Stand Age

  

Figure 5: Risk pattern as a function of stand aging. Hosts are denoted by disks sized in proportion to DBH. Trees at high risk have black disks, trees at medium risk are dark grey, and trees at low risk are light grey. The location of the focus tree is indicated by the shaded square at the origin.

To examine changes in the risk of MPB attack as a stand ages we generate virtual LPP stands using Roberts' stem model (see [31], summarized in the appendix). Snapshots of a stand at 50, 70, 120 and 200 years were used as input to the risk assessment procedure. Ages were chosen to represent the maximum number of >20 cm DBH hosts (at 50 years), a variety of hosts at maximum size ($\sim$50 cm at 200 years), and two samples of spatial host structure in intervening years. These stands span the likely extremes of induced and constituitive resin responses which may be expected to arise due to intra-stand competition and secession.

Results are illustrated in figure 5, with all parameters held constant except stand age. The total number of hosts at high and medium risk declines with stand age, but so does the total number of hosts. No host which survives the full 200 years changed its risk category, and the spatial limits of the risk profile are apparently the same. This is consistent with the idea that host vigor, per se, is less critical than micro-climate factors in controlling stand risk.