Linking field performance to initial morphological traits of thuja seedlings in a semiarid Mediterranean area

Forest seedling size is still a subject of debate in plant production for reforestation programs in a semiarid Mediterranean area. The aim of the study was to assess the effect of initial morphological traits of Tetraclinis articulata (Vahl.) Mast. containerized seedlings on survival rate and growth performance after 11 months of outplanting in a semiarid Mediterranean site. The rooting depth of dead and alive seedlings was also explored to determine the effective depth for survival optimization and consequently to characterize the critical minimal soil water content necessary for seedlings to overcome drought season. The survival probability of T. articulata seedlings was strongly influenced by the initial seedling collar diameter, showing an increasing pattern with a critical minimum of 3.24 mm. The sturdiness index affected the survival probability, showing a trend downwards with a critical maximum of 5.93 cm mm⁻¹. The shoot height relative growth rate of the seedlings was significantly affected by the initial morphological class, giving more advantage to small seedlings, while the collar diameter relative growth seemed to be insensitive to the initial seedling size. The evaluation of the rooting depth in relation to seedling survival allowed the characterization of a minimum depth of 57.74 cm that the roots must exceed in order to reach the wettest soil layers; this soil depth corresponds to a minimum critical gravimetric water content of 12% below which the roots cannot survive. These results suggest that in semiarid regions, where rainfall is scarce and soil moisture is uncertain, planting shortstemmed seedlings with a large collar diameter can enhance survival and shoot growth during the first year after planting.