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Review article Ecophysiology and field performance of black spruce (Picea mariana): a review MS Lamhamedi PY Bernier Natural Resources Canada, Canadian Forest Service, Quebec Region, 1055 du PEPS, PO Box 3800, Sainte-Foy, Quebec G1V 4C7, Canada (Received 7 January 1994; accepted 5 May 1994) Summary — This paper presents a literature review of black spruce (Picea mariana [Mill] BSP) eco- physiology concerning the response of net photosynthesis and stomata to changes in environmental factors. Current knowledge on root growth, mineral nutrition and response to high temperature, CO 2 enrichment and climate change, frosts, water stress and flooding are also covered. The review ends with an overview of stand establishment and field performance of planted seedlings. The authors highlight the need for research on the long-term effects of multiple stresses, such as climate change and air pollution on the black spruce ecosystem. Picea mariana / ecophysiology / photosynthesis / environmental stress Écophysiologie Résumé — et performances des plants de l’épinette noire. Revue. Cet article pré- sente une revue de littérature de l’écophysiologie de l’épinette noire (Picea mariana [Mill] BSP) met- tant l’accent sur les facteurs environnementaux qui affectent la photosynthèse nette et la réponse des stomates. Cette revue offre une mise à jour sur l’état actuel des connaissances sur la croissance racinaire, sur la nutrition minérale, ainsi que sur la réponse de la plante aux températures élevées, à l’augmentation en CO atmosphérique et aux changements climatiques, aux gels, au stress hydrique, 2 et à l’engorgement des sols. Finalement, l’article rapporte différents résultats concernant la régénération naturelle et la performance des plants de l’épinette noire en site de reboisement. Les auteurs soulignent l’importance de poursuivre les recherches sur les effets à long terme de stress multiples comme la pol- lution de l’air et les changements climatiques sur l’écosystème de la pessière noire. Picea mariana / écophysiologie / photosynthèse / stress environnemental * Correspondence and reprints t Present address: of and Department Forestry, Agronomic Hassan II Institute, 6202, Veterinary Rabat-Instituts, Morocco
As with all plant species, the growth of INTRODUCTION black spruce seedlings or trees is a function of how physiological processes respond to Black spruce, Picea mariana (Mill) BSP, is the physical environment. Knowledge about the principal constituent of the North Amer- such responses is important for the contin- ican boreal forest. Although slow growing, uing improvement of forestry practices in the it is an important source of high-quality fibre boreal forest and for the assessment of the for the Canadian pulp and paper industry. impact of climatic changes that are predicted Its range includes most of Canada and the to take place in that ecosystem. northern United States (fig 1),where it grows Black spruce physiology has been rela- on a wide variety of mineral and organic soils well studied in Canada, with a more tively (Heinselman, 1957; Morgenstern, 1978; limited number of ecophysiological studies Cauboue and Malenfant, 1988; Sims et al, of the species under natural conditions car- 1990). Black spruce is moderately shade ried out in the last few years. To our know- tolerant (Sims et al, 1990) and is less aggres- ledge, the last review on black spruce phys- sive than other boreal species such as bal- iology dates back to the Black Spruce sam fir (Abies balsamea L [Mill]) or white Symposium held in 1975 (Canadian Forestry birch (Betula papyrifera Marsh). It can grow Service, 1975). Although genetic research under conditions of low nutrient availability, has been and is still actively being carried and can therefore outcompete other species out on black spruce, we decided to omit detailed coverage of this topic from our on nutrient-poor sites (Lafond, 1966).
concentration, soil water availability and phe- review. Several studies have reported nology (Kozlowski et al, 1991).Some fac- genetic variations in black spruce regard- tors, such as atmospheric humidity deficit, ing clinal variation (Morgenstern, 1975; affect photosynthesis indirectly through sto- 1978; Fowler and Mullin, 1977; Park and matal effects. Others, like temperature, have Fowler, 1988; Chang and Hanover, 1991), a more direct effect on the biochemistry of characters and foliar flavonoids (Parker cone photosynthesis. However, many factors have al, 1983; Stoehr and Farmer, 1986), et both a direct and an indirect effect, making allozyme variation (Yeh et al, 1986; cause and effect interpretation more uncer- Desponts and Simon, 1987), heterozygos- tain. We have retained 3 factors that act ity (Park and Fowler, 1984), genotypic sta- directly photosynthesis: light, tempera- bility of provenances (Khalil, 1984), inher- on ture and the age of the needles. ent variation in ’free’ growth in relation to number of needles (Pollard and Logan, Measured maximum rates of net photo- 1976), heat tolerance (Colombo et al, 1992) synthesis for black spruce, all units converted and mineral nutrition (Maliondo and Krause, (table I), vary from about 0.03 μmol g (nee- -1 1985; Mullin, 1985). Additional work has dle dry weight) s for trees in the field, to -1 failed to find evidence of ecotypic variation 0.036 μmol g s for seedlings in the field, -1 -1 in black spruce (Wang and Macdonald, to 0.1 μmol g s for seedlings in the -1 -1 1992, 1993; Zine El Abidine, 1993; Zine El greenhouse, to 0.17 μmol g s for-1 -1 Abidine et al, 1994). The reader should refer seedlings in irrigated and fertilized exterior to the specific studies for additional infor- sand beds (table I). Most measurements mation on these topics. Details on the aut- reported here were performed on unshaded ecology and silviculture of black spruce are 1-year-old or current-year needles. given in Black and Bliss (1980), Cauboue and Malenfant (1988), Sims et al (1990) and Jeglum and Kennington (1993). The objective of the current review is to provide an update on research results on the ecophysiology and field performance of black spruce, with an emphasis placed on the regeneration phase. The major topics of this review are the response of net photo- synthesis and stomatal conductance to cer- tain environmental parameters, such as light and temperature. Also covered are transpi- ration, root growth, mineral nutrition, overall responses to specific environmental stresses. The last section covers field per- formance. NET PHOTOSYNTHESIS As in all tree species, the rate of photosyn- thesis in black spruce is influenced by envi- ronmental factors such as light, tempera- 2 CO atmospheric humidity, ture,
(Vowinckel et al, 1975) and on greenhouse as 100 μmol m s has been mea- -2 -1 high seedlings (Black and Bliss, 1980). Vow- sured under warm conditions in actively inckel et al (1975) reported light saturation at growing young stock (table II). Yue and Mar- 1 000 μmol m s for mature trees in the -2 -1 golis (1993) reported a significant effect of field. Work on seedlings under controlled or temperature on this value with measure- semi-controlled conditions has yielded values ments ranging from 5 μmol m s at 5°C to -2 -1 ranging from about 1 000 μmol m s to -2 -1 27 μmol m s at 30°C in rooted black -2 -1 as low as 200 μmol m s for very young -2 -1 cuttings. spruce stock under optimal growth conditions (table II). This variability in response shows that the light response curve of photosynthesis in Temperature black spruce is dependent on the amount of chlorophyll per unit of illuminated leaf Figure 3 show the temperature response of area (Leverenz, 1987). Growth conditions net photosynthesis and dark respiration in evidently play a major role in the level at black spruce. Net photosynthesis stays at which photosynthesis becomes light satu- 90% of optimal or above at temperatures rated. between 15 and 25°C. Zine El Abidine The light compensation point for black (1993) found optimal temperatures for net spruce is reached around 35-50 μmol photosynthesis of around 24 to 27°C for fer- -2 -1 m s although , tilized seedlings in sand beds. High opti- compensation point as a
values can be found in seedlings mum reared under high temperatures (Manley and Ledig, 1979). Although dark respiration decreases with decreasing temperature, cool nights (10 versus 20°C) have been found to reduce overall growth in green- house seedlings (Lord et al, 1993), sug- gesting a carry-over effect of cool tempera- tures either on the photosynthesis apparatus or on the stomata. Age of needles Needle retention on black spruce varies from 5 to 7 years in southerly reaches of the boreal forest in Quebec (CH Ung, Cana- dian Forest Service, Quebec Region, per- sonal communication) to 13 years in cen- tral Alaska (Hom and Oechel, 1983), and up to 30 years under subarctic conditions (Chapin and Van Cleve, 1981).Different needle age classes differ in their photosyn- thetic capacity. Using 14 labelling on whole C branches of P mariana trees of interior Alaska, Hom and Oechel (1983) showed that needles maintained 40% of maximum photosynthetic rate after 13 seasons of growth. The nutrient use efficiency (the amount of CO fixed per unit nutrient con- 2 tent) decreased with needle age and was more pronounced for nitrogen than for phos-
phorus (Hom and Oechel, 1983). The seedlings in exterior sand beds fertilized decrease in the photosynthetic activity of (table I). Stomatal conductance influences older needles has been attributed to net photosynthesis by controlling the amount decreased stomatal and mesophyll con- of CO that can enter the mesophyll. 2 ductances, accumulation of wax in stomatal Recent work with black spruce seedlings cavities, and nonreversible winter chloro- has shown that this effect is not linear, with plast degradation (Jeffre et al, 1971; Lud- stomatal limitation to net photosynthesis low and Jarvis, 1971). Increasing needle becoming important only at low values of longevity appears to maximize the photo- stomatal conductance (Stewart et al, 1994). synthetic return per unit of nutrient invested in the needles (Chapin and Van Cleve, 1981; Hom and Oechel, 1983). Light response In many tree species, maximal stomatal STOMATAL CONDUCTANCE ) s (g conductance is reached when the light level reaches about 10% of full sunlight, or Stomatal conductance is influenced by sev- about 200 μmol m s (Hinckley et al, -2 -1 eral environmental factors, the most impor- Measurements on fertilized black 1978). tant being light, atmospheric humidity deficit, spruce seedlings in outside sand beds needle temperature and soil water avail- (Zine El Abidine, 1993) show near maxi- ability (Grossnickle and Blake, 1986; mum conductance at light levels closer to Roberts and Dumbroff, 1986; Blake and 100 μmol m s The rise in conductance -2 -1 . Sutton, 1987; Zeiger et al, 1987; Gross- with increasing light level is also much more nickle, 1988; Blake et al, 1990; Zine El rapid in black spruce than in either white Abidine, 1993). It was formerly thought that spruce (Picea glauca [Moench] Voss) or these environmental factors controlled sto- jack pine (Pinus bankslana Lamb) (Gross- matal opening solely via hydraulic signals nickle and Blake, 1986), indicating the that could be quantified by measuring the greater shade tolerance of this species. xylem water potential. We now know from interacts with other environmental Light recent research that stomata integrate sig- as well in its influence of the parameters nals from a wider variety of sources, includ- stomata. The slope and maxima of the sto- ing hormonal fluxes from drying roots (Davies matal conductance-light relationship of black and Zhang, 1991),in such a way as to pre- spruce is influenced by atmospheric humid- vent large fluctuations in the plant water sta- ity deficit (Grossnickle and Blake, 1986) and tus (Meinzer and Grantz, 1991). However, soil dryness (Wang and Macdonald, 1993) this expanded view of stomatal function has as these parameters appear to control the yet to shed light on how internal water status maximum value of stomatal conductance. information is translated into stomatal responses, as well as which physical mea- sure of plant water status is most physio- Effect of atmospheric humidity logically significant (Schulte, 1992). Maximum reported values of stomatal The atmospheric humidity deficit, or more conductances to water vapour for black accurately the difference between atmos- spruce, all units converted, range from 0.58 mmol g s for mature trees in the -1 -1 pheric humidity inside the needle and in the field to 1.5 mmol g s for seedlings in the -1 -1 outside air, has a major influence on the field to 3.0 mmol g s for irrigated and -1 -1 stomatal opening of black spruce and other
boreal conifers and Zhang, 1991). Increases in needle ABA (Grossnickle and Blake, 1986). Stomata are usually open under low content in relation to high water stress have humidity deficits and close as the air been negatively correlated with stomatal becomes drier. Reported responses of conductance or transpiration in several tree black spruce stomata are quite variable (eg, species (Blake and Ferrell, 1977; Hinckley et Grossnickle and Blake, 1986; Blake and al, 1978; Newville and Ferrell, 1980; John- Sutton, 1988; Zine El Abidine, 1993), and and Ferrell, 1982; Hogue et al, 1983; son highly dependent other physiological Johnson, 1987), including black spruce on or physical parameters (Blake and Sutton, (Roberts and Dumbroff, 1986). 1988). Overall, however, absolute humidity ABA concentration is sensitive indica- a deficits (AHD) greater than 12-14 g m -3 tor of stress intensity and can reach cause significant closure of the stomata. 3.63 μg g dry weight during severe water -1 stress in black spruce (Roberts and Dum- broff, 1986). Even after rewatering, the Xylem water potential (ψ ) x delay of a few days in the recovery of stom- atal conductance suggests the presence of residual ABA or ABA metabolites in the Under low levels of AHD (2.0-10 g m ), -3 vicinity of the guard cells (Roberts and Dum- stomatal conductance decreases as ψ x broff, 1986; Johnson 1987). Such a residual becomes more negative. At higher AHD effect can be exploited with exogenous ABA. levels, there is little relation between ψ xand Pretreatment of black spruce seedlings with conductance as AHD itself becomes limiting. ABA or synthetic analogs (Blake et al, 1990) In the field, Blake and Sutton (1988) has been shown, through its effect on sto- observed that values of stomatal conduc- matal conductance, to promote more tance in newly planted black spruce declined favourable water potentials, enhanced water rapidly as water potential fell below retention and increased survival after out- -0.5 MPa. Stomatal closure of black spruce planting (Marshall et al, 1991). trees can occur at a ψ of about -1.3 MPa x (Wolff et al, 1977; Grossnickle and Blake, 1986; Blake and Sutton, 1987), although Water stress preconditioning Zine El Abidine (1993) measured stomatal conductance of up to 2.4 mmol g s at -1 -1 that level of ψ In that study, extrapolation . x When subjected to successive episodes of of the boundary line suggests a stomatal water stress, stomata of black spruce closure around -2 MPa. Although they grow seedlings will undergo changes in naturally in moist soils and cool humid boreal behaviour. Zwiazek and Blake (1989) found forests, black spruce seedlings or trees can that water stress preconditioning of black reach a midday xylem water potential of -2 spruce seedlings increased stomatal sen- MPa or lower (Wolff et al, 1977; Bernier, sitivity to subsequent water stress. Zine El 1993; Zine El Abidine, 1993). Abidine (1993), however, found the oppo- site, ie a decrease in stomatal sensitivity to water stress following preconditioning, a Soil drought and growth regulators result similar to what has been found for Douglas-fir (Pseudotsuga menziesii [Mirb] Root tips in drying soils produce abscisic Franco) (van den Driessche, 1991).This acid (ABA), a growth regulator that influ- apparent contradiction in results may stem ences stomatal conductance and regulates from differences in the length or in the inten- different developmental processes (Davies sity of the preconditioning stress, or from
differences in other uncontrolled variables. ROOT GROWTH What is clear, however, is that stomatal mechanisms in black spruce are dynamic general, root growth of black spruce In and are able to acclimate to a changing seedlings is slower than that of other boreal environment. conifers (Grossnickle and Blake, 1986). Mature trees appear to maintain similar char- acteristics: fine root production has been TRANSPIRATION measured at 113 g m for black spruce -2 compared with 366 gm for white spruce -2 (Van Cleve et al, 1983). Root biomass in Transpiration rates of plants are governed by an old black spruce site was estimated at leaf-to-air conductances and humidity gra- 1 230 g m and comprised only 15% of -2 dients, as well as by total leaf area at the total tree biomass (Tryon and Chapin, 1983). plant or canopy level and root-level hydraulic Root growth is usually superficial with long conductances. Current theories suggest that trailing roots progressing at the mineral internal physiological processes link with soil-organic layer interface, or in the sur- external physical processes to regulate face organic layers in organic soils (Sims water loss and plant water status (Meinzer et al, 1990). Mechanical stability of single and Grantz, 1991).Such structural regula- trees is poor (Sims et al, 1990), but that of tion leads to canopy-level values of tran- dense stands is good because of the inter- spiration that appear decoupled from sto- locked architecture of the root system (Smith matal dynamics (Meinzer and Grantz, 1991). et al, 1987). on well-watered black Measurements Root growth declines during the period inside a well-ventilated seedlings spruce of shoot growth, as shoot growth itself uses (minimal boundary-layer resistance) cuvette most of the stored and current photosyn- show maximum rates between 50 and thates. At other times of the year, soil tem- 90 μmol g s (D’Aoust, 1978a; Zine El -1 -1 perature is the major regulator of root growth Abidine, 1993). Midday values from natu- (Lawrence and Oechel, 1983a,b) although ral and planted seedlings on a boreal clear- its effect on growth is more pronounced in cut averaged 20 μmol g s with a maxi- -1 -1 , large roots than in fine ones (Tryon and mum value of 50 μmol g s (PY Bernier, -1 -1 Chapin, 1983). For root diameters ranging unpublished data). We could find no data from 0.5 to 1.5 mm, root growth of black on daily water use by black spruce seedlings spruce reaches its optimum at 20°C and or trees. Our best estimate for seedlings stops when soil temperature drops below based on peak rates cited above would be 5°C (Tryon and Chapin, 1983). Black about 5 g H g d under warm sunny 2 -1 -1 O spruce appears to maintain active root conditions. At the canopy level, Lafleur growth later in the fall in peatlands than east- (1992) measured evapotranspiration rates of ern larch (Larix laricina [DuRoi] K Koch), about 0.1 mm h from a subarctic black -1 although it is unclear whether this difference spruce stand. McCaughey (1978) obtained is due to a greater tolerance to cold tem- peak values of about 1 mm h over a bal- -1 peratures or to flooding (Conlin and Lief- sam fir stand located at a slightly lower ele- fers, 1993). vation than nearby black spruce stands in the Laurentian highlands, north of Quebec Several other factors also affect root can City. On-going experiments under the large- growth of black spruce trees. Prévost and scale BOREAS program (Sellers et al, 1993) Bolghari (1990) found that root penetration should yield values over a broader range decreased with increasing soil bulk densi- of sites and environmental conditions. ties. Bulk densities of 0.85 and 1.05 g cm -3
concentration in needles of the planted deep root penetration, whereas favoured seedlings reflected early dilution in the nutri- densities of 1.25 and 1.45 g cm restricted -3 ent-rich tissues, and, later in the growing root elongation. Bernier (1993) reported season, growth limitation. Nutrient use effi- that, in containerized seedlings planted in ciency of planted seedlings tended to mineral soil, most of the increase in root increase with acclimation to the site. mass during the first field season took place inside the low-density peat plug, with only In the field, growth of black spruce 10% of the new root mass developing out- appears largely N-limited. The cool and side the plug. In forested bogs, rooting humid conditions of the boreal forest, plus depth is strongly correlated with depth to the presence of tanins in the needle litter, water table (Lieffers and Rothwell, 1987). favour the accumulation of organic matter Seed provenance, needle damage, or other and the slow decomposition by soil micro- factors influencing tree vigour also affect organisms (Waring and Schlesinger, 1985). root growth. Root C/N for black spruce stands ranges from 303 to 347 gC/gN (Van Cleve et al, 1981; Auclair and Rencz, 1982). In addi- MINERAL NUTRITION tion, within the boreal forest, black spruce grows on sites with greater nutrient limita- tions than either white spruce or white birch In the nursery, black spruce seedlings (Van Cleve and Harrison, 1985). Site-to-site respond very well to nitrogen fertilization. variations in nitrogenase activity in a sub- Optimal growth of the seedlings has been arctic black spruce forest depend largely on observed at a substrate nitrogen concen- lichens with nitrogen-fixing phycobionts and tration of 250 to 350 ppm (D’Aoust, 1980). on the moss cover (Billington and Alexander, Weekly fertilization of containerized black 1983). Mosses in particular have a high spruce seedlings is usually determined by retention capacity for nutrients, particularly the target biomass. Recommended final phosphorus, and compete effectively with needle concentrations (% oven dry weight) black spruce for that resource (Chapin et for 2-year-old containerized seedlings are al, 1987). 1.61%, 0.27%, and 1.00% in N, P, and K, Minimum crit- Treatments that increase nitrogen avail- respectively (Langlois, 1990). ability in the forest, such as drainage, thin- ical needle concentrations have been esti- ning or fertilization increase the growth of mated at 1.20%, 0.14%, 0.30%, 0.10%, and black spruce. In a 50- to 60-year-old black 0.09%, for N, P, K, Ca, and Mg, respectively spruce stand, the N-fertilization treatments (Morrison, 1974). Increased N supply accompanied by thinning and drainage increases amino-acid concentrations such increased foliar N concentration and con- as proline, glutamine acid, and arginine (Kim tent of current needles (Mugasha et al, et al, 1987). Improved nutritional status 1991).In another trial 15 years after N-fer- through exponential fertilization in the nurs- tilization, the total volume increases ranged ery also increases growth of black spruce from 3 to 9 m for an application of 112 kg 3 seedlings after outplanting (Timmer et al, N/ha and from 11.5 to 12.5 m for 448 kg/ha 3 1991. ) (Weetman et al, 1980). Older needles of P Once outplanted, nursery-grown mariana can act as a sink for nutrient and seedlings must adapt to a much poorer soil carbon storage during nongrowth periods environment. Comparing natural and planted (Chapin and Kedrowski, 1983). black spruce seedlings during 2 growing In nature, black spruce forms mycorrhizal seasons, Munson and Bernier (1993) found associations with several ectomycorrhizal patterns of N, P, and K that the seasonal
gen availability, low nutrient availability, and fungi such as Hebeloma crustuliniforme (Bull low root zone temperature (Van Cleve et al, ex St Am), Laccaria bicolor (Maire) Orton, 1981; Lieffers and Rothwell, 1986). Toler- Hebeloma cylindrosporum Romangnési, and Telephora terrestris Ehrh ex Fr. The ance to flooding and low soil temperatures presence of H crustuliniforme in the rhizo- are ecological characteristics that allow black sphere helps black spruce seedlings use spruce to dominate lowland boreal forests (Crawford, 1976; Larsen, 1982). Studies (Abuzinadah protein nitrogen as a source and Read, 1986). Mycorrhiza also help black examining the tolerance of boreal conifers to spruce compete with the moss cover for flooding show that black spruce seedlings nutrients (Chapin et al, 1987). Inoculation are more tolerant to flooded soils than white of containerized black spruce seedlings with spruce, Sitka spruce (P sitchensis [Bong] L bicolor improves growth when the Carr), Scots pine (P sylvestris L) and Euro- seedlings are supplied with limited amounts pean larch (Larix decidua Mill) (Zinkan et of nitrogen (Gagnon et al, 1988). Short-root al, 1974; Crawford, 1976; Levan and Riha, density of black spruce is also improved by 1986). inoculation with L bicolor, H cylindrospo- Although black spruce is more tolerant rum, and T terrestris (Stein et al, 1990; to flooding than most other boreal conifers, Browning and Whitney, 1991).Changes in its survival and growth are negatively the architecture of root systems by ecto- affected by flooding in peatlands (Payan- mycorrhizal fungi can improve mineral nutri- deh, 1975; Dang and Lieffers, 1989). Root tion and drought tolerance of host plants tips do not survive prolonged flooding and (Lamhamedi etal, 1991, 1992a,b), The show little growth into flooded soil (Levan extramatrical phase of ectomycorrhizal fungi and Riha, 1986), where oxygen concentra- has also been shown to act as a link for car- tions can drop below an apparently critical bohydrate and nutrient transfer between level of 2.0 ppm (Zinkan et al, 1974). Craw- adjacent trees or seedlings of various ford (1976) observed an increase in accu- species (Newman, 1988). Such interplant mulation of ethanol and malic acid in flooded transfers plays a role in the establishment of production of malic acid and tree roots. The black spruce regeneration. the use of starch enable the roots to respire at low oxygen concentrations through gly- colysis (Crawford, 1976). RESPONSES TO ENVIRONMENTAL the diurnal Flooding greatly influences STRESSES of water relations of black spruce. pattern Grossnickle (1987) found reduced diurnal ecosytems, black spruce seedlings In boreal fluctuations of g and ψ in flooded black sx subjected to different environ- trees or are spruce compared with nonflooded seedlings. including flooding, heat mental stresses The reduction in g in response to flooding s stress, water stress, and frost. This section is accompanied by a decrease in photo- looks at whole plant responses to specific synthesis and transpiration (Zaerr, 1983; stresses rather than focussing on a specific Levan and Riha, 1986). The flooding of roots physiological function mechanism. or reduces root hydraulic conductivity, which can increase water stress and xylem injury. Flooding also decreases mineral nutrition Flooding and hormonal levels in trees (Kozlowski and Pallardy, 1979; Kozlowski 1984; Gross- nickle, 1987). Recovery of g after flooding In the boreal forest, flooding imposes a triple s constraint on tree growth, that of low oxy- may take several days (Grossnickle, 1987).
high temperatures (Koppenaal et erance to Drainage of peatlands improves rates of al, 1991). assimilation, foliar nitrogen concentra- net tion, water use efficiency, and mesophyll conductance (g (Macdonald and Lieffers, ) m Water stress 1990). Drainage of peatlands can increase soil temperatures and improve substrate aeration, changes that can influence the Black spruce seedlings are more sensitive to early timing of photosynthetic start-up and water stress than other boreal conifers the growth of trees. Wang and Macdonald and Blake, 1986; Blake and (Grossnickle (1993) found that seedlings grown at low Sutton, 1988; Grossnickle, 1988). In the substrate temperatures (8°C at 5 cm below field, part of this sensitivity is due to the shal- the surface) were smaller and showed lower low and slow-growing root system (Gross- P g and g than those at higher substrate , ns m nickle and Blake, 1986; Bernier, 1993), while temperatures (20°C). another part is related directly to physio- logical processes. Sensitivity to water stress water relations time is not static as over Heat stress components change in concert with shoot phenology both in seedlings and in mature (Colombo, 1987; Zine El Abidine et trees High temperatures at the soil surface can al, 1994). Sensitivity to water stress occur for brief periods on boreal planting increases dramatically from bud break to sites during the summer, reducing physio- the middle of the period of shoot elonga- logical processes of young seedlings and tion, and decreases progressively thereafter possibly causing serious damage (Seidel, (Zine El Abidine et al, 1994). During the 1986; Lopushinsky and Max, 1990). The period of maximum sensitivity, a high evap- exposure of seedlings to high temperatures orative demand can induce turgor loss even causes cell membrane damage, protein and under conditions of high soil water avail- enzyme denaturation and the accumulation ability (Zine El Abidine, 1993). Drought tol- of toxic nitrogenous compounds that can erance mechanisms of black spruce have cause mortality (Stathers and Spittlehouse, been related to the phenological state of the 1990; Colombo et al, 1992). The sensitiv- seedlings (Buxton et al, 1985; Roberts and ity of black spruce seedlings varies with tis- Dumbroff, 1986; Colombo, 1987; Blake et sue age and ontogeny. Current-year shoots al, 1991). are more sensitive than older shoots; actively growing seedlings are more sensi- Maintenance of turgor during drought is tive than dormant ones (Koppenaal and achieved mainly through osmoregulation, Colombo, 1988). The susceptibility of black the passive and sometimes active accu- mulation of osmotically active molecules spruce seedlings to direct and indirect dam- within the cell in response to water stress age increases exponentially with increas- (Turner and Jones, 1980; Morgan, 1984). ing temperature and length of exposure Sugars and amino acids are the major con- (Colombo and Timmer, 1992). The expo- stituents of osmoregulation in expanded sure of plants to high temperatures (47°C leaves of many species with sugars being for 30 min) induces the synthesis of heat apparently dominant in black spruce shock proteins (HSP) which play a role in (Zwiazek and Blake, 1990a; Tan et al, the acquisition of thermotolerance (Colombo 1992a,b). Concentrations of several amino et al, 1992). Preconditioning black spruce acids in the free amino-acid pool also vary seedlings to heat shock (pretreated for 6 d greatly during drying (Cyr et al, 1990). Large at 38°C for 3 h per d) can increase their tol-
increases in the concentration of proline in controls to 0.83 mg g dry weight in treated -1 response to both moderate and severe seedlings. The sterols identified were sito- drought were also observed in black spruce, sterol, stigmasterol and campesterol. suggesting a role for proline in the stress Zwiazek and Blake (1990b) showed that adjustment mechanism protecting enzymes sterol/phospholipid ratios varied markedly from heat denaturation (Paleg et al, 1981; depending on the severity of water stress Cyr et al, 1990). and suggested that the decrease of this ratio during water stress could indicate an The degree of active osmotic adjustment important mechanism contributing to stress is influenced by the degree of stress pre- tolerance. In a later study, these same conditioning. Zwiazek and Blake (1990a) authors showed that the electrolyte leak- found that the osmotic potential of precon- age method detected membrane injury in ditioned black spruce seedlings was lower black spruce with greater sensitivity than than that of unconditioned plants before and other methods such as the electrical during subsequent exposure to osmotic impedance and xylem sap methods stress with polyethylene glycol (PEG). The (Zwiazek and Blake, 1991). difference in osmotic potential between non- stressed preconditioned and unconditioned plants was attributed to an active accumu- Frosts and frost tolerance lation of soluble carbohydrates in the pre- conditioned seedlings. In addition to carbo- hydrate and amino-acid accumulation in Black spruce trees in the boreal forest must response to water stress, Zwiazek and Blake be able to grow in the summer in spite of (1990a) also observed an increase in major occasional radiative frosts, and survive the organic acids in drought-stressed black winter in spite of air temperatures that can spruce. Tan et al (1992a,b) revealed that drop to -40°C or below. Cellular membranes faster- and slower-growing black spruce in black spruce are very permeable and progenies differed in osmotic adjustment permit rapid transport of water out of living and changes of soluble carbohydrates and cells, preventing formation of intracellular amino acids under osmotic stress. Recently, ice crystals (Glerum, 1976). however, Zine El Abidine (1993) found no Although black spruce is capable of tol- such active osmoregulation in his precon- erating some level of frost during the grow- ditioned black spruce seedlings. The use of ing season, photosynthesis is impaired. Field 3 prolonged drying cycles by Zine El Abidine experiments show that air temperatures of (1993) instead of the short preconditioning -4°C kill actively growing shoots and cause with polyethylene glycol (PEG) of Zwiazek large reductions in net photosynthesis and and Blake (1990a) might explain the differ- stomatal conductance in 1-year-old needles; ences in results. PEG can also be absorbed air temperatures of -3°C produce no visi- by treated plants and affect their physio- ble damage and impact only moderately on logical processes (Lawlor, 1970). net photosynthesis and stomatal conduc- Zwiazek and Blake (1990b) found that tance (Dang et al 1991, 1992). Full recovery black spruce seedlings stressed with PEG of the photosynthetic apparatus after a sum- leaked more electrolytes than unstressed mer frost may take several weeks (Pharis plants. Leakage under stress was less in et al, 1970). Recovery of stomatal control preconditioned than in unconditioned after a frost is accelerated by the exposure seedlings. Preconditioning and osmotic of the foliage to direct sunlight through stress also halved sterol concentrations in mechanisms that remain unclear (Dang et the shoots from 1.96 mg g dry weight in -1 al, 1992).
1993), and therefore increased susceptibil- Growth, bud formation and freezing tol- ity to late frosts. Such a problem has in black spruce seedlings are regu- erance prompted a search for rapid evaluators of lated by photoperiod and temperature, as frost hardiness. Frost hardiness has been with most boreal conifers, through the correlated with shoot moisture content involvement of the phytochrome (Pollard (Colombo, 1990; Calmé et al, 1993), mitotic and Logan, 1977; D’Aoust, 1978b, 1981; index of shoot primordia (Colombo et al, D’Aoust and Trudel, 1984; Grossnickle and 1989), and electrical impedance (Glerum, Blake, 1985; D’Aoust and Hubac, 1986; 1973). The relationship between frost tol- Colombo et al, 1989; Bigras and D’Aoust, erance and shoot water content appears to 1992). Generally, the hardening phase of be independent of cultural practices (Calmé shoot components coincides with the accu- et al, 1993). None of these methods has yet mulation of food reserves (Glerum, 1976). emerged as operationally suitable for nurs- Roots, on the other hand, harden only in ery operators. response to low temperature (Bigras and D’Aoust, 1992). Shoots and roots of black spruce differ greatly in their winter frost har- Increasing atmospheric CO 2 diness. For example, Bigras and D’Aoust and climate change (1992) achieved a frost tolerance of -30°C in black spruce shoots, but of only -6.4°C in roots during a controlled experiment. The the effect of elevated Most of the work on limited frost hardiness of roots coupled with CO on black spruce has been done at the 2 the superficial rooting habit of black spruce nursery level, looking at elevated CO as a 2 suggests that this species would be vulner- cultural treatment. In all studies, CO enrich- 2 able to episodes of mid-winter snowmelt fol- ment has resulted in increased needle, stem lowed by severe frosts. and root biomass and nitrogen use efficiency (Bégin, 1986; Campagna and Margolis, influence frost hard- Other factors can 1989; Lord et al, 1993). Increases in growth ening in black spruce. Cold temperatures have been as high as 41 % (Bégin, 1986), during growth reduce needle cuticulariza- although the response of specific morpho- tion during needle maturation (Vanhinsberg logical parameters depends on the devel- and Colombo, 1990), predisposing seedlings opment stage (Campagna and Margolis, to desiccation damage during winter. Alter- 1989). CO enrichment does not appear to and cool temperatures dur- of 2 nances warm affect sugar, starch or total nonstructural ing hardening, the other hand, increase on carbohydrates, suggesting that black spruce frost hardiness in both roots and shoots can maintain a strong sink to avoid accu- (Colombo, 1994). Provenance, soil mois- mulation of nonstructural carbohydrates ture, and plant nutrient levels also appear under high CO concentrations. CO enrich- 2 to influence frost hardiness of black spruce 2 ment has been shown to reduce the late (Pollard and Logan, 1979; Glerum, 1985). summer frost hardiness of black spruce Early and late frosts are one of the lead- seedlings (Margolis and Vézina, 1990), but ing causes of seedling mortality in nurseries, to increase the growth of drought-stressed particularly in containerized production. Even seedlings (Johnsen, 1993). short-day treatments, commonly used in Our knowledge on possible responses nurseries to induce growth cessation, bud of the boreal forest ecosystem to climate setting, and frost hardening (D’Aoust, 1981; warming has been derived mostly from Bigras and D’Aoust, 1992), can result in ear- mathematical modelling under various sce- lier spring dehardening and needle flush narios. In general, climate-change models (Colombo, 1986; Bigras and D’Aoust, 1992,
in the nutrient turnover predict an increase Ideal seedbeds for germination vary rates of the boreal forest (Pastor and Post, according to site moisture conditions (Flem- 1988), the general movement of tree species ing and Mossa, 1994). Germination is gen- towards northern latitudes (Gates, 1990) erally lower on thick organic matter or moss and an increased potential for forest fire cover (St-Pierre et al, 1992; Fleming and (Stocks, 1993). As far as we know, the only Mossa, 1994). During and after germina- field experiment on possible effects of cli- tion, mosses can act as a reservoir for water mate change on the boreal forest has been and nutrients during drought periods, pre- that of Van Cleve et al (1990) who showed venting seedling desiccation (Marek, 1975). that a warming of the root zone by 8-10°C During germination, black spruce is more above ambient temperature in late summer sensitive to water stress than either jack increased the rate of decomposition of the pine or balsam fir (Thomas and Wein, 1985). forest floor and elevated foliar nutrient con- Following initial establishment, young centrations. Predictions of northward move- black spruce trees can differ greatly in ments of species are still affected by an growth and survival as constraints typical apparent lack of knowledge on the exact of boreal sites such as cold, waterlogged nature of factors defining the current range or nutrient-poor soils, and competition from of species such as black spruce (Bonan and other vegetation limit growth. Local micro- Sirois, 1992). Further appreciation of effects climate might also increase the probability of of climate changes will probably come from extreme events such as frosts that can physically-based ecosystem modelling reduce annual growth increments and cumu- (Bonan, 1993). lative growth (Glerum and Paterson, 1989). As with most tree species, final tree vigour and root/stem/needle carbon partitioning in STAND ESTABLISHMENT mature black spruce are closely related to AND FIELD PERFORMANCE site quality (Robichaud and Methven, 1991). One of the major constraints to black Black spruce forests are by and large even- spruce establishment following a perturba- aged, originating from large-scale pertur- tion is competing vegetation. Different types bations such as fires. Over the past few of interactions between black spruce decades, in portions of the boreal forest, seedlings and competitive species have clear-cutting has partially replaced such nat- been demonstrated. Allelopathic effects on ural agents. Natural regeneration of fire-per- germination and root and shoot growth of turbed areas takes place through the ger- black spruce have been shown with the eri- mination of seeds originating from caceous shrub Kalmia angustifolia L (Peter- semi-serotinous cones that have survived 1965; Mallik, 1987), causing son, severe the fire in scorched tree tops (Millar, 1939). forest regeneration problems in Newfound- In clear-cuts, regeneration can take place land. On moist sites, fast-growing species of through seeds rained in from adjacent Sphagnum can smother young seedlings, stands or residual trees, or through the or force them to produce adventitious roots growth of layers. Although black spruce ever higher up their stems (Sims et al, layers have long been considered as ’sec- 1990). Usually, however, shrubs or herba- ond-class’ regeneration, we now know that ceous vegetation reduce the growth and their growth following release from the survival of natural or outplanted seedlings mother tree can be as good as that of through their effects on water relations, gas seedlings (Morin and Gagnon, 1992; Paquin exchange, mineral nutrition, soil and air tem- and Doucet, 1992a,b). perature, and light quantity and quality
The shallow and slow growth of roots of (Brand and Janas, 1988; Brand, 1990, 1991; newly planted black spruce seedlings (Arm- Jobidon, 1992). Light is the dominant fac- son, 1975; Bernier, 1993) coupled with low influencing the performance of outplanted tor soil water availability or stomatal closure at seedlings under competitive black spruce high atmospheric humidity deficits can stress (Brand and Janas, 1988; Jobidon, severely limit water and mineral absorption 1992). Shrubs are usually regarded as more (Grossnickle and Blake, 1986; Blake and serious competitors for light than are herba- Sutton, 1987; Grossnickle, 1988). ceous vegetation, although both types of vegetation use site resources, making them Pre-planting RGC has been correlated unavailable to seedlings. performance of black spruce with the field (Sutton, 1983, 1987), as well as that of other In Canada, the control of competing veg- conifers such as lodgepole pine (Pinus con- etation has been done extensively by using torta Dougl), Douglas-fir, western hemlock glyphosate, a foliage-active herbicide (Arnup and MacKintosh, 1989). The use of herbi- (Tsuga heterophylla [Raf] Sarg), interior spruce (P glauca - P engelmannii complex) cides as a tool for controlling competition has recently come under scrutiny for envi- and Scots pine (Ritchie, 1985; Simpson, ronmental reasons. Different strategies such van den Driessche, 1990; Mattsson, 1991; relation could be found as biological control, integrated silviculture, 1991).However, no companion species and the use of large between RGC and field performance of Nor- seedlings are now under laboratory and way spruce (P abies [L] Karst) (Mattsson, field investigation for improving the field per- 1991). In addition, not all types of RGC formance of planted black spruce stock indices correlate equally well with field per- while minimizing the use of herbicides. formance. Sutton (1987), for example, found poor correlations between field performance Black spruce is widely used in artificial and RGC indices based on new root num- regeneration programs across Canada and bers, and good correlations between field research has gone into relating seedling performance and RGC indices based on quality in the nursery to performance in the mean length of new roots >1 cm. field. Although operational grading of nurs- ery seedlings is usually based on morpho- Although the relation between RGC and logical attributes such as height, root collar performance is conceptually simple, its field diameter, and height/diameter ratio, the application, and the physiological justification relation between such grading and initial for using 1 arbitrary index over the other, or field performance of planted black spruce 1 set of test conditions over the other, even stock is not always clear (Paterson, 1991). appear more uncertain. In general, RGC can be considered an index of seedling Most research into physiological grading of vigour. Final seedling survival and perfor- nursery black spruce seedlings has centered mance will rest, however, on how the on root growth capacity (RGC) (Sutton, 1980, seedling can overcome site-specific stresses 1983,1987,1990; Burdett, 1987). The growth (Grossnickle, 1988), stresses for which RGC of new roots increases the absorption of may or may not be critical. In that sense, water (Colombo and Asselstine, 1989) and the integrated assessment approach devel- thus improves the establishment of black oped by Grossnickle et al (1991) makes spruce seedlings after planting. In the field, ecophysiological sense by recognizing black spruce seedling survival and growth more implicitly that no one single test will ever has been correlated with growth of new roots correlate well with seedling survival and per- (Prévost and Bolghari, 1990). Black spruce broad range of field formance cannot develop new roots rapidly over the con- over a ditions. first growing season, especially in dry sites.
RESEARCH NEEDS in with black spruce of its general, as one components. The boreal forest is a major component of terrestrial ecosystems, and Several lie ahead in the major challenges itscoupling with atmospheric processes is management of boreal forests in general, presently being assessed through BOREAS, and of black spruce stands in particular. The large-scale Canada-US research program a first and most immediate of these is the ade- (Sellers et al, 1993). Although we do not quate regeneration of harvested sites. A know the exact extent of the changes that second one, more long term and hypothet- will take place, we must estimate the effects ical, and yet possibly more taxing on our of such changes on the components of the resources, is the sensitivity of the boreal boreal forest and their repercussions on the forest ecosystem to climatic changes and atmosphere. atmospheric pollution (Harrington, 1987; The second level of research requirements Zayed et al, 1991; Stocks, 1993). created by the current and predicted changes Growth of planted or natural seedlings in atmospheric composition and behaviour is influenced by the presence of woody or is that concerning their impact on forestry herbaceous vegetation. Traditional treat- practices. For example, over 100 million black ments for competition control have relied spruce seedlings were planted in 1992 in the heavily on the application of herbicides, a province of Quebec. Questions regarding practice that has come under intense public the long-term impact of climate change on criticism over the past few years. Better black spruce plantation should be monitored management of black spruce regeneration by establishing field trials. These trials could will require increased knowledge of the make it possible to choose genotypes that medium- and long-term impact of herbicide are best adapted to any changed climatic use on seedling growth, either through direct conditions. Impacts of multiple stresses effects on the plant, or through indirect must be assessed, and improved selection effects such as the impact of herbicides on methods for tolerance or resistance to the microflora and microfauna of the soil. stresses must be designed and imple- Research is also much needed on the pro- mented. posed alternatives to herbicide application. Large planting stock, companion species, biological herbicides, increased reliance on ACKNOWLEDGMENTS advanced regeneration, and all improved silvicultural systems in general are alterna- Postdoctoral support for MS Lamhamedi was pro- tives that will require a greater input of vided by the Natural Sciences and Engineering knowledge for their application. Research Council of Canada and by the Cana- The world’s environment is not static as dian Forest Service, Natural Resources Canada. humans have made a significant impact on The authors thank A D’Aoust for a critical review of the manuscript and P Cheers for her editorial all its components. The atmosphere in par- comments. ticular is being profoundly altered as CO2 concentrations increase beyond recorded levels. Global circulation models, however REFERENCES imperfect, all appear to lead to the conclu- sion that boreal areas will become increas- ingly warmer and drier (IPCC, 1990; Kurz Abuzinadah RA, Read DJ (1986) The role of proteins in the nitrogen nutrition of ectomycorrhizal plants. et al, 1992). Such a prospect creates III. Protein utilization by Betula, Picea and Pinus in research needs at 2 levels. The first con- mycorrhizal association with Hebeloma crustilini- cerns the fate of the boreal forest ecosystem forme. New Phytol 103, 507-514
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