Temperature thresholds for the onset of xylogenesis in alpine shrubs on the Tibetan Plateau

Key message

The threshold minimum air temperature driving xylem growth of alpine  Rhododendron aganniphum is lower than that commonly observed at the treeline of conifers.

Abstract

Understanding how alpine shrubs grow and which environmental factors drive their biomass gain could help to functionally differentiate trees and shrubs. The cambium is the main meristem responsible for wood formation in trees and shrubs. Thus, a better knowledge of cambium growth dynamics in alpine shrubs would allow explaining why shrubs displace trees above the treeline. Here, we aim to investigate the timings and dynamics of xylogenesis and to identify the thermal thresholds controlling the onset of xylem growth of Rhododendron aganniphum, a tall shrub growing above the alpine treeline on the Tibetan Plateau. Timings of xylogenesis and radial growth rates were assessed from anatomical observations of the developing xylem during three growing seasons (2011, 2012, and 2013). The threshold temperature at which xylogenesis had a 0.5 probability of being active was calculated with logistic regressions. The onset of xylogenesis was observed between mid and late June, whereas the end of xylogenesis lasted from mid to late September. Overall, the duration of xylem growth lasted 88–101 days, and 94 % of the ring was formed from June to August. The threshold for the onset of xylem growth was observed at 2.0 ± 0.6 °C for the minimum air temperature, lower than that commonly observed for treeline conifers (ca. 6 °C). This low thermal threshold allows alpine shrubs to have a growing season long enough to complete xylem production and maturation during the warmest summer months. Our results suggest that the time required to complete xylogenesis is critical to understand why shrubs displace trees above the treeline.

     

Critical temperatures for xylogenesis in conifers of cold climates

Aim To identify temperatures at which cell division and differentiation are active in order to verify the existence of a common critical temperature determining growth in conifers of cold climates. Location Ten European and Canadian sites at different latitudes and altitudes. Methods The periods of cambial activity and cell differentiation were assessed on a weekly time‐scale on histological sections of cambium and wood tissue collected over 2 to 5 years per site from 1998 to 2005 from the stems of seven conifer species. All data were compared with daily air temperatures recorded from weather stations located close to the sites. Logistic regressions were used to calculate the probability of xylogenesis and of cambium being active at a given temperature. Results Xylogenesis lasted from May to October, with a growing period varying from 3 to 5 months depending on location and elevation. Despite the wide geographical range of the monitored sites, temperatures for onset and ending of xylogenesis converged towards narrow ranges with average values around 4–5, 8–9 and 13–14 °C for daily minimum, mean and maximum temperature, respectively. On the contrary, cell division in the cambium stopped in July?August, when temperatures were still high. Main conclusions Wood formation in conifers occurred when specific critical temperatures were reached. Although the timing and duration of xylogenesis varied among species, sites and years, the estimated temperatures were stable for all trees studied. These results provide biologically based evidence that temperature is a critical factor limiting production and differentiation of xylem cells in cold climates. Although daily temperatures below 4?5 °C are still favourable for photosynthesis, thermal conditions below these values could inhibit the allocation of assimilated carbon to structural investment, i.e. xylem growth.     

Predicting xylem phenology in black spruce under climate warming

In the next century, the boreal ecosystems are projected to experience greater rates of warming than most other regions of the world. As the boreal forest constitutes a reservoir of trees of huge ecological importance and only partially known economic potential, any possible climate‐related change in plant growth and dynamics has to be promptly predicted and evaluated. A model for assessing xylem phenology in black spruce [Picea mariana (Mill.) B.S.P.] using daily temperatures and thermal thresholds was defined and applied to predict changes in onset, ending and duration of xylem growth under different warming scenarios with temperatures rising by up to 3 °C. This was achieved by collecting and analyzing a dataset obtained from a 7‐year monitoring of cambium phenology and wood formation on a weekly time‐scale in trees growing in four sites at different latitudes and altitudes in the Saguenay‐Lac‐Saint‐Jean region (Quebec, Canada). The onset of xylem growth occurred between mid‐May and early June while the end ranged between mid‐September and early October, resulting in a growing season of 101–141 days. The model predicted longer duration of xylem growth at higher temperatures, with an increase of 8–11 days/ °C, because of an earlier onset and later ending of growth. With an increase of 3 °C in the mean temperature during the year, the duration of xylem growth changed on average from 125 to 160 days. The predicted changes in cambial phenology could significantly affect future wood production of the boreal ecosystems.     

Temporal dynamic of wood formation in Pinus cembra along the alpine treeline ecotone and the effect of climate variables

We determined the temporal dynamic of cambial activity and xylem development of stone pine (Pinus cembra L.) throughout the treeline ecotone. Repeated micro-sampling of the developing tree ring was carried out during the growing seasons 2006 and 2007 at the timberline (1,950 m a.s.l.), treeline (2,110 m a.s.l.) and within the krummholz belt (2,180 m a.s.l.) and the influence of climate variables on intra-annual wood formation was determined. At the beginning of both growing seasons, highest numbers of cambial and enlarging cells were observed at the treeline. Soil temperatures at time of initiation of cambial activity were c. 1.5°C higher at treeline (open canopy) compared to timberline (closed canopy), suggesting that a threshold root-zone temperature is involved in triggering onset of above ground stem growth. The rate of xylem cell production determined in two weekly intervals during June through August 2006–2007 was significantly correlated with air temperature (temperature sums expressed as degree-days and mean daily maximum temperature) at the timberline only. Lack of significant relationships between tracheid production and temperature variables at the treeline and within the krummholz belt support past dendroclimatological studies that more extreme environmental conditions (e.g., wind exposure, frost desiccation, late frost) increasingly control tree growth above timberline. Results of this study revealed that spatial and temporal (i.e., year-to-year) variability in timing and dynamic of wood formation of P. cembra is strongly influenced by local site factors within the treeline ecotone and the dynamics of seasonal temperature variation, respectively.     

Cambial sensitivity to rising temperatures by natural condition and artificial heating from late winter to early spring in the evergreen conifer Cryptomeria japonica

Differences in the timing of cambial reactivation and the initiation of xylem differentiation in response to the sum of daily maximum temperatures were studied in two Cryptomeria japonica trees with cambium of different ages under natural and locally heated conditions. In addition, we observed the effects of low temperature on cambial activity. The timing of cambial reactivation and of the initiation of xylem differentiation differed between 55- and 80-year-old cambium under natural conditions. In the 55-year-old cambium, cambial reactivation occurred when the cambial reactivation index (CRI), calculated on the basis of daily maximum temperatures in excess of 10°C, was 94 and 97°C in 2007 and 2008, respectively. In 80-year-old cambium, cambial reactivation occurred when the CRI, calculated on the basis of daily maximum temperatures in excess of 11°C, was 69 and 71°C in 2007 and 2008, respectively. After cambial reactivation in 2007, normal cell division was evident in the cambium even though the minimum temperature had fallen between −2 and −3°C. Under natural conditions, xylem differentiation started 38–44 days after cambial reactivation. In heated stems, the time between cambial reactivation and the initiation of xylem differentiation ranged from 14 to 16 days, a much shorter time than under natural conditions, indicating that continuous exposure to an elevated temperature had induced earlier xylem differentiation. Our observations indicate that the sensitivity to reactivation inducing stimuli of the cambium depends on both the stage of dormancy and tree age of the cambium.     

Intra-annual tracheid formation of Norway spruce provenances in southern Finland

We studied the intra-annual wood formation in a Norway spruce provenance experiment in southern Finland from 2004–2008. Two Finnish provenances, northern and southern, as well as German and Hungarian provenances were included. Timing of tracheid formation and differentiation, and tracheid dimensions were determined from periodically extracted microcores. The aim was to determine the differences between the years and provenances in the timing of the xylogenesis and in the xylem characteristics. Year-to-year variation was high both in timing of tracheid formation and xylem characteristics, while between-provenance differences were small. The onset of tracheid formation varied from early May to late June in different trees in different years. The onset of tracheid formation was not closely related to the annual variations of temperature sum. In all the years, daily temperatures exceeded the threshold +5°C for several weeks before the onset of tracheid formation. The highest tracheid formation rate occurred after the summer solstice in all years and generally coincided with the highest daily temperatures during the growing season. Tracheid production ceased early in 2006 due to a mid-summer drought. Cell differentiation continued late in autumn as non-mature tracheids were still observed around mid-September. No clear differences between the provenances in the timing of tracheid formation were observed, although the Finnish provenances tended to initiate tracheid formation slightly earlier than the other provenances. The tree-ring widths of the Finnish provenances were also wider, while tracheid diameter of the German provenance was slightly smaller. Our results indicate that between-tree variation in the timing of wood formation is high compared with the latitude effect of seed source.     

A re-assessment of high elevation treeline positions and their explanation

In this review I first compile data for the worldwide position of climate-driven alpine treelines. Causes for treeline formation are then discussed with a global perspective. Available evidence suggests a combination of a general thermal boundary for tree growth, with regionally variable “modulatory” forces, including the presence of certain taxa. Much of the explanatory evidence found in the literature relates to these modulatory aspects at regional scales, whereas no good explanations emerged for the more fundamental global pattern related to temperature per se, on which this review is focused. I hypothesize that the life form “tree” is limited at treeline altitudes by the potential investment, rather than production, of assimilates (growth as such, rather than photosynthesis or the carbon balance, being limited). In shoots coupled to a cold atmosphere, meristem activity is suggested to be limited for much of the time, especially at night. By reducing soil heat flux during the growing season the forest canopy negatively affects root zone temperature. The lower threshold temperature for tissue growth and development appears to be higher than 3°C and lower than 10°C, possibly in the 5.5–7.5°C range, most commonly associated with seasonal means of air temperature at treeline positions. The physiological and developmental mechanisms responsible have yet to be analyzed. Root zone temperature, though largely unknown, is likely to be most critical. Received: 3 October 1997 / Accepted: 14 April 1998     

Notes on the Effect of Cutting Bracken (Pteris aquilina L.)

Background: Undisturbed high elevation treelines follow a common growing season isotherm, irrespective of latitude. Small stature plants thrive at much higher elevations because they grow in a favourable microclimate near the ground, whereas trees are aerodynamically coupled to free atmospheric circulation, hence the uniform treeline elevation in a given region.

Aims: I argue that the treeline results from tree architecture and not from a tree-specific inferior physiology. At tissue level, all cold adapted higher plants (including winter crops) face similar temperature related limitations and are constrained by similar thermal thresholds.

Methods: In order to explore this hypothesis, winter rape and winter oat were grown in the field and the daily rate of leaf expansion was measured during early winter, with temperatures similar to those at treeline at the beginning of the growing season (monthly mean temperature of 5 °C).

Results: Leaves of these winter crops only grew when the degree hours above 5 °C for a given day exceeded zero. Rape and oats showed very similar responses. The data support a common temperature threshold for tissue formation in winter crops and treeline trees.

Conclusions: A literature survey revealed lower temperature thresholds (close to freezing point) in arctic and Antarctic algae. It appears that the formation of complex, partly lignified tissue does not happen below 5 °C, whereas such temperatures exert little constraints on photosynthesis. The study illustrates that tissue growth in winter crops can help understand growth of treeline trees, given that both exhibit similar minimum temperature requirements for meristem functioning.     

Early season temperature controls cambial activity and total tree ring width at the alpine treeline

Background: Temperature directly affects xylogenesis at high-elevation treelines. The low-temperature limitation of meristematic processes is thus key to understand treeline formation.

Aims: We aimed to experimentally test in situ the direct low-temperature effect on wood tissue formation at the alpine treeline.

Methods: We applied controlled Peltier-mediated cooling and warming (±3 K) to branch segments in Pinus uncinata at the treeline in the Swiss Alps. In addition, we studied xylogenesis in untreated trees during the growing season by sequential micro-coring.

Results: Micro-cores indicated that the cambial zone was fully developed by the time the cooling and warming treatment started, shortly after snowmelt. Presumably, because of this, experimental cooling of branches did not significantly reduce the number of cells produced per season. Warming extended the formation of early wood into the late season, and thus reduced the fraction of late wood.

Conclusions: We conclude that temperatures very early in the season determine the width of the cambial zone which, in turn, strongly controls the number of tracheids produced during the remaining growing season. Temperatures later in the season mainly determine the early wood to late wood ratio. These data provide an empirical basis for the mechanistic understanding of tree growth at the treeline in response to temperature.     

Lengthening of the duration of xylogenesis engenders disproportionate increases in xylem production

In cold climates, the expected global warming will lead to earlier cambial resumptions in spring, with a resultant lengthening of the growing season but unknown consequences on forest productivity. The phenological traits of cambium activity and xylem formation were analyzed at a short time scale along a thermal gradient represented by an alti‐latitudinal range from the 48th to 53rd parallels and covering the whole closed black‐spruce [Picea mariana (Mill.) BSP] forest in Quebec, Canada. A hypothesis was tested that warmer temperatures influence cambium phenology, allowing longer duration and higher intensity of growth, and resulting in proportionally increased xylem production. From April to October 2012, cell division in cambium and post‐cambial differentiation of xylem were observed on anatomical sections obtained from microcores collected weekly from the stem of fifty trees. The southern and warmer site was characterized by the highest radial growth, which corresponded to both the highest rates and longest durations of cell production. The differences in terms of xylem phenology and growth were marginal between the other sites. Xylem growth was positively correlated with rate and duration of cell production, with the latter explaining most variability in growth. Within the range analyzed, the relationship between temperature and most phenological phases of xylogenesis was linear. On the contrary, temperature was related with cell production according to an exponential pattern. Periods of xylogenesis of 14 days longer (+13.1%) corresponded to a massive increase in cell production (33 cells, +109%). This disproportionate change occurred at a May–September average temperature of ca. 14 °C and a snow‐free period of 210–235 days. At the lower boundary of the distribution of black spruce, small environmental changes allowing marginal lengthening of the period of cell division could potentially lead to disproportionate increases in xylem cell production, with substantial consequences for the productivity of this boreal species.     

Pattern of xylem phenology in conifers of cold ecosystems at the Northern Hemisphere

The interaction between xylem phenology and climate assesses forest growth and productivity and carbon storage across biomes under changing environmental conditions. We tested the hypothesis that patterns of wood formation are maintained unaltered despite the temperature changes across cold ecosystems. Wood microcores were collected weekly or biweekly throughout the growing season for periods varying between 1 and 13 years during 1998–2014 and cut in transverse sections for assessing the onset and ending of the phases of xylem differentiation. The data set represented 1321 trees belonging to 10 conifer species from 39 sites in the Northern Hemisphere and covering an interval of mean annual temperature exceeding 14 K. The phenological events and mean annual temperature of the sites were related linearly, with spring and autumnal events being separated by constant intervals across the range of temperature analysed. At increasing temperature, first enlarging, wall‐thickening and mature tracheids appeared earlier, and last enlarging and wall‐thickening tracheids occurred later. Overall, the period of wood formation lengthened linearly with the mean annual temperature, from 83.7 days at ?2 °C to 178.1 days at 12 °C, at a rate of 6.5 days °C^?1. April–May temperatures produced the best models predicting the dates of wood formation. Our findings demonstrated the uniformity of the process of wood formation and the importance of the environmental conditions occurring at the time of growth resumption. Under warming scenarios, the period of wood formation might lengthen synchronously in the cold biomes of the Northern Hemisphere.     

Is precipitation a trigger for the onset of xylogenesis in Juniperus przewalskii on the north-eastern Tibetan Plateau?

Background and Aims A series of studies have shown that temperature triggers the onset of xylogenesis of trees after winter dormancy. However, little is known about whether and how moisture availability influences xylogenesis in spring in drought-prone areas.Methods Xylogenesis was monitored in five mature Qilian junipers (Juniperus przewalskii) by microcore sampling from 2009 to 2011 in a semi-arid area of the north-eastern Tibetan Plateau. A simple physical model of xylem cell production was developed and its sensitivity was analysed. The relationship between climate and growth was then evaluated, using weekly wood production data and climatic data from the study site.Key Results Delayed onset of xylogenesis in 2010 corresponded to a negative standardized precipitation evapotranspiration index (SPEI) value and a continuous period without rainfall in early May. The main period of wood formation was in June and July, and drier conditions from May to July led to a smaller number of xylem cells. Dry conditions in July could cause early cessation of xylem differentiation. The final number of xylem cells was mainly determined by the average production rate rather than the duration of new cell production. Xylem growth showed a positive and significant response to precipitation, but not to temperature.Conclusions Precipitation in late spring and summer can play a critical role in the onset of xylogenesis and xylem cell production. The delay in the initiation of xylogenesis under extremely dry conditions seems to be a stress-avoidance strategy against hydraulic failure. These findings could thus demonstrate an evolutionary adaptation of Qilian juniper to the extremely dry conditions of the north-eastern Tibetan Plateau.     

Adaptive latitudinal variation of the duration and thermal requirements for development in the ground beetle Amara communis (Panz.) (Coleoptera, Carabidae)

The data are obtained on development time at six constant temperatures (12, 14, 16, 18, 20, 22°C) and thermal requirements for preimaginal development in a ground beetle Amara communis from Arkhangelsk (64°34′N) and St. Petersburg (59°53′N). The larval and pupal development times were found to be significantly shorter in the Arkhangelsk than in the St. Petersburg population under all temperatures. As a result, total preimaginal development appeared to be shorter by 6.2–6.6% in the Arkhangelsk population. The regression lines of the larval, pupal and total (egg-to-adult) development rate on temperature for the Arkhangelsk population run above and steeper than the respective lines for the St. Petersburg population. Both populations share the similar values of the thermal thresholds (7.2–8.2°C). This explains faster preimaginal development in the northern population under all temperatures above the threshold. Thus, the slope of the regression lines increases, i.e., the sum of degree-days decreases, whereas the thermal threshold for development exhibited no distinctive changes from south to north in this species. Adults from Arkhangelsk reared in the experiments appeared heavier on the average in comparison with those from St. Petersburg, especially at 18–22°C. Temperature did not significantly affect adult weight, except the fact that the beetles were slightly heavier at 20 and 22°C. Consequently, the well-known “temperature-size rule” is violated in this species. Relative growth rate in larvae of A. communis increased considerably with temperature rise from 14 to 22°C. It was significantly higher in the beetles from Arkhangelsk at 18–22°C. There were no differences in larval growth rate between the two populations at 14 and 16°C.     

Influences of Culture Temperature on the Growth, Lipid Content and Fatty Acid Composition of Aurantiochytrium sp. Strain mh0186

The growth, lipid content, and fatty acid composition of Aurantiochytrium sp. strain mh0186 at different temperatures were investigated. Strain mh0186 grew well at 15–30°C, but weakly at 10°C. The biomass at 15–30°C was significantly higher than at 10 and 35°C, and the total lipid at 15–35°C was significantly higher than that at 10°C. The amount of DHA in the total fatty acid was highest at 10°C and decreased in response to temperature increase. The content of DHA (mg/g-dry cell weight) at 15–30°C were significantly higher than those at 35°C and those at 15–25°C were significantly higher than those at 10 and 35°C. The DHA yield at 15–35°C was significantly higher than those at 10 and 35°C. Unsaturation of fatty acid was regulated by temperature and was enhanced in response to temperature decrease. The ratio of DHA to DPA varied at different temperatures.     

Recent climate warming forces contrasting growth responses of white spruce at treeline in Alaska through temperature thresholds

Northern and high‐latitude alpine treelines are generally thought to be limited by available warmth. Most studies of tree‐growth–climate interaction at treeline as well as climate reconstructions using dendrochronology report positive growth response of treeline trees to warmer temperatures. However, population‐wide responses of treeline trees to climate remain largely unexamined. We systematically sampled 1558 white spruce at 13 treeline sites in the Brooks Range and Alaska Range. Our findings of both positive and negative growth responses to climate warming at treeline challenge the widespread assumption that arctic treeline trees grow better with warming climate. High mean temperatures in July decreased the growth of 40% of white spruce at treeline areas in Alaska, whereas warm springs enhance growth of additional 36% of trees and 24% show no significant correlation with climate. Even though these opposing growth responses are present in all sampled sites, their relative proportion varies between sites and there is no overall clear relationship between growth response and landscape position within a site. Growth increases and decreases appear in our sample above specific temperature index values (temperature thresholds), which occurred more frequently in the late 20th century. Contrary to previous findings, temperature explained more variability in radial growth after 1950. Without accounting for these opposite responses and temperature thresholds, climate reconstructions based on ring width will miscalibrate past climate, and biogeochemical and dynamic vegetation models will overestimate carbon uptake and treeline advance under future warming scenarios.     

Morphological,cultural, and biodestructive peculiarities of <Emphasis Type="Italic">Chaetomium</Emphasis> species

Chaetomium is a fungus species that inhabits different cellulose substrates. Some species of this genus destroy books, textile, and wood. This genus is of particular interest due to its antagonistic and enzyme peculiarities. Studies on morphological and cultural features of Chaetomium species have both theoretical and practical output. Growth rates and development of C. globosum, C. funicola, C. elatum, and C. spirale were studied under different temperatures (17–20°C, 25°C, 27°C, 30°C, and 33–35°C) and carbon source media (glucose, saccharose, mannite, lactose, amylum, and cellulose). The optimal growth temperature was 25–27°C for all the studied species, while the temperature range of 33–35°C inhibited the colonies’ growth. Growth dynamics and colony shape and morphology, as well as development of overhead mycelium and ascocarps, varied greatly in different Chaetomium species in regard to the carbon source media. When comparing the destructive effect of four studied species and C. murorum on craft paper, the highest activity was registered for C. globosum, C. funicola, and C. elatum.     

Temperature profiles of cellular growth and exopolysaccharide synthesis by Botryococus braunii Kütz. UC 58

Temperature profiles (range 20–33 °C) were obtained for growth and exopolysaccharide (EPS) biosynthesis of the microalga Botryococcus braunii strain UC 58 under photoautotrophic conditions. The maximum temperature for growth was 32 °C and the temperature dependence of the specific growth rate was described by the Hinshelwood equation based on the Arrhenius relationship. The optimal range of temperatures for growth and extracellular EPS synthesis (25–30 °C) concurred and production of 4.5–5 g l⁻¹ of EPS was obtained routinely, leading to high broth viscosities. Below 23 °C EPS biosynthesis was negligible, although the specific growth rate maintained high values. At supraoptimal temperatures EPS biosynthesis decreased, accompanying the increase in doubling time. The polymers formed at temperatures within the optimal range for production, when dissolved in water, produced solutions (2 gl⁻¹) with the highest viscosity, suggesting that their molecular weight showed the highest values. The degree of polymerization of the EPS synthesized at suboptimal and supraoptimal temperatures was significantly below the values within the optimal range.     

Temperature requirements ofScytosiphon lomentaria (Scytosiphonales,Phaeophyta) from the Gulf of Thessaloniki,Greece, in relation to geographic distribution

Temperature requirements for growth, reproduction and formation of macrothalli of a day-neutral strain ofScytosiphon lomentaria from the Gulf of Thessaloniki were experimentally determined and correlated with the geographic distribution in the North Atlantic Ocean. The microthallus grew in a wider temperature interval and better at higher temperatures than did the macrothallus. Germlings acclimated to 5 or 15°C grew sufficiently (>20% of maximum rate) and developed into macrothalli at 5–25°C and 5–27°C. Macrothalli acclimated to 10 or 15°C grew sufficiently at 5–20°C. Macrothalli acclimated to 15°C survived at −1°C and reproduced at 5 to 23°C. Regardless of the acclimation temperature, germlings and macrothalli grew optimally (>80% of maximum rate) at 15–25°C and at 10–15°C. The experimental data explain only the southern distribution boundary ofScytosiphon in the North Atlantic. This boundary is composite in nature: on the European coasts it is a growth boundary, whereas on the American coasts it is a lethal one.     

The effects of artificial soil frost on cambial activity and xylem formation in Norway spruce

We studied the effects of artificial soil frost on cambial activity and xylem formation on 47-year-old Norway spruce [Picea abies (L.) Karst.] trees grown on medium fertile site type (with moraine soil) in eastern Finland (62°42′N; 29°45′E). Different soil frost treatments applied were: (1) natural snow accumulation and melting (control, CTRL); (2) artificial removal of snow from soil surface during two consecutive winters (OPEN); and (3) snow clearing and insulation (FROST), which was in other ways similar to OPEN, but the ground was insulated in early spring to delay soil thawing. Each treatment was replicated in three blocks, and two sample trees in each plot were repeatedly microcored during growing seasons of 2006–2007 for the analysis of the onset, cessation and the duration of xylem formation. The phases of tracheid differentiation (tracheids in radial enlargement, secondary cell wall formation, and mature tracheids) were measured from the microcores of 2007. The intra-ring growth and wood density variables were analysed based on X-ray densitometry. In FROST in 2006, xylem formation started a week later than in the other treatments. In 2007, no difference was found between the treatments. The discrepancy in results between the two study years may be explained by between-years variation in weather, i.e., the winter was colder in 2005/2006 than in 2006/2007. No effects of soil frost treatments on tracheid differentiation and on most of the intra-ring growth and density variables were discovered. Our results suggest that the delayed thawing of moraine soil may slightly affect the onset, timing and duration of xylem formation in Norway spruce. However, the effects of delayed soil frost may depend also on the soil type and become more evident with increasing water holding capacity of the soil.     

Effects of elevated temperature on bacterial community structure and function in bioreactors treating a synthetic wastewater

The impact of elevated temperature on bacterial community structure and function during aerobic biological wastewater treatment was investigated. Continuous cultures, fed a complex growth medium containing gelatin and α-lactose as the principal carbon and energy sources, supported mixed bacterial consortia at temperatures ranging from 25–65°C. These temperature- and substrate-acclimated organisms were then used as inocula for batch growth experiments in which the kinetics of microbial growth and substrate utilization, efficiency of substrate removal, and mechanism of substrate removal were compared as functions of temperature. Bacterial community analysis by denaturing gradient gel electrophoresis (DGGE) revealed that distinct bacterial consortia were supported at each temperature. The efficiency of substrate removal declined at elevated temperatures. Maximum specific growth rates and the growth yield increased with temperature from 25–45°C, but then decreased with further elevations in temperature. Thus, maximum specific substrate utilization rates did not vary significantly over the 40°C temperature range (0.64 ± 0.04 mg COD mg⁻¹ dry cell mass h⁻¹). A comparison of the degradation of the protein and carbohydrate portions of the feed medium revealed a lag in α-lactose uptake at 55°C, whereas both components were utilized simultaneously at 25°C. Journal of Industrial Microbiology & Biotechnology (2000) 24, 140–145. Received 09 August 1999/ Accepted in revised form 12 November 1999