ISSN:0012-9658    

DOI:10.2307/1941717

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941718

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941719

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941720

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

To test the prediction that a mutation causing greater reproductive effort will result in reduced vegetative vigor, we compared the seed production, growth, and carbohydrate status of normal and pistillate genotypes of the grass Tripsacum dactyloides differing in seed production by up to fourfold. We evaluated the costs of reproduction by two methods: experimental manipulation of reproductive effort and comparison of high and low—yielding genotypes. Despite the large difference in seed production, normal and pistillate (high—yielding) genotypes did not differ in growth rate over a 1—yr period. Contrary to predictions, carbohydrate reserves in the high—yielding genotype were significantly higher at the peak of the seed ripening period. Stalk defoliation and complete removal of seed stalks reduced plant growth rate, total aboveground biomass, and biomass of vegetative storage organs, especially when plants were also under stress from biweekly defoliation. However, stalk removal caused increased rates of growth in the year after experimental manipulations. Reproductive tillers were costly to the plant in terms of lost meristems and therefore future plant size, but seed costs were contained within reproductive tillers of both genotypes, explaining the lack of trade—off in the high—yielding, pistillate genotpye. Although experimental reduction of reproductive effort revealed a cost of reproduction, this result could not be used to correctly predict the consequences of a gene for greater seed production.

ISSN:0012-9658    

DOI:10.2307/1941721

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Intraspecifically, plant seed production is a function of both seed size (mean mass per seed) and plant size. In this paper we examined the interspecific relationship between the size of seeds and plants and the mean long—term annual seed production per tree. For canopy trees, we show that seed production is highly (inversely) correlated with the mean seed mass as a power law argument. Tree size (basal area or leaf mass) is directly proportional to seed production over a limited range. Analysis of seed production for herbaceous plants indicates a relationship similar to that for trees. As the exponent relating seed size to seed production is>°1.0, it follows that large—seeded plants produce more total annual crop mass than do small—seeded species. However, this is balanced by the greater investment in ancillary reproductive tissue by smaller seeded species. The results obtained here ought to be of theoretical and applied interest in, for example, stand—level simulations of population dynamics or for planning the optimal size and shape of clearcuts intended to be regenerated naturally.

ISSN:0012-9658    

DOI:10.2307/1941722

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Some species of coastal sage and chaparral shrubs of California are extremely tolerant of tissue dehydration, surviving water potentials as low as —9 MPa during dry summer months. Such low water potentials (high tensions on xylem water) are known to cause severe embolism formation in the xylem vessels of woody plants, blocking water transport and potentially causing shoot dieback. Thus drought hardly species of coastal sage and chaparral are either extremely resistant to water stress—induced embolism or they become severely embolized during summer drought. We compared the seasonal changes in xylem water potential and xylem embolism (percent loss in hydraulic conductivity of stem segments due to air emboli) between co—occurring Salvia mellifera (coastal sage) and Ceanothus megacarpus (chaparral) growing in the Santa Monica Mountains of southern California. We also determined the relative sensitivity of each species to water stress induced embolism by artificially dehydrating branches and measuring percent loss in hydraulic conductivity of xylem tissues at a given water potential. We found that both species experienced the same minimum in seasonal water potentials (—8MPa) but the xylem of S. mellifera lost 78% in hydraulic conductivity whereas the xylem of C. megacarpus lost only 17% in hydraulic conductivity. These values for a natural plant community were within 10% of those predicted by our artificial dehydration curves. Our estimate of susceptibility to water stress—induced embolism indicated that 50% loss in hydraulic conductivity would occur at —4.5 MPa for S. mellifera but at ‐11 MPa for C. megacarpus. Irrigation of S. mellifera for one summer reduced loss in conductivity from 78 to 38% and increased leaf areas 10—fold, indicating that xylem embolism and leaf drop were drought induced. Our results show that xylem tissues of S. mellifera are more sensitive to water stress and tissue dehydration than those of co—occurring C. megacarpus. The observed ability of S. mellifera to inhabit drier sites than C. megacarpus may result from drought deciduousness in summer and high growth rates in spring that facilitate rapid construction of new xylem and leaf tissues. It may be that facultative drought deciduousness in coastal sage is tightly coupled to drought—induced embolism of xylem tissues.

ISSN:0012-9658    

DOI:10.2307/1941723

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We studied the variation in growth of individual trees in a population of Pinus sylvestris near Zurich in relation to tree age, size, and local competition. This population established naturally when the drainage pattern in a peat bog was altered after the Second World War. An unusual opportunity to study stand development was afforded when the trees were harvested in 1987 as part of a restoration program. All trees stumps were mapped, and growth since establishment was measured as cross—sectional area of wood in 5—yr intervals, using disks from the base of each tree. The distribution of cross—sectional area was lognormal over most of the period of growth. Size at the beginning of a time interval was the primary determinant of individual growth during that interval, but age and local competition also had significant effects. Younger, smaller trees were better fitted by an exponential growth model, whereas older, larger trees were better fitted by a Gompertz (sigmoidal) model. Some of the result were very different from previous studies: (1) The spatial distribution of trees was not significantly different from random. (2) Size variability decreased during the course of stand development, even though there was no self—thinning. This was because (3) the density of natural establishment was low, and competition, although clearly detectable, was still relatively weak and symmetric after 45 yr of stand development. Thus, there was noinitial advantage in competition, and the younger trees, which were still growing exponentially, were able to catch up with the older trees, whose relative growth rates were declining.

ISSN:0012-9658    

DOI:10.2307/1941724

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Density dependence was examined in two species of neotropical treelets, Faramea occidentalis and Desmopsis panamensis, in a 50—ha plot on Barro Colorado Island in Panama. Survival and growth probability of plants larger than 1 cm dbh (diameter at breast height), and recruitment into the 1—cm class, were assessed as a function of the number of conspecific neighbors in various distance and size classes. Density—dependent effects on survival and growth were strong in Faramea. Performance of 1—8 cm dbh plants declined with increasing numbers of adult neighbors within 1 m, 1—2 m, and 2—4 m, but neighbors at 4—6 had no effect. Performance also declined with increasing numbers of juvenile neighbors<4f cm dbh, but the effect was less pronounced. Saplings of Faramea grew poorly in areas of high juvenile density within 30 m, independent of the effects caused by neighbors within 4 m. In contrast to Faramea, Desmopsis showed no density dependence in survival nor in growth. For recruitment, however, the two species showed similar patterns: recruit density was lower in regions with an adult conspecific within 1 or 2 m distance, but higher in regions with high densities of adult or juvenile conspecifics between 2 and 30—60 m. Desmopsis must have suffered density—dependent effects at stages<1 cm dbh, but the effect vanished by the 1 cm dbh stage. In Faramea, strong neighborhood effects were evident in all size classes<8 cm dbh.

ISSN:0012-9658    

DOI:10.2307/1941725

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The relative importance of interspecific interference and facilitation in natural plant communities is controversial, in part due to the difficulty of manipulating these interactions in the field. I investigated the relative intensity of facilitative and interfering effects of Arthrocnemum subterminale, a subshrub common in the upper zones of California salt marshes, on three species of winter annuals by measuring spatial associations, manipulating Arthrocnemum canopy cover and root distributions, and comparing aspects of the physical microhabitat under Arthrocnemum to that in the open. The annuals Hutchinsia procumbens and Parapholis incurva were positively associated with Arthrocnemum, and survival and aboveground biomass of these species declined when shrub canopies were removed. Spergularia marina was negatively associated with Arthrocnemum, and survival increased when shrub canopies were removed. Combined effects of facilitation and interference were observed for Parapholis survival and biomass, which were reduced by root interference even though the overall effect of Arthrocemum on Parapholis was positive. Arthrocnemum canopies elicited direct effects on the winter annuals, but also altered substrate characteristics. Soil was mounded under the shrubs, averaging 3.14 cm higher in elevation than the surrounding open soil. Soil salinity was 27% lower under Arthrocnemum canopies, soil moisture was 13% higher under shrubs, and shrub understories received less light than open marsh soil. Although Arthrocnemum had species—specific interfering and facilitative effects, and interference and facilitation operated simultaneously, my results suggest that facilitative effects of Arthrocnemum were more significant than interference in the transition zone of the upper marsh. Physical conditions in the open reduced overall annual biomass and survival, but were ameliorated by Arthrocnemum, which altered the species composition and productivity of the upper marsh plant community.

ISSN:0012-9658    

DOI:10.2307/1941726

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY