Showing papers on "Growing season published in 1995"

Effects of fire regime and habitat on tree dynamics in north florida longleaf pine savannas

Abstract: Frequent, low intensity fire was an important component of the natural dis- turbance regime of presettlement savannas and woodlands in the southeastern USA dom- inated by longleaf pine (Pinus palustris), and prescribed burning is now a critical part of the management of these endangered habitats. Fire season, fire frequency, and fire intensity are three potentially important, though still little understood, components of both natural and managed fire regimes. In this long-term (8-yr) study, we experimentally (through the use of prescribed burning) tested for effects of fire season (eight different times throughout the year) and fire frequency (annual vs. biennial burning), on population dynamics (re- cruitment, growth, mortality, change in density, and change in basal area (the total basal area of all stems in a plot)) and species composition of trees in two quite different types of longleaf-pine-dominated habitats (north Florida sandhills and flatwoods). Limited fire temperature and intensity data were also collected during one year to examine the rela- tionship between fire behavior (temperature and intensity) and tree mortality. Contrary to prior hypotheses, our results showed few systematic or predictable effects of season or frequency of burning on dynamics of longleaf pine. Instead, variability in the population dynamics of this species appeared to be related largely to variation in fire behavior, regardless of the season of burning. Consistent with prior hypotheses, we found that deciduous oak species (Quercus laevis, Q. margaretta, and Q. incana) were least vulnerable to dormant-season burning and most vulnerable to burning early in the growing season. This was shown particularly by seasonal trends in the effect of burning on oak mortality (both topkill and complete kill) and, to a lesser extent, on oak recruitment. Oak densities and basal areas also declined in the spring- burned plots, resulting in a shift away from oaks and towards increased dominance by longleaf pine. Detrimental effects of spring burning on oaks were partly explained by fire behavior, but there appeared also to be an important residual effect of burning season, particularly on complete kill. Though longleaf pine population dynamics did not differ markedly as a result of burning season and frequency, we did find important differences in pine dynamics between the two habitats (i.e., sandhills and flatwoods). In general, populations of longleaf pines in the sandhills appeared to be density regulated, while flatwoods pine populations were declining regardless of the level of intraspecific competition. This suggests that long-term persistence of longleaf pine, and perhaps other fire-adapted species in frequently burned longleaf-pine- dominated communities, may be determined by complex interactions between habitat factors and fire regimes.

Use of a Chlorophyll Meter to Monitor Nitrogen Status and Schedule Fertigation for Corn

Abstract: The SPAD 502 chlorophyll meter 1 (Minolta Camera Co., Osaka, Japan) is a new instrument that has been introduced as a tool to improve N management. This study was conducted to evaluate the ability of the chlorophyll meter to detect plant N deficiencies in corn (Zea mays L.) by identifying when it would be appropriate to supply N fertilizer in irrigation water. Nitrogen response studies were conducted on five irrigated sites in central Nebraska in 1991. Crop N status was monitored during the growing season and additional N was added to simulate fertigation when apparent N deficiencies were detected with the meter. Changes in N status over the season were determined relative to an adequately fertilized in-field reference plot. Earlyseason (V6) N deficiencies were poorly correlated with yield because factors such as nitrate leaching, organic matter mineralization, and nitrate present in irrigation water modified the crop N supply during the growing season. Nitrogen deficiencies detected late (R4-R5) in the season were more highly correlated with yield than early season N stresses. Treatments that started with adequate fertilizer and then became deficient were corrected without yield loss. Young plants in a deficient state could not be corrected to full yield potential. Chlorophyll meters can be a valuable tool for N management of irrigated corn production when used to assess crop N status in the irrigated Great Plains

Phenotypic plasticity of the phenology of seven European tree species in relation to climatic warming

Abstract: To evaluate the potential responses of individual trees to climatic warming, phenological observations of clones of Larix decidua (Mill.), Betula pubescens (Ehrh.), Tilia cor-data (Mill.), Populus canescens (Ait.), Quercus robur (L.), Fagus sylvatica (L.) and Picea abies (L.) relocated over a large latitudinal range in Europe were analysed. The magnitude of the response of the clone was compared to that of genetically different trees of the same species in part of the latitudinal range, which were assumed to have adapted to their local climates. It was found that the responses of the date of leaf unfolding and the date of leaf fall in the clones to temperature were similar in magnitude to those in the genetically different trees. This demonstrates that trees possess considerable plasticity and are able to respond phenotypically to a major change in their local climate. For the clones of Larix decidua and Quercus robur the duration of the growing season may decrease with increasing temperature, because leaf fall is advanced more than leaf unfolding. In Betula pubescens and Populus canescens, leaf unfolding and leaf fall are advanced equally, whereas in Tilia cordata and Fagus sylvatica the date of leaf fall seems to be unaltered but the date of leaf unfolding advances with increasing temperature. These differences in the duration of the growing season at increased temperature may alter the competitive balance between the species. Descriptive dynamic models showed that most of the variance in the date of leaf unfolding can be accounted for by temperature. However, a generally applicable model of leaf fall based on temperature and/or photoperiod could not improved the null model, i.e. the mean date of leaf fall, because of variability in other environmental factors. The lowest temperatures around the dates of leaf unfolding and leaf fall differed among the clones. The hypothesis that the survival of the clones is curtailed by spring frosts was supported. Thus, these lowest temper-tures around leaf unfolding may represent thresholds below which the species cannot survive. It is argued that these thresholds may be a particularly sensitive means to evaluate the impact of climatic warming on the geographical distribution of tree species.

Carbon dioxide exchange in a peatland ecosystem

Abstract: Micrometeorological measurements of carbon dioxide exchange were made in an open peatland in north central Minnesota during two growing seasons (1991 and 1992). The vegetation at the site was dominated by Sphagnum papillosum, Scheuchzeria palustris, and Chamaedaphne calyculata. The objective of the study was to examine the diurnal and seasonal variations in canopy photosynthesis (P) and develop information on the net ecosystem CO2 exchange. The two seasons provided contrasting microclimatic conditions: as compared with 1991, the 1992 season was significantly wetter and cooler. Canopy photosynthesis was sensitive to changes in light, temperature, and moisture stress (as indicated by water table depth and atmospheric vapor pressure deficit). Under moderate conditions (temperature 18–28°C, vapor pressure deficit 0.7–1.5 kPa, and water table near the surface) during the peak growth period, midday (averaged between 1000–1400 hours) P values ranged from 0.15 to 0.24 mg m−2 s−1. Under high-temperature (30°–34°C) and moisture stress (water table 0.16–0.23 m below the surface and vapor pressure deficit 2.2–3.0 kPa) conditions, midday P was reduced to about 0.03–0.06 mg m−2 s−1. There was a high degree of consistency in the values of P under similar conditions in the two seasons. Seasonally integrated values of the daily net ecosystem CO2 exchange indicated that the study site was a source of atmospheric CO2, releasing about 71 g C m−2 over a 145-day period (May-October) in 1991. Over a similar period in 1992, however, this ecosystem was a sink for atmospheric CO2 with a net accumulation of about 32 g C m−2. These results are consistent with previous investigations on CO2 exchange in other northern wetland sites during wet and dry periods.

Influence of organic matter incorporation on the methane emission from a wetland rice field

Abstract: Methane (CH4) emission from Philippine rice paddies was monitored with a closed chamber technique during the 1992 dry and wet season. CH4 emissions were significantly higher in the dry season. Application of green manure stimulated CH4 emissions. In plots that received more than 11 t ha−1 of fresh green manure, CH4 emission was highest during the first half of the growing season. Significant amounts of CH4 may evolve during or immediately after transplanting, if the organic amendments are incorporated 1 to 3 weeks before transplanting. Laboratory incubations of soil cores show that CH4 production is highest near the soil surface. CH4 production in green manure treated fields is higher than in urea-fertilized fields, but toward the end of the season this difference is less pronounced. Around panicle initiation, the fraction of CH4 produced, which was emitted to the atmosphere, is lower than at tillering or ripening. The impact of organic amendments on CH4 emissions at different locations of the world can be described by a dose response curve, if CH4 emission from organically amended plots is expressed relative to CH4 emission from mineral fertilized plots of the same location and season. Various organic amendments (e.g., straw, fermented residues) have a similar effect on CH4 emissions after correction for differences in easily decomposable carbon content.

Acacia karroo invasion of grassland: environmental and biotic effects influencing seedling emergence and establishment.

Abstract: Acacia karroo Hayne is the most important woody invader of grassland in South Africa, and can greatly reduce the productivity of grassland A field experiment was conducted to test the hypotheses that emergence, growth and the 1st year's survival of Acacia karroo would be enhanced by (1) defoliation of the grass sward, (2) increased irradiance, (3) increased moisture availability and (4) its germination within cattle dung pats The study was conducted on one site above and one below the natural altitudinal treeline of this species in grassland of the eastern Cape, South Africa Not one seedling emerged from dung pats Neither location nor the other treatments affected the density of emerging seedlings, although only 404 seedlings m−2 emerged of the 200 seeds m−2 planted Shading dramatically increased the density of surviving seedlings In the open, only 3 and 15 seedlings m−2 remained respectively at the end of the growing season or the beginning of the next, compared to 233 and 195 seedlings m−2 under shading for these respective times This was attributed to the effect of shade on moisture availability in a season which received only 54% of average rainfall Seedling survival until the end of the growing season was enhanced (30%) by shade at both sites, but also by supplemental water (24%) and defoliation of the sward (7%) at the site above the treeline Across sites and treatments, seedling survival was related to moisture availability, with no or poor survival for < 500 mm rainfall, indicating this species can only establish in certain rainfall years Seedling survival over winter was not influenced by treatment, but was greater for larger seedlings Treatments affected seedling size, in particular seedlings growing under shade and within a dense grass canopy were etiolated A karroo seedlings are capable of establishing and surviving within a dense grass sward for at least a year, tolerant of low irradiance and of interference, which, because most seeds do not persist for much longer than a year, suggests this species forms predominantly a seedling bank This has implications for the invasion of grassland by woody species

Interactive effects of fire and grazing on structure and diversity of Mediterranean grasslands

Abstract: . The separate and combined effects of fire and cattle grazing on structure and diversity of productive Mediterranean grasslands in northern Israel were examined within a set of climatically and edaphically similar sites. Cover and height of green and dry plants in winter, and species richness and diversity in spring, were measured in paired transects on both sides of cattle fences, and on both sides of boundaries of both incidental and experimentally lit fires. Early in the first growing season after a fire, plant cover as well as height of green plants were reduced, compared to unburnt grassland. These structural effects of fire were similar to the effects of grazing, but they were greater in ungrazed than in grazed grasslands, indicating a fire-grazing interaction. The effects of fire were considerably attenuated in the second growing season after the fire. Species richness and diversity tended to be higher in grazed than in adjacent ungrazed grasslands. Richness consistently increased after a fire only in grazed grasslands with a strong perennial component. In ungrazed grasslands, and in predominantly annual grasslands, fires reduced species richness and diversity at least as often as they increased it. Fire and grazing should be regarded as two agents with distinct and interactive effects on the community, rather than as two alternative mechanisms of a general disturbance factor.

The variability of vessel size in beech (Fagus sylvatica L.) and its ecophysiological interpretation

Abstract: The vessel areas of ten beech trees growing on a dry site were measured separately for all tree rings using automatic image analysis. These data were correlated with the monthly amount of precipitation from the July prior to the growing season until the August of the current growing season. It is evident that vessel formation at the beginning of cambial activity is mainly controlled by internal factors. The rainfall in the previous summer and autumn and in the contemporary May had only a slight influence. Vessel formation towards the end of the cambial activity is strongly influenced by the July rainfall and is thus determined to a greater degree by external factors. These results are discussed on the basis of hypotheses of tree physiology.

Succession and Fire Season In Experimental Prairie Plantings

Abstract: Fire season influenced the cover of species and flowering guilds of plants in replicated grass and forb plantings of tallgrass prairie species in Wisconsin. Over two burn cycles at 3-yr intervals, cover increased in the rhizomatous perennials Andropogon gerardii, Aster simplex and Solidago altissima in plots burned in spring, or summer, or left unburned, but cover showed dramatic positive or negative responses to spring or summer burns in Agropyron repens, Erigeron annuus, Panicum virgatum, Phalaris arundinacea, and Rudbeckia hirta. Aggregate response of flowering guilds was stronger. By the last (1993) census, the phenological guild flowering before mid-July (summed cover values for nine species) accounted for 4% cover in unburned plots and <2% cover in plots burned in early spring, but 32% cover in plots burned in midsummer. The late-flowering guild (summed values for 21 species) accounted for 93% cover in unburned plots and 97% cover in plots burned in early spring, but 66% cover in plots burned in midsummer. Remaining space was taken by a mid-season guild (11 species), only one of which (E. annuus) was common enough for individual statistical analysis. These results have implications for succession of prairie vegetation under different seasonal fire regimes. Over 8 yr, summer fires that simulated the timing of lightning fires retarded progression to dominance of large, late-flowering C 4 grasses, allowing early-flowering species eliminated in other treatments to persist or even prosper. Primaeval lightning fires in midsummer may have produced quite different communities than anthropogenic fires, which are rarely set during the middle of the growing season.

Water Limitations and Plant Community Development in a Polar Desert

Abstract: Polar deserts of the High Arctic contain vast areas of minimal plant cover and low primary productivity. Significant development of polar desert plant communities is largely restricted to areas with considerable cover of a cryptogamic soil crust, which develops in sites exposed to continued surface runoff from melting snow for some of the short growing season. Thus, soil drought and plant water stress have often been assumed to be major constraints to plant community development in polar deserts. To examine this issue, water availability and plant water relations of common herbaceous perennial plants were studied over three growing seasons in a typically barren ("noncrusted") site and a site with a well—developed cryptogamic crust and vascular plant community ("crusted"). Soil water content was consistently higher in the crusted site than the noncrusted site through all growing seasons. These differences had limited biological relevance because subsurface soils at both sites remained effectively saturated...

Effects of habitat and growing season fires on resprouting of shrubs in longleaf pine savannas

Abstract: The effects of habitat and timing of growing season fires on resprouting of shrubs were studied in second-growth longleaf pine savannas of the west Gulf coastal plain in the southeastern United States. Within the headwaters of three different drainages of the Calcasieu River in the Kisatchie National Forest in western Louisiana, replicated permanent transects were established that extended from xeric upland longleaf pine savannas into downslope hydric seepage savannas. All shrubs were mapped and tagged, and numbers of stems were counted prior to any fires. Replicated prescribed fires were set early (June) and late (August) during the 1990 growing season; maximum fire temperatures were measured within both upland and seepage habitats within each transect. Shrubs were relocated; stems were recensused two and twelve months after the fires. At least some shrubs of all species resprouted from underground organs; none regenerated solely from seed banks in the soil. There was no reduction in total numbers of stems one year after fires compared to before fires, either in the upland or in seepage savannas. In addition, there was no reduction in total numbers of stems one year after early or late growing season fires. Fire-related mortality was restricted to small shrubs (< 18 stems) and was not associated with high fire temperatures. The rate of resprouting varied among species and between habitats. Resprouting occurred more rapidly in seepage than upland savannas, but more resprouts were produced in upland than seepage savannas one year after fires. In contrast to other upland species, Vaccinium arboreum and V. elliottii delayed resprouting more than two months following fire. Stems of Rhus copallina and Pyrus arbutifolia, species with long rhizomes, increased more after fires in June than fires in August. We suggest that growing season fires may block further recruitment of shrubs into longleaf pine savannas, but reduction in numbers of large shrubs may require additional management.

Influence of trees on above-ground production dynamics of grasses in a humid savanna.

Abstract: . Above-ground grass biomass, necromass and tree litter were measured monthly over a vegetation cycle under tree clumps and in the open, in a humid savanna in Cote d'Ivoire. Grass production was calculated using several methods to better discriminate the contribution of the different grass compartments. Above-ground grass biomass is higher in the open than under canopies during the second part of the growing season, but there is no difference in grass necromass dynamics. Physical protection of grass necromass by tree litter against decaying under tree canopies was assumed to explain this discrepancy. Grass production, calculated as the sum of positive increments of biomass and necromass, equals 1073 g m-2 yr-1 in the open, against 74 % underneath trees. However, basal ground cover is only 50 % of that in the open. Comparison with other savanna studies as a whole does not show any significant effect of rainfall on the relationship between under-canopy and outside-canopy grass production. However, in arid conditions, grass production tends to increase under light-canopied trees (mostly Acacia legumes) which hardly affect grass photosynthesis, but add high quality litter to the soil surface.

Limited uptake of summer precipitation by bigtooth maple (Acer grandidentatum Nutt) and Gambel's oak (Quercus gambelii Nutt)

Abstract: Winter and spring precipitation that saturates to deep soil layers precedes summer droughts in the Intermountain West. Occasional summer convection storms relieve summer drought, but are infrequent and unreliable from year to year, leading to the hypothesis that dominant tree species might not invest limited carbon reserves to surface roots to take up summer precipitation in these regions. We compared the hydrogen (δD) and oxygen (δ18O) isotope ratios of winter, spring and summer precipitation to that of xylem sap water in Acer grandidentatum and Quercus gambelii, two dominant trees of this region. By this method we could identify water sources utilized throughout the growing season. Xylem δD and δ18O values changed significantly when each species leafed-out; this change was not associated with changes in either soil or plant water status (as measured by predawn and midday water potentials). This shift is apparently related increased transpirational flux, which may flush out residual stem water from the previous growing season. δD values of xylem sap of both species matched winter precipitation input values throughout most of the summer, indicating a reliance on deep-soil moisture sources throughout the growing season. Mature Q. gambelii did not take up summer precipitation, whereas A. grandidentatum responded slightly to the largest summer rain event. Small trees of both species, particularly A. grandidentatum, showed a limited uptake of summer rains.

Ecosystem gas exchange in a California grassland : seasonal patterns and implications for scaling

Abstract: We used the eddy covariance technique to measure exchanges of water vapor, energy, and CO 2 between California serpentine grassland and the atmosphere. Even though the system was built around an inexpensive, one-dimensional sonic anemometer and a closed-path CO 2 analyzer, energy balance closure was accurate to ± 20% at a net radiation of 500 W/m 2 , and the spectra and cospectra indicated only modest information loss from incompletely resolved high-frequency turbulence. In the early and middle parts of the growing season, net radiation, latent heat, and sensible heat all had similar diurnal dynamics, with latent heat accounting for 60% of the net radiation. Late in the growing season, energy dissipation by latent heat dropped dramatically, even though the vapor pressure gradient remained high. The Ω factor, an index of the role of canopy conductance in regulating transpiration (Jarvis and McNaughton 1986), decreased from 0.8 early in the growing season (indicating predominant control of transpiration by net radiation) to 0.1 late in the growing season (indicating a shift to control of transpiration by canopy conductance and vapor pressure deficit). Canopy conductance was a linear function of the product of net photosynthesis and relative humidity, divided by the CO 2 concentration, as predicted by Ball (1988). The slope of the relationship, however, was greater early in the growing season than at other times. Whole-ecosystem carbon exchange rates were modest, with midday net photosynthesis reaching maximum values of 6-8 μmol.m -2 .s -1 in early April. Diurnal variation in photosynthesis roughly paralleled variation in photosynthetically active photon flux density (PFD), but with the daily maximum increasing with canopy development early in the growing season and decreasing with drought at the end of the growing season. Photosynthesis did not clearly saturate at high levels of PFD. Ecosystem dark respiration increased strongly (Q 10 =4.6) with increasing soil surface temperature. The efficiency with which absorbed radiation was used in ecosystem photosynthesis, integrated over entire days, was 0.0115 ± 0.0015 mol CO 2 /mol PFD (20% of the values measured for healthy, single leaves under low light conditions) until late in the growing season, when the efficiency fell sharply. Using simple assumptions to extrapolate measurements from 11 d to the entire growing season, we estimate ecosystem annual gross primary production to be 11.1 mol CO 2 /m 2 , yielding an annual net primary production of 133 g biomass/m 2 . This is near the center of the range of published values from aboveground harvest studies.

Yield and Nitrogen Use Efficiency of Irrigated Corn in the Northern Great Plains

Abstract: Nitrogen and water are the two most common limitations to crop production in the semiarid northern Great Plains. Little is known about N use efficiency by irrigated corn (Zea mays L.) in this region. A study was conducted to determine how irrigation and N fertility levels affect growth and N use efficiency by corn. Corn was grown under three irrigation levels : precipitation plus irrigation equal to one, two, and three times the calculated evapotranspiration (ET) rate. Fertilizer use efficiency was determined using 15 N-enriched fertilizer applied at rates equivalent to 100 and 200 kg N ha -1 . Grain and dry matter yields, N content, and utilization of fertilizer N all exhibited yearly variations, probably the result of annual weather patterns, especially temperature. For years when temperatures during the growing season were below the 30-yr average and affected corn growth, there were no differences in yields and N content between the two fertility levels. For years when temperatures during the growing season were warm enough for favorable growth, corn responded to increasing N fertility with 60% greater yields, 75% greater N content, and 60% greater percentage N derived from fertilizer with the higher N fertility treatment. Averaged across rates, grain utilized 35% and stover an additional 15% of the applied fertilizer, while 30% remained in the upper 0.6 m of the soil profile at the end of the growing season. Twenty percent of the applied fertilizer could not be accounted for, lost to leaching or denitrification. Supplemental irrigation and N fertilization are viable management practices available to producers in the semiarid northern Great Plains.

Phenological Plasticity as an Adaptation by Common Bean to Rainfed Environments

Abstract: Under rainfed conditions in the semi-arid highlands of Mexico, time to first flower and to maturity of some cultivars of common bean (Phaseolus vulgaris L.) is strongly reduced with late plantings. Consideration of rainfall patterns in the region suggested that this response might be an adaptation to growing seasons of variable length, where length is determined by time of onset of summer rains and by risk of water deficit or low temperatures at the end of the season. Analysis of weather patterns at two sites in the highlands suggested that when rains commenced in early June, there was a high probability of adequate precipitation during the season. When rains commenced in mid- to late-July, total precipitation was lower. For a third site, total precipitation showed no relation with date of onset of rains. Evaluations of 20 bean genotypes planted at five dates each in 2 yr at Pabellon, Aguascalientes, (20 degrees 11' N latitude, elevation 1910 m) demonstrated large differences in plasticity of the pre-flowering and reproductive periods. Cultivars Garbancillo Zarco and Tlaxcala-475 were late to flower and mature under early plantings, but were similar to other genotypes in late plantings. Other genotypes of highland origin showed little or no plasticity, suggesting that this trait is not universal among cultivars currently grown there. Furthermore, cultivars and breeding lines from other regions showed no plasticity. Marked phenological plasticity occurred only in genotypes from the highlands which were highly photoperiod sensitive, but several highly sensitive genotypes showed no or little plasticity. Similar patterns were noted in a survey of 81 genotypes that included a wider range of highland germplasm.

Wood (delta)^(13)C as a Measure of Annual Basal Area Growth and Soil Water Stress in a Pinus Strobus Forest

Abstract: The relationships between annual wood tissue δ 13 C, growing season soil water potential, and basal area growth were studied in a mature, white pine (Pinus strobus) stand at the Coweeta Hydrologic Laboratory, in western North Carolina. In 1992, four bolewood cores that spanned the years from 1980 to 1989 were extracted from each of ten equal-size, co-dominant white pine trees within the stand. The 1980s were a time of extreme climate with some of the hottest, driest, and wettest years recorded at Coweeta. Annual basal area growth ranged from 14.5 to 25.0 cm 2 .tree -1 .yr -1 , and modeled values of average growing season soil water potential ranged from -0.21 to -5.58 MPa, when measured to a depth of 60 cm. After correcting annual wood tissue δ 13 C for atmospheric changes in δ 13 C, carbon isotopic discrimination (A) ranged from 18.52 to 19.62‰. The A of annual wood tissue was positively correlated with average growing season soil water potential (r 2 = 0.74, P = 0.0005, n = 10 growing seasons) and average annual basal area growth (r 2 = 0.78, P = 0.0002, n = 10 seasons). Basal area growth and growing season soil water potential were also correlated (r 2 = 0.64, P = 0.002, n = 10 seasons). These results suggest that annual wood tissue δ 13 C could potentially be useful in estimating historic changes in soil water potential and basal area growth in mature forest ecosystems.

Growth response of subalpine fir (Abies lasiocarpa) to climate in the Olympic Mountains, Washington, USA

Abstract: Growth response of subalpine fir (Abies lasiocarpa) to climate was studied across its local geographical and elevation range in the Olympic Mountains, Washington. A dendroecological analysis of subalpine fir across a range of elevations (1350-1850 m) and annual precipitation (125-350 cm y−1), was used to compare environmental factors affecting growth. Climate-growth relationships were explored using Pearson product-moment correlation coefficients; partial correlation analysis was used to assess relationships among site chronologies and climatic variables. Radial growth is negatively correlated with winter precipitation at high elevation and wet sites, but not at low and middle elevation dry sites. Growth is positively correlated with current growing season temperature at all sites; however, growth is negatively correlated with previous year August temperature, indicating that climate affects growth in subsequent years. Positive correlations between growth and summer precipitation during the growing season at low and middle elevation dry sites suggest that soil moisture is partially limiting to growth on these sites. If the climate of the Pacific Northwest becomes warmer and drier, then subalpine fir growth may increase at high elevation and wet sites, but may decrease at lower elevation dry sites in the Olympic Mountains. However, the growth response of subalpine fir to potentially rapid climate change will not be uniform because subalpine fir grows over a wide range of topographic features, habitats, and local climates at different geographical scales. A comparison of growth response to current growing season temperature suggests that the temperature-related growth response of subalpine fir is not adequately described by the parabolic curve used in JABOWA-based models.

Growth and dry matter partitioning in loblolly and ponderosa pine seedlings in response to carbon and nitrogen availability

Abstract: summary We grew loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings in a factorial experiment with two CO2 partial pressures (35 and 70 Pa) and two nitrogen treatments (1-0 and 3–5 mM NH4+) for one growing season in a ‘common garden’ experiment designed to examine the extent that dry matter and nitrogen accumulation and partitioning are environmentally controlled. Ponderosa pine seedlings grown in 35 Pa CO2 and 3.5 mM NH4+ showed symptoms of nitrogen toxicity, characterized by greatly reduced growth, and moderately reduced total plant N. With the exception of this treatment combination, there were no significant differences between species in total plant dry matter or total plant nitrogen, suggesting that responses of growth to environmental conditions were stronger than heritable responses. There were however large differences in dry matter and N partitioning between the two species. Increases in leaf mass were largest in loblolly pine, whilst ponderosa pine tended to have higher root: shoot (R:S) ratios. R:S ratio of loblolly increased in response to C availability and decreased in response to N availability, whilst R: S ratio of ponderosa pine was much less responsive to resource availability. Total plant N varied with N supply, and N partitioning was related to plant growth and carbon partitioning. Carbon and N were interactive, such that an increase in the accumulation of either resource was always accompanied by an increase in the other. Over several seasons the different patterns of resource acquisition and biomass allocation that we observed in a uniform environment could potentially result in different growth rates at most resource levels. In the first season, contrary to our expectations, heritable differences in growth rate did not appear.

A landscape analysis of the candelaria watershed in mexico: Insights into paleoclimates affecting upland horticulture in the southern yucatan peninsula semi-karst

Abstract: Biocultural systems adapt to global climate change through its regional manifestations. Subsistence customs are the cultural interface between regional climate and culture at large. Swidden horticulturalists in Campeche, Mexico report that dry April followed by early onset of the wet season enhances the productivity of upland tropical gardens, or milpas. To relate regional seasonality of moisture to global climate, growing season discharge for Mexico's Candelaria River from 1958 to 1990 was analyzed relative to global average temperature. Analysis of covariance revealed a statistically significant relationship (p < 0.001). Further analysis showed that hot global climate eliminates the dry season, which lowers milpa productivity by preventing burning of the slash. Cold global climate delays the wet season and planting, also at cost of productivity. Intermediate global temperature fosters optimal wet—dry season combinations. Productivity of milpas is therefore directly related to global climate through the intervening mechanism of seasonality of moisture. A regression model reflecting these findings is used to retrodict paleohydrology for the last 3000 years. The pattern of ascendancy and decline of ancient southern Maya lowland urban centers is reviewed in the perspective of changing hydrological conditions. The model indicates that fluorescence occurred with optimal balance of wet and dry season duration and catastrophes unfolded during extended wet or dry periods. We suggest that the southern Maya lowlands have had a precipitous record of urban development and collapse in part because of complex interactions of global climate and upland horticulture. The most productive conditions for milpa issue from an inherently unstable overlay of global climate on a relatively narrow band of partially developed karst (semikarst) geological formations.

Effects of elevated CO 2 and N fertilization on soil respiration from ponderosa pine ( Pine ponderosa ) in open-top chambers

Abstract: We measured growing season soil CO2 evolution under elevated atmospheric CO2 and soil nitrogen (N) additions. Our objectives were to determine treatment effects, quantify seasonal variation, and de...

Photosynthetic and biomass allocation responses of liquidambar styraciflua(hamamelidaceae) to vine competition

Abstract: This 2-year field study examined stomatal conductance, photosynthesis, and biomass allocation of Liquidambar styraciflua saplings in response to below- and aboveground competition with the vines Lonicerajaponica and Parthenocissus quinquefolia. Vine competition did not affect stomatal conductance of the host trees. The leaf photosynthetic capacity and photosynthetic nitrogen-use efficiency were significantly reduced by root competition with vines, either singly or in combination with aboveground competition, early in the second growing season. However, such differences disappeared by the end of the second growing season. Trees competing below ground with vines also had lower allocation to leaves compared with stems. Aboveground competition with vines resulted in reduced photosynthetic capacity per unit leaf area, but not per unit leaf weight, in trees. No correlation was found between single leaf photosynthetic capacity and tree growth. In contrast, a high positive correlation existed between allocation to leaves and diameter growth. Results from this study suggest that allocation

Persistence of desertified ecosystems: explanations and implications *

Abstract: Studies of rainfall partitioning by shrubs, responses of shrub-dominated ecosystems to herbicide treatment, and experiments using drought and supplemental rainfall were conducted to test the hypothesis that the shrub-dominated ecosystems that have replaced desert grasslands are resistant and resilient to disturbance. Between 16 and 25% of the intercepted rainfall is channelized to deep soil storage by stemflow and root channelization. Stemflow water is nutrient enriched and contributes to the "islands of fertility" that develop under desert shrubs. Drought and rainfall augmentation experiments during the growing season after 5 consecutive years of summer drought found that (1) growth of creosotebushes, Larrea tridentata, was not significantly affected, (2) perennial grasses and forbs disappeared on droughted plots, (3) nitrogen mineralization increased in the short term, and (4) densities and biomass of spring annual plants increased on the droughted plots. Doubling summer rainfall for 5 consecutive years had less-significant effects. Coppice dunes treated with herbicide in 1979 to kill mesquite (Prosopis glandulosa) had the same frequency of occurrence of the shrub as the untreated dunes when remeasured in 1993. These data indicate that the shrub-dominated ecosystems persist because they are resistant and resilient to climatic and anthropogenic stresses.

Yielding ability of potato crops as influenced by temperature and daylength

Abstract: Today, potato is grown commercially in almost all climates of the world, except in tropical lowlands. The highest tuber yields are obtained in areas with temperate climates in North western Europe and the North-West of the United States. In tropical and subtropical climates tuber yields are lower and less stable. To determine if this is due to the lack of adaptation of potato to the conditions prevailing in the tropics, the effect of climate on potential yield was examined. The study presented in this thesis explored the potential and attainable yields of potato. It aimed to determine and quantify the effect of climate on potential tuber dry matter production and to assess the genetic variation in the sensitivity for this climate. Potential dry matter production in potato is governed by the genetic characteristics of a cultivar and the climatic factors, radiation, temperature and daylength. Hitherto, research into the effect of temperature and daylength on the potato crop has been done in two types of studies. The tuber yield was either correlated directly with these climate factors, or separate processes in the plant were related to temperature and daylength. The general introduction demonstrates that to understand the behaviour of the crop and the variation in tuber yields, the crop and its relations with the environment must be studied as an integrated system. The subsequent chapters describe a study in which experimental data are incorporated into a simulation model. In the experiments the relations of the individual crop characteristics to temperature and daylength were obtained and the model was used to calculate the consequences of variation in daylength and temperature for tuber dry matter production. First a series of models constructed in the Netherlands was tested with weather data from a series of years in Scotland (Chapter 2). There appeared to be no difference in the ability of the models to simulate tuber dry matter production after reparameterization. It was easier to parameterize for simple models than complex models because the latter included more crop processes. To improve the model's general applicability, the key processes determining tuber dry matter production had to be identified. In all previous models, dry matter allocation was driven by a temperature-dependent development. This was an oversimplification for the purposes of this study and therefore a new relation describing dry matter allocation had to be developed. It was hypothesized that dry matter allocation is governed by a dominant tuber sink and that this dry matter allocation determines the earliness and thus the yielding ability of a potato crop. This hypothesis was confirmed in a series of experiments on early and late cultivars in the Netherlands (Chapter 3). The difference in dry matter allocation between cultivars however, was only partly explained by the differences in dry matter production. Leaf longevity also played a role in explaining earliness and yielding ability. To assess the effect of temperature, daylength and radiation on yield formation in potato, field trials were carried out in Rwanda (2 altitudes), Tunisia (Spring, Autumn and Winter) and the Netherlands. To study the interaction between environment and genotype, eight cultivars differing in earliness when grown in the Netherlands were planted. Total dry matter production and tuber yields were analysed in terms of light interception and light use efficiency. Variation in total and tuber dry matter production was mainly explained by the differences in light interception. Light interception was divided into average light intensity, length of the growing period and in maximum proportion of light intercepted. The light use efficiency was found to be inversely related to an increased radiation intensity. Variation in length of the growing season was the most important factor explaining the differences in total light interception. Shorter days at emergence and higher temperatures throughout the season resulted in a shorter growth cycle. The extent of the response differed between cultivars and was mainly explained in terms of duration of ground cover. To quantify the relation between tuber yield and the climatic factors; (temperature, daylength and radiation) the growth cycle of the potato crop was divided into three phases (Chapters 5). By relating the length of each phase to temperature, daylength and radiation, the influence of these factors on crop growth and development processes was determined. The variation in the length of all three phases contributed to the variation in the duration of the growth cycle and thus to the variation in tuber yield. Both higher temperatures and shorter daylengths hastened the development in the phase between emergence and tuber initiation, and the degree of the change depended on the cultivar. In the second phase, from start of tuber growth to end of leaf growth, temperature and daylength had similar effects but these were less than in phase 1. The last phase, from end of leaf growth to end of crop growth was shortened by high temperatures and high radiation. The shift in sink priorities between tubers and leaves in phase 1 affected phases 2 and 3, so part of the variation in these phases could be explained from the variation in phase 1. The information obtained in the experiments was combined with data from literature to obtain a complete overview of temperature and daylength reactions to the various growth and developmental processes. These relations were introduced in the simulation model developed in chapter 3. The model (LINTUL-POTATO described in Chapter 6) was used to explore different climate and temperature situations for a standard potato cultivar. Integrating the effects of temperature and daylength of the separate processes resulted in simulated tuber dry matter production levels that were agreeing with those reported in the literature. It is shown that as daylength increases, potato can tolerate a broader range of temperatures. In chapter 7 the model is verified against the set of experiments used to build the model and validated against independent data. The model explains the differences in observed tuber dry matter production between locations by incorporating the effects of temperature and daylength. The differences between cultivars were smaller than the differences between location and therefore explained less well. The values observed in experiments carried out in climates that were most suitable for potato production were simulated better than those from experiments carried out in less suitable conditions. Finally the yielding ability of a potato crop for a range of climates between the equator and 60°N was evaluated. The potential total dry matter production, potential tuber dry matter production and the ideotype were determined for each climate. The potential total dry matter was dependent on the length of the growing season. Tuber dry matter production was dependent on the length of the growing season and the length of the growth cycle: tuber yields were restricted when the growing season was too short to fit the growth cycle or when tuber initiation was too early and the conditions favoured tuber growth at the expense of leaf growth. In chapter 8 the advantages of the approach and the application of systems analysis in breeding and introduction of new cultivars are discussed. Combining observations at the process level with the explanatory capacity of the simulation model reveals how tuber dry matter production is affected by the climatic factors temperature, daylength and radiation. Temperature and daylength together affected the development before tuber initiation; further development was affected by temperature only. Radiation was found to influence growth rate only. Integrating the effects revealed that especially the period between emergence and tuber initiation were determinative for further growth and development of the crop. This insight can be used to design ideotypes in breeding, to improve the efficiency of selection procedures and to analyse yield gaps in potato production.

Growth responses of Calamagrostis lapponica to simulated environmental change in the sub-arctic

Abstract: A sub-arctic heath community dominated by dwarf shrubs has been subjected to three components of environmental change (temperature, water and fertiliser) in a factorial experiment over three growing seasons. Above-ground biomass of the most abundant grass in the ecosystem, Calamagrostis lapponica, was measured after one, two and three seasons, shoot height and flowering frequency were recorded after two and three seasons. There was up to 50 times more above-ground biomass at the end of the second season compared with the end of the first season. Fertiliser had the greatest effect on all parameters, resulting in greater biomass, increased flowering and a greater shoot height. The mean biomass of C. lapponica was greatest in the third season, but still contributed less than 8% of the above-ground biomass in the fertilised plots; the dominant species being ericaceous dwarf shrubs. The response of the grass to temperature and water was not as marked as the response to fertiliser. Temperature increase led to greater shoot height in both seasons whilst in the first season both warmer temperature and water additions resulted in greater biomass. All treatment effects were less in the third season than in the second, perhaps because of competition from the dominant dwarf shrubs which respond more slowly to the perturbations than the grass. The responses of the C. lapponica and the dwarf shrubs to the treatments are compared. If climate change results in greater solute availability in this ecosystem, there may be a marked change in species composition with C. lapponica becoming more prevalent

Vegetation and soil response to grazing simulation on riparian meadows.

Abstract: Riparian areas have not responded consistently to grazing systems, suggesting that more knowledge is needed to explain how different areas respond to specific stresses. Several studies were conducted to determine herbaceous plant response to simulated grazing on riparian areas. One low-elevation redtop (Agrostis stolonifera L.) site in Oregon and 2 high-elevation sedge (Carex spp. L.) sites in Idaho were studied for 3 years. Several combinations of defoliation, compaction, nutrient return, and season of use were examined. The redtop community responded to spring, fall, or spring-fall defoliations by maintaining or increasing the following year's aboveground biomass production. The sedge communities maintained or decreased the following years's biomass production after spring, mid summer, or late summer defoliations. An increase in forbs occurred in 1 sedge community following spring defoliations to 1- or 5-cm residual stubble heights. The most consistent plant response among areas was reduction in height growth and biomass production following compaction treatments. When both defoliation and compaction are considered, it appears that spring, fall, or spring and fall grazing to a 5-cm stubble height on the redtop site would not decrease riparian herbage production. In contrast, when defoliation, compaction, and nutrient return effects are considered in the mountain meadow sedge-dominated communities, grazing once annually during the growing season to a 5-cm stubble height in the spring, or to a 10-cm stubble height in late summer, or at a utilization rate exceeding 30% of the total annual biomass production can reduce herbage production significantly. Results suggest that many of the land management agency riparian guidelines would maintain biomass productivity in these sedge-dominated communities.