Pinus ponderosaalters nitrogen dynamics and diminishes the climate footprint in natural ecosystems ofPatagonia

Abstract

SummaryUnderstanding climate effects on plant–soil interactions in terrestrial ecosystems remains challenging due to the fact that floristic composition covaries with climate, particularly along rainfall gradients. It is difficult to separate effects of precipitationper sefrom those mediated indirectly through changes in species composition. As such, afforestation (the intentional planting of woody species) in terrestrial ecosystems provides an ecological opportunity to assess the relative importance of climate and vegetation controls on ecosystem processes.We investigated the impacts of 35 years of afforestation on ecosystemNdynamics, in ecosystems ranging from arid shrub‐steppe to closed‐canopy forest inPatagonia,Argentina. Sites of natural vegetation and adjacent sites planted with a single exotic species,Pinus ponderosa,were identified in five precipitation regimes along a continuous gradient of 250–2200 mm mean annual precipitation (MAP). We evaluatedCandNparameters of vegetation and soil, as well as natural abundance of13Cand15Nin leaves, roots, ectomycorrhizae (EcM) and soils.In natural vegetation, most leaf traits (%N,C:Nratios, leaf mass per area, δ15Nvalues) demonstrated strong significant relationships withMAP, while these relationships were nearly absent in afforested sites. In addition, theEcMof native southern beech and pine trees were significantly enriched in15Nrelative to leaves at all sites where they were present. While soilCandNpools in both vegetation types increased withMAP, overall pool sizes were significantly reduced in afforested sites.Synthesis. Observed relationships between leaf traits and precipitation in natural vegetation may be driven largely by shifts in species composition and plant–soil interactions, rather than direct effects of precipitation. Our results suggest that a change in the species composition of the dominant vegetation is sufficient to alterCandNcycling independently of climate constraints: pine afforestation homogenizedNdynamics across sites spanning an order of magnitude ofMAP. These results highlight the important control of ectomycorrhizal associations in affectingCandNdynamics. Additionally, they serve to demonstrate that altering natural species composition alone is sufficient to cause large, detectable impacts onNturnover independently of direct climate effects.