Abstract
Temperature fluctuation (TF) in an 18-year-old Fraxinus pennsylvanica var. subintegerrima and its surrounding environment was monitored using HOBO Pro temperature sensors recording every 15 min from December 2001 to February 2003 at The Morton Arboretum, Lisle, Illinois, U.S. There were significant differences (P < 0.05) between TF in 2001, mild cold temperatures, and 2003, severe record-breaking cold temperatures. In mid-December 2001, TF range in soil 30 cm (12 in) was 4°C (39.2°F) to 4.5°C (40.1°F), sod was 3°C (37.4°F) to 4°C (39.2°F), and soil surface was 2°C (35.6°F) to 2.5°C (36.5°F), whereas canopy and mulch ranged from −1°C (30.2°F) to 10°C (50°F). The south side of the trunk had the highest fluctuation of 1°C (33.8°F) to 14°C (57.2°F) followed primarily by the west side with occasional peaks in the east. However, the west side had the highest temperature peak in mid-June. The temperature difference between south and north sides during mid-December were approximately 7°C (44.6°F). In April, the TF inside the trunk ranged from 2°C (35.6°F) to 5.5°C (41.9°F) compared with the canopy, which varied between −0.5°C (31.1°F) and 8°C (46.4°F). The west side was 2°C (35.6°F) to 3°C (37.4°F) higher in mid-July than the south, east, and north sides. On 15 February 2003, which was the coldest day recorded, the soil 30 cm (12 in) temperature (under the mulch) reached ≈−1°C (≈30.2°F), whereas sod and soil surface were ≈−2°C (≈28.4°F). Mulch and base temperature ranged from −1°C (30.2°F) to −5°C (23°F) and −2.5°C (27.5°F) to −7.5°C (18.5°F), respectively. Root core temperature was ≈−1°C (≈30.2°F), the trunk temperature range was −2.5°C (27.5°F) to −3.5°C (25.7°F), whereas the canopy was −2.5°C (27.5°F) to −7.5°C (18.5°F). The south TF range was between −0.5°C (31.1°F) and −7.5°C (18.5°F) from midday to midnight. The TF difference between south and north sides was ≈2.5°C (≈36.5°F). This freeze and thaw of the south side during winter months has been attributed to sunscald in some trees. Based on temperature observations during the coldest and warmest week, a temperature fluctuation factor (TFF), a difference between weekly minimum and maximum temperature, was introduced. During the coldest week, the TFF for canopy to trunk was 2×, trunk to root or soil was 10×, and canopy to root or soil was 20×. During the warmest week, the TFF for canopy to trunk was 2×, trunk to root or soil was 7.5×, and canopy to root or soil was 15×. The stem water content was higher throughout the year; however, the bud water content was significantly higher when approaching budbreak in April to May. In a companion study, the effect of mulch depth on TF was reexamined showing that the temperature of mulch varies dependent on the time of year. In October, 15 cm (6 in) mulch was several degrees warmer than ground, 7.5 cm (3 in) mulch, and 30 cm (12 in) mulch (P < 0.05); however, in December and February, 30 cm (12 in) of mulch was significantly warmer (P < 0.05). There are many factors other than temperature that affect tree growth and development. The dynamics of TF give a greater understanding of the role temperature plays in tree physiology as well as improving horticultural and arboricultural understanding in urban environments, resulting in improved landscape management.
Higher plants are ectothermic and as such are unable to maintain a constant optimum temperature in their tissues (Fitter and Hay 1987); therefore, their growth, development, and performance are all affected by environmental factors of which none are as vital as temperature (Weiser 1970). Ability of temperate woody plants to withstand freezing temperatures is affected by temperature fluctuation (TF) within the tree and its surrounding environment before freezing temperatures (Levitt 1980; Sakai and Larcher 1987). Temperature fluctuation, a frequent freezing and thawing during winter in temperate woody plants, may contribute to reduced cold hardiness (Sakai and Larcher 1987) depending on the stage of dormancy (Shirazi and Fuchigami 1995) and poststress temperature (Shirazi and Fuchigami 1993). The extreme variability of air and soil temperatures forces plants to adapt, tolerate, or avoid temperature extremes (Levitt 1980). In temperate climates, roots begin to grow after soil is thawed compared with milder climates in which roots grow all year round. The ability of roots to grow is dependent on species and genotype but is also affected by soil temperature, soil moisture, and oxygen availability (Kozlowski and Pallardy 1997).
Huttunen and Soveri (1993) reported that soil freezes to a mean annual depth of 15 (6 in) to 150 cm (60 in) depending on the location, soil moisture, soil texture, air temperature, and snow depth (Sakai 1970; Solantie 2000). The same factors also determine the time of soil thaw (Solantie 2003). Root zone temperature is dependent on soil depth and properties such as color, bulk density, moisture, texture, and type of vegetative cover. Extensive study on TF in wintering trees was first reported by Sakai in 1966. He found that the bark temperature on the south side of a tree increased sharply at noon to ≈21°C(≈69.8°F), whereas the temperature on the north side stayed nearly constant ≈−1°C(≈30.2°F) to −2°C (28.4°F) in Kalopanaz septemlobus koidz in Sapporo, Japan. He also reported that TF in the center of a stem is dependent on stem diameter. In the center of a tree (Ulmus davidiana planch var. japonica nakai) with a diameter of 86 cm (34.4 in), the temperature remained nearly constant throughout the day, from −0.5°C (31.1°F) to −2°C (28.4°F) in the winter. With advanced computerized sensors, we monitored TF at 14 locations in an 18-year-old Fraxinus pennsylvanica var. subintegerrima located at The Morton Arboretum, Lisle, Illinois, U.S. The objective of this study was to understand more about how trees sense their surrounding environment and how temperatures fluctuate within a tree. This information gives a greater understanding of the role temperature plays in tree physiology as well as improving horticultural and arboriculture practices in urban environments.
MATERIALS AND METHODS
HOBO Pro (Onset Corp., Bourne, MA) temperature sensors were placed at 14 different locations: east, west, north, south (1.37 m [4.5 ft] above the soil), trunk (10 cm [4 in] inside the trunk of the tree, 1.41 m [4.7 ft] above the soil layer), root (1.25 cm [0.5 in] inside the root, 30 cm [12 in] under the soil, 20 cm [8 in] from the tree), canopy (7.5 m [24.8 ft] inside the tree canopy), soil surface (under the mulch, approximately 15 cm [6 in]), mulch (7.5 cm [3 in] inside the mulch layer), base (approximately 15 cm [6 in] above mulch layer on the west side), soil 30 cm (12 in) (30 cm [12 in] into the soil under mulch), ground (placed 45 cm [18 in] aboveground, 1.8 m [5.9 ft] from the trunk), sod (placed just under the sod 1.8 m [5.9 ft] from the trunk), and snow (2.5 cm [1 in] above the mulch at trunk flair on the south side). All locations were positioned by using a handheld GPS receiver (eTrex Summit; Garmin, Olathe, KS) (Figure 1; Table 1). The temperature was recorded every 15 min from December 2001 through February 2003. Data were downloaded with an Onset HOBO Shuttle at the site and transferred to a computer in the laboratory (The Morton Arboretum, Lisle, Illinois, U.S.).
Location of the HOBO Pro temperature sensors on the Fraxinus pennsylvanica var. subintegerrima (red ash) tree and its surrounding environment.
Weekly mean of maximum and minimum temperatures observations from the 14 HOBO Pro temperature sensorsz.
The water content of the stem and buds was measured by weighing the fresh tissues (FW) and placing them in a conventional oven at 75°C (167°F) for 48 hr. Samples were then weighed again for dry weight (DW). Percent water content was calculated by subtracting the DW from FW, dividing by FW, and multiplying by 100 (percent water content = ((FWDW)/FW) × 100). Three samples were taken weekly from October 2002 through May 2003.
For the mulch study, mixed wood mulch was placed around four Acer saccharum (sugar maple) 60 cm (24 in) from the trunk and HOBO Pro temperature sensors were placed approximately 20 cm (8 in) from the trunk and 10 cm (4 in) below the surface of the mulch layer. Mulch depths were 7.5, 15, and 30 cm (3, 6, and 12 in) and a bare ground area served as the control. A stake secured the HOBO Pro temperature sensors for all locations. Statistical analysis was performed using SAS software 2003 (SAS Institute Inc, Cary, NC). Analysis of variance for data were performed using time interval as the replication.
RESULTS AND DISCUSSION
There was a significant difference (P < 0.05) in TF among locations and years. Total mean temperature observation for all 14 HOBO Pro temperature sensor locations from December 2001 through April 2002, which was considered a mild year in Northern Illinois, are shown in Figure 2. Root total TF was slightly higher than soil 30 cm (12 in). Sod and soil surface were the same as well as east and south. Canopy and base were also the same as well as the west and north sides. Total mean temperature observation from October 2002 through March 2003, which was considered a severe, record breaking cold year in Northern Illinois, are shown in Figure 2B. Root and soil 30 cm (12 in) total TF was the same as well as sod and soil surface followed by mulch, trunk, east, and canopy TF. West, north, and base were all similar with a total mean of −0.5°C (31.1°F) to 0°C (32°F). The ground location above the sod (45 cm [18 in] above sod, outside the canopy) had the lowest total mean temperature (≈−1°C[≈30.2°F]). The minimum and the maximum weekly temperature observation for all the sensor locations are shown in Table 1. The lowest minimum temperature occurred in the root −0.87°C (30.4°F) and the highest occurred in the ground −12.34°C (9.8°F), which was observed in January 2003. The root also had the lowest maximum temperature of 21.95°C (71.5°F) occurring in the first week of August 2002. The highest maximum temperature of 26.16°C (79.1°F) occurred in the canopy. The comparison of the coldest and warmest week in root, trunk, soil 30 cm (12 in), and canopy are shown in Figure 3. During the coldest week, the canopy had a TF of 20°C (68°F) (2°C [35.6°F] to −18°C[−0.4°F]) followed by the trunk with a TF of 10°C (50°F) (0°C [32°F] to −10°C [14°F]). Soil and root core had a TF of 1°C (33.8°F) (−0.5°C [31.1°F] to 0.5°C [32.9°F]). During the warmest week, the canopy experienced TF of 15°C (59°F) (17°C [62.6°F] to 32°C [89.6°F]) followed by the trunk with a TF of 7.5°C (45.5°F) (17.5°C [63.5°F] to 25°C [77°F]). Soil and root core had a TF of 1°C (33.8°F) (22.5°C [72.5°F] to 25.5°C [77.9°F]). The same pattern of TF was observed for both the coldest and the warmest weeks. The canopy showed twice the TF when compared with the trunk. The soil and root experienced only 1°C (33.8°F) TF both in the coldest and the warmest week (Figure 3). Based on this information, a temperature fluctuation factor (TFF) was developed, reflecting the difference between weekly minimum and maximum temperature. The following TFF ratios are the outcome of TF for the coldest and warmest week, in which C = canopy, T = trunk core, R = root core, and S = soil.
Mean weekly temperature at 14 locations in Fraxinus pennsylvanica var. subintegerrima (red ash) during 2001 through 2003. Mean separation by Duncan’s multiple range test (P < 0.05). Data are means of ≈16,000 observations.
Comparison of coldest and warmest week temperature fluctuations in root, trunk, and canopy of Fraxinus pennsylvanica var. subintegerrima (red ash).
Considering other variables such as trunk and root diameter, soil moisture, soil textures as well as altitude and latitude may contribute to prediction of the TFF for tree management practices.
Dynamics of TF in 14 locations are shown in Figure 4 for the 15th day of each month (January 13, February 18, May 16) from January through December 2002. The first graph for each month (Figure 4) shows TF in the soil surface, sod, mulch, base, and soil 30 cm (12 in). The second graph for each month (Figure 4) shows TF in the root, trunk, and canopy. The third graph for each month (Figure 4) shows TF in the north, south, east, and west. The color codes represent each HOBO Pro temperature sensor. We observed dynamic changes in temperature every 15 min in one day of each month. April 15 shows the most classic graph regarding the TF in this study, which indicates the TF in the soil surface, soil 30 cm (12 in), sod, mulch, and base. Budbreak at this time of the year as well as the greening of grasses is often an indication that soil is warming up with a steady increase in soil temperature ≈10°C(≈50°F) to 15°C (59°F) during the spring. Peak of the soil temperatures are reached at ≈22°C (≈71.6°F) in mid-August and gradually decreases monthly to 20°C (68°F) in September, 17°C (62.6°F) in October, 12°C (53.6°F) in November, and 4°C (39.2°F) by December. The optimum temperature for root growth depends on species, genotype, stage of development, and availability of oxygen and water. The minimum temperature for root growth for temperate zone woody plants is between 0°C (32°F) and 5°C (41°F), whereas the optimum temperature is between 20°C (68°F) and 25°C (77°F). Lyr and Garbe (1995) reported the occurrence of lower root temperature optima in species of northern origin compared with species of southern origin. The impact of soil temperature on root growth is affected by soil moisture. Teskey and Hinckley (1981) demonstrated in white oak in Missouri that root elongation is affected by soil temperature below 17°C (62.6°F); however, above 17°C (62.6°F), it is soil moisture that is the dominant factor. Based on our results, September to November are the most appropriate times for planting trees in northern latitudes because the temperature of the root zone makes an ideal situation for root growth. TF for January and mid-February are shown in Figure 5. On 15 February 2003, which was the coldest day recorded, the soil 30 cm (12 in) temperature reached approximately −1°C (30.2°F), whereas sod and soil surface were ≈−2°C(≈28.4°F). Mulch and base temperature range was −1°C (30.2°F) to −5°C (23°F) and −2.5°C (27.5°F) to −7.5°C (18.5°F), respectively. Root core temperature was ≈−1°C (≈30.2°F), the trunk temperature was −2.5°C (27.5°F) to −3.5°C (25.7°F), whereas the canopy TF was −2.5°C (27.5°F) to −7.5°C (18.5°F). The south HOBO Pro temperature sensor TF range was between −0.5°C (31.1°F) and −7.5°C (18.5°F) from midday to midnight. The fact that soil temperature does not fluctuate and that there is a lack of cold freezing temperatures in the root zone when compared with aboveground and canopy temperatures explains the lack of cold hardiness induction in roots. Overall, the underground organs are more sensitive to frost than the shoot organs. Root tip low temperature tolerance is between −1°C (30.2°F) and −3°C (26.6°F) (Sakai and Larcher 1987). Depending on the stage of development and species, the permanent roots freeze at −5°C (23°F) to −20°C(−4°F) (Sakai and Larcher 1987). In this study, the minimum root core temperature recorded was nearly −1°C (30.2°F). The location of the sensor in the root core was under mulch, making it less likely that the fine roots in the root zone would be affected by frost. Exposure of the roots to cold temperatures resulted in double the cold hardiness in both roots and shoots in Lonicera tatarica, Cotoneaster horizontalis, and Euonymus europaeus (Pellet 1971). However, this effect was not achieved in Ligustrum obtusifolium. Root cold hardiness is species-specific as a result of different seasons of growth in root and shoot (Sakai and Larcher 1987). Repo et al. (2005), using simulated winter to summer conditions, reported that 2 weeks of delayed soil thawing caused death in the saplings of Pinus sylvestris L., whereas no delay or a short delay caused only minor damage and reversible recovery.
Daily temperature fluctuation (TF) in Fraxinus pennsylvanica var. subintegerrima (red ash) tree and its surrounding environment for the 15th day of each month from January through December 2002. The first graph for each month shows TF in soil surface, sod, mulch, base, and soil 30 cm (12 in). The second graph for each month shows TF in root, trunk, and canopy. The third graph for each month shows TF in north, south, east, west, and canopy.
Daily temperature fluctuation (TF) in Fraxinus pennsylvanica var. subintegerrima (red ash) tree and its surrounding environment for the 15th day of each month from January through mid-February 2003. The first graph for each month shows TF in soil surface, sod, mulch, base, and soil 30 cm. The second graph for each month shows TF in root, trunk, and canopy. The third graph for each month shows TF in north, south, east, west, and canopy.
There were significant temperature differences depending on time of day between the south and west sides compared with the north and east sides. These higher temperatures, during the winter months, have been associated with sunscald, which often results in low temperature-induced cracks on the southwest side of the trunk (Sakai 1966). A higher south side temperature was observed in December and February, but not in January, because we had a very mild January in 2002. This higher peak on the south side also was observed in September and October as well as in December.
When approaching budbreak, bud water content was significantly higher than stem water content (Figure 6). The stem water content and budbreak have a direct relationship; however, root zone temperature and soil moisture also contribute to earlier budbreak, with the exception of genetically controlled species. These TF observations and TFF determinations may be useful for a better understanding of how a tree can sense its surrounding environment and how temperatures fluctuate in and around a tree. This data are also useful for freeze protection as well as horticultural and arboricultural practices in urban environments. More research is needed for understanding the interaction of temperature and other environmental factors such as soil moisture, soil structure, and availability of oxygen in the root zone. Regarding mulch depth, more research is warranted for better landscape management and healthier trees in urban environments.
Percent water content in stems and buds of Fraxinus pennsylvanica var. subintegerrima (red ash) tree. Arrow indicates date of budbreak. Curves were fitted (polynomial third order) through weekly observation from October 2002 to May 2003.
Having higher TF than expected in mulch prompted the reinvestigation of TF with regard to mulch depth. The TF of mulch at different depths is shown in Figure 7. TF varies depending on time of year. In October, 15 cm (6 in) mulch was several degrees warmer (P < 0.05) than ground, 7.5 cm (3 in) mulch, and 30 cm (12 in) mulch. However, in December and February, 30 cm (12 in) mulch was significantly warmer (P < 0.05) (Figure 7). The microbial activity in the mulch may contribute to the higher temperature. Borges and Chaney (1989) indicated that the development and efficiency of mycorrhizal fungi is higher when soil temperature is between 8°C (46.4°F) and 27°C (80.6°F). Our results indicated that the higher the mulch depth, the warmer the temperature. However, overmulching may cause problems in urban landscapes such as adventitious root formation at the base of the trees, which may result in root girdling.
Temperature fluctuation in different mulch depths in four Acer saccharum (sugar maple) during October and December 2004 and February 2005. Mean separation by Duncan’s multiple range test (P < 0.05). Data are means of ≈3,000 observations.
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