated with changes in forest cover fraction. At the same time, it was not necessarily the case with other environmental variables, such as air temperature and water availability at the bivariate level. We also used a model attribution method to demonstrate that atmospheric conditions were the dominant control of forest carbon changes (56% of the total study area) followed by water‐related (29% of the total study area) and vegetation (15% of the total study area) conditions. Regionally, we find evidence that carbon gains from long‐term forest growth covary with long‐term carbon sinks inferred from atmospheric inversions. Our results describe the contributions from the atmosphere, water‐related and vegetation conditions to forest carbon changes and provide new insights into the underlying mechanisms of the coupling between forest growth and the global carbon cycle.

Plant canopies intercept, process and potentially assimilate atmospheric nitrogen (N) additions, but the forest‐scale effects of canopy processes on N cycling and plant nutrition are not clear. Substantial method artefacts and scaling issues exist in previous experimental studies which measure relevant N fluxes either at (a) natural abundance, (b) via a ¹⁵N tracer or (c) by incorporation of additional ¹⁵N into measured biomass, meaning these processes are often overlooked or discounted. Here, working in a mature Sitka spruce plantation and under ambient conditions, we used all three of these methods independently at stand, tree and branch scale to assess the effects of canopy interception on capture, assimilation and incorporation of inorganic N deposition by canopy biomass. We consistently found that above 70% of N deposition was unaccounted for (i.e. assimilated, processed beyond detectability, volatilised or retained) when passing through the canopy, independent of the assessment method. Using short‐term ¹⁵N tracers, we found that this unaccounted N was retained in canopy tissues. Apparent uptake from monitoring of ecohydrological fluxes was sustained and consistent, suggesting that the tree canopies were a sink for N deposition. Seasonal variation in recovery also suggested biological activities influenced fluxes and transformations of this ion within the canopy. The total amount of this canopy N uptake was greater than the mean annual N in litterfall, implying that this flux was supporting the turnover of the canopy. When we used ¹⁵N to directly assess uptake into branch biomass we also did not find evidence for immediate relocation to other parts of the tree. Our findings suggest that at our N‐limited site, canopy uptake of N deposition is likely to be as important as root uptake of N but may be limited to supplying N to the canopy rather than the whole tree. Further research in this area is crucial to better understand the interactions of future changes in N deposition on primary production and carbon storage in forests. A free Plain Language Summary can be found within the Supporting Information of this article.