Background/Question/Methods Oro-Iberian Festuca indigesta grasslands are alpine ridge-crest grasslands mainly on acidic soils to be found in the mountains of the Iberian Peninsula. It has been reported that this Mediterranean, endemic-rich habitat is declining due to global warming, but there is a lack of relevant, basic knowledge for conservation purposes on the ecological factors controlling its species diversity. A randomized, mensurative field experiment using twenty mountain peaks in two mountain ranges of the Northern Iberian Mountain System, analyzed using GLMs, was carried out with the aim of answering the question: What are the main factors controlling plant diversity? Therefore, the objectives were: to describe patterns of plant species diversity (Simpson’s diversity index, Whittaker plots), and ii) to explain the above patterns by means of the hypothesis that plant diversity can be explained by chemical and physical soil properties and by habitat descriptors.
Results/Conclusions The obtained values of the Simpson’s index ranged from 0.42 to 0.92. The fitted model explained c.60% of the variation of plant species diversity: it included i) log-transformed topsoil available phosphorus, and ii) gradient of slope, as ecological factors explaining plant diversity. Available phosphorus, ranging from 1.6 to 28.1 ppm (mg kg^-1), was the most influential factor and diversity and available phosphorus covaried linearly: an increase in phosphorus meant a decrease in plant diversity. Moreover, frequency of sheep droppings was negatively correlated with plant diversity (r = -0.446, p = 0.049) which suggests that sheep could be acting as a driver of phosphorus. Gradient, ranging from 2.5% to 23.5%, covaried linearly with plant diversity but, in this case, an increase in slope meant an increase in plant diversity. Furthermore, slope was positively correlated with the cover of stones and rocks (r = 0.547, p = 0.013) as well as with the cover of bare soil (r = 0.465, p = 0.039), suggesting that steeper mountains are more heterogeneous, and probably offer better colonization opportunities.