seasonal nutrient transfers by foliar resorption, …lowest of 17 forest sites participating in the...

Seasonal nutrient transfers by foliar resorption,leaching, and litter fall in a northern hardwoodforest at Lake Clair Watershed, Quebec, Canada

Louis Duchesne, Rock Ouimet, Claude Camiré, and Daniel Houle

Abstract: A descriptive temporal model was used to evaluate the flow of macronutrients (N, P, K, Ca, and Mg) be-tween the forest canopy and incident precipitation for the Lake Clair Watershed (LCW) located in the northern hard-wood forest region of Quebec, Canada. The model also quantified the resorption mechanism. Wet precipitation,throughfall, foliage, and litter fall data for 1997 were used to quantify the following: (1) dry deposition intercepted byforest cover (0.38, 0.07, 0.07, and 0.03 kg·ha–1 for Ca, K, Mg, and P, respectively); (2) leaching from foliage (1.81,6.46, 0.48, and 0.13 kg·ha–1 for Ca, K, Mg, and P, respectively); and (3) foliar resorption (N = 65%, P = 65%, K =42%, Mg = 30%, and Ca = 10%). Foliar N, P, and K pools increased after bud break and remained constant until mid-September when they decreased rapidly. The foliar Ca pool increased until leaf fall, while the foliar Mg pool reached amaximum in early July and decreased slowly until leaf senescence. Phosphorus, K, Ca, and Mg were leached from thecanopy whereas N from wet precipitation was retained by the canopy. The relatively high Mg and Ca resorption ratesare consistent with the low soil Ca and Mg availability reported at the LCW. Consideration of leaching and dry deposi-tion, as well as the temporal dimension, demonstrated the importance of each of these parameters for increasing the ac-curacy of the foliar nutrient resorption estimates.

Résumé: Un modèle temporel descriptif a été construit afin d’évaluer les principaux flux des macro-nutriments (N, P,K, Ca et Mg) entre le couvert forestier et la précipitation incidente pour le bassin versant du lac Clair, situé dans lazone forestière de feuillus nordiques du Québec, au Canada. Le modèle a aussi permis de quantifier le mécanisme derésorption. Les précipitations sous forme humide, les pluvio-lessivats, le feuillage et la litière ont été utilisés afind’évaluer les paramètres suivants: (1) les dépositions sèches interceptées par le couvert forestier (0,38, 0,07, 0,07, et0,03 kg·ha–1 pour Ca, K, Mg et P respectivement); (2) le lessivage à partir du feuillage (1,81, 6,46, 0,48, et 0,13 kg·ha–1

pour Ca, K, Mg et P respectivement) et (3) la résorption foliaire (N = 65%, P = 65%, K = 42%, Mg = 30% et Ca =10%). Le réservoir du feuillage en N, P et K augmente jusqu’à la mi-septembre pour ensuite diminuer rapidement. Leréservoir en Ca augmente après le débourrement et demeure constant jusqu’à la chute des feuilles alors que celui enMg atteint un maximum au début du mois de juillet et diminue lentement jusqu’à la sénescence des feuilles. Le P, K,Ca et Mg sont lessivés du couvert forestier alors que le N déposé par les précipitations est absorbé par ce dernier. Lestaux relativement élevés de résorption de Mg et Ca, en accord avec la faible disponibilité de Mg et Ca dans le sol dece bassin versant, démontrent l’importance de considérer le lessivage foliaire, les dépôts secs ainsi que la dimensiontemporelle afin d’accroître la précision de l’évaluation de la résorption foliaire.

Duchesne et al. 344

Introduction

Foliar nutrient resorption is an important component ofnutrient cycling dynamics in northern hardwood ecosystems.This type of resorption involves the movement of inorganicor organic substances from senescing leaves and the subse-quent transportation of these substances to surviving tissues

(Killingbeck 1986). This process plays a primary role in nu-trient conservation by deciduous tree species because nutri-ents following this pathway are not lost through litter fall.

Published studies of resorption have assessed the impactof species (Killingbeck 1985; Escudero and del Arco 1987;Jonasson 1990; Escudero et al. 1991), successional stage(Ryan and Bormann 1981; Gholz et al. 1985; Potter et al.1987; Baker and Attiwill 1985; Helmisaari 1995; Lodhiyalet al. 1995), and site characteristics (Fife and Nambiar 1982;Chapin and Kedrowski 1983; Stachurski and Zimka 1975;Staff 1982; Nambiar and Fife 1987; Del Arco et al. 1991) onnutrient fluxes from leaves to twigs. However, few studieshave considered foliar resorption at a stand level in naturalecosystems. These studies have been limited to conifer eco-systems (Helmisaari 1992a, 1995) or sampling during au-tumn senescence (Ryan and Bormann 1981).

Nutrient losses from the foliage pool can arise from threeprincipal pathways: litter fall, leaf leaching, and resorption.The relative size of these fluxes has a direct impact on nutri-

Can. J. For. Res.31: 333–344 (2001) © 2001 NRC Canada

333

DOI: 10.1139/cjfr-31-2-333

Received December 22, 1999. Accepted November 22, 1000.Published on the NRC Research Press website onFebruary 13, 2001.

L. Duchesne, R. Ouimet,1 and D. Houle. Direction de larecherche forestière, Forêt Québec, ministère des Ressourcesnaturelles du Québec, 2700, rue Einstein, Sainte-Foy, QCG1P 3W8, Canada.C. Camiré. Centre de recherche en biologie forestière,Faculté de foresterie et de géomatique, Université Laval,Sainte-Foy, QC G1K 7P4, Canada.

1Corresponding author. e-mail: [email protected]

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Color profile: DisabledComposite Default screen

ent conservation by trees (Chapin 1980). Estimation of leafleaching requires accurate quantification of the interactionstaking place in the tree canopy. Previous studies showed thatan important part of atmospheric inorganic N depositionmay be retained within the forest canopy (Neary and Gizyn1994; Dillon and Molot 1990; Houle et al. 1999a). In con-trast, the flux of base cations (Ca, Mg, and K) is generallymuch higher by throughfall than by incident precipitation.The flux enrichment can originate from washing off of drydeposition from the canopy or from internal leaf leaching. Itis important to distinguish these two sources of enrichmentbecause dry deposition represents an input to the ecosystem,while leaf leaching is an intrasystem transfer resulting in nu-trient losses from the foliage pool. Intensive field measure-ments are necessary to accurately quantify resorption andproduce accurate nutrient budgets.

The soil at the Lake Clair Watershed (LCW) experimentalstation is characterized by low levels of exchangeable Caand Mg, resulting partly from the granitic origin of the localtill and significant soil leaching of these cations (Houle et al.1997). Moreover, Houle et al. (1997) mentioned that the res-ervoir of exchangeable cations at the LCW was one of thelowest of 17 forest sites participating in the “Integrated For-est Study.” The site also receives very low amounts of Cathrough atmospheric deposition. Previous studies haveshown that resorption may be important on any nutrient-poorsite (Stachurski and Zimka 1975). These considerations sug-gest that resorption in the stand of LCW may be an impor-tant mechanism that would minimize nutrient losses.

The main objective of this study was to quantify the an-nual resorption of macronutrients (N, P, K, Ca, and Mg) forthe LCW within the context of an intensive measurementstudy. To reach this aim, the seasonal patterns of foliargrowth, precipitation, throughfall, and litter fall were quanti-fied to assess their influence on seasonal nutrient fluxes inthe resorption process. We evaluated the hypothesis that theresorption of basic cations must be quantitatively important

to minimize nutrients losses given the low availability of ex-changeable basic cations in the soil.

Materials and methods

Study areaThe study was carried out at the Lake Clair Watershed (LCW,

226 ha), 50 km northwest of Québec, Que. (46°57′, 71°40′; 285 ma.s.l.), where the average annual precipitation is 1225 mm and theaverage annual temperature is 3.7°C (data since 1950, ministère del’Environnement du Québec 1990). This site is one of three inten-sively monitored watersheds within the Forest Ecosystem Researchand Monitoring Network («Réseau d’étude et de surveillance desécosystèmes forestiers») in Quebec. The catchment included astudy plot of 0.5 ha used for sampling activities. The underlyingbedrock in the study area consists mainly of gneiss and granite.The sandy loam soil is classified as a Ferro-Humic Podzol, accord-ing to The Canadian System of Soil Classification(AgricultureCanada Expert Committee of Soil Survey 1987). The soil isstrongly acidic (pH CaCl( )2

3.1 for organic layer) and poor in avail-able Ca and Mg (Houle et al. 1997). The stone content lies be-tween 25% (Houle et al. 1997) and 40% (C. Camiré, recentobservations). The stand is uneven aged and dominated by sugarmaple (Acer saccharumMarsh.) in association with yellow birch(Betula alleghaniensisBritt.), and American beech (Fagusgrandifolia Ehrh.) (basal area of 21.4, 3.0, and 3.2 m2·ha–1, respec-tively, for diameter at breast height (DBH) >9.0 cm), with a totalbasal area of 28 m2·ha–1. Dominant and codominant sugar mapletrees are 85–130 years old, with an average height of 20 m andDBH of 28 cm. The watershed was selectively harvested in 1948.Since the early 1980s, there has been evidence of sugar maple de-cline (missing foliage, leaf necrosis, hasty leaf fall) (Gagnon andRoy 1993). The general severity of the maple decline in terms offoliage loss has been estimated to be 16–25% (Gagnon and Roy1993). Beech represents 80% of the regenerating pole size stratum(1.1 cm < DBH < 9.1 cm, 2300 stem/ha, mean height 3.1 m; 1993survey), while sugar maple represents 5% (156 stem/ha), and yel-low birch represents 8% (232 stems/ha).

SamplingFoliage of the three dominant tree species was sampled periodi-

cally throughout the 1997 growing season, using a foliar samplingmethod adapted from Ellis (1975). In early March 1997, 30 treeswere randomly chosen in close vicinity of the study plot andtagged for repeated sampling. Tree selection was based on the rela-tive basal area of tree species in the monitoring plot (20 maples, 5beeches, and 5 birches). Twice a month during the 1997 growingseason (April 15 to October 15), two twigs of about 1.25 cm in di-ameter were collected with telescopic shears from the second orthird quarter of each crown height. All leaves or leaf primordiawere immediately manually stripped from each twig, counted,dried at 60°C, and weighed. Samples from the same tree were thenpooled, grounded to 500µm, and a subsample was taken for chem-ical analysis.

Wet-only and bulk precipitation were collected weekly in 1997at the center of a forest clearing (1 ha wide) located at the catch-ment boundary, using an automatic wet-only collector and 2-Lplastic bottles (n = 2, 5 m apart), fitted with 577-cm2 plastic fun-nels. Throughfall was sampled weekly in the study plot, using 36collectors at 1.5 m height, which were systematically distributedevery 10 m within a 40 m × 90 mgrid (four rows of nine collec-tors). The collectors consisted of 2-L plastic bottles fitted with577-cm2 plastic funnels. During the winter months (January toApril and November to December 1997), bulk precipitation andthroughfall collectors were replaced, respectively, by two and

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334 Can. J. For. Res. Vol. 31, 2001

Fig. 1. Model framework to estimate foliar resorption. Rectan-gles are used to represent stock, and pipes, with spigot and cir-cle attached, represent flow. The flows fill and drain the stocks.The lines with arrows indicate variables used to estimate accre-tion (or resorption) and dry deposition on the canopy surface.



twelve 708-cm2 pails for snow collection. At each sampling date,the throughfall collectors were cleaned with demineralized waterand samples were pooled prior to analysis. A previous studyshowed that these samplers provide interception data that are in ac-cordance with the literature (Houle et al. 1999a). The spatial vari-ability of throughfall in this stand has also been studied (Houle etal. 1999b).

Litter fall was collected biweekly from August to October inclu-sively, with five 1-m2 litter traps randomly distributed in the studyplot. The traps consisted of a nylon mesh screen enclosed by awooden frame (20 cm height to prevent loss of leaves), positioned30 cm above the ground. At each litter sampling, leaves weresorted by species. Leaf dry mass of each species was determinedprior to analysis. Six weeks of data were missing for precipitation(April 22 to May 13, June 16, and October 27) and 8 weeks forthroughfall (April 22 to May 13, June 16, October 27, January 29,and February 4). However, the impact of these missing observa-tions were considered negligible on the evaluation of resorption be-

cause they occurred when foliage was absent or when precipitationevents and subsequent throughfall volumes were insignificant.

Laboratory analysesWater samples were filtered (0.45µm, Nucleopore) prior to the

analyses, which were performed within 1–2 days after collection.Cations were occasionally fixed with ultrapure HNO3 (final con-centration, 0.5%), frozen, and analyzed 1–2 months later. Tissuesamples were digested by the Kjeldahl method (H2SO4 98%). TotalN was measured by flow injection spectrometer (FIA, type LachatAE), and other major elements (Na, P, K, Ca, and Mg) were mea-sured by plasma emission spectrometry (ICP Jarrel Ash). Analysesfor NO3

– were done by ion chromatography (Dionex 2120) andanalyses for NH4

+ were done by colorimetry (Technicon AA2).Certified standard reference (apple leaves No. 1515; pine needlesNo. 1575) were analyzed jointly to independently assess the qual-ity of the analyses; the results were always within 5% of the certi-fied value.

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Duchesne et al. 335

Fig. 2. Foliar nutrient concentrations of sugar maple (dashed line), yellow birch (dotted line), and American beech (dashed and dottedline) during 1997. Vertical lines indicate 2 standard errors of the mean.



EquationsThe canopy is out of reach from the herbivorous mam-

mals, and since 1988 no sign of pest infestation has been re-ported. For these reasons, we assumed that the net flux ofnutrients from foliage to atmosphere, along with losses re-sulting from herbivores and foliar parasites, were negligible.Temporal estimations of nutrient fluxes were realized withthe model shown in Fig. 1. Measured temporal fluxes werewet and bulk precipitation, throughfall, and leaf fall, alongwith change in foliage content. Net canopy exchange, drydeposition, leaf leaching, and nutrient transportation fromstems to leaves (accretion) and from leaves to stems (resorp-tion) were estimated from a set of equations described be-low. Total deposition was the sum of wet and dry deposition.

Net accretion and resorption were calculated as follow:

[1] X X Xt t ttres lit leachleaf= − −∆

whereXtres is the flux of elementX between twigs and fo-

liage at timet (kg·ha–1), ∆leaft is the change in foliar contentof elementX at timet (kg·ha–1), X

tlit is the leaf fall of elementX at timet (kg·ha–1), andX

tleach is the leaching or absorptionof elementX at time t (kg·ha–1).

The change in foliar pool of an elementX at time t wasevaluated by the difference of the canopy foliar pool be-tween two successive samplings:

[2] ∆leaf leaf leaft X Xt t

= −−1

where Xtleaf is the foliar pool of an elementX at time t

(kg·ha–1).The foliar nutrient pools (X

tleaf ) were estimated by sum-ming, across species, the product of the average leaf concen-tration, mean leaf mass, and the total number of leaves forthe species found in the litter traps in 1997:

[3] X X M Nt i t i t i

ileaf leaf leaf leaf= × × ×

=

−∑ ([ ] ), ,

1

31210

where[ ],

Xi tleaf is the mean foliar concentration of elementX

(mg·kg–1) for speciesi (sugar maple,i = 1; yellow birch,i =2; American beech,i = 3) at timet, M

i tleaf ,is the mean leaf

mass (mg/leaf) for speciesi at time t, andNileaf is the mean

total number of leaves found in the litter traps (leaves/ha)for speciesi in 1997.

To evaluate leaf leaching, wet deposition (Xwet) was sub-stracted from total throughfall (Xtf) to obtain the net canopyexchange (Xnce). Secondly, dry deposition was substractedfrom net canopy exchange to obtain leaching fluxes. Usually,the contribution of dry deposition to the net canopy ex-change is estimated by subtracting wet precipitation frombulk precipitation. However, given the surface area of leaves,dry deposition of particulate measures to inert surface mustbe scaled for the full forest canopy. Here, a method based onthroughfall measurements was used. Previously applied bysome researchers (Gosz 1980; Gosz et al. 1983; Ulrich 1983;Beier 1991; Hultberg and Ferm 1995; Houle et al. 1999a),this method uses measurements of Na+ deposition as a modelsubstance for dry deposition of particles:

[4] X Xt

t t

t

tdrytf wet

wetwet

Na Na

Na=

−

whereXtdry is the dry deposition on the canopy surface of el-

ementX at timet (kg·ha–1), Natf tis the total throughfall flux

of sodium at timet (kg·ha–1), Nawettis the wet deposition of

sodium at timet (kg·ha–1), andXtwet is the wet deposition of

elementX at time t (kg·ha–1).In eq. 4, we assumed that foliar leaching of Na was negli-

gible, as confirmed by Houle et al. (1999a), for the stand un-der study. Considering foliage interception and the widevariety of N compound forms in the atmosphere (gaseous,particulate, and dissolved) (Johnson and Lindberg 1992), theX/Na ratio method was not adapted for the evaluation of drydeposition of N. For this reason, bulk-precipitation N wasconsidered as total N deposition.

The stemflow flux was considered negligible in the calcu-lations because it accounted for less than 5% for every ele-ment considered, except for K for which it contributed to anestimated 9% (Houle et al. 1999a). For every calculation, fo-liar and leaf litter samplings were reported on a weekly basisby dividing biweekly results in two equal parts. Subsequently,

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336 Can. J. For. Res. Vol. 31, 2001

Fig. 3. Evolution of foliar nutrient pool on a stand basis during 1997. Vertical lines indicate 2 standard errors of the mean.



weekly results were linked to precipitation and throughfallvolume to evaluate all parameters on a 1-week time step.

Annual budget and error estimationThe annual budget was determined by summing weekly

data. The error associated with foliar sampling was assessedby evaluating the standard error of each sampling mean, andthen by integration of the standard error in the calculationfor each variable for every time step. This was done by re-specting variance properties associated with the above equa-tions. No error estimation could be applied to the single wetcollector. Error estimation associated with weekly through-fall data was obtained from Houle et al. (1999b). Finally, er-ror estimates were calculated for the entire year with respectto variance properties (Zar 1974).

Results

Foliar nutrient concentrationsSugar maple, yellow birch, and American beech followed

similar seasonal patterns of foliar nutrient concentrations(Fig. 2). Nitrogen, P, K, and Mg concentrations reached amaximum at the end of May and slowly decreased until theend of the growing season. In contrast, foliar Ca concentra-tions reached a minimum early in spring and increased untilleaf senescence. Concentrations of basic cations (K, Ca, andMg) were highest in yellow birch.

Seasonal variation in the stand was similar for foliar N, P,and K pools (Fig. 3). In 1997, buds began to break on theweek of May 14, followed by a rapid increase of leaf nutri-ents until the mid-June, which then remained relatively con-

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Duchesne et al. 337

Fig. 4. Nutrient contribution by leaf fall of sugar maple (dashed line), yellow birch (dotted line), and American beech (dashed and dot-ted line) during 1997.



stant during the growing season, and decreased rapidly afterSeptember 17. The stand foliar Ca pool increased until theend of the season, whereas the Mg pool reached a maximumearly on the week of June 25, and then decreased slowly un-til leaf senescence.

Leaf litter fallLitter fall began in August and was essentially completed

by October 22 (Fig. 4). Although abscission of maple leavesbegan earlier than for the other species, the process for alltree species ended at the same time at the end of October1997. The greatest proportion of leaf fall occurred within2 weeks, between October 2 and 20, during which the per-centage of total foliar litter that reached the ground was 61%for maple, 90% for beech, and 80% for birch. Total leaf lit-ter fall was 2.95 Mg·ha–1 (Table 1). Twenty-six percent ofthe the leaves were sugar maple, but this represented 41% oftotal leaf biomass. Seasonal trends in leaf litter fall in 1997were similar to those from 1990 to 1994 (Fig. 5).

Precipitation and net canopy exchangeFluxes of P, K, Ca, and Mg in throughfall were greater

than those in wet precipitation, particularly from June to Oc-tober when foliage was present (Fig. 6). In contrast, therewas 28% less N in throughfall than in wet precipitation dur-ing this period.

Temporal trends of estimated dry deposition and leafleaching or absorption for N, P, K, Ca, and Mg are shown inFig. 7. The calculations indicate that dry deposition of theseelements occurred throughout the year, and even during win-

ter. On an annual basis, about 30% of the estimated total de-position of these elements (except N) was attributed to drydeposition.

Based on the difference between total deposition andthroughfall fluxes, P, K, Ca, and Mg nutrients were leachedfrom foliage whereas N was retained by the foliar canopy(Fig. 7). A small enrichment of N in throughfall was ob-served only when low precipitation events occurred duringthe summer. Leaching and interception processes took placelargely during the growing season when foliage was present,despite evidence of leaching during the winter months. Onan annual basis, 57, 96, 59, and 68% of the total throughfallflux of P, K, Ca, and Mg, respectively, originated from can-opy leaching. On an annual basis the canopy intercepted14% of the N deposition, which is a conservative estimateconsidering that dry deposition is not accounted for in thisestimation.

Based on the comparison of bulk and wet-only precipita-tion data, the interception of dry deposition by bulk collec-tors did not have a major influence onX/Na element ratios(Fig. 8), as illustrated by the proximity of the linear regres-sion line to the 1:1 line. Because of the lower ratios mea-sured in bulk precipitation as compared with the ratiosobserved in wet-only collectors, the P, Ca, and Mg dry depo-sitions were slightly overestimated. In contrast, dry deposi-tion of K was slightly underestimated.

Foliar accretion and resorptionWeekly nutrient fluxes between leaves and twigs are illus-

trated in Fig. 9. At the beginning of the growing season, nu-

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338 Can. J. For. Res. Vol. 31, 2001

Fig. 5. Annual trends in cumulative leaf litter fall from 1990 to 1994 and in 1997 for the Lake Clair Watershed.

Species Leaves/ha Dry mass (kg·ha–1) N (kg·ha–1) P (kg·ha–1) K (kg·ha–1) Ca (kg·ha–1) Mg (kg·ha–1)

Sugar maple 4 740 (975) 1220 (4.1) 10.84 (0.22) 0.62 (0.02) 4.81 (0.12) 6.41 (0.11) 0.72 (0.02)American beech 7 050 (215) 840 (6.1) 6.96 (0.19) 0.49 (0.02) 3.85 (0.11) 5.71 (0.22) 0.83 (0.02)Yellow birch 6 160 (627) 890 (6.1) 8.43 (0.62) 0.66 (0.05) 3.61 (0.23) 6.71 (0.52) 1.07 (0.08)Total 17 950 (1 179) 2950 (86.5) 26.23 (0.68) 1.77 (0.06) 12.27 (0.28) 18.83 (0.58) 2.62 (0.09)

Note: Values within parentheses indicate standard error.

Table 1. Biomass and nutrient flux in foliar litter in a sugar maple stand at the Lake Clair Watershed, Quebec, Canada in 1997.

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Duchesne et al. 339

trients accumulated rapidly in leaves until the completeformation of foliage. An equilibrium was maintained duringsummer despite small fluctuations. Resorption into twigs be-gan at the end of September and was active until the mid-October. Foliar N, P, and K were resorbed in greater propor-tion than Ca and Mg during the senescence period. Cumula-tive values in Fig. 9 indicate the total accretion (whenpositive) and resorption (when negative) flux between twigsin foliage. Net resorption flux was 65% of the maximum Nleaf pool and 42% of the maximum P leaf pool.

Annual budgetThe 1997 annual nutrient budget based on fluxes is pre-

sented in Table 2. For P, Ca, and Mg, 25, 20, and 10% of theenrichment observed in throughfall flux was estimated tooriginate from dry deposition. In contrast, only 1% of the Kenrichment was attributed to dry deposition. Canopy leach-ing of P, K, Ca, and Mg increased the throughfall flux from

atmospheric depositions by 1.9, 18.6, 1.8, and 2.8 times, re-spectively. If leaching was taken into account, the estimatedrequirements for foliar production were, respectively, 0.99,1.02, 1.25, 1.09, and 1.12 time greater than estimates not in-corporating leaching losses (Table 2). The requirement forannual foliage production (accretion) (kg·ha–1·year–1) was72.3 for N, 5.4 for P, 31.9 for K, 22.8 for Ca, and 4.4 forMg. Foliar resorption conserved a major percentage of nutri-ents, 65% N, 65% P, 42% K, 10% Ca, and 30% Mg. Lessthan 40% of the requirement for foliar accretion of N, P, andK reached the soil by litter fall. Calcium and Mg were theleast resorbed nutrients, leaving, respectively, 84 and 59% offoliar requirement as litter fall.

Discussion

Representativeness of the resultsThe duration of senescence determines the amount of nu-

Fig. 6. Nutrient flux in wet precipitation (dashed line) and throughfall (dotted line) during 1997. Vertical bars in the top panel repre-sent the amount of wet precipitation.



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340 Can. J. For. Res. Vol. 31, 2001

trient resorption (Escudero et al. 1991). However, it is influ-enced by environmental conditions. Even though the presentstudy reports only 1 year of monitoring, we are confidentthat 1997 is representative of the general yearly trend, basedon the timing of leaf litter fall (Fig. 5) and the annual nutri-ent requirement that has been determined over several years(Table 3). The annual requirement evaluated for 1997 is inthe range of the annual requirement of the 1990–1994 period.

As mentioned before, sugar maple at the LCW show signsof decline estimated to a maximum of 25% of foliage loss.These observation are consistent with the measured leaf lit-ter fall. Only 41% of the total leaf biomass in litter trapsoriginated from sugar maple trees while they accounted for76% of the total basal area. However the total foliage bio-mass observed in this study is greater than values reportedfor other tolerant hardwoods (Table 4), suggesting a greaterleaf biomass from partner species. Consequently the foliageloss from sugar maple decline does not seem to have a major

impact on the estimation of foliar resorption at the stand levelexcept for specific differences associated to tree species.

Atmospheric deposition and canopy interactionThe X/Na ratio method separated the net throughfall flux

of P, K, Ca, and Mg in two parts: leachate from the canopyand dry deposition on the canopy. Contrary to some reports,dry deposition may have a major influence on calculationsof resorption (Chapin and Kedrowski 1983; Killingbeck etal. 1990). If dry deposition was not subtracted from net can-opy exchange, leaching fluxes would have been overesti-mated by 23.1, 1.1, 21.0, and 14.6%, for P, K, Ca, and Mg,respectively, resulting in an overestimation of resorption.Particularly for Ca, the resorption would have been overesti-mated by 17% by omitting dry deposition estimates.

Nitrogen and phosphorusEstimates of resorption based on changes in nutrient con-

Fig. 7. Nutrient contribution from dry deposition on forest canopy (dotted line) and nutrient fluxes by leaching (if positive) or absorp-tion (if negative) by the forest canopy (dashed line) during 1997.



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Duchesne et al. 341

tent in foliage are essentially estimates of net resorption,rather than absolute resorption. For N, the net resorptionflux accounted for 65% of the maximum leaf N pool. Drydeposition of N was not evaluated, causing an underestima-tion of total N deposition. This situation yielded a conserva-tive evaluation of N interception and, consequently, aconservative evaluation of N resorption.

The results of this study demonstrated that the decline inpool size of N and P in senescing leaves was due more to re-sorption from leaves to stems than to leaching and litter fallloss. This corresponds to the general observation that N andP are not readily leached from leaves (Cole and Rapp 1981;Ryan and Bormann 1981; Parker 1983) and that N and P are

proportionally more resorbed (Fife and Nambiar 1982; Staff1982; Escudero et al. 1992). At the LCW, the resorbed N andP pools can potentially supply the major part of the amountneeded for production of new foliage in the following sea-son. Nitrogen resorption varies from 21% in hard beech(Nothofagus truncata(Colenso) Cockaine) (Miller 1963) to79% in larch (Larix laricina (Du Roi) K. Koch) (Chapin andKedrowski 1983). For P, resorption estimates vary from 7%in red oak (Quercus rubraL.) (Sampson and Samisch 1935)to 89% in eastern cottonwood (Populus deltoidesBart. exMarsh. ssp.deltoides) (Baker and Blackmon 1977). Method-ological differences (including content vs. concentrationdata, degree of senescence when leaves were collected, and

Fig. 8. Relation betweenX/Na ratios of bulk and wet-only collectors at the Lake Clair Watershed. Data were collected from 1988 to 1997.

Fluxes N P K Ca Mg

Atmospheric deposition 5.76 (0.05) 0.10 (0.004) 0.25 (0.02) 1.26 (0.04) 0.23 (0.05)Wet deposition 5.27 (ne) 0.07 (ne) 0.18 (ne) 0.88 (ne) 0.16 (ne)Dry deposition ne 0.03 (0.002) 0.07 (0.01) 0.38 (0.03) 0.07 (0.004)Bulk deposition 5.76 (0.053) 0.07 (0.004) 0.30 (0.019) 1.28 (0.023) 0.20 (0.045)Leaching –0.78 (0.07) 0.13 (0.01) 6.46 (0.17) 1.81 (0.06) 0.48 (0.05)Throughfall 4.98 (0.05) 0.23 (0.01) 6.71 (0.17) 3.05 (0.05) 0.71 (0.02)Foliar accretion 72.29 (3.01) 5.43 (0.25) 31.94 (1.17) 22.83 (0.80) 4.44 (0.19)Foliar resorption 46.84 (1.46) 3.53 (0.12) 13.21 (0.79) 2.19 (0.09) 1.34 (0.06)Leaf litter fall 26.23 (0.68) 1.77 (0.06) 12.27 (0.28) 18.83 (0.58) 2.62 (0.09)Net requirement for foliar production 25.45 (3.35) 1.90 (0.27) 18.74 (1.41) 20.63 (0.81) 3.10 (0.19)

Note: Atmospheric deposition = wet deposition + dry deposition (estimated by Na+ ratio method). Leaching or interception = wet deposition + drydeposition – throughfall. Foliar resorption = foliar accretion – leaching – litter fall. Net requirement = foliar accretion – foliar resorption = litter fall +leaching. Values within parentheses indicate standard error. ne, not evaluated.

Table 2. Nutrient budget (kg·ha–1) for the Lake Clair Watershed for 1997.



© 2001 NRC Canada

342 Can. J. For. Res. Vol. 31, 2001

taxonomic and ecological differences) among studies that re-port N and P resorption, make it difficult to make more de-tailed comparisons with our results.

Base cationsThe estimate of net resorption of Mg (30%) in our study

is higher than reported values, which range from 0 to 20%(Potter et al. 1987; Helmisaari 1992b). Calcium resorption

(10%) is similar to Ostman and Weaver’s (1982) estimate of11.5% for a northern hardwood forest dominated byQuercus prinusL. (chestnut oak), using a method similar toours (integration of major fluxes of the biochemical cycle ina temporal way).

Table 4 shows a comparison of foliar nutrient and leaf lit-ter fall pools at the LCW with data reported for other north-ern hardwood ecosystems, notably the Huntington Forest(Johnson and Lindberg 1992), the Turkey Lakes watershed(Johnson and Lindberg 1992), and the Hubbard Brook Ex-perimental Forest (Likens et al. 1995). Compared to othernorthern hardwood stands, the total leaf Ca and Mg pool atour study site is the lowest reported. This is consistent withthe low soil Ca and Mg availability at the LCW (Houle et al.1997). Differences between foliar and litter fall Ca poolswere also higher at our site than in any of the reported stud-ies, suggesting a comparatively higher Ca and Mg resorptionrate.

The LCW has low soil Ca availability (Houle et al. 1997),

Fig. 9. Foliar accretion (if positive) and resorption (if negative) during 1997. Dashed lines indicate cumulative values.

N P K Ca Mg

1990 27.19 1.89 32.23 22.13 2.871991 23.13 1.58 23.06 21.75 2.831992 22.27 1.68 33.52 18.15 2.331993 23.41 1.79 28.46 19.70 2.731994 22.88 1.41 15.08 18.77 2.251997 25.45 1.90 18.74 20.63 3.10

Table 3. Annual nutrient requirement (kg·ha–1) for foliage pro-duction at the Lake Clair Watershed.



low leaf Ca concentration (Moore et al. 2000), and a rela-tively high resorption rate (this study). Calcium and Mg re-sorption fluxes are also quantitatively important since theyare higher than mineral soil weathering rates, evaluated at2.4, 0.1, and 0.1 for Ca, K, and Mg, respectively (Houle etal. 1997). Compared with the Huntington Forest, TurkeyLakes watershed, and the Hubbard Brook ecosystem, Ca ismore tightly cycled at LCW. This may lead to a lower de-pendence on soil as a Ca and Mg source and to reduced nu-trient transfer by litter fall.

Acknowledgments

Research supported by the ministère des Ressourcesnaturelles du Québec, Forêt Québec. We would like to thankJ.-G. Laflamme, J. Gagné, M. Saint-Germain, B. Toussaint,and J. Martineau for field assistance and the chemistry labo-ratory of the Direction de la recherche forestière for the nu-merous chemical analyses. We also thank M. Coyea for herdedication in editing the manuscript.

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seasonal nutrient transfers by foliar resorption, …lowest of 17 forest sites participating in the...

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