Rootable depth controls height growth of Pinus halepensis Mill. in gypsiferous and non-gypsiferous soils

• Height growth of P. halepensis is mainly controlled by soil rootable depth.
• Phosphorus availability also plays a minor role.
• Penetration resistance over 6.2 MPa restricts root development

Pinus halepensis is generally considered a species adapted to soils with gypsum but there is hardly any data available to support such statement nor to assess the degree to which soil gypsum may constrain tree development. We studied fifty five 200 m2-plots in a P. halepensis plantation in NE Spain, 23 on soils with gypsum and 32 on soils without gypsum. Trees were measured to estimate site index at age 40 years (SI40). A soil pit was described in each plot to a depth of 1 m or to a root-limiting layer, and samples of the various horizons analysed for pH, organic carbon (C), total nitrogen, Olsen phosphorus (P), exchangeable potassium (extracted with NH4OAc), calcium carbonate (calcimeter method), and gypsum concentration (Artieda method), and texture. We studied root development in the soil horizons of 15 of these plots by counting root numbers at depths of 0–30 cm, 30–55 cm, and 55–80 cm in three 100 cm2-squares per depth. Penetration resistance and bulk density were also measured in these horizons. Soils with gypsum were frequently less than 25 cm deep, and had negligible concentrations of Olsen phosphorus. Values of SI40, with a maximum of 15.5 m, were primarily determined, in all types of soils, by a positive effect of soil rootable depth, indicating the dominant influence of water availability, and to a lesser extent by the negative effect of the C/P ratio and rock fragment content in the upper 30 cm of soil. Density of fine and very fine roots decreased in deeper soil horizons from a maximum value of 97 roots·dm− 2 in the surface horizons. Mechanical impedance by increased penetration resistance was the main limitation for root development. Soil gypsum does not have a direct influence on growth but constrains the volume of soil that may be explored by roots.