Showing papers on "Growing season published in 2000"

Separating root and soil microbial contributions to soil respiration: A review of methods and observations

Abstract: Forest soil respiration is the sum of heterotrophic (microbes, soil fauna) and auto- trophic (root) respiration. The contribution of each group needs to be understood to evaluate implications of environmental change on soil carbon cycling and sequestration. Three primary methods have been used to distinguish hetero- versus autotrophic soil respiration including: integration of components contributing to in situ forest soil CO2 efflux (i.e., litter, roots, soil), comparison of soils with and without root exclusion, and application of stable or radioactive isotope methods. Each approach has advantages and disadvantages, but isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots. Pub- lished data from all methods indicate that root/rhizosphere respiration can account for as little as 10 percent to greater than 90 percent of total in situ soil respiration depending on vegetation type and season of the year. Studies which have integrated percent root contribution to total soil respiration throughout an entire year or growing season show mean values of 45.8 and 60.4 percent for forest and nonforest vegetation, respectively. Such average annual values must be extrapolated with caution, however, because the root contribution to total soil respiration is commonly higher during the growing season and lower during the dormant periods of the year. Abbreviations: TScer -t otal soil CO 2 efflux rate; f - fractional root contribution to TS cer; RC - root contribution to TScer

Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress.

Abstract: The extension of growing season at high northern latitudes seems increasingly clear from satellite observations of vegetation extent and duration1,2. This extension is also thought to explain the observed increase in amplitude of seasonal variations in atmospheric CO2 concentration. Increased plant respiration and photosynthesis both correlate well with increases in temperature this century and are therefore the most probable link between the vegetation and CO2 observations3. From these observations1,2, it has been suggested that increases in temperature have stimulated carbon uptake in high latitudes1,2 and for the boreal forest system as a whole4. Here we present multi-proxy tree-ring data (ring width, maximum late-wood density and carbon-isotope composition) from 20 productive stands of white spruce in the interior of Alaska. The tree-ring records show a strong and consistent relationship over the past 90 years and indicate that, in contrast with earlier predictions, radial growth has decreased with increasing temperature. Our data show that temperature-induced drought stress has disproportionately affected the most rapidly growing white spruce, suggesting that, under recent climate warming, drought may have been an important factor limiting carbon uptake in a large portion of the North American boreal forest. If this limitation in growth due to drought stress is sustained, the future capacity of northern latitudes to sequester carbon may be less than currently expected.

Climate change is affecting altitudinal migrants and hibernating species.

Abstract: Calendar date of the beginning of the growing season at high altitude in the Colorado Rocky Mountains is variable but has not changed significantly over the past 25 years. This result differs from growing evidence from low altitudes that climate change is resulting in a longer growing season, earlier migrations, and earlier reproduction in a variety of taxa. At our study site, the beginning of the growing season is controlled by melting of the previous winter's snowpack. Despite a trend for warmer spring temperatures the average date of snowmelt has not changed, perhaps because of the trend for increased winter precipitation. This disjunction between phenology at low and high altitudes may create problems for species, such as many birds, that migrate over altitudinal gradients. We present data indicating that this already may be true for American robins, which are arriving 14 days earlier than they did in 1981; the interval between arrival date and the first date of bare ground has grown by 18 days. We also report evidence for an effect of climate change on hibernation behavior; yellow-bellied marmots are emerging 38 days earlier than 23 years ago, apparently in response to warmer spring air temperatures. Migrants and hibernators may experience problems as a consequence of these changes in phenology, which may be exacerbated if climate models are correct in their predictions of increased winter snowfall in our study area. The trends we report for earlier formation of permanent snowpack and for a longer period of snow cover also have implications for hibernating species.

Responses of tree fine roots to temperature

Abstract: Soil temperature can influence the functioning of roots in many ways. If soil moisture and nutrient availability are adequate, rates of root length extension and root mortality increase with increasing soil temperature, at least up to an optimal temperature for root growth, which seems to vary among taxa. Root growth and root mortality are highly seasonal in perennial plants, with a flush of growth in spring and significant mortality in the fall. At present we do not understand whether root growth phenology responds to the same temperature cues that are known to control shoot growth. We also do not understand whether the flush of root growth in the spring depends on the utilization of stored nonstructural carbohydrates, or if it is fueled by current photosynthate. Root respiration increases exponentially with temperature, but Q10 values range widely from c. 1.5 to > 3.0. Significant questions yet to be resolved are: whether rates of root respiration acclimate to soil temperature, and what mechanisms control acclimation if it occurs. Limited data suggest that fine roots depend heavily on the import of new carbon (C) from the canopy during the growing season. We hypothesize that root growth and root respiration are tightly linked to whole-canopy assimilation through complex source–sink relationships within the plant. Our understanding of how the whole plant responds to dynamic changes in soil temperature, moisture and nutrient availability is poor, even though it is well known that multiple growth-limiting resources change simultaneously through time during a typical growing season. We review the interactions between soil temperature and other growth-limiting factors to illustrate how simple generalizations about temperature and root functioning can be misleading.

Constraints to growth of boreal forests.

Abstract: Understanding how the growth of trees at high latitudes in boreal forest is controlled is important for projections of global carbon sequestration and timber production in relation to climate change. Is stem growth of boreal forest trees constrained by the length of the growing season when stem cambial cells divide1, or by the length of the period when resources can be captured2? In both cases, the timing of the thaw in the spring is critical: neither cambial cell division nor uptake of nutrients and carbon dioxide can occur while the soil is frozen. Here we argue, on the basis of long-term observations made in northern Saskatchewan and Sweden, that the time between the spring thaw and the autumn freeze determines the amount of annual tree growth, mainly through temperature effects on carbon-dioxide uptake in spring and on nutrient availability and uptake during summer, rather than on cambial cell division.

Changes in North American spring

Abstract: Onset of the growing season in mid-latitudes is a period of rapid transition, which includes heightened interaction between living organisms and the lower atmosphere. Phenological events (seasonal plant and animal activity driven by environmental factors), such as first leaf appearance or flower bloom in plants, can serve as convenient markers to monitor the progression of this yearly shift, and assess longer-term change resulting from climate variations. We examined spring seasons across North America over the 1900–1997 period using modelled and actual lilac phenological data. Regional differences were detected, as well as an average 5–6 day advance toward earlier springs, over a 35-year period from 1959–1993. Driven by seasonally warmer temperatures, this modification agrees with earlier bird nesting times, and corresponds to a comparable advance of spring plant phenology described in Europe. These results also align with trends towards longer growing seasons, reported by recent carbon dioxide and satellite studies. North American spring warming is strongest regionally in the northwest and northeast portions. Meanwhile, slight autumn cooling is apparent in the central USA. Copyright © 2000 Royal Meteorological Society

Altering Rainfall Timing and Quantity in a Mesic Grassland Ecosystem: Design and Performance of Rainfall Manipulation Shelters

Abstract: Global climate change is predicted to alter growing season rainfall patterns, potentially reducing total amounts of growing season precipitation and redistributing rainfall into fewer but larger individual events Such changes may affect numerous soil, plant, and ecosystem properties in grasslands and ultimately impact their productivity and biological diversity Rainout shelters are useful tools for experimental manipulations of rainfall patterns, and permanent fixed-location shelters were established in 1997 to conduct the Rainfall Manipulation Plot study in a mesic tallgrass prairie ecosystem in northeastern Kansas Twelv e9x1 4 ‐mfixed-location rainfall manipulation shelters were constructed to impose factorial combinations of 30% reduced rainfall quantity and 50% greater interrainfall dry periods o n6x6 ‐mplots, to examine how altered rainfall regimes may affect plant species composition, nutrient cycling, and above- and belowground plant growth dynamics The shelters provided complete control of growing season rainfall patterns, whereas effects on photosynthetic photon flux density, nighttime net radiation, and soil temperature generally were comparable to other similar shelter designs Soil and plant responses to the first growing season of rainfall manipulations (1998) suggested that the interval between rainfall events may be a primary driver in grassland ecosystem responses to altered rainfall patterns Aboveground net primary productivity, soil CO2 flux, and flowering duration were reduced by the increased interrainfall intervals and were mostly unaffected by reduced rainfall quantity The timing of rainfall events and resulting temporal patterns of soil moisture relative to critical times for microbial activity, biomass accumulation, plant life histories, and other ecological properties may regulate longerterm responses to altered rainfall patterns

Rodents, plants, and precipitation: spatial and temporal dynamics of consumers and resources

Abstract: Resource/consumer dynamics are potentially mediated by both limiting resources and biotic interactions. We examined temporal correlations between precipitation, plant cover, and rodent density, with varying time lags using long-term data from two sites in the Chihuahuan desert of North America: the Sevilleta Long-term Ecological Research site (LTER), New Mexico, USA and a site near Portal, Arizona, USA. We also calculated the spatial correlations in precipitation, plant cover, and rodent dynamics among six sites, five at Sevilleta and one at Portal. At Sevilleta, all three variables were temporally correlated, with plant cover responding to precipitation during the same growing season and rodent populations lagging at least one season behind. At Portal, plant stem count was also correlated with precipitation during the same growing season, but there was no significant correlation between rodents and either precipitation or plant growth. Spatial correlations in plant cover and rodent populations between sites reflected the localized nature of summer rainfall, so that sites with highly correlated summer precipitation exhibited higher correlations in plant cover and rodent populations. In general, our results indicate that limiting resources influence consumer dynamics, but these dynamics also depend crucially on the biotic interactions in the system.

Trading forage quality for quantity? Plant phenology and patch choice by Svalbard reindeer

Abstract: Plant phenology of Luzula heathland plots in Spitsbergen (78°N) was manipulated by adding or removing snow, which altered the time for plots (2 m×2 m; n=10) to become snow-free A 2-week difference in snowmelt, equivalent to approximately one-sixth of the growing season, was achieved between advanced (first to be snow-free) and delayed (last to be snow-free) treatments, which influenced plant biomass and plant quality Nitrogen content of the forage species decreased with time after snowmelt, whereas C:N ratio increased Manipulation of snowmelt led to a shift in ”phenological time”, without altering these plant quality parameters as such Early in the growing season, Svalbard reindeer (Rangifer tarandus platyrhynchus) selected the advanced plots which had been snow-free for longest, presumably because of the greater biomass of both Luzula confusa and Salix polaris, major components of reindeer diet at that time of the year Moreover, the proportion of live Luzula leaves was highest in advanced plots, relative to both unmanipulated control and delayed plots In contrast, plant quality, measured as nitrogen content and C:N ratio of leaves, was lowest in the preferred plots Phenolic content did not differ among treatments, and is therefore unlikely to play a role in reindeer selection for plots with early snowmelt Unlike in temperate regions, where selection for plant quality seems to be of major importance, selection for plant quantity might be an outcome of generally low levels of plant biomass and high forage quality during the growing season in the high Arctic Reindeer selection for high plant biomass is likely to lead to a more favourable nitrogen and energy return than selection for high plant quality

Measuring and modelling seasonal variation of carbon dioxide and water vapour exchange of a Pinus ponderosa forest subject to soil water deficit.

Abstract: Summary We conducted ecosystem carbon and water vapour exchange studies in an old-growth Pinus ponderosa forest in the Pacific North-west region of the United States. The canopy is heterogeneous, with tall multiaged trees and an open, clumped canopy with low leaf area. Carbon assimilation can occur throughout relatively mild winters, although night frosts can temporarily halt the process and physiological factors limit its efficiency. In contrast, carbon assimilation is often limited in the ‘growing season’ by stomatal closure associated with high evaporative demand (D) and soil water deficits. All of these factors present a challenge to effectively modelling ecosystem processes. Our objective was to generate an understanding of the controls on ecosystem processes across seasonal and annual cycles from a combination of fine-scale process modelling, ecophysiological measurements, and carbon and water vapour fluxes measured by the eddy covariance method. Flux measurements showed that 50% and 70% of the annual carbon uptake occurred outside the ‘growing season’ (defined as bud break to senescence, ∼ days 125–275) in 1996 and 1997. On a daily basis in summer, net ecosystem productivity (NEP) was low when D and soil water deficits were large. Whole ecosystem water vapour fluxes (LE) increased from spring to summer (1.0–1.9 mm d−1) as conducting leaf area increased by 30% and as evaporative demand increased, while evaporation from the soil surface became a smaller portion of total LE as soil water deficits increased. The models underestimated soil evaporation, particularly following rain. In the SPA model, varying the temperature optimum for photosynthesis seasonally resulted in overestimation of carbon uptake in winter and spring, showing that in coniferous forests, assumptions about temperature optima are clearly important. Daily estimates of soil surface CO2 flux from measurements and site meteorological data demonstrated that modelling of soil CO2 flux based on an Arrhenius-type equation in CANPOND overestimated CO2 respired from the soil during drought and when temperatures were low.

Simulating the effects of climate change on the carbon balance of North American high‐latitude forests

Abstract: Summary The large magnitude of predicted warming at high latitudes and the potential feedback of ecosystems to atmospheric CO2 concentrations make it important to quantify both warming and its effects on high-latitude carbon balance. We analysed long-term, daily surface meteorological records for 13 sites in Alaska and north-western Canada and an 82-y record of river ice breakup date for the Tanana River in interior Alaska. We found increases in winter and spring temperature extrema for all sites, with the greatest increases in spring minimum temperature, average 0.47 °C per 10 y, and a 0.7-day per 10 y advance in ice breakup on the Tanana River. We used the climate records to drive an ecosystem process model, BIOME_BGC, to simulate the effects of climate change on the carbon and water balances of boreal forest ecosystems. The growing season has lengthened by an average of 2.6 days per 10 y with an advance in average leaf onset date of 1.10 days per 10 y. This advance in the start of the active growing season correlates positively with progressively earlier ice breakup on the Tanana River in interior Alaska. The advance in the start of the growing season resulted in a 20% increase in net primary production for both aspen (Populus tremuloides) and white spruce (Picea glauca) stands. Aspen had a greater mean increase in maintenance respiration than spruce, whereas spruce had a greater mean increase in evapotranspiration. Average decomposition rates also increased for both species. Both net primary production and decomposition are enhanced in our simulations, suggesting that productive forest types may not experience a significant shift in net carbon flux as a result of climate warming.

Water conservation in urban landscapes

Abstract: other seasonal water uses, such as evaporative coolers and swimming pools, but evidence indicates that landscape irrigation accounts for most of the seasonal increase in municipal water use. When a severe water shortage in Seattle, Wash., in Summer 1992 resulted in the banning of turfgrass irrigation, consequent municipal water use did not deviate from winter levels (Fig. 1, bottom), indicating that increased seasonal water use in Seattle indeed goes to landscapes. Water consumption for landscape use varies with rain and ET (Table 1). Applying the subtraction method described above to data from six cities around the United States, those in the summer-rainfall region east of the Mississippi River increase water use about one-third during the spring to fall growing season. Summing the increased seasonal water use showed that landscapes account for approximately 10% of total seasonal water consumption for these cities, the rest going to indoor and other, nonirrigation, uses. In the arid Mountain West, seasonal water use increases nearly 3to 4-fold during the growing season, and landscapes can account from a third to nearly half of the total municipal yearly water use (Vickers, 1991). The amount of water applied to landscapes can be divided into three levels of usage. The first level is water needed to meet baseline physiological plant water needs. The second level is water needed to compensate for system nonuniformity to ensure that the all plants receive the baseline level, particularly in turf. The third is water applied in excess of that needed by plants or for system uniformity, which is potentially conservable.

Hydrochory, seed banks, and regeneration dynamics along the landscape boundaries of a forested wetland

Abstract: Following the environmental sieve concept, the setting in which the recruitment of Taxodium distichum occurs in, becomes increasingly restrictive from the seed to seedling stage in an impounded forested wetland. Although a wide elevational band of dispersing seed moves across the boundary of a swamp-field in the water sheet, the zone of germination is relegated to that portion of the forested wetland that draws down during the growing season. Seedling recruitment is further restricted to the uppermost zone of the winter water sheet. These patterns are likely applicable to other species of dominant swamp species, e.g., Cephalanthus occidentalis crossed the boundary of a forested wetland and abandonded field in winter flooding (November–December and November–March, respectively) in Buttonland Swamp. The elevation of the boundary was 101.3 m NGVD. While the seeds of at least 40 swamp species were dispersed across the boundary, few viable seeds were dispersed after the winter season. Kriged maps showed seeds of T. distichum and C. occidentalis dispersed in patches in the water depending on the position of the water sheet. Most species of both water- and gravity-dispersed species had a localized pattern of seed distribution (either spherical or exponential) and this indicated that seeds may not be dispersed for great distances in the swamp. Water-dispersed T. distichum and C. occidentalis had larger dispersal ranges (A0=225 and 195 m, respectively) than Bidens frondosa and B. discoidea (A0=14 and 16 m, respectively). Seed dispersal varied with season depending on the availability of seeds. In Buttonland Swamp, viable seeds typically were dispersed for T. distichum in November–June, and for C. occidentalis in November-July. Low water occurred in August 1993 and high in February 1994 (99.8 and 101.6 m NGVD, respectively). The seed banks along the landscape boundary varied in species composition according to elevation (r2 = 0.996). While the similarity of species richness between water-dispersed seeds and the seed bank at elevations that flooded (during June 1993 through May 1995) was high (10–17%), it was low between water-dispersed seeds and the seed bank at elevations that did not flood (5%). T. distichum seeds had a short germination window in that seeds germinated within a year following their production in zones that were flooded in the winter followed by drawdown during the next growing season. After 1 year, less than 5% of the T. distichum seeds remained viable on the surface of the soil. Germination of T. distichum was confined to specific elevations (above 99.3 but below 101.6 m NGVD) during this study with 4.1% of the seedlings surviving for more than 2 years at a mean of 101.4 m NGVD. All seedlings below this elevation died. To maximize natural regeneration along the boundaries of swamps in abandoned farm fields targeted for restoration, this study suggests a flood pulse regime consisting of high water in the winter to maximize dispersal of live seeds followed by low water in the summer to facilitate seed germination and seedling recruitment. Hydrologic restoration could assist in the natural recovery of damaged wetlands if a seed source exists nearby.

A transient, nutrient‐based model of arctic plant community response to climatic warming

Abstract: We developed a nutrient-based, plant community and ecosystem model (ArcVeg) designed to simulate the transient effects of increased temperatures on the biomass and community composition of a variety of arctic ecosystems. The model is currently parameterized for upland, mesic ecosystems in high Arctic, low Arctic, treeline, and boreal forest climate zones. A unique feature of ArcVeg is that it incorporates up to 18 plant functional types including a variety of forbs, graminoids, shrubs, and nonvascular plants that are distinguished by a set of five parameters. Timing and rate of growth, as well as nutrient use, are particularly important in defining competitive interactions in the model and in explaining coexistence in complex communities. Simulations of climatic warming, which increase nitrogen mineralization and growing season length, suggest an increase in total biomass for high and low Arctic zones over 200 yr, and an increase in shrub biomass at the expense of other plant functional types. The initial community response to warming was a function of the initial dominance structure, whereas the long-term response reflected adaptations of plant functional types to the new environment. Therefore, long-term responses (decades to centuries) differed in both direc- tion and magnitude from initial responses. In addition, warming resulted in the formation of novel, stable plant communities after 200 simulation years that were not typical of current zonal vegetation types in the Arctic of northwestern North America.

Methane emission from rice fields in China: Measurements and influencing factors

Abstract: Methane emissions from rice fields in China were measured at eight sites in five provinces under conditions representative of local practices for rice cultivation. Methane emission rates during the rice growth period varied greatly from site to site and with treatments at the same site, ranging from 0.3 to 205 g CH4/m2. Flooded or waterlogged rice fields in the nonrice growth season continuously emitted CH4 substantially. The average CH4 emission rate from a rice field in Chongqing was as high as 36.2 g CH4/m2 in the nonrice growing season. Furthermore, flooding in the nonrice growth season also significantly stimulated CH4 emission during the rice growth period in the next year. Increases in the rate of CH4 flux after rice transplanting were less when the number of consecutive upland crops grown before rice transplanting was greater. CH4 emissions from rice fields located on downslope was larger than from those on midslope and upslope in hilly areas due to poor drainage of the former. Application of rice straw in fall when winter wheat was sown did not increase CH4 emission significantly during the following rice growth period. CH4 emission was depressed by the application of ammonium sulfate but was, in general, not significantly affected by urea application.

Plant carbon–nutrient interactions control co2 exchange in alaskan wet sedge tundra ecosystems

Abstract: We explored the long-term (8-yr) effects of separate field manipulations of temperature and nutrient availability on carbon balance in wet sedge tundra near the Arctic Long Term Ecological Research (LTER) site at Toolik Lake, Alaska. Our goals were (1) to assess the relative importance of chronic warming (with field greenhouses) and increased N and P availability (by fertilization) in controlling gross ecosystem photosynthesis, ecosystem respiration (plant plus heterotrophic respiration), and ultimately ecosystem C balance; and (2) to attempt to partition ecosystem responses to these treatments between plant and soil contributions. We present results of the effects of these manipulations on whole-system CO2 exchange over seasonal and diel cycles, and on nonrhizosphere soil microbial respiration using in situ soil incubations. Wet sedge control plots were, at best, a weak sink for carbon even during the peak growing season. Chronic nutrient additions of N + P shifted wet sedge carbon balance to a strong sink throughout the growing season; nutrient availability regulated seasonal and diel CO2 exchanges in these two wet sedge ecosystems. The N + P plots had significantly higher photosynthesis and ecosystem respiration in spite of the unanticipated effect of ∼30% reduction in thaw depth in these plots, apparently due to a twofold increase in litter accumulation insulating the soil surface and/or possible shading from greater plant cover in these plots. These results highlighted the prevailing importance of nutrient–carbon interactions in controlling ecosystem processes and ecosystem C balance in arctic tundra. In contrast, warming had only subtle effects on CO2 exchanges. Increased temperatures in the warmed plots had little effect on instantaneous rates of photosynthesis or respiration. After eight years of chronic warming with an average 5.6°C higher air temperature over the growing season and a 40–200% increase in net N mineralization rate, it was surprising that warming did not have more profound effects on CO2 exchange and plant cover. If there were an effect of warming, increased temperatures might cause early canopy development and lengthen the growing season, rather than directly affect instantaneous rates of photosynthesis. Based on photosynthesis–light response curves developed from the early- and late-season diel measurements, we demonstrated that the main effect of warming was to accelerate the development of the canopy early in the season. By midseason, however, there were no significant differences in C exchange between warmed and control plots. Perhaps the most important and novel result emerging from this study is the prevailing importance of plant C exchange, not soil processes, in driving ecosystem C fluxes. First, nonrhizosphere soil microbial respiration as estimated CO2 flux from in situ soil incubations was a small fraction of whole-system respiration and did not vary among treatments. This suggests that anaerobic conditions or some other factor may limit soil microbial respiration more than do temperature or nutrients. Second, plant respiration contributed most (90%) of the ecosystem respiration in fertilized plots. This unanticipated and large contribution from plant respiration highlights the critical importance of understanding the response of plant respiration to global environmental change in these wet sedge ecosystems.

Interannual variability of net ecosystem CO2 exchange at a subarctic fen

Abstract: Landscape-scale net ecosystem CO 2 exchange (NEE) and the energy balance of a subarctic fen were studied during five growing seasons near Churchill, Manitoba. Interannual variability in NEE was large and ranged from a net sink of -235 g CO 2 m -2 in 1996 to a net source of +76 g CO 2 m -2 in 1994. Annual estimates of CO 2 exchange indicate that during the present period the fen is losing carbon nearly 3 times faster than its long-term historical gain of about 11 g CO 2 m -2 yr -1 . Our estimates suggest that gross ecosystem photosynthesis may be more variable than ecosystem respiration on diurnal, seasonal, and interannual timescales. Our data strongly indicate that an early snowmelt combined with wet and warm conditions during the spring period lead to large carbon acquisition even when drier conditions were experienced over the majority of the growing season. The phenological stage of the vegetation relative to the climatic conditions experienced is an important cause of the interannual variability in NEE. An accurate representation of phenology in climate models is, therefore, critical to the success of forecasting the carbon budgets of northern wetlands.

Effects of water regime and competition on the establishment of a native sedge in restored wetlands

Abstract: Summary 1. Restoration of depressional wetlands in mid-continental North America typically involves reflooding formerly drained basins. There have been several thousand restorations in this manner over the past 10 years. Deliberate revegetation is rarely attempted as it is assumed that native vegetation will eventually establish naturally. Carex (sedges), which occur commonly on the periphery of native wetlands and in adjacent saturated meadows, do not readily re-establish in reflooded basins. It is unlikely that Carex will reappear without deliberate reintroduction. Criteria for introduction and establishment of native sedges do not currently exist because environmental constraints are not well understood. 2. In this study we investigated the environmental constraints that might limit the establishment of the native wetland sedge, Carex lacustris, in reflooded wetland basins. Immature sedges were planted in three experimental wetlands with different water-level fluctuation regimes (i.e. falling, rising, static) along an elevational gradient, to determine the effects on survival and stand development of changing water level, water depth, competition from naturally recruited vegetation, and planting density. 3. The response of immature sedges to water-level fluctuation in the first growing season did not indicate the outcome of mature stand development by the end of three growing seasons. Whereas initial survival and growth were lowest in the falling water basin, stem density, height and biomass were greatest under this type of water-level fluctuation by the third growing season. 4. Interspecific competition was most intense at the upper elevations, particularly in the rising water basin. Although sedge growth was limited by increasing water depth in the absence of competition, growth was uniformly low across all elevations in the presence of competition. Competition reduced sedge growth to differing degrees in the rising vs. falling and static water basins. 5. Results suggest that C. lacustris can produce dense stands under a primarily annual weed community within two to three growing seasons, but that Phalaris arundinacea (reed canary grass) can preclude successful establishment of C. lacustris. 6. Control of water-level conditions and competition is most crucial during the first growing season to ensure appropriate conditions for successful stand establishment. The potential for growth as well as mortality is greatest during this first year. 7. Competition can be a significant constraint on the successful establishment of C. lacustris and should be considered explicitly when developing a restoration plan.

Complex interactions in a streamside plant community

Abstract: Ecologists are increasingly finding that complex combinations of competitive and facilitative interactions influence the distribution and abundance of plants. I conducted a two-year field experiment to explore these processes in a streamside community lining the South Fork Eel River in northern California. Specifically, I tested the hypothesis that the sedge Carex nudata provides critical stable substrate for other plants during winter floods and protection from herbivores over the growing season. In addition to these facil- itative effects, Carex is also hypothesized to compete with the associated species, and thus limit their size and reproduction. To evaluate these hypotheses, I followed the performance of transplanted individuals of Mimulus guttatus, M. cardinalis, Juncus covillei, Conocephalum conicum, and Brach- ythecium frigidum and naturally occurring individuals of Epipactis gigantea on Carex tussocks with dense, thinned, pinned back, or completely clipped Carex stems. The five transplanted species were also planted directly onto the emergent streambed. Though stream- bed transplants grew as well as those on tussocks over the summer, they experienced significantly greater winter mortality, up to 100%, supporting the hypothesis that tussocks provide a critical stable substrate. In contrast, growing season competition by Carex reduced biomass by over 50% for five of the six species and reduced reproductive performance by over 60%. Also, over the growing season, Carex protected M. guttatus and Epipactis from insect larvae and deer, respectively, reducing herbivory by >75%. Additional results from a deer exclosure treatment showed that the positive effects of this "associational defense" were equal in magnitude to the negative effects of Carex competition on Epipactis biomass. The mechanisms underlying these associational defenses and the implications of my results for the relationship between disturbance and facilitation are discussed. I suggest that re- garding plant interactions as combinations of facilitative and competitive components may enhance our understanding of natural communities.

Seasonal CO2 assimilation and stomatal limitations in a Pinus taeda canopy

Abstract: Net CO(2) assimilation (A(net)) of canopy leaves is the principal process governing carbon storage from the atmosphere in forests, but it has rarely been measured over multiple seasons and multiple years. I measured midday A(net) in the upper canopy of maturing loblolly pine (Pinus taeda L.) trees in the piedmont region of the southeastern USA on 146 sunny days over 36 months. Concurrent data for leaf conductance and photosynthetic CO(2) response curves (A(net)-C(i) curves) were used to estimate the relative importance of stomatal limitations to CO(2) assimilation in the field. In fully expanded current-year and 1-year-old needles, midday light-saturated A(net) was constant over much of the growing season (5-6 mmol CO(2) m(-2) s(-1)), except during drought periods. During the winter season (November-March), midday A(net) of overwintering needles varied in proportion to leaf temperature. Net CO(2) assimilation at light saturation occurred when daytime air temperatures exceeded 5-6 degrees C, as happened on more than 90% of the sunny winter days. In both age classes of foliage, winter carbon assimilation accounted for approximately 15% of the daily carbon assimilation on sunny days throughout the year, and was relatively insensitive to year-to-year differences in temperature during this season. However, strong stomatal limitations to A(net) occurred as a result of water stress associated with freezing cycles in winter. During the growing season, drought-induced water stress produced the largest year-to-year differences in seasonal CO(2) assimilation on sunny days. Seasonal A(net) was more drought sensitive in current-year needles than in 1-year-old needles. Relative stomatal limitations to daily integrated A(net) were approximately 40% over the growing season, and summer drought rather than high temperatures had the largest impact on summer A(net) and integrated annual CO(2) uptake in the upper crown. Despite significant stomatal limitations, a long duration of near-peak A(net) in the upper crown, particularly in 1-year-old needles, conferred high seasonal and annual carbon gain.

Effects of lengthened growing season and soil warming on the phenology and physiology of Polygonum bistorta.

Abstract: Summary The phenological and physiological responses of arctic tundra plant species are key to predicting their survival in a warmer climate. One of the consequences of a warmer climate in the Arctic will be a longer growing season. We examined the effects of lengthened growing season and soil warming on the widely distributed forb, Polygonum bistorta L. Three treatments were established near Toolik Lake, Alaska in 1995 and 1996: extended season, extended season with soil warming, and an unmanipulated control. The season was extended by removing the snow load in the spring and keeping the treatments free of snow in the autumn. The spring snow removal extended the snow-free period over that of controls by 8 d in 1995 and 24 d in 1996. As a result, the number of accumulated soil thaw days and consequently the depth of soil thaw increased on the treatment plots. Polygonum bistorta responded to the treatments by becoming active earlier and senescing earlier, resulting in a growth period of similar duration to that of the controls. Leaf size and leaf number were unaffected by the treatments, as were leaf photosynthetic assimilation rates and nutrient concentrations. The results indicate that internal constraints limit the response of this species to lengthened growing season, suggesting that it is a determinant or periodic species. With climate warming, this periodic growth will put P. bistorta at a competitive disadvantage relative to plants that can respond to lengthened growing season.

Multi-variable model for identifying crop response zones in a field

Abstract: An computer implemented apparatus and method are disclosed for defining areas of a field in which a crop or other vegetation is grown based on their selective ability to grow such vegetation through a growing season, or some shorter preselected time period. The method includes making a number of temporally separated measurements through air borne imaging of a field, registering the data to the geography of the field and each other, normalizing the data including converting the data to a vegetative index indicative to the presence of vegetation in the field, comparing the data to identify clusters of like value, and classifying the clusters and images to learn how the different field areas responded in growing vegetation through the season. With this method, the field may be segregated into a number of like areas called crop response zones which exhibit similar vegetative growth characteristics as an aid to a grower in his decision making regarding how to maximize yield in his field.

Reducing the abundance of leafhoppers and thrips in a northern California organic vineyard through maintenance of full season floral diversity with summer cover crops

Abstract: 1 Maintenance of floral diversity throughout the growing season in vineyards in the form of summer cover crops of buckwheat (Fagopyrum esculentum Moench) and sunflower (Helianthus annus Linnaeus), had a substantial impact on the abundance of western grape leafhoppers, Erythroneura elegantula Osborn (Homoptera: Cicadellidae), and western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), and associated natural enemies. 2 During two consecutive years, vineyard systems with flowering cover crops were characterized by lower densities of leafhoppers and thrips, and larger populations and more species of general predators, including spiders. 3 Although Anagrus epos Girault (Hymenoptera: Mymaridae), the most important leafhopper parasitoid, achieved high numbers and inflicted noticeable mortality of grape leafhopper eggs, no differences in egg parasitism rates were observed between cover cropped and monoculture systems. 4 Mowing of cover crops forced movement of Anagrus and predators to adjacent vines resulting in the lowering of leafhopper densities in such vines. 5 Results indicate that habitat diversification using summer cover crops that bloom most of the growing season, supports large numbers of predators and parasitoids thereby favouring enhanced biological control of leafhoppers and thrips in vineyards.

Trace gas exchange in a high‐Arctic valley: 3. Integrating and scaling CO2 fluxes from canopy to landscape using flux data, footprint modeling, and remote sensing

Abstract: Within the framework of the European Land Arctic Physical Processes project and as part of the Danish Research Council's Polar Program, a study on trace gas exchange in a high-arctic ecosystem was conducted in NE Greenland, May-August 1997 On the basis of carbon dioxide flux measurements from three dominant surface types, this paper reports on the upscaling of such measurements from canopy to landscape level Over a three-week period starting in mid-July, the different surfaces revealed large differences in the CO2 flux with uptake rates ranging from 07 g C m(-2) d(-1) over the dwarf shrub heath to 30 g C m(2) d(-1) over denser parts of the fen, while willow snowbed revealed intermediate uptake rates The carbon dioxide exchange could be simulated by a CO2 model, combining photosynthesis and soil respiration routines, for which the parametrization depended on the vegetation type Results from the simulation were supported by a sensitivity analysis based on a three-dimensional footprint model where it was shown that the CO2 uptake was strongly related to the measured leaf area index The CO2 model was used to calculate the spatial distribution in Net Ecosystem Exchange (NEE) on the basis of Landsat satellite data acquired at the peak of the growing season and stratified according to vegetation type It was found that there was a reasonable agreement between the satellite-based flux estimate (-077 g C m(-2) d(-1)) and the CO2 flux found by areal weighting of the eddy correlation measurements (-088 g C m(-2) d(-1)) for Me specific study day Finally, the summer season NEE was calculated for the whole Zackenberg Valley bottom In June, there was a valley-wide carbon loss of 84+/-26 g C m(-2) month(-1), whereas the valley system accumulated 188+/-67 g C m(-2) season(-1) during the growing season (July-August) (Less)

Use of false rings in Austrian pine to reconstruct early growing season precipitation

Abstract: As a consequence of dry conditions, coniferous trees may produce radially smaller diameter tracheids within their tree rings before regular latewood formation starts. The resulting structures, whic...

Contrasting fine-root production, survival and soil CO 2 efflux in pine and poplar plantations

Abstract: Tree root activity, including fine-root production, turnover and metabolic activity are significant components of forest productivity and nutrient cycling. Differences in root activity among forest types are not well known. A 3-year study was undertaken in red pine (Pinus resinosa Ait.) and hybrid poplar (Populus tristis X P. balsamifera cv `Tristis no. 1') plantations to compare belowground root dynamics. We measured fine-root production, mortality and standing crop, as well as soil CO2 efflux. Pine fine-root production was only 2.9% of that of poplar during three years; 85 pine roots were observed in minirhizotron tubes compared with 4088 poplar roots. Live-root density oscillated seasonally for both species with late winter minimum and autumn maximum. Poplar reached constant maximum live-root length within the first growing season, but pine continued to increase observed fine-root length for three growing seasons. Within the first 100 days following initial appearance, 22% of the pine roots disappeared and 38% of the poplar roots disappeared. Median fine-root longevity of pine was 291 days compared with 149 days for poplar roots. Fine-root longevity increased with depth in the soil, and was greater for roots with initial diameter >0.5 mm. The probability of poplar root death from late February to May was more than three times that in any other season, regardless of root age. Despite the greater poplar root production and live-root length, fine-root biomass and soil CO2 efflux was greater in pine. Greater metabolic activity in the pine stand may be due to greater fine-root biomass or greater heterotrophic respiration.