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Factors Affecting Annual Forage Yield in Oak Woodlands

Oaks ’n’ Folks – Volume 11, Issue 2 – September, 1996

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

An obvious factor is the yearly variation in weather. A recent study at the University of California Sierra Foothill Research and Extension Center (SFREC) showed forage levels at the end of the growing season were 80% greater during the most productive year of the study when compared to the least productive year (table 1). Forage yield was moderately correlated with annual precipitation totals. Observations were taken at four sites over five years and 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 of rainfall throughout the growing season points out the importance of precipitation distribution as a factor in annual forage yield. Other research has shown temperature at critical points in the growing season to be important in determining 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 by different letters are significantly different (p<0.05)

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

In the SFREC study, we examined the effect of several blue oak canopy levels on forage yield. The data suggested that all levels of canopy (the lowest was 25%) reduced forage growth under trees, as compared to open grassland,but results were only statistically significant in two of the five years studied (table 2). Previous research, in areas of similar rainfall and latitude,indicated that reduced forage yields could be expected under most of the canopy 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 by different letters are significantly different (p<0.05)

As discussed above, a large year-to-year variation exists in forage production in oak woodlands. In the SFREC study, as well as in earlier research in a 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 low forage 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. The sites were located in similar oak woodland, with the most distant being about four miles apart. They ranged in elevation from 600 to 1350 feet and varied in slope from 18 to 40%. Forage yield differed greatly among sites:the most productive site, with an average yield of slightly over 2000 pounds per acre, provided twice the yield of the least productive site. Differences among sites were consistent over years and did not appear to be related to the obvious physical factors: elevation, slope, slope aspect or soil.The large difference among four sites located within relative proximity demonstrates the large degree of variability that can occur in forage production in oak woodlands.

Mike Connor
Superintendent, UC Sierra Foothill Research and Extension Center