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Original article Analysis of growth and light interception of balsam fir and white birch saplings following precommercial thinning D Pothier A Margolis Centre de Recherche en Biologie Forestière, Faculté de Foresterie et de Géomatique, Université Laval, Ste-Foy, Québec, Canada G1K 7P4, (418) 656-7120 (Received 14 May 1990; accepted 15 January 1990) Summary — A precommercial thinning was conducted on young balsam fir (Abies balsamea (L) Mill) and white birch (Betula papyrifera Marsh) trees. Changes in light environment and growth re- sponse of the trees were followed during the next 2 growing seasons. The relative growth rate (RGR) of thinned balsam firs increased during both the first and the second growing season. This in- in growth was attributed to a greater net assimilation (NAR) which was associated with rate crease a higher level of light availability. Thinning tended to positively affect the RGR of white birch during the first summer following treatment but not during the second growing season. Similar fluctuations were noted for NAR even though light levels remained high for thinned white birch trees during both the first and the second growing season. Balsam fir produced more sapwood per unit of additional leaf area than controls during the first summer following treatment but no differences were observed dur- ing the second. The sapwood area growth to leaf area growth ratios of thinned and control white birches were similar during both the first and the second summer following thinning. Thus the sap- wood area-leaf area relationship appears to be more stable following abrupt changes in environ- mental conditions for the indeterminate growth species, white birch, than for the determinate growth species, balsam fir. growth analysis / thinning / net assimilation rate / area-leaf light efficiency / sapwood use ratio area Résumé — Analyse de la croissance et de l’interception de la lumière du sapin baumier et du bouleau à papier à la suite d’une éclaircie précommerciale. Une éclaircie précommerciale a été réalisée autour de jeunes tiges de sapin baumier (Abies balsamea (L) Mill) et de bouleau à papier (Betula papyrifera Marsh) et leur croissance de même que les changements de conditions lumi- neuses ont été étudiés pendant les deux saisons de croissance suivant le traitement. Le taux de croissance (RGR) des sapins baumiers éclaircis a augmenté pendant les 2 saisons de croissance suivant le traitement. Cette hausse de croissance a résulté d’une augmentation du taux d’assimila- tion net (NAR) qui a été associée à une plus grande disponibilité de la lumière. L’éclaircie a eu ten- dance à affecter positivement le RGR des bouleaux à papier pendant la première année suivant le traitement mais l’effet contraire a été observé pendant la deuxième année. Cette même tendance a aussi été observée pour le NAR quoique le niveau de lumière soit resté plus élevé pour les bouleaux * Correspondence and reprints
à papier éclaircis pendant la deuxième année suivant le traitement. Les sapins baumiers éclaircis ont produit plus d’aubier par unité de croissance en superficie foliaire que les témoins pendant le premier été suivant le traitement, mais aucune différence n’a été observée pendant le second été. La relation entre la croissance en superficie d’aubier et la croissance en superficie foliaire des bouleaux à papier dégagés a été semblable à celle des témoins pendant les 2 saisons de croissance suivant le traite- ment. Il semble donc que, à la suite d’un changement des conditions environnementales, la relation entre la superficie d’aubier et la superficie foliaire des arbres soit plus stable dans le cas de l’espèce à croissance indéterminée, le bouleau à papier, que pour l’espèce à croissance déterminée, le sapin baumier. analyse de croissance / éclaircie / taux d’assimllation net / efficacité d’utilisation de la lumière / relation superf d’aubier-superficie foliaire icie INTRODUCTION (Brand, 1986). This higher light intensity modify leaf morphology by increasing can Intra- and inter-specific competition in the thickness of leaf mesophyll and the dense stands can limit the availability of amount of chlorophyll per unit leaf area environmental resources such as light, wa- which can lead to increased photosynthetic ter, and mineral nutrients. Reducing com- rates (Nygren and Kellomäki, 1983; Oren petition through thinning is a common silvi- et al, 1986). Higher photosynthetic rates cultural practice which increases the per unit leaf area were thought to be re- supply of these environmental resources sponsible for the increased stemwood to selected crop trees. The increased light growth observed soon after thinning of intensity is often associated with a higher Douglas fir (Brix, 1983). The contribution of transpiration rate on a unit leaf area basis net assimilation rate to the growth re- (Black et al, 1980; Whitehead et al, 1984). sponse began to decline 2 yr after treat- Following thinning, this increased water ment, and was gradually replaced during loss per unit leaf area is normally compen- subsequent years by the effect of in- sated for by lower transpiration and rainfall creased foliage biomass (Brix, 1983). interception rates per unit of ground area, The growth response of a tree following often resulting in higher soil water content thinning is determined primarily by its pho- (Whitehead et al, 1984; Aussenac and tosynthetic capacity (Brix, 1983). Conse- Granier, 1988). Nevertheless, if little or no quently, this study aims to examine the re- osmotic adjustment occurs, leaves of sponse of thinned trees in terms of foliage thinned trees can reach zero turgor, a quantity, foliage efficiency and the relation- symptom associated with thinning shock ship between sapwood area and leaf area. (Pothier and Margolis, 1990). Thinning can Since photosynthetic photon flux density also increase the availability of mineral nu- (PPFD) was expected to be the major envi- trients by reducing root competition if ronmental component affecting tree sprouting does not occur. Moreover, addi- it measured around sample growth, was tional nutrients can be released by faster integrated into the trees and then growth litter decomposition caused by the in- analysis. Two common competitors in the crease in temperature (Piene, 1978). forests of eastern Canada, balsam fir Light availability is usually the most im- (Abies balsamea (L) Mill), and white birch portant factor affecting tree growth follow- (Betula papyrifera Marsh) were selected ing removal of competing vegetation for this study.
at least 48 h. Shoot subsamples were also dried MATERIALS AND METHODS for the determination of fresh weight-dry weight conversion factors. The same procedure was used for balsam fir shoots and needles of the Study area previous growing season. Leaves, shoots and stems from the rest of the crown section were weighed separately and another set of subsam- The study took place at Forêt Montmorency ples was taken for determination of fresh (47.3°N 71.2°W) located ≈80 km north of weight-dry weight conversion factors. Further- Québec City, Québec, Canada. Mean annual more, the basal area growth for 1988 and 1989 precipitation is≈1 430 mm,≈ 66% (950 mm) in was calculated for each of the 3 crown sections the form of rain. The mean annual temperature and its percentage in relation to the total basal is 0.3 °C with monthly averages ranging from area was calculated. This factor was applied to -15.8 to 14.8 °C for January and July, respec- the total biomass of the stem and branches of tively. The growing season typically extends each crown section in order to estimate stem from the beginning of June to the end of August. and branch growth for each of the 2 years. A mixed balsam fir-white birch stand was se- lected for study. The stand, established from natural regeneration following a 1975 clearcut, Light measurements was located on an east-facing exposure with a 12% slope. The soil was a well-drained humic orthic podzol derived from till. The stand con- On 5 cloudless days during each growing sea- tained ≈ 30 000 stems/ha in 1987,≈ 70% of son, the percentage of photosynthetic photon which were white birch. At the beginning of the flux density (PPFD) reaching each sample tree experiment, the average diameter at breast was estimated using a Li-Cor quantum sensor height and total height of balsam fir were 1.4 cm (Li-190SB, Li-Cor Ltd, Lincoin, NE, USA). The and 2.2 m, respectively, while the values for quantum sensor, which was connected to an in- white birch were 1.6 cm and 3.3 m, respectively. tegrating millivoltmeter, was uniformly scanned During the autumn of 1987, 5 blocks were up and down and from side to side of the sun- randomly established in the stand. Within each facing side of each sample tree for ≈ 20 s. Just 6 balsam fir and 6 white birch trees block, were prior to each of these measurements, another randomly selected. For 3 trees of each species, integrated measure of PPFD was taken in an all trees within a distance of 1.5 m from a select- opening under full sun. By dividing these 2 val- ed tree were cut down. The other 3 trees of ues, we were able to assess the percentage of each species were left as controls. PPFD reaching our study trees. Photosynthetic photon flux density measurements were taken at randomly selected times between 9:00 and 15:00 for each tree. Thus, these PPFD meas- Biomass measurements urements represent average values from the main part of the day and do not include any At the end of August in 1988 and 1989,1 tree of measurements taken when the sun was near each species-treatment combination was cut per the horizon. block. The 20 trees (2 species x 2 treatments x The total incident PPFD for each growing 5 blocks) were divided by height into 3 crown was estimated from 1-August 31) season (June sections, placed in separate plastic bags, and located in an opening a Bellani pyranometer stored in a refrigerator. site. The daily readings of near the experimental the Bellani pyranometer were corrected for For each crown section, all current year changes in temperature and then transformed shoots and leaves were removed and their fresh into net short-wave radiation (150-4 000 nm) us- weights were determined. Subsamples of≈ 30 ing the equation of Bernier and Plamondon balsam fir needles and 5 white birch leaves (1983). Total PPFD was calculated assuming were then collected, and leaf areas were meas- that 50% of the net short-wave radiation lay ured with a leaf area meter (Model Li-3000, Li- within the waveband 400-700 nm (Hunt et al, Cor Ltd, Lincoln, NE, USA). These subsamples, 1984). and all remaining leaves, were dried at 70 °C for
The product of incident PPFD for each grow- and the percentage of PPFD availa- ing season ble to the trees was taken as an estimate of light availability (I for each sample tree. More- ) o where J is the amount of PPFD energy inter- over, the amount of PPFD intercepted by trees cepted by a plant per unit time (MJ yr Thus, ). -1 (J) was estimated by the product of I times the o I corresponds to the light availability, ie the o projected leaf area of each tree (Hunt et al, amount of incident light per unit area per unit 1984). time (MJ m yr and LUE is the light use effi- -2 -1), ciency ie the amount of wood (g dry weight) pro- duced per unit of intercepted light (g MJ ). -1 Growth analysis Since RGR, NAR and LUE are average values over periods of 1 yr, the product of SLA x LWR x NAR (Eq 1) and I x LUE (Eq 2) only approxi- o Growth analysis was performed using the rela- mate RGR and NAR, respectively. These prod- tive growth rate (RGR) and its classical subdivi- ucts will equal RGR and NAR only if the values sion into specific leaf area (SLA), leaf weight ra- are calculated instantaneously (Radford, 1967). tio (LWR), and net assimilation rate (NAR): Since the values shown in tables I and II are yearly averages, multiplying these average val- ues together does not necessarily give the same result that one would expect from the equations. In fact, fairly large discrepancies did occur. where W is the total dry mass of stemwood and Statistical analysis branches (g), LA is the projected leaf area of the tree (m LW is the leaf weight of the tree ), 2 Data (g), and ∂W/∂t is the instantaneous rate of were subjected to analyses of variance for split-plot design where species was the main change in plant dry mass. The calculations of a plot, and the thinning treatment was the split RGR, SLA, LWR, and NAR were made accord- plot. Least significant differences at the 95% lev- ing to Margolis and Brand (1990). el were computed using the Waller-Duncan mul- These calculations used estimates of LA of tiple comparison test. Logarithmic data transfor- the previous growing season because direct mation was applied when the variances of measurements would have required defoliating groups were found to be proportional to their the trees at the beginning of the experiment. For means. balsam fir, projected leaf area of the previous year was assessed by subtracting the current year leaf area production from the total leaf area RESULTS adjusted to account for the 12.5% turnover of balsam fir foliage per year (Bakusis and Han- sen, 1965). The same procedure was applied Precommercial thinning increased RGR of for the determination of the previous year leaf balsam fir by 43% the first growing season weight. For white birch, the previous year leaf following treatment and 65% the second area was estimated from the sapwood area to leaf area ratios determined for the current year growing season (table I). Thinning did not leaf area and subtracting the current year sap- significantly affect LWR of balsam fir but wood growth. Previous growing season leaf rather resulted in a decreased SLA. Net weight for white birch was computed as the assimilation rate (NAR) was increased product of the previous year leaf area and the 78% and 92% by the treatment during the specific leaf weight (g/m of the control white ) 2 first and the second growing seasons fol- birches only. lowing thinning, respectively (table I). Net assimilation rate (NAR) was further sub- a given amount of leaf area, Thus, for divided into light availability and light use effi- thinned balsam fir produced more wood ciency (Margolis and Brand, 1990):
than controls. The increase in NAR was the second growing season, however, the associated with higher light availability I o LWR of the thinned birches increased sig- (table I), whereas light use efficiency (LUE) nificantly but NAR was no longer statisti- was inversely related to I (fig 1). o cally significant and thinned trees even showed a tendency toward a lower NAR The overall effect of the thinning on the than the controls. The increase in the LWR growth rate of white birch was less clear (table II). Whereas the NAR increased sig- was compensated for by a decrease in the nificantly the first season, the compensat- SLA and thus the RGR of thinned and con- ing decrease in SLA resulted in no signifi- trol birches showed no significant differ- cant differences between RGRs. During (table II). ences
The sapwood area-leaf area ratio of neither balsam fir nor white birch differed between treatments (table III). However, the slope of the sapwood area growth-leaf area growth relation of thinned balsam firs was different from that of controls (P < 0.10) during the first year following treat- ment (fig 2). During this first summer, thinned balsam firs tended to produce more sapwood area per amount of leaf area growth than did control trees. During the second year following thinning, howev- er, both treated and control balsam fir trees had similar slopes for the sapwood area growth to leaf area growth relation- ship (fig 2). On the other hand, thinning did not significantly affect the sapwood area Thinning did not affect the relative growth to leaf area growth relationship of height growth rate of either species (table white birch during either the first or the III). The diameter growth rate of balsam fir second growing season (fig 3) even though there was a significant increase in was stimulated by thinning for both years of growth, but no statistical differences LWR for thinned trees 2 yr after treatment were detected for white birch (table III). (table II).
im- DISCUSSION Consequently, the growing seasons. provement in growth of balsam fir following be attributed to leaves which thinning can The relative growth rate of thinned balsam have higher efficiency for producing a fir was increased during both the first and wood, ie trees with a higher NAR. the second growing seasons following Net assimilation rate can be partitioned treatment (table I). On the other hand, the ratio of photosynthetic to nonphotosynthet- into light availability and light use efficiency (Eq 2). The thinning increased I of balsam ic tissues (LWR) was not affected by treat- o fir 420% and 123% during the first and the ment (table I). Neither were there any sig- nificant differences in total leaf area second growing seasons following treat- ment, respectively (table I). These increas- between treatments during either of the 2
in I probably increased phototsynthetic o to leaf production (table II), to root produc- es by increasing the period of time that rates tion, to respiration or to storage are possi- leaves were ligth-saturated as well as in- ble reasons for the low above-ground creasing PPFD levels when leaves were stemwood growth of the thinned white birches during the second summer after not light-saturated. treatment. Another way in which NAR can be in- creased is by increasing the efficiency of During the first summer following treat- transforming a given unit of light into ment, thinned balsam firs produced more wood. This would be seen as an increased sapwood per unit of additional leaf area LUE at a given value of I Although the than controls (fig 2). This situation prob- . o maximum photosynthetic rate per unit leaf ably occurred because the determinate area can be increased with increasing light growth habit of balsam fir placed restric- levels, it is normally reached below 50% of tions on the number of foliage elements full sunlight for temperate tree species of and thus on the potential leaf area growth small size (Leverenz and Jarvis, 1979; Ny- whereas cambial growth was not restricted gren and Kellomäki, 1983). Since the thin- (Lanner, 1985). Consequently, thinned and ning increased PPFD beyond 80% of full control balsam firs produced similar in- sunlight, a significant part of this energy creases in leaf area during the first grow- was probably not used by the leaves for ing season whereas the thinning increased photosynthesis because factors other than sapwood area growth relative to controls. This led to differences in the sapwood area light availability became limiting. This would explain the curvilinear nature of the growth to leaf area growth relationship (fig relation between LUE and I (fig 1) as well 2). As proposed by Jarvis (1975), a higher o as the lower LUE values calculated for sapwood area growth to leaf area growth thinned trees in comparison to controls (ta- ratio following thinning could potentially re- ble I). duce the resistance to water flow from roots to leaves and thus help trees accli- Thinning tended to positively affect mate to a higher rate of transpiration. RGR of white birch during the first summer Thinned and control balsam fir no long- following treatment whereas the reverse showed different sapwood area growth effect was observed during the second (ta- er to leaf area growth ratios during the sec- ble II). Similar fluctuations were noted in ond growing season following treatment NAR (table II). The decreased RGR and (fig 2). This can be explained by the in- NAR of the thinned white birch during the creased leaf area growth of thinned trees second growing season was accompanied due to favorable light conditions during bud by similar trends in diameter and height formation in the first growing season fol- growth rates (table III). Such growth re- lowing the treatment. Since both sapwood sponses after thinning are often referred to area growth and leaf area growth were as thinning shock (Harrington and Reuke- stimulated by the treatment during this sec- ma, 1983). However, the thinned white ond summer, the relation between them for birches were still able to respond to treat- thinned trees equilibrated with that of the ment by decreasing SLA during both sum- controls (fig 2). mers (table II) and this suggests that their leaves were still more photosynthetically The area growth to leaf area sapwood active than controls (Nygren and Kel- ratio of thinned and control white growth birches were similar during both the first lomäki, 1983; Oren et al, 1986). An in- crease in the allocation of photosynthates and the second growing seasons following
manuscript. This re- comments on the helpful thinning (fig 3). Thus the sapwood area- Natural Science and search supported by was leaf area relationship appeared to be more Engineering Research Council of Canada and stable following abrupt changes in environ- the US National Science Foundation. mental conditions for white birch than for balsam fir. This faster adjustment of white birch following new environmental condi- REFERENCES tions is most likely due to its indeterminate growth habit. Aussenac G, Granier A (1988) Effects of thin- The morphological differences between ning on water stress and growth in Douglas balsam fir and white birch have been fir. Can J For Res 18, 100-105 shown to influence their physiological re- Bakuzis EV, Hansen HL (1965) Balsam fir Abies sponse to precommercial thinning. Pothier balsamea (L) Mill. A Monographic Review. and Margolis (1990) demonstrated that the The University of Minnesota Press, Minneap- deciduous habit of white birch versus the olis, USA evergreen habit of balsam fir can affect Bernier PY, Plamondon AP (1983) Estimating leaf water relations following thinning. net short-wave radiation with the Bellani py- Agric Meteorol 30, 175-184 They also demonstrated that differences in ranometer. the wood anatomy of the 2 species (ie ves- Black TA, Tan CS, Nnyamah JU (1980) Transpi- ration rate of Douglas fir trees in thinned and sels versus tracheids) permitted the main unthinned stands. Can J Soil Sci 60, 625-631 stem of white birch to maintain a sapwood Brand DG (1986) Competition-induced changes permeability = 2 times greater than that of in developmental features of planted Douglas balsam fir (Pothier and Margolis, 1990). fir in southwestern British Columbia. Can J Assuming similar differences in the resis- For Res 16, 191-196 tance to water flow through minor branch- Brix H (1983) Effects of thinning and nitrogen es, white birch would be able to maintain a fertilization on growth of Douglas fir: relative transpiration rate nearly 2 times greater contribution of foliage quantity and efficiency. than balsam fir for the same leaf water po- Can J For Res 13, 167-175 tential. The results reported in this current Harrington CA, Reukema DL (1983) Initial shock paper show that balsam fir responded to and long-term stand development following thinning with greater NAR during both the thinning in a Douglas fir plantation. For Sci first and the second growing seasons fol- 29, 33-46 lowing treatment while white birch re- Hunt R, Warren Wilson J, Hand DW, Sweeney sponded with a greater NAR the first year DG (1984) Integrated analysis of growth and light interception in winter lettuce 1. Analyti- and a greater LWR the second year. Final- cal methods and environmental influences. ly, we suggest that the indeterminate Ann Bot 54, 743-757 growth habit of white birch permitted it to Jarvis PJ (1975) Water transfer in plants. In: reestablish an equilibrium between sap- Heat and Mass Transfer in the Biosphere (Af- wood growth and leaf area growth area gan NH, De Vries, eds) John Wiley and more rapidly than did the determinate Sons, N Y, 369-394 growth species balsam fir. Lanner RM (1985) On the insensitivity of height growth to spacing. For Ecol Manage 13, 143- 148 ACKNOWLEDGMENTS Leverenz JW, Jarvis PG (1979) Photosynthesis in Sitka spruce. VIII. The effects of light flux density and direction on the rate of net photo- We thank Y Gagnon, J Bernier and E Pothier for assistance in the field and the laboratory and T synthesis and the stomatal conductance of Hinckley and one anonymous reviewer for their needles. J Appl Ecol 16, 919-932
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