Bringing the concept of ammonia critical levels into managing cork-oak woodland for conservation

Agricultural activities often emit excessive nitrogen concentrations to nearby habitats, impacting ecosystems structure and functioning. To protect them, critical levels were established as the concentration above which direct adverse effects on ecosystems may arise. However, due to lack of tools, critical levels are seldom applied in management strategies for conservation. Our objective was to provide a spatial-explicit tool to incorporate the concept of ammonia critical levels in management practices of atmospheric nitrogen impacts. To do that, lichens diversity was sampled in 19 plots at a cork oak woodland (Montado), a High Nature Value Farmland where multiple agricultural activities, surrounded by high-intensity agricultural activities, coexist. We inferred and selected the best performing trait-based metrics and determined the main environmental drivers of change. The most responsive metric was the abundance of oligotrophic lichen species, which was likely responding to nitrogen incoming from nearby high-intensity agricultural areas. The low intensity agricultural activities, cattle grazing within the study area, had no effect. Using the known ammonia critical levels for that ecosystem, we modelled and mapped the long-term critical levels exceedance for the entire study area. This allowed us to signal high intensity agriculture as the source of impact and to map the areas where management practices need further attention under a conservation perspective: either decreasing fertilization in nearby fields or by considering these as buffer areas, outside the scope of conservation.