Root growth dynamics linked to above-ground growth in walnut (Juglans regia)

Background and Aims Examination of plant growth below ground is relatively scant compared with that above ground, and is needed to understand whole-plant responses to the environment. This study examines whether the seasonal timing of fine root growth and the spatial distribution of this growth through the soil profile varies in response to canopy manipulation and soil temperature.Methods Plasticity in the seasonal timing and vertical distribution of root production in response to canopy and soil water manipulation was analysed in field-grown walnut (Juglans regia ‘Chandler’) using minirhizotron techniques.Key Results Root production in walnuts followed a unimodal curve, with one marked flush of root growth starting in mid-May, with a peak in mid-June. Root production declined later in the season, corresponding to increased soil temperature, as well as to the period of major carbohydrate allocation to reproduction. Canopy and soil moisture manipulation did not influence the timing of root production, but did influence the vertical distribution of roots through the soil profile. Water deficit appeared to promote root production in deeper soil layers for mining soil water. Canopy removal appeared to promote shallow root production.Conclusions The findings of this study add to growing evidence that root growth in many ecosystems follows a unimodal curve with one marked flush of root growth in coordination with the initial leaf flush of the season. Root vertical distribution appeared to have greater plasticity than timing of root production in this system, with temperature and/or carbohydrate competition constraining the timing of root growth. Effects on root distribution can have serious impacts on trees, with shallow rooting having negative impacts in years with limited soil water or positive impacts in years with wet springs, and deep rooting having positive impacts on soil water mining from deeper soil layers but negative impacts in years with wet springs.     

Asynchronicity in root and shoot phenology in grasses and woody plants

Phenology is central to understanding vegetation response to climate change, as well as vegetation effects on plant resources, but most temporal production data is based on shoots, especially those of trees. In contrast, most production in temperate and colder regions is belowground, and is frequently dominated by grasses. We report root and shoot phenology in 7‐year old monocultures of 10 dominant species (five woody species, five grasses) in southern Canada. Woody shoot production was greatest about 8 weeks before the peak of root production, whereas grass shoot maxima preceded root maxima by 2–4 weeks. Over the growing season, woody root, and grass root and shoot production increased significantly with soil temperature. In contrast, the timing of woody shoot production was not related to soil temperature (r=0.01). The duration of root production was significantly greater than that of shoot production (grasses: 22%, woody species: 54%). Woody species produced cooler and moister soils than grasses, but growth forms did not affect seasonal patterns of soil conditions. Although woody shoots are the current benchmark for phenology studies, the other three components examined here (woody plant roots, grass shoots and roots) differed greatly in peak production time, as well as production duration. These results highlight that shoot and root phenology is not coincident, and further, that major plant growth forms differ in their timing of above‐ and belowground production. Thus, considering total plant phenology instead of only tree shoot phenology should provide a better understanding of ecosystem response to climate change.     

Mechanisms of Riparian Cottonwood Decline Along Regulated Rivers

Decline of riparian forests has been attributed to hydrologic modifications to river flows. However, little is known about the physiological and structural adjustments of riparian forests subject to modified flow regimes, and the potential for forest restoration using historic flow regimes is poorly understood. In this paired river study, we compared hydrology, water relations, and forest structure in cottonwood-dominated floodplains of the regulated Green River to those of the unregulated Yampa River. We measured floodplain groundwater levels, soil water availability, cottonwood xylem pressure (Ψ_xp), and leaf-level stomatal conductance (g_s) to assess current impacts of river regulation on the water status of adult cottonwoods. We also simulated a flood on the former floodplain of the regulated river to evaluate its impact on cottonwood water relations. Canopy and root structure were quantified with estimates of cottonwood leaf area and percent live canopy and root density and biomass, respectively. Regulation of the Green River has lowered the annual peak flow yet raised minimum flows in most years, resulting in a 60% smaller stage change, and lowered soil water availability by as much as 70% compared to predam conditions. Despite differences in water availability, daily and seasonal trends in Ψ_xp and g_s were similar for cottonwoods on the regulated and unregulated rivers. In addition, soil water added with the experimental flood had little effect on cottonwood water relations, contrary to our expectations of alleviated water stress. Green River cottonwoods had 10%–30% lower stand leaf area, 40% lower root density, and 25% lower root biomass compared with those for Yampa River cottonwoods. Our results suggest that water relations at the leaf and stem level are currently similar for Yampa and Green River trees due to structural adjustments of cottonwood forests along the Green River, triggered by river regulation.     

Latitudinal Variations in Seasonal Activity of Influenza and Respiratory Syncytial Virus (RSV): A Global Comparative Review

Background

There is limited information on influenza and respiratory syncytial virus (RSV) seasonal patterns in tropical areas, although there is renewed interest in understanding the seasonal drivers of respiratory viruses.

Methods

We review geographic variations in seasonality of laboratory-confirmed influenza and RSV epidemics in 137 global locations based on literature review and electronic sources. We assessed peak timing and epidemic duration and explored their association with geography and study settings. We fitted time series model to weekly national data available from the WHO influenza surveillance system (FluNet) to further characterize seasonal parameters.

Results

Influenza and RSV activity consistently peaked during winter months in temperate locales, while there was greater diversity in the tropics. Several temperate locations experienced semi-annual influenza activity with peaks occurring in winter and summer. Semi-annual activity was relatively common in tropical areas of Southeast Asia for both viruses. Biennial cycles of RSV activity were identified in Northern Europe. Both viruses exhibited weak latitudinal gradients in the timing of epidemics by hemisphere, with peak timing occurring later in the calendar year with increasing latitude (P<0.03). Time series model applied to influenza data from 85 countries confirmed the presence of latitudinal gradients in timing, duration, seasonal amplitude, and between-year variability of epidemics. Overall, 80% of tropical locations experienced distinct RSV seasons lasting 6 months or less, while the percentage was 50% for influenza.

Conclusion

Our review combining literature and electronic data sources suggests that a large fraction of tropical locations experience focused seasons of respiratory virus activity in individual years. Information on seasonal patterns remains limited in large undersampled regions, included Africa and Central America. Future studies should attempt to link the observed latitudinal gradients in seasonality of viral epidemics with climatic and population factors, and explore regional differences in disease transmission dynamics and attack rates.     

Seasonal patterns of fine root demography in a cool-temperate deciduous forest in central Japan

In forest ecosystems, fine roots have a considerable role in carbon cycling. To investigate the seasonal pattern of fine root demography, we observed the fine root production and decomposition processes using a minirhizotron system in a Betula-dominated forest with understory evergreen dwarf bamboo. The length density of fine roots decreased with increasing soil depth. The seasonal patterns of each fine root demographic parameter (length density of visible roots, rates of stand-total fine root production and decomposition) were almost the same at different soil depths. The peak seasons of the fine root demographic parameters were observed in the order: stand-total fine root production rate (late summer) > length density of the visible roots (early autumn) > stand-total fine root decomposition rate (autumn, and a second small peak in spring). The fine root production rate was high in the latter part of the plant growing season. Fine root production peaked in late summer and remained high until the end of the tree defoliation season. The higher stand-total fine root production rate in autumn suggests the effect of understory evergreen bamboo on the stand-total fine root demography. The stand-total fine root decomposition rate was high in late autumn. In the snow-cover period, the rates of both fine root production and decomposition were low. The fine root demographic parameters appeared to show seasonal patterns. The fine root production rate had a clearer seasonality than the fine root decomposition rate. The seasonal pattern of stand-total fine root production rate could be explained by both overstory and understory above-ground productivities.     

Soil water potential and temperature sum during reproductive growth control seed dormancy in Alopecurus myosuroides Huds.

The sustainable management of unwanted vegetation in agricultural fields through integrated weed control strategies requires detailed knowledge about the maternal formation of primary seed dormancy, to support the prediction of seedling emergence dynamics. This knowledge is decisive for the timing of crop sowing and nonchemical weed control measures. Studies in controlled environments have already demonstrated that thermal conditions and, to some extent, water availability during seed set and maturation has an impact on the level of dormancy. However, it is still unclear if this applies also under field conditions, where environmental stressors and their timing are more variable. We address this question for Alopecurus myosuroides in south‐western Sweden. We quantified the effects of cumulated temperature and precipitation as well as soil water potential during the reproductive growth phase of A myosuroides on primary seed dormancy under field conditions. Empirical models differing in focal time intervals and, in case of soil water potential, focal soil depths were compared regarding their predictive power. The highest predictive power for the level of primary dormancy of A. myosuroides seeds was found for a two‐factorial linear model containing air temperature sum between 0 and 7 days before peak seed shedding as well as the number of days with soil water potential below field capacity between 7 and 35 days before peak seed shedding. For soil water potential, it was found that only the top 10 cm soil layer is of relevance, which is in line with the shallow root architecture of A. myosuroides. We conclude that for this species the level of dormancy depends on the magnitude and timing of temperature and water availability during the reproductive growth phase. Water availability appears to be more important during maternal environmental perception and temperature during zygotic environmental perception.     

Incubation behavior adjustments,driven by ambient temperature variation,improve synchrony between hatch dates and caterpillar peak in a wild bird population

For organisms living in seasonal environments, synchronizing the peak energetic demands of reproduction with peak food availability is a key challenge. Understanding the extent to which animals can adjust behavior to optimize reproductive timing, and the cues they use to do this, is essential for predicting how they will respond to future climate change. In birds, the timing of peak energetic demand is largely determined by the timing of clutch initiation; however, considerable alterations can still occur once egg laying has begun. Here, we use a wild population of great tits (Parus major) to quantify individual variation in different aspects of incubation behavior (onset, duration, and daily intensity) and conduct a comprehensive assessment of the causes and consequences of this variation. Using a 54‐year dataset, we demonstrate that timing of hatching relative to peak prey abundance (synchrony) is a better predictor of reproductive success than clutch initiation or clutch completion timing, suggesting adjustments to reproductive timing via incubation are adaptive in this species. Using detailed in‐nest temperature recordings, we found that postlaying, birds improved their synchrony with the food peak primarily by varying the onset of incubation, with duration changes playing a lesser role. We then used a sliding time window approach to explore which spring temperature cues best predict variance in each aspect of incubation behavior. Variation in the onset of incubation correlated with mean temperatures just prior to laying; however, incubation duration could not be explained by any of our temperature variables. Daily incubation intensity varied in response to daily maximum temperatures throughout incubation, suggesting female great tits respond to temperature cues even in late stages of incubation. Our results suggest that multiple aspects of the breeding cycle influence the final timing of peak energetic demand. Such adjustments could compensate, in part, for poor initial timing, which has significant fitness impacts.     

A Shift in Seasonal Rainfall Reduces Soil Organic Carbon Storage in a Cold Desert

Shifts in the seasonal timing of rainfall have the potential to substantially affect the immense terrestrial stores of soil organic carbon (C, SOC). It remains unclear, however, how changes in the timing of rainfall are influencing SOC storage. We hypothesized that a sustained shift in rainfall timing from winter to a spring-summer regime would reduce desert SOC stores by creating moist and warm soil conditions, thus promoting decomposition. To investigate this, we evaluated how an 11-year seasonal shift in rainfall (winter to spring-summer regime) affected SOC storage (that is, dissolved organic C, light SOC, and heavy SOC) in soils beneath dominant shrub and perennial grass species in a cold desert sagebrush-steppe ecosystem. We also measured the soil C to nitrogen (N) ratios, standing litter stocks, and root biomass C to help interpret the long-term changes in SOC stores. As predicted, a seasonal shift in rainfall caused heavy SOC to decline beneath Artemisia tridentata ssp. wyomingensis by 14%, from 3.1 to 2.7 kg C m⁻², and Pseudoroegneria spicata by 19%, from 3.0 to 2.4 kg C m⁻². Neither dissolved organic C, nor the light fraction, responded to changes in rainfall. The C to N ratio of heavy SOC beneath Artemisia declined by at least 6% under the warmer and moister conditions of the spring-summer regime, suggesting that alterations in decomposition dynamics contributed to the loss of SOC. Unexpectedly, coarse litter and root C in Artemisia soils were lower under the spring-summer than winter rainfall regime, suggesting that a decline in litter inputs may also have contributed to the loss of SOC. The C to N ratio of heavy SOC, litter stores (that is, coarse litter and thatch), and root C in Pseudoroegneria soils demonstrated similar responses as in Artemisia soils, but these variables were at best only marginally significant. Our results suggest that a sustained seasonal shift in rainfall from winter to spring-summer will reduce heavy SOC across cold deserts, and that this reduction will stem from alterations in decomposition dynamics and net primary production by plants. Further, as global temperatures rise we may see more overlap of moist and warm soil conditions, especially in ecosystems with winter rainfall regimes (for example, Mediterranean-climate ecosystems and temperate forests), that may reduce SOC in the absence of rainfall changes.     

Changes in grape phylloxera abundance in ungrafted vineyards

To examine seasonal changes in the abundance of grape phylloxera Daktulosphaira vitifoliae (Fitch), several sampling methods were tested at vineyards in Victoria, Australia. At a recently infested site, changes detected by root assessment, trunk trapping, and emergence trapping were closely correlated, although the largest numbers of grape phylloxera were obtained using traps that collected phylloxera emerging from soil. This trapping technique was further used to investigate changes in grape phylloxera numbers across three different sites from southeastern Australia as well as in three consecutive seasons at the same vineyard. Grape phylloxera numbers decreased as vines deteriorated; a single peak of emergence occurred in every summer. Size and timing of emergence peaks varied between sites and also between vine blocks within a site. The number of grape phylloxera trapped was correlated with degree-days. Monitoring soil temperature may provide a way of timing control options against grape phylloxera and a way of identifying peak periods when phylloxera detection surveys should be completed or when grape phylloxera are at the highest risk of spreading among vineyards.     

Cytokinin Profiles in the Conifer Tree <Emphasis Type="Italic">Abies nordmanniana</Emphasis>: Whole-Plant Relations in Year-Round Perspective

Conifer trees are routinely manipulated hormonally to increase flowering, branching, or adjust crown shape for production purposes. This survey of internal cytokinin levels provides a background for such treatments in Abies nordmanniana, a tree of great economic interest. Reference points in the crown and root system were sampled destructively in 4- and 6-year-old trees and analyzed for a range of cytokinins by LC-MS/MS. No seasonal patterns were detected in the root samples, and a major portion of cytokinin was in conjugated forms. Dramatic and consistent seasonal changes occurred in the crown, at levels 17–65 times higher than in the root. Predominant among crown cytokinins was ZR, except in the needles where IPR was also prominent. Within the crown, cytokinin profiles in different organs differed consistently. The leader bud showed a pronounced mid-June minimum, and a maximum later in summer. Subapical buds showed the same June minimum but peaked in mid autumn at a much lower level. Maxima in these buds were preceded by peaks in the subapical stem. Parallel patterns were observed in homologous tissues on branches.This pattern is consistent with two surges beginning in the uppermost stem tissues leading to subsequent accumulation or stimulated production within the buds. Strong differential hormonal profiles between adjacent buds with different fates agree with recent evidence of localized cytokinin production. The data suggest a reduced role of root-derived cytokinins in crown development. Practical cytokinin treatments for crown-shape regulation require close attention to dosage as well as precise timing and positioning.     

Adenylate control contributes to thermal acclimation of sugar maple fine‐root respiration in experimentally warmed soil

We investigated the occurrence of and mechanisms responsible for acclimation of fine‐root respiration of mature sugar maple (Acer saccharum) after 3+ years of experimental soil warming (+4 to 5 °C) in a factorial combination with soil moisture addition. Potential mechanisms for thermal respiratory acclimation included changes in enzymatic capacity, as indicated by root N concentration; substrate limitation, assessed by examining nonstructural carbohydrates and effects of exogenous sugar additions; and adenylate control, examined as responses of root respiration to a respiratory uncoupling agent. Partial acclimation of fine‐root respiration occurred in response to soil warming, causing specific root respiration to increase to a much lesser degree (14% to 26%) than would be expected for a 4 to 5 °C temperature increase (approximately 55%). Acclimation was greatest when ambient soil temperature was warmer or soil moisture availability was low. We found no evidence that enzyme or substrate limitation caused acclimation but did find evidence supporting adenylate control. The uncoupling agent caused a 1.4 times greater stimulation of respiration in roots from warmed soil. Sugar maple fine‐root respiration in warmed soil was at least partially constrained by adenylate use, helping constrain respiration to that needed to support work being performed by the roots.     

Synchrony of Seed Dispersal, Hydrology and Local Climate in a Semi-arid River Reach in California

The temporal availability of propagules is a critical factor in sustaining pioneer riparian tree populations along snowmelt-driven rivers because seedling establishment is strongly linked to seasonal hydrology. River regulation in semi-arid regions threatens to decouple seed development and dispersal from the discharge regime to which they evolved. Using the lower Tuolumne River as a model system, we quantified and modeled propagule availability for Populus fremontii (POFR), Salix gooddingii (SAGO), and Salix exigua (SAEX), the tree and shrub species that dominate near-channel riparian stands in the San Joaquin Basin, CA. A degree-day model was fit to field data of seasonal seed density and local temperature from three sites in 2002–2004 to predict the onset of the peak dispersal period. To evaluate historical synchrony of seed dispersal and seasonal river hydrology, we compared peak spring runoff timing to modeled peak seed release periods for the last 75 years. The peak seed release period began on May 15 for POFR (range April 23–June 10), May 30 for SAGO (range May 19–June 11) and May 31 for SAEX (range May 8–June 30). Degree-day models for the onset of seed release reduced prediction error by 40–67% over day-of-year means; the models predicted best the interannual, versus site-to-site, variation in timing. The historical analysis suggests that POFR seed release coincided with peak runoff in almost all years, whereas SAGO and SAEX dispersal occurred during the spring flood recession. The degree-day modeling approach reduce uncertainty in dispersal timing and shows potential for guiding flow releases on regulated rivers to increase riparian tree recruitment at the lowest water cost.     

Influence of nitrogen application rate on the magnitude of root and shoot growth flushes of Viburnum odoratissimum Ker-Gawl

Most trees and shrubs have cyclic, rather than continuous shoot and root growth. Growth cycles have important implications with regards to transplant and fertilization timing. Although growth cycles of several woody species have been characterized, no information is available on sweet viburnum (Viburnum odoratissimum). A series of experiments was conducted to study shoot and root growth flushes of sweet viburnum and the influence of nitrogen fertilization on flushes. Plants were grown in observation tubes and kept under greenhouse conditions. Sweet viburnum exhibited alternating periods of root and shoot elongation, and the root elongation peak preceded shoot elongation peak by 6–18 days. Increased nitrogen fertilization rate negatively impacted the magnitude and number of root growth flushes. Further research is needed to determine when maximum nitrogen uptake is occurring, relative to root and shoot growth cycles. Section Editor: P. J. Gregory     

Root anatomy of <Emphasis Type="Italic">Pinus taeda</Emphasis> L.: seasonal and environmental effects on development in seedlings

The environmental and seasonal effects on anatomical traits of Pinus taeda L. seedling roots were studied in the laboratory in three contrasting root growth media and also in typical outdoor nursery culture. Growth media with lower water regimen and high penetration resistance caused a reduction in lengths of the white and condensed tannin (CT) zones and acceleration of development of suberin lamellae in the endodermis. As a possible counter to this reduction in zone lengths, second-order laterals were produced closer to the tips of first-order laterals. This suggested there may be an advantage to producing more shorter roots under stressful conditions. Under outdoor nursery conditions (June to mid-December) the white zone was always a rather small part of the root system surface area (4.5% in December), but it dominated as a provider of cortical plasmalemma surface area (CPSA) in contact with modified soil solution (65% in December) because of its live cortex and capacity to increase nearly three fold the amount of CPSA per unit root length. The CT zone always provided most of the total root surface area (80% in December). Although it had no live cortex, a few cells of the CT zone endodermis remained non-suberized passage cells, perhaps giving this major part of the root system some capacity for ion and water absorption. A late summer increase in CPSA was due largely to the rapid production of mycorrhizae. Root systems were capable of very rapid replacement of roots lost due to undercutting and lateral root pruning. The great variation in CPSA per unit root length contained in the white, mycorrhizal and CT zones suggested a capacity to adapt rapidly to changing conditions.     

Influence of temperature and soil drying on respiration of individual roots in citrus: integrating greenhouse observations into a predictive model for the field

In citrus, the majority of fine roots are distributed near the soil surface – a region where conditions are frequently dry and temperatures fluctuate considerably. To develop a better understanding of the relationship between changes in soil conditions and a plant’s below‐ground respiratory costs, the effects of temperature and soil drying on citrus root respiration were quantified in controlled greenhouse experiments. Chambers designed for measuring the respiration of individual roots were used. Under moist soil conditions, root respiration in citrus increased exponentially with changes in soil temperature (Q₁₀ = 1·8–2·0), provided that the changes in temperature were short‐term. However, when temperatures were held constant, root respiration did not increase exponentially with increasing temperatures. Instead, the roots acclimated to controlled temperatures above 23 °C, thereby reducing their metabolism in warmer soils. Under drying soil conditions, root respiration decreased gradually beginning at 6% soil water content and reached a minimum at <2% soil water content in sandy soil. A model was constructed from greenhouse data to predict diurnal patterns of fine root respiration based on temperature and soil water content. The model was then validated in the field using data obtained by CO₂ trapping on root systems of mature citrus trees. The trees were grown at a site where the soil temperature and water content were manipulated. Respiration predicted by the model was in general agreement with observed rates, which indicates the model may be used to estimate entire root system respiration for citrus.     

The long-term trends and seasonal variation of the aeroallergen Alternaria in Derby, UK

Very little is known in the UK about long term trends of theAlternaria spore although it is known to trigger asthma. It hasrecently become apparent that Alternaria spore levels areincreasing in Derby and a detailed study of Alternaria wasundertaken to investigate the increase in numbers, seasonal variationand diurnal periodicity. The seasonal (June—October)Alternaria spore concentrations show a distinct upward trendand there is evidence of an earlier seasonal start and an increase inthe seasonal duration. There has been a dramatic rise in the number ofdays with an Alternaria spore count above 50 spores per cubicmetre, with the peak daily count usually occurring in August butoccasionally in late July or early September. August generally has thehighest monthly total and for 1991–1998 there was a positivecorrelation with monthly rainfall and average temperature. Day to dayspore levels show a positive correlation between Alternariaspore concentrations and maximum temperature but a slight negativecorrelation with daily rainfall. The peak time for spore capture is14.00–22.00, and more than half the daily Alternariacatch is caught between 18.00 and 24.00 hours. The upward trend inAlternaria spore concentrations may be responsible forincreasing levels of respiratory disease, especially during harvesttime.     

Diaspore dispersal season influences root growth and seed traits,but not germination responses,of sympatric Piper species in a seasonal forest

• Seeds may differ in terms of dormancy, longevity, sensitivity to desiccation and dry mass, according to the timing (dry season/rainy season) of diaspore dispersal. In addition, seasonal variations in temperature and water availability can act as signals of the season during seed development, influencing germination responses and root growth. We evaluated the effects of temperature variations and water availability on germination parameters, root growth and seed traits of four coexisting Piper species in seasonal vegetation that differed in diaspore dispersal timing.
• Eight temperature treatments (15, 20, 23, 25, 28, 30, 35 °C, and alternate 30 °C–20 °C) and four induced water potentials (0, −0.3, −0.6 and −1.2 MPa) were used. The parameters germination onset, germination percentage (G%), mean germination time (MGT), root elongation, seed longevity during ex situ storage and dry mass of seeds were evaluated.
• Germination responses observed were independent of the diaspore dispersal timing, such as variations in germination onset, G% and MGT, both in temperature and water availability treatments. In contrast, root elongation, longevity and dry mass of seeds varied according to the time of diaspore dispersal.
• Our results corroborate the hypothesis that the timing of diaspore dispersal is an important factor in controlling the initial development of seedlings in seasonal vegetation, but not in germination responses. The predominance of negative effects of temperature increases and water deficit on root growth shows that the initial stages of plant development can be strongly impacted by these environmental factors.

     

Correction to: The biogeochemical consequences of litter transformation by insect herbivory in the Subarctic: a microcosm simulation experiment

Northern forest ecosystems are projected to experience warmer growing seasons and increased soil freeze–thaw cycles in winter over the next century. Past studies show that warmer soils in the growing season enhance nitrogen uptake by plants, while soil freezing in winter reduces plant uptake and ecosystem retention of nitrogen, yet the combined effects of these changes on plant root capacity to take up nitrogen are unknown. We conducted a 2-year (2014–2015) experiment at Hubbard Brook Experimental Forest in New Hampshire, USA to characterize the response of root damage, nitrogen uptake capacity, and soil solution nitrogen to growing season warming combined with soil freeze–thaw cycles in winter. Winter freeze–thaw cycles damaged roots, reduced nitrogen uptake capacity by 42%, and increased soil solution ammonium in the early growing season (May–June). During the peak growing season (July), root nitrogen uptake capacity was reduced 40% by warming alone and 49% by warming combined with freeze–thaw cycles. These results indicate the projected combination of colder soils in winter and warmer soils in the snow-free season will alter root function by reducing root nitrogen uptake capacity and lead to transient increases of nitrogen in soil solution during the early growing season, with the potential to alter root competition for soil nitrogen and seasonal patterns of soil nitrogen availability. We conclude that considering interactive effects of changes in climate during winter and the snow-free season is essential for accurate determination of the response of nitrogen cycling in the northern hardwood forest to climate change.     

Morphological plasticity of wheat and barley roots in response to spatial variation in soil strength

Root systems of individual crop plants may encounter large variations in mechanical impedance to root penetration. Split-root experiments were conducted to compare the effects of spatial variation in soil strength on the morphological plasticity of wheat and barley roots, and its relationship to shoot growth. Plants of spring barley (Hordeum vulgare cv Prisma) and spring wheat (Triticum aestivum cv Alexandria) were grown for 12 days with their seminal roots divided between two halves of a cylinder packed with sandy loam soil. Three treatment combinations were imposed: loose soil where both halves of the cylinder were packed to 1.1 g cm^–3 (penetrometer resistance 0.3 MPa), dense soil where both halves were packed to 1.4 g cm^–3 (penetrometer resistance 1 MPa), and a split-root treatment where one half was packed to 1.1 and the other to 1.4 g cm^–3. In barley, uniform high soil strength restricted the extension of main seminal root axes more than laterals. In the split-root treatment, the length of laterals and the dry weight of main axes and laterals were increased in the loose soil half and reduced in the dense soil half compared with their respective loose and dense-soil controls. No such compensatory adjustments between main axis and laterals and between individual seminal roots were found in wheat. Variation in soil strength had no effect on the density of lateral roots (number per unit main axis length) in either barley or wheat. The nature and extent of wheat root plasticity in response to variation in soil strength was very different from that in response to changes in N-supply in previous experiments. In spite of the compensatory adjustments in growth between individual seminal roots of barley, the growth of barley shoots, as in wheat, was reduced when part of the root system was in compacted soil.