lichen growth rates?

[Fred Rhoades]

aztlan at primenet.com (Joseph Howell) writes:

>What is the general growth rate of lichens? Say for example, how long does it 
>take for a lichen to cover the exposed surface of a rock? Or is this question 
>moot, due to different growth rates between species? I'm asking mainly because 
>archaeologists (of which I am one) often use the presence of a lichen-covered 
>cobble as an indicator of a sites general age. But I've never actually had 
>anyone claim to know just how fast lichens grow, so I've always been a bit 
>suspicious of this - it sounds more like "outdoor lore". 

>Any info? Thanks.

The question is somewhat moot.  Different species have different growth 
rates and even individuals of the same species have different rates in 
different locales.  Rates have been measured ranging from fractions of 
mm/year for many crustose species to over 10 cm/yr for some fruticose 
species.  I worked on Lobaria oregana, a large, arboreal species in 
conifers of the pacific northwest:  its linear growth rate varies 
depending on the size of the lichen, reaching about 2 cm / year in the 
larger thalli.  For each locale, lichen species' growth rates need to be 
standardized by measuring the rates for several years or by estimating 
them from thalli on surfaces of known age.  The latter technique of 
lichenometry is well documented in the geological literature.  It has 
been criticized by many because of several assumptions.

However, the presence of lichens on rocks is evidence that the rocks have
been undisturbed for some time, and in one locale, the different average
sizes of the largest thalli of the same species on rocks at different
locales, assuming everything else is equal, is good evidence of the
relative lengths of time ago the various sites have been disturbed in the
past.  For a ball-park estimate of the time, use 0.1 mm / year (radial 
growth rate) for crustose species.

Fred Rhoades
Biology Department, WWU
Bellingham, WA  98225
USA
FAX: (360) 650-3148
Voice: (360) 733-9149

email: fredr at henson.cc.wwu.edu