Germination ecology in mountain hemlock (Tsuga mertensiana (Bong.) Carr.)

Forest Ecology and Management 144 (2001) 183±188

Germination ecology in mountain hemlock (Tsuga mertensiana (Bong.) Carr.) Y.A. El-Kassabya,*, D.G.W. Edwardsb a

Department of Forest Sciences, Faculty of Forestry, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 b FTB Forest Tree Beginnings, 4018 Cavallin Court, Victoria, British Columbia, Canada V8N 5P9 Received 6 December 1999; accepted 14 February 2000

Abstract The effect of variable heat sums (440, 400, 320 and 2808 h days), expressed as alternating temperatures (25/15, 20/15, 20/10 and 15/108C over 8/16 h (day/night) regimes), and various strati®cation treatments (0, 4, 8 and 12 weeks) and their interactions on germination, were investigated using eight natural-stand seedlots of mountain hemlock (Tsuga mertensiana [Bong.] Carr.). The alternating temperature regimes were obtained using a computerized thermogradient system. Temperature/ heat sum differences and seedlot differences had major impacts on germination. The most rapid and complete germination occurred under the highest two heat sums. Strati®cation showed no effect on germination under any of the experimental conditions. Signi®cant interactions in germination parameters were observed only between temperaturesseedlots. All other interactions were minor and judged to have little biological and/or practical importance. A prerequisite for good natural regeneration depends on the presence of favorable germination environments coupled with good seed crops. These requirements are infrequent, and explain the commonly-observed poor stocking levels achieved by natural regeneration alone in this species. Thus, arti®cial regeneration is recommended to augment natural regeneration to reach acceptable stocking levels and ensure gene pool conservation of mountain hemlock. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Seed germination; Heat sums; Regeneration

1. Introduction Mountain hemlock (Tsuga mertensiana (Bong.) Carr.) is a long-lived, montane conifer usually found on cold, snowy sites. It is most abundant in the subalpine coastal regions of Alaska and British Columbia, but its range extends to central California, with isolated populations found in the interior of British Columbia, Idaho and Montana (Means, 1990). The species is characterized by disjunct populations due the physical separation of the high elevation sites where they occur. * Corresponding author. Fax: ‡250-381-0252. E-mail address: [email protected] (Y.A. El-Kassaby).

Because of this disjunct nature and low frequency in some areas, mountain hemlock has been added to the world list of threatened species (Farjon et al., 1993). In British Columbia, the species is a minor component of the high elevation harvest and is being arti®cially regenerated with an average of 320,000 seedlings planted per year (1995±1999: British Columbia Ministry of Forests, 1999, personal communication). As well as its timber, it is valued for watershed protection and wildlife habitat (Harestad, 1980). As a minor species, little is known about the genetics, physiology and natural regeneration potential of mountain hemlock. A major effort to investigate the genetic diversity, mating system, adaptive and

0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 3 7 0 - 4

184

Y.A. El-Kassaby, D.G.W. Edwards / Forest Ecology and Management 144 (2001) 183±188

quantitative attributes and regeneration (germination) ecology for the purpose of developing a suitable conservation strategy for the species is underway. The present study represents one component of this larger effort. Understanding the species' natural seed production and germination ecology in relation to the microenvironment created by the harvest method will increase the chances for in situ conservation. Mountain hemlock seed production can be abundant on 175±250year-old trees (Means, 1990), but heavy cone crops usually occur only at 3-year intervals, with complete failure in the intervening years (Franklin et al., 1974). Natural regeneration of mountain hemlock is slow on disturbed sites (i.e., after logging) (Means, 1990) and clear cut areas in Oregon have been found with only 60% stocking after 5±11 years (Minore and Dubrasich, 1981). Adequate stocking may not be achieved within 55±88 years (Agee and Smith, 1984). Early seedling development is best in partial shade, but even so it is reported to be slow (Dahms and Franklin, 1965). Thus, the natural regeneration success of mountain hemlock depends on intensity and timing of natural seed production, method and timing of harvesting, and the environmental conditions during germination and early seedling development. It has been shown that mountain hemlock germinates best in the absence of light (Edwards and ElKassaby, 1996) and that the seeds rapidly deteriorate during simulated long-term storage (El-Kassaby and Edwards, 1998). Thus, successful regeneration requires a supply of fresh, viable seeds, favourable germination temperatures and shade for early seedling development. This study was designed to investigate the effects of temperature and strati®cation and their interactions on mountain hemlock seed germination. Eight natural-stand seedlots were used to evaluate a suite of four temperature regimes in combination with four strati®cation durations, chosen to approximate natural germination environments. 2. Materials and methods 2.1. Materials Eight natural-stand bulk seedlots (Table 1) were obtained from the British Columbia Ministry of For-

Table 1 Location of the mountain hemlock seedlots used in the germination ecology study Seedlot No.a

Location

Latitude

Longitude

Elevation (m)

2780 3854 3936 4515 7404 9783 9838 10904

Parksville Hodoo Ck. Wakeman Zeballos Copper Can. Kearsley Ck. Ashlu Ck Sale Mtn.

498160 518200 518100 508100 488560 498190 508010 518100

1248330 1258320 1268250 1268470 1248130 1228220 1238330 1188100

824 1250 1100 700 1100 1280 1000 1700

a

Registration number used by the British Columbia Ministry of Forests.

ests' Tree Seed Centre. Germination tests were carried out on a computerized thermogradient system (Price and Leadem, 1993) consisting of four modules each containing 16 cells arranged in a 44 matrix. This permitted the simultaneous testing of germination under several temperature regimes. Each module could be programmed to accommodate a total of 16 temperature-strati®cation combinations. These combinations comprised: (a) four alternating temperature 8/16 h (day/night) regimes (25/15, 20/15, 20/10 and 15/108C) providing four different daily heat sums (440, 400, 320, and 2808 h above a threshold of 08C); and (b) four strati®cation periods (0, 4, 8, and 12 weeks) at 1±28C. Each treatment±seedlot combination was replicated four times in a factorial arrangement. Thus, the experimental design comprised a total of 128 (448) treatments (temperature±strati®cationseedlot combinations). Each cell accommodated four replications. Replications were of 50 seeds each, for a total of 3200 (44450) seeds per seedlot. Replications within each treatmentseedlot combination were randomly assigned to all cells over the four modules. Seeds were hydrated for 24 h to bring them to full imbibtion, drained, lightly surface dried, and refrigerated at 1±28C for the speci®ed duration. Chilling treatments were scheduled beginning with the 12week strati®cation, so that all germination tests began on the same date. Seeds were counted as germinated when the radicle and hypocotyl equaled or exceeded four times the length of the individual seed. Germinants were assessed at 2-day intervals for the ®rst two weeks, at 3±4 days for the following two weeks, then

Y.A. El-Kassaby, D.G.W. Edwards / Forest Ecology and Management 144 (2001) 183±188

weekly up to 90 days, and were classed as normal or abnormal according to the International Seed Testing Association (1993). 2.2. Statistical analysis Germination was analysed as (i) germination capacity (GC), the percentage of seeds that had germinated normally at the end of the test; (ii) germination speed (R050 ), the number of days for 50% of the germinating seeds to germinate (Thomson and El-Kassaby, 1993), but germination rate (R50), the number of days required for 50% of all (germinating‡non-germinating) seeds to germinate was not calculated because many treatment combinations failed to achieve 50%; (iii) peak value (PV), an index of germination speed that expresses germination rate as the maximal quotient derived by dividing daily the accumulated number of normal germinants by the corresponding number of days (Czabator, 1962); this is the mean daily germination of the most vigorous components of a seedlot, and a mathematical expression of the tangent drawn through the origin of a sigmoid curve representing the typical course of germination, and (iv) germination value (GV), which combines germination capacity and speed into a single index (Czabator, 1962). Germination data were analysed using ANOVA after appropriate data transformation (to normalize the calculated response variables and to achieve

185

homogeneity of variances). Transformations were as follows: GC by arcsine, R050 by (1ÿ[1/x‡1]), GV by p … ‰x ˆ 0:5Š†, where x is the variable in question; PV was not transformed. Transformed data were analyzed using ANOVA based on the following linear model: Yijkl ˆ m ‡ Tempi ‡ Stj ‡ Slk ‡ Temp Stij ‡ Temp Slik ‡ St Sljk ‡ Temp St Slijk ‡ e…ijk†l where Yijkl is the measurement of the lth replication (random effect) in the ith temperature in the jth stratification treatment in the kth seedlot, m the overall mean, Tempi the effect due to the ith temperature (fixed effect), Stj the effect due to the jth stratification treatment (fixed effect), Slk the effect of the kth seedlot (random effect), Temp Stij the effect due to the interaction between the ith temperature and the jth stratification treatment, Temp Slik the effect due to the ith temperature and the kth seedlot interaction, St Sljk the effect due to the jth stratification treatment and the kth seedlot interaction, Temp St Slijk the effect due to the ith temperature, the jth stratification, and the kth seedlot interaction, and e(ijk)l the residual term. 3. Results From the results of ANOVA it was evident that the temperature, strati®cation and population effects impacted germination with various magnitudes (Table 2). Across eight seedlots and four strati®cation

Table 2 ANOVA and percent contribution of the variance components (Var. Comp.) for the germination parametersa of eight mountain hemlock seedlots SOV

d.f.

Temperature (T)

3

Stratification (S)

3

Population (P)

7

TS TP

9 21

SP

21

TSP

63

Residual

511

a

EMSb

Var. Comp.

R050

s2e ‡4 s2TSP ‡16 s2TP ‡yT

yT

93.18

yS

0.49

s2P yTS s2TP

s2e ‡4 s2TSP ‡16 s2SP ‡yS

s2e ‡4 s2TSP ‡16 s2SP ‡16s2TP ‡64s2P

s2e ‡4 s2TSP ‡yrs s2e ‡4 s2TSP ‡16 s2TP s2e ‡4 s2TSP ‡16 s2SP s2e ‡4 s2e

s2TSP

s2SP

s2TSP s2e

GC

GV

PV

9.78

73.64

74.46

0.00

0.22

0.38

0.07

27.11

3.37

1.86

0.50 0.20

0.69 3.76

1.31 2.36

1.96 2.29

0.34

0.00

0.00

0.00

0.06

0.47

0.88

0.90

5.16

58.20

18.20

18.16

Germination parameters are (i) germination speed (R050 ), (ii) germination capacity (GC), (iii) germination value (GV), and (iv) peak value p  (PV). Data transformation are as follows: R050 by (1ÿ[1/x‡1]), GC by arcsin, GV by … ‰x ˆ 0:5Š†, and PV is not transformed. b yP is variance due to temperature (fixed), yS the variance due to stratification (fixed), s2P the variance due to population (random), yTS the variance due to temperaturestratification interaction, s2TP the variance due to temperaturepopulation interaction, s2SP the variance due to stratificationpopulation interaction, s2TSP the variance due to temperaturestratificationpopulation interaction, s2e the residual effect.

186

Y.A. El-Kassaby, D.G.W. Edwards / Forest Ecology and Management 144 (2001) 183±188

periods, temperature (as expressed by heat sums) exerted the greatest effect, accounting for between 10 (GC) to 93% (R050 ) of the observed total variation (Table 2). Germination speed attributes (R050 and PV) responded the most to change in temperature/heat sum. Germination speed (R050 ) steadily declined from 73.8, 44.1, 19.2±15.7 days for 280, 320, 400 and 440 daily heat sums, respectively. Peak values of germination (PV) mirrored R050 , declining from 4.05, 2.54, 1.02±0.91 over the same heat sum range. Germination capacity (GC) was the parameter least effected by heat sum, accounting for 9.78% of the observed total variation (Table 2). Although the effects on GC were signi®cant, differences in total germination were <7% from the highest to the lowest heat sum. Germination value (GV), which is a product of total germination

and germination speed, also was greatly affected by heat sum, accounting for 73.6% of the total observed variation (Table 2), with the same declining order of values with heat sums (3.9, 2.4, 0.9 and 0.8). This impact on GV is largely the result on germination speed rather than germination totality. Across all temperature±strati®cation regimes, seedlot differences (populations) were signi®cant for GC, GV and PV, but not R050 . Seedlot effects were most pronounced for GC, accounting for 27.1% of the total observed variation (Table 2). The totality of germination differences ranged from 90 to 77% among the eight seedlots. Signi®cant effects were observed for GV and PV also (Table 2), but the percent of total observed variation (GV: 3.4 and PV: 1.9) was too small to have any biological and/or practical importance.

Fig. 1. Germination course of seedlot No. 10904 (Sale Mtn.) for (a) unstratified seeds, (b) seeds stratified for 4 weeks, (c) seeds stratified for 8 weeks and (d) seeds stratified for 12 weeks, under four alternating temperature regimes (heat sums).

Y.A. El-Kassaby, D.G.W. Edwards / Forest Ecology and Management 144 (2001) 183±188

Across all temperature±seedlot combinations, strati®cation effects were not signi®cant and accounted for a very small percentage of the total observed variation (range: 0±0.5%) (Table 2). Notable is that unstrati®ed seeds germinated well under the 25/158C (4408 h day) regime. This suggests that seed dormancy in a montane environment would be an added cost to reproductive ®tness. These observations were consistent with earlier work indicating that strati®cation has a slight effect on germination of this species (Edwards and El-Kassaby, 1996). The laboratory data con®rmed that rapid and high germination could be achieved under appropriate temperature/heat sum conditions that differ from the usual prescription (constant 208C, 4808 h day) for this species (International Seed Testing Association, 1993). Of the interactions among the three sources of variation (temperature, strati®cation and seedlots) only temperatureseedlot produced signi®cant effects (Table 2). This was expected because temperature and seedlot effects were themselves signi®cant. However, the percentages of total observed variation (GC: 3.8, GV: 2.4 and PV: 2.3) once more were too small to have any biological and/or practical importance. Graphical comparisons of these three interactions failed to reveal meaningful trends (data not shown). Without exception, the germination course of all eight seedlots over the various temperature/heat sumstrati®cation regimes exhibited similar trends. Germination in seedlot No. 10904, which is a typical representation of all seedlots, is illustrated in Fig. 1a±d. General conclusions can be drawn as follows: 1. irrespective of strati®cation period, including unstrati®ed seeds, germination increased with increasing heat sum; 2. the most rapid germination occurred at the highest two heat sums (440 and 4008 h days); 3. a decrease in heat sum from 400 to 3208 h days caused a major reduction in germination speed; 4. a similar additional reduction in germination speed occurred with a further decrease to 2808 h days; 5. thus, it appears that the threshold heat sum for mountain hemlock germination lies between 320 and 4008 h days and is speculated to be close to 400. In an unrelated study on the effects of temperature cycling within specified heat sums, heat sum per se Ð

187

and not temperature Ð played the overriding role and, as observed here, the effect was almost entirely on germination rate of mountain hemlock and other coniferous seeds (Leadem and Edwards, in preparation). 4. Discussion Data on stand microclimates under which natural germination for mountain hemlock occurs are unavailable. However, in a study of a high elevation coastal site near Campbell River, British Columbia, Benton (1999) estimated and compared soil surface temperatures in old growth and under several harvesting regimes: clear cut; seed tree; patch cut; and shelterwood over a 3-year period (1996±1999). For April he determined that daily accumulated heat sums at the soil surface averaged 4, 26, 78, 20 and 908 h for old growth, clear cut, seed tree, patch cut and shelterwood, respectively. These average heat sums were far below any of the heat sums provided experimentally in this study. Thus, no mountain hemlock germination would be expected. Daily heat sum accumulations for May averaged 59, 216, 317, 250 and 220 for old growth, clear cut, seed tree, patch cut and shelterwood, respectively. Again, these heat sums do not achieve the threshold levels determined experimentally (i.e., between 320 and 4008 h days) for mountain hemlock germination to occur. The daily heat sum accumulations during July were more favourable for germination. Under clear cut, seed tree and patch cut heat sums of 334 (range: 245±427), 458 (range: 422±509) and 365 (range: 336±389), respectively, were found. In contrast, old growth and shelterwood had heat sums of 309 (range: 274±367) and 294 (range: 228±360), respectively, during which some germination might begin. These heat sums and their ranges indicate that mountain hemlock germination will occur during July of certain years when environmental conditions are favourable. In August, all logging methods produced above the minimum daily heat sums required for mountain hemlock to germinate with averages of 335, 319, 372, 350 and 380 for old growth, clear cut, seed tree, patch cut and shelterwood, respectively. Thus, the majority of mountain hemlock germination at high elevation may be expected during the month of

188

Y.A. El-Kassaby, D.G.W. Edwards / Forest Ecology and Management 144 (2001) 183±188

August. However, a marked decrease in daily-accumulated heat sums occurred during September, range 213±280, across all logging treatments. This was followed by even sharper declines, as might be expected, during October (range: 89±137) and November (range: 17±48). Therefore, even if mountain hemlock seed germination is successful during July and August, germinants will encounter rapidly decreasing temperatures during September, October and November, months characterized by snowfall. This clearly indicates that the growing season for such high elevation sites is very compact because germination and early seedling development is delayed, and rapidly followed by bud set. To secure mountain hemlock seed germination and early seedling development leading to adequate stocking, good seed crops associated with favourable microclimates (shade and warm soils) and minimal site disturbances are required. The use of suitable harvesting methods that provide these requirements early in the growing season (i.e. July at high elevation) would favour increased chances for natural regeneration. As previously noted, even under these favourable conditions adequate stocking may take many years. Thus, arti®cial regeneration would appear to be necessary to augment natural regeneration in reaching acceptable stocking levels. The reliance on a combination of natural and arti®cial regeneration, with local seed sources, would better secure conservation of the species' gene pools.

Acknowledgements This study was funded by Forest Renewal British Columbia Grant number HQ96059-RE. Dr. C.L. Leadem (Research Branch, British Columbia Ministry of Forests) provided access to the thermogradient system. R. Benton (Paci®c Forestry Centre, Canadian Forest Service) provided the raw data on which the soil surface heat sums were calculated.

References Agee, J.K., Smith, L., 1984. Subalpine tree reestablishment after fire in the Olympic Mountains, Washington. Ecology 65, 810±819. Benton, R.A., 1999. Impacts of montane alternative silvicultural practices on within stand and soil microclimate/microenvironment. Forest Renewal British Columbia, Lands and Resources Research Programme 1998/1999 Year-end Progress Report, FRBC Ref. No. PA-96574-RE, 9 pp. Czabator, F.J., 1962. Germination value: an index combining speed and completeness of pine seed germination. For. Sci. 8, 386± 396. Dahms, W.G., Franklin, J.F., 1965. Mountain hemlock (Tsuga mertensiana (Bong. Carr.). In: Fowells, H.A. (comp. and Ed.), Silvics of Forest Trees of the United States. USDA, Agric. Handbk. 271, Washington, DC, pp. 712±716. Edwards, D.G.W., El-Kassaby, Y.A., 1996. The effect of stratification and artificial light on the germination of mountain hemlock seeds. Seed Sci. Technol. 24, 225±235. El-Kassaby, Y.A., Edwards, D.G., 1998. Genetic control of germination and the effects of accelerated aging in mountain hemlock seeds and its relevance to gene conservation. For. Ecol. Manage. 112, 203±211. Farjon, A., Page, C.N., Schellevis, N., 1993. A preliminary world list of threatened conifer taxa. Biodiv. Conserv. 2, 304±326. Franklin, J.F., Carkin, R., Booth, J., 1974. Seeding habits of upperslope species. I. A 12-year record of cone production. USDA For. Serv., Res. Note PNW-213. Pacific Northwest Forest and Range Experiment Station, Portland, OR, 12 pp. Harestad, A.S., 1980. Seasonal movement of black-tailed deer on northern Vancouver Island. Dissert. Abstr. Int. 40, 5088±5089B. International Seed Testing Association, 1993. International rules for seed testing. Seed Sci. Technol. 21 (Suppl.), 284. Means, J.E., 1990. Tsuga mertensiana (Bong.) Carr. Mountain hemlock. In: Burns, R.M., Honkala, B.A. (tech. coords.). Silvics of North America. Vol. 1, Conifers. Agric. Handbk. 654. Washington, DC. USDA Forest Service, pp. 623±634. Minore, D., Dubrasich, M.E., 1981. Regeneration after clearcutting of subalpine stands near Windigo Pass, Oregon. J. For. 79, 619± 621. Price, D.T., Leadem, C.L., 1993. A computerized, solid-state, controlled temperature gradient system for determining optimal seed germination temperatures. In: Edwards, D.G.W. (comp. and Ed.), Dormancy and Barriers to Germination, Proc. Internat. Symp. IUFRO Project Group P2.04-00 (Seed Problems), April 1991, Victoria, British Columbia. Forestry Canada, Pacific Forestry Centre, Victoria, BC, pp. 103±112. Thomson, A.J., El-Kassaby, Y.A., 1993. Interpretation of seed germination parameters. New For. 7, 123±132.