Temperature regulation of bud-burst phenology within and ... · year. Freezing degree-hours were the summed product of neg- ative degrees; the maximum contribution of any winter day

Tree Physiology 26,42 1-430 O 2006 Heron Publishing-Victoria, Caiznda

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

JOHN D. BAILEY and CONSTANCE A. HARRINGTON~

' Oregon State University, Department of Forest Resources, 280 Peavy Hall, CorvaUis, OR 97331, USA

Corresponding author (john. [email protected]) '

USDA Forest Service, Olympia Forestry Sciences Laboratory, 3625 93rd Avenue SW, Olympia, WA 98512-9193, USA

Received May 18,2005; accepted August 13,2005; published online January 16,2006

Summary 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 "C; once chilled, temperatures > 5 OC 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 arnoun t 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 receiv- ing 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. Wq modified the basic heat-forcing model to initialize, or reset to zero, the accumulation of heat units whenever significant freezing temperature events (2 3 degree-hours day-' < 0 OC) 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 predctions of bud burst. Past models of bud burst have re- lied heavily on data from controlled experiments with s w l e 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 in- terpret the implications of climate-change scenarios. Develop- ing and testing new models will require data from both field and con trolled-etzvironrnen t experiments.

Keywords: bud break, chilling, dormancy, freezing, heat forcing, heat sum, photoperiod.

Introduction

The phenology of bud burst is fundamental to tree survival and growth in temperate and boreal regions of the world (Sakai and Larcher 1987). Early expansion of vegetative tissue is advanta- geous for producing biomass and maintaining site dominance; however, early tissue expansion also increases the risk of frost damage from late-spring freezing temperatures (Heide 2003), and spring frosts are two- to three-times more likely than fall frosts to damage Douglas-fir in the Pacific Northwest (Timrnis et a]. 1994). Plants have evolved mechanisms to use photoperiod and temperature cues to balance the benefits from early bud burst with the probability of significant damage from spring frost (Sakai and Larcher 1987, Hannerz 1999): Changes in microclimate around plants or prevailing landscape climate (TPCC 2001 ) will a1 ter that balance within and among species (Murray et al. 1989, Hiinnhen 1995, Guak et al. 2998).

Standard phenological models for temperate and boreal trees include a minimum chilling requirement during the rest phase of donnancy (late autumn and winter) that leads to full growth competence of buds. This period is followed by peri- ods of heat forcing, during the quiescence phase of dormancy (in early spring), that ultimately produces bud. burst (Fuchi- gami et al. 1982, H t e n 1995, Hamerz 1999, H i n e n and Hari 2002, Tanja et al. 2003). Chuine et al. (1999) and Hbninen and Hari (2002) concluded that there is little hfference in accuracy among these various models, but the best models (1) address chilling and forcing with functions more sophisticated than simple degree-days and (2) consider forcing temperatures sequential to the onset of quiescence, once chilling requirements are fulfilled. Cannell and Smith (1983) and

422 BAILEY AND H

Murray et al. (1989) showed that a heat-forcing function for boreal conifers may need to be modified by the duration of chilling, but the need for this type of model has not been demonstrated for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco).

Wornmack (1960,1964) first demonstrated that Douglas-fir has a chilling requirement, and greenhouse and growth-chamber studies have established that chilling Douglas-fir seedlings for 1200 h at 0 to 5 "C will result in rapid bud burst and proper growth following planting for most seed sources (van den Driessche 1975, 1977, Ritchie 1984, Lavender and Stafford 1985). In the forests of the Pacific Northwest, this chilling requirement is naturally met in most years by early February and can be enhanced for seedlings by artificial cold storage (Ritch- ie 1984, Lavender and Stafford 1985). In the Pacific North- west, there are typically less than 100 h below 0 "C, and such sub-freezing temperatures are less efficient than temperatures of 0-5 "C in meeting the chilling requirements for Douglas-fur (Lavender 1981). Heat forcing of Douglas-fu buds comes from the cumulative heat sum of temperatures > 5 "C (Camp- bell and Sugano 1975, Hiinninen 1995) with bud burst typically occurring in May. Photoperiod has been shown important in growth chamber and greenhouse settings (Heide 1993a, 2003, Partanen et al. 1998), but relatively unimportant in predicting the timing of bud burst in the field (Hiinninen 1995, Chuine et al. 1999, Osbourne et al. 2000, Hiinninen and Hari 2002).

We tested the simple sequential, bud-burst model with Douglas-fir saplings and three natural winter-spring temperature profiles. Trees were in a field study with large, replicated plots that exhibited small but consistent differences in air and soil temperature. TVe hypothesized that: (I) buds of young Douglas-fir saplings in warmer microsites would burst sooner than those in cooler microsites, because they would experience greater heat forcing in the spring after achieving the minimum 1200 h of chilling; and (2) dfferences in timing of bud burst across years would be consistent with such sequential model predictions. We also explored the usefulness of soil temperature in addition to, or instead of, air temperature in predicting the sequence of bud burst within and among years.

Materials and methods

The Fall River Long-Term Site Productivity study area was established in 1999 on a 20-ha, highly productive, low-elevation (300 m), gently-sloping site in the Coast Range Mountains 60 km southwest of Olympia, WA, USA (Terry et al. 2001). Mean annual precipitation is 226 cm falling mostly as rain from September through May; temperatures are mild (9.2 OC mean annual) with a mean January minimum of -0.1 "C (US- DA NRCS 1999). Soils are a deep, well-drained silt loam over silty clay loam (Typic Fulvudand in the Boistfort series) developed from highly weathered basalt with ash influences in the upper horizons (Steinbrenner and Gehrke 1973).

Fall River was established to examine the responses of soil processes, microclimate and Douglas-fir growth to varying amounts of logging slash and residual coarse wood retention

and vegetation control following commercial timber harvest. Harvest treatments in 1999 ranged from a bole-only harvest (small-end log diameter of 10 cm) that left - 130 Mg ha-' of slash and coarse woody debris uniformly scattered across plots, to a total-tree harvest that also included the removal of historic coarse woody residuals, leaving only woody material smaller than 0.6 cm diameter in the plots (< 3 Mg ha- '). Seed- lings from a mixed seed lot of 23 local, half-sib families were raised in a nursery bed for one year and then transplanted and grown a second year, The 2-year-old seedlings were planted at 2.5-m spacing in spring 2000 on 48,0.25- ha plots in a random- ized complete block design (Terry et al. 2001). We sampled the young saplings in 15 plots during three years (200 1,2002 and 2004); the specific plots sampled each year varied slightly based on the needs of other research activities.

Air temperature was monitored 25 cm above the soil surface with temperature probes and HOBO H8 data loggers (Onset Computer Corp., Pocasset, MA) with 4-8 shielded probes in each plot located midway between planted rows of seedlings; the data loggers recorded at 30-minute intervals. Chlling hours were identified as each hour when temperature was in the 0-5 "C range and then summed beginning October 15 each year. Freezing degree-hours were the summed product of negative degrees; the maximum contribution of any winter day was 48 freezing degree-hours, equivalent to 12 h at -4 OC, recorded on March 7,2002. Heat forcing was the summed product of degree-hours above a range of threshold temperatures from 4 to 10 OC (we present results based on 5 "C as the threshold temperature in this paper). We weighted heat-forcing hours with a function that varied from a minumum weight of 0.4 at 5.1 OC to a maximum weight of 1 at 30 "C (after Cleary and Waring 1959). The maximum daily cantribution to heat forcing was 255 degree-hours, equivalent to just over 10 h at or above 30 "C, recorded on April 26, 2004. We also recorded soil temperature at a I O-crn depth with temperature probes and HOBO H8 data loggers logging at 30-minute intervals and cal- culated soil heating above a range of threshold temperatures from 2 to 6 "C unweighted and summed after chilling requirements were fulfilled.

We monitored bud development on 30 to 55 seedlings per plot in the first 2 years, 2001 and 2002, using three codes for bud condition (Table 1). In 2001, seedlings averaged 0.6 cm in basal diameter (measured 15 cm above groundline) and 0.4 m in total height (corresponding values for 2002 were 1.6 cm in diameter and 0.9 m in height). Bud development was not monitored during spring 2003; however, we measured and monitored bud development for a third year (2004) on 40 saplings in each of 13 plots selected to cover the full range of temperatures across the study area. Jn 2004, we used an expanded five-code system (Table 1) more similar to the index used by Murray et al. (1989) and Hannerz (1999). Photo guidelines were used for training and in the field to acheve consistency between two observers, who rotated their observations by plot on a weekly basis and jointly rated 5% of the total sample each week. Saplings averaged 5.4 cm in basal diameter (2.2 cm &- ameter at breast height) and 2.3 m in total height in spring, 2004.

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WITHIN- AND AMONG-YEAR REGULATION OF BUD BURST 423

Table 1. Development codes describing terminal buds of Douglas-fir ary 10,2001 (the accumulation of 1200 h of chilling on aver- saplings at: the Fall River Long-Term Site Productivity study. Integer age), whereas the four warmest plots logged freezing tempera- coding (O,1,2) was used in 2001 and 2002; all five codes were used in mes for only 12.2% of that time period (P = 0.02). The 2004. maximum one-day contribution to freezing (44 degree-hours)

Code Description

0 Retracted, dormant winter bud; dark reddish brown color

0.5 Slightly swollen with some elongation, minor separation of terminal bud scales

1 Elongated, swollen bud with scale separation throughout the bud; lighter color overall

1.5 Extremely-swollen whitish buds, green tissue visible between scales near terminus

2 ' Fully-ruptured bud with new green foliage visible and protruding

We used plot means of temperature, chilling hours, heat sums, bud condilion code at given dates and mean date of terminal bud burst as the response variables for each year and performed simple t tests withln each year, assuming unequal sample size and variance (SAS 8.2, SAS Institute Inc., Cary, NC). We tested for differences in each response variable between the four warmest and four coolest plots (based on air and soil temperature profiies) for each year (the maximum number consistently available) to maintain a balanced design. Plot selection varied slightly among years because of move- ment of sensors to address other objectives of the Fall ~{ver s~udy, occasional failure of sensors and changing microcli- mates as the stand developed; however, five of the eight plots were used every year and the same plot was used from year to year more than 80% of the time. The small but consistent differences in temperature ( w m versus cool) were created by variable amounts of harvest residue present across treatments, variability in landform producing cold area drainages and proximity to neighboring mature stands that cast shade. We present specific information from a single, meman plot (Plot 10) for the study area to illustrate differences across years.

Results

Cool versus warm plots

Lower temperatures were consistently associated with shading from neighboring mature stands along the southern and western edges of the plantation, with a slight depression in the area near the southwest corner and with slash accumulation left by the bole-only removal treatment across the plantation. For example, the lowest recorded air temperature in 2001 (-5.8 "C) was in the bole-only treatment plot at a shaded western edge of the plantation and the average hourly minimum temperature was lower across all bole-only removal treatment plots relative to total-tree harvest plots (P < 0.01). The four coolest plots across the study site, on average, logged 13.9% of total winter hours at or below 0 "C between October 15,2000 and Febru-

was from a bole-only harvest plot in a shaded corner of the study area; the maximum one-day contribution to heat forcing (255 degree-hours) was from a total-tree harvest plot in an unshaded portion of the study area. Overall, chfferences in mean daily temperature between cool and warm plots were typically less than 0.5 "C, difficult to discern (Figure la), and significant at P c 0.05 for no more than about 10 days per month. Only when freezing degree-hours, chilling hours, and heat forcing were accumulated over the winter and spring &d the differences between cool and warm microsites become readily apparent and statistically significant (Table 2, Fig- ures lb and lc).

Contrary to what would have been predicted from the simple sequential heat-forcing model, however, mean terminal bud burst of young Douglas-fir saplings on cool-microsite plots was on average 2 and 3 days earlier, respectively, (P < 0.03) relative to that on warm plots during 2001 and 2004 (Figure 2a). This earlier bud burst occurred despite similar or lower springtime heat forcing (Table 2). Differences in bud burst between the coolest and warmest plots were 3 and 4 days, respectively, in 2001 and 2004. Furthermore, bud developmenl was more advanced in cool plots for 2-3 weeks before mean terminal bud burst in both of those years (Figure 2a). This pattern of bud burst among the four warmest and four coolest plots was consistent in 20bl and 2004 (2001 shown in Fig- ure 2b); there were no differences in either the rate of terminal bud development or ultimate date of mean terminal bud burst in 2002.

Annual variation

There were important differences among the three years of tlus study in terms of temperature profiles and resulting chilling hours and heat forcing (Figure 3). The winter of 2000 through the spring of 2001 approximated the 50-year average for this area (National Weather Service, Silver Spring, MD) and 1200 h of chilling were achieved on February 12,200 1 for the median plot at Fall River (Figure 3a). Winter temperatures were lower before 2002 bud burst and the chilling requirement was fulfilled nine days earlier than the previous year (February 3, 2002). Conversely, winter months before 2004 bud burst were warmer than average and the chilling requirement was not met until nine days later than the average year (February 21, 2004). Theoretically, earlier fulfillment of 1200 chlling hours translates to an earlier start of quiescence and accumulation of heat forcing units; however, the relatively cool temperatures of any February contribute little to heat forcing and all years showed similar heat forcing through much of March (Figure 3b).

Our characterization of 2002 as a cool year (particularly a cold winter) and 2004 as a warm year was supported during the ensuing spring heat-forcing months of each year. March 2002 averaged 2.7 "C cooler than March 2001 and spring tem-

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BAILEY AND HARRINGTON

-Cool plot (23) - - - Warm plot (43)

16 peratures produced an adhtional965 chilling hours beyond the minimum 1200 h (Figure 3a), 5% more than 2001. April 2004 averaged 3.8 "C warmer than April 2001 and spring tem-

12 peratures produced only 545 h of chding beyond the 1200 h requirement (15% less than 2001). However, total heat forcing reflects this cool versus warm year characterization only for 2004 (Table 2). During 2001 (a year with winter and spring

9 8 temperatures close to the 50-year average) plots accumulated on average 3200 weighted degree-hours based on our calculations. The spring months of 2002 had a slightly cooler temper-

4 ature profile, but the length of the heat-forcing period was longer, producing 3800 (+18%) weighted degree-hours despite being a cooler year. During the warm 2004 spring, the same plots totaled 4800 (+5 1%) weighted degree-hours (Fig-

0 ure 3b shows data for Plot 10). I-Feb-01 1 -Apr-Ol 30-May-01 Although 2002 accumulated sigruficantly more heating

units than 2001, mean date of terminal bud burst was 14 days -, b. Chilling

-Cool plot (23) - - - - - Warm plot (43)

later than in the previous year. Clearly, a simple sequential heat-forcing model could not account for these observed differences. Our sequential heat-forcing model was not improved across the years with changes to threshold temperatures (e.g., 5 versus 4.4 "C for chilling) nor by various weighting functions (or unweighted data) in our heat-forcing calculations. Such adjustments produced earliedlater dates to fulfilled chilling requirements and highedlower heat sum values; however, none of these adjustments altered the prediction of earlier bud burst with higher temperatures, which was exactly the op- posite of what we observed.

Soil temperature data, alone or in combination with air temperature, was not useful in predicting date of bud burst. Soil temperatures were correlated with air temperatures; thus, the plots with lower air temperatures were the plots with lower soil

1 -Apr-Ol 30-May-01 temperatures. Like air heat sums, accumulated soil heating at

c. Heat forcing Cool plot (23) - - - - - Warm plot (43)

bud burst was 22% greater in 2002 relative to 2001; thus, soil temperature did not help explain why bud burst was later in 2002 than in 2001. Soil temperatures varied much less than air temperatures and no freezing temperatures were recorded in the soil.

Based on an examination of temperature data across years, we hypothesized that cold weather or freeze events in the spring (after 1200 chilling hours had been achieved) were playing a role. We evaluated the possible effect of these events by initiahzing (i.e., resetting to zero) the sum of heat forcing units under several criteria. There were numerous dates each year when air temperature was slightly below freezing (0.5 "C) for up to 1 h. These minor freeze events &d not seem useful in our modeling exercises because they occurred so frequently. We found, however, that if we used "three or more hours per

I-Feb-01 1 -Apr91 30-May-01 day at least 1 "C below freezing" to define a significant freeze

Date event, and, thus, the criterion for initializing the accumulation of heat forcing, we could reverse the order of predicted bud

Figure 1 . The 2001 air temperature profiles (a), accumulation of chill- burst for 2001 and 2002. The date of the last s i ~ f i c a n t freeze - ing hours (b) and a . . ~ ~ m ~ l a t i o n of heat-forcing units (c) for two con- event varied by more than a month across the three years (Ta-

plots at FallRive~ 23, classified as ble 2). Initializing heat forcing in bud burst acool plot, receives morning shade during the winter and has accumulated harvest residue on the soil surface; Plot 43. a w- lot. is at the same number heat forcing units in 2002 and

I

unshaded with bare soil exposed. Instantaneous differences are minor 2001 (Figure 4) and in the correct sequence relevant to ob- (a) but cumulative differences become significant (b and c). served bud burst. The accumulated number of heat-forcing

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WITKIN- AND AMONG-YEAR REGULATION OF BUD BURST 425

Table 2. Air temperature and bud burst (means with ranges) for four wann and four cool plots at the Fall River study site, western Washington. Temperatures were measured 25 cm above the soil surface. Chilling hours were summed from October 15 to bud burst.,Heat forcing is summed from date of minimum chilling (1200 h) to bud burst as weighted degree-hours adapted from Cleary and Waring (1969). The last freeze event is de- fined as the last date, after 1200 chilling hours have been achieved, with 2 3 degree-hours day- ' below 0 "C.

Warm plots Cool plots P value

Winter-Spriizg 2001 Date 1200 chilling hours achieved February 15 (Feb 14-16) Total chilling hours 2036 (2000-2067) Total heat forcing (degree-hours) 3212 (3143-3349) Date of terminal bud burst May 17 (May 16-19) Date of last freeze event: April 14,2001 (-32 days before bud burst)

Winter-Spring 2002 Date 1200 chilling hours achieved February 4 (Feb 4-5) Total chilling hours 21 58 (2128-2203) Total heat forcing (degree-hours) 3868 (3559-4127) Date of terminal bud burst May 28 (May 25-30) Date of last freeze event: May 9,2002 (-20 days before bud burst)

Winter-Spring 2004 Date 1200 chilling hours achieved February 22 (Feb 20-24) Total chilling hours 1733 (1709-1784) Total heat forcing (degree-hours) 5026 (4828-5179) Date of bud burst May 9 (May 7-9) Date of last freeze event: April 6,2004 (-32 days before bud burst)

February 9 (Feb 5-14) 2 1 13 (2040-2209) 3227 (3088-3398) May 15 (May 14-15)

January 3 1 (Jan 26-Feb 3) 2275 (2233-2352) 3797 (3768-3826) May 29 (May 28-3 1)

February 18 (Feb 15-21) 1774 (1 557-1 807) 4656 (4509-4814) May 6 (May 5-8)

units was much higher at the date of bud burst in 2004 than in the other two years; however, the modified model predicted the sequence of bud burst correctly across the three years.

Differences in photoperiod when various thresholds were reached may be important in determining the rate of development; however, because of differences in temperature patterns among years, the effect of photoperiod is confounded with differences in number of chilling hours and heat-forcing units. Photoperiod on the date of the last freeze event ranged from 13.11 h on April 6,2004 to 14.76 h on May 9,2002. Thus, the photoperiod was 99 minutes longer on the date when we started accumulating heat forcing in 2002 than in 2004. Photo- period at the date of 50 or 100% bud burst differed much less (less than 30 minutes) across the three years.

Discussion

The additional chilling hours associated with cooler microsites apparently caused earlier bud burst in ths young Doug- las-fir plantation for two of our three study years. We had a well-replicated design with dozens of precise temperature sensors and hundreds of young saplings of similar genetic compo- sition, common in industry plantations. Genetic regulation of bud burst has been well documented (Campbell and Sugano 1975, White et al. 1979, Li and Adam 1993, Myking and Hei de 1 995), but our data set contained this genetic variability within the enor structure of the experiment and still yielded significant differences among treatments. There were no photoperiod hfferences among cool and warm microsites withn years and no consistent hfference in foliar nitrogen (N) con- centration between our cool and warm plots (Roberts et al.

2005). Our cooler sites may have had a slightly greater N availability because of leaching from accumulations of logging slash (B. Strahm, data on file, University of Washington) and N has been shown to promote earlier bud burst in Picea abies (L.) Karst. (Flcbistad and Kohmann 2004). However, it seems unlikely that the small hfferences in N concentrations in soil solution would accelerate bud development because foliar N did not hffer significantly between the warmer (open, total-tree harvest) and cooler (partially-shaded, bole-only harvest) plots. We suspect that the modest gain in growing season (2-4 days) resulted from some benefit of adhtional chilling. Active management that fosters cooler microsites (e.g., Lang- vall andLiifvenius 2002), at least in the absence of other major growth-limiting factors, could result in earlier bud burst. Of course, such practices could also increase the risk of spring frost injury by lowering mean temperature in the weeks following bud burst.

Several authors have documented that additional chlling promotes earlier bud burst in greenhouses and growth chambers in several forest species (Cannell and Smith 1983, Murray et al. 1989, Heide 19938, Myking and Heide 1995, Partanen et al. 1998). Van den Driessche (1975,1977) and Xhtchie (I 984) found that additional chilling (more than the minimum 1200 hours) of Douglas-fir seedhngs led to earlier bud burst in controlled-environment studies, particularly with cooler forcing temperatures that more closely simulated those in our field study. In one field experiment on conifer bud burst, Hanninen (1 995) found an unexplained discrepancy in his models exam- ining the effects of major increases in temperature (3-6 "C) on Pinus sylvestris L. saplings. f i s models predicted bud burst -50 days earlier than it actually occurred with such warmer

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426 BAILEY AND HARRJNGTON

8 - Apr 28 - Apr 18 - May 7 - Jun I -Feb 1-Apr 31 -May 2.0 --- -

5000 2004 b. Heat forcing I

1 I 2002

?

I 0

#

Date Day of year

Figure 2. Bud development over successive weeks of observation at Figure 3. Annual differences in: (a) chilling hours above 1200 and (b) Fall River, western Washington. See Table 1 for bud codes. Mean val- heat-forcing degree hours during the quiescent period before date of ues for warm and cool plots are shown (a) for three study years. The bud burst for Plot 10, the median plot at Fall River, western Washing- rate of bud burst in 2001 and 2904 was consistently higher for cooler ton. Heat forcing is the sum of weighted degree-hours (see text for de- plats relative to warmer plots (shown by lines with one standard error tails). The line for each year begins the day 1200 chilling hours is bars placed at monitoring times demonstrating significant differences; achieved and ends when bud burst is complete. n = 4). In 2002, rates of bud burst did not differ significantly between cool and warm plots. Values for individual plots (b) are shown for 2001.

temperatures; it is possible that a lack of complete chilling slowed bud development despite substantial heat forcing, or that photoperiod played an undocumented role in the observed effects. Though his model was inaccurate, major temperature increases still correctly predicted earlier bud burst overall (Hiinninen 3993, a result supported by Osbourne et al. (2000) as well as the results from the third, very warm year (2004) in this study. La~lgvall and Ufvenius (2002) found delayed bud burst under a range of Picea abies shelterwood densities that effectively warmed microsites relative to clearcut areas over

several years following harvest; however, these authors did not present chlling-hour data because of the low chilling requirement of the species. In summary, it will require a more complex model than we currently have to accommodate both the negative influence of slight warming (due to delay in achieving, or to a decrease in, chilling hours) and the positive influence of major warming through increases in heat forcing (Heide 1993b, Osbourne et d. 2000, Pop et al. 2000).

Though 1200 h at 0-5 O C is sufficient for normal bud development in Douglas-fir following planting (van den Driessche 1977, Lavender 198 I), it appears that the upper limit of benefi- cial chilling for coastal Douglas-fir tree buds experiencing nat-


WITJXIN- AND AMONG-YEAR REGULATION OF BUD BURST 427

Day of year

Figure 4. Revised heat forcing trajectories by year at Fall River, westem Washington, based on initializing the accumulation of heat-forcing units whenever a freezing event (> 3 degree-hours day- ' below 0 "C) occurred. The line for each year ends when bud burst was complete, When heat forcing is initialized for each freeze event, it rises most rapidly after the last such event.

ural temperature profiles is actually closer to 2000 h. We documented more rapid bud burst on cooler plots in two years when chilling hours averaged 1750 and 2080 across plots (Ta- ble 2), suggesting an effect of additional chilling up to 880 h beyond the minimum required to break bud dormancy; however, we observed no marginal effect of increasing chilling to 2200 h. These observations are consistent with nursery and greenhouse observations on Douglas-fir seedlings by Ritche (1984), who showed a 2-week decrease in days to bud burst with 2100 versus 1200 chilling hours, and with the observations by van den Driessche (1975) that bud burst at 12.5-13 O C

was slow when Douglas-fir seedlings received 1250 chilling hours and rapid when plants had received 2070 chilling hours. Tile optimum chilling requirement is longer (2856 hours suggested by Wells 1979) for Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca); thus, it may vary with ge- notype or geographic location as well.

Freezing events

unknown. Short-term non-lethal freezing events can s ip f i - cantly impact recovery of photosynthetic capacity in Piaus sylvestris and Picea abies in boreal regions (Htinninen and Hari 2002, Tanja et al. 2003). Rinne et al. (1997) explored the role of abscisic acid in Betula spp. bud burst, Bergervoet et al. (1999) identified the role of microtubule formation in the regulation of growth hormones in Pinus sylvestris, Arnasino (2004) proposed competing enzymes with different Qlo values and several authors have speculated that there is a role for c y t o h n s and gibberellins (Reid and Burrows 1968, Laven- der et al. 1973). Further research on last-freeze initialization under field temperature profiles may provide insight into one or more of these mechanisms. Our last-freeze initialization model consistently places an-

nual bud burst in its observed sequence and roughly aligns the total heat forcing required for bud burst in 2001 and 2002, but it does not explain the shorter, 20day interval to bud burst observed in 2002 (Figure 4). This 40% greater "efficiency" in heat forcing later in the spring after the last freeze may reflect: (1) the advantages of 5% additional chilling, (2) the compres- sion of similar and abundant heat forcing units into a shorter time interval (Figure 4), or (3) some predsposition to more rapid bud development later in the season (e.g., a longer photoperiod or changes in light quality as spring progresses or both). Adjusting the weighting function for heat forcing changed the values of various calculations but did not change the order of predicted bud burst across years, nor the percent differences in amount of heat forcing at time of bud burst. Our addition of last-freeze initialization to the sequential

bud-burst model did not predict or explain the much greater heat forcing associated with bud burst in 2004 (the wannest year). Similar to the model-predicted results in Hanninen (1 993, our heat-forcing function predicted a much earlier bud burst in 2004 than we actually observed (Figure 4). In other words, mean terminal bud burst should have occurred 10 days earlier, near April 28 (at -2000 weighted degree-hours), if bud burst were solely controlled by the amount of heat forcing following the last significant freeze. Again, adjusting the weighting functions for heat forcing uniformly changed values in all years but &d not affect the pattern. Slower bud development in 2004 may be explained by reduced chlling hours or much shorter photoperiod at the time of the last freeze event or bud burst. Simple length of time since the last significant freezing event may also play some role in regulating bud burst. The time between last freezing event and bud burst ranged from 20

Variation in bud burst among years in ths 3-year a p to 32 days (Figure 4), perhaps indicating the existence of a ba-

peared to be regulated by the amount of heat forcing accuinu- sic ~h~siolo@cal recovery period similar to that suggested by lated since the last significant freezing event (> 3 degree-hours Hianinen and Hari (2002) or Tmja et al. (2003)- below freezing per day). Initializing heat forcing units at each such freezing event led to more accurate prediction of bud lempemture

burst between the fmt two very mssimilar thermal years (Fig- Soil temperatures did not improve our models of bud burst. ure 4), and may be the only way to predict the 2-week delay in Soil temperatures were much less variable than air tempera- bud burst during 2002. Such an incorporation of last-freeze tures and &d not fall below the freezing point; thus, they were initialization might explain the anomalous bud burst pattern less effective than air temperatures in predicting bud break. documented by Hannerz (1999). Physiological responses to Cold soil has been shown to delay bud burst of Douglas-fir minor freezing within bud tissues llkely delays phenological (Lavender 1973, Lopushnsky and Max 1990) but there is no development, although the specific mechanism of response is evidence that soil temperature is the primary driver of bud

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428 BALLEY AND HARRLNGTON

burst. Because the viscosity of water is directly proportional to water temperature (Kramer 1934), cold soil will reduce water transport (Carlson and Miller 199 1) and, thus, retard cell elongation; however, there is no evidence that the actual sensing mechanism driving bud burst is outside the buds themselves. Furthermore, soil temperatures did not explain the order of bud burst between microsites or among years in our study or in the study by Tanja et al. (2003).

Climate warming

Any climate warming in temperate and boreal latitudes (IPCC 2001) will lead to winter temperature profiles that provide, on average, fewer chilling hours per year for Douglas-fir in the Pacific Northwest; therefore, trees, especially those at the pe- riphery of the species' range, may experience a less than optimum number of chilling hours for bud development. If temperature variability does not increase, however, the same warmer profiles would produce earlier dates for the last signrf- icant non-lethal freezing temperature and greater cumulative heat forcing after that last freeze. As in the very warm third year of this study (2004), the latter two effects would more than compensate for delayed or reduced cumulative chilling hours, leading overall to earlier bud burst (Hiinninen 1995, Pop et al. 2000). However, current heat-forcing components of standard, sequential bud-burst models consistently over-predict bud bwst and may miss the rank order of bud burst among various climate scenarios as they do not incorporate the effects of freezing temperatures. Additional research with climate- controlled chambers programmed to mimic natural temperature profiles should emble rapid developme~t of more accurate models of bud burst. The importance of additional chilling, timing of freezing events and photoperiod could be evaluated with trials designed to test specific hypotheses.

Whether a warmer climate in temperate and boreal latitudes produces a longer and more productive growing season de- pends on many assumptions about future climate (Spittlehouse 2003) and is well beyond the scope of this paper. However, two phenological issues are worth considering. (1) The 50-year average winter-spring temperature profile for our study area, which is in the central portion of the species' range, results in about 2100 chilling hours, whereas 2004, which averaged 2 OC warmer during the winter and spring, produced only 1745 chlling hours. Another 3 OC increase in temperature will reduce chilling hours to the demonstrated minimum for Douglas-fir (i-e., 1200 chilling hours), below wbch odd growth patterns will likely develop (Lavender 198 1). If insuf- ficient chilling occurs, long photoperiods will presumably trigger bud burst (as demonstrated for Pinus taedu seedlings by Garber 1983). However, it remains to be determined what photoperiod will trjgger bud burst with low chilling accumulation, and whether the timing of that photoperiod will effectively shorten the growing season from its current length. (2) The more erratic weather patterns likely to occur with climate warming (IPCC 2001) could increase the likelihood of lethal freezing events for buds and newly emergent tissues (Hamin-

en 1996), although short photoperiods could retard bud burst beyond the time of such events.

Tree stress, damage and mortality caused by changes in phenology could become as important as pests and fire (Tnnes and Peterson 2003, Spittlehouse 2003), or interact with pests and fire in shaping temperate and boreal ecosystems to match future climates. For example, Clancy et al. (2004) suggested spring phenology was the most important factor associated with stand-level susceptibility of Douglas-fu to western spruce budworm (Choristoneura occidentalis Freeman) because early bud burst in Douglas-fir would coincide with instar emergence. Our study was located on a relatively homogeneous slope in

a simple stand type (plantation) with relatively subtle differences in microclimate within years. Much larger variability in temperature patterns can be anticipated if more complex stand structures are considered and different natural temperature patterns occur; thus, future models of bud burst will need to be more complex. Determining the mechanisms by which trees sense and respond to winter and spring temperatures by exper- imentation under a wide range of natural conditions as used in this study will be fundamental in developing and testing future models.

Acknowledgements

We thank Weyerhaeuser Company for providing access to the study area and data on tree size. Partial funding was provided by the USDA Forest Service-American Forest and Paper Association Agenda 2029 research program and Northern Arizona University.

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Temperature regulation of bud-burst phenology within and ... · year. Freezing degree-hours were the summed product of neg- ative degrees; the maximum contribution of any winter day