Pocket Gopher Damage to Oak on the Santa Rosa Plateau
The average stand age of Engelmann Oak (Quercus engelmannii Greene) on the Santa Rosa Plateau, Riverside County, California, is 80-130 years, with trees older than this being less frequent in number, and very few tree reaching 300 years of age. Likewise, there are relatively sparse numbers of saplings and young oaks compared to the middle age classes. There are many factors responsible for the lack of recruitment of young Engelmann oaks, just as there are for the natural mortality of older oaks in the stand.
Cattle and deer grazing has been reported as one of the major limiting factors for regeneration. Both livestock and deer damage oak seedlings and saplings by browsing on the saplings and inadvertently trampling on the seedlings. Surface browsing of seedlings by small mammals (deer mice [Peromyscus maniculatus, P. boylii, P. californicus], pocket mice [Perognathus californicus], ground squirrels [Spermophilus beecheyi], harvest mice [Microtus californicus], woodrats [Neotoma fuscipes]) is also a significant limiting factor in oak regeneration. Our studies in southern oak woodland on the Santa Rosa Plateau, also indicate extensive pocket gopher (Thomomys bottae) damage to roots of Engelmann and coast live oak (Quercus agrifolia Nee) seedlings and saplings, up to several centimeters basal diameter.
In order to compare pocket gopher damage with other kinds of mortality to seedlings and saplings, e.g. browsing by non pocket gophers; dehydration; fire; insects, a total of 4,597 Engelmann oak seedlings and saplings in 39 study sites were tagged and monitored spring and fall seasons for cause of mortality (or top-kill) from 1987-1989. The rates of resprouting from stem death mortalities and resprouts from other causes of mortality were also recorded. Likewise, the extent of pocket gopher activity near to, and in the vicinity of Engelmann oaks, was estimated by running a series of line transects (0.33 m wide) under 55 randomly selected trees in the study area. Four transects, placed in the four compass directions, were run under each tree from the trunk outward to the canopy's dripline and 1.5 m beyond. The total length of of all transects under the 55 trees was 1,862 m (1,422 m from trunk to the inner edge of the dripline and 440 m under the dripline plus 1.5 m beyond). The number of pocket gopher holes/mounds and the number of living oak seedlings and saplings found along the transects were recorded. To compare effects of sun and shade, the distance on the transect from trunk to the inner part of the tree's dripline was considered "shade" and the dripline area plus 1.5 m beyond was considered "sun".
Pocket gopher activity
The main manner of killing oak seedlings and saplings by pocket gophers is girdling or gnawing completely through the roots, often at a slant cut (Fig. 1). Pocket gophers have also been observed to come out of their holes and surface cut vegetation. The average number of gopher mounds/holes per m of transect line was .96 from trunk to dripline and 1.8 within the dripline plus 1.5 m beyond. The average number of living seedlings/saplings was .14/m of transect line from trunk to dripline and .11/m within the dripline plus 1.5 m.
The overall percent survival for total numbers of Engelmann oak seedlings and saplings tagged in the study sites from fall 1987 to fall 1989 was 72% and 71% for sun and shade respectively. The causes for the 28% mortality in the sun were: 43% dehydration; 20% non-pocket gopher browsing (deer, ground squirrels, deer mice, harvest mice, etc.); 18% pocket gophers; 17% controlled burn; and 3% insects. The causes for the 29% mortality of seedlings/saplings in the shade were: 57% dehydration; 15% non-pocket gopher browsing; 12% pocket gopher; 13% controlled burn; and 3% insects.
The total overall rate of resprouting of seedling/sapling mortalities by pocket gopher activity during the study period was 7%, compared to 32% of seedling/sapling mortalities by non-pocket gopher damages such as dehydration and browsing by animals other than pocket gophers. The difference is statistically significant.
Resprouting of Engelmann oak seedling and sapling mortalities is less likely to occur with deaths caused by pocket gophers because of the damage to the roots which need to remain alive if resprouting is to occur. It is generally thought that pocket gopher activity is greater under the canopy shade compared to the clearings. However, our study showed slightly higher activity in the transect lines under the canopy drip line zone plus 1.5 m beyond compared to the more shady areas under the canopy. Since there tends to be a relatively higher concentration of saplings in sun clearings, compared to canopy shade, saplings in the sun are most likely to replace older trees as they die. This extra pocket gopher activity in the dripline zone may result in reducing survivorship of the more healthy, vigorous and usually larger seedlings and saplings at or beyond the margin of the canopy of mature trees. Thus, measures for the population control of pocket gophers and other rodents would be needed, particularly in the potential regeneration sites of Engelmann oak.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc