INTERNAL CLOCK OF TREES. “Endogenous rhythmic growth in oak trees is regulated by internal clocks rather than resource availability. The results indicate that rhythmic growth involves dramatic oscillations in plant metabolism and gene regulation between below- and above-ground parts. Ectomycorrhizal symbiosis may play a previously unsuspected role in smoothing these oscillations without modifying the rhythmic growth pattern.”
http://jxb.oxfordjournals.org/content/66/22/7113.full