NATIONAL PARK SERVICE Giant Sequoia Ecology: Fire and Reproduction

Introduction

In order for forest trees to perpetuate themselves they must not simply produce seeds in sufficient number, but the seeds must germinate and the developing seedlings must be able to tolerate the environmental conditions into which the seeds have fallen. Thus, extant species have evolved a variety of strategies to insure adequate seed production and subsequent survival of offspring. In terms of its strategy of seed production that involves serotinous cones, the giant sequoia differs dramatically from other members of the Taxodiaceae. The other members of the family produce cones that normally shed their seeds in less than two years after starting development (Buchholz 1938). Giant sequoia cones start forming in the summer of a given year and are pollinated the following spring. Fertilization takes place by that fall, and the cones mature the following summer (Buchholz 1937), thus the whole process takes more than two years. These serotinous cones may remain green and closed for over twenty years (Buchholz 1938). The giant sequoia is dependent on the seeds from these mature cones for its sole method of reproduction. Although its closest living relative, the coast redwood (Sequoia sempervirens), is noted for propagation through stump sprouting (Stebbins 1948), no sprouts from roots or stumps are known to occur in the giant sequoia (Schubert 1962).

These two native California sequoias also differ markedly in the number of seeds per cone. The coast redwood yields only about 60 seeds per cone while the giant sequoia cone averages about 200 seeds (Buchholz 1939). Slightly higher averages of seeds per cone have been reported for the giant sequoia, namely about 230 (Schubert 1962) and 234 (Fry and White 1930). Both of these latter sources report a maximum of 329 seeds per cone, thus the average number of about 230 seeds per cone reported by Schubert (1962) may be based on the Fry and White (1930) determinations. Schubert (1962) reported that the seeds fall at a rate of about 120 cm/sec (4 ft/sec) and may be blown laterally as much as 177 m (580 ft). This would enable the groves to gradually expand, providing conditions on the periphery are favorable for seed germination and seedling survival.

The number of cones produced per tree has been reported by Schubert (1962) to be 2000 per year for mature trees. The total number of cones on a large mature tree that fell in Whitaker's Forest was estimated to be about 50,000 (Zinke pers. comm.). At what age trees first produce cones with germinable seeds is still uncertain, with Schubert (1962) reporting that trees about 20 years old produced infertile seeds.

Although cones may remain closed for over 20 years, natural forces may open them before then. Douglas squirrels have long been known to feed on the cones and thus may release seeds in the process (see Chapter 9). The convective movement of hot air in hot surface fires may dry cones and cause heavy seed fall. Kilgore and Biswell (1971) reported that hot air rising to about 36 m (100 ft) during a prescribed burn on Redwood Mountain dried numerous cones which subsequently released many seeds.

Although most tree species have some kind of seed dormancy (Kramer and Kozlowski 1960), the giant sequoia seeds germinate as soon as conditions are favorable. Those seeds falling on litter and humus are usually unable to establish seedlings (Metcalf 1948). Stark (1968b) reports that litter, if wet, is a good germination medium but in nature litter rapidly dries out, thus desiccating the seed or seedling dependent upon it. This is in contrast with most coniferous seeds which can tolerate air-dry conditions (Kramer and Kozlowski 1960).

Hartesveldt and Harvey (1967) reported giant sequoia seeds on the ground after a prescribed fire at a concentration of 7,500 per m². This high concentration was attributed to the heating of the cones of a large sequoia near an adjacent burn pile.

Seed germination of the giant sequoia begins in February or March and proceeds throughout the summer as long as conditions are suitable (Schubert 1962). Giant sequoia seeds germinate best in moist soil about 1 cm below the surface, at 10°C to 20°C, pH 6 to pH 7, and reduced light (5,000 f.c.) (Stark 1968b). Stark also reported that selected large seeds (8 mm average length) germinated 153% better than a mixture of normal seeds, while small seeds (under 4 mm average length) germinated only 6.9% as well as the controls.

Seed germination is highly variable even when seeds are treated to the best of conditions. Stark (1968b) tested 12,000 seeds from 42 sequoia groves and obtained an average of 22.5% germination. The highest germination she obtained was 55.5%. Metcalf (1948) obtained 42% germination from seeds removed from an old lichen covered cone. Fry and White (1938) reported that 15% germination was a fair average, but estimated that only one in a million seeds germinates under natural conditions.

Hartesveldt and Harvey (1967) reported that all test seeds scattered on the surface of the forest floor in a sequoia grove failed to germinate after 20 days exposure. They further stated that under conditions produced by surface fires, soil surface temperatures may be as high as 48.9°C (120°F) to 69.4°C (157°F) which would rapidly desiccate the seeds and the seedbed.

The microenvironment of the forest ground surface first influences seed survival, then germination, and upon germination, the development and survival of the seedlings. The early seedling stage is probably one of the most critical as far as the survival of the species is concerned. Hartesveldt and Harvey (1967) followed over 2,000 seedlings which had naturally seeded in after the prescription burning of two test plots. At the end of the first summer 45% were alive. By the end of October, 30% remained alive, and by the next summer only 10%. Only 1.4% of the seedlings were still alive after two summers of growth, thus in 18 months 98.6% mortality had occurred.

Fire is among the physical factors that are considered beneficial to sequoia seedling survival through indirect means. Large trees such as white fir may be killed, thus opening up the canopy and letting in additional light which is required by the seedlings. Also the soil may be affected in several ways.

If the temperature is lethal to seeds buried in the soil, then potential competitors to the giant sequoia seedlings will be eliminated. High temperatures make the mineral soil friable and wettable (Donaghey 1969). Friable soil allows falling sequoia seeds to penetrate, particularly during a rain storm. Wettability was observed by Donaghey (1969) to increase with increased temperature, and thus it allows greater water penetration. Friable soils may allow easier root penetration and thus enhance the establishment of young seedlings.

High temperatures from hot fires may also eliminate potential pathogens such as Penicillium sp. which produces necrosis of the hypocotyl in giant sequoia seedlings (Swift, W. 1975). Bega (1964) reported several fungi as potential pathogens of seedlings. Heat may also volatilize any ectocrines that may be present. Therefore if any or all of the above are true, then giant sequoia seedlings should survive best where the hottest fires have occurred, provided the fires do not destroy the seed source. One favorable factor that may be adversely affected by hot fires is the endomycorrhizal fungi associated with giant sequoia. S. Swift (1975) found that seedlings could survive for at least 3 months under sterile conditions without my corrhizae and thus may survive until mycorrhizal relationships are established. Many conifers require mycorrhizae for survival, and the giant sequoia does not appear to be an exception.

Grove expansion and longevity of remnants

The giant sequoia is considered a relict species which persists in relatively restricted groves at the mid-elevations in the Sierra Nevada (Hartesveldt 1962). Rundel (1971, 1972a) has stated that there is no evidence of any change in grove boundaries during the last 500 years or longer. No remnants of sequoias between the present disjunct populations have been found, although individuals or only a few trees exist a kilometer or so beyond the closest grove. There is a lone 150 year old tree, for example, at Rabbit Meadow over a kilometer north of Redwood Mountain Grove.

The question of how long downed giant sequoia may persist is related to the above question of grove expansion or contraction. If dead giant sequoias may persist standing for over 2000 years, as determined by Hartesveldt (1964), then evidence should be available to infer grove contraction in this case.

Methods and materials

Seed production

Four major aspects of seed production were examined. These were 1) the number of mature trees producing cones per hectare, 2) number of cones per tree, 3) number of seeds per cone, 4) and number of, and timing of, seeds reaching the ground.

The number of trees per hectare was determined by mapping all giant sequoias in the study areas. In addition, two surveys by commercial forestry companies for the National Park Service of the Redwood Mountain Grove and the Giant Forest were analyzed to determine the density of mature giant sequoias.

The number of cones per tree was estimated by two methods. Stecker (Chapter 7) hand-counted cones within the crown of the 88.4 m (290 ft) Castro Tree. Using binoculars from the ground, Shellhammer (Chapter 9) devised a cone load system for estimating cone numbers in mature trees.

The number of seeds per cone was determined by taking individual cones and placing them either in small paper bags and allowing them to air dry, or placing them in aluminum foil and oven-drying them. After drying, each cone was examined, any remaining seeds were teased from the cone, and then all seeds were counted. Almost 200 cones were examined in this fashion. Additional data were gathered on number of scales per cone, size of cone and variation between trees as to the number of seeds per cone.

The number of seeds falling per unit was determined by placing 29 catch panels each 1 m² on four transects through North Area. Two 1 m² catch panels were placed among 11 large giant sequoias and monitored daily during one summer. On two occasions 10 randomly selected dm² plots were examined for seeds that had fallen to the ground in an apparent heavy concentration.

Seed germinability

Two methods were employed to determine seed germinability. Seeds were either planted in loamy sand at a depth of 1 cm or placed on moist filter paper in petri dishes. The depth of 1 cm was considered optimal by Stark (1968), and loamy sand is the prevalent type of soil in giant sequoia groves. In addition, the snap test was used to rapidly ascertain whether a batch of seeds appeared to be viable and thus probably germinable

Perhaps the distinction between viable and germinable needs to be clarified. A seed may be viable but unable to germinate for various reasons when placed under conditions optimal for germination, i.e. alive but unable to germinate. For example, seeds in first year cones are viable (alive) but are not capable of germinating. If a seed did germinate it of course was viable. In the snap test a seed was snapped (broken) across the long axis of the seed and the condition of the embryonic area assessed. If the area was cream colored and full, it was considered viable and thus potentially germinable. if the area was white, brown or black and/or shrunken, then it was considered nonviable. Parallel tests were run to determine the similarity of results from petri dish or soil germination tests and snap tests.

In order to test the effect of natural environmental conditions on germination, seeds were collected from one seed source and placed on the ground in North Area and thus exposed to natural conditions during the summer. At intervals a sample of 100 seeds was selected for both soil germination and snap test examination to evaluate the effect of light, moisture and temperature.

In order to ascertain the effect of the cone pigment on germination three thousand seeds were tested in five concentrations of the cone pigment, i.e. 22%, 18%, 9%, 4.5%, and 2.2% by weight. The seeds were tested for germination with 10 replicates of 50 seeds each, maintained at 26°C to 30°C in petri dishes.

Seedling establishment and mortality rates

The period of seedling establishment, as stated earlier, is one of the most critical stages in the life cycle of the giant sequoia. Even though seed germination conditions may be optimal, the species will not survive if the seedlings are quickly decimated. In order to determine the rate of survival and the factors involved, all seedlings that could be found in the treated sections during two or three summers were numbered and staked and then followed as individuals throughout the study. The numbered stakes were placed approximately 10 cm north of each seedling in order to minimize such unnatural effects as shade and root disturbance. During the study period 7,666 giant sequoia seedlings were so marked. The condition of the seedling was noted as to development at the time each was discovered. Namely, the presence of cotyledons, secondary leaves and branches was recorded, as was the condition of the substrate. Records were kept as to the combination of treatments that may have occurred at the seedling's site, such as scarification due to the heavy equipment, scarification plus surface fire, surface fire alone, or burn pile. In addition, the microhabitat of the seedlings was identified. Seedlings next to fallen twigs or next to rocks were recorded, as was the substrate if it was a standing or fallen rotten log. Hence the mortality that occurred to the seedlings in succeeding years on certain substrates or in certain micro habitats could be evaluated and correlated with such environmental conditions.

Meter wide transects were run in all treated and control areas during the summer of 1969. These surveys, plus random searches for seedling giant sequoias in untreated areas, yielded data which help evaluate the significance of fire and/or manipulation in giant sequoia regeneration. The transects were run in mid-July and then repeated in late August in order to determine the mortality rate.

Mortality rates for the large individualized study of 7,666 seedlings were determined by both weekly and monthly surveys during 1966 and 1967, or by yearly surveys, generally run during the third week in October. During the weekly surveys in 1966 and 1967 each dead giant sequoia seedling was exhumed and analyzed for cause of death. The specimen was then preserved. The weekly determinations of mortality helped define the causes of death. The monthly determination helped identify the period of the year when mortality was greatest. The yearly surveys assisted in determining the annual rate of mortality.

Growth rate

The growth of trees is a constellation of activities, as suggested by Kozlowski (1962). We concentrated, however, on vertical and diameter growth of individual trees. These measurements were made during three stages in the giant sequoia's life history. Particular interest was taken in the early seedling stage, where height measurements were taken and relative lengths of shoots versus roots were made on exhumed dead seedlings. Height and dbh (diameter at breast height) were measured on sapling-sized trees at Cherry Gap and Converse Basin (Fig. 3) in 1964, 1969 and 1974

On large trees (over 4 feet in diameter) dbh was taken, and often increment cores were also removed. Increment coring was done on three trees inside each treated section and three trees in each control section in 1974 in order to ascertain the effect of fire and/or manipulation on their respective rates of growth.

Seedling survival and growth

Several of the factors measured as relevant to succession were also involved in assessing giant sequoia survival and growth. Specific attention was paid to soil moisture and light availabilities using the methods described in Chapter 3. The amount of moisture at the individual sites of certain giant sequoia seedlings was monitored, and a correlation coefficient determined between percent of full sunlight and height of seedlings.

Two projects were carried out involving the association of fungi with giant sequoia seedlings. Methods were developed to grow seedlings in sterile conditions on White's medium and then to inoculate them with fungal isolates. In studies on mycorrhizal relationships, other aseptic seedlings were examined for mycorrhizae and inoculated with isolates from other sequoia roots shown to have endomycorrhizae. The results of these studies are reported briefly in the introduction to this chapter.

Grove expansion and remnants

Grove expansion was investigated by inspecting the maps of grove inventories and selecting sites where apparently young trees were at the edge of the grove. Increment cores were taken of the suspected young trees and the nearest mature sequoia for age determination by basal area regression and fire history to determine whether the date of a fire correlated with the age of the young sequoias.

To determine the longevity of fallen sequoias, particularly in meadows, cores were taken from trees growing in the root pits of fallen sequoias, and basal area regression curves were used to extrapolate to the beginning of the tree in question.

Results

Seed production

The reproductive capacity of a plant that relies entirely on seeds for propagation depends on basic characteristics, e.g. the number of reproductive individuals per unit area. For the giant sequoia, the total density of such trees may be about 37 per hectare. However, if only mature trees greater than 4 ft dbh are considered, the value is significantly lower. The decision to use 4 ft dbh for designating mature giant sequoias was based on several lines of evidence. First, the 4 ft diameter class is the size at which population curves for two large mature giant sequoia groves level off (Fig. 35) and second, this is the diameter of trees about 40 to 86 m (110 to 240 ft) tall and capable of bearing large numbers of cones. As a general rule, a tree that height will be about 400 years old (Fig. 36), and for the first 800 years of growth the ratio of 1 ft in diameter growth equals 100 years is a reasonable estimate (Fig. 37). After 800 years the radial growth slows so that by 2000 years the average tree measured was 14.5 ft in diameter. (These data were derived from the measurements on cut stumps taken by Huntington in Converse Basin. Copies of his field notes served as the basis of a computer program from which radius and basal area were calculated for each 10 year period on 97 trees.)

Fig. 35. Number of giant sequoias in two mature giant sequoia groves with respect to dbh.

Fig. 36. Regression of height of giant sequoias to diameter (n=227).

Fig. 37. Regression of diameter of giant sequoias to their age (n=97).

Taking mature giant sequoia as ones larger than 4 ft in diameter, and using the tree surveys done in the two largest groves as a basis of density determination, the mean density was 5.3 trees per hectare. According to the surveys, there were 11,890 trees larger than 4 ft in diameter in 2,247 hectares. A comparison of diameters of 19 trees in Trail Area, between our measurements taken above the butt swell and those of the Western Timber Service Tree Inventory of the same trees, revealed a larger value on the average by the latter. The average increase was about 5 inches per tree, which may mean that a few trees may have been included in a class size they should not have been. The discrepancy was least in the small diameter classes. The error introduced by the arbitrary decision of where to take the diameter reading above the butt swell was of little consequence for those about 4 ft in diameter.

Given a mean of 5.3 mature trees per hectare, the next question was how many cones do they produce per tree per year? Through in-tree determinations by Stecker (see Chapter 7) and on-the-ground calculations by Shellhammer (see Chapter 9), we estimated that the typical mature tree may have about 14,000 cones. The number of new cones added per year was estimated to be about 1,500 on the average, although Stecker has counted over 20,000 new cones in the Castro Tree, added in one seemingly exceptional year.

Assuming that a mature giant sequoia puts on approximately 1,500 new cones in the average year, then a basic question is how many seeds are produced per cone? From the 196 cones examined, a total of 39,684 seeds were counted yielding an average of 202.4 seeds per cone with a range of 47 to 393 seeds. The number of seeds per average cone varied significantly from tree to tree (Fig. 38). Earlier disagreements as to the number of seeds per cone may be due to collections of cones all from one tree. If 1,500 new cones per tree are added and there are 5.3 mature trees per hectare and about 200 seeds per cone, then approximately 1,590,000 seeds would be produced annually in a hectare.

Fig. 38. Variation in number of seeds per cone for different trees. Paired t-tests showed that AC, AE and BC differed (p<.001) and AD and BE differed (p<.05).

Cone scale counts on 183 cones showed a total of 6,136 scales, for an average number of scales per cone of 33.5, and an average of about 6 seeds per scale. This is slightly less than the 36 scales per cone reported by Beidleman (1950). The maximum number of scales per cone reported by him was 56. We found one cone with 61 scales, unusual because all other cones we have examined (several thousand) have a 3/5 Fibonacci series pattern while this one has a 5/8 pattern of cone scale spirals. This ratio still fits the series.

The factors affecting seed fall are varied but generally are of two major types, namely those affecting cones in the trees so that they release seeds, and those that cause cones to fall. The work of Phymatodes nitidus, a cerambycid beetle, is of prime importance in the tree (see Chapter 8). In addition, the Douglas squirrel may feed upon cones in the tree and release seeds. We inferred this happened in 1966 in Trail Area when numerous seeds were found on the ground. Finally, other miscellaneous factors may cause browning of cones in the trees, such as cone bearing branches breaking but not falling from the tree. Fires which cause or increase burn scars at the base may also contribute to the numbers of browning cones that release seeds by reducing water flow through the vascular system.

Some cones that were closed and green in the tree were observed to fall to the ground from storm damage, and thus to release seeds on the ground. Douglas squirrel cut cones often yield seeds on the ground, whether through the feeding activities of the squirrels or from having been forgotten by them (see Chapter 9).

A nine month catchment of fallen seeds and cones revealed a highly variable number from one sample plot to the next. Twenty-five plots, each 1 m², had 983 seeds on them for a mean of 39.3 (SD=57.2) seeds per m² (range 0 to 182). The plot with 182 seeds also had 15 cones present. In another seedfall study of 1.5 months (Sept. & Oct. 1964), 203 seeds were collected on the 25 m² plots. When combined, these data yield a mean of 54.2 seeds per m² per year or 542,000 seeds per ha. The same area (North Area) produced 2,419 seeds in 28 m² plots for an average of 86.4 seeds per m². Pooling data from both years (1964 and 1965), an annual seed fall of about 860,000 seeds per hectare is estimated. The density of the mature giant sequoias in the area is about 6 per hectare.

In tests for rodent predation on seeds, ten screened plots 1/4 m² collected 63 seeds, while the adjacent 25 1 m² plots collected 327 seeds. The 1/4 m² plots, if adjusted for their smaller size, would have collected approximately 252 seeds per 25 m². Apparently predation was not effective in reducing seed numbers on the exposed m² plots inasmuch as they had more seeds per m² than the exclosure plots. Those seeds which come from cones on the ground probably play a small role in disseminating the species because the cones fall beneath existing trees. The seeds released in the tree tops have the potential of being carried laterally great distances. Stecker calculated that seeds fall at about 1.8 m (6 ft) per second and may be dispersed laterally as much as 502 m (1647 ft).

In a detailed ground study, we found an estimated 7520 seeds per m² had fallen by July after a fire the previous September. Only about 1% of these seeds appeared viable in July via the snap test.

The 2 m² plots, under 11 giant sequoias that were monitored daily from August 1 through September 6, 1967, yielded numbers that varied from 2 to 9 seeds per day with an average of 2.4 seeds per m² per day. It appears that seeds fall from trees at a fairly consistent rate during the late summer. If this rate was representative, and it is probably high because it was under a dense stand of mature sequoias, over 8 million seeds per hectare could fall per year.

Once seeds reach the ground, by whatever means, viability must be sustained until proper germination conditions are present. Seeds falling during the summer are often exposed to desiccation and other problems on the forest floor. In a study of 1000 seeds recently removed from fresh cones and placed on the ground, the percent apparently viable in snap tests dropped from 45% on the first day of exposure to 0% on the 20th day. A parallel study of 100 seeds per test for germination in native soils revealed similar results. After one day the germination rate was 38% and after 10 days germination had dropped to only 19%.

Seeds tested for germination in petri dishes with various concentrations of the cone pigment showed a statistically significant (p<.01) delay and reduction in germination at the two highest concentrations of 22 and 18% when compared to the other concentrations (Fig. 39). The data were statistically treated as a regression of y on x with t-tests of standard deviations of means of y (Grindeland pers. comm.).

Fig. 39. Germination of giant sequoia seeds in a gradient of pigment solution concentrations and distilled water.

Seedling establishment

The pattern of seedlings established in a given area was highly variable (Fig. 40). The densest patterns were southeast of mature giant sequoias, thus suggesting that the prevailing winds had been a factor (Fig. 41). Also, it seemed evident that the fires may have played a part. In one case, heat from a burn pile may have been sufficient to brown cones and thus release substantial numbers of seeds, while in another situation the fire was hot enough at the base to cause sloughing off of the bark and cambium, and may have also interrupted water transport to the cones high in the tree, drying them and releasing seeds.

Fig. 40. Concentrations of giant sequoia seedlings adjacent to mature trees. See Chapter 9 for cone load rating.

Fig. 41. Dense stand of staked seedlings (in 1 m2 area.)

Although the first manipulation and use of fire was in September of 1964 in North Area, no seedling sequoias were observed in the treated section. Possibly, however, low seedfall and adverse environmental conditions produced this unexpected result. A bulldozed fireline in Trail Area the same year produced 30 individuals which were numbered and staked as of August 23, 1965. None of these seedlings survived until 1974.

In a survey of the presence of first year giant sequoia seedlings in control versus test sections it was apparent that the fires and manipulations had greatly encouraged establishment of giant sequoias. In the dry year of 1966, 1,565 first year seedlings were located in the Trail Area test section, while only 10 seedlings were found in the control section. Almost 30 times as many giant sequoia seedlings were present in the treated sections of the areas as in the control sections of the meter wide transect survey by Shellhammer, made during the wet year of 1969 (Table 6). The sequoia seedling population had dropped by 86% when surveyed again five weeks later in the last week in August. The treatments also seemed to be favorable to the other tree species, but to a much lesser degree as far as number of seedlings was concerned (Table 7). White fir and sugar pine seedlings showed a 5.3 and 2 times higher density, respectively, in the treated sections than in the control sections (Table 7).

Table 6. Giant sequoia seedlings per hectare (July 1969) in treated (1902m² sample) vs. control (2495m²).

Table 7. Tree seedlings per hectare (July 1969) in treated (1902 m² sample) vs. control (2495 m²).

It seems, therefore, that the "window" opened in the forest floor by fire to encourage the giant sequoia to become established was extended an additional year by the high precipitation. That is to say, if normal or lower than normal precipitation had occurred in 1969, the abundance of giant sequoia seedlings would probably have been very low in both treated and control sections. As a general observation, giant sequoia seedlings do come up each year on undisturbed sites, but only in exceptionally wet years are they at all numerous. We therefore infer that conditions are most favorable to giant sequoia reproduction for a period of two or three years after a disturbance to the forest floor. After that time very few seedlings manage to survive even though some seeds germinate each spring.

Seedling mortality rates

During the study years, the 7,666 live first year seedlings that had been staked in 1964-1967 decreased to only 418 live seedlings by 1974. Thus the treatments, after 10 years, have induced a density of about 90 small trees per hectare while there were none in the control areas. An additional 44 unstaked seedlings of unknown age were observed in the treated sections. These may either be ones that have somehow lost their identification stakes or were not staked at all. They may thus account for part of the loss which is discussed later under miscellaneous factors.

A population of 163 ten-year-old sequoias in Converse Basin (Fig. 3) was followed from 1964 to 1974. During that period there was only a 21.5% mortality, or 2.2% per year on the average. The population was located on three sites identified as good (in a drainage way), moderate (in a swale), and poor (near a ridge top). The sequoia seedlings on various substrates yielded a clear picture as to the importance of hot fires to seedling survival (Table 8). The survival percentage of seedlings growing where there had been a burn pile was 7.7 times greater than the percentage survival of seedlings on other substrates. The data for the 1968 South Area population are biased because the seedlings were found during the last week in August and the first week in September rather than during early summer. Selective die-off of those on other than the burn pile substrate had already occurred, therefore it is possible that a higher percent figure of survival was obtained for this population than the other three populations. If the 1968 South Area population is excluded, the seedlings on burn pile soils survived at 11.5 times those on other substrates.

Table 8. Percent survival of giant sequoia seedlings on burn pile substrate vs. other substrates, as of 1974.

Desiccation during the summer months appears to be the major factor in mortality of seedlings (Table 9). More than half (52.8%) of the seedlings discovered during the dry summer of 1966 died due to desiccation. The only other discernible factor accounting for more than 1% mortality was insect predation at 3.5%. Total mortality at the end of the summer was 62.5% for 1966 but only 26.3% for the 1967 seedlings. Inasmuch as the precipitation was only 78.1 cm (30.75 in) in the 1965-1966 season while it was 172.7 cm (68 in) in the 1966-1967 season, a drop in desiccation mortality might be expected. In contrast, the mortality due to arthropod damage increased from 1966 to 1967 (5.7% to 25.4%). The major mortality (67%) for both 1966 and 1967 seedling populations was desiccation.

Table 9. Factors involved in giant sequoia seedling mortality in Trail Area (1966 population N = 1565, 1967 population N = 3641).

Whereas the average of 39.4% mortality was observed during the three summer months of 1966 and 1967, only 25% died during the nine months of fall, winter and spring of 1967-68. The highest weekly mortality rate for the 1966 population of seedlings was 13% during the month of August. The weekly rate dropped off dramatically to only 4% by September. A population of 1378 seedlings in October of 1967 dropped to 1040 by June of 1968. The summer mortality averaged about 13% per month but mortality was only 3% per month for the rest of the year. These data support the contention that summer desiccation is the major mortality factor for giant sequoia seedlings.

Giant sequoia seedling survival was not only dependent on conditions of substrate and such mortality factors as insect predation, but also varied according to developmental stages (Table 10). Those seedlings which had reached only the cotyledon stage in the summer had only a 20% survival by October, while those which had developed secondary leaves and also started branching, were able to survive at about 75%. inasmuch as seedlings grow roots proportionally larger than shoots, it seems reasonable to suggest that the larger shoots supported larger roots which may have penetrated to greater depths. They had reached the receding soil moistures which helped them survive beyond the summer months into October.

Table 10. Giant sequoia seedling survival with respect to developmental stage during summer (N = 1753).

Growth of giant sequoias

The four populations of giant sequoia seedlings that were measured for shoot growth had an average height of only 31.8 cm in 1974 (Table 11). The shoot height of seedlings in South Area were significantly greater statistically (p<.00l) than those in Trail Area. Those growing in burn pile substrates in South Area were significantly taller statistically than those in other substrates. The mean growth per year in Trail Area was 3.2 cm, while seedlings in South Area grew at about 4.5 cm per year.

Table 11. Average heights (cm) of giant sequoia seedling shoots on burn pile substrate versus other substrates, as of November 1974.

The tallest seedling in 1974 was 135 cm and was growing in a burn pile substrate in South Area. As it was only 7 years old when measured, it had grown at the rate of almost 20 cm a year. The tallest seedling in the other substrates was only 71 cm tall. The smallest seedling still alive in 1974 was only 4 cm tall and was observed in Trail Area on other than burn pile substrate. It was 10 years old, thus averaging only 0.4 cm growth per year.

Seventy first-year sequoia seedlings were measured for height at the end of summer after light determinations had been made at their sites in mid-July. The correlation coefficient for height with percent of full sunlight was positive, but only 0.34 and therefore not significant.

When seedling shoot growth for a summer was analyzed with respect to root growth and site, it was determined that seedlings produced roots 2 to 2.5 times as long in depth as shoots. The amount of growth for each segment of the plant was not significantly different when seedlings in Trail Area were compared with seedlings from the apparently drier Ridge Area. However, the proportions were 34.6% shoot growth and 65.4% root growth at Trail Area, while Ridge Area seedlings had 28.7% shoot growth and 71.3% root growth; thus roots were longer in proportion to the total length of the main axis of the plant in the drier site.

The growth of 10 to 20 year old giant sequoias was ascertained by following three populations in Converse Basin. The good site (drainage way) trees grew at the rate of 17 cm per year (Fig. 42). The moderate site (mesic slope) trees grew at about 7 cm per year. And the poor site (dry ridge) trees grew at only about 1 cm per year. The overall mean growth was only about 5 cm per year. The maximum growth rate for the 10 tallest trees on the good site was about 25 cm per year. The fastest growing sequoia on the good site grew at the rate of 56 cm (1.8 ft) per year.

Fig. 42. Site quality and growth rate of giant sequoia saplings.

The growth rate of giant sequoias older than 20 years was investigated by analyzing the data of the Evans Survey of the South Calaveras Grove in the central Sierra Nevada of California. The analysis of a Converse Basin population described earlier indicates that giant sequoias increase in diameter for the first 800 years at an average of 1 ft per 100 years. The height data suggest that from 100 years of age until 400 years the annual vertical growth rate is about 12.2 cm. After reaching 4 ft in diameter, or 400 years of age, the vertical growth rate declines and typically levels off near 76 m (250 ft) high at between 800 and 1500 years of age. The maximum height obtained by a mature giant sequoia is probably about 110 m (310 to 320 ft).

One impetus for the investigation of the effect of fire on giant sequoia regeneration was the apparent greater growth rates of trees in heavy use areas than in low use areas (Fig. 43). Highly significant differences (p<.00l) between the two areas were noted for average annual increment. The statistical test used was Wilcoxon's rank sum test (Hollander and Wolfe 1973). Those trees subject to heavy use (N=20) included individuals with substantial portions of their root systems covered by roads and/or buildings. Sequoias (N=19) far removed from such activities were cored and found to be growing at a rate not only considerably slower than those subject to heavy impact, but slightly less than growth rates fifty to one hundred years earlier. The question thus raised was, were giant sequoias growing slower in the remote areas due to the lack of natural disturbances such as fire? The experimental burns were designed to see if growth would increase in those trees subject to treatment, as well as to remove accumulated forest floor fuel, and thus induce sequoia reproduction. Five of 15 examined sequoias in 1974 had grown at a significantly faster rate (p<.05) after the treatment in the treated sections than before treatment. The remaining 10 trees showed no significant change in growth rate. None of the 12 trees examined in the control sections showed significant change in growth rate before versus after treatment.

Fig. 43. Growth response of large sequoias to two levels of human activity. Mean annual increments showed significant difference, p<.001.

Grove expansion and longevity of remnants were examined in Redwood Mountain Grove, Grant Grove and Lost Grove (Fig. 3). A casual inspection of the North Calaveras Grove was also made. All of these groves have young trees over 30 m (100 ft) beyond the last large mature tree in at least one point along the perimeter. The Giant Forest inventory shows small trees to be on the periphery in several places. When cored, the young trees were determined to be about 100 years old and coincided with fire release patterns (i.e. dramatic increased growth) in the nearest large giant sequoias in Lost Grove and Redwood Mountain Grove.

The length of time that fallen sequoias have been down in a moist habitat was investigated by coring 6 trees of various species growing in the root pits of sequoias that had fallen into meadows. The trees were at the edges of Crescent, Round, and Log Meadows in the Giant Forest grove in Sequoia National Park. The trees cored at Crescent and Round Meadows were giant sequoias and thus there is a high probability that they came in immediately after the large trees fell because soil conditions are optimal for sequoia regeneration for only a few years after a disturbance. The trees cored at Log Meadow were two sugar pines and three white fir. As these two species produce relatively large seeds which can tolerate thick duff, they may have come in several years after the sequoias had fallen. However, it is most likely that they seeded in a year or so after the large trees fell and opened up a favorable substrate. The oldest tree examined growing in a sequoia root pit was an estimated 135 years of age.

Discussion and summary

Seed production

The giant sequoia produces seeds at a prodigious rate. Not only does it produce seeds at over three times the rate of its closest relative, the coast redwood, on a seeds-per-cone basis, but we determined that it sheds approximately 1,000,000 seeds per hectare per year. Furthermore, when hot fires burn through a giant sequoia forest, seed fall may increase to over 20,000,000 per hectare. This increased seedfall would thus occur at a propitious time when forest floor conditions were optimal for seed germination and seedling survival.

Seeds falling at times other than shortly after a fire are subject to adverse conditions, such as thick litter and duff, and quickly die or fail to produce seedlings. Minor disturbances to the surface stratum, however, enable a few seeds to germinate under satisfactory conditions. An uprooted tree or a river deposit may provide a suitable substrate. Muir (1878) contended that the falling of giant sequoias alone would provide enough suitable substrate for sequoia regeneration. However, the fact that at least a few young sequoias should then be found in almost every fallen giant sequoia root pit is not borne out by field inspection of such root pits. Young sequoias are occasionally found in large giant sequoia root pits, but they are the exception not the rule.

Although exposed mineral soil favors seedling survival in contrast to duff and litter covered surfaces, it is not necessary for seed germination. This concept that mineral soil is required for seed germination continues to persist (Schuft 1972) even though ample evidence was provided by Stark (1968b) that wet litter provides a good germination medium. In 1969 we observed several giant sequoia seedlings in litter and duff that were over 30 cm (1 ft) in depth. In nature however, this medium rapidly dries out in the summer dry period and the seedlings die (Stark 1968a). When wet, the duff may also enable pathogenic fungi to attack the seedlings.

The seeds produced in the cones are released to become potential seedlings in two major ways. There appears to be two complementary reproductive strategies that have evolved in the giant sequoia. One is the persistent constant release of seeds throughout the year and throughout the decades in the absence of fire. The other strategy is the dramatic evulsive event that fire invokes, where seeds are released in tremendous numbers. The constant rain of seeds and cones released by Douglas squirrel and Phymatodes nitidus activity provides a seed inoculum which may find suitable ground conditions in the root pits of fallen trees or the action of streams and avalanches. However intermittent, fire, particularly hot fires, may provide an unusually heavy seedfall. The fires need not be intense throughout the entire area burned, but may, due to the uneven presence or absence of heavy fuel, vary greatly in temperature (Kilgore 1972). These hot spots, as was shown in our burn pile areas, induce exceptional increase in seed release and provide the most suitable seedling substrate.

An additional factor in seed germination may be the allelopathic effects of the cone pigment. Our tests indicated that the higher concentrations of the cone pigment delay and reduce germination. Inasmuch as the pigment is a mixture of organic compounds (Kritchensky and Anderson 1955), it is probable that the high temperatures of fires would destroy it. It is also water soluable and thus would fit the characteristic of other allelopathic substances which prevent germination until sufficient water dissolves and carries them away. The increased number of sequoia seedlings observed in nontreated sections in 1969 after a winter of exceptionally heavy precipitation supports this hypothesis. In addition, it corresponds with the increased survival of seedlings in burn pile soils inasmuch as the higher temperatures there could destroy the pigment. The pigment may function, therefore, to retard some germination conditions for the seeds. The low (less than one percent) germination of seeds taken from the forest floor suggests that only those seeds that escape desiccation are likely to be involved in allelopathy.

Seedling establishment

The two strategies of continuous and intermittent seedfall due to fire have their ramifications in seedling establishment. The continuous stratagem is faced with modest substrate conditions, at best, for germination of the seeds and the survival of the seedlings, while the intermittent stratagem will confront high seed release, favorable seed germination, and exceptional seedling growth and survival. Giant sequoia seedlings in the hot burn pile soils, excluding South Area, survived at over 11 times the rate of those in disturbed or lightly burned soils. Several characteristics of previously highly heated soils may contribute to the outstanding success, survival and establishment of giant sequoia seedlings on that substrate. Highly heated soils are more wettable. The friable nature of the soil readily permits seed and root penetration. The latter is apparently critical for survival of sequoia seedlings because summer drought appears to be the most critical factor in their mortality. Although nutrient levels may be down after a hot fire, St. John and Rundel (1976) have found that seedlings with subadequate nitrogen had a greater root to shoot ratio than those with adequate levels. When one links the above with the shade intolerance of the giant sequoia, an interesting hypothesis emerges.

The greater the sunlight the greater the root penetration even though dry soil may be deeper in sunlit areas (Daubenmire 1974). Thus giant sequoias which survive best in sunlit areas should have greater root length in soils which are subject to rapid desiccation. Burn piles with friable soils and low nitrogen would also aid root penetration and provide the optimal site for seedling survival. Thus it is hypothesized that intermittent fires open the canopy which in turn provides greater sunlight that induces increased root penetration in the friable soil and therefore greater sequoia seedling survival and growth.

Seedling mortality

The substrate in which the test seedlings grew greatly affected their mortality. The relatively high survival of seedlings on burn pile soils was extremely significant. Overall the data indicated a 7.7 times greater survival for those on the soil subject to the hottest fires. In fact, if the South Area data were eliminated on the grounds of the bias due to the late summer finding of seedlings, the difference was 11.1-fold. The mortality factors of the sites helped focus on the unique attributes of highly heated soils.

Desiccation was the most important among all of the factors causing mortality in the summer when mortality rates were at their highest. Apparently when seasonal precipitation was increased the mortality owing to desiccation dropped, while that due to insect predation increased to as much as 25%. An increase in mortality due to fallen branches was noted after a winter of increased precipitation. Of minor importance were heat canker, bird and mammal predation and fungal attacks.

Growth of giant sequoias

Seedling giant sequoias grow at a highly variable rate under natural conditions. The measured extremes were 20 cm and 0.4 cm per year. The substrate, with respect to fire, appears to make a significant difference. In the extreme, seedlings on burn pile substrates grew at over twice the rate of those on substrates only mechanically disturbed. This suggests that not only was seed germination and seedling survival best on burn pile soils, but also that either root penetration and/or nutrient levels over a few years were improved by hot fires, even though their nutrients may be low immediately after a fire (St. John and Rundel 1976).

Radial growth was observed to increase in some giant sequoias which were subjected to treatment by fire and manipulation around them. Since not all of them showed a significant increase in average annual increment the question of determining past frequency of fires on analyses of deviation in radial growth is not conclusive. However, none of the trees in the control areas (and thus not subject to fire) showed a statistically significant increase in radial growth after the dates of fires in their manipulated paired plots.

Radial growth of mature giant sequoias growing where their root surfaces were either covered by buildings or roads showed significantly greater rate of growth when compared to those not subject to such circumstances. The factors which may account for this apparent contradiction rest mainly on the apparently better soil moisture regime produced by roads and buildings. In a similar situation in the Mariposa Grove, Hartesveldt (1964) has suggested that removal of competing vegetation and reduction in evaporation may be major factors. Increased warming of the soil beneath roadways may also facilitate water uptake.

Grove expansion

Although giant sequoia grove perimeters may be relatively stable, as suggested by Rundel (1972a), there is evidence of some expansion. Analyses of tree surveys of Giant Forest and Redwood Mountain Grove indicate that almost all large fallen trees are within the grove perimeters, but several young trees are beyond the large mature trees. Coring of some of the young trees showed that their ages were about 100 years and correlated positively with fire-released growth in the closest large giant sequoias. One young tree was 89 m (275 ft) beyond the large mature sequoias.

Fire adaptations of giant sequoia

Many plant species exhibit adaptations to fire. Chaparral plants have seeds that germinate best when heated and have latent buds that may develop after fire. The giant sequoia also possess several characteristics that are considered as fire adaptations.

Five major criteria of fire adaptation are listed by Daubenmire (1974). Of the five criteria, the giant sequoia clearly qualifies on four of them, and to a lesser extent on the fifth. Specifically and briefly the criteria are: rapid growth, fire resistance, elevated canopies and evanescent lower branches, latent buds and serotinous cones. Rapid growth is generally acknowledged to occur in the giant sequoia, and we have recorded vertical growth approaching 60 cm (2 ft) per year in 10 to 15 year old sequoias under natural conditions. Fire resistance is apparent in the thick bark which lacks pitch and burns poorly. The bark flutes are often 30 cm (1 ft) or more in thickness near the base of the tree, and one we have measured was almost 80cm (2.5 ft) in thickness. Seventy year old sequoias amid white firs of the same age survived the intense McGee burn of 1955, while most of the white firs died.

Giant sequoias are known for their shade intolerance and for the evanescent nature of their lower branches. These characteristics lead to an elevated canopy which in turn reduces the chance of crown fires and death of the trees even though lower branches may be killed by a fire.

Although giant sequoias apparently lack latent buds at the base of the trunk or on the roots, they are capable of sending out branches from the bole of the tree when the old branches are lost. We have observed this on trees of all ages starting with trees only a few years old. This is the one criterion that is not met to the extent of other fire adapted species such as those of the chaparral, where latent buds exist in the root crown.

Finally, the giant sequoia produces serotinous cones which may serve a dual purpose as an adaptation to rodent predation and fire. This concept will be developed further in Chapter 10.