Fertilization and allelopathy modify Pinus halepensis saplings crown acclimation to shade

Pinus halepensis Mill., is a Mediterranean pioneer forest species with shade-intolerant features. The purpose of this study is to better understand how stand fertility and allelopathic properties of adult trees influence shade acclimation of saplings. Crown growth and morphological plasticity were studied under different light, fertilization, and allelopathic conditions in a nursery experiment. We tested whether shade-acclimation capacity increases with fertilization, and is affected by autotoxicity due to pine leachates. We examined stem diameter, and crown characteristics (length, width, shape, and density) in a factorial experiment with two levels for each tested factor: light (full and 20% reduced light), fertilization (low and high rate of NPK fertilizer) and allelopathy (control and allelopathic leachates uptake). In our study, shading induced a significantly higher crown length, width, and surface. Fertilization strongly increased crown length and vertical expended crown shape (the ratio crown length/crown width). Leachates uptake reduced crown length and density, highlighting an autotoxicity phenomenon. We concluded that P. halepensis saplings presented a shade-avoiding syndrome and that the crown shade-acclimation response increased with fertilization but was severely compromised by autotoxicity. We finally discuss the role of fertilization and allelopathy in early P. halepensis acclimation ability.