Are spruce
seedlings starved of soil-based sustenance?
What factors might hinder the regeneration of an apparently
dominant species within a plant community? It is this question that Castells et al., Barcelona, Spain and Fairbanks,
Alaska (pp. 1247–1252) have attempted to answer.
Their research centred on Picea glauca (white spruce), which regenerates very poorly, if at all, in
managed forests in North America. It has been
suggested previously that compounds leaching from plants of the understorey are
directly harmful to seeds or seedlings of P. glauca, i.e. that failure to
regenerate is a result of allelopathy. The authors prepared leachates with high
concentrations of phenolics from two understorey shrubs, Ledum palustre and
Empetrum hermaphroditum, and leachates with very low phenolic contents from two mosses, Sphagnum sp.
and Hyloconium splendens (although
the Sphagnum extract
was expected to be high in phenolics because the moss itself is). Direct
application of the moss leachates to seeds had no effect on germination whilst
leachates from the shrubs delayed germination by up to a day and reduced
percentage germination from approx. 70 % to approx. 60 %; there was no direct
effect of leachates on the growth of seedlings. These data do not support the
idea that regeneration failure may be ascribed to an allelopathic effect on
germination. Instead it is probable that the interaction between the leachates
and soil nitrogen is a major factor. Addition of leachates of L. palustre and
E. hermaphroditum to
soil resulted in a significant decrease in mineralization of nitrogen. The
reason for this effect on the nitrogen cycle is not clear—it may possibly be
attributed to increased microbial growth (and hence sequestering of N in an
organic form)—but what is clear is that N availability is reduced and this is
likely to affect seedling growth after endogenous N reserves have been
exhausted. Whether this alone can explain the poor regeneration is a matter for
further investigation.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
