IHRMP Oak Fact Sheets No. 60

Factors Affecting Annual Forage Yield in Oak Woodlands

Hardwood rangelands are appreciated for the important non-commodity valuesthey provide, including open space, rich wildlife habitats, watersheds,and recreation areas. Many of these values are reduced or lost as rangelandsare developed. Most rangeland in the state is held by private individuals,and these owners usually are interested in obtaining a financial returnfrom their property to pay taxes and show some return on investment. Thus,if rangelands are going to continue to remain in open space, commoditiesneed to be produced, along with the non-commodity values. Meat from grazinglivestock is the most important commodity produced from rangelands, butproductivity varies considerably from location to location, partly due tovariation in range forage yield . This article will discuss several factorsaffecting forage productivity.

An obvious factor is the yearly variation in weather. A recent studyat the University of California Sierra Foothill Research and Extension Center(SFREC) showed forage levels at the end of the growing season were 80% greaterduring the most productive year of the study when compared to the leastproductive year (table 1). Forage yield was moderately correlated with annualprecipitation totals. Observations were taken at four sites over five yearsand included a very wet (178% of the normal precipitation of 28.5 inches)and a very dry (66% of normal) year. One relatively dry year (84% of normal)still produced average forage growth. Its remarkably even distribution ofrainfall throughout the growing season points out the importance of precipitationdistribution as a factor in annual forage yield. Other research has showntemperature at critical points in the growing season to be important indetermining yield.

Table 1. Forage Yield by Year

Year Normal Precipitation (pct.) Annual Forage Yield (lbs./ac)

1990-91 87 1100 c

1991-92 84 1694 b

1992-93 145 1522 b

1993-94 66 1135 c

1994-95 178 1975 a

(note: values in the same row followed bydifferent letters are significantly different (p<0.05)

Oak canopy has also been shown to be a major factor affecting understoryforage growth, but the effects of canopy vary greatly throughout the state.In more arid parts of California's oak woodlands, with less dense canopyand generally larger trees, deciduous oak canopy enhances forage yield.In areas of higher precipitation and dense canopy, research indicates thatoak canopy reduces forage production. Some studies have identified the dividingpoint in terms of precipitation effects on understory forage growth as 20inches annually; i.e., in locations of greater than 20 inches annual rainfall,canopy reduces yield with the opposite effect occurring in areas of lessthan 20 inches.

In the SFREC study, we examined the effect of several blue oak canopylevels on forage yield. The data suggested that all levels of canopy (thelowest was 25%) reduced forage growth under trees, as compared to open grassland,but results were only statistically significant in two of the five yearsstudied (table 2). Previous research, in areas of similar rainfall and latitude,indicated that reduced forage yields could be expected under most of thecanopy levels we studied.

Table 2. Forage Yield by Canopy Level and Year

Year Normal precipitation (pct.) Canopy Level

0 pct. 25 pct. 50 pct. 75 pct.

(pounds per acre)

1990-91 87 1557 a 1178 b 816 c 848 c

1991-92 84 1651 a 1872 a 1702 a 1551 a

1992-93 145 1704 a 1316 c 1467 bc 1599 ab

1993-94 66 1255 a 1147 a 1061 a 1082 a

1994-94 178 1989 a 1877 a 1993 a 2041 a

Average 1631 a 1478 a 1408 a 1424 a

(note: values in the same row followed bydifferent letters are significantly different (p<0.05)

As discussed above, a large year-to-year variation exists in forage productionin oak woodlands. In the SFREC study, as well as in earlier research ina similar area, the yearly variability was much less under low canopy levels.This difference in yield variability is important to livestock producers.Not only is stable forage production desirable, but forage produced in lowforage years is more valuable than that available in a high forage year,due to its relative scarcity.

The SFREC study also looked at yield differences among four sites. Thesites were located in similar oak woodland, with the most distant beingabout four miles apart. They ranged in elevation from 600 to 1350 feet andvaried in slope from 18 to 40%. Forage yield differed greatly among sites:the most productive site, with an average yield of slightly over 2000 poundsper acre, provided twice the yield of the least productive site. Differencesamong sites were consistent over years and did not appear to be relatedto the obvious physical factors: elevation, slope, slope aspect or soil.The large difference among four sites located within relative proximitydemonstrates the large degree of variability that can occur in forage productionin oak woodlands.