Seedling survival responses to irradiance are differentially influenced by low-water availability in four tree species of the Iberian cool temperate–Mediterranean ecotone

Abstract

Abstract Inter-specific differences in seedling survival responses along a sun-shade gradient and the influence of low-water availability were examined for four Iberian tree species (Q uercus robur L., Quercus pyrenaica Willd., Pinus sylvestris L. and Pinus pinaster Ait.) typical of the cool temperate–Mediterranean transition zone. Seedlings were grown under controlled conditions in a factorial experiment with four levels of irradiance (1%, 6%, 20% and 100% of full sunlight) and two levels of water availability. Five censuses (from late spring to autumn) leading to four regular intervals (T 0 → T I ; T I → T II ; T II → T III ; T III → T IV ) were established. Statistical models of seedling survival as a function of irradiance were calibrated throughout the whole experiment (T 0 → T IV ) and also for each time interval and water availability level. Seedling survival responses among different species diverged both in the type of functional response to irradiance and in their response to water stress. Ranking of species according to shade tolerance ( Q. pyrenaica > Q. robur > P. sylvestris > P. pinaster ) contrasted with tolerance of high irradiance and conformed to a hypothetical sun-shade trade-off for survival (i.e. species having higher survival in low irradiance—oaks—had poorer survival at high irradiance and vice-versa). Low-water availability also differentially affected each species, with pines being more drought tolerant than oaks. At an intra-specific level, low-water availability decreased survival of Q. pyrenaica under both high and low irradiance. For Q. robur , however, low-water availability exerted a relatively stronger effect under low irradiance. Consequences of the interplay between irradiance and water availability for explaining segregation and coexistence of forest tree species at the ecotone between cool temperate and Mediterranean forests are discussed.