Seasonal and Site Effects on Oak Fine Root Production and Ectomycorrhizal Colonization in California Oak Woodland

At three sites in a blue oak woodland in northeast California, we studied seasonal patterns of blue oak (Quercus douglasii Hook and Arn.) fine root (< 2 mm diameter) production and ectomycorrhizal (ECM) colonization using root ingrowth cores. In a short-term study, ingrowth cores were installed and harvested in 3-month intervals for one year. Oak fine root production was lower in fall and winter, higher in spring, and intermediate in summer. ECM roots were found only in spring and fall. In a long-term study, ingrowth cores were installed in January 1998 and harvested after 6, 9, and 12 months, respectively. ECM colonization was higher after the 6-month (winter, spring) interval, lower after the 9-month (winter, spring, summer) interval and intermediate after the 12-month (winter, spring, summer, fall) interval. Both long-term and short-term studies demonstrated that ECM fungi were active in spring and fall seasons. ECM colonization was much higher in an undisturbed six-month interval (winter + spring, long-term study) than in a disturbed 6-month interval (winter + spring, short-term study), suggesting that disturbance decreased mycorrhizal colonization. Introduction In northern California blue oak woodlands, plant growth is affected by the typical Mediterranean climate—cool, wet winters and hot, dry summers. Aboveground, oak leaves usually emerge in late March and senesce in October, while annual grasses begin to grow in November after first fall rains and senesce in early May. However, there is little information about belowground root growth. Three studies have quantified the biomass and spatial distribution of oak roots (Callaway and others 1991, Millikin and Bledsoe 1999, Millikin and others 1997), but information about oak fine root production and temporal distribution in California oak woodlands is lacking. The importance of ectomycorrhizas (ECM) to ecosystem processes has been well documented. However there is little information about the timing of ECM occurrence and the longevity of ECM roots. In a Norway spruce stand in Sweden, there were seasonal effects on ECM formation; the average lifespan of ECM roots was 300-1,000 days, depending on soil characteristics and root branch order (Majdi 1 An abbreviated version of this paper was presented at the Fifth Symposium on Oak Woodlands: Oaks in California’s Changing Landscape, October 22-25, 2001, San Diego, California. 2 Graduate Student and Professor, respectively, Department of Land, Air and Water Resources, University of California, Davis, CA 95616 (e-mail: [email protected]; [email protected]) USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 99 Oak Fine Root Production and Ectomycorrhizal Colonization—Cheng and Bledsoe and others 2001). In addition, about 45 percent ECM roots were dead after 200 days (Majdi and Nylund 1996). In this study, using root ingrowth cores, we measured oak fine root production and ECM colonization at three sites in the Sierra Nevada foothills. The purpose of our study was to examine seasonal effects on oak and grass fine root production and ECM colonization in both short-term and long-term studies. This paper focuses on oak fine root production and ECM colonization; grass root production was reported elsewhere (Cheng and Bledsoe [In press]). Methods A short-term and a long-term study were conducted at three sites at the University of California Sierra Foothill Research and Extension Center (near Browns Valley, California) in 1998. For the 4-year period 1996-2000, mean annual temperature was 18°C (summer 24°C, winter 11°C) and mean annual precipitation was 90 cm. In 1998 the temperature was 16°C, while precipitation was 124 cm. In general, precipitation begins in October or November and ends in May. Soils are classified as fine, mixed, thermic Typic Haploxeralfs (Dahlgren and Singer 1991). The dominant trees include winter-deciduous blue oaks (Quercus douglasii) and foothill pines (Pinus sabiniana). Common annual grasses include Bromus spp. and Avena barbata. Additional characteristics are found in Cheng and Bledsoe [In press]. The three sites (River, Upland and Hilltop) we selected for this experiment were very similar in vegetation and soil but differed slightly in elevation. At each site, we randomly selected three open-grown blue oak trees. Around each tree, we installed ingrowth cores under the oak canopy and in the open area. Ingrowth cores (40 cm long and 7.5 cm diameter) were made of 3 mm nylon mesh and filled with sieved (2 mm) root-free soil from each site by horizon (A: 0-5 cm; B: 5-40 cm). Before filling cores, soils were lightly moistened with water so that soils remained within the ingrowth cores. In the short-term study, 18 ingrowth cores (3 sites × 3 trees × 2 replicates) were installed every 3 months beginning in January 1998 and harvested 3 months later. The short-term study lasted one year, thus there were four seasonal sampling periods: winter, spring, summer and fall. In the long-term study, we installed 54 cores in January 1998 and harvested 18 cores after 6, 9, 12 months, respectively. In both studies, oak fine root production and ECM colonization were evaluated. At harvest, each ingrowth core was removed from the soil and divided into four segments (0-10 cm, 10-20 cm, 20-30 cm and 30-40 cm). In each segment, roots were separated from soil using a root elutriator (Gillison’s Variety Fabrication, Inc., Benzonia, MI). Roots were stored in water at 4°C and sorted within one month. Dead oak roots, defined as desiccated, dark brown, shrunken and highly fragile, were separated from live oak fine roots. After sorting, ECM oak root colonization was examined under a dissecting microscope using the grid-line intersect method (Brundrett and others 1996). Colonization was calculated as ECM tips per meter of total root length. Root samples were dried at 70°C and weighed. Fine root production was estimated by measuring new fine root growth into ingrowth cores. 3 Mention of trade names or products is for information only and does not imply endorsement by the U.S. Department of Agriculture. USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 100 Oak Fine Root Production and Ectomycorrhizal Colonization—Cheng and Bledsoe Results There were no significant differences in oak fine root production between cores located under the canopy and in the open area, so data were combined. At all three sites, seasonal effects were significant (fig. 1). Oak fine root production was lower in fall and winter than in spring. Surprisingly, there was significant oak fine root growth in summer, even though soils were very dry during that time. 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 W inter Spring Summer Fall oa k fin e ro ot p ro du ct io n (g m -2 m on th -1 ) River Upland Hilltop Figure 1—Seasonal effects on oak fine root production at three blue oak woodland sites. Values are means of four to six cores for each season: winter (Jan–Mar 1998), spring (Apr–Jun 1998), summer (Jul–Sep 1998), and fall (Oct–Dec 1998). For each core, data from all four soil depths were combined. Bars are standard errors of means. Data were modified from Cheng and Bledsoe [In press]. Site effects on oak fine root production were also significant. Overall oak fine root production was greater at the Upland site than at the River and Hilltop sites (fig. 1). Annual oak fine root production, which was estimated by adding total root production from each season, was highest at the Upland site (63 g m y), and less at the River (15 g m y) and Hilltop (16 g m y) sites. Oak fine root production did not differ significantly with soil depth, but did differ with site and season (fig. 2). At the River site, root production was greater in spring than in other seasons at all soil depths (0-40 cm). At the Upland and Hilltop sites, root production was significantly greater in spring than in other seasons in the upper soil (0-20 cm), but was similar in spring and summer in the lower soil depths (20-40 cm). In both short-term and long-term studies, we examined oak fine roots and determined ECM colonization. No ECM oak roots were found at the Hilltop sites in two studies (table 1). In the short-term study, ECM oak roots were found only in spring and fall. The colonization rates were similar between the River and Upland sites. In the long-term study, ECM roots were found at all time intervals. The USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 101 Oak Fine Root Production and Ectomycorrhizal Colonization—Cheng and Bledsoe colonization rate was higher in 6-month interval (75 and 53 tips/meter, respectively) than in 9-month (21 and 9 tips/meter) and 12-month intervals (42 and 24 tips/meter).