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

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