Metatranscriptomics sheds light on the links between the functional traits of fungal guilds and ecological processes in forest soil ecosystems.

Soil fungi belonging to different functional guilds, such as saprotrophs, pathogens, and mycorrhizal symbionts, play key roles in forest ecosystems. To date, no study has compared the actual gene expression of these guilds in different forest soils. We used metatranscriptomics to study the competition for organic resources by these fungal groups in boreal, temperate, and Mediterranean forest soils. Using a dedicated mRNA annotation pipeline combined with the JGI MycoCosm database, we compared the transcripts of these three fungal guilds, targeting enzymes involved in C- and N mobilization from plant and microbial cell walls. Genes encoding enzymes involved in the degradation of plant cell walls were expressed at a higher level in saprotrophic fungi than in ectomycorrhizal and pathogenic fungi. However, ectomycorrhizal and saprotrophic fungi showed similarly high expression levels of genes encoding enzymes involved in fungal cell wall degradation. Transcripts for N-related transporters were more highly expressed in ectomycorrhizal fungi than in other groups. We showed that ectomycorrhizal and saprotrophic fungi compete for N in soil organic matter, suggesting that their interactions could decelerate C cycling. Metatranscriptomics provides a unique tool to test controversial ecological hypotheses and to better understand the underlying ecological processes involved in soil functioning and carbon stabilization.

ENSO and NAO affect long-term leaf litter dynamics and stoichiometry of Scots pine and European beech mixedwoods.

Litterfall dynamics (production, seasonality and nutrient composition) are key factors influencing nutrient cycling. Leaf litter characteristics are modified by species composition, site conditions and water availability. However, significant evidence on how large-scale, global circulation patterns affect ecophysiological processes at tree and ecosystem level remains scarce due to the difficulty in separating the combined influence of different factors on local climate and tree phenology. To fill this gap, we studied links between leaf litter dynamics with climate and other forest processes, such as tree-ring width (TRW) and intrinsic water-use efficiency (iWUE) in two mixtures of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) in the south-western Pyrenees. Temporal series (18 years) of litterfall production and elemental chemical composition were decomposed following the ensemble empirical mode decomposition method and relationships with local climate, large-scale climatic indices, TRW and Scots pine's iWUE were assessed. Temporal trends in N:P ratios indicated increasing P limitation of soil microbes, thus affecting nutrient availability, as the ecological succession from a pine-dominated to a beech-dominated forest took place. A significant influence of large-scale patterns on tree-level ecophysiology was explained through the impact of the North Atlantic Oscillation (NAO) and El Niño-Southern Oscillation (ENSO) on water availability. Positive NAO and negative ENSO were related to dry conditions and, consequently, to early needle shedding and increased N:P ratio of both species. Autumn storm activity appears to be related to premature leaf abscission of European beech. Significant cascading effects from large-scale patterns on local weather influenced pine TRW and iWUE. These variables also responded to leaf stoichiometry fallen 3 years prior to tree-ring formation. Our results provide evidence of the cascading effect that variability in global climate circulation patterns can have on ecophysiological processes and stand dynamics in mixed forests.

Thinning affects nutrient resorption and nutrient-use efficiency in two Pinus sylvestris stands in the Pyrenees.

Needle chemical composition was measured, and nutrient resorption, nutrient-use efficiency (NUE), and other indexes were estimated for 24 months in two contrasting natural Pinus sylvestris L. forests in the western Pyrenees in Spain. For each location (Aspurz, 650 m elevation, 7% slope; Garde, 1335 m elevation, 40% slope), there were three reference plots (P0), three plots with 20% of the basal area removed (P20), and three with 30% of the basal area removed (P30). Needle P, Ca, and Mg concentrations were higher in Garde, but N concentration was higher for Aspurz, without differences for K. Nutrient-resorption efficiency of P was higher in Aspurz, of Mg higher in Garde, and there were no differences between sites in N and K. Nutrient-resorption proficiency was significantly higher in the site with lower soil nutrient availability, i.e., for P, Ca, and Mg in Aspurz, but N in Garde (no differences in K); this may be an indicator of nutrient conservation strategy. Annual nutrient productivity (A) was higher for all nutrients in Aspurz, whereas the mean residence time (MRT) was higher in Garde in all nutrients but P. NUE was significantly higher in Garde for all nutrients but P, which was more efficiently used in Aspurz. In both sites, N, P, and K concentrations were higher in the 2002 cohort, Ca in the 2000 cohort, and maximum Mg was found in the 2001 cohort. Thinning caused a reduction of Mg concentration in the 2001 cohort in Aspurz, an increase of Ca resorption proficiency in Aspurz and Mg resorption at both sites, and reduction of P, K, and Mg nutrient response efficiency (NRE) in Garde. Thinning may have caused an increase of the C:Mg ratio through facilitating the development of more biosynthesis apparatus in a more illuminated canopy, but it seemed not to affect resorption in a significant way. Changes in NRE in Garde after thinning show that forest management can affect how trees use nutrients. Our results indicate that the strategy to optimize NUE is different in each stand. In Aspurz (a Mediterranean ecosystem), pine trees carried out resorption more efficiently, while in Garde (a continental forest), trees used nutrients for longer periods of time and reduced their efficiency in using the available soil nutrients after reduced competition by thinning.