|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
Repression is a biological phenomenon whereby tree growth and stand development fail to exploit the potential productivity of the site. The impact is widespread in fire-origin stands of lodgepole pine (Goudie 1996) but not in other species.
Repression reduces height and volume increment in lodgepole pine shortly after the growing space is fully utilized in stands established at extreme densities. The process usually begins before trees reach a height of 2 m, although stands with 1 000 000 trees/ha may be affected when only 0.2 m tall (Mitchell and Goudie 1980).
Analysis of espacement trials (Carlson and Johnstone 1983) shows that the height growth of plantations with more than about 15 000 planted trees/ha are affected (Figure 1), and future measurements may indicate minor repression at lower densities. The pattern of growth and development of repressed stands resembles that of stands growing on sites of much lower productivity; consequently, merchantable yields will be achieved considerably later than had repression been avoided. Repression does not, however, cause stands to stagnate or cease development, as was once believed.
Figure 1. The height growth of lodgepole pine is reduced at high establishment densities.
Long-term growth losses are particularly dramatic in repressed fire-origin stands of lodgepole established with more than 50 000 trees/ha (Mitchell and Goudie 1980), particularly on sites of relatively low productivity. At extreme densities (500 000+ trees/ha), stand production may be reduced by as much as 60%.
Regeneration survey data indicate that repression is not likely to be a serious problem in stands that regenerate after logging because establishment densities are much lower, trees seed-in over five to 10 years (instead of two to three years), and clumped (less uniform) tree distributions are more common. The resultant lower densities and greater tree-size diversity of post-logging stands minimizes the risk of repression losses.
Repression is also unlikely in plantations, unless supplemented by concurrent, natural, in-fill regeneration. The impact of dense, but delayed, natural regeneration on planted lodgepole pine stands is not known.
Response to treatment
Stand density interventions are an effective means of preventing repression in lodgepole pine if treatment occurs before the onset of repression. Stands that are thinned after the onset of repression do not show a consistent height growth response to treatment (J.S. Thrower and Assoc. 1993). However, there is evidence of an independent response in diameter growth.
Until more is known about treatment response it is reasonable to assume that the early height growth of repressed stands is indicative of future productivity.
Copyright 1999 Province of British Columbia