Effects of Vaccinium myrtillus on spruce regeneration: Testing the notion of coevolutionary significance of allelopathy

Failure of natural regeneration of conifers, such as subalpine spruce (Pice a abies) and black spruce (Picea mariana), has been reported in the presenc e of dominant ericaceous understory plants of boreal forests of North Ameri ca, Fino-Scandinavia, and northern Europe. Among other factors such as comp etition for light and nutrients, conifer regeneration failure has been attr ibuted to allelopathic effects of the understory ericaceous plants. Rabotno v theorized that (the manifestation of) allelopathy is a result of long-ter m coevolution within established plant communities and that it may have max imum inhibitory effects on introduced species. Our objectives were to deter mine what components of the understory ericaceous plant, Vaccinium myrtillu s, affect spruce regeneration and to test Rabotnov's hypothesis. Field expe riments were complemented with laboratory studies in which seed germination and primary growth of the two spruces were used as response variables. We found that P mariana was generally more affected than P. abies by V myrtill us allelochemicals, both in field and in vitro experiments. Field germinati on of P. abies was only 2% and 3% in the undisturbed sowed plots and in Vac cinium-removed sowed plots, respectively, but P. marana did not germinate a t all in these treatments. In humus-removed sowed plots, P. abies had 27% g ermination, while P. marian had only 15%. In a controlled experiment, P. ma riana had the highest decrease in dry weight of primary root in the fresh l eaf treatment of V myrtillus (77%), followed by its leaf leachate (71%), hu mus (29%), and humus leachate (13%). The decreases in root dry weights of P . abies due to these treatments were 67, 47, 30, and 10%, respectively. Our results provide support for Rabotnov's hypothesis. It is possible that bot h V. myrtillus and Kalmia angustifolia, involved in the growth inhibition p rocess of P abies and P. mariana, respectively adopted similar "strategies" of allelopathic inhibition of conifers, by allocating a large part of thei r carbon pool to the production of secondary metabolites.