Rapid juvenile development in old forest lichens

Generation time and juvenile growth rates are important but poorly known parameters in lichen population biology. By using a noninvasive method, we aimed to quantify these variables in juvenile thalli of Lobaria pulmonaria (L.) Hoffm., L. scrobiculata (Scop.) D.C., and Pseudocyphellaria crocata (L.) Vain., in situ, on twigs of Picea abies (L.) H. Karst in boreal rainforests. Growth was monitored during the summer months (May-August, 106 d), as well as in the remaining part of the year (259 d), for each of two sequential years, by means of repeated photography and imaging analysis. The mean relative thallus-area growth rates were 0.53 +/- 0.02, 0.41 +/- 0.02, and 0.57 +/- 0.04 mm(2).mm(-2).year(-1) (mean +/- SE), respectively, in the three species, equivalent to a yearly growth of 101 +/- 5%, 70 +/- 6%, and 121 +/- 12%. Growth was much slower during the winter (0.09-0.12 mm(2).cm(-2).d(-1)) than in summer (0.19-0.27 mm(2).cm(-2).d(-1)). Relative growth rates significantly declined with increasing thallus size. Estimated generation times in L. scrobiculata and P. crocata, based upon the first observed formation of reproductive structures, were 15-22 and 9-13 years, respectively. Studied L. pulmonaria thalli produced no diaspores during the experiment, consistent with a generation time > 17 years. The relative growth rates we measured and our estimated generation times are faster than those earlier recorded. Thus, our noninvasive method can estimate parameters needed to model population growth within a reasonable period of time. The rapid juvenile development implies that the growth rates and generation times are unlikely to be the limiting factors that exclude these lichens from young forests.