Climatic effects on litter decomposition from arctic tundra to tropical rainforest

Transcription

1 Global Chage Biology (2003) 9, 575±584 Cliatic effects o litter decopositio fro arctic tudra to tropical raiforest JARI LISKI*,ARI NISSINEN{, MARKUS ERHARD{ adolli TASKINEN *Europea Forest Istitute, Torikatu 34, FIN Joesuu, Filad, ad The Departet of Forest Ecology, Uiversity of Helsiki, Filad, {Fiish Eviroet Istitute, PO Box 140, FIN Helsiki, Filad, {Potsda-Istitute for Cliate Ipact Research, PO Box , D Potsda, Geray, The Departet of Forest Ecology, PO Box 27, FIN Uiversity of Helsiki, Filad Abstract Cliatic effects o the decopositio rates of various litter types i differet eviroets ust be kow to predict how cliatic chages would affect key fuctios of terrestrial ecosystes, such as utriet ad carbo cyclig ad plat growth. We developed regressio odels of the cliatic effects o the first-year ass loss of Scots pie eedle litter i boreal ad teperate forests across Europe (34 sites), ad tested the applicability of these odels for other litter types i differet ecosystes fro arctic tudra to tropical raiforest i Caada (average three year ass loss of 11 litter types at 18 sites), the USA ad Cetral Aerica (four litter types at 26 sites). A teperature variable (aual ea teperature, effective teperature su or its logarith) cobied with a suer drought idicator (precipitatio ius potetial evapotraspiratio betwee May ad Septeber) explaied the first-year ass loss of the Scots pie eedle litter across Europe with a higher R 2 value tha actual evapotraspiratio (8±0.74 vs. 1) ad with less systeatic error for ay sub-regio. The odel with teperature su ad the suer drought idicator appeared best suited to the other litter types ad eviroets. It predicted the cliatic effects o the decopositio rates i North ad Cetral Aerica with least systeatic error ad highest R 2 values (0.72±0.80). Copared with Europe, the decopositio rate was sigificatly less sesitive to aual ea teperature i Caada, ad to chages i actual evapotraspiratio i the USA ad Cetral Aerica. A siple odel distiguishig teperature ad drought effects was able to explai the ajority of cliatic effects o the decopositio rates of the various litter types tested i the varyig eviroets over the large geographical areas. Actual evapotraspiratio suarizig the teperature ad drought effects was ot as geeral cliatic predictor of the decopositio rate. Keywords: boreal forest, decopositio, drought, litter, teperate, teperature forest Received 22 Jauary 2002; revised versio received 5 Septeber 2002 ad accepted 4 October 2002 Itroductio Decopositio of plat litter is a key process i utriet ad carbo cyclig i terrestrial ecosystes. It akes utriets available for growth agai ad releases carbo to be retured to the atosphere. Ay chages i the Correspodece: J. Liski, Europea Forest Istitute, c/o Departet of Forest Ecology, PO Box 27, FIN Uiversity of Helsiki, Filad, fax , e-ail: jari.liski@efi.fi rate of decopositio ay thus chage the utriet availability, plat growth ad the carbo budget of terrestrial ecosystes. The decopositio rate depeds o the quality of the litter, as well as favourable teperature ad oisture coditios; therefore, cliatic chages would chage its rate. To predict these chages, the effects of teperature ad oisture o the decopositio rate ust be kow. Actual evapotraspiratio is ofte raked as the best cliatic idex of the decopositio rate (Meeteeyer, ß 2003 Blackwell Publishig Ltd 575

2 576 J. LISKI et al. 1978; Berg et al. 1993; Aerts 1997). It suarises teperature ad oisture effects, icreasig with teperature as log as there is water to be evaporated (Roseberg et al., 1983). Actual evapotraspiratio values are low where teperatures are low or soils are dry. Thus, cool orther forests ay have siilar evapotraspiratio values as dry souther forests, but for differet reasos. Actual evapotraspiratio has ot, however, always proved to be the best cliatic idicator of the decopositio rate. Aual ea teperature ad precipitatio explaied the cliatic effects o the average three-year ass loss of 11 litter types across Caada so well that actual evapotraspiratio provided little additioal explaatio (Moore et al., 1999). Alog a trasect fro orther Feoscadia to the west coast of Portugal i Europe, the first-year ass loss of Scots pie (Pius sylvestris L.) eedle litter was closely related to actual evapotraspiratio but, i easter cetral Europe ad aroud the Mediterraea, this ass loss was lower tha expected (Berg et al., 1993). Nevertheless, the authors associated the decopositio rate to actual evapotraspiratio cocludig that the relatioship was differet i these two regios. Such discrete regio-specific relatioships would be difficult to use i predictig the possible chages i the decopositio rate i respose to cliatic chages. Moreover, these differeces suggest that there ay be a ore direct ad geeral way to relate the decopositio rate to cliate. The objective of this study was to evaluate how accurately ad geerally the effects of cliate o the decopositio rate ca be quatified by distiguishig the effects of teperature ad drought copared to suarisig these effects i actual evapotraspiratio. First, we quatified the effects of teperature, drought ad actual evapotraspiratio o the first-year ass loss of Scots pie eedle litter i boreal ad teperate uplad forests i Europe; the, secod, we tested the applicability of these quatified effects o the decopositio rates of other litter types i other ecosyste types fro arctic tudra to tropical raiforest i North ad Cetral Aerica. Materials ad ethods Modellig ass loss i Europe We developed regressio odels of the first-year ass loss of Scots pie eedle litter usig data fro 34 forest sites across Europe (Berg et al., 1993). This data was collected specifically to study the effects of cliate o the ass loss values. The experietal desig ad laboratory aalyses were stadardised to esure that data fro all sites were coparable. These sites icluded ost of the variatio i teperature, seasoality, ad cotietality i Europe, extedig fro orther Feoscadia to the Mediterraea ad fro the Atlatic coast to easter Polad. Aual ea teperature at the sites raged fro 1.7±16.6 8C, aual precipitatio varied fro 402 to 1500 ad aual actual evapotraspiratio raged betwee 328 ad 654. At each site, seescet Scots pie eedles were allowed to decopose i 25 esh bags for a year, ad the decopositio rate was easured as a average ass loss fro these bags. Accordig to the easured decopositio rates ad cliatic variables explaiig these rates, Berg et al. (1993) grouped these sites ito four groups: Scadiavia-NW-cotietal trasect ( sites), the Europea west coast (w sites), the Mediterraea ( sites) ad the Cetral Europea (c sites) sites. We calculated effective teperature su of the year (0 8C threshold), ad precipitatio ad potetial evapotraspiratio fro May to Septeber for these study sites to eable our aalyses. With referece to geographical coordiates ad altitude, we derived ea othly teperature ad precipitatio for each site usig 10' logitude-latitude-refereced cliate data ad the BIOM odel (Sykes et al., 1996). Effective teperature su was calculated by assuig that ea othly teperatures occurred i the iddle of each oth with daily values liearly iterpolated; ad potetial evapotraspiratio was calculated fro ea othly teperatures usig the algoriths of the BIOM odel. To evaluate how accurately our estiates represeted the cliate at the study sites, we also calculated cliate variables reported for these sites by Berg et al. (1993). Liear correlatio coefficiets for estiated ad observed aual ea teperature, aual actual evapotraspiratio ad total aual precipitatio were 0.98, 0.88 ad 0.77, respectively. The lower correlatio coefficiet for aual precipitatio was due to larger tha observed estiates for two sites i south-west Spai ad saller estiates for two sites i south-west Swede; without these sites the coefficiet was Because the results of this study were ot sesitive to precipitatio estiates of these four sites, we cocluded that our estiates were adequate. Liear regressio odels were used to explai the ass loss, k, k ˆ a bt cdor k ˆ a be: 1a 1b We used aual ea teperature, teperature su or logarith of teperature su to describe teperature (T), the differece betwee precipitatio ad potetial evapotraspiratio fro May to Septeber to describe suer drought (D), or actual evapotraspiratio (E) to describe the cobied effects of teperature ad drought as idepedet variables i these odels. At ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584

3 CLIMATIC EFFECTS ON LITTER DECOMPOSITION 577 three sites, the suer drought idicator had positive values, which we set equal to zero, assuig o drought effect. Logarith of teperature su was icluded i the aalyses to study if ass loss was ore sesitive to teperature uder cold tha war coditios, a pheoeo suggested based o soil respiratio ad itroge ieralisatio easureets (Raich & Schlesiger, 1992; Kirschbau, 1995). Testig the odels i North ad Cetral Aerica We tested the regressio odels, which we developed usig the Europea data, agaist data collected fro 18 sites across Caada (Moore et al., 1999) ad 26 sites across the USA ad Cetral Aerica (Gholz et al., 2000). These data covered ad exceeded variatio i teperature ad suer drought foud i the Europea data (Fig. 1). The aual precipitatio of the Caadia sites varied fro 261 to 1782 (Trofyow & CIDET Workig Group, 1998) ad that of the US-Cetral Aerica sites fro 260 to 3914 (Gholz et al., 2000). At each Caadia site, 11 litter types were set to decopose i esh bags ad the average ass reaiig after three years was reported (Moore et al., 1999). For the US-Cetral Aerica sites, data o the ass reaiig Suer drought idicator () Aual ea teperature ( C) Europea data (Berg et al., 1993) Caadia data (Moore et al., 1999) US ad Cetral Aerica data (Gholz et al., 2000) Fig. 1 Aual ea teperature ad suer drought (precipitatio ius potetial evapotraspiratio fro May to Septeber) at the study sites across Europe (Berg et al., 1993) used to developregressio odels of the cliatic effects o litter decopositio rate, ad at the study sites across Caada (Moore et al., 1999) ad the USA ad Cetral Aerica (Gholz et al., 2000) used to test these odels. after the first year of decopositio i esh bags were available for the eedles of a pie species (Pius resiosa), fie roots of aother pie species (Pius elliottii), ad leaves ad fie roots of a tropical deciduous tree (Dypetes glauca) (Gholz et al., 2000). We derived values for the idepedet variables of our regressio odels (Eqs 1a, b) at the Caadia ad the US-Cetral Aerica study sites fro cliate data reported (Trofyow & CIDET Workig Group, 1998; Forest Sciece Data Bak, 2000). We calculated teperature su ad the accuulated potetial evapotraspiratio betwee May ad Septeber fro the ea othly teperatures like we did for the Europea study sites. Because the quality of the experietal litter i Caada, the USA ad Cetral Aerica differed fro that i Europe, we tested our regressio odels i relative istead of absolute ters. Cosequetly, we refored Eqs 1a ad b for k ˆ k 0 1 b T T 0 g D D 0 or 2a k ˆ k 0 1 b E E 0 2b where k 0 expresses ass loss uder teperature, suer drought or actual evapotraspiratio coditios T 0, D 0 or E 0, ad b ad g express the proportioal chage i ass loss whe chagig fro these cliate coditios to other teperature, suer drought or actual evapotraspiratio coditios T, D or E. The values of k 0, b ad g were derived fro Eq 1a as k 0 ˆ a bt 0 cd0, b ˆ b/k 0 ad g ˆ c/k 0 or fro Eq 1b as k 0 ˆ a be 0 ad b ˆ b/k 0. We tested this odel i two ways. First, we calculated ass loss values for the Caadia ad the US-Cetral Aerica study sites usig Eq 2 with coefficiets b ad g deteried fro the Europea data, ad copared these estiates to easured values. The ass loss value (k 0 ) of the Caadia ad the US-Cetral Aerica litter types at certai teperature (T 0 ), suer drought (D 0 ) ad actual evapotraspiratio coditios (E 0 ) was estiated by usig the averages of these variables for the Caadia ad the US-Cetral Aerica data; a useful feature of liear regressio odels is that they always iclude this average poit. The Caadia average three year ass loss was 60 (3 years) 1, aual ea teperature was C, teperature su was C days, logarith of teperature su was 3.31 log (8C days), suer drought idicator was 61.9 ad actual evapotraspiratio was 466. The US-Cetral Aerica average first-year ass losses were 00, 84, 11 ad 28 year 1 for Pius resiosa eedles, Drypetes leaves, Pius elliottii roots ad Drypetes roots, respectively, aual ea teperature was C, teperature su was C days, ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584

4 578 J. LISKI et al. logarith of teperature su was 3.58 log (8C days), suer drought idicator was 93.9 ad actual evapotraspiratio was 642. We estiated the first-year ass loss of the Europea litter at these sae coditios usig Eq 1. The, we calculated the values of coefficiets b ad g usig this value for k 0 i Eq 2. These b ad g-values expressed how uch the decopositio rate of the Europea litter chaged i relative ters as cliate diverged fro the Caadia or the US-Cetral Aerica average coditios. Fially, we used Eq 2 with these b ad g-values ad the Caadia or the US-Cetral Aerica average ass loss values (see above) to calculate the ass loss estiates for the Caadia ad the US-Cetral Aerica study sites ad litter types. To evaluate the reliability of the calculated values, i additio to visually ispectig scatterplots, we fitted a liear regressio betwee the calculated ad the easured ass loss values. Deviatio of the slopes of these regressios fro oe idicate a systeatic differece betwee the calculated ad the easured values while the R 2 values of these regressios easure the proportio of variatio i the easureets explaied by the odels. The secod approach to testig was to fit Eq 2 to the Europea, the Caadia ad the US-Cetral Aerica data separately, ad to copare the resultig estiates of b ad g. Because these coefficiets depeded o T 0, D 0 ad E 0, we did the fittigs for the Caadia ad the US- Cetral Aerica average teperature, drought ad actual evapotraspiratio coditios. To estiate the statistical sigificace of the differece i the coefficiets betwee Europe ad the other regios, we estiated oral distributios for the coefficiets based o their 95% cofidece liits ad calculated the overlappig areas. Results Modellig ass loss i Europe Teperature explaied the first-year ass loss of Scots pie eedle litter alog the Scadiavia-NW-cotietal trasect ( sites) as well as did actual evapotraspiratio (Table 1, Fig. 2). This was expected because water deficit did ot liit actual evapotraspiratio at these sites, ad so actual evapotraspiratio depeded oly o teperature. For these sites, the liear correlatio coefficiet betwee actual evapotraspiratio ad aual ea teperature was 0.99, ad that betwee actual evapotraspiratio ad teperature su was Soe of the sites o the Europea west coast (w sites) followed the sae relatioships but the ass loss values were lower at ost of the cetral Europea (c sites) ad the Mediterraea sites ( sites) (Fig. 2). We foud that these low ass loss values at the cetral Europea (c sites) ad the Mediterraea sites ( sites) were associated with suer drought. Drought severity was estiated as othly precipitatio ius potetial evapotraspiratio fro May to Septeber. The drier the suer the saller the ass loss copared to expectatios based o teperature or actual evapotraspiratio relatioships alog the Scadiavia-NW-cotietal trasect ( sites) (see Fig. 2). Addig this suer drought idicator to the regressio odels as aother idepedet variable explaied the ass loss values across Europe with cosiderably less systeatic error for ay site groupad R 2 values fro 8 to 0.74, depedig o the teperature variable (Table 1, Fig. 3). The systeatic error of the ass loss estiates alog the Scadiavia-NW-cotietal trasect ( sites) calculated usig the odel with teperature su ad the suer drought idicator (Fig. 3b) was further Table 1 Regressio odels for the first-year ass loss (year 1 ) of Scots pie eedle litter i Europe (data fro Berg et al., 1993) Model Model # R 2 Sites icluded T at (1) sites T dd0 (2) sites T log(dd0) (3) sites ± E (4) sites T at D (5) all sites T dd D (6) 8 34 all sites T log(dd0) D (7) 9 34 all sites E (8) 1 34 all sites T at, aual ea teperature (8C); T dd0, effective teperature su with 0 8C threshold (8C days); T log(dd0), the logarith of the teperature su (log (8C days)); D, the differece betwee precipitatio ad potetial evapotraspiratio fro May to Septeber (); E, aual actual evapotraspiratio (); ad ` sites', a subgroub of the sites o a Scadiavia-NW-cotietal trasect. ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584

5 CLIMATIC EFFECTS ON LITTER DECOMPOSITION 579 (a) (b) Fig. 2 The first-year ass loss of Scots pie eedle litter easured across Europe plotted agaist (a) aual ea teperature ad (b) aual actual evapotraspiratio (data fro Berg et al., 1993). The letter sybols idicate groupig of the sites by Berg et al., ito the Scadiavia-NW-cotietal trasect (), the Europea west coast (w), the Mediterraea () ad Cetral Europea (c) site groups. The lies show regressio odels (a) #1 ad (b) #4 show i Table 1 for to the sites. Mass loss (year 1 ) w c cc c c w w w Mea aual teperature (8C) w Mass loss (year 1 ) c c ww w c c w w w Aual actual evapotraspiratio () (a) T at, D (b) T dd0, D Measured ass loss (year 1 ) w w w w w c c c c Measured ass loss (year 1 ) w w w w c c c c c w Modelled ass loss (year 1 ) (c) T log(dd0), D (d) E Modelled ass loss (year 1 ) Fig. 3 The first-year ass loss of Scots pie eedle litter easured across Europe (Berg et al., 1993) plotted agaist the values calculated usig the regressio odels: (a) #5, (b) #6, (c) #7 ad (d) #8 show i Table 1. The letter sybols idicate groupig of the sites by Berg et al., ito the Scadiavia-NW-cotietal trasect (), the Europea west coast (w), the Mediterraea () ad Cetral Europea (c) site groups. Measured ass loss (year 1 ) w w ww w c c cc c Modelled ass loss (year 1 ) Measured ass loss (year 1 ) ww w w w c c c c Modelled ass loss (year 1 ) decreased by usig log-trasfored teperature su values (Fig. 3c). Testig the odels i North ad Cetral Aerica The effects of teperature ad suer drought o ass loss were ot statistically differet betwee Europe ad Caada whe teperature su or log-trasfored teperature su were used together with the suer drought idicator as cliate variables (Table 2). The effect of actual evapotraspiratio was ot differet either. Mass loss estiates for the Caadia study sites, which were calculated usig odels with these cliatic effects quatified based o the Europea data, were coparable to easured values (Figs 4b, c, d). The slopes of the regressios betwee these calculated ad easured values were close to oe, idicative of sall systeatic differece (Table 4). The odels with teperature su or ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584

6 580 J. LISKI et al. Table 2 95% cofidece liits for the effects of teperature or actual evapotraspiratio (b) ad suer drought (g) o litter ass loss rate i Europe ad Caada. b ad g are coefficiets of Eqs 2a ad b, which were fitted to data o the first-year ass loss of Scots pie eedle litter across Europe ( ˆ 34, Berg et al., 1993) ad data o the average three-year ass loss of 11 litter types across Caada ( ˆ 18, Moore et al., 1999) b g Cliate variables i the odel Europe Caada Europe Caada T at, D 91±99 48±86** 2.5± ± ** T dd0, D 7± ± ± ± T log(dd0), D 1.7± ± ± ± E 1.8± ± Statistically sigificat differeces i the cliatic effects (P < 1) are idicated by **. The abbreviatios of the cliate variables are explaied i Table 1. (a) T at, D Measured ass loss ((3 years) 1 ) Modelled ass loss ((3 years) 1 ) (c) T log(dd0), D Measured ass loss ((3 years) 1 ) Modelled ass loss ((3 years) 1 ) (b) T dd0, D Measured ass loss ((3 years) 1 ) (d) E Measured ass loss ((3 years) 1 ) Modelled ass loss ((3 years) 1 ) Modelled ass loss ((3 years) 1 ) Fig. 4 Average three-year ass loss of 11 experietal litter types, as easured across Caada (Moore et al., 1999), plotted agaist the values calculated usig regressio odels with the effects of cliate deteried fro easureets take i Europe (Berg et al., 1993). Cliate variables used i the odels were (a) aual ea teperature ad suer drought (precipitatio ius potetial evapotraspiratio fro May to Septeber), (b) teprature su ad suer drought, (c) log-trasfored teperature su ad suer drought ad (d) aual actual evapotraspiratio. The odels were adjusted to pass through the average easured ass loss value (6 (3 years) 1 ). The lies idicate liear regressios fitted to the easured ad the odel-calculated values; the slopes ad the R 2 values of these regressios are give i Table 4. log-trasfored teperature su ad the suer drought idicator explaied a little ore of the variatio i the data (R 2 ˆ 0.71 ad 0.76) tha the odel with actual evapotraspiratio aloe (R 2 ˆ 4). Whe aual ea teperature was used to idicate teperature, ass loss was sigificatly ore tolerat of it ad suer drought i Caada tha i Europe (Table 2), ad this odel clearly uderestiated the low ass loss values ad overestiated the high values (Fig. 4a). The effects of cliate o ass loss were ot statistically differet betwee Europe ad the USA-Cetral Aerica either whe log-trasfored teperature su was used together with the suer drought idicator as cliate variables (Table 3). Whe the other teperature variables ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584

7 CLIMATIC EFFECTS ON LITTER DECOMPOSITION 581 Table 3 95% cofidece liits for the effects of teperature or actual evapotraspiratio (b) ad suer drought (g) o litter ass loss rate i Europe ad the USA ad Cetral Aerica. b ad g are coefficiets of Eqs (2a) ad (2b), which were fitted to data o the first-year ass loss of Scots pie eedle litter across Europe ( ˆ 34, Berg et al., 1993) ad data o the first-year ass loss of four litter types across the USA ad Cetral Aerica ( = 26, Gholz et al., 2000) b g USA ad Cetral Aerica USA ad Cetral Aerica Europe Europe Drypetes glauca roots Pius elliottii roots Drypetes glauca leaves Pius resiosa eedles Pius sylvestris eedles Drypetes glauca roots Pius elliottii roots Drypetes glauca leaves Pius resiosa eedles Pius sylvestris eedles Cliate variables used i the odel Tat, D 42±55 34±68 28±53 27±49 27±50 1.1± ± * 0.82± ± ± Tdd0, D 4± ± ± ± * 0± ± ± * 0.90± ± ± T log(dd0), D ± ± ±1.5 5±1.4 6± ± ± ± ± ± E 1.5± ± * 5± ** 4± ** 1± ** Statistically sigificat differeces i the cliatic effects, P < 1 or P < 5, are idicated by ** or *, respectively. The abbreviatios of the cliate variables are explaied i Table 1. were used, statistical differeces i the cliatic effects were foud for two litter types. Mass loss of Pius resiosa eedle litter was slightly ore sesitive to suer drought whe aual ea teperature (P-value ˆ 28) or teperature su (P-value ˆ 32) was used as a teperature variable, ad the risk level for the differece i the teperature effects o the ass loss of Pius elliottii root litter was just below 5% (P-value ˆ 48) whe teperature su was used as a teperature variable. I cotrast, ass loss of all four litter types was statistically sigificatly ore tolerat of chages i actual evapotraspiratio i the USA ad Cetral Aerica tha i the Europe (P < 01±19). Mass loss of the litter types at the US-Cetral Aerica study sites were ost accurately calculated whe teperature su ad the suer drought idicator were used as cliate variables. The slopes of the regressios betwee the calculated ad the easured values were 0.71 or 0.91 for the foliar litter types ad 4 or 6 for the root litter types (Table 4, Fig. 5b). The R 2 values of these regressios raged fro 0.72 to Although the slopes were a little closer to oe for both foliar litter types (0.73 ad 0.97) whe log-trasfored teperature su was used as a teperature variable, the scatterplots of the calculated ad the easured values revealed aother kid of systeatic error i the calculated values. The lowest ad the highest calculated values were uderestiatios of the easured values while the calculated values i the iddle were overestiatios (Fig. 5c). Whe aual ea teperature was used as a teperature variable, ass loss of each litter type at the three coldest sites was uderestiated (Fig. 5a). Whe actual evapotraspiratio was used aloe as a cliate idex, the sesitivity of ass loss of three litter types, Drypetes glauca leaves ad the both root litter types, to cliate was substatially overestiated, ad the R 2 values were low for the root litter types (Table 4, Fig. 5d). Discussio Aog the cliate variables tested, teperature su ad suer drought were those that best explaied cliatic effects o litter decopositio rates i various coditios. They explaied the first-year ass loss of Scots pie eedle litter across Europe essetially as well as did aual ea teperature or logarith of teperature su cobied with suer drought (Table 1, Fig. 3), ad, i additio, a regressio odel, which was fitted to the Europea data ad had these cliate variables, predicted variatio i the ass loss of differet litter types across Caada, the USA ad Cetral Aerica with least systeatic error ad, o average, highest R 2 values (Table 4, Figs 4 ad 5). We cosidered these advatages ore iportat tha the weak statistical differeces ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584

8 582 J. LISKI et al. Table 4 Reliability of regressio odels fitted to data o the first-year ass loss of Scots pie eedle litter across Europe (data fro Berg et al., 1993) i predictig the cliatic effects o ass loss values easured across Caada (average three-year ass loss of 11 litter types, Moore et al., 1999) ad across the USA ad Cetral Aerica (first-year ass loss of four litter types, Gholz et al., 2000) Cliate variables used i the odel T at, D T dd0, D T log(dd0), D E Regio Litter type Slope R 2 Slope R 2 Slope R 2 Slope R 2 Caada Average of litter types USA ad Cetral Aerica Pius resiosa eedles Drypetes glauca leaves Pius elliottii roots Drypetes glauca roots The slopes ad the R 2 values are fro regressios fitted betwee the easured ad the odel-calculated values (Figs 5, 8). The slopes idicate how uch the easured value chaged, o average, for a uit chage i the odel-calculated value, ad the R 2 values idicate the share of variatio i the easured values explaied by the odels. The abbreviatios of the cliate variables are explaied i Table 1. (a) T at, D Measured ass loss (year 1 ) Modelled ass loss (year 1 ) (c) T log(dd0), D Measured ass loss (year 1 ) Modelled ass loss (year 1 ) (b) T dd0, D Measured ass loss (year 1 ) (d) E Measured ass loss (year 1 ) Modelled ass loss (year 1 ) Modelled ass loss (year 1 ) Fig. 5 The first-year ass loss of Pius elliottii root litter, as easured across the USA ad Cetral Aerica (Gholz et al., 2000), plotted agaist the values calculated usig regressio odels with the effects of cliate deteried fro easureets take i Europe (Berg et al., 1993). Cliate variables used i the odels were (a) aual ea teperature ad suer drought (precipitatio ius potetial evapotraspiratio fro May to Septeber), (b) teperature su ad suer drought, (c) log-trasfored teperature su ad suer drought ad (d) aual actual evapotraspiratio. The odels were adjusted to pass through the average easured ass loss value (11 year 1 ). The lies idicate liear regressios fitted to the easured ad the odel-calculated values; the slopes ad the R 2 values of these regressios are give i Table 4. I ters of these slopes ad R 2 values, this litter type represets a average of the four litter types tested. ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584

9 CLIMATIC EFFECTS ON LITTER DECOMPOSITION 583 betwee Europe ad USA-Cetral Aerica i the teperature su effect o Pius elliottii root litter ad the suer drought effect o Pius resiosa eedle litter (Table 3). O the other had, decopositio rate was foud substatially ore tolerat of aual ea teperature i Caada tha i Europe (Table 2, Fig. 4a). This was probably because ay of the Caadia sites had a very cotietal cliate. Although cold witers decreased aual ea teperature at these sites, suers were still war eough to provide favourable coditios. Logtrasfored teperature su was icluded i the aalyses to study if decopositio was ore sesitive to teperature i cold coditios, like suggested based o soil respiratio ad itroge ieralisatio easureets (Raich & Schlesiger, 1992; Kirschbau, 1995). The results were cotroversial. Replacig teperature su with its logarith iproved the ass loss estiates for the coldest sites i Europe (Fig. 3) but had little effect i Caada (Fig. 4) ad led to uderestiatios of ass loss at the coldest sites i the USA (Fig. 5). Our results suggest that a siple odel ca explai the ajority of cliatic effects o the decopositio rates of differet litter types i widely varyig eviroets fro arctic tudra to tropical raiforest. The applicability of such a odel does ot see to be liited to ay specific year i the start of decopositio, sice odels developed usig the first-year ass loss data fro Europe predicted the three-year ass loss values accurately across Caada (Table 4, Fig. 4). At later stages, decopositio ay becoe ore tolerat of cliate (Johasso et al., 1995), ad decopositio of old soil orgaic atter has bee foud to be rather isesitive to teperature (Liski et al., 1999; Giardia & Rya, 2000). Such a wide applicability of our siple odels was ot expected. Earlier, whe actual evapotraspiratio had bee used as a cliate idex, it had bee cocluded that cliatic effects o litter decopositio were specific to geographical regios (Berg et al., 1993), litter types (Berg et al., 2000) ad decopositio years (Johasso et al., 1995). We also foud that the relatioshipbetwee actual evapotraspiratio ad the decopositio rate was sigificatly differet i Europe tha i the USA ad Cetral Aerica (Table 3). It sees possible to explai cliatic effects o litter decopositio rates ore geerally ad accurately whe teperature ad drought effects are distiguished tha whe they are suarised i actual evapotraspiratio. This coclusio cotradicts the prevailig view that cliatic effects o litter decopositio are best explaied by actual evapotraspiratio at regioal or larger scales (e.g. Meeteeyer, 1978; Berg et al., 1993; Aerts, 1997). Our coclusio is probably differet, because we used differet criteria to evaluate the cliate idexes. I additio to the R 2 values of the odels, we paid attetio to systeatic differeces betwee regios ad litter types to see how widely applicable the odels were. The earlier studies have evaluated the odels based o the R 2 values oly. Despite a high R 2 value, a odel ay be systeatically ivalid for idividual regios or eviroetal coditios. I the preset study, we ivestigated the direct effects of cliate o decopositio rate. Cliate ay also affect decopositio rate idirectly by chagig litter cheistry (Aerts, 1997). To predict the total effects of cliate o litter decopositio, the regressio odels developed here ust be coupled with iforatio o the possible chages i litter quality. Ackowledgeets We thak Taru Palosuo, Ari Pussie, Raija Laiho, Raisa MaÈkipaÈaÈ ad Tio Karjalaie for coets, ad three aoyous reviewers for costructive criticis o a earlier versio of this paper. This study was partly fuded by the Europea Coissio through ATEAM (EVK2-CT ) ad CASFOR-II (ICA4-CT ) projects. Refereces Aerts R (1997) Cliate, leaf litter cheistry ad leaf litter decopositio i terrestrial ecosystes: a triagular relatioship. Oikos, 79, 439±449. Berg B, Berg MP, Botter P et al. (1993) Litter ass loss rates i pie forests of Europe ad Easter Uited States: soe relatioships with cliate ad litter quality. Biogeocheistry, 20, 127±159. Berg B, Johasso MB, Meeteeyer V (2000) Litter decopositio i a trasect of Norway spruce forests: substrate quality ad cliate cotrol. Caadia Joural of Forest Research, 30, 1136±1147. Forest Sciece Data Bak (2000) Data Set Provided by the Forest Sciece Data Bak, a Partershipbetwee the Departet of Forest Sciece, Orego. State Uiversity, ad the US Forest Service Pacific Northwest Research Statio, Corvallis, Orego. available at td23/td2304.txt. Gholz HL, Wedi DA, Sithera SM et al. (2000) Log-ter dyaics of pie ad hardwood litter i cotrastig eviroets: toward a global odel of decopositio. Global Chage Biology, 6, 751±765. Giardia CP, Rya MG (2000) evidece that decopositio rates of orgaic carbo i ieral soil do ot vary with teperature. Nature, 404, 858±861. Johasso M-B, Berg B, Meeteeyer V (1995) Litter ass-loss rates i late stages of decopositio i a cliatic trasect of pie forests. Log-ter decopositio i a Scots pie forest. IX. Caadia Joural of Botay, 73, 1509±1521. Kirschbau MUF (1995) The teperature depedece of soil orgaic atter decopositio, ad the effect of global ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584

10 584 J. LISKI et al. warig o soil orgaic C storage. Soil Biology ad Biocheistry, 27, 753±760. Liski J, Ilvesiei H, MaÈkelaÈ A et al. (1999) CO 2 eissios fro soil i respose to cliatic warig are overestiated ± the decopositio of old soil orgaic atter is tolerat of teperature. Abio, 28, 171±174. Meeteeyer V (1978) Macrocliate ad ligi cotrol of litter decopositio rates. Ecology, 59, 465±472. Moore TR, Trofyow JA, Taylor B et al. (1999) Litter decopositio rates i Caadia forests. Global Chage Biology, 5, 75±82. Raich JW, Schlesiger WH (1992) The global carbo dioxide flux i soil respiratio ad its relatioship to vegetatio ad cliate. Tellus, 44B, 81±99. Roseberg NJ, Blad BL, Vera SB (1983) Microcliate: the Biological Eviroet. Wiley, New York. Sykes MT, Pretice IC, Craer W (1996) A biocliatic odel for the potetial distributios of North Europea tree species uder preset ad future cliates. Joural of Biogeography, 23, 203±233. Trofyow JA, CIDET Workig Group(1998) The Caadia Itersite Decopositio Experiet (CIDET): Project ad Site Establishet Report. Iforatio Report BC-X-378. Natioal Resources Caada, Caadia Forest Service, Pacific Forestry Cetre, Victoria, BC, Caada. ß 2003 Blackwell Publishig Ltd, Global Chage Biology, 9, 575±584