Influence of bird-induced soil fertility gradients on oomycete distribution in a threatened Quercus suber population

Abstract

Cork oaks are considered keystone structures in the ecosystem of the stabilised sands of the Doñana National Park. These remnant big trees are threatened by nesting of colonial waterbirds (whose debris induce deep soil chemical changes) and by two pathogenic oomycetes, Phytophthora cinnamomi and Pythium spiculum. We analyzed the distribution of seven oomycete species in soils along a bird influence gradient. Canonical analysis revealed a significant relationship between the composition of the oomycete community and soil chemical variables closely related to bird debris accumulation (soil N and P contents). Some Pythium spp. were able to live in heavily fertilized soils, while pathogenic species did not thrive in that soils. A separate analysis showed that the presence and abundance of P. cinnamomi and P. spiculum in the rizosphere was significantly lower in trees with moderate or high bird influence. We concluded that 1) natural fertilization by bird faeces induced soil changes affecting pathogenic oomycetes behaviour; 2) as long as these bird-induced soil changes were no too extreme to cause dieback by themselves, they could prevent the spread and tree decline events caused by soil pathogens.