High autumn temperature delays spring bud burst in boreal trees, counterbalancing the effect of climatic warming

The effect of temperature during short-day (SD) dormancy induction was examined in three boreal tree species in a controlled environment. Saplings of Betula pendula Roth, B. pubescens Ehrh. and Alnus glutinosa (L.) Moench. were exposed to 5 weeks of 10-h SD induction at 9, 15 and 21 degrees C followed by chilling at 5 degrees C for 40, 70, 100 and 130 days and subsequent forcing at 15 degrees C in a 24-h photoperiod for 60 days. In all species and with all chilling periods, high temperature during SD dormancy induction significantly delayed bud burst during subsequent flushing at 15 degrees C. In A. glutinosa, high temperature during SD dormancy induction also significantly increased the chilling requirement for dormancy release. Field experiments at 60 degrees N with a range of latitudinal birch populations revealed a highly significant correlation between autumn temperature and days to bud burst in the subsequent spring. September temperature alone explained 20% of the variation between years in time of bud burst. In birch populations from 69 and 71 degrees N, which ceased growing and shed their leaves in August when the mean temperature was 15 degrees C, bud burst occurred later than expected compared with lower latitude populations (56 degrees N) in which dormancy induction took place more than 2 months later at a mean temperature of about 6 degrees C. It is concluded that this autumn temperature response may be important for counterbalancing the potentially adverse effects of higher winter temperatures on dormancy stability of boreal trees during climate warming.     

Models of the spring phenology of boreal and temperate trees: Is there something missing?

According to prevailing theory, air temperature is the main environmental factor regulating the timing of bud burst of boreal and temperate trees. Air temperature has a dual role in this regulation. First, after the cessation of growth in autumn, prolonged exposure to chilling causes rest completion, i.e., removes the physiological growth-arresting conditions inside the bud. After rest completion, prolonged exposure to warm conditions causes ontogenetic development leading to bud burst or flowering. During the past three decades, several simulation models based on chilling and forcing have been developed and tested. In recent modeling studies of the timing of bud burst in mature trees, the simpler thermal-time models that assume forcing starts on a fixed date in the spring have outperformed the chilling-forcing models. We hypothesize that this discrepancy may be due to some element missing from the chilling-forcing models. We tested two new model formulations by introducing reversing, temperature-driven elements that precede forcing and by fitting the models to seven historical time series of data of flowering and leaf bud burst of common boreal tree species. In these tests, both of the new models were generally more accurate in predicting the timing of bud burst than a classical chilling-forcing model, but less accurate than the simple thermal-time model. We therefore conclude that besides chilling, other environmental factors are involved in the regulation of the timing of bud burst. Further work is needed to determine if the regulatory factors derive from air temperature or from some other environmental condition such as changes in light conditions, like day length or night length.     

Dormancy release and chilling requirement of buds of latitudinal ecotypes of Betula pendula and B. pubescens

Bud burst and dormancy release of latitudinal ecotypes of Betula pendula Roth and B. pubescens Ehrh. from Denmark ( approximately 56 degrees N), mid-Norway ( approximately 64 degrees N) and northern Norway ( approximately 69 degrees N) were studied in controlled environments. Dormant seedlings were chilled at 0, 5 or 10 degrees C from October 4 onward and then, at monthly intervals from mid-November to February, batches of seedlings were held at 15 degrees C in an 8-h (SD) or 24-h (LD) photoperiod to permit flushing. A decline in days to bud burst occurred with increasing chilling time in all ecotypes. In November, after 44 chilling days, time to bud burst was least in plants chilled at 0 and 5 degrees C. The difference diminished with increasing chilling time, and in February, after 136 chilling days, bud burst was earliest in plants chilled at 10 degrees C. Long photoperiods during flushing significantly reduced thermal time after short chilling periods (44 and 74 days), but had no effect when the chilling requirement was fully met after 105 or more chilling days. No significant difference in these responses was found between the two species. In both species, chilling requirement decreased significantly with increasing latitude of origin. Bud burst was normal in seedlings overwintered at 12 degrees C, but was erratic and delayed in seedlings overwintered at 15 and especially at 21 degrees C, indicating that the critical overwintering temperature is between 12 and 15 degrees C. We conclude that there is little risk of a chilling deficit in birch under Scandinavian winter conditions even with a climatic warming of 7-8 degrees C. The likely effects of a climatic warming include earlier bud burst, a longer growing season and increased risk of spring frost injury, especially in high latitude ecotypes.     

Climatic control of bud burst in young seedlings of nine provenances of Norway spruce

Detailed knowledge of temperature effects on the timing of dormancy development and bud burst will help evaluate the impacts of climate change on forest trees. We tested the effects of temperature applied during short-day treatment, duration of short-day treatment, duration of chilling and light regime applied during forcing on the timing of bud burst in 1- and 2-year-old seedlings of nine provenances of Norway spruce (Picea abies (L.) Karst.). High temperature during dormancy induction, little or no chilling and low temperature during forcing all delayed dormancy release but did not prevent bud burst or growth onset provided the seedlings were forced under long-day conditions. Without chilling, bud burst occurred in about 20% of seedlings kept in short days at 12 degrees C, indicating that young Norway spruce seedlings do not exhibit true bud dormancy. Chilling hastened bud burst and removed the long photoperiod requirement, but the effect of high temperature applied during dormancy induction was observed even after prolonged chilling. Extension of the short-day treatment from 4 to 8 or 12 weeks hastened bud burst. The effect of treatments applied during dormancy development was larger than that of provenance; in some cases no provenance effect was detected, but in 1-year-old seedlings, time to bud burst decreased linearly with increasing latitude of origin. Differences among provenances were complicated by different responses of some origins to light conditions under long-day forcing. In conclusion, timing of bud burst in Norway spruce seedlings is significantly affected by temperature during bud set, and these effects are modified by chilling and environmental conditions during forcing.     

Temperature regulation of bud-burst phenology within and among years in a young Douglas-fir (Pseudotsuga menziesii) plantation in western Washington, USA

Past research has established that terminal buds of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings from many seed sources have a chilling requirement of about 1200 h at 0-5 degrees C; once chilled, temperatures > 5 degrees C force bud burst via accumulation of heat units. We tested this sequential bud-burst model in the field to determine whether terminal buds of trees in cooler microsites, which receive less heat forcing, develop more slowly than those in warmer microsites. For three years we monitored terminal bud development in young saplings as well as soil and air temperatures on large, replicated plots in a harvest unit; plots differed in microclimate based on amount of harvest residue and shade from neighboring stands. In two of three years, trees on cooler microsites broke bud 2 to 4 days earlier than those on warmer microsites, despite receiving less heat forcing from March to May each year. A simple sequential model did not predict cooler sites having earlier bud burst nor did it correctly predict the order of bud burst across the three years. We modified the basic heat-forcing model to initialize, or reset to zero, the accumulation of heat units whenever significant freezing temperature events (> or = 3 degree-hours day(-1) < 0 degrees C) occurred; this modified model correctly predicted the sequence of bud burst across years. Soil temperature alone or in combination with air temperature did not improve our predictions of bud burst. Past models of bud burst have relied heavily on data from controlled experiments with simple temperature patterns; analysis of more variable temperature patterns from our 3-year field trial, however, indicated that simple models of bud burst are inaccurate. More complex models that incorporate chilling hours, heat forcing, photoperiod and the occurrence of freeze events in the spring may be needed to predict effects of future silvicultural treatments as well to interpret the implications of climate-change scenarios. Developing and testing new models will require data from both field and controlled-environment experiments.     

Effects of photoperiod and temperature on the timing of bud burst in Norway spruce (Picea abies)

We examined the effects of several photoperiod and temperature regimes imposed during the winter-spring period on the timing of bud burst in rooted cuttings of Norway spruce (Picea abies (L.) Karst.) grown in a greenhouse in Finland. The treatments were initiated in November and December after the cuttings had been exposed to natural chilling and freezing events. Irrespective of the treatments applied, time to bud burst decreased with increased duration of previous exposure to natural chilling and freezing events. Fluctuating day/night temperatures and continuous lengthening of the photoperiod hastened bud burst. Shortening the photoperiod delayed bud burst, suggesting that little or no ontogenetic development toward bud burst takes place during mild periods before the winter solstice. In the case of climatic warming, this phenomenon may prevent the premature onset of growth that has been predicted by computer simulations with models that only consider temperature regulation of bud burst.     

Development and growth of primordial shoots in Norway spruce buds before visible bud burst in relation to time and temperature in the field

The timing of bud development in ecodormancy is critical for trees in boreal and temperate regions with seasonally alternating climates. The development of vegetative buds and the growth of primordial shoots (the primordial shoot ratio) in Norway spruce were followed by the naked eye and at stereo and light microscopic levels in fresh-cut and fixed buds obtained by regular field samplings during the spring of 2007, 2008 and 2009. Buds were collected from 15 randomly selected trees (all 16 years old in 2007) of one southern Finnish half-sib family. The air temperature was recorded hourly throughout the observation period. In 2008 and 2009, initial events in the buds, seen as accumulation of lipid droplets in the cortex area, started in mid-March and were depleted in late April, simultaneously with the early development of vascular tissue and primordial needles. In mid-April 2007, however, the development of the buds was at least 10 days ahead as a result of warm spells in March and early April. Variation in the timing of different developmental phases within and among the sample trees was negligible. There was no clear one-to-one correspondence between the externally visible and the internal development of the buds. The dependence of the primordial shoot ratio on different types of temperature sum was studied by means of regression analysis. High coefficients of determination (R(2)?≈?95%) were attained with several combinations of the starting time (beginning of the year/vernal equinox), the threshold value (from -3 to +5 °C), and the time step (hour/day) used in the temperature summation, i.e., the prediction power of the primordial shoot ratio models turned out to be high, but the parameter estimate values were not unambiguous. According to our results, temperature sums describe the growth of the primordial shoot inside the bud before bud burst. Thus, the results provide a realistic interpretation for the present phenological models of bud development that are based on temperature sums and external observations of bud burst only, and they also provide new tools for improving the models.     

Predicting spring phenology and frost damage risk of Betula spp. under climatic warming: a comparison of two models

Timing of bud burst and frost damage risk for leaves of Betula spp. in response to climatic warming in Finland was examined with two models. In the first model, ontogenetic development in spring was triggered by an accumulation of chilling temperatures. The second model assumed an additional signal from the light climate. The two models gave radically different estimates of frost damage risk in response to climate warming. The chilling-triggered model forecast a significant and increasing risk with increased warming, whereas the light-climate-triggered model predicted little or no risk. The chilling-triggered model is widely applied in phenological research; however, there is increasing experimental evidence that light conditions play a role in the timing of spring phenology. Although it is not clear if the light response mechanisms are appropriately represented in our model, the results imply that reliance on a light signal for spring development would afford a degree of protection against possible frost damage under climate warming that would not be present if chilling were the sole determinant. Further experimental tests are required to ascertain the light-related mechanisms controlling phenological timing, so that credible model extrapolations can be undertaken.     

Temperature sum accumulation effects on within-population variation and long-term trends in date of bud burst of European white birch (Betula pendula)

Within-population variation in phenology of boreal trees indicates their adaptability to climatic variations. Although interannual variations in date of bud burst have been widely discussed, little is known about within-population variation, the key determinants for this variation and the effects of this variation on estimates of trends in bud burst date. Over a period of nine years, we monitored timing of bud burst daily in 30 mature white birch (Betula pendula Roth) trees in a naturally regenerated stand. Our results revealed not only large interannual variation but also considerable intraannual variation among individual trees in date of bud burst, the maximum within-population variation being four weeks. Bud burst can be accurately predicted by the date when a threshold value of temperature sum in spring is reached (base temperature +5 degrees C). Based on this temperature sum and past temperature records, we estimated the trend in date of bud burst. The linear trend estimate based on the years 1926-2005 is an advancement of 1.2 days per decade (95% confidence interval, +/- 0.7 days), which is much less than that predicted by time series based on coarser time intervals. We conclude that, because of large interannual differences, and large annual within-population variations in bud burst, estimates of bud burst date based on measurements made over a period of only a few decades are unreliable.     

Slash pine bud dormancy as affected by lifting date and root wrenching in the nursery

Bud dormancy of root wrenched and unwrenched slash pine (Pinus elliottii Engelm.) seedlings growing in a forest nursery was measured on five lifting dates. Determination of bud dormancy was based on days to budbreak (DBB) under optimal growing conditions, mitotic activity in the apical meristem, chilling hours accumulated, and bud morphology. Based on DBB, seedlings were most dormant at Lift 2 on November 24 after exposure to 189 hours below 10 degrees C and 93 hours below 6.7 degrees C. Mitotic activity in the apical meristem was at its lowest 23 days later at Lift 3, possibly indicating the period when seedlings are most resistant to transplanting stresses. Multiple wrenching resulted in a slight shift in the dormancy cycle as wrenched seedlings set bud sooner in the nursery and broke bud sooner at the planting site in the spring than control seedlings. This implies that wrenched seedlings can be successfully lifted from the nursery earlier and will initiate spring shoot growth earlier than control seedlings.     

Effects of dormancy and environmental factors on timing of bud burst in Betula pendula

We tested three theories predicting the timing of bud burst in mature birch (Betula pendula Roth) trees utilizing a 60-year phenological time series together with meteorological temperature observations. Predictions of the timing of bud burst based on light conditions in addition to temperature were more accurate than predictions based on dormancy development and temperature (prediction standard error of 2.4 days versus 4.3 days). The signal from light conditions, represented by fixed calendar date, determined the start of bud ontogenesis rather than dormancy release. We suggest that models developed to predict the timing of bud burst be utilized in the analysis of plant responses to climate change and of climate change itself.     

树莓温室栽培技术

对树莓温室生产的几个重要因子,如品种、温湿度、肥水管理、疏枝和修剪、病虫害防治等进行了探讨。经过树莓栽培品种的比较筛选后．2000年9月3日将正常产果的红莓2年生优壮植株移入温室,研究其温室栽培特性。经过一定寒冷期后,于当年12月20日开始扣棚、扣棚105d开花,127d果实开始成熟．比露地栽堵结果期提前40d。扣棚前和采收后进行合理修剪,实施有效的花果管理,严格调控温湿度．及时防治病虫害,生产的果实品质优良。2001年春季平均产果达300kg／666．7m^2,第2年即2002年春季达500kg／666．7m^2。     

Influence of light quality on the germination of Betula papyrifera seeds

The influence of light quality on germination of unstratified birch seeds (Betula papyrifera) was investigated in a greenhouse study. Different light regimes were created with five plastic filters of different colour in combination with basic illumination. The filters were adjusted to equal levels of photosynthetically active radiation. The difference in light quality in terms of R : Fr ratio (red : far‐red light ratio) varied from 0.23 to 1.03. Germination was recorded 7, 10, 14, 21, 25, 31 and 36 days after sowing. Response to light quality was significant from 25 days after sowing (p < 0.043), and it was found that differences increased with time. Filters producing light regimes with low R : Fr ratios were found to reduce and delay germination of birch seeds. Germination after 36 days varied from 55% for the control treatment to 11% for the lowest R : Fr ratio. The relationship between seed germination percentage and R : FR ratio was found to be significant (p < 0.017) with an R‐sq value of 88.6%.     

Nitrogen sources for current-year shoot growth in 50-year-old sessile oak trees: an in situ (15)N labeling approach

We used long-term in situ (15)N labeling of the soil to investigate the contribution of the two main nitrogen (N) sources (N uptake versus N reserves) to sun shoot growth from bud burst to full leaf expansion in 50-year-old sessile oaks. Recovery of (15)N by growing compartments (leaves, twigs and buds) and presence of (15)N in phloem sap were checked weekly. During the first 2 weeks following bud burst, remobilized N contributed ~90% of total N in growing leaves and twigs. Nitrogen uptake from the soil started concomitantly with N remobilization but contributed only slightly to bud burst. However, the fraction of total N due to N uptake increased markedly once bud burst had occurred, reaching 27% in fully expanded leaves and 18% in developed twigs. In phloem sap, the (15)N label appeared a few days after the beginning of labeling and increased until the end of bud burst, and then decreased at full leaf expansion in June. Of all the shoot compartments, leaves attracted most of the absorbed N, which accounted for 68% of new N in shoots, whereas twigs and new buds accounted for only 28 and 3%, respectively. New N allocated to leaves increased from unfolding to full expansion as total N concentration in the leaves decreased. Our results underline the crucial role played by stored N in rapid leaf growth and in the sustained growth of oak trees. Any factors that reduce N storage in autumn may therefore impair spring shoot growth.     

Effects of light quality on gas exchange and dry matter partitioning in Eucalyptus grandis W. Hill ex Maiden

Stockplants of Eucalyptus grandis were pruned to a height of 7–10 cm and after 3 weeks were placed in growth cabinets set at a photon flux density (PFD) of 200 μmol m⁻² s⁻¹ and red to far-red ratios of 0.4, 0.7, 1.3, 3.5 or 6.5. Experiments tested the effects of light quality on growth and gas exchange of stockplants. Light quality did not affect the total shoot dry weight (DW), root DW or shoot to root ratio of stockplants or their total leaf area. However, there were significant effects of light quality on: (i) plant height, which was greatest at red:far-red (R:FR) ratios of 0.4 and 0.7; (ii) partitioning of DW between leaves and stems, with greater stem DW and less leaf DW at low R:FR ratios (0.4 and 0.7); (iii) partitioning of DW and leaf area between the most dominant shoot and all other (non-dominant) shoots; (iv) specific leaf area, which was greatest at low R:FR ratios. In the above characters, the dominance ratio (ratio of most dominant shoot to sum of all other shoots) was greatest at low R:FR ratios and least at ratios of 3.5 and 6.5. Photosynthetic rate per unit leaf area and leaf chlorophyll concentration significantly increased with increasing R:FR ratio. However, photosynthesis per unit chlorophyll concentration was significantly greater at low R:FR ratios. Generally, light quality had no significant effect on photosynthetic rate per leaf or per unit dry weight, but rates of transpiration, stomatal conductance and water use efficiency increased with an increase in R:FR ratio. These data indicate that compensatory changes in plant morphology and gas exchange caused equality in total dry weight per plant between treatments. The above effects of light quality on dry matter partitioning and gas exchange had important effects on the size, number, morphology and physiology of subsequently collected cuttings for vegetative propagation.     

Effects of elevated CO(2) and temperature on cold hardiness and spring bud burst and growth in Douglas-fir (Pseudotsuga menziesii)

We examined effects of elevated CO(2) and temperature on cold hardiness and bud burst of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings. Two-year-old seedlings were grown for 2.5 years in semi-closed, sunlit chambers at either ambient or elevated (ambient + ~ 4 degrees C) air temperature in the presence of an ambient or elevated (ambient + ~ 200 ppm) CO(2) concentration. The elevated temperature treatment delayed needle cold hardening in the autumn and slowed dehardening in the spring. At maximum hardiness, trees in the elevated temperature treatment were less hardy by about 7 degrees C than trees in the ambient temperature treatment. In general, trees exposed to elevated CO(2) were slightly less hardy during hardening and dehardening than trees exposed to ambient CO(2). For trees in the elevated temperature treatments, date to 30% burst of branch terminal buds was advanced by about 6 and 15 days in the presence of elevated CO(2) and ambient CO(2), respectively. After bud burst started, however, the rate of increase in % bud burst was slower in the elevated temperature treatments than in the ambient temperature treatments. Time of bud burst was more synchronous and bud burst was completed within a shorter period in trees at ambient temperature (with and without elevated CO(2)) than in trees at elevated temperature. Exposure to elevated temperature reduced final % bud burst of both leader and branch terminal buds and reduced growth of the leader shoot. We conclude that climatic warming will influence the physiological processes of dormancy and cold hardiness development in Douglas-fir growing in the relatively mild temperate region of western Oregon, reducing bud burst and shoot growth.     

Dormancy release of Norway spruce under climatic warming: testing ecophysiological models of bud burst with a whole-tree chamber experiment

Ecophysiological models predicting timing of bud burst were tested with data gathered from 40-year-old Norway spruce (Picea abies (L.) Karst.) trees growing in northern Sweden in whole-tree chambers under climatic conditions predicted to prevail in 2100. Norway spruce trees, with heights between 5 and 7 m, were enclosed in individual chambers that provided a factorial combination of ambient (365 micromol mol-1) or elevated (700 micromol mol-1) atmospheric CO2 concentration, [CO2], and ambient or elevated air temperature. Temperature elevation above ambient ranged from +2.8 degrees C in summer to +5.6 degrees C in winter. Compared with control trees, elevated air temperature hastened bud burst by 2 to 3 weeks, whereas elevated [CO2] had no effect on the timing of bud burst. A simple model based on the assumption that bud rest completion takes place on a fixed calendar day predicted timing of bud burst more accurately than two more complicated models in which bud rest completion is caused by accumulated chilling. Together with some recent studies, the results suggest that, in adult trees, some additional environmental cues besides chilling are required for bud rest completion. Although it appears that these additional factors will protect trees under predicted climatic warming conditions, increased risk of frost damage associated with earlier bud burst cannot be ruled out. Inconsistent and partially anomalous results obtained in the model fitting show that, in addition to phenological data gathered under field conditions, more specific data from growth chamber and greenhouse experiments are needed for further development and testing of the models.     

Translocation of nitrogen in the xylem of field-grown cherry and poplar trees during remobilization

Studies of small trees growing in pots have established that individual amino acids or amides are translocated in the xylem sap of a range of tree species following bud burst, as a consequence of nitrogen (N) remobilization from storage. This paper reports the first study of N translocation in the xylem of large, deciduous, field-grown trees during N remobilization in the spring. We applied 15N fertilizer to the soil around 10-year-old Prunus avium L. and Populus trichocharpa Torr. & Gray ex Hook var. Hastata (Dode) A. Henry x Populus balsamifera L. var. Michauxii (Dode) Farwell trees before bud burst to label N taken up by the roots. Recovery of unlabeled N in xylem sap and leaves was used to demonstrate that P. avium remobilizes N in both glutamine (Gln) and asparagine (Asn). Sap concentrations of both amides rose sharply after bud burst, peaking 14 days after bud burst for Gln, and remaining high some 45 days for Asn. There was no 15N enrichment of either amide until 21 days after bud burst. In the Populus trees, nearly all the N was translocated in the sap as Gln, the concentration of which peaked and then declined before the amide was enriched with 15N, 40 days after bud burst. Xylem sap of clonal P. avium trees was sampled at different positions in the crown to assess if the amino acid and amide composition of the sap varied within the crown. Sap was sampled during remobilization (when the concentration of Gln was maximal), at the end of remobilization and at the end of the experiment (68 days after bud burst). Although the date of sampling had a highly significant effect on sap composition, the effect of position of sampling was marginal. The results are discussed in relation to N translocation in adult trees and the possibility of measuring N remobilization by calculating the flux of N translocation in the xylem.