The changing flowering phenology of Crinum lilies in arid and semi-arid regions: implications for phenological responses to climate change

Herbarium data have availed the much-needed long-term data that have constrained long-termand real-time phenological studies. Using herbarium specimens from the National Herbarium of Namibia, species, region, rainfall, latitude and longitude were regressed against the day of flowering to determine the effect of each variable on the day of flowering. This study used parameter estimates and the odds ratios from ordinal regression analysis to interpret the influence of the explanatory variables on the day of flowering. The results revealed that flowering dates of C. lugardiae have advanced by six days/decade during the period 1950 to 2010. Average summer rainfall decreased by approximately 3 mm per year from 1960 to 2015 in Namibia. Consequently, significant rainfall effects were recorded on the day of flowering for C. buphanoides, C. stuhlmannii subsp. delagoense and C. paludosum which have advanced by approximately 1 day/decade/mm, 1 day/decade/mm and 2 day/decade/mm during the period studied, respectively. Increase in latitude delayed day of flowering whilst an advancement of the day of flowering with an increase in longitude was observed. Substantial interaction effects among the explanatory variables were demonstrated. Combined effects of rainfall and latitude had a negative effect on the day of flowering whereas interaction effects of average summer rainfall and longitude had a positive combined effect on day of flowering. Our results, therefore, reveal the complex effects of rainfall and geographical location on flowering phenology of different Crinum species. The potential impacts of the phenological responses of different Crinum species on other organisms are discussed.