摘要:

Tissue water potential is the most important factor throughout the seasons controlling phenological events, photosynthesis, and productivity of Larrea divaricata growing in Deep Canyon near Palm Desert, California. Growth of reproductive structures was initiated at the time of highest tissue water potential and ceased as water potential decreased. Percentage foliation correlated strongly with dawn water potential (r = 0.89). The elongation rate of stems and the rate of node production were both dependent on tissue water potential. Leaf growth and node growth proceeded at varying rates throughout the year, providing a continuous sink for photosynthates. Photosynthesis rates ranged from 9.02 mg CO2incorporated per day per gram dry weight of leaf tissue in September to an estimated 74.7 mg CO2in early February. Net photosynthesis and relative productivity correlated very strongly with dawn water potential (r = 0.93 and r = 0.97, respectively). Larrea plants were labeled at 1— to 2—month intervals with photosynthetically incorporated14CO2to determine the utilization in growth and storage of photosynthate fractions produced at various times throughout the year. Tissue was subsampled at similar intervals, and the activity in various metabolic compounds (sugar, starch, lipid, organic acid, amino acid, protein, cellulose, and cell—wall materials) was analyzed. The utilization of photosynthates in the various fractions was similar in all seasons. No appreciable mobilization into and out of storage materials was apparent. Never dormant, Larrea remains metabolically active and forms new tissue throughout the year. This growth pattern may be an important adaptation allowing Larrea to exist in a wide range of geographical and climatic areas, and, perhaps owing to the species' tropical affinities, it might have been a preadaptation to the desert environment.

ISSN:0012-9615    

DOI:10.2307/1942260

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY

摘要:

The modern forest of the Catskill Mountain region, New York, particularly the area within the New York State Forest Preserve, was studied and compared with the presettlement forest as determined from surveyors' records. The most notable changes were a marked drop in the proportion of beech (Fagus grandiflora) and hemlock (Tsuga canadensis) and increases in sugar maple (Acer saccharum), red oak (Quercus borealis), and chestnut oak (Quercus prinus). Reproduction and size—class data suggest that in old—growth hemlock stands hemlock is replaced by hardwoods and is not a part of a homogeneous climax forest in spite of its prominence in the presettlement forest. Beech does not give evidence, in the dynamics of the modern forest, of returning to its presettlement dominance. Some pioneer stands are as rich in species and as diverse as are those usually thought of as climax. The comparison of presettlement and recent forests is related to recent discussion of climax concept.

ISSN:0012-9615    

DOI:10.2307/1942261

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY

摘要:

The alpine flora of the Sierra Nevada has developed relatively recently and largely in situ from western American sources. The Sierra thus provides a good site for an attempt to answer the question: "How does an alpine flora originate?" The primary study area was a transect from the desert near Bishop, California (1,400 m), to Piute Pass in the Sierra Nevada (3,540 m). Upward along the transect the vegetational gradient is Ephedra nevadensis—Tetradymia spinosa desert shrub, Pinus monophylla—Artemisia tridentata open woodland, Pinus jeffreyi open forest, Pinus murrayana forest, Pinus albicaulis—subalpine herbaceous vegetation, and scattered alpine communities. Only 19% of the alpine species at Piute Pass occur in the Arctic, whereas 38% are held in common with the Rocky Mountains. Species endemic to the Sierra (17%) are in genera predominantly from the Californian or Great Basin floras of lower elevations. A number of species have populations in the desert and also at high elevations near the alpine zone. Most of the alpine flora consists of perennials, but several annual species are also present. Annuals are rare in other arctic and alpine floras. Environmental monitoring stations were maintained at several locations along the transect during two summers. Air temperature at 5 cm above the ground decreased with increasing elevation at a rate of 0.74°C/100 m. Summer precipitation increased nonlinearly with elevation at a rate of 0.16—0.82 cm/100 m. Long—term annual average precipitation for Bishop is 14.6 cm; the annual precipitation for Piute Pass estimated from this study is greater than 78 cm with a strong winter maximum in the form of snow. Soil moisture during the summer is low for all sites. Strong vegetation patterning occurs in both alpine and desert areas along drainageways from snowbanks or perennial streams. Solar, sky, and net radiation at 1 m above the soil are greater for alpine than for desert areas. Air and plant—tissue temperatures near the surface of the alpine soil are higher than those in most other alpine areas. Laboratory experiments showed several physiological responses to be characteristic of Sierran alpine populations. These may be important in plant evolution and migration into an alpine habitat. (1) Germination of seeds from alpine plants occurred maximally between 20° and 30°C. Constitutive dormancy is a minor factor; the low winter temperatures of the alpine zone appear to operate as an exogenous dormancy control. Desert and low—elevation species of this area are predominantly winter—germinators and have maximum germination at low temperatures. (2) Mature alpine plants have strong dormancy control by short photoperiod; dormancy in lower elevation populations may be induced by either short or long photoperiods. (3) Temperatures of the upper photosynthetic compensation point and maximum net photosynthesis are lower in plants of alpine species. (4) Low temperature regimes cause plants to shift dark respiration higher rates when compared with plants of the same species from high temperature regimes; this change appears to have both genotypic and phenotypic causes. (5) Considerable acclimation of dark respiration to a change in temperature can occur in as little as 8—10 hr, beginning 1—2 hr after the change. The speed of acclimation increases with increasing temperature and is genetically based. Populations from higher elevations have faster acclimation rates than those from lower elevations. This characteristic could allow alpine plants to adapt rapidly to changing weather conditions. (6) Acclimation of dark respiration does not appear to be related to changes in leaf water potential, diffusion resistance, or isozymes of three dehydrogenases. It is related to changes in the rate of mitochondrial oxidation (7) Translocation of starch from chloroplasts at low temperatures was impaired in a desert species, but was maintained in an alpine species. Although a number of physiological processes in alpine plants are adapted to low temperatures, there is no indication that any single process, or adaptation to temperature alone, is responsible for the evolution and success of an alpine species. Efficient utilization of a short, clod growing season may be the most important selective characteristic in the origin of an alpine flora. Some genera of lower elevations near the Sierra are already preadapted, in their winter and spring growth patterns, to such a low—temperature regime and may have provided a part of the present Sierran alpine gene pool.

ISSN:0012-9615    

DOI:10.2307/1942262

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY

摘要:

The salt marsh at Barnstable, Massachusetts, occupies an embayment into which it has spread during the past 4,000 years. It exhibits all stages of development from the seeding of bare sand flats through the development of intertidal marsh to the formation of mature high marsh underlain by peat deposits more than 20 ft deep. Observations and measurements of the stages of its formation are presented. The geomorphology of the marsh is considered in relation to the factors which have influenced its development, i.e., the ability of halophytes to grow at limited tide levels, the tidal regime, the processes of sedimentation, and the contemporary rise in sea level. The rates at which the early stage of development takes place have been determined by observations during a period of 12 years and the time sequence of later stages by radiocarbon analyses.

ISSN:0012-9615    

DOI:10.2307/1942263

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY

摘要:

Factors contributing to breeding success of puffins were studied on Great Island, Newfoundland, in 1968 and 1969. Puffin burrow density was negatively correlated with distance from the cliff edge and positively correlated with angle of slope. These correlations are biologically significant in that close to the cliff edge, where the angle of slope was steep, breeding success was significantly higher than on adjacent level habitat. In spring both habitats were occupied simultaneously, and nest—site tenacity was equally strong in them. During settlement the frequency of fighting was higher and the peak was reached earlier on slope habitat. Males were heavier on slope than on level habitat just after peak egg laying, although wing lengths were similar. Females were similar on the two habitats. Measurements of eggs from both habitat were the same. Egg—laying dates were also similar, but annual variation was greater on level than on slope habitat, as was variation within a single year. Hatching success was higher on slope habitat, mainly because the incidence of egg disappearance was lower during incubation; also the frequency of infertile eggs was greater on level habitat. Fledging success was higher on slope habitat, and higher on both habitats in 1969. Frequency of chick deaths in the nest and disappearance before fledging was higher on level habitat in both years. Fledging success was higher for early—hatched chicks in both habitats. Total breeding success was higher on slope habitat, and higher in both habitats in 1969. Fledging condition of chicks varied according to the habitat and time period in which they were raised. On the average, body weight at fledging was greater and less variable for birds on slope habitat; early—hatched chicks were heaviest in both habitats. Wing length of young at fledging did not differ. Early—hatched chicks on slope habitat fledged quickest (also quicker than early—hatched chicks on level habitat); late—hatched birds on slope fledged slower than late—hatched birds on level habitat. Age at fledging was greater in 1968 than in 1969 on both habitats. Meal size delivered to chicks by parents was the same on the two habitats, but frequency of feeding was greater on slope than level habitat. Breeding pairs on slope habitat were unable to raise two chicks (artificial twins) to fledging; similarly, one adult could not rear a single chick. Adults feeding chicks on level habitat were attacked and robbed more frequently by gulls than birds on slope habitat, probably because escape (take—off) from an attack was quicker on slope than on level ground. Experiments performed to determine causes of the differential egg and chick loss before fledging in the two habitats showed that during incubation the proportion of eggs displaced to the burrow entrance by incubating birds when leaving the burrow in a hurry was the same, but the rate of panic flights was higher on level habitat. Also, chicks when starved spent more time at the burrow entrance than when fed regularly. It is concluded that the difference in breeding success on the two habitats was due to a higher exposure of eggs and chicks to gull predation on level habitat. The primary cause for this differential exposure was that adults on slope habitat were less vulnerable to gull disturbance during incubation and gull robbery when feeding their chicks. Thus breeding failures resulted from the interactions of food shortage and gull interference. This conclusion was tested by comparing breeding performance of birds at Great Island with birds at two islands where gull interference was absent. Egg and chick survival was greater under "gull—free" conditions. In addition, body weights at fledging were higher and less variable at the colonies without gull interference than at Great Island. The ways in which natural selection acts upon puffins at the breeding colony are considered; a model relating puffin nest distribution and habitat features is presented, and predictions which might be tested are outlined.

ISSN:0012-9615    

DOI:10.2307/1942264

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY