摘要:

Productivity of Agave lechuguilla, a commercially harvested plant that occurs over vast areas of the Chihuahuan Desert, was measured using conventional dry mass changes in the field and was predicted based on physiological responses to environmental variables in the laboratory. An environmental productivity index (EPI) was constructed as the product of indices for water status, leaf temperature, and photosynthetically active radiation (PAR). Each of these component indices was assigned a maximum value of unity when that variable was not limiting net CO2uptake over a 24—h period. Soil water potential, daily air and leaf temperatures, and PAR in the planes of the leaves at the field site in Coahuila, Mexico, could thus be quantitatively described in terms of their effect on net CO2uptake. Water status proved to be the most important of the three variables considered, nearly all leaf unfolding occurring during the relatively wet summer and early fall. The rosette habit and low leaf area index (1.23) for the plant groups monitored led to a relatively uniform and high level of PAR over the leaves. Seasonal changes in PAR proportionally affected both the modest daytime net CO2uptake and the predominant nighttime net CO2uptake for this crassulacean acid metabolism plant, while seasonal variations in temperature had relatively small effects on net CO2uptake over a 24—h period. EPI was highly correlated with the number of new leaves unfolding each month in the field (°2°0.83); counting unfolding leaves in a nondestructive method of estimating productivity. For the 1—yr study period EPI averaged 0.28, which led to a predicted annual dry mass gain per unit leaf area of 0.68 kg/m2. Field measurements indicated that the actual dry mass gain was about half this value, the difference representing photosynthate needed for constructing and maintaining folded leaves, stem, and roots. The productivity of A. lechuguilla per unit ground area explored by its roots was 0.38 kg.m—2.yr—1, which although much less than for agricultural crops, is still much greater than the average productivity for desert ecosystems.

ISSN:0012-9658    

DOI:10.2307/1938497

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

In the Rocky Mountains, USA, year—round wind exposure has been correlated with widespread needle dehydration and death in winter for alpine timberline conifers. Needle death may be due solely to the drying effects of winter wind or may also result from predisposition to winter injury associated with suboptimal summer growth conditions. Our experiments showed that in the lower timerbline ecotone in southwest Wyoming, needle mortality was primarily due to winter wind and cuticle abrasion; death was frequent only in wind—exposed needles of flagged trees and surface needles of krummholz mats. At higher elevations where only krummholz mats and a few flagged trees exist, mortality averaged ≥75% for needles unprotected by snow, regardless of wind exposure. Snow covered needles had low mortality throughout the timberline ecotone. Winter death of naturally wind—exposed needles of Picea engelmannii occurred at ≤—4.5 MPa water potential (≤60% relative water content). Cuticular resistance of wind—exposed needles declined from 100—250 ks/m in autumn to ≤30 ks/m by midwinter. Experimentally reversing the windward—leeward orientation of small, flagged trees in early winter resulted in lower xylem pressure potentials and needle viabilities for newly wind—exposed (originally leeward) compared to newly sheltered (originally windward) shoots. Also, sheltering exposed branches from winter wind on flagged trees in the lower timberline ecotone (3200 m) increased mean overwinter needle survival from near 0 to ≥50% in both Picea engelmannii and Abies lasiocarpa. Scanning electron micrographs of dehydrate wind—exposed needles collected in March at this site showed little cuticular surface wax, probably because of windborne ice crystal abrasion. However, winter dehydration and death in both experimentally and naturally wind—sheltered needles at higher elevation may have been due to inadequate needle maturation during summer, which could act to exclude flagged trees from the upper timberline ecotone.

ISSN:0012-9658    

DOI:10.2307/1938498

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Four populations of Sedum wrightii were studied with respect to their photosynthetic carbon metabolism. This leaf—succulent species exhibits crassulacean acid metabolism (CAM). Plants that possess CAM have the potential to vary their rates of transpirational water loss greatly in response to drought. Leaf thickness and biomass °13C values were measured in mid growing season for the populations in their native habitats. The populations exhibited significant differences in both leaf thickness and °13C values ranged from —13.8% to 22.9%. In the field, populations differed in the proportion of day vs. night CO2uptake during growth. Three of the populations were compared in a controlled—environment study. It was found that there are both environmental and genetic determinants of the differences in photosynthetic carbon metabolism. We observed a significant correlation between plant growth and biomass °13C value. The largest plants exhibited the greatest proportion of day vs. night CO2uptake. These finding suggest that in this species there may be an inverse correlation between the ability to converse water and the ability to gain carbon. The unusually wide range of phostosynthetic phenotypes exhibited in this species may well explain its presence over a wide range of environments.

ISSN:0012-9658    

DOI:10.2307/1938499

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Survivorship and allocation to seed and ramets of sexual and apomictic Antennaria parlinii were studied to determine whether these forms differed in colonizing and competitive ability. Apomictic plants produced more seeds per inflorescence, but their mean mass per seed was lower compared to sexual plants. However, because sexual seedling survivorship was higher than that of apomictic seedlings, the estimated numbers of offspring recruited, when male seedlings were included, were not different. Thus the primary difference in colonizing ability between sexuals and apomicts appears to be in the potential for dispersal provided by the greater number of lighter seeds produced by apomictic plants. The pattern of allocation to ramet production, one indication of potential competitive ability, was different among males, females, and apomicts. In general, both mean length of a stolon and total stolon length per plant were lower in apomicts than in males and females. Apomicts also had the highest frequency of ramet mortality, while males had the lowest. This pattern of ramet production in sexuals results in a growth form of a solid mat of long—lived, wandering stolons. Males consistently had the lowest mortality and also the highest total allocation to ramet production. Although all of an apomict's offspring bear the costs of maternity, the sons of a sexual female do not. Therefore, sexuality may be advantageous in competitive environments because of the higher adult survivorship and increased opportunity for future reproduction in those offspring that can reproduce as males.

ISSN:0012-9658    

DOI:10.2307/1938500

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The relative success of six populations of the annual Phlox drummondii raised under conditions of intraspecific competition was studied in five field sites and an experimental garden located in central Texas. All pairwise combinations of seedlings from six Phlox populations and pure stands of each were planted at a density of eight plants per styrofoam cup. The cups were taken to the field sites, the bottoms were removed, and the cups sunk into the soil. Five replicates of each population pair were transplanted into each of the six sites from which the seeds had been collected. After the flowering season the pots were collected and the number of flowers (or fruits) and amount of biomass per plant were determined. Flower production was strongly related to biomass (° =0.96, P<.0001). In general, pure stands had higher mean flower production than mixed stands, and in mixtures there was a significant tendency for plants grown at their native site to outcomplete aliens in most sites. The production differential between natives and aliens was greater in mixed stands than in pure stands. No one population performed best in all sites and all populations performed best in all sites and all populations performed best in the experimental garden. The results indicate that Phlox drummondii is locally adapted to its microenvironment. The specific outcome of competition is dependent on the site in which competition occurs and the orgin of the competitors.

ISSN:0012-9658    

DOI:10.2307/1938501

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Within a grassland in southern Arizona, Stipa neomexicana occurs only on dry ridge crests with low total grass cover, while total grass cover is greater below the ridge crests in moister, low—lying areas. I hypothesized that Stipa neomexicana was limited to these dry ridges by competitive exclusion. This hypothesis was tested by removal experiments conducted at three positions along the topographic gradient. The responses of Stipa were compared with those of Aristida glauca and other neighboring species. The predictions made under the hypothesis were confirmed; competitors limited seedling establishment, seeding survival, flower production, and the growth of mature plants. The response to the removal of competitors increased from the ridge crest to the downslope positions. Competition depressed estimated finite rates of population increase for Stipa neomexicana, particularly on the lower slope. Thus, increasing competition from neighboring grasses along the topographic gradient was responsible for restricting Stipa neomexicana to the unfavorable ridge crest sites.

ISSN:0012-9658    

DOI:10.2307/1938502

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Sediments in three ponds between 1300—1500 m on the south side of Mt. Rainier were examined for plant macrofossils and pollen. Macrofossils of seral species such as Abies lasiocarpa, Pseudotsuga menziesii, Pinus monticola, Abies procera, and Pinus contorta are conspicuous from 6000 to 3400 BP. These species suggest a climate that was warmer/drier than today and favored frequent fires. Neoglacial cooling may have begun 3700—3400 BP, as species typical of higher elevations became prominent; a decline in seral species after 3400 BP suggests less frequent fires. In the last 100 yr, Tsuga heterophylla became abundant and then declined at the highest elevation site. General trends in pollen percentages are similar to the macrofossil curves. Tephra deposition from Mt. Rainier and Mt. St. Helens did not produce conspicuous changes in forest composition. Few major fires are evident from charcoal and macrofossils at these sites.

ISSN:0012-9658    

DOI:10.2307/1938503

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The effects of nitrogen availability on N cycling and N use efficiency (NUE) were examined in natural and fertilized loblolly pine stands on the upper coastal plain of South Carolina. Indices of N availability, based on potential rates of N mineralization, ranged from 1.6 to 11 kg°ha—1(8 wk)—1in the stands, and concentrations of N in foliage, wood, fine roots, and needlefall increased with greater N availability. Litterfall dry masses, net aboveground production, and litterfall nitrogen were all positively correlated with N availability, while indices of NUE decreased with increased N availability. Mechanisms that could explain increased NUE in low—N sites were examined in the field and in a phytotron study. First, nitrogen retranslocation on a per needle basis did not change significantly across the N—availability gradient, and thus could not account for the change in NUE efficiency. For the stands as a whole, however, substantially more N was retranslocated at the highest levels of N availability. Second, an increase in N uptake efficiency could not account for an increase in NUE with low N availability, since phytotron—grown seedlings fertilized with N had significantly higher rates of N uptake per unite root mass and lower root: shoot ratios than N—limited seedlings. Third, net production per unit N within pine seedlings was significantly higher in the N—limited plants, suggesting that an increase in the amount of carbon fixed per unit of tissue N could account for the observed increased in NUE in loblolly pine stands at low levels of N availability.

ISSN:0012-9658    

DOI:10.2307/1938504

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Patterns of nitrogen cycling were studied in N—fertilized (N: 95 kg/ha, P: 45 kg/ha, K: 135 kg/ha) and “unfertilized” (no N, but the same levels of P and K) agroecosystems with heavy weed infestations. To calculate monthly standing crop nitrogen levels, nutrient concentrations were multiplied by biomass values for each month. Soils were also sampled monthly at three depths; precipitation was sampled weekly and analyzed for N inputs. Nitrification and denitrification enzyme activities were measured, as well as rates of denitrification from unaltered soil cores. Nitrate leaching was measured with porous cup lysimeters. Weeds completed more vigorously with the crop in fertilized treatments than in unfertilized treatments, resulting in greater crop growth in unfertilized treatments. Nitrification, denitrification, and leaching losses of N were greater in fertilized treatments than in unfertilized treatments, especially after residue input following harvest of the summer crop. Soil organic nitrogen tended to be lower in unfertilized treatments than in fertilized treatments after summer cropping, whereas soil organic carbon showed an opposite trend. As a result the carbon: nitrogen ratio of soil organic matter was significantly higher in unfertilized treatments than in fertilized treatments. Fertilized treatments showed a positive N balance (47 kg/ha) while unfertilized treatments showed a net loss of N (57 kg/ha). Nitrogen use efficiency by plants was higher in unfertilized treatments than in fertilized treatments, but was low relative to many unmanaged ecosystems.

ISSN:0012-9658    

DOI:10.2307/1938505

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The dominant microbial transformations of ammonium and nitrate in the sediments of a tidal freshwater marsh estimated from measured rates of isotope dilution of ammonium [15N] added to sediment and litter samples, reduction of nitrate[15N] to ammonium[15N] in fresh sediments, and net exchange of ammonium and nitrate between the marsh and river water in flumes built in the field. In two separate laboratory experiments, nitrification in mixed, fresh, surface sediments was 2.8 and 3.4 nmol°cm—3.h—1. In undisturbed field sediments, nitrification rates are probably ≤1 nmol°cm—3°h—1. Additions of nitrate[15N] to fresh sediments showed that nitrate produced from ammonium by nitrification was reduced very quickly. Less than 10% of this nitrate was reduced back to ammonium by dissimilation. The net transfer of nitrate in this marsh is to the sediments, and since nitrate does not accumulate in these sediments, the remaining nitrate produced by nitrification must be reduced by denitrification or by assimilatory nitrate reduction. Ammonium was removed quickly from filtered and aerated river water that was mixed with plant litter in laboratory incubations. Ammonium—N removal from experimental flasks (215 nmol°g—1°h—1) was dependent on the presence of litter and was biologically mediated, but was not due to nitrification since nitrate concentrations were constant in the same experiments. Under anaerobic conditions, ammonium—N was released from litter to river water (93 nmol°g—1°h—1). The estimated total rate of ammonium—N immobilization by microbes on aerobic litter (130 μmol°m—2°h—1) agrees well with measured losses of ammonium from river water in field flume studies (156 μmol°m—2°h—1). An aerobic litter layer may enable marshes to regulate the loss of ammonium that is regenerated in reduced, subsurface sediments and that would otherwise be lost by diffusion or advection. For example, during portions of the year, excess ammonium may be immobilized on plant litter in aerobic surface layers. Some of this litter is incorporated into reduced subsurface sediments where, later, microbial mineralization may regenerate the immobilized nitrogen. The importance of the nitrogen recycling process in a particular marsh will depend on the degree to which litter is mineralized in place or exported from the marsh before it is incorporated into the reduced sediment, as well as the extent to which plant roots can take advantage of the regenerated nitrogen. If conditions are favorable (low export fraction, low litter C:N ratio, and advantageous root distribution) then mineralization may supply a major portion of the annual plant requirement for nitrogen in excess of that stored in perennial tissues. Both internal recycling and translocation make the plant community less dependent on nitrogen imported from the river.

ISSN:0012-9658    

DOI:10.2307/1938506

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY