Phosphorus limitation and competitive capacities of Pinus halepensis and Quercus ilex subsp. rotundifolia on different soils

Abstract

Aleppo pine (Pinus halepensis) and the evergreen holm oak (Quercus ilex) dominate forest areas of the Mediterranean Basin. Both species regenerate abundantly after fires: pine through seedlings and holm oak through resprouts. Cumulative nutrient losses caused by frequent fires may have decreased soil nutrient availability in such areas. To assess the role of nitrogen and phosphorus as limiting factors for growth of these species during post-fire recovery, a field fertilisation and competition experiment was conducted in a 5-year post-fire shrubland on calcareous soil, where naturally-regenerated saplings of Aleppo pine and resprouts of interior holm oak (Quercus ilex subsp. rotundifolia) coexist. Three years after fertilisation, relative basal area increment was 56% greater in pines fertilised with 250 kg P ha–1 than in non fertilised ones. N fertilisation had small or no effects. Interactions between N and P fertilisation were not observed. Growth of Aleppo pine only increased with P fertilisation when neighbours were removed. Hence, the negative effect of neighbours on growth was greater when P availability was enhanced by fertilisation. In contrast, holm oak was able to grow more (110%) in response to increased P supply even without neighbour removal. A common garden experiment was then conducted with potted seedlings to investigate whether the suggested higher competitive capacity of holm oak for P held under a range of P amendments on different soils and competitive situations. P fertilisation increased seedling biomass yield of both species. When P availability increased, a negative effect of neighbours on growth was observed for holm oak and in 70 a lesser extent for Aleppo pine. In conclusion, in the field, holm oak resprouts showed higher competitive ability for P uptake compared to Aleppo pine saplings, but in potted seedlings in common garden conditions this trend was not observed. Therefore holm oak is not always competitively superior to Aleppo pine for P. Potted seedlings of both species had a notable plasticity in shoot/root biomass allocation, but only holm oak increased its proportional allocation to roots when neighbours were present. P availability can be a key factor in growth and competitive relations of these two species, but effects differ depending on soil type, individual age, regeneration type (i.e., seedling versus resprouts), and competitive situation.