Towards the interior of the peninsula, these forests become progressively more scarce: as the continental characteristics of the climate become stronger, the species most sensitive to cold become steadily more scarce. The continental groves, on soils lacking lime (calcium oxide), tend to be rich in junipers (Juniperus oxycedrus) and are superseded at higher altitudes and on cooler slopes by Pyrenean Oaks. This phenomenon is apparent in the Sierra de Guadarrama: when the oak forests have been destroyed, the soil is so poor and the environmental conditions so unfavourable, that it leads to ragged thickets dominated by common rock rose, Spanish lavender and rosemary. On limy soils something similar takes place, above all at altitudes of over 900 metres, oaks are accompanied by Spanish juniper (Juniperus thurifera) and the scarcity of shrubs is such that the same Holm oak (Q. ilex subsp. rotundifolia or ballota) dominates almost entirely on its own the first phases of deterioration of the forest. The degradation caused by burning or felling leads to thickets of Scorpion's thorn (Genista scorpius), thyme and common lavender (Lavandula angustifolia).