NATIONAL PARK SERVICE Giant Sequoia Ecology: Fire and Reproduction

Objectives of the study

Fire has long been a part of the ecology of giant sequoia groves. We hypothesized that it could be used as a tool to manage present-day groves, but we recognized that much more of the ecology of the species involved must be known to do so effectively. Therefore we felt we needed to:

1. determine specifically the role of fire and other disturbance factors in seed distribution and germination, and in seedling establishment and survival;

2. understand more adequately the position of the giant sequoia in the ecological successional pattern;

3. determine the role of arthropods, birds and mammals in the life history of the giant sequoia;

4. provide the National Park Service and other agencies that manage giant sequoias with data upon which to establish ecologically sound management and interpretive action programs.

Design

The general design of our studies was as follows:

1. Four study areas were established. These were chosen so as to differ in patterns of fuel accumulation, substrate, plant composition of the groundcover, and, to some degree, exposure and soil moisture. All areas were similar in that they generally lacked recent sequoia regeneration and were near fire roads and water sources.

2. Half of each study area was divided into manipulated and control portions. Each portion was studied before, to some extent, during and in the first, third and ninth years after manipulation.

3. One to three different management techniques were used in the manipulated areas, depending upon their topography and pattern of fuel accumulation.

4. The major thrusts of our studies were the changes in vegetation associated with manipulation, survival of giant sequoia seedlings and the interactions of invertebrates, birds and mammals with the various stages in the life cycle of the giant sequoia. Each seedling produced in the manipulated portions of the study areas was to be individually identified and followed throughout the period of the study, allowing us to correlate substrate conditions and seedling survival.

The study areas

The four study areas were located in the Redwood Mountain Grove in Kings Canyon National Park (Fig. 3) at elevations between 1646 and 2042 meters (5400 and 6699 ft). Each area was in an advanced state of plant succession and contained little or no recent reproduction by giant sequoias. Most areas had about 15 giant sequoias per hectare (6 per acre) and most of the trees exceeded 3 meters (10 feet) in diameter at breast height (dbh). Although fire had been excluded from the Redwood Mountain Grove for 70 to 80 years, the grove was disturbed by a small shake-shingle operation that was carried out until 1940 when the grove became part of the park.

Fig. 3. Location of Redwood Mountain Grove, the site of these studies.

An abundant understory of white fir (Abies concolor) and some incense cedar (Calocedrus decurrens) existed in most parts of the grove. Large canopy trees, in addition to the giant sequoia, included white fir, sugar pine (Pinus lambertiana) and Ponderosa pine (P. ponderosa). A thick layer of litter existed over most of the grove, with sweet cicely (Osmorhiza chilensis), white hawkweed (Hieracium albiflorum), and trail plant (Adenocaulon bicolor) as dominant species of ground cover.

A small stream, Redwood Creek, ran beside or near three of the four areas and provided a normal mid-Sierran riparian association of willow (Salix spp.), western azaleas (Rhododendron occidentale), creek dogwood (Cornus californica), mountain alder (Alnus tenuifolia), and California hazelnut (Corylus rostrata). The soil material in the study areas was derived primarily from metamorphic schists, although granitic parent material was present in other parts of the grove. The soil was generally of a gray-brown podzolic type, and its texture varied from fine sand to sandy loam.

The annual precipitation in the grove ranged from 46 to about 230 cm (18 to about 90 inches) depending upon altitude, exposure or the year. Summer storms were light and infrequent. Most of the precipitation occurred in the form of snow between October and April. The average depth of the snow in the grove was 1.5 to 1.8 meters (5 to 6 ft). Temperatures varied during the period of the study from average highs and lows of 4.4°C and -1.1°C (40°F and 30°F) in the coldest months to 26°C and 10°C (80°F and 50°F) in the warmest months. The coldest temperature recorded in a field area during the study was -11.6°C (11°F)

Access to each of the areas was by a fire road which descends from Redwood Saddle and parallels Redwood Creek (Fig. 4). Although this road is used as a trail, it is closed to public vehicular traffic, thus the public access to the areas was slight.

Fig. 4. Study areas in the Redwood Mountain Grove, Kings Canyon National Park.

Two study areas, Ridge and North (Fig. 4), measured 1.8 hectares (4 acres) each and the other two, Trail and South, were each 3.6 hectares (8 acres) in size. Ridge Area was located on the upper portion of the east-facing slope of Redwood Mountain near Redwood Saddle. It was the steepest (34% slope) and the driest area. North Area was situated on a small, flat, old flood plain on the west side of Redwood Creek and was relatively mesic. It had the greatest initial understory of young white fir and incense cedar of any of the areas.

The area with the second steepest slope (17%) was Trail Area. It contained the greatest amount of fallen trees and litter.

South Area, located on a southwesterly facing slope across Redwood Creek from Trail Area, was the most mesic site, with a seep occurring near the middle of the manipulated area. For a tabular comparison of the four areas see Table 1.

Table 1. Comparison of the four study areas

Methods

The vegetation of Ridge, North and Trail Areas was sampled by means of regularized rectangular plots. Woody vegetation taller than one meter was sampled by 5 x 20 meter plots, while all other vegetation was sampled by 1 x 2 meter plots. The initial inventories were carried out in these three areas in the summers of 1964 and 1965. South Area was sampled by the line intercept method for small vegetation and the random pairs method for tree species in 1966 (see Chapter 4).

Surface burning of the litter and ground vegetation was possible only in Ridge Area. North, Trail and South Areas contained so many dead snags and downed trees as to preclude a similar use of fire throughout most of each of those areas. After felling the snags, the debris in each of these areas was piled or windrowed and then burned. These experimental fires were the first such fires in National Park forests in the West.

A cleat-tread bulldozer was employed in North Area to move and pile logs. To minimize the physical disturbance to the soil, a rubber-tired Hough Payloader was employed for the same purpose in Trail Area. A bulldozer with a brush blade was used to move logs into windrows for burning in South Area. Surface burning was carried out wherever possible after mechanical manipulation in each of the areas. During these treatments the land surface of each area was both burned and physically disturbed, and the crown canopy was partially opened.

Vegetational changes were assessed during the year after burning (1965 or 1966, depending on the area), then in 1969 and again in 1974 (Chapter 4). Seedling giant sequoias were individually identified and monitored semiannually for nine years (Chapter 5). Cone loads of giant sequoias i.e. the numbers of cones per tree, in the areas were assessed (Chapter 5 and 9), as were the number of seeds per cone and the rate of seed fall (Chapter 5). Measurements of physical and edaphic factors affecting seed germination and seedling survival were carried out throughout the study (Chapter 3). Increment borings of giant sequoias were made to ascertain growth responses to the manipulations.

Arthropods associated with giant sequoias were identified and studied both from the ground and in two large giant sequoias in Redwood Canyon (Chapter 6). In the later phases of the project arthropods identified as being involved with some stage of giant sequoia reproduction were studied more intensively both on the ground and in the trees (Chapter 7).

Birds and mammals in the areas were censused by direct observation and by mark and release methods of live trapping before and after manipulations (Chapter 8). Seed spot tests carried out on the forest floor were used to assess the preferences of birds and mammals for conifer seeds (Chapter 8). The Douglas squirrel (Tamiasciurus douglasi) was identified in the early phases of the study as the one vertebrate that directly affects reproduction of the giant sequoia. It received intensive study during the latter years of the project (Chapter 9).

Numerous other studies on various aspects of giant sequoia ecology were carried out between 1964 and 1974. They are reported in various parts of Chapters 3 through 9.

Implications of this study for the management and interpretation of giant sequoias are discussed in Chapter 10.