Table 1. Sampling days in 2012 and 2014. Days from 0 indicate the number of days after beginning of short day (SD) treatment. The day number from the beginning of the year (DOY, day of the year) indicates the sampling day. It is calculated as the midpoint of the sampling dates of the two study years. Day 0 was the beginning date of SD treatment, which was July 16 in 2014 and July 21 in 2014. Samples were not taken on day 0 in 2014. The degree day (d.d.) temperature sums were calculated with a threshold value of 0 °C from the day of snow melt until the day preceding the sampling day.
DOY Year 2012 Year 2014
Date d.d. Days from 0 Date d.d. Days from 0
198 16 July 995 0 18 July 1121 –3
202 19 July 1037 3 23 July 1227 2
208 26 July 1144 10 28 July 1344 7
211 30 July 1224 14 30 July 1383 9
214 2 Aug 1273 17 1 Aug 1425 11
218 6 Aug 1342 21 6 Aug 1534 16
225 13 Aug 1412 28 13 Aug 1669 23
228 16 Aug 1480 31 15 Aug 1700 25
232 20 Aug 1539 35 20 Aug 1768 30
235 23 Aug 1575 38 22 Aug 1794 32
246 4 Sept 1719 50 1 Sept 1907 42
252 10 Sept 1771 56 8 Sept 1999 49
259 17 Sept 1852 63 15 Sept 2084 56
266 24 Sept 1912 70 22 Sept 2154 63
274  2 Oct 1977 78 29 Sept 2198 70
Table 2. Changes during bud development in the height (mm ± SD) of primordial shoots, whole bud and bud scale complex as well as the most common estimation for bud development phases and stages in springs before short day (SD) treatments in 2012 and 2014. The degree-day (d.d.) temperature sums with a threshold value of 0 °C were calculated from snow-melt each year until the day preceding the sampling day. The development stages are 0: no changes; 1: slightly swollen; 2: swollen, transparent bud scales; 3: bud scales loosened; and 4: bud burst and shoot elongated. Development phases are 0: all needle primordia have rounded tips, and the apex is wholly visible; 1: the tips of the needle primordia are pointed, beginning from the lowest part of the primordial shoot nearly up to the apex; 2: the needle primordia have elongated and are bending over the apex, and the elongation of the primordial shoot has started; 3: the apex can no longer be seen due to the elongation of the primordial shoot and needles; 4: needle primordia and shoot are elongating. Nm = not measured.
Year Date d.d. Height of
primordial shoot 		Height of whole bud Height of bud scale complex Development phase of bud Development stage of bud
2012 24 April 0 1.4 ± 0.1 2.5 ± 0.2 1.1 ± 0.2 1 0
26 April 0 1.4 ± 0.2 2.3 ± 0.2 1.0 ± 0.2 1 0
7 May 60 2.0 ± 0.3 2.5 ± 0.2 0.5 ± 0.2 1 1
14 May 117 2.9 ± 0.4 3.1 ± 0.2 0.2 ± 0.3 4 2
16 May 142 3.6 ± 0.5 3.8 ± 0.4 0.2 ± 0.3 4 3
18 May 173 5.6 ± 0.8 5.6 ± 0.8 0.0 ± 0.0 4 3
2014 24 March nm 1.5 ± 0.1 2.6 ± 0.1 1.1 ± 0.1 0 0
10 April 18 1.6 ± 0.2 2.7 ± 0.2 1.1 ± 0.2 1 1
23 April 83 2.2 ± 0.3 3.0 ± 0.3 0.9 ± 0.2 2 1
8 May 146 3.3 ± 0.5 3.6 ± 0.3 0.3 ± 0.3 3 2
Table 3. Changes during bud development in the height (mm ± SD) of primordial shoots, whole bud and bud scale complex as well as the most common estimation for bud development phases and stages in the spring after previous summer short day (SD) treatments in 2013 and 2015. The degree-day (d.d.) temperature sums with a threshold value of 0 °C were calculated from snow-melt each year until the day preceding the sampling day. The bold text in the table indicates the statistically significant difference between SD and untreated control (CTRL) in a given day (p < 0.05, t-test). The development stages are 0: no changes; 1: slightly swollen; 2: swollen, transparent bud scales; 3: bud scales loosened; and 4: bud burst and shoot elongated. Development phases are 0: all needle primordia have rounded tips, and the apex is wholly visible; 1: the tips of the needle primordia are pointed, beginning from the lowest part of the primordial shoot nearly up to the apex; 2: the needle primordia have elongated and are bending over the apex, and the elongation of the primordial shoot has started; 3: the apex can no longer be seen due to the elongation of the primordial shoot and needles; 4: needle primordia and shoot are elongating.
Height of primordial shoot Height of whole bud Height of bud scale complex Development phase of bud Development stage of bud
Year Date d.d. CTRL SD CTRL SD CTRL SD CTRL SD CTRL SD
2013 6 May 85 1.8 ± 0.2 1.9 ± 0.2 2.8 ± 0.4 2.7 ± 0.3 1.1 ± 0.3 0.7 ± 0.3 2 2 0 0
13 May 142 2.6 ± 0.5 2.7 ± 0.5 3.4 ± 0.5 3.1 ± 0.5 0.8 ± 0.2 0.4 ± 0.3 3 3 1 2
15 May 168 3.4 ± 0.3 3.4 ± 1.0 4.0 ± 0.5 3.8 ± 0.9 0.6 ± 0. 4 0.4 ± 0.3 3 3 1 2
17 May 195 4.5 ± 1.0 4.1 ± 1.0 4.8 ± 0.9 4.3 ± 1.0 0.3 ± 0.2 0.2 ± 0.2 4 4 2 3
20 May 245 7.1 ± 2.0 7.2 ± 1.7 7.3 ± 1.9 7.2 ± 1.6 0.1 ± 0.1 0.0 ± 0.1 4 4 2 4
2015 21 April 18 1.4 ± 0.6 1.4 ± 0.2 2.9 ± 0.4 2.3 ± 0.8 1.5 ± 0.3 0.9 ± 0.6 1 1 0 0
5 May 83 1.7 ± 0.1 1.8 ± 0.4 2.9 ± 0.3 2.5 ± 0.4 1.2 ± 0.4 0.7 ± 0.2 1 1 0 0
10 May 122 1.9 ± 0.3 2.2 ± 0.2 3.1 ± 0.5 2.8 ± 0.5 1.2 ± 0.7 0.5 ± 0.3 2 2 1 1
15 May 162 2.0 ± 0.3 2.8 ± 0.3 3.0 ± 0.2 3.3 ± 0.6 1.0 ± 0.4 0.5 ± 0.2 3 3 1 2
18 May 180 3.0 ± 0.3 3.2 ± 0.3 4.0 ± 0.8 3.5 ± 0.8 1.0 ± 0.5 0.3 ± 0.3 3 3 1 2
20 May 191 3.2 ± 0.3 4.0 ± 0.3 4.0 ± 0.8 4.0 ± 1.0 0.8 ± 0.4 0.1 ± 0.1 3 3 1 2
25 May 242 5.2 ± 0.4 5.1 ± 0.3 5.1 ± 1.0 5.3 ± 1.2 0.1 ± 1.0 0.1 ± 0.3 3 4 2 3
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Fig. 1. Longitudinally cut untreated control (CTRL) (a,b) and short day (SD) treated (c,d) buds sampled on 21 April 2015, as well as bud scales removed from CTRL (e) and SD (f) buds on 22 September 2014. The development phase of the primordial shoots (a,c), as well as the external phenological stage of buds (b,d), is 0 (no changes/all needle primordia have rounded tips and the apex is wholly visible) in both treatments. Observe the higher scale complex (a) and, thus, longer outside measure (b) in CTRL buds compared to those in SD buds (c,d). Instead, the primordial shoots are equally long (a,c). Observe the typical conic shape of the CTRL bud and bud scales with combined needle-scale structure and thorns compared to more rounded shape of SD bud and scales (d,f). The lines on the buds show how the height of the primordial shoot (a) and that of the whole bud (b,d) was measured. Similarly, the lines in (e,f) show how the height was measured from four medium size scales (e,f). Asterisk = apex, bar = 2 mm in each picture.

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Fig. 2. Innermost neo-formed needles around the apex in a control seedling (CTRL) sampled on 23 May 2014 (a). Innermost bud scales around the apex (b,c) and uppermost bud scales (d,e) in early bud formation in CTRL (b,d) and short day (SD) treated (c,e) seedlings sampled on 30 July (SD) and on 4 August (CTRL) 2014. Observe the conic CTRL scales (b, d) and cup-shaped SD scales (c), as well as a combined scale-needle structure of the outermost scales in SD-treated buds (e). The lines in (d) and (e) show how the height of bud scales was measured. Asterisk = apex, bar = 0.2 mm in each picture.

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Fig. 3. Formation of new buds in untreated control (CTRL) and short day (SD) exposed seedlings in summers 2012 and 2014. The number of bud scales (a) was counted only in 2014, whereas number of rows of needle primordia (b), height (mm) of primordial shoot (c) and that of the whole bud (d) were followed in both 2012 and 2014. The calculated height of the bud scale complex is shown in (e). The graphs were started on the day of year (DOY) 198, i.e. some days before the appearance of first forming structures. Observe in (a) that the formation of bud scales was started before the DOY 214, but the scales were not rigid enough to remove until the day 214.

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Fig. 4. Longitudinally cut untreated control (CTRL) bud sampled on 20 May (a,b) and short day (SD) treated (c,d) bud sampled on 18 May, in 2015. The inner development phase in both CTRL and SD buds is the phase 3 (the apex can no longer be seen due to the elongation of the primordial shoot and needles) (a,c), whereas the external phenological stage of the buds is the stage 1 (slightly swollen) in CTRL (b) and the stage 2 (swollen, transparent bud scale) in SD (d) buds. Asterisk = apex, bar = 1 mm in each picture.