I too have reservations about using certain tree - ring proxies when they are known to breakdown at times but the cause is not understood. (realclimate.org)
Not freezing cold, but certain trees need between 600 and 900 hours of temperatures below 45 degrees in order to produce the best fruit. (redeeminggod.com)
When certain trees dominate a street, they can raise the ozone level considerably. (scientificamerican.com)