Partial support of this work by the US Forest Service under contract no. INT-92660-PJVA;
amendment no. 1;
is acknowledged. The principal investigator for this project was D.S. Bowles. Mike Mann's help in providing and installing the multi-taper method routines is thankfully acknowledged. We thank the three anonymous reviewers and Dr David Maidment;
for comments that helped improve the content and style of the manuscript;