Figure 2 - Understory Herbage Yield versus Crown Canopy versus Spacing

Figure 2 gives some insight into relationships of D+ and herbage production from McConnell and Smith, and Pearson and Jameson. In figure 2, McConnell and Smith's and Pearson and Jameson's data show wide spacing to be beneficial in additional herbage production. Exceptionally wide spacing does not apparently increase herbage yield.

Additional Notes ¹

Data in figure two from McConnell and Smith come from north-central Washington. Comon understory species of the ponderosa pine stands were balsam root (Balsamorhiza sagitata), pine grass (Calamagrostis rubescens), silky lupine (Lupinus sericeus), beardless bluebunch wheatgrass (Agropyron inerme), prairie junegrass (Koelaria christata) , Idaho fescue (Festuca idahoensis), and bitterbrush (Purshia tridentata). Understory vegetation from the Pearson study was Arizona fescue. (Festuca arizonica), mountain muhly (Muhlenbergia cuspidata), bottlebrush squirreltail (Sitanion hystrix), and sedge (Carex geophila). Location of the study was near Flagstaff , Arizona. The data from the two areas is in good agreement even though it comes from two areas far apart with different understory composition. This suggests the curve is applicable to Montana situations where the ponderosa pine stands are dominated in the understory by grasses and forbs.

It is not clear how quickly herbage production increases can occur after thinning a ponderosa pine from the data presented in figure two of the technical note. However, the values presented in figure two from McConnell and Smith were obtained 3 years after the initial thinning to the respective D+ spacings (values on X axis). Data presented by Pearson at the 20th meeting of the American Society of Range Management, Seattle, February 14 to 17, 1967 shows high levels of herbage production are attained two years after treatment.

In a follow up study ² to the one from which the data in table two was taken, McConnell and Smith found that herbage production seven years after thinning had continued to increase. They found for the 8-year period a 6-pound increase in understory yield for each percent decrease in tree canopy and a 9-pound increase in grass yield for each 1-foot increase in pine spacing.

They also reported in this study increases in yield of grasses, forbs and shrubs with decreasing canopy percent. Average increase in yield (air-dry pounds per acre) at 30 percent canopy (D+8 or D+9) over control (90 percent canopy) is: 200 pounds for grasses, 175 pounds for forbs, 35 pounds for shrubs.

¹ Prepared by H. E. Hunter, State Staff Forester, SCS, Bozernan, Montana.

² McConnell, Burt R., and Smith, Justin G., 1970: Response of understory vegetation to ponderosa pine thinning in Eastern Washington. Journal of Range Management, Vol. 23:208-212.

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Last Modified: 07/05/2007