Growth phenology adjusts to seasonal changes in water availability in coexisting evergreen and deciduous mediterranean oaks

Different leaf(evergreen vs.deciduous habit)and xylem(diffuse-vs.ring-porous wood)traits represent contrasting strategies to face seasonal changes in water availability and temperature.However,how contrasting leaf and xylem habits of coexisting tree species affect stem wood formation and tree-ring development remains poorly understood.Here,we investigated the spatio-temporal patterns of wood formation in two deciduous oaks(Quercus faginea and Quercus petraea)and two evergreen oaks(Quercus ilex and Quercus suber)coexisting in seasonally dry Mediterranean forests along an aridity gradient in Spain.We hypothesized that growth responses to drought and intra-and inter-annual growth patterns would differ between functional groups.We simulated intra-and interannual growth using a modified version of the Vaganov-Shashkin(VS)process-based,growth model.The VS model simulations were used to estimate growth changes under a high emission scenario(RCP 8.5)for the current distribution of the study oak species and to forecast their future performance under warm(4.8℃)conditions in the Iberian Peninsula.Our simulations indicate that climate warming would induce a shortening of the ringgrowth season and a reduction of radial growth in evergreen and deciduous Mediterranean oaks,particularly in dry sites from southern and eastern Iberia currently occupied by Q.ilex and Q.faginea.Evergreen oaks may better recover after dry periods than deciduous oaks by resuming growth after the summer drought.Low soil water availability in spring would be more detrimental to growth of deciduous oaks.Process-based growth models should be refined and validated to better forecast changes in tree growth as a function of climate.

Climatic and anthropogenic factors explain the variability of Fagus sylvatica treeline elevation in fifteen mountain groups across the Apennines

Background:Fagus sylvatica forms the treeline across the Apennines mountain range,with an average elevation of 1589 ma.s.l.Previous studies evidenced that the current position of the treeline in the Apennines is heavily depressed as a result of a complex interaction between climatic factors and the past human pressure.In this study we correlated treeline elevation in the fifteen major mountain groups in the Apennines with selected climatic,geomorphological,and human disturbance variables in order to investigate in detail the site-specific features affecting the current treeline distribution.Results:Treeline elevation was lowest in the North Italy(Apuan Alps),while the highest treeline was found in Central Italy(Simbruini).An absolute maximum treeline elevation of F.sylvatica exceeding 2000ma.s.l.was found on 13 mountain peaks in Central and Southern Italy.Noteworthy,treeline elevation was largely lower on warmer south-facing slopes compared to northern slopes,with values several hundred meters lower in the Gran Sasso and Velino-Sirente.Although the causes of this pattern are still unknown,we argue that treeline elevation on southfacing slopes may be limited by the combination of climatic constraints(i.e.summer drought)and human disturbance.Evidence of a pervasive anthropogenic effect depressing treeline elevation was found in the North(Apuan Alps)Central(Gran Sasso,Velino-Sirente,Sibillini)and Southern part of Apennines(Pollino).By contrast,treeline elevation of the Laga,Simbruini,and Orsomarso mountain groups appears less affected by past anthropogenic disturbance.Finally,we recorded in the several mountain groups(i.e.Majella,Marsicani and Pollino)the coexistence of very depressed treelines just a few kilometers away from much higher treelines,among the highest ever recorded for F.sylvatica.Conclusions:Finally,we argue that F.sylvatica treeline across the Apennines is locally shaped both by the interaction of low temperatures experienced by the species in its earliest life stages in snow-free open spaces with summer soil water depletion and human disturbance.