The above graph plotted expected stand age against road distance to the nearest sawmill, holding other variables constant at sample means or modes. We see a generally positive relationship between stand age and distance to the nearest sawmill. The government relationship is increasing for all relevant observations, while industry-managed and NIPF-managed stands show a curvilinear relationship. Note that the number of FIA plots that are more than 20 miles from their nearest sawmill are very few (64 out of 1,536, or 4.1% of NIPF plots, and 18 out 338, or 5.3% of industry plots), so less confidence should be attached to the downward-sloping portion of the distance curves.
For both NIPF and industry, at sample average and modal values for other variables in each ownership, through the first 15 road miles, each road mile of distance adds about 0.8 years to the expected age of the stand. For government-owned stands, each road mile adds 0.36 years.
