Effect of nitrogen fertilization in the nursery on the drought and frost resistance of Mediterranean forest species

The objective of this study was to assess the effect of N fertilization in the nursery on frost and water stress resistance of seedlings in Mediterranean forest species. We reviewed the data of six independent fertilization experiments that were performed between 1997 and 2003 in six Mediterranean forest species: three oaks (Quercus suber L., Q. ilex L., and Q. coccifera L.), and three conifer (Pinus pinea L., P. halepensis Mill., and Juniperus thurifera L.). Plants were cultivated under two contrasting N fertilization regimes and at the end or during the cultivation period several parameters related to drought and frost resistance were measured. N fertilization affected more the morphological than the physiological characters. Changes in most morphological traits in response to N fertilization tended to have the same variation pattern, whereas physiological traits had variable responses. N Fertilization reduced frost hardiness in Pinus species and increased the osmotic potential both at full turgor and at turgor loss point in J. thurifera. High-fertilized (HN) seedlings in all species were larger and had greater shoot to root mass ratio than low-fertilized or unfertilized (LN) plants. HN pine seedlings also had higher stomatal conductance than LN plants. These characteristics might impair the water balance of HN plants if soil water content remains low immediately after transplanting. In contrast, HN plants showed higher new root growth capacity than LN seedlings and the proportion of new roots emerging from the plug with respect to shoot size did not differ between fertilization treatments or it was greater in HN plants than in LN seedlings. These responses could improve the efficiency of roots to explore the soil and, therefore, the drought avoiding capacity of HN plants.