Special Section Insert: Lessons from a record-breaking freeze: Some olives show damage; many, coldhardiness
A once-in-a-century cold front, expressed as an advective freeze, damaged ‘Manzanillo’ extensively statewide. ‘Ascolano,’ ‘Sevillano’ and ‘Mission’ received minor damage. Damage included tip burn, defoliation, bark splitting and limb dieback. The next growing season some new leaves were deformed and flower bud damage could be found. Outbreaks of olive knot had also been expected, but few occurred.
Freeze damage and coldhardiness in olive: findings from the 1990 freeze
‘Manzanillo’ trees in an orchard near Visalia show the extensive freeze damage characteristic of young trees.
During the last 2 weeks of 1990, a mass of dense, frigid Arctic air moved into California, and temperatures plunged to record lows along the Pacific Coast and in the Central Valley. Not only were temperatures record breaking, but the cold spell's duration and its widespread effect were noteworthy. This “Arctic Express,” as it was called by the media, was a once-in-a-lifetime experience for most Californians accustomed to mild winters. Once immediate concerns were dealt with, such as frozen water pipes, attention turned to the cold front's effects on California's agriculture. One major crop adversely affected was olive (Olea europaea L.). Responding to concerns of olive growers, the California Olive Committee (COC) requested the University of California's Department of Pomology at Davis to study the olive under freeze conditions.
Late in December 1990, a once-in-a-century cold front moved through California, threatening olive, citrus and nursery plantings across the state. Responding to olive growers’ concerns, the California Olive Committee funded a UC study of olive freeze damage – yielding a wealth of data on olive performance under exceptional freeze conditions. Printing of this special section was funded by the California Olive Committee.
Cover photos illustrate olive damage seen throughout the state after the December 1990 freeze.
The study has been broad based, from botanical and horticultural aspects of freeze damage and recovery to climatic and economic questions. Our goals were: (1) to understand the nature of freeze damage in olive, (2) to explore the question of olive cultivar coldhardiness, (3) to learn the frequency of such climatic events in California, (4) to estimate the potential economic impact on producers and the state economy, and (5) to develop for California's olive producers guidelines for minimizing the effects of future freezes.
Our study included (1) field examination of olive orchards, (2) microscopic examination of freezed-damaged olive tissue, (3) laboratory experiments to look at coldhardiness in California cultivars, (4) climatological research, (5) questionnaires sent to olive growers and (6) economic studies.
Field work was carried out in California's two major olive-producing regions: Butte, Tehama, and Glenn counties in the Sacramento Valley and Tulare and Madera counties in the San Joaquin Valley. Assisting were UC Cooperative Extension farm advisers and olive growers. The COC mailed approximately 1,400 questionnaires regarding the freeze to growers; 311 responses were received and used for data processing and analysis. The questionnaire dealt not only with damage symptoms and severity but also with production levels and cultural practices.
December 1990 and olive climates
Early in the third week of December 1990, a low-pressure trough moved through California bringing light precipitation. Behind the trough came cooler air that caused temperatures to drop, followed shortly by a large mass of dense, frigid Arctic air. By early morning on December 18, most of the Central Valley experienced freezing temperatures. The air mass continued to build and, on the nights of December 22, 23 and 24, temperatures plummeted. New record minimum temperatures were established. Subsequent damage to olive occurred with this combination of cold and its duration: Central Valley cities stretching from Redding to Bakersfield recorded lows below 20°F (-6.7°C) on these first 3 days of winter, and freezing nighttime temperatures persisted into the first week of the New Year.
Record low temperature data for Davis and for several olive-growing sites in the Sacramento and San Joaquin valleys are given in table 1. The December 1990 freeze set new record lows for all sites, except Davis and Visalia. At some sites temperatures were possibly either lower or higher than those given in table 1. For example, official National Climatic Data Center (NCDC) records show Visalia's lowest temperature was 21°F (-6.1°C) on December 24, but data gathered by the California Irrigation Management Information System's (CIMIS) automated station in Visalia show the lowest temperature was 17.3°F (-8.1°C) on December 23.
Low temperatures were accompanied by persistent winds, bringing about a condition known as an advective freeze. Because wind mixes air so that colder, dense air has no chance to accumulate at ground level or to drain to low-lying areas, these winds may have prevented even lower temperatures. Advective freezes also have a very low dew point (the temperature at which air becomes saturated and produces dew). The lower the dew point, the more potentially damaging. The lowest dew points of interest recorded by CIMIS during the 1990 freeze were 15°F (-9.4°C) at Visalia December 23, 0°F (-17.8°C) at Davis December 21, and -9°F (-22.8°C) at Orland, also on December 21.
Advective freezes may be contrasted with radiation frosts, which take place under still or very low wind conditions. Colder air accumulates near the ground; warmer air can be found at altitudes of 25 to 50 feet (7.6 to 15.2 m). Cold air is heavy and therefore migrates to valleys and other low-lying areas. In such frosts dew point is at freezing or a few degrees below it. Radiation frosts usually occur in early to mid-autumn or late winter to early spring. These frosts rarely seriously affect olive trees, although unharvested fruit may be damaged in autumn, as sometimes occurs in ‘Mission’ olives. Such frosts may damage other fruit crops, especially those characterized by early spring flowering.
Although temperatures at which olive trees can be damaged vary, depending on climatic conditions, temperatures at or below 20°F (-6.7°C) are often critical. The frequency of such episodes in California's olive-growing areas varies considerably (table 2). Generally, the San Joaquin Valley is less threatened by such freezes than is the Sacramento Valley. For example, Willows in Glenn County may see such temperatures once every 5 years, whereas Visalia in Tulare County may experience such freezes once every 17 years. Timing of such freezes also varies. The Sacramento Valley (Orland, Willows, Oroville, and Davis) is more susceptible to damaging freezes in early and mid-December before trees have had time to harden off than is the San Joaquin Valley (Visalia and Porterville) (table 3). Normally, cool, wet weather in November and December in the Sacramento Valley acclimates olive trees so that they can resist freeze damage – but not always. Such acclimation is normally less likely in the San Joaquin Valley. However, according to CIMIS data, by the time of 1990 freeze, Visalia in the south had received 198 hours ≤ 33.8°F (1°C), whereas Orland in the north had received only 111 hours of such temperatures.
Although new records were set at many California sites during the 1990 freeze, the Central Valley had previously experienced cold fronts. According to NCDC records, serious freezes in which temperatures dipped below 20°F (-6.7°C) occurred at several sites in 1913, 1919, 1930, 1932, 1937, 1948, 1949, 1950, 1972, and 1978. Of the freezes before 1990, the 1913 and 1932 freezes were the most severe and extensive (table 1). The frequency and geographical extent of such freezes varied. Temperature records before 1913 are sketchy for many sites in California, but what records exist indicate another extensive freeze, capable of damaging olive trees, occurred in 1888.
California olive-producing regions are not alone in experiencing damaging freezes. Olive trees in northern Italy have been damaged at least five times in this century, the most recently in 1989. Records for Italy also indicate at least three freezes in the 19th century and four in the 18th century. The olive-growing regions of Azerbaijan along the Caspian Sea also report several episodes of damaging temperatures. Trees were reportedly exposed to freezes of 17.6°F (-8°C) in 60% of the years studied, of 14°F (-10°C) in 40%, of 8.6°F (-13°C) in 5%, and of 3.2°F (-16°C) in 1%. Indeed, most northerly growing areas, from Spain in the west to China's Turkic regions in the east, are subjected fairly regularly to low temperatures that may damage olive trees. Freeze damage to olive is not always limited to the north. Defoliation and limb dieback have been observed on olive trees on the warm island of Crete as a result of a freeze during the winter of 1991–1992
Factors affecting coldhardiness
Although olive, as a species, is the most cold-hardy of the subtropical fruit trees, certain olive cultivars are more cold-hardy than others. For example, in the 1990 California freeze, ‘Manzanillo’ trees suffered more damage than did any other cultivars grown in the state. Where ‘Manzanillo’ was grown alongside ‘Mission,’ ‘Ascolano,’ or ‘Barouni’ in the same orchard, the ‘Manzanillo’ trees sustained obvious damage, but the other cultivars sustained little or none. Responses of growers to the questionnaire clearly support differences in coldhardiness among the five major California cultivars (table 4). ‘Manzanillo’ was the most damaged. Other cultivars appeared to be hardier; ‘Ascolano’ emerged as the hardiest. Table 5 provides an evaluation of the coldhardiness of olive cultivars from various countries held in the USDA collection at Winters and observed following December 1990. Italian scientists suggest that differences in olive coldhardiness may be related to the stomatal density on leaves of different cultivars with hardy cultivars having fewer stomata per unit of area.
Hardiness increases when trees are exposed to cold temperatures as autumn proceeds into winter, a process called “acclimation.” The importance of acclimation can be illustrated by an example of a mature ‘Manzanillo’ orchard in Glenn County, chosen for a study specifically because it had been damaged in a freeze in November 1985. As a consequence, defoliation, bark split, and olive knot infestation were so great that the trees were cut to within 2 feet (0.6 m) of the ground, and new scaffolds and canopies were developed from shoots that sprouted on each stump. Although temperatures reached only 22°F (-5.6°C) at nearby Willows during this episode, the experience came so early in the acclimation process that little hardening off had occurred.
TABLE 1. Low temperatures recorded in California olive-producing regions,* from National Climatic Data Center, 1913–1990
TABLE 2. Number and frequency (%) of years reporting at least one occurrence of minimum low temperatures below given temperature thresholds in olive-producing regions in California, 1913–1990
TABLE 3. Timing of lowest yearly minimum temperature of ≤ 20°F (-6.7°C) in California olive-producing regions, 1913–1990
TABLE 4. Statewide freeze damage estimation for five olive cultivars as reported on grower questionnaires
Table 5. Subjective rating* of December 1990 cold (lowest temperature: 15°F [-9.4°C]) damage to olive varieties in the U.S. Department of Agriculture Germplasm Repository in Winters, California
TABLE 6. Freeze damage rating by county as reported on grower questionnaires
By contrast, in December 1990, the trees, now some 12 feet (3.7 m) tall, had had more time to harden off, and damage was relatively minor, even though the temperature reached 11°F (-11.7°C) at Willows. Increased tree age perhaps also contributed to the better outcome in 1990. Differences in damage to California cultivars from place to place may be partly explained by differences in acclimation (table 6). However, differences in cold severity also affect levels of damage. Acclimation differences cannot be used to explain the perceived greater extent of damage to trees in the San Joaquin Valley as compared with those in the Sacramento Valley because, as mentioned, Visalia in the south had, in fact, received more hours at acclimating temperatures than had Orland in the north. The greater total damage in the south is probably due to the extensive ‘Manzanillo’ acreage.
Pruning, irrigation and fertilization affect tree growth and thus influence coldhardiness. For example, in a ‘Sevillano’ orchard near Corning, trees pruned in the fall before the freeze produced vegetative growth that did not have enough time to harden off before December's severe cold. Although ‘Sevillano’ trees are normally rather cold-hardy, pruned trees were damaged by the low temperatures; nearby unpruned ‘Sevillano’ trees escaped injury (fig. 1). This pattern was repeated in numerous locations statewide and with other cultivars. The open canopies on pruned trees probably exposed tree parts to frigid air and wind stress. Wind stress may also account for increased damage seen in trees along orchard perimeters.
Freeze damage was evaluated on ‘Manzanillo’ trees in a Madera County orchard in which UC researchers conducted irrigation trials. As measured by the extent of defoliation, damage was greater on trees receiving more water than on those receiving less. Excessive irrigation, especially when continued after harvest, may elicit more vegetative growth and may make trees more subject to damage. Conversely, limited water may promote tree desiccation, thereby increasing hardening off as well as retarding vegetative growth. In terms of tree recovery, however, well watered trees responded quickly and began strong growth once temperatures warmed in spring 1991; recovery of low-irrigation trees was less vigorous. Moreover, leaf loss was greater on high-irrigation trees immediately following the freeze, but defoliation from stress under high temperatures in summer 1991 was greater than on low-irrigation trees. Defoliation on all ‘Manzanillo’ trees was more extensive than normal throughout the growing season, but low-irrigation trees lost many more leaves as the season progressed.
The bottom line with respect to irrigation seems to be this: (1) little or no irrigation after harvest encourages hardening off before cold weather and decreases chances of freeze damage, and (2) adequate irrigation in spring encourages recovery from freeze damage.
Clear evidence of damage from fertilization did not emerge in our study. Nitrogen applied late in the season (after July), or in amounts or formulations that persist into fall, may account for continued vegetative growth that will not harden off before winter arrives. Trees, therefore, are more susceptible to damage from low temperatures.
What affects olive coldhardiness in California mirrors what has been noted in Italy. There, duration of freezing temperatures and tree age were listed as important factors - the same factors probably operative in California's 1990 freeze. Repeated freezing and thawing over more than a week no doubt contributed to physical stress and strain on trees. Growers’ responses to the questionnaire indicated that young trees were more severely affected than were older trees, regardless of cultivar (table 7).
Temperatures ranging from 19.4°F (-7°C) to -4°F (-20°C) have reportedly damaged olive. The 1932 freeze damaged Sacramento Valley olive trees at 17°F (-8.3°C); others were not damaged until temperatures dropped to 11°F (-11.7°C). In the former Soviet Union, temperatures from 17.6° to 14°F (-8° to -10°C) slightly damaged olive trees, whereas temperatures of 1.4° to -7.6°F (-17° to -22°C) killed trees down to the ground. Damaging temperatures are sometimes given as known thresholds.
TABLE 7. Average statewide freeze damage by tree age as reported on grower questionnaires
Fig. 1. This ‘Sevillano’ tree in Tehama County was pruned in November before the freeze and was severely damaged because of a loss of hardiness. The damage was made more severe by an infestation of olive knot disease.
The critical temperature parameters given here are based largely on field observations, not on experimentation. This partly explains the reason for the great range in temperatures suggested as damaging to olive. Controlled laboratory experiments on the freezing characteristics of olive are limited. In one study using cold-acclimated olive tissues, 50% injury was seen on leaves at 10.4°F (-12°C), on leaf buds at 8.6°F (-13°C), on stem cambium at -4°F (-20°C), and on stem xylem at -0.4°F (-18°C). Experiments in Japan on an unnamed cultivar of olive, as part of a mass screening of more than 100 species, revealed damage to leaves and buds at 8.6°F (-13°C) and damage to twigs at 5°F (-15°C).
We examined coldhardiness in California olive cultivars in laboratory experiments at the Washington State University Tree Fruit Research and Extension Center in Wenatchee, Washington. Samples were processed within 30 hours of harvest. Sampling was repeated at 1-month intervals beginning October 28, 1991 and ending April 13, 1992. Leaves of these cultivars were processed beginning in December rather than in October. Samples of shoots from ‘Ascolano,’ ‘Sevillano,’ and ‘Barouni’ cultivars were analyzed, along with ‘Manzanillo’ and ‘Mission,’ once on January 27, 1992.
In the Wenatchee laboratory, basal internodes and leaves of the current season's growth were cooled at a rate of 27°F per hour (15°C/hr) from room temperature to -40°F (-40°C) to determine the differential response of each sample. In this type of analysis, the target tissue's internal temperature is measured. Freezing events are noted by a rise in temperature as the latent heat of the fusion of water is released; this is called an “exotherm.” Computer-assisted collection of sensor signals was performed at a frequency of 20 Hertz (Hz).
The onset of freezing events under laboratory conditions may or may not indicate the survival temperature of a particular tissue under field conditions. Because of the great proportion of water in plant tissue, freezing always severely challenges the viability of affected cells. It cannot be assumed, however, that freezing will inevitably kill frozen tissue or the entire plant. To associate freezing temperatures with survivability, on two dates plant tissue samples were cold-treated to noted exotherm temperatures in separate experiments, then exposed to viability tests, and further observed over extended periods to assess plant survivability or functionality. In the viability tests on olive tissue, a modified form of the tetrazolium chloride (TTC) procedure described for apple was used.
Thermograms of the freezing of olive tissues showed from one to three exothermic peaks, depending on tissue morphology and the season. Each peak corresponded to the freezing of distinct cell aggregates. The area under each peak was proportional to the quantity of cell water and/or cell mass involved in the transition from water to ice.
In a typical olive shoot, the bark is composed of the cork cells of the exterior and the living phloem cells inside (which transport sugars and other substances to organs that need them). The wood is composed of xylem cells (which transport water and minerals from the roots), ray parenchyma cells (which relate the xylem to the phloem), and the pith (an area of undifferentiated cells at the center). The area between the xylem and the phloem is the cambium. The cambium is the source of all new cells, whether xylem or phloem, for lateral growth. In most species, freezing of pith cells almost always kills them, but does not impair plant survival. In contrast, freezing of xylem parenchyma or ray parenchyma cells kills these cells and the entire branch or shoot. Freezing kills any actively growing tissues. The first exotherm seen in olive stem tissue is that of the extracellular water and of bark cells. The second exotherm is that of the xylem ray parenchyma cells in the wood. A third exotherm has been observed only in stem tissue at the time of and after the onset of growth in spring. The third exotherm apparently has no connection with plant survivability, as indicated by the TTC viability test; however, it may be useful as a physiological marker of the beginning of growth. Leaf tissue exhibits one exotherm.
Fig. 2. The freezing temperatures for the bark as winter progressed do not indicate a capacity for acclimation, whereas the freezing temperatures for the wood and leaves show an unmistable pattern of acclimation and deacclimation for both of these cultivars.
Fig. 3. Tip burn (here on ‘Barounl’) is the death and blackening of tissues at or near the succulent growing tips. This breaks apical dominance, and two or more axillary buds will begin growing in the spring, thereby creating a bushier canopy.
In the current year's olive shoots, freezing in the bark accompanied freezing of extracellular water at temperatures well above such events inside the xylem and pith cells of the wood. Viability tests show that bark cells did not survive this freezing. Bark cells did not exhibit any acclimation as the season progressed (fig. 2). The fact that bark cells froze at higher temperatures than wood cells may be a significant factor by itself in determining the extent of damage to olive trees. Once it froze, the bark became rigid. The later freezing, in turn, of xylem water and xylem cells caused a volume expansion leading to splitting of the bark. This splitting could then lead to desiccation of bark and wood. The split could also be a wound allowing for entry of various pathogens.
Not only did the xylem and pith cells of the wood freeze at much lower temperatures than did the bark cells and extracellular water, but the temperatures of freezing in the xylem and pith cells also varied according to the time of season in a pattern, suggesting acclimation capability (fig. 2). This acclimation capability was also borne out by the fact that the xylem and pith cell nucleation temperatures could be significantly lowered by storage of unacclimated tissue in a moist environment at 36° to 40°F (2.2° to 4.4°C) for 7 days. Viability tests indicated that this freezing was associated with the death of these xylem tissues and that the consistently lower nucleation temperatures for ‘Mission’ xylem, compared with ‘Manzanillo’ xylem, mean that ‘Mission’ is a hardier cultivar. The one-time test on January 27, 1992 of ‘Ascolano,’ ‘Barouni,’ and ‘Sevillano’ with ‘Manzanillo’ and ‘Mission’ confirmed the existence of cultivar differences in coldhardiness (table 8). ‘Barouni’ and ‘Sevillano’ showed xylem freezing similar to that seen in &lsqu