NATIONAL PARK SERVICE Giant Sequoia Ecology: Fire and Reproduction

Introduction

Several physical factors were measured in the study plots and in adjacent areas during the study period. Soil texture and moisture regimes were assessed during the summer. Soil temperatures during experimental burning were also explored, as were normal soil temperatures. Typical weather measurements included temperature and relative humidity, while precipitation data were obtained from nearby sites.

The amount of sunlight striking the forest floor was determined at specific sites and related to plant species present. Also litter and duff accumulations were assessed and related to plant species.

Methods and materials

Soils of the study sites were assessed as to composition and soil moisture. Soil composition was determined by taking samples systematically from a depth of from 2.5 to 10.2 cm (1 to 4 in). This depth was selected because it represents the rooting zone of giant sequoia seedlings. In addition to the 12 samples taken from the study areas, 115 samples were taken from 12 different giant sequoia groves. The samples were taken as opportunity permitted. The samples were analyzed by Curtis and Tompkins, Ltd. of San Francisco for total nitrogen, potassium (K₂O), phosphorous pentoxide (P₂O₅), and pH.

Soil moisture determinations were made by using gypsum soil moisture blocks (Bouyoucos and Mick 1940). The blocks were first soaked in water and then placed systematically at various sites within the treated and control sections. Blocks were placed at depths of 7.6, 15.2 and 30.5 cm (3, 6 and 12 in). They were monitored by the use of a Delmhorst ohmmeter with readings converted to percent total weight after standardization for the type of soil, namely loamy sand. Gypsum soil moisture blocks were also carefully placed in a regularized grid in two of the thick stands of giant sequoia seedlings which came up after treatment, at 7.6 cm (3 in) below the surface of the mineral soil, and monitored throughout the summers.

Soil temperature was also monitored, both as a seasonal variant in control soils and as a characteristic of soils exposed to fire. Soil temperatures were measured at the 30.5 cm (1 ft) level by a recording soil probe max.-min. thermometer placed systematically in North Area. Soil temperatures at or near the surface were measured by a thermistor with constant recording on a Rustrac tape recorder. These measurements were made at sites selected for their extreme characteristics of open charred soil or deeply shaded conditions.

The soil temperatures during a fire were determined by using Tempilaq° (Fenner 1960). Nineteen fusion temperature paints of Tempilaq° with melting points from 65.6°C (150°F) to 644.6°C (1200°F) at 27.7°C intervals were used. Each temperature sensitive paint was applied to strips of mica 4 or 6 inches in length which were then stapled to a supportive sheet of asbestos. These test strips were carefully placed in the soil with the mica side against the undisturbed soil face. Soil was filled in behind the asbestos sheet and the surface of the mineral soil was marked on the sheet (Fig. 5).

Fig. 5. Tempilaq° temperature recording device in place.

Tempilaq° test sheets were placed at 12 regularized sites in the surface burn in Ridge Area and in two burn piles in North Area. After the prescribed burns both temperature and depth to which it occurred were read off the fused Tempilaq° on the mica sheets.

Weather measurements were made using standard weather equipment. A recording hygrothermograph was placed in a standard weather shed and standardized periodically using a sling psychrometer. Three-pen remote recording thermographs were used, one at each of two locations, to obtain temperature profiles at the soil surface and at depths of 15 cm (6 in) and 1.8 m (6 ft). Temperatures were also periodically taken through out the height of a giant sequoia. Maximum—minimum thermometers were placed in all study areas and in six other giant sequoia groves. Temperatures in the northernmost grove, the Placer County Grove, and the southernmost grove, the Deer Creek Grove, as well as the highest elevation tree and the lowest elevation tree were also monitored. Temperatures at near ground level were measured using a field portable thermistor which recorded on a Rustrac two-pen recorder powered by a 12 volt wet battery.

Precipitation data were extrapolated from records kept throughout the year at nearby weather stations.

Light was measured as sunlight striking the forest floor. A chemical light meter in the form of anthracene in benzene was utilized with the change in optical density being read with a Bausch and Lomb Spectro-photometer 20. The method is essentially that suggested by Dore (1958) and modified by Waring and Major (1966). Two vials containing the liquid mixture were placed at each site over a 24 hour period. To assess the relationship of various ground cover plants to light, the pairs of vials were placed on grey felt in the centers of each regularized ground cover plot. After exposure the vials were collected and the optical density of the fluid was determined.

The accumulation of litter and duff under a forest canopy are of considerable ecological significance. To measure rate of fall, 1 meter square catch panels were placed at 25 sites on the regularized grid of North Area and measured after 9 months of litter fall (Fig. 6). Litter and duff depths in situ were measured at 50 of the 1 x 2 m plots. A clean cut was made at the four corners and in the middle of each plot. The depth of litter and duff was then measured in each profile, and an average of the five measurements was used as the depth of litter and duff for each plot. Six classes of duff and litter depth were determined in 2 cm increments running from 0 to 12 cm (0 to 30 in).

Fig. 6. Litter and seedfall panels with adjacent exclusion section.

Results

The surface soils of the study areas were essentially sandy loam with only two samples in the sandy loam category. The silt and clay fractions averaged 13% and 18% respectively, which is in the sandy loam soil type designation, using the U.S.D.A. Soil Textural Class triangular diagram (USDA 1957). The majority (60%) of those samples of soil were in the loamy sand category, 10% were sandy loam and 30% were sand.

The average pH value was 7.2 for the study area soils, whereas in the study of other sequoia groves the average pH was 6.9. The range of pH for the study area soils was from 6.9 to 7.3, while those of the other groves were from 6.0 to 7.5. Therefore it appears that the acidity of soils in which giant sequoias grow is near neutrality. The major nutrients at the study areas were within the range of those found in other sequoia groves.

Total nitrogen averaged 0.28% in the samples from the study area, while it was 0.20% in the 115 samples taken from other sequoia groves. The total nitrogen showed a significant drop in the highly heated soils. From two samples under burn piles only 0.01% and 0.02% total nitrogen were measured, whereas the average for 10 samples outside burn piles was 0.28%. The phosphorous level was 11.5 ppm for the study area samples and 15.5 ppm for the 12 grove survey. The phosphorous level of the study site samples averaged 0.7 ppm while the larger study samples mean was 0.87 ppm.

Soil moisture varied from year to year. For example, the soil moisture at a depth of 7.6 cm (3 in) in a dense stand of giant sequoia seedlings in Trail Area, was significantly higher for a longer period in 1969 than in 1966 (Fig. 7). Similarly, when soil moistures at varying depths from 30.5 cm (1 ft) to 7.6 cm (3 in) were compared in North Area during the summers of 1966 and 1969 the depletion was greatest in 1966 in rate and amount of decrease.

Fig. 7. Soil moisture at 7.6 cm (3 in) among giant sequoia seedlings.

The highest soil surface temperature recorded was 70°C (158°F) on sunlit ground char. Over a three year period the recording 30.5 cm (1 ft) probe thermometer recorded a maximum of 16.6°C (62°F) and a minimum of 10°C (50°F) at the one foot depth. A summer profile of temperature from near the ground to 85.3 m (280 ft), near the top of a giant sequoia, is shown in Fig. 8.

Fig. 8. Summer mean maxima and minima in giant sequoia canopy.

Soil temperatures during fires varied considerably from site to site depending on the amount of fuel present (Fig. 9). Temperatures under burn piles ranged as high as 399°C (750°F) 2.5 cm below the surface (Hartesveldt and Harvey 1967). Some surface burns resulted in Tempilaq° readings up to 260°C (500°F) at mineral soil surface and to as deep as 5 cm (2 in). In some spots soil surface temperatures exceeded 649°C (1200°F) while in others the fire was not hot enough to consume the fuel (Fig. 10).

Fig. 9. Light ground fuel in foreground, heavy fuel in background.

Fig. 10. Patchy fuel consumption by surface fire.

In order to characterize the mesoclimatic conditions at the study areas, comparisons were made with longer periods of weather records at Grant Grove and Whitaker's Forest. Grant Grove is at an elevation of 1980 m (6500 ft) and 4.8 km (3 miles) from the study sites. Whitaker's Forest is at 1645 m (5400 ft) and about 1.6 km (1 mile) from the study sites. The study areas range from 1646 m (5400 ft) to 1890 m (6200 ft) in elevation and thus are very similar to the above reference locations.

In Fig. 11 and Fig. 12 the three locations are compared as to their mean maximum and minimum temperatures throughout several years. By inspection of the charts it is evident that Grant Grove and Redwood Canyon (study site) were most alike, while Whitaker's Forest was warmer on the average. This was probably due to its lower elevation and the fact that it is on a west facing slope, not in a canyon. The lowest temperature recorded for the Redwood Canyon sites was -11.6°C (11°F)

Fig. 11. Mean maxima for Redwood Mountain Grove, and Whitaker's Forest.

Fig. 12. Mean minima for Redwood Mountain Grove, Grant Grove, and Whitaker's Forest.

During July and August of 1966 temperatures were recorded at three levels: 2.54 cm (1 in) below the soil surface, 15 cm above, and 1.8 m above, among dense stands of giant sequoia seedlings in Trail and Ridge Areas (Table 2). The soil at 2.54 cm below the surface reached the highest temperatures, except for the maximum temperature at Ridge Area. The 15 cm temperatures at Trail Area were as low or lower than those at the other two elevations. Ridge Area is located almost 244 m (800 ft) higher in elevation than Trail Area and is not subject to the same cold air drainage. The maxima at the 1.8 m level on Ridge Area were lower than those at Trail Area, while the minima were higher.

Table 2. Microclimate temperature (°C) gradients at Trail and Ridge Areas during July and August 1966.

Precipitation at the study sites is probably very close to that of the reference locations due to the similar elevations and close proximity. A comparison of precipitation data from Whitaker's Forest and Grant Grove revealed a high correlation (r = .94). In twelve months for which there were data, and in which over 2.54cm (1 in) fell per month, a total of 209.1 cm (82.33 in) fell at Grant Grove while 209.5cm (82.50 in) fell at Whitaker's Forest. This was over a four year period during which the greatest difference between months was less than 7 cm (2.75 in), and all but two of the months showed a difference of less than 3 cm (1.2 in). It was assumed to be adequate to use the precipitation data from either of these areas to elucidate the response of plants to precipitation at the study sites.

Fig. 13 presents the precipitation patterns at Grant Grove for the four years at the beginning of our study. It is of interest to note that there were alternate wet and dry years and that in the wet years considerable precipitation occurred during the later winter and spring, whereas in the dry years most of the precipitation fell in the early winter. The total seasonal precipitation for each period was: 1965-66, 78.1 cm (30.75 in); 1966-67, 172.7 cm (68 in); 1967-68, 56.8 cm (22.4 in), and 1968-69, 222.1 cm (87.45 in). The mean annual precipitation for Grant Grove during the study period of 11 years was 113 cm (45 in). The mean seasonal precipitation was 109 cm (43 in) and the unusual heavy precipitation during the 1968-69 season was over twice normal.

Fig. 13. Precipitation at Grant Grove (data from National Park Service) (4.8 km (3 mi) from study sites).

Although long range data are not available for the study site, it is located near Grant Grove, for which there is a long range record. Fig. 14 presents the climograph for Grant Grove. As with much of California, the climograph indicates warm, dry summers and relatively cold, wet winters and it is assumed that the study site would have a climograph very similar to Grant Grove.

Fig. 14. Climograph of Grant Grove. (4.8 km (3 mi) from study sites).

Relative humidity measurements showed typical high morning readings and relatively low afternoon readings. The monthly means for maxima and minima were typical for a mesic forest habitat (Fig. 15). The lowest values were in late summer and early fall, with August having the lowest mean values. Of special interest was the extreme low relative humidity recorded for August during four consecutive years (1967-70). The mean for July was 27% and for September it was 26%. The range of values did not overlap for August and the other two months. The lowest relative humidity recorded was 16% in August of 1970. These data are particularly important in evaluation of giant sequoia seedling survival as discussed in Chapter 5.

Fig. 15. Monthly average maxima and minima relative humidities, North Area, 1966-69.

Sunlight striking the sample plots in the study areas varied from 0 to 41% of full sunlight. The average percentages for the various areas are of dubious value because of the relatively few points sampled, i.e. less than 50 per hectare. However, they do help quantify the general impression that North Area is more open than Ridge or Trail (Table 3).

Table 3. Mean percentages of sunlight striking the forest floor at sample plots.

Discussion and summary

The range of physical attributes within which the giant sequoia ecosystem occurs appears to be moderate. The climate is one in which precipitation varies from 46 cm (18 in) to 152 cm (60 in) with a mean of 112 cm (44 in) per year (Schubert 1962). The minimum temperature recorded during the study period was -11.6°C (11°F) however, Schubert (1962) reports -24°C (-12°F) as the occasional low for giant sequoia communities. Maxima rarely exceed 38°C (100°F)

The substrate in which giant sequoias and their associated plants grow was determined to be a loamy sand in 60% of the cases, the remaining were sand or sandy loam. In addition to our studies, Zinke and Crocker (1962) found sandy loam soils in the Merced Grove, the Nelder Grove and Giant Forest. As with many other factors, there is both an advantage and disadvantage to plant growth in these extreme types of soil. The more sand in the soil the less tightly water is held and the less work the plant must do to gain soil moisture (Lutz and Chandler 1946). The disadvantage of these sandy soils is that of rapid percolation beyond the root zone of seedlings. The soil moisture curves for the study areas confirm this tendency, since the 1966 soil moisture levels fell almost to zero by the end of July in Trail Area.

The pH level, both in the study areas and in 12 other giant sequoia groves, was essentially neutral. General absorption of nutrients is maximized by soil pH values near 7 (Daubenmire 1974). Therefore, even if such soils may be relatively poor in nutrients the pH levels are optimal for absorption. The mean total nitrogen level in the study sites was 0.3%, which is within the range of 0.24-0.45% determined by Zinke and Crocker (1962) as occurring in other giant sequoia groves. Although nitrogen may be reduced immediately after a fire, it is probable that the available nitrogen is actually increased in time (Agee 1973; Viro 1974; St. John and Rundel 1976). Although fire may aid plant reproduction by removing duff and litter so that seeds can reach mineral soil the black char on the surface may increase soil surface temperature to a high degree. This increased heating can lead to desiccation and death of seeds and seedlings in such sites. During fires the penetration into the soil of high temperatures was highly variable. Under burn piles, temperatures of about 400°C (750°F) were recorded at 2.5 cm below the surface. Agee (1973) reported a temperature of 206°C (403°F) in a white fir—giant sequoia prescribed burn in which litter and duff were the fuel. High temperatures such as these consume surface seeds and small plants, and in the case of the hottest fires, lethal temperatures can penetrate as much as 20 cm (8 in) below the surface. This essentially sterilizes the soil as far as seed germination of many plants is concerned.