Size Structure in Populations of Two Threatened Endemic Plant Species of the Canary Islands: Cistus osbaeckiaefolius and Helianthemum juliae

According to the new IUCN categories, Cistus osbaeckiaefolius Webb ex Christ. is Vulnerable (VU) and Helianthemum juliae Wildpret is Critically Endangered (CR); both of these plant species are endemic to the Canary Islands (Tenerife). In order to develop appropriate management measures and plan future actions for these two species, we analyzed the size structure of their populations using two easily measured parameters that correlate with age—height and biovolume. Simple mathematical models were fitted to describe the population structure of each species. Cistus osbaeckiaefolius occurs in five populations in the high mountains of Tenerife Island. Four of the populations have 184, 134, 189, and 210 individuals. In a partial sample of the fifth population we counted 195 adults and 12,426 seedlings in a 400-m² area. Populations size structure (based on both height and biovolume) differed significantly among the five populations (p < 0.05, F= 0.00). Size structure of three of the populations was best described by a negative exponential function; for the other two a power function gave a better result. In all five populations the height parameter was the better independent variable. Assuming a relationship between height and age, all populations of C. osbaeckiaefolius are stable with a constant recruitment rate. The two populations of Helianthemum juliae, occurring in the high mountains of Tenerife Island, consist of 238 and 24 individuals. Size structure of the two populations differed significantly in height (p < 0.05, F= 0.04) but not in biovolume (p < 0.05, F= 0.08 ). Population structures of both populations were best described by a negative exponential function, and height classes gave better results. Again assuming a relationship between height and age, we suspect that the population of 24 individuals is unstable; we observed a small correlation coefficient (r = 0.61, p = 0.000), which suggests the existence of some limiting factors in recruitment.