Frost Hardiness of Nothofagus procera and Nothofagus obliqua in Britain

The frost hardiness of four seedlots of Nothofagus proccra andsix seedlots of Nothofagus obliqua was measured experimentallyduring three winters. Shoots were taken from saplings growingat the Bush Estate in Scotland. All seedlots set buds in lateSeptember, hardened very slowly in the autumn, were damagedto some extent by temperatures below –14°C in mid-winter,and dehardened during frosty weather in February/March priorto budburst in mid to late April. The range of mid-winter temperaturesgiving 0%, 50% and 100% kill (LT₀, LT₅₀ and LT₁₀₀) were 8–14°C,13%20°C and 14–22°C, respectively. By contrast,British Fagus sylvatica hardened off rapidly in September, wasundamaged by frosts well below –20°C in mid-winter,and did not deharden until late April, prior to budburst atthe end of May. Nothofagus seedlots from Nuble in Chile (the most Equatorialsource) were the most frost susceptible: unfortunately, seedof this origin was supplied to many British nurseries between1976 and 1W9. Seedlots from Neuquen in Argentina, and from maturetrees of Malleco (Chile) origin growing in Britain, were themost host hardy. N. procera tended to be hardier than N. obliqua,and the trees became hardier with age. Past temperature records for Britain suggested that all theNothofagus seedlots had a high risk of suffering severe frostdamage at least once during a timkr rotation in all but mildcoastal regions. Spring and autumn frosts may be more damagingthan winter frosts. However, if it were possible to select individualswithin populations that were 3–6°C more frost hardythan the population means, such trees would be sufficientlyhardy to avoid frost damage in most lowland regions.     

Winter Frost Hardiness of two Chilean Provenances of Nothofagus procera in Scotland

The frost hardiness of the shoots of individual trees withintwo Chilean provenances of Nothofagus procera (Poepp & Endl.)Oerst. was measured once in each of the months January, February,November and December 1989 and January and February 1990. Therewere significant (P<0.05) differences of frost hardinessbetween provenances but only one tree could be shown to be significantlymore frost hardy than the others within the same provenance.During the winter of 1989/90 both provenances were hardy toabout –14°C (temperature killing 50 per cent of shoots)in December, but the shoots dehardened to about –9°Cin January before hardening again in February. This patternof alternate hardening and dehardening seemed to mirror changesin air temperature and could render N. procera liable to frostdamage where (as happened in 1988/9 in the UK) mild spells occurin winter followed by severe frosts.     

Seasonal Changes in the Frost Hardiness of Provenances of Picea sitchensis in Scotland

Changes in the natural level of frost hardiness of shoots offour provenances of Picea sitchensis were monitored over twogrowing seasons by detaching shoots from 7 to 10-year-old treesgrowing in a nursery in Scotland, and subjecting them to freezingtemperatures under conditions which simulated night frosts. Six seasonal phases of frost hardiness were identified (Fig.3).

1. During each autumn, killing temperatures (the level of hardiness)decreased from –5°C to below –20°C, beginningseveral weeks after shoot elongation ceased. Alaskan provenanceshardened in September, apparently in response to shorteningday lengths alone, whereas an Oregon provenance did not hardenuntil November, after repeated frosts. Queen Charlotte Islandsprovenances were intermediate.
2. From November to March allprovenances were hardy to below –20°C,which is adequateto prevent direct freezing injury at mostplantation sites.
3. In March-April, several weeks before bud-burst, old shootsdehardenedto killing temperatures of about –10°Cin responseto warm temperatures, and southerly provenancesdid so beforenortherly ones.
4. During bud-burst the newly-emergingshoots were hardy to only–3°C to –5°C untilthey were about 3.5 cmlong. All provenances burst bud at thesame time and were equallyfrost susceptible at this time.
5. DuringMay-July the elongating shoots fluctuated in hardinessbetween–5°C and –10°C apparently in responsetofluctuating ambient temperatures.
6. In August 1980 there wasa period of late summer dehardeningto killing temperaturesof about –3°C.

Seasonal changes in hardiness are discussed in relation to changesin shoot growth and environmental factors. The main opportunitiesfor selecting frost hardy genotypes seem to be in the rate ofautumn hardening, the time of pre-bud burst dehardening, andthe time of bud-burst.     

Frost Hardiness of Red Alder (Alnus rubra) Provenances in Britain

The phenology and frost hardiness of shoots of 15 provenancesof Alnus rubra growing in Scotland were measured over one autumn,winter and spring. Dates of budset (in September) and the onsetof rapid frost hardening (in October-November) occurred about2 days earlier for each degree latitude of origin northwards,except for an Idaho provenance. However, all provenances dehardenedat about the same time in March and burst their buds between8 and 14 April. Assuming that rapid frost hardening in the autumnwas triggered primarily by shortening daylengths, Alaskan provenancesof A. rubra seemed better adapted to British conditions thansouthern British Columbian provenances, which have been mostcommonly planted. However, even Alaskan provenances are proneto spring frost damage. Scottish A. glutinosa and Alaskan A.sinuata set buds and frost hardened 1–2 weeks before eventhe Alaskan A. rubra, and burst their buds 2–3 weeks laterin April-May. All three species were hardy to below –30°Cfrom December to mid-March.     

Effects of nitrogen fertilization and temperature on frost hardiness of Aleppo pine (Pinus halepensis Mill.) seedlings assessed by chlorophyll fluorescence

In a 14-week study, 1-year-old Aleppo pine seedlings were grownin two growth chambers. Seedlings were artificially hardenedby decreasing photoperiod and temperature. In each chamber halfof the seedlings were fertilized with nitrogen (8.4 mg seedling^–1).In order to determine the relative importance of the hardeningenvironment versus fertilization, each chamber was programmedto decrease night temperatures down to a low of 8 or 4°C.Chlorophyll fluorescence and frost hardiness was measured fivetimes during the experiment. A sample of seedlings from eachtreatment was exposed to an artificial frost at –5°Cand the freezing effects were assessed by measurements of chlorophyllfluorescence and visual evaluation of needle damage. Seedlingsincreased their frost hardiness during the experiment in allthe treatments but the ratio of variable to maximal chlorophyllfluorescence (F_v/F_m) measured before freezing did not vary duringthe experiment. This indicates that Aleppo pine maintains itsphotosynthetic ability during hardening in contrast to otherconiferous species from colder climates. The effect of nitrogenfertilization on frost hardiness was small in comparison withchamber effect. Nitrogen fertilization slightly delayed theacquisition of hardening in the coldest chamber. Seedlings inthe warmest chamber did not become fully resistant to –5°C,but in the coldest chamber, where night temperature reached4°C, all the seedlings were resistant to the frost. Severedamage caused by frost could be related to a rapid rise of minimalfluorescence (F₀) but the best index of damage was the dropof F_v/F_m after freezing.     

Morphological and physiological differences in Scots pine seedlings of six seed origins

One-year-old seedlings of Scots pine (Pinus sylvestris L.) offour native seed origins (Loch Maree Islands, Glengarry/GlenMorriston, Glen Affric and Abernethy), a commercial Britishseedlot, and a seedlot from Hedesunda, in middle Sweden, werecompared at monthly intervals from October 1993 to April 1994.Seedling morphology, root condition, root frost hardiness andbud dry matter were determined at each date. There were clear morphological differences among seed origins.Seedlings raised from the commercial seedlot (A70) were largerbut had a poorer root:shoot ratio than the other seed origins.Of the native pines tested, the Loch Maree Islands origin allocateda larger proportion of its photosynthate to fine roots and needlesand smaller proportion to woody structures. Seedlings raised from the commercial British seedlot tendedto have poorer bud lignification than the other origins andalso, in autumn, higher electrolyte leakage rates from its fineroots. During winter, the Swedish origin had the lowest fineroot electrolyte leakage. Seedlings of all origins showed aprogressive increase in fine root hardiness towards mid-winterwith maximum hardiness (–7°C) in January. Dehardeningoccurred over subsequent months reaching –3°C in April.Differences among origins were evident. The Swedish seedlotdeveloped greater frost resistance than the other origins, hardeningbegan earlier in autumn and dehardening began later in spring.The commercial seedlot hardened later than the other originsbut reached a similar level of frost hardiness by January. Ofthe native pines, seedlings of the Loch Maree Islands originwere slowest to develop root hardiness.     

Frost hardening and dehardening in Abies procera and other conifers under differing temperature regimes and warm-spell treatments

Frequent bud frost damage in cultivation of Abies procera Rehderand pending climate changes are the background for this studyof cold hardiness under varying acclimation regime (in closed-topchambers) and experimental warm spells during the cold season.LT₅₀ values were established by freezing tests at differenttimes of year. Damage and deaths were assessed on leader buds,subapical lateral buds, needles and cambium. Minor parallelexperiments involved Abies nordmanniana, Picea abies and Piceasitchensis. Lower acclimation temperatures resulted in deeperfrost hardiness during late autumn but less during spring, comparedwith ambient temperature controls. Elevated temperatures resultedin less deep frost resistance. Apical buds generally developeddeeper frost hardiness than lateral buds but less deep thanthe cambium, varying with species, however. Frost damage inbuds ranged from death over partially destroyed bud contentsresulting in distorted shoots to buds seemingly remaining dormant.Responses to warm spells differed with duration, timing andspecies, ranging from dramatic decrease in frost hardiness withor without subsequent recovery to no reaction. Furthermore,the reactions did not show any clear relation to dormancy level.For A. procera, exposure to fluctuating temperatures appearedto be particularly problematic. This explains why this speciesdevelops best in coastal climates, and in sites sheltered fromtemperature extremes either by hedging, a winter snow cover,or topography. The Christmas tree production will suffer severelyon sites with harsh temperatures due to losses of lateral andterminal buds, which destroy the crown symmetry. Clipping ofgreenery is less influenced by frost damages, although the developmentof normal branch whorls is often disturbed.     

Sitka Spruce and Douglas fir Seedlings in the Nursery and in Cold Storage: Root Growth Potential, Carbohydrate Content, Dormancy, Frost Hardiness and Mitotic Index

Seedlings (transplants) of 2+1 Sitka spruce (Picea sitchensis(Bong.) Carr.) and 1 + 1 Douglas fir (Pseudotsuga menziesii(Mirb.) Franco) were grown in a nursery at the Bush Estate,Scotland. Batches were lifted and cold stored at 0.5°C inNovember, December and January. Changes in growth, shoot apicalmitotic index, root growth potential (RGP), carbohydrate content,bud dormancy and shoot frost hardiness were monitored throughoutthe winter by taking samples at intervals from the nursery andfrom cold storage. Frost hardening occurred during the later stages of bud development(as mitotic indices decreased); autumn hardening was arrestedwhen seedlings were put in cold store, and some dehardeningoccurred in cold storage, especially in spring. Bud dormancystarted, and was greatest, just after bud growth (mitotic activity)virtually ceased; chilling in cold store was almost as effectivein releasing dormancy as natural chilling. The concentrationof total nonstructural carbohydrates stayed more or less constantat 100–150mg g^–1 from September to April in thenursery; in cold storage carbohydrates were depleted at 0.4–0.6mgg^–1 d^–1 (corresponding to respiration at 0.03–0.05mgCO₂ g^–1 h^–1) until there was only 40–50mgg^–1. Root growth potentials in the nursery increased in December,once the buds ceased growth, became dormant and had receivedsome chilling. Sitka spruce was ‘storable’ in November,before RGPs increased, but they then failed to achieve maximalfrost hardiness or ROP. Winter RGPs were high in Sitka spruceand were increased or maintained in cold storage, whereas RGPswere low in Douglas fir and decreased immediately after storage(except when stored in January). By the end of April, the RGPof cold stored Sitka spruce was much higher than that of directlifted plants. ROP changes in the nursery and in cold storagewere not consistently related to changes in seedling carbohydratecontents, shoot frost hardiness or bud dormancy. In practical terms, it was concluded that (1) the optimum dateto start lifting bare- rooted conifer transplants in the autumnis when their shoot apical mitotic indices have decreased tonear zero, and their RGPs have risen sharply; (2) high RGPsmay depend as much on the morphology of the roots (e.g. numberof undamaged root apices) as on the physiology of the shoots(e.g. carbohydrate status, dormancy and frost hardiness); and(3) in spring, transplants kept in cold storage since November,December or January are more frost hardy, slightly more dormant,and (in May) have higher RGPs than transplants lifted from thenursery.     

Autumn Frost Damage on Young Picea sitchensis 2. Shoot Frost Hardening, and the Probability of Frost Damage in Scotland

The natural increase in frost hardiness of detached shoots ofPicea sltchensis during August to November was measured usinga programmable freezing chamber. Oregon, Queen Charlotte Islandsand Alaskan provenances were compared, and the effects on hardeningof long days, warm temperatures and frosts were determined.A computer model was constructed to mimic the observed patternsof autumn frost hardening, as functions of air minimum temperatures,daylengths and the occurrence of frosts. The model was used(a) to describe the pattern of autumn frost hardening at differentsites in northern Britain, using past meteorological records,and hence (b) to determine when frosts occurred that might havedamaged young trees. The model accurately predicted known instancesof autumn frost damage at Kirroughtree and Carnwath. The predicted probability of autumn frost damage on young treesof P. sitchensis in upland areas of Scotland was much lowerthan that previously predicted for spring frost damage. Theestimated return time for autumn frost damage to an Oregon provenanceat Eskdalemuir was 8.3 years, and the return time for a Q.C.I.provenance was longer than 10 years. Most damaging frosts occurredin October, but frosts like those on 13–15 October 1971,which followed warm weather and caused wide spread damage inScotland, have been quite rare. Alaskan provenances would rarelybe damaged by autumn frosts, nor would trees of Q.C.I. provenancegrowing in lowland areas of Scotland, or at Masset on the QueenCharlotte Islands.     

Spring Frost Damage on Young Picea sitchensis 1. Occurrence of Damaging Frosts in Scotland Compared with Western North America

Air frosts (in Stevenson screens) of –2.5°C or belowin Scotland were judged to be potentially damaging to the newlyemerging shoots on young trees of Picea sitchensis at the timeof budburst. This was based on a knowledge of the killing tissuetemperature and the relationship between air and grass minimain May. Dates of last spring air frosts of –2.5°C and –4.5°Cwere obtained for 42 meteorological stations in northern Britainwith runs of 19 to 116 years. Frequency distributions of lastfrost dates were approximately normal. About 80% of the variationbetween stations could be attribuoted to altitude, distancefrom the sea and latitude. The variance in last dates of –2.5°Cfrosts decreased from mild to cold sites. Multiple regressionswere used to produce maps of last frost dates in 20x20 km gridsquares in Scotland. Dates of last 28° F (–2.2°C) and 24°F (–4.4°C)frosts were obtained for 20 sites in western North America,spanning the natural range of P. sitchensis. Mean dates increased,and variances decreased, from south to north. Most Scottish upland plantation sites (e.g. Glentress and Eskdalemuir)experience –2.5°C air frosts until later in the yearthan all coastal stations in western North America south ofCordova, Alaska. On average, Eskdalemuir (242 m altitude) experiences–2.5°C air frosts about 4 weeks later in the springthan Masset (3 m) on the coast of the Queen Charlotte Islands,which has been the source of much of the P. sitchensis plantedin Scotland. Masset has a spring frost climate somewhat milderthan Durham.     

Shading Reduces both Visible and Invisible Frost Damage to Norway Spruce Seedlings in the Field

Two-year-old cuttings of Norway spruce were subjected to nightfrosts in spring on an exposed site in southern Sweden. Shadingwas used to assess the influence of sunlight on the extent ofdamage resulting from night frost. Chlorophyll fluorescencewas measured in needles in flushing shoots, and in shoots atthe stage of bud burst. The F_v:F_m ratio was significantly lowerfor plants exposed to light, compared with shaded plants onthe days following the night frost (minimum temperature –6°C).The effect was similar both in 1-year-old and current year needles.The low F_v:F_m ratios indicate damage to photosystem H, causedby an interaction between sub freezing temperatures and highlight intensity. Shading also increased the survival of flushingshoots. It is suggested that regeneration of Norway spruce onsites exposed to frost should be carried out in partial shade,for example under a shelterwood.     

Dieback Disease of Pines with Special Reference to Corsican Pine Pinus nigra var. Calabrica Schn.: II. The Relationship between Frost Resistance, Microclimate, and.Disease

The winter frost hardiness level of Corsican Pine buds in northernEngland is found to range from –16° C to –20C (+3° F to –4° F). Buds in the lower parts ofthe tree are more susceptible to frost damage than those nearerthe leader, and buds at a given position in the canopy on anorth-facing slope are less hardy than those in a correspondingposition on a tree in a south aspect. The symptoms produced by artificial freezing are distinct fromthose found in dieback disease, and temperature measurementsin diseased and healthy stands suggest that frosts are not severeenough to account for the disease. The lower hardiness levelsin disease susceptible areas are therefore considered to beof secondary importance and are probably a result of a weakenedphysiological condition in the tree. Diseased slopes are chiefly characterized by high humidities,low light intensities, and only slightly reduced air and soiltemperatures when compared with nearby healthy areas.     

Performance and Frost Hardiness of Picea sitchensis x Picea glauca Hybrids in Scotland

The growth of seven Picea sitchensis x Picea glauca hybridswas compared with the growth of two P. sitchensis provenancesand Picea glauca var.albertiana at two sites in northern Scotland.The sites were at Aultmore (an exposed, dry site with a mineralsoil) and Shin (a frosty, wet site with deep peat). They wereof the type considered more suited to Pinus contorta than P.sitchensis. At age 10, in 1984, most of the hybrids, at both sites, wereabout 10 and 20 per cent taller than P. sitchensis of Masset(Q.C.I.) and Ketchikan (Alaska) provenance, respectively. P.glauca var. albertiana grew very poorly, especially at Aultmore. At Aultmore, the frost hardiness of three of the tallest hybrids,the two P. sitchensis provenances, and P. glauca var.albertiana,was tested at about 3-weekly intervals throughout 1982 and 1983.Detached shoots were subjected to artificial frosts in a programmablechamber. P. glauca var. albertiana was frost susceptible atbudburst, but at all other times it was relatively very frosthardy (eg. to^–10°C in mid-August). Also, the hybridswere consistently more hardy than P. sitchensis of even Ketchikan(Alaska) provenance from July onwards. However, the hybridswere less frost hardy than P. sitchensis of Masset (Q.C.I.)provenance in early spring (they dehardened a week earlier inMarch-April) and their buds were equally as frost susceptibleat the time of budburst. In 1983, trees of P. glauca var. albertianaburst their buds about a week sooner than P. sitchensis. It was concluded that P. sitchensis x P. glauca hybrids canperform better than P. sitchensis at sites considered ‘marginal’for P. sitchensis, and that their good performance may be partlyattributed to, or associated with, their greater summer andautumn frost hardiness. A programme of inter-specific hybridizationis being pursued.     

Growth, cold-hardiness, protein content, and digestibility of 70 Leucaena seedlots on three sites in Texas, USA

Seventy seedlots of Leucaena leucocephala, L. pallida, L. diversifolia, L. retusa, L. esculenta, L. confertifolia, L. greggii and L. pulverulenta and various hybrid combinations were examined for survival to freezing weather, biomass production, leaf protein content and in vitro dry matter digestibility in Texas. Three sites along a 600 km north-south transect were chosen to provide a wide range in exposure to freezing weather. The minimum temperature on the warmer Three Rivers site was −7 °C while the minimum temperatures on the colder Vernon site was −14 °C. At the Three Rivers site where −7 °C temperatures occurred and where forage production was a priority and some winter frost damage was not a problem, L. leucocephala K636 and hybrids containing L. diversifolia and L. pallida have the most promise. If total lack of damage to −7 °C is essential, species such as L. retusa, L. pulverulenta (seedlot 61, 62) and hybrids such as L. retusa × L. greggii (53) and L. retusa × L. pulverulenta (55) have the greatest potential. This study examined only one family from each of the cold-hardy species as a parent in the promising hybrids. As extensive intraspecific genetic variation is present within these leucaena species there is great potential to find hybrids with much better combinations of cold hardiness, forage production, leaf protein and dry matter digestibility. This revised version was published online in June 2006 with corrections to the Cover Date.     

Frost hardiness gradients in shoots and roots of picea mariana seedlings

Frost hardiness of tissues along the length of the stem and the root was investigated in first‐year black spruce (Picea mariana (Mill.) B.S.P.) seedlings. Frost hardiness of 1 cm long stem and root segments was evaluated based on Index of Injury, calculated from post‐freezing electrolyte leakage. Frost hardiness was tested approximately weekly beginning seven weeks after seedlings were transferred from an 18 to a 10 h photoperiod, both at day/night temperatures of 26°C/16°C. Trees were transferred to temperatures of 10°C day and 5°C night at a 10 h photoperiod after a further 18 days. Frost hardiness was greater at the terminal bud and least at the root tips. Although shoots were generally more frost hardy than roots, differences in hardiness along the stem and root axes were gradual, rather than abruptly differing at the shoot‐root interface. All tissues, including root tips, increased in frost hardiness after conditioning for 18 days under short photoperiods (10 h) and warm temperatures (26?C/16°C, day/night). Under cold temperatures (10°C/5°C, day/night) all tissues, excepting the root tips, tolerated — 16°C with little subsequent electrolyte leakage.     

Spring Frost Damage on Young Picea sitchensis 2. Predicted Dates of Budburst and Probability of Frost Damage

The probability of frost damage at the time of budburst on thelateral shoots of young trees of Picea sitchensis in Scotland,and at Masset on the Queen Charlotte Islands, was determinedfrom (a) the dates of budburst, predicted from past temperaturerecosds using a thermal time / chilling model, and (b) the datesof last air frosts of –2.5°C. The mean date of budburst occurred between 6 and 13 May at arange of Scottish sites up to about 250 m altitude, but it occurredabout 6 days later for each further 100 m increase in altitudeup to 550 m. This resulted from an interplay between chillingand thermal time requirements for budburst. In different years,budburst dates ranged from mid-April to early June. From 1920to 1960 there was a trend towards warmer springs and earlierdates of budburst, which has been reversed since 1960. Potentially damaging frosts have occurred near the date of budburstonce every 3 years (on average) at Eskdalemuir and Braemar duringthe last 66 to 107 years, and once every 3 to 5 years in manyupland plantation regions of Scotland. The probability of damagingfrosts was greatest in the period 1930 to 1960 when budburstoccurred relatively early, and has been relatively small since1960, because budburst has occurred relatively late. Renewedwarming of the climate in future may increase the probabilityof damaging frosts coinciding with the time of budburst. Masset, on the Queen Charlotte Islands, from where Britain hasimported most P. sitchensis seed, has relatively cool springsso that budburst occurs relatively late (averaging 18 May).Also there are few damaging frosts at Masset after late April.Consequently, potentially damaging spring frosts have occurredthere only once in 65 years.     

Autumn Frost Damage on Young Picea sitchensis 1. Occurrence of Autumn Frosts in Scotland compared with Western North America

Dates of first autumn air frosts (in Stevenson screens) of –2.5°Cand 4.5°C were obtained for 42 meteorological stations innorthern Britain with runs of 18 to 116 years. Frequency distributionsof first frost dates were approximately normal. Altitude, distancefrom the sea, latitude and distance from the NWSE axis of Britaintogether accounted for 75% and 81% of the variation in datesof first –2.5°C and –4.5°C frosts, respectively,at the 42 stations. The variance in dates of first frosts decreasedfrom lowland coastal to upland inland sites. Multiple regressionswere used to produce maps of first frost dates in 20 x 20 kmgrid squares of Scotland. Dates of first 28°F (–2.2°C) and 24°F (–4.4°C)frosts were obtained for 20 sites in western North America,spanning the natural range of P. sitchensis. Mean dates wereearlier, and variances decreased, from south to north. Upland sites like Eskdalemuir and Kielder Castle experience–2.5°C and –4.5°C frosts earlier in theautumn than all coastal stations in western North America southof about latitude 58°N (between Cordova and Sitka), andabout 4 weeks earlier than at Masset on the Queen CharlotteIslands. Thus, P. sitchensis from Q.C.I. and further south mayoften experience autumn frosts in Scotland before the treeshave experienced the cool/short days that they require to inducefrost hardening.     

The Influence of Desiccation, Rough Handling and Cold Storage on the Quality and Establishment of Sitka Spruce Planting Stock

In January 3-year-old Sitka spruce were lifted and treated eithercarefully or roughly. Plants in each treatment were either transferredto the laboratory for testing, or transferred to cold storageat –1°C. In the laboratory, half of the plants ineach treatment were desiccated to shoot water potentials ofless than –2.0 MPa. Plants were then tested for root growthpotential (RGP), bud dormancy, frost hardiness and carbohydratecontent before transplanting at a field site, Additional plantswere removed from the nursery and cold store at approximatelymonthly intervals until late April, and then treated as above.In April, plants in each treatment were transferred to on-sitefield storage (sheughs), for 2 or 4 weeks prior to testing asabove. Carefully handled plants maintained large RGP which was positivelycorrelated with plant water potential, root water content, shootrelative growth rate and field survival. Overall RGP was reduced: 59 per cent by desiccation; 85 percent by rough handling; and 98 per cent by desiccation and roughhandling in combination. On site storage for 4 weeks in April/May reduced RGP. Whereplants produced more than 30 new roots > 1 cm long in RGPtests, field survival exceeded 90 per cent. Cold storage at–1°C maintained RGP, (of carefully handled plants),bud dormancy and shoot frost hardiness at approximately pre-storagevalues. In contrast, total nonstructural carbohydrate contentdeclined in cold store but increased in March/April for plantsin the nursery.     

Frost hardiness of Picea rubens growing in spruce decline regions of the Appalachians

It has been proposed that pollutants predispose Picea rubens Sarg. growing in the high Appalachians to frost damage. The pattern of autumn hardening of P. rubens growing at Whiteface Mountain, NY, and Newfound Gap, NC, was monitored by detaching shoots at 1-3 weekly intervals, air freighting them to Scotland, and freeze-testing them. The temperatures that produced freezing injury from August 1986 to January 1987 were compared with minimum air temperatures recorded in those months at nearby meteorological stations over 22 previous years. There was only weak evidence that the onset or degree of frost hardening was inadequate to protect the trees from direct freezing injury (as opposed to winter desiccation). Historically, minimum air temperatures occasionally fell below the lethal temperature for a 10% kill (LT(10)), but they rarely fell below the LT(50). The trees hardened rapidly in the autumn (max. 2.2 degrees C day(-1)) to between -30 degrees C and -40 degrees C by January (LT(50)), including trees showing visible decline on Clingman's Dome, TN. Individual trees differed in hardiness by up to 10 degrees C. It is concluded that any pollutant-induced susceptibility to freezing injury is insufficient, on its own, to account for forest decline in the Appalachians.     

Assessment of frost damage to leafless stem tissues of Quercus petraea: A reappraisal of the method of relative conductivity

Leafless Quercus petraea stems 1.4–5.0 mm in diameterwere exposed to freezing temperatures before assessing frostdamage visually and by two methods where electrolyte leakagefrom tissues was examined. Visual assessment could not discriminatelethal from non-lethal damage, whereas electrolyte leakage could.Assessments based on the rate of electrolyte leakage took almosttwice as many man hours to obtain, but were more consistentthan those produced using the method of relative conductivity.Full equilibration of electrolyte leakage following exposureto freezing temperatures required at least 5 days at 4°C. Autoclaving the tissues at 121°C for 15 min failed to releaseall diffusible electrolytes even after 10 days equilibration,but complete release was achieved 24 h after autoclaving for120 min. Unless full equilibration is achieved, it is impossibleto define a value of relative conductivity that is indicativeof lethal damage. The use of predefined lethal values of relativeconductivity is inad visable because the relative conductivityof undamaged (unfrozen) shoots varies substantially with thephysiological state of the tissues. Allowing 5–7 days after freezing to permit full electrolyteequilibration, followed by autoclaving for 90–120 minat 121°C and allowing 24 h before measurement provides areliable method for assessing freezing injury by the methodof relative conductivity in small pieces of leafless stem tissues.Differences between treatments can then be assessed using standardstatistical procedures.