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Abstract:Studying species turnover along gradients is a key topic in tropical ecology. Crucial drivers, among others, are fog deposition and soil properties. In northern Peru, a fog-dependent vegetation formation develops on mountains along the hyper-arid coast. Despite their uniqueness, these fog oases are largely uninvestigated. This study addresses the influence of environmental factors on the vegetation of these unique fog oases. Accordingly, vegetation and soil properties were recorded on 66 4 × 4-m plots along an altitudinal gradient ranging from 200 to 950 m asl. Ordination and modelling techniques were used to study altitudinal vegetation belts and floristic composition. Four vegetation belts were identified: a low-elevation Tillandsia belt, a herbaceous belt, a bromeliad belt showing highest species richness and an uppermost succulent belt. Different altitudinal levels might reflect water availability, which is highest below the temperature inversion at around 700 m asl. Altitude alone explained 96% of the floristic composition. Soil texture and salinity accounted for 88%. This is in contrast with more humid tropical ecosystems where soil nutrients appear to be more important. Concluding, this study advances the understanding of tropical gradients in fog-dependent and ENSO-affected ecosystems.

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Studies on the effect of high atmospheric N deposition report inconsistent results on forest productivity and N cycling which might be related to P availability in soil and subsequently affect tree P nutrition. We wanted to test the effects of (i) site i.e., a P-poor versus a P-rich site and of (ii) fertilization (N, P, N+P) on inorganic P (Pi) and organic P (Po) concentrations as well as on biologically cycled phosphate (inferred from the O isotope signature after adding an 18 O-enriched label) in xylem sap. We measured Pi and Po concentrations and the O isotope signature in phosphate (δ 18 O Pi) in xylem sap of beech (Fagus sylvatica L.) trees two and 14 days after addition of 18 O-enriched water to the organic layer in a full factorial fertilization experiment (control, +N, +P, +NP) at two sites differing in P availability. Higher P concentrations in xylem sap at the P-rich than at the P-poor site originated from accelerated biological P cycling indicated by incorporation of 18 O from the isotope label into phosphate in xylem sap shortly after labeling. At this site, δ 18 O W values of xylem sap after label application remained close to background δ 18 O W values of soil solution. We speculate that in contrast to P uptake, trees took up water from deeper (non-18 O-labeled) soil layers. At the P-poor site, the 18 O label was recovered both in xylem sap water and phosphate in xylem sap, the latter only after 14 days. These results imply that trees relied on the organic layer for P acquisition and water uptake. However, biological processes associated with an incorporation of 18 O from the label were slower at the P-poor than at the P-rich site. P addition (P, NP) increased Pi concentrations in xylem sap at the P-rich site. Based on δ 18 O Pi values in xylem sap, the additional P originated both from the fertilizer and from accelerated biological P cycling in soil. We conclude that P-poor sites likely suffer more from climate change in case of an increased frequency of droughts because as opposed to P-rich sites both water and nutrient uptake will be affected.