Inter-annual and inter-specific variability in response to natural variation in ambient solar radiation
There was a substantial difference in p CA concentrations in pollen grains from the same individual tree samples in 2013 and 2014, and between Pinus spp. (Table 2 and Fig. 2 a). On average, pollen from 2014 contained 31% more p CA than pollen from the same trees in 2013. The 95% credible interval of the differences between the two years means did not cross zero, providing strong statistical support that p CA abundance was different between the years for the individuals of all species tested (Pinus nigra, P. pinaster, P. sylvestris and P. uncinata ) (Fig. 2 b).
The measured solar radiation shows variation between the two collection years, particularly during the pollen development period, but not during the preceding growing season (Fig. 1). The total amount of solar radiation during the month (28 days) before pollen collection in May 2013 was 5214 W/m2 compared to 6401 W/m2 in 2014 and two months before was 9429 W/m2 in 2013 and 12191 W/m2 in 2014. Across both years, there was strong and consistent species-level variation, which can be partially accounted for by pollen size (Fig. 2 c, d). However, differences between species remained even after correcting for pollen size corrections, and Pinus pinaster still showed higher p CA ratios compared with the other species (Fig. 2 c, d).