摘要:

LaMarche et al. (1984) hypothesized that recent trends of increasing ring widths in subalpine conifers may be due to the fertilizing effects of increased atmospheric CO2. Five tree—ring series from foxtail pine (Pinus balfouriana), lodgepole pine (P. murayana), and western juniper (Juniperus occidentalis) collected in the Sierra Nevada, California, were analyzed to determine if the temporal and spatial patterns of recent growth were consistent with the hypothesized CO2—induced growth enhancement. Specifically, I address the following questions: (1) Can growth trends be explained solely in terms of climatic variation? (2) Are recent growth trends unusual with respect to long—term growth records? For three of the five sites, 20th—century growth variation can be adequately modeled as a function of climatic variation. For the remaining two sites, trends in the residuals from the growth/climate models indicate systematic underestimation of growth during the past decade that could be interpreted as either CO2fertilization or as a response to extreme climatic events during the mid 1970s. At all five sites, current growth levels have been equalled or exceeded during some preindustrial periods. Taken together, these results do not indicate that CO2—induced growth enhancement is occurring among subalpine conifers in the Sierra Nevada. While the results presented here offer no support for the hypothesized CO2fertilization effect, they do provide insights into the response of subalpine conifers to climatic variation. Response surfaces demonstrate that precipitation during previous winter and temperature during the current summer interact in controlling growth and that the response can be nonlinear. Although maximum growth rates occur under conditions of high winter precipitation and warm summers for all three species, substantial species—to—species variation occurs in the response to these two variables.

ISSN:0012-9658    

DOI:10.2307/1938895

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We studied carbon dioxide concentrations in a mixed deciduous forest in New England, USA by making continuous measurements at 0.05, 0.2, 3, and 12 m above the soil surface. The measurements began in early March and continued until the end of November 1985; therefore, they spanned the growing season and parts of the dormant seasons both before and afterwards. The data were compared with those from Mauna Loa, Hawaii, which represent global atmospheric CO2levels in the Northern Hemisphere. The results show strong seasonal and daily variations in CO2concentrations at all heights in the forest. On average, during the growing season, CO2levels were generally higher in the forest than in bulk air at Mauna Loa. The highest level of CO2was found near the forest floor and the lowest at the 12—m level. Daily levels of CO2were constant throughout the day in the dormant season and were the same for all heights in the forest. However, during the growing season, the CO2concentrations were lowest during the middle of the day, especially at the 12 m height. Thus, this study shows that the CO2concentrations in the forest may be quite different than those in bulk air and that seedlings, saplings, and mature trees may experience different CO2environments for which they may show different responses in photosynthesis, growth, and water use. Moreover, a tree may experience different CO2environments as it grows towards the canopy, and different modules of an individual may also be growing in different CO2atmospheres.

ISSN:0012-9658    

DOI:10.2307/1938896

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Leaves that show paraheliotropic movement are inclined at a shallow angle with respect to the horizontal near dawn and dusk, and at a steep angle near solar noon. However, the mean inclination angle is only a first approximation to the distribution of leaf angles, and, as such, is not a good measure of the complexity of the underlying distribution. In order to ascertain the accuracies of statistical summaries of angular data used to measure the orientation of plant leaves for intercepting light, I used cellulation plots to analyze the distribution of leaf angles or cosines of the angle of incidence. Examination of these plots shows that inclinations of leaves in the top canopy layer of individual heliotropic plants are distributed over a wide range of angles during a period of several hours before and after solar noon, but that there is a sharp lower limit to this distribution of angles. Experimental observations show that near midday leaves are inclined at least at some minimum angle with respect to the horizontal. In order to interpret these data, I used a simple model of plant canopy photosynthesis and optimized the distribution of leaf angles to maximize total canopy photosynthesis. The results of this model suggest that the existence of a limit angle may be a function primarily of stress factors, and that the distribution of leaf angles more vertical than the limit angle may be important primarily for maximization of whole—plant photosynthesis.

ISSN:0012-9658    

DOI:10.2307/1938897

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1938898

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

This paper analyzes the correlation between leaf orientation and the environmental conditions that prevail within the biogeographic range of each of the four South American Larrea species (L. ameghinoi, L. cuneifolia, L. divaricata, and L. nitida; Zygophyllaceae). Data on the distribution of Larrea species were gathered from herbarium specimens. Measurements of leaf orientation were made throughout the Monte Desert, and in more detail in northern Patagonia, where the four species coexist. The direct solar radiation intercepted by the unshaded leaves of each species was estimated through a computer model and plotted as a function of the hourly time for the summer and winter solstices. L. ameghinoi presents horizontal leaves and prostrate growth, characters which allow its development on sites that are exposed to the Patagonian westerlies. The species, however, is an inefficient light interceptor in winter and early spring, when moisture conditions are adequate in Patagonia. Its architecture is the result of selection for cushion—type, wind resistant forms, at the expense of light interception. It is restricted to windy, open areas of the Patagonian steppe. L. cuneifolia shows erect, east—facing leaves and branches, which maximize interception in the early morning and late afternoon, keeping noon interception at a minimum. It can tolerate very hot environments by physically evading the midday sun and intercepting more early morning and late afternoon light. It colonizes the hotter and drier parts of the Monte Desert. L. divaricata has divaricate leaves with folioles uniformly distributed in all azimuthal directions, and showing an inclination of around 70°. Although it never shows maximum light interception efficiencies, it preforms relatively well in all seasons and at all hours of the day. Its distribution is wide, not only in the arid Monte, but reaching also the Chaco woodlands and the Pacific coastal deserts. L. nitida shows erect, north—facing leaves and branches. Its leaf orientation distribution allows the gradual warming of the leaf surfaces during the morning, with a maximum light interception near winter noons. The species grow in the Patagonian Monte and on the slopes of the Andes. Its general affinity with the colder west side of the Patagonian and Monte Deserts links its distribution with winter—type rains of Pacific origin. The results suggest that the contrasting leaf orientations of the four South American Larrea species reflect the prevalent selective conditions endured under long periods in isolation, and that leaf orientation is an adaptive character that influences the habitat specificity of the different species.

ISSN:0012-9658    

DOI:10.2307/1938899

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Treefall gaps are through to contribute to the diversity of plants in tropical forests by providing opportunities for niche differentiation in modes of regeneration. To examine this hypothesis, we studied the survival, diameter growth, and recruitment of saplings in ≥100 species of woody plants in a 50—ha permanent plot of moist tropical forest on Barro Colorado Island, Panama, from 1982 to 1985. The performance of saplings in low—canopy sites (10 saplings/ha) species appeared to be generalists. Many rare (<1 sapling/ha) or occasional (1—10 saplings/ha) species survived significantly (P ≤ .05) less well than the average survivorship of saplings, while many common species survived significantly better than average. Some rare or occasional species grew rapidly, either in low—canopy sites or in both canopy—height categories, while most common species grew slowly in both situations. Rare and occasional species had significantly more recruits per adult than did common species, but often this did not balance their higher mortality. Large differences in survival, growth, and recruitment between canopy—height categories were found only among rare and occasional species.

ISSN:0012-9658    

DOI:10.2307/1938900

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Using several adjacent Great Basin desert and Sierran montane trees and shrubs, we tested the prediction that water—use efficiency (WUE) is lower in more drought—tolerant species. We also measured maximum net photosynthetic rates and photosynthetic nitrogen—use efficiency (PNUE) to determine if these attributes decrease at high WUE. Differences in the physiological responses to resource availability were more clearly evident between growth forms, trees vs. shrubs, than between vegetation types, montane vs. desert. The shrubs (Anelanchier alnifolia, Arctostaphylos patula, and Artemisia tridentata) maintained higher rates of net photosynthesis at lower predawn water potential but had lower WUE than the trees (Juniperus osteosperma, Pinus monophylla, P. jeffreyi, and P. ponderosa). PNUE was highest in the shrubs that also had the highest foliage N content, and PNUE was inversely related to WUE. However, trees had higher nutrient retranslocation efficiency and growth—based NUE than shrubs. Low WUE and high drought tolerance of shrubs suggests that maintaining high WUE under competitive water—limited conditions may not be advantageous. In contrast, conservative use of water (high WUE) and high growth—based NUE by trees lengthens the season of net carbon assimilation and more closely matches growth with low rates of nutrient availability on nutrient—poor sites.

ISSN:0012-9658    

DOI:10.2307/1938901

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We studied the dynamics of a serpentine annual grassland in northern California in relation to disturbance by pocket gophers (Thomomys bottae) and interannual variation in rainfall over the period 1982—1988. Mapping of gopher mound formation indicated that the probability that gophers would disturb any given area during the 6—yr study period had a near normal distribution with a peak at two disturbances during that period. Disturbance levels varied considerably from year to year and spatially, and thus the disturbance regime is complex. Exclosure experiments indicated that gopher disturbance had a significant effect on the abundances of many of the grassland annuals, perennial grasses, and cormaceous species. Aboveground herbivores had no detectable effects. Over the period of the study annual rainfall varied threehold and plant community dynamics were strongly affected by this. In particular, Plantago erecta decreased and Lasthenia californica increased in abundance with increasing rainfall. Abnormally high rainfall in 1982—1984 allowed the buildup of populations of Bromus mollis, the only nonnative species that invades the serpentine significantly. B. mollis was, however, subsequently virtually eliminated from the serpentine grassland by two consective years of severe drought. Invasion of B. mollis was found only on gopher mounds formed in 1983. In subsequent years recolonization of gopher mounds was predominantly by other species, in particular Lotus subpinnatus. Interannual variability in species abundances was most apparent on disturbed microsites. Our results provide a longer term perspective from which to view shorter term experimental and modelling work and highlight the need for long—term observations of ecological systems.

ISSN:0012-9658    

DOI:10.2307/1938902

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Transport of resources among connected ramets can increase the growth of clonal plants when ramets are located in microsites with contrasting light, water, or total soil nutrient availability. To test whether this is also true for soil nitrogen availability alone, two greenhouse experiments were conducted using the stoloniferous herb Fragaria chiloensis (beach strawberry). First, pairs of ramets were prepared in which the two ramets were connected by a stolen but rooted in separate pots. Ramets in a pair were: (1) severed from each other and given contrasting levels of soil nitrogen (high vs. low); (2) left connected and given contrasting levels of soil nitrogen; or (3) left connected and given a uniform level of soil nitrogen (both high or both low). Second, single ramets were given the high, the low, or one of three intermediate levels of soil nitrogen. Maintaining the vascular connection between ramets given contrasting levels of the soil nitrogen resulted in a greater production of new stolons and new ramets by the ramet given low nitrogen level, and in slightly greater leaf growth of the ramet given the low nitrogen level if it was also younger than the connected ramet. However, a ramet given high soil nitrogen grew much more than a ramet given low soil nitrogen, even when connected. Maintaining the vascular connection between ramets given a uniform level of soil nitrogen had no significant effect on growth. Single ramets given a higher soil nitrogen had greater total and component masses, more stolons and new ramets, higher nitrogen concentrations, higher specific leaf area, and a higher proportional mass of leaves and of stolons and new ramets. Experiments suggested that: (1) transport of nitrogen among ramets can increase growth in F. chiloensis, but to a limited extent; (2) transport of nitrogen through a stolon may be either acropetal or basipetal but tends to promote the growth of younger ramets; (3) higher soil nitrogen availability alters plant architecture by increasing allocation to leaves and to new stolons and ramets; and (4) physiological integration among ramets modifies colonal architecture because nitrogen acquire by roots promotes growth of both a rooted ramet and its new ramets, whereas nitrogen acquired from another ramet promotes the growth of new ramets almost exclusively.

ISSN:0012-9658    

DOI:10.2307/1938903

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

In clonal plants, genetically identical ramets arise from a common stolon or rhizome. Anatomical connection often allows physiological integration, the translocation of resources from a larger mother ramet to a developing daughter ramet. Translocation of a limiting resource can reduce the mother's growth while increasing the daughter's growth. Our models predict patterns in resource translocation; the models assume that fitness increases with the expected biomass a genet attains over a season of vegetative growth in a stochastic environment. In each model a ramet's growth depends nonlinearly on its level of a limiting resource. If resource availability varies both spatially and temporally, and a ramet's growth does not depend on its size, an analytical approximation for total genet growth leads to several new predictions. If a ramet's growth increases as a concave function (i.e., a function with decreasing positive slope) of resources level, physiological integration should increase when spatial variance increases and spatial convariance is negative. If a ramet's growth increases as a convex function (increasing positive slope) of resource level (the less likely case), spatial variance—convariance in resource availability has the opposite effect on translocation. Independently of the concavity or convexity of the growth function, increasing temporal variance in the mother ramet's resource availability reduces translocation, and increasing temporal variance in the daughter ramet's resource availability increases translocation. When a ramet's growth increases with both its resource level and its size, translocation and growth in one time interval influence the value of future physiological integration. For this case a stochastic dynamic programming model demonstrates how translocation can depend on time and the sizes of the mother and daughter ramets, as well as on spatiotemporal resource variability. The predictions qualitatively match those deduced from the model of size—independent growth, although translocation often declines late in the season of vegetative growth. The dynamic model also indicates that a large mother ramet should always share resource with a daughter ramet. But a smaller mother ramet should often abandon a daughter and allocate all its available resource to its own growth.

ISSN:0012-9658    

DOI:10.2307/1938904

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY