John Muir: The Mountains of California. [4]
Chapter 3: The Snow.
"The first snow that whitens the Sierra, usually falls about the end of October or early in November, to a depth of a few inches, after months of the most charming Indian summer weather imaginable. But in a few days, this light covering mostly melts from the slopes exposed to the sun and causes but little apprehension on the part of mountaineers who may be lingering among the high peaks at this time.
The first general winter storm that yields snow that is to form a lasting portion of the season’s supply, seldom breaks on the mountains before the end of November.
(...)
The first heavy fall is usually from about two to four feet in depth. Then, with intervals of splendid sunshine, storm succeeds storm, heaping snow on snow, until thirty to fifty feet has fallen. But on account of its settling and compacting, and the almost constant waste from melting and evaporation, the average depth actually found at any time seldom exceeds ten feet in the forest region, or fifteen feet along the slopes of the summit peaks."
"It often happens that while one side of a lake basin is hopelessly snow-buried and frozen, the other, enjoying sunshine, is adorned with beautiful flower-gardens."
USDA Forest Service Database [3]
Seedling development
"Giant sequoia seeds germinate best when totally buried in disturbed mineral soil. April, May, September, and October temperatures are best for early development."
"Soil moisture conditions and seedling survival are generally better in spring than during any other season. Light conditions are generally best for growing at one-half full sunlight."
[Note micha: Because of the clean mountain air and the southern latitude, energy input in the Sierra Nevada is considerably higher than in lowlands of north-western Europe, where i would suggest to expose SeqGig to full sun as soon as possible.]
Upon germination, the seedling stands 3/4 to 1 inch (1.9-2.5 cm) high, usually with four cotyledons. By autumn, seedlings have up to six branches and are 3 to 4 inches (8-10 cm) tall. After the second year, the seedling attains a height of 8 to 12 inches (20-30 cm) with a taproot penetrating to a depth of 10 to 15 inches
(25-38 cm) [28]."
R. J. Hartesveldt et al: The Giant Sequoia of the Sierra Nevada [1]
Chapter 5
Conditions for germination
"Yet, in some sequoia groves, there are extensive areas of almost bare ground within the seeding range of mature sequoias on which we find no sequoia seedlings year after year. In other instances, mineral soil plays absolutely no role in germination and small sequoia seedlings growing from rotting stumps and other masses of organic debris such as thick leaf litter."
"Mineral soil, then, is only one of several influences upon germination of sequoia seeds. Statistically, it is the most important substrate for germination, but is not an absolute requirement."
"When lying on the exposed soil surface, the seeds not only quickly lose their viability but also seem to germinate poorly even when the soil is moist. Fry and White (1930) claim that seeds pressed against the soil surface by heavy snow germinate well, but recent experimentation indicates that germination is greatest when the seed is completely surrounded by moist soil, as when burried. That seeds germinate on the surface is well documented; but again, survival is extremely low."
"Stark's field experiments indicate that although germination actually occurs over a very wide range of temperatures, namely, from 30° to 92°F (-1.6°C to 34°C), optimum temperatures are most common during the months of April, May, September, and October. Soil moisture conditions and seedling survival are generally better in the spring than during any other season. High summer temperatures and the resulting desiccation of the soil greatly reduce germination."
"Experiments indicate that sequoia seeds will germinate in full sunlight and also in the dark, but that optimal germination occurs during the growing season when the light is approximately one-half full strength of the sun. When stronger, light is converted into excessive heat energy and thus dries the soil."
"Closely allied is the pH or degree of acidity or basicity of the soil, Stark (1968) found that a slightly acid soil (pH 6-7) produced the highest germination percentage at a temperature of 68°F and concluded that pH was not a limiting factor in natural sequoia habitats. She found that strongly basic soils (pH 9) stunted the seedling growth, but did not retard germination. It did, however, alter the color of the foliage to an intense blue-green."
"We have already mentioned that seeds on the surface generally do not germinate because insufficient moisture is transmitted to the embryo. Burial of the seed is important, but the seed must not be buried too deeply. While seeds placed deeper than 1 inch may germinate, the developing shoot will seldom reach the surface and survive. The optimum depth, which seeds rarely exceed in normal circumstances, is about 0.25 inch (Beetham 1962)."
"Seeds will often become wedged in a small crack in the soil alongside a partially buried rock or piece of wood, which provides the necessary protection against radiation and proper soil moisture conditions. This is also an advantage in seedling survival, the next most delicate stage in the sequoia life cycle (Hartesveldt and Harvey 1967)."
Conditions for seedling survival:
"Muir (1878) records that not one seed in a million germinates, and that not one seedling in 10,000 attains maturity. These figures, widely repeated, may be figurative rather than literal, but the frailty of the species during this stage is no myth."
"Because of the small amount of food stored in sequoia seeds, the newly germinated seedlings must become rapidly self-sufficient. Fry and White (1930) state that the earliest stage of germination (extension of the radicle or primary root) takes place beneath the snow, and that the seed roots are as much as 1 or 2 inches long before the snow melts. This may affect the survival of the emerging cotyledons, or seed leaves. As soon as the protective seed coat is shed from the new leafy shoot, a root system must be functional to supply the cotyledons with the necessities for photosynthetic activity, and because sequoias apparently produce few, if any, root hairs, root length becomes the more essential."
"Beetham (1962) has amply demonstrated that seedlings grow best in full sunlight where the soil is protected by at least a light layer of leaf litter."
"One of the more difficult forms of seedling death to assess is that from reduced light brought about by canopy shading that may starve the plant. Although Baker (1949) lists sequoia as having intermediate tolerance to shade, Beetham (1962) indicates clearly that it is very sensitive to low light intensity. This is supported by the fact that sequoia seedlings are seldom found in areas densely populated with taller vegetation. A striking example of death influenced by shading was found by Hartesveldt (1963) in Yosemite's Mariposa Grove. At the end of a 25-year period, of the several thousand seedlings established there and recorded on a park map dated 1934, only 13.8% remained alive in a 1959 resurvey of the same area. Hundreds of dead saplings, twisted and contorted in dense shade, demonstrated the effect of the heavy overtopping crown canopy composed largely of white fir (Fig. 10).(...) Undoubtedly, excessive shading may be coupled with another agent such as root fungi or poor soil-moisture conditions."
"Excessive moisture is a factor which limits gas exchange at the root surface because it usurps the pore space normally occupied by gases. Low soil oxygen content reduces root respiration which reduces water intake and photosynthesis, eventually to the point of cessation. This is probably a common cause of seedling death along the edges of meadows where seeds of sequoias are often scattered abundantly, but where seedlings seldom survive."
H. Thomas Harvey et al: Giant Sequoia Ecology - Fire and Reproduction [2]
Introduction
"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."
"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."
"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."
"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."
Discussion and summary: Seed production
"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."
Seedling establishment:
"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."
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Thema: Bergmammutbaum (Sequoiadendron): Ökologie des Keimlings (Gelesen 24053 mal)