Onset of reproduction in rhododendron lapponicum shoots: The effect of shoot size, age, and nutrient status at two subarctic sites

Abstract

Potential factors that trigger the onset of reproduction in shoot modules were studied in the dwarf shrub Rhododendron lapponicum in subarctic Swedish Lapland. The following questions were addressed: (1) Can the large size effect that commonly is found on onset of reproduction be explained as an effect of the amount of nitrogen and/or phosphorus accumulated? (2) Is there an effect of age when controlling for size and amount of nutrient reserves? (3) Does the relationship between size or nutrient reserves and onset of reproduction vary between high- and low-altitude sites? Shoots from two different sites, a subalpine heath (345 m altitude) and an alpine heath (900 m), were compared. The reproductive status of the shoots (non-reproductive or with an inflorescence bud) was related to shoot age, size (shoot leaf area), and nitrogen and phosphorus concentration by means of logistic regression analysis. The analysis indicates that an index of shoot nitrogen pool size was the most important factor; however, we could not strictly differentiate between the effect of this index and that of shoot leaf area and nitrogen concentration. A large shoot leaf area and nitrogen concentration (i.e. a large nitrogen pool index) increases the likelihood of turning reproductive. When controlling for shoot leaf area and nitrogen concentration a negative effect of increasing age was found. We suggest that this age effect may be related to a declining shoot resource economy with increasing age. Triggering of reproduction in R. lapponicum is thus complex and all factors included in our analysis appear to be involved. Shoots at the high-altitude site were more likely to enter the reproductive phase than shoots with a similar leaf area at low altitude. The only factor for which we found support for the hypothesis about decreasing importance of nutrients with increasing altitude was for phosphorus concentration. The mean age at onset of reproduction did not differ between altitudes.