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Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes., This work utilised the network of dendrometer observations established by the COST Action network STReESS (grant FP1106). We acknowledge the involved networks TreeNet, Swiss Long-term Forest Ecosystem Research Programme LWF, French National Network for Long-term FOrest ECOsystem Monitoring RENECOFOR, the German Long Term Ecosystem Research Network LTER-D, the Italian Long Term Ecosystem Research Network ILTER, the Integrated Carbon Observation System ICOS and Tree-Watch.net. R.L.S. acknowledges funding from the Special Research Fund (BOF) of Ghent University for Postdoctoral Fellowships and the Spanish Ministry of Science, Innovation, and Universities (Juan de la Cierva Programme, grant IJC2018-036123-I). R.L.P., R.Z., P.F., and G.v.A. acknowledge funding from the Federal Office for the Environment FOEN (00.0365.PZ I 0427-0562, 09.0064.PJ/R301-0223; project treenet.info) and the Swiss National Science Foundation SNF (20FI21_148992, 20FI_173691, P2BSP3_184475; project LOTFOR 150205 and Grant 20FI20_173691; project ICOS-CH). J.M.V. and M.M. acknowledge funding from the Spanish MINECO via competitive grants CGL2013-46808-R and CGL2017-89149-C2-1-R. R.P. acknowledges funding from the grant RTI2018-095297-J-I00 (Spain) and by a Humboldt Research Fellowship for Experienced Researchers (Germany).

Infection by multiple pathogens of the same host is ubiquitous in both natural and managed habitats. While intraspecific variation in disease resistance is known to affect pathogen occurrence, how differences among host genotypes affect the assembly of pathogen communities remains untested. In our experiment using cloned replicates of naive Plantago lanceolata plants as sentinels during a seasonal virus epidemic, we find non-random co-occurrence patterns of five focal viruses. Using joint species distribution modelling, we attribute the non-random virus occurrence patterns primarily to differences among host genotypes and local population context. Our results show that intraspecific variation among host genotypes may play a large, previously unquantified role in pathogen community structure., The factors that determine whether pathogens co-occur in a host are poorly understood, especially for plant viruses. Here the authors conduct field experiments with the plant Plantago lanceolata and its viruses, showing that viral co-occurrences are driven predominantly by environmental context and host genotype rather than viral interactions.

Yellow rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a devastating fungal disease threatening much of global wheat production. Race-specific resistance (R)-genes are used to control rust diseases, but the rapid emergence of virulent Pst races has prompted the search for a more durable resistance. Here, we report the cloning of Yr15, a broad-spectrum R-gene derived from wild emmer wheat, which encodes a putative kinase-pseudokinase protein, designated as wheat tandem kinase 1, comprising a unique R-gene structure in wheat. The existence of a similar gene architecture in 92 putative proteins across the plant kingdom, including the barley RPG1 and a candidate for Ug8, suggests that they are members of a distinct family of plant proteins, termed here tandem kinase-pseudokinases (TKPs). The presence of kinase-pseudokinase structure in both plant TKPs and the animal Janus kinases sheds light on the molecular evolution of immune responses across these two kingdoms., Yellow rust fungus severely limits global wheat production and breeding of durable resistance is challenging. Here Klymiuk et al. isolate the broad-spectrum Yr15 resistance gene from wild emmer wheat and show that it is a member of a distinct tandem kinase-pseudokinase family of plant proteins.

Co-infections by multiple pathogen strains are common in the wild. Theory predicts co-infections to have major consequences for both within- and between-host disease dynamics, but data are currently scarce. Here, using common garden populations of Plantago lanceolata infected by two strains of the pathogen Podosphaera plantaginis, either singly or under co-infection, we find the highest disease prevalence in co-infected treatments both at the host genotype and population levels. A spore-trapping experiment demonstrates that co-infected hosts shed more transmission propagules than singly infected hosts, thereby explaining the observed change in epidemiological dynamics. Our experimental findings are confirmed in natural pathogen populations—more devastating epidemics were measured in populations with higher levels of co-infection. Jointly, our results confirm the predictions made by theoretical and experimental studies for the potential of co-infection to alter disease dynamics across a large host–pathogen metapopulation., Co-infection of plants with multiple pathogen strains is predicted to alter disease dynamics. Here, Susi et al. use experimental and natural population data to show that co-infected host plants spread more disease and cause more devastating epidemics than singly infected hosts.

The microbial communities living in the human intestine can have profound impact on our well-being and health. However, we have limited understanding of the mechanisms that control this complex ecosystem. Here, based on a deep phylogenetic analysis of the intestinal microbiota in a thousand western adults, we identify groups of bacteria that exhibit robust bistable abundance distributions. These bacteria are either abundant or nearly absent in most individuals, and exhibit decreased temporal stability at the intermediate abundance range. The abundances of these bimodally distributed bacteria vary independently, and their abundance distributions are not affected by short-term dietary interventions. However, their contrasting alternative states are associated with host factors such as ageing and overweight. We propose that the bistable groups reflect tipping elements of the intestinal microbiota, whose critical transitions may have profound health implications and diagnostic potential., Intestinal microbes can have important effects on our health. Here, the authors analyse the gut microbiota composition in 1,000 western adults and find that certain bacteria are either abundant or nearly absent, and that these alternative states are associated with ageing and overweight.

While the negative effects that pathogens have on their hosts are well-documented in humans and agricultural systems, direct evidence of pathogen-driven impacts in wild host populations is scarce and mixed. Here, to determine how the strength of pathogen-imposed selection depends on spatial structure, we analyze growth rates across approximately 4000 host populations of a perennial plant through time coupled with data on pathogen presence-absence. We find that infection decreases growth more in the isolated than well-connected host populations. Our inoculation study reveals isolated populations to be highly susceptible to disease while connected host populations support the highest levels of resistance diversity, regardless of their disease history. A spatial eco-evolutionary model predicts that non-linearity in the costs to resistance may be critical in determining this pattern. Overall, evolutionary feedbacks define the ecological impacts of disease in spatially structured systems with host gene flow being more important than disease history in determining the outcome.

The role of trees in the nitrous oxide (N2O) balance of boreal forests has been neglected despite evidence suggesting their substantial contribution. We measured seasonal changes in N2O fluxes from soil and stems of boreal trees in Finland, showing clear seasonality in stem N2O flux following tree physiological activity, particularly processes of CO2 uptake and release. Stem N2O emissions peak during the vegetation season, decrease rapidly in October, and remain low but significant to the annual totals during winter dormancy. Trees growing on dry soils even turn to consumption of N2O from the atmosphere during dormancy, thereby reducing their overall N2O emissions. At an annual scale, pine, spruce and birch are net N2O sources, with spruce being the strongest emitter. Boreal trees thus markedly contribute to the seasonal dynamics of ecosystem N2O exchange, and their species-specific contribution should be included into forest emission inventories. Forest soil is known to be a source of the greenhouse gas N2O, but the impact of what is planted in that soil has long been overlooked. Here Machacova and colleagues quantify seasonal N2O fluxes from common boreal tree species in Finland, finding that all trees are net sources of this gas.

Boreal forests are ecosystems with low nitrogen (N) availability that store globally significant amounts of carbon (C), mainly in plant biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms controlling boreal C and N pools are not well understood. Here, using a three-year field experiment, we compare SOM decomposition and stabilization in the presence of roots, with exclusion of roots but presence of fungal hyphae and with exclusion of both roots and fungal hyphae. Roots accelerate SOM decomposition compared to the root exclusion treatments, but also promote a different soil N economy with higher concentrations of organic soil N compared to inorganic soil N accompanied with the build-up of stable SOM-N. In contrast, root exclusion leads to an inorganic soil N economy (i.e., high level of inorganic N) with reduced stable SOM-N build-up. Based on our findings, we provide a framework on how plant roots affect SOM decomposition and stabilization., Understanding mechanisms of soil organic matter (SOM) decomposition and stabilisation improves soil-climate feedback predictions. Here the authors show that roots in boreal forest promote organic nitrogen economy and provide a framework on how roots affect decomposition and stabilisation of SOM.