Comparison of carbon dynamics and water use efficiency following fire and harvesting in Canadian boreal forests

agricultural and forest meteorology 149 (2009) 783–794

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Comparison of carbon dynamics and water use efficiency following fire and harvesting in Canadian boreal forests M.S. Mkhabela a, B.D. Amiro a,*, A.G. Barr b, T.A. Black c, I. Hawthorne c, J. Kidston c, J.H. McCaughey d, A.L. Orchansky e, Z. Nesic c, A. Sass a, A. Shashkov f, T. Zha b a

Department of Soil Science, University of Manitoba, Winnipeg, MB, Canada R3T 2N2 Climate Research Division, Environment Canada, 11 Innovation Blvd., Saskatoon, SK, Canada S7N 3H5 c Faculty of Land and Food Systems, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 d Department of Geography, Queen’s University, Kingston, ON, Canada K7L 3N6 e 5903 109A Street, Edmonton, AB, Canada T6H 3C4 f Air Quality Research Division, Environment Canada, Toronto, ON, Canada M3H 5T4 b

article info

abstract

Article history:

Fire and harvesting are major forest renewal processes in the Canadian boreal forest. The

Received 3 June 2008

eddy covariance method was used to compare ecosystem fluxes of carbon dioxide between

Received in revised form

harvested and burned boreal forest sites in Saskatchewan, Canada. The harvest chron-

30 October 2008

osequence had sites harvested in 2002 (HJP02), 1994 (HJP94) and 1975 (HJP75), whereas the

Accepted 31 October 2008

fire chronosequence sites were burned in 1998 (F98), 1989 (F89), 1977 (F77) and 1929 (OJP). All sites were dominated by jack pine prior to the disturbance. During 2004 and 2005, net ecosystem production showed an average carbon gain (g C m2 year1) at F89 = 84,

Keywords:

HJP75 = 80, HJP94 = 14 and OJP = 20. The other sites lost carbon (g C m2 year1) at

Boreal forest

HJP02 = 139, F98 = 20, and F77 = 58. Gross ecosystem production (GEP), ecosystem

Eddy covariance

respiration (Re) and evapotranspiration tended to be greater at the burned sites than

Fire

the harvested sites. The F89 and F77 sites had the strongest response of GEP to photo-

Harvesting

synthetically active radiation, and the strongest response of Re to soil temperature at the 2-

Carbon exchange

cm depth. HJP02 had the weakest responses, followed by HJP94. This apparent greater

Evapotranspiration

ecosystem activity at the burned sites is likely caused by local differences in soil moisture

Water use efficiency

and nutrients, differences in vegetation development, and differences in the decomposi-

Net ecosystem production

tion of coarse woody debris. # 2008 Elsevier B.V. All rights reserved.

1.

Introduction

The boreal forest is potentially a large sink or source for atmospheric carbon dioxide, the major greenhouse gas implicated in global warming, because of its large size and huge carbon stocks (Dixon et al., 1994). The carbon dynamics of this biome are largely driven by periodic disturbances of fire, insects, disease and harvesting (Kurz and Apps, 1999; Chen et al., 2000; Kurz et al., 2008a,b). In the Canadian boreal forest,

harvesting activities are mostly confined to the southern parts, whereas fire affects the broader forest, with more fire in the west central regions than in the east (Stocks et al., 2002). In most of the Canadian boreal forest, harvesting is a relatively new practice with forested areas experiencing their first cut. This contrasts with Europe, where several harvesting rotations have been experienced and fire plays a much smaller role in forest renewal. The ecological differences between fire and harvesting have been the subject of many investigations,

* Corresponding author. Tel.: +1 204 474 9155; fax: +1 204 474 7642. E-mail address: [email protected] (B.D. Amiro). 0168-1923/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2008.10.025

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especially as harvesting practices are evolving to maintain ecological integrity, often through emulating natural disturbances (Hunter, 1993; Attiwill, 1994; Bergeron et al., 1999). For example, boreal harvesting patterns are being developed to emulate fire and the potential ecological effects are being evaluated (McRae et al., 2001; Work et al., 2004; Harrison et al., 2005). Within this context, much site inventory work has been done to understand the carbon dynamics following harvesting (Howard et al., 2004; Martin et al., 2005) and fire (Wang et al., 2002; Hicke et al., 2003; Bond-Lamberty et al., 2007) in the Canadian boreal forest. However, direct whole ecosystem carbon flux measurements are scarce, and direct comparisons between the disturbance types are lacking. Currently, the international FLUXNET community is synthesising disturbance effects on ecosystem carbon balances, and disturbance has been one of the foci of the Fluxnet Canada Research Network (Margolis et al., 2006). Most previous studies of ecosystem carbon fluxes using eddy covariance towers have focused on a single disturbance. In the boreal forest, flux measurements have been made at fire chronosequences in Manitoba (Litvak et al., 2003; Goulden et al., 2006), Alaska (Randerson et al., 2006) and Saskatchewan (Amiro, 2001; Sass, 2007). Harvesting flux measurements have been reported for Alberta (Amiro, 2001), Saskatchewan (Amiro et al., 2006a; Grant et al., 2007) and Quebec (Giasson et al., 2006; Bergeron et al., 2008). There are few comparisons of harvesting and fire effects, although Schulze et al. (1999) summarised a range of findings from sites from Europe, Asia and North America. The challenges to compare the processes of fire and harvesting are related to establishing treatments that have similar climates, soil conditions, and species. This is confounded by the requirement for establishing experimental treatments of sufficient scale (>100 ha) to allow for eddy covariance measurements of whole ecosystem exchange, and by the availability of historical conditions and management practices that include both fire and harvesting. The Boreal Ecosystem Research and Monitoring Sites (BERMS) area of central Saskatchewan provides one such laboratory where experiments have been set up to allow multiple year comparisons of fire and harvesting effects on carbon fluxes. In this area, three mature sites, representing jack pine, black spruce, and trembling aspen forests were established during the BOREAS experiment in 1994 (Sellers et al., 1997). Measurements at these sites have continued with the addition of three recently burned sites and three recently harvested sites over the last decade. This employs the chronosequence concept, where we assume that we can gain knowledge about the sequential development of forest stands through studies of a range of ages at some point in time. We recognise the limitations to this concept, especially when it is very difficult to set up treatments that are matched, and it is almost impossible to set up replicated sites. However, within these practical limitations, the experiment at BERMS is likely the most closely matched set of treatments within the global flux community. Previously, we reported on some of the initial data on carbon fluxes from two of the fire sites and one harvested site for 2001 and 2002 (Amiro et al., 2006a). These data showed that a 4-year-old burned site (F98) and an 8-year-old harvested site (HJP94) were moderate carbon sources of between 50 and

130 g C m2 year1. This contrasted to a 14-year-old burned site (F89) that was a carbon sink of 68 g C m2 year1. However, there were insufficient sites available to allow a true comparison between fire and harvesting. In the present paper, we analyse data from 2004 to 2005, which is the only common period where flux towers were operating at three young harvested sites, three young fire sites, and a mature jack pine site. Our objective was to investigate the differences among the sites as a function of stand age and disturbance history (i.e., fire and harvesting), and to identify possible causes for these differences.

2.

Materials and methods

2.1.

Sites location and description

The study sites are located in central Saskatchewan (about 548N, 1068W) (Table 1), Canada and are within 100 km of each other with relatively similar climates. Net ecosystem CO2 exchange (NEE) data collected from three post-fire sites (F77: burned in 1977, F89: burned in 1989, F98: burned in 1998), three post-harvest sites (HJP75: harvested in 1975, HJP94: harvested in 1994, HJP02: harvested in 2002) and one mature site (OJP: often dated to a burn in 1929, although new records show that it might have last been burned in 1916) were used in this study. The harvested sites and OJP are dominated by relatively pure stands of jack pine (Pinus banksiana Lamb). Tree regeneration at the harvested sites was from seeds (cones) left behind following the whole-tree harvest; no additional planting was conducted. The fire sites, however, have a mixture of jack pine, black spruce (Picea mariana (Mill.) BSP), and trembling aspen (Populus tremuloides Michx.), even though they were largely dominated by jack pine prior to the fire. This mixture is a typical successional response following fire in this area where these tree species develop together with trembling aspen often leading, jack pine next, and black spruce last in a successional trajectory of canopy dominance. In addition, the soils at the harvested sites are sandier than at the fire sites, even though all of these soils are classified as brunisols (Table 1). It is very difficult to match exactly the site characteristics among treatments, even in the same geographical area. The carbon inventories for each site are given in Table 1, derived from data reported in the Fluxnet-Canada data information system (Fluxnet-Canada, 2008). For live aboveground biomass, the sites are ranked as OJP > HJP75 > F77 > F89 > HJP94 > F98 > HJP02. However, F98, F89 and F77 all have much greater amounts of dead above-ground biomass and biomass in the LFH (top forest floor organic layer) and mineral soil layers than the other sites. It is important to note that the amount of dead above-ground biomass is much less at the harvested sites because of the removal of the tree boles during harvesting. Many of the smaller branches remained on the sites with cones attached for tree regeneration. In contrast, the dead material at the fire sites is much greater because of the inventory of dead trees killed by the fires. The differences in the carbon in the LFH layer are most likely caused by site differences than by management history. For example, the OJP site can be interpreted as the undisturbed baseline for the three harvested sites, being in very close proximity and on similar soils. The LFH

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Table 1 – Site location and characteristics. F77 Year of origin Latitude Longitude Elevation (m) Soil texture Total soil N (0–50 cm) (t N ha1) Total leaf N (%) Live above ground C (t C ha1) Dead above ground C (t C ha1) Root C (t C ha1) LFH (t C ha1) Mineral soil C (0–50 cm) (t C ha1) Canopy height (m) LAI Dominant over-storey tree species Dominant under-storey tree species Sonic anemometer model IRGA Flux height (m) u* threshold (m s1)

F89

F98

OJP

HJP75

HJP94

HJP02

1977 54.485N 105.818W 563 Sandy loam 1.43  0.22

1989 54.254N 105.877W 540 Sandy loam 1.93  0.46

1998 53.917N 106.078W 548 Sandy loam 1.70  0.27

1929 53.916N 104.690W 579 Sandy 1.00  0.12

1975 53.876N 104.645W 534 Sandy 0.97  0.17

1994 53.908N 104.656W 580 Sandy 1.01  0.32

2002 53.945N 104.649W 580 Sandy 0.82  0.16

0.95  0.06 20.4  6.9

1.07  0.03 8.4  3.4

1.35  0.14 0.6  0.4

1.24  0.01 52.1  9.5

1.20  0.00 21.1  1.0

1.55  0.01 1.7  0.4

1.36  0.01 0.3  0.2

13.4  2.2

13.7  8.0

108.2  22.3

6.2  1.3

3.6  0.9

5.6  1.0

8.8  1.3

9.1  2.4 16.9  1.4 31.2  2.9

5.7  1.6 28.9  5.7 22.6  4.2

1.4  0.6 27.4  4.4 33.4  3.2

15.0  2.9 5.3  0.6 15.8  1.2

9.1  1.0 8.8  1.5 18.4  2.7

1.3  0.3 1.0  0.3 14.0  2.7

– 1.0  0.7 17.4  3.3

6 2.8 Jack pine

8 3.1 Jack pine

2 0.8 Jack pine

– – Jack pine

None

None

None

None

CSAT3 LI-7500 12 0.25

CSAT3 LI-7500 6, 10 0.25

18 dead, 1 live 1.3 Jack pine, Black spruce Jack pine, Trembling aspen, Black spruce CSAT3 LI-7500 10, 20 0.25

18 2.0 Jack pine

Black spruce

4 3.0 Jack pine, Trembling aspen Black spruce

CSAT3 LI-6262 29 0.35

Gill R3-50 LI-7000 16 0.35

SAT-550 LI-6262 6 0.1

CSAT3 LI-6262 5 0.1

Adapted from Amiro et al. (2006a) and Coursolle et al. (2006); Carbon stocks from Fluxnet-Canada (2008). Standard error.

carbon at OJP was about 5 t C ha1, which is much less than the fire sites, especially considering that forest floor carbon was lost during the fires. The interpretation is that there are two groups of sites based on location. One group consists of OJP and the harvested sites on sandier soils with small amounts of carbon in the soil; the other group is the fire sites with slightly heavier soils with greater amounts of soil carbon. This is complicated by the history of fire at OJP and the fire sites compared to the recent harvesting at the other sites, resulting in different pools of dead organic material.

2.2.

Flux measurements

Details of instrumentations used at each site are provided in Table 1, and are further described by Amiro et al. (2006a). At all sites, the eddy covariance (EC) technique was used to measure turbulent fluxes of carbon dioxide (CO2), latent heat (lE) and sensible heat (H) densities continuously throughout the year. At the fire sites, the instrumentation consisted of sonic anemometers (model CSAT3 Campbell Scientific, Logan, UT, USA and Edmonton, Canada) and open-path infrared gas analysers (IRGA, model LI-7500 LI-COR Inc., Lincoln, NE, USA) mounted within 30 cm of the sonic array. At the harvested and OJP sites, the instrumentation consisted of a sonic anemometer (model R3-50, Gill Instruments Ltd., Lymington, UK, at HJP75; model SAT-550, Kaijo Co., Tokyo, Japan, at HJP94; model CSAT3, Campbell Scientific Inc., at HJP02 and OJP), and a closed-path infrared gas analyzer (IRGA, model LI-6262, LICOR Inc, Lincoln, NE, USA, at HJP02, HJP94, and OJP; model LI7000, LI-COR Inc., at HJP75). The closed-path IRGAs were housed in temperature-controlled housings allowing year-

round sampling. Air samples were drawn into the IRGAs at 10 L min1 using 3–4 m long heated sampling tubes. The IRGAs at all sites were calibrated frequently using gases of known CO2 concentration. The instruments at each site were mounted above the canopy on scaffolding or triangular towers. NEE was calculated from the 30-min flux and storage below the flux measurement height. Net ecosystem production (NEP) was calculated as negative NEE (NEE). Positive NEP corresponds to C gained by the forest whereas negative NEP indicates C lost to the atmosphere. Air temperature above and within the canopy was measured using HMP45C temperature/humidity probes (Campbell Scientific Inc.). Soil temperature (Ts) at various depths was measured using either chromel-constantan or copper-constantan thermocouples. Soil heat flux density (G) was measured using heat flux plates Thornthwaite Model 610 (Pittsgrove, NJ, USA) except at the OJP site where Middleton plates (model CN3, Middleton Solar, Yarraville, Victoria, Australia) were used. Volumetric soil water content in the top 30 cm was measured using reflectometers model CS615 (Campbell Scientific Inc.), while photosynthetically active radiation (PAR) was recorded using either ML-020P (Eko, Co., Ltd., Tokyo, Japan) or LI-190 (LI-COR Inc., Lincoln, NE, USA) quantum sensors.

2.3.

Data processing and gap-filling procedures

Precipitation events occasionally cause instrument malfunctions, thus data quality control included removal of spikes caused by instruments malfunction and other causes. Nighttime flux data below a site-specific friction velocity (u*)

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threshold were also removed (Table 1). The u* threshold for each site was defined after binning night-time CO2 flux measurements into 10 u* bins with equal sample numbers, and then identifying the u* value corresponding to 80% of the CO2 flux averaged over the last three bins. After screening, more than 60% of the data remained at all sites. Missing data were gap-filled using standard methods developed by the Fluxnet-Canada Research Network (Barr et al., 2004; Amiro et al., 2006a). The methods used the relationship between ecosystem respiration (Re) and soil temperature at a 5-cm depth to fill missing respiration data and photosynthetic uptake was filled through a relationship between gross ecosystem production (GEP) and above-canopy incoming PAR. Measurements at the F77, F89 and F98 site were made with open-path IRGAs. Our experience has been that these instruments do not give reliable measurements during cold temperatures, with some of the issues likely caused by instrument heating (Amiro et al., 2006b; Grelle and Burba, 2007). Hence, we excluded flux measurements when the air temperature <0 8C, and filled these winter gaps using sitespecific regressions between soil temperature at the 2-cm depth and night-time NEE for an air temperature dataset between 0 and 10 8C (Sass, 2007). Note that the development of these regressions included conditions when the soil temperature was <0 8C but when the air temperature was >0 8C. In each year, both spring and autumn conditions were used to develop the regression.

2.4.

selected GEP instead of NEP for the WUE calculation to avoid issues with Re that would arise from decomposition of coarse woody debris and other heterotrophic processes in the younger forest sites. Data manipulation and statistical analyses were done using Matlab (Version 7.3.0, The MathWorks, Natick, MA, USA).

3.

Results and discussion

3.1.

Gross ecosystem production

At all sites, GEP increased in spring reaching a peak during mid-summer, about day of the year (DOY) 190 and thereafter declined in response to changes in air temperature and solar radiation (Fig. 1). Generally, the fire sites had higher GEP compared to the harvested sites. During both 2004 and 2005, F89 had the highest GEP (maximum 10 g C m2 d1) followed by F77 (maximum 7 g C m2 d1) while HJP02 had the lowest GEP (maximum 1 g C m2 d1). The OJP site had relatively similar GEP to the much younger harvested sites (HJP75 and HJP94) and the youngest fire site F98, with a maximum GEP of 4 g C m2 d1. The generally higher GEP at the fire sites compared to the harvested sites may be attributed to the presence of both coniferous and deciduous species on the fire

Environmental controls on CO2 exchange

Regression analyses using non-gap-filled data were performed to relate Re to Ts at the 2-cm depth, and to relate GEP to PAR. We used a two-parameter exponential equation to relate Re to Ts as follows: Re ¼ A exp ðB  Ts Þ

(1)

where Re is ecosystem respiration (mmol m2 s1), A and B are fitted parameters, and Ts is the soil temperature at the 2-cm depth. The relative change in respiration rate for a 10 8C change in soil temperature (i.e., temperature sensitivity coefficient; Q10 = exp(B10)) and the respiration rate (mmol m2 s1) at a reference soil temperature of 10 8C (R10 = AQ10) were calculated using the derived parameters. Only night-time data recorded when the u* was greater than a site-specific threshold (Table 1) and soil temperature was above zero for the period 1 May to 30 September were used. The data were bin-averaged with bin widths of 2 8C for Ts. To relate GEP to PAR, we used non-gap-filled bin-averaged (bin widths 50 mmol m2 s1) day-time data (PAR > 5 mmol m2 s1) and u* greater than a site-specific threshold (Table 1) for the period 1 June to 31 August of each year using the following equation: G¼

CQ# DþQ#

(2)

where G is GEP (mmol m2 s1), C and D are fitted parameters, and Q# is PAR. Average water use efficiency (WUE) at each site was calculated as the ratio of total GEP to total evapotranspiration (ET) using data for the period 1 June to 31 August each year. We

Fig. 1 – Weekly average of daily gross ecosystem production (GEP) (g C mS2 dS1) for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005.

agricultural and forest meteorology 149 (2009) 783–794

sites resulting in higher photosynthesis at the fire sites during the summer. Goulden et al. (2006) have clearly shown the changes in the species mix of a fire chronosequence in northern Manitoba from deciduous to coniferous with stand age. This affects both the maximum flux and the period of GEP, with deciduous forests having a shorter growing season. When comparing CO2 exchange for a large number of FluxnetCanada research network sites for August 2003, Coursolle et al. (2006) also observed that F89 and F77 had relatively high values of maximum GEP compared to many other forest ecosystems, even those of greater age.

3.2.

Ecosystem respiration

Weekly averages of daily Re fluxes during both 2004 and 2005 for all the study sites are shown in Fig. 2. Similar to GEP, Re increased during spring, reached a peak in late summer and then declined; however, Re reached the maximum later in the season, about DOY 230, compared to GEP, indicating a lag in Re. Dunn et al. (2007) and Bergeron et al. (2007) reported a similar observation from Canadian boreal black spruce forests. This lag in Re is likely caused by low soil temperatures during spring and the fact that the forest may initially replenish carbohydrate reserves prior to resumption of growth (Dunn et al., 2007; Goulden et al., 1997), plus higher soil temperatures in late summer that likely enhance heterotrophic respiration. In general, Re fluxes were higher for the fire sites compared to the

Fig. 2 – Weekly average of daily ecosystem respiration (Re) (g C mS2 dS1) for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005.

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harvest sites. During both years F77 and F89 had the highest Re fluxes (maximum 8 g C m2 d1), while HJP02 had the lowest fluxes (maximum 2 g C m2 d1). The other sites OJP, HJP75, HJP94 and F98 had relatively similar Re fluxes (maximum 4 g C m2 d1). The higher Re fluxes from F77 and F89 may be caused by decomposing coarse woody debris. However, these two sites also have greater soil respiration than F98, which is due in part to higher root respiration (Singh et al., 2008). Previous studies have shown that in the Canadian boreal forest, soil respiration contributes up to 70% of the total ecosystem respiration (Gaumont-Guay et al., 2006; Lavigne et al., 1997), but can be as high as 88% (Khomik et al., 2006). We suspect that the more vigorous successional vegetation at these young fire sites also has greater Re than the less diverse vegetation at the recently harvested sites.

3.3.

Net ecosystem production

Cumulative NEP shows that over the 2004–2005 period the youngest harvested site HJP02 was a C source throughout the year, while the other sites became C sinks from DOY 140 until the end of the year (Fig. 3). The only exceptions were F77, HJP94 and OJP which became relatively C neutral after about DOY 310. In 2005, only HJP02 was a C source throughout the year, while F89 and HJP94 became C sinks by DOY 130, and HJP75 and OJP became C sinks by DOY 140, remaining so until the end of the year. F77 became a C sink by DOY 140 and then became a C source by DOY 240 until the end of the year. F98 was relatively C neutral by DOY 180 and remained so until DOY 280 when it became a C source until the end of the year. In 2004, F89 reached a maximum cumulative NEP of 160 g C m2 by DOY 240 and then declined to 115 g C m2 by the end of the year, while in 2005, it reached a maximum of 210 g C m2 and dramatically declined to 50 g C m2 by the end of the year. Similar to F89, F77 reached a maximum of about 75 g C m2 by mid summer in both years and then collapsed to about 40 and 80 g C m2 by the end of the year in 2004 and 2005, respectively. In the months of July and August (mid-summer) NEP in the boreal forest is often depressed due to warmer soils, high respiration rates, and dry soil conditions (Barr et al., 2007; Bergeron et al., 2007; Dunn et al., 2007). It is very clear from Fig. 3 that NEP at some sites, such as F77, peaks in early summer and there is a carbon loss over the rest of the year. The dramatic decline in NEP at both the F77 and F89 sites is most probably a result of decaying coarse woody material contributing to Re. However, we have some additional uncertainty with the winter data for F98, F89 and F77 because of the exclusion of winter data collected by the open-path eddy covariance systems. Notably, our estimate of winter respiration at F77 is greater than that from the other sites (Fig. 2), contributing to the overall carbon loss from this site. To assess whether this winter uncertainty changes the conclusions for F77, we gap-filled the F77 winter data using the winter data from F89. This site was chosen because it is in closest proximity to F77 and showed a positive NEP for the year, providing an upper limit for comparison. For 2005, this changed the annual NEP total for F77 from 78 to 24 g C m2. Although this decreased the loss, it demonstrates that F77 lost carbon even considering a broader range of uncertainty.

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Fig. 3 – Cumulative daily net ecosystem production (NEP) for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005.

3.4.

Annual carbon balance

The age of the stand influences the annual C balance, particularly following harvest (Table 2). Although GEP is important, Re is often the main determinant of the net annual C balance in forests (Valentini et al., 2000). During both years, the youngest harvest site (HJP02) was the strongest C source of any of the sites, losing more than 120 g C m2 year1. The Re/ GEP ratio for this site was 2.75 and 2.24 in 2004 and 2005,

respectively (Table 2), clearly indicating that overall C exchange at this site was dominated by Re. Over the 2 years, HJP94 was close to C neutral accumulating 27 g C m2 with an average Re/GEP ratio of 0.97 whereas HJP75 was a moderate C sink totalling 159 g C m2, with an average Re/GEP ratio of 0.86. The OJP site was close to C neutral or a small sink with Re/GEP ratios of 0.99 and 0.94 in 2004 and 2005, respectively. Among the three youngest fire sites, stand age seemed to have no major role. The F77 site was a moderate C source with Re/GEP ratios of 1.05 and 1.09 in 2004 and 2005, respectively. This contrasts to F89, which was the largest C sink of any of the sites, totalling 168 g C m2 over the 2 years. The F98 site was C neutral (NEP = 3 g C m2 year1) in 2004 but was a small C source (NEP = 43 g C m2 year1) in 2005, with Re/GEP ratios of 0.99 and 1.06 in 2004 and 2005, respectively. The greater source strength at F77 was surprising considering that F89 and HJP75 (closer in age) were moderate C sinks. This may be partially attributable to actively decaying woody material contributing to higher Re compared to the other sites, particularly the harvested sites. Even though the amount of dead aboveground material is similar at F77 and F89 (Table 1), the coarse woody debris at F77 is in more contact with the ground and is visibly decaying. Hence, we suspect that the respiration flux from coarse woody debris is greater at F77, based on observations along other fire chronosequences (Bond-Lamberty et al., 2003). This may indicate that C dynamics following fire go through four phases compared to three phases for harvested sites: soon after fire, burned sites become C sources; then become C sinks; then become C sources again when the dead woody material starts decaying; and thereafter become C sinks or neutral. In contrast, harvested sites are C sources soon after harvest; C sinks at intermediate age; and then C neutral or a small sink at maturity. This hypothesised pattern is still uncertain because of only three points in each chronosequence at ages <50 years. This could be universally true, or perhaps the F77 and F89 sites are peculiarities that are atypical. Such questions indicate the need for more sites to be added at other ages, and at different spatial locations for fires of the same age.

3.5.

Evapotranspiration and water use efficiency

During both years, cumulative ET for June to August was generally higher for the fire sites ranging from 192 to 313 mm compared to 117 to 194 mm at the harvested sites (Table 3).

Table 2 – Annual total ecosystem respiration (Re), gross ecosystem production (GEP), net ecosystem production (NEP) and the ratio of Re to GEP for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005. Site

F77 F89 F98 HJP75 HJP94 HJP02 OJP

Re (g C m2 year1)

GEP (g C m2 year1)

NEP (g C m2 year1)

Re/GEP ratio

2004

2005

2004

2005

2004

2005

2004

2005

791 787 385 464 360 245 556

984 849 499 513 376 222 558

752 903 388 544 353 89 560

906 902 456 592 410 99 594

39 115 3 80 7 155 4

78 53 43 79 34 123 36

1.05 0.87 0.99 0.85 1.02 2.75 0.99

1.09 0.94 1.09 0.87 0.92 2.24 0.94

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Table 3 – Total growing season rainfall (May–September), evapotranspiration (ET), gross ecosystem productivity (GEP) and water use efficiency (WUE) during the period June to August for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005. Site

2004

WUE (g C kg1 water)

2005

Rainfall (mm)

ET (mm)

GEP (g C m2)

Rainfall (mm)

ET (mm)

GEP (g C m2)

2004

2005

441

247 253 192 194 177 124 183

507 656 271 435 241 64 376

422 491 424 407 481

281 313 245 177 143 117 156

563 631 319 369 236 67 373

2.05 2.59 1.41 2.24 1.36 0.52 2.05

2.00 2.02 1.30 2.08 1.65 0.57 2.39

F77 F89 F98 HJP75 HJP94 HJP02 OJP

* 502 * 568 * 491

* 433

* = missing data.

The youngest harvested site (HJP02) had the lowest ET, while F89 had the highest. Total annual ET at the F89 site also tends to be higher than other nearby sites (Amiro et al., 2006a). The higher ET at the fire sites compared to the harvested sites was likely, in part, a result of the presence of both deciduous and coniferous trees. In addition, the fire sites had higher soil water content than the harvested sites, which might have enhanced ET (Fig. 4). We calculated WUE as the ratio of the sum of GEP to the sum of ET for the June to August period (Table 3). The selection

Fig. 4 – Weekly average of volumetric soil water content in the 0–30-cm depth (SWC; m3 mS3) for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005.

of the summer period ensures that deciduous leaves are developed so that canopy processes dominate the forest flux. Average WUE for the 2 years was lowest for HJP02 at 0.62 g C kg1 water, followed by F98 at 1.36 g C kg1 water and HJP94 at 1.51 g C kg1 water. All the other sites had relatively similar WUE values ranging from 2 to 2.6 g C kg1 water, with the highest value being for F89. McCaughey et al. (2006) estimated an average daily WUE of 2 to 2.5 g C kg1 water for a boreal mixed-wood forest in Ontario, while Coursolle et al. (2006) recorded average WUE values for August 2003 ranging from 0.05 to 6.6 g C kg1 water for several Canadian forests and peatlands. The latter study also reported that HJP02 had the lowest WUE at 0.05 g C kg1 water. They estimated WUE values for F89, F77, F98, HJP94 and OJP at 3.3, 2.5, 1.1, 1.6 and 1.5 g C kg1 water, respectively, which are comparable to our values for the same sites. Kljun et al. (2007) found that WUE was 4.62, 3.12, and 3.12 g C kg1 water for an old boreal aspen stand, a boreal black spruce stand and OJP, respectively. Regardless of the method of disturbance, recently disturbed sites tend to use water less efficiently, likely because of greater relative surface evaporation compared to whole ecosystem evapotranspiration, without carbon uptake through leaves. Our sites with essentially full canopy cover (LAI > 2, Table 1) all have WUE values in the range of 2– 2.6 g C kg1 water, and it is only the most open sites (HJP02, HJP94, F98) that have WUE <2 g C kg1 water.

3.6.

Environmental factors affecting Re and GEP

3.6.1.

Relationship between Re and soil temperature

The relationship between Re and Ts at the 2-cm depth was investigated using non-gap-filled night-time data when u* was greater than the specified site friction velocity (Table 1) and PAR<5 mmol m2 s1. At all the sites, Re increased exponentially with increasing soil temperature at the 2-cm depth (Fig. 5). The F89 site had the greatest temperature response as shown by Q10 values that exceeded 5 (Table 4). The other sites had Q10 values between 2.5 and 4. In the absence of water stress, larger Q10 values indicate greater temperature sensitivity (Khomik et al., 2006). We also calculated mean respiration rates scaled to a reference temperature of 10 8C (R10) for each site to remove the influence of different soil temperatures among sites. In both years, F77 and F89 had the highest R10 values (5–6 mmol m2 s1), whereas HJP02 had the lowest (0.8–1 mmol m2 s1) (Table 4). The Q10 and R10 values reported

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Fig. 5 – Relationship between ecosystem respiration (Re) and soil temperature at the 2-cm depth for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005. Data were binned by temperature bin widths of 2 8C.

in the current study are similar to those reported by other researchers for boreal forest ecosystems (Griffis et al., 2003; Giasson et al., 2006; Bergeron et al., 2007, 2008). At all sites and both years, the relationships between Re and Ts were highly significant ( p < 0.001) with R2 values ranging from 0.84 to 0.98 (Table 4). McCaughey et al. (2006) found that Ts at 2-cm depth accounted for 93% of the variation in Re in a boreal mixedwood forest, whereas Barr et al. (2007) reported that Ts at the 2-cm depth accounted for 69% of the variation in Re in a boreal aspen forest.

3.6.2.

Relationship between GEP and PAR

The relationship between GEP and PAR was investigated using non-gap-filled June-to-August data, which includes the period of full leaf development for deciduous species. At all sites and both years, GEP increased with PAR explaining from 77 to 96% of variation in GEP depending on the site (Fig. 6; Table 5). This is a higher portion than observed at some other sites, such as coastal Douglas-fir stands in British Columbia (56–78%; Humphreys et al., 2006). The response of GEP to PAR was highest at F89 followed by F77 and lowest at HJP02. The F98, HJP75 and OJP sites had similar light responses, with HJP94

Fig. 6 – Relationship between gross ecosystem production (GEP) and photosynthetically active radiation (PAR) for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005. Data were binned by PAR bin widths of 50 mmol mS2 sS1.

being slightly lower. The two fire sites (F89 and F77) and the mature site (OJP) showed a decline in GEP at PAR 1400 mmol m2 s1, probably a result of stomatal closure. A decline in GEP at PAR > 1000 mmol m2 s1 is common in other forest and tundra sites (Humphreys et al., 2005; Bergeron et al., 2008; Lafleur and Humphreys, 2008). The relationship between GEP and PAR would be expected to be a function of LAI. This tends to be true for HJP02 and HJP94, which have the lowest LAI (Table 1) and the lowest GEP response to PAR. However, this is not strictly true for the remaining sites. For example, HJP75, F89 and F77 all have LAI values between about 2.8 and 3.1 (Table 1), but there is a clear difference in response with F89 > F77 > HJP75. Some of this could be explained by species differences, with F89 having a larger deciduous component. However both HJP75 and F77 are almost pure jack pine. Part of the difference could be caused by differences in canopy development with possible nutrient implications. In particular, HJP75 has a tree density of about 7000 stems ha1 measured in 2002, which has decreased from about 10,000 stems ha1 measured in 1994 (Gower et al., 1997). This contrasts to F77, of relatively similar age, with about 3100 stems ha1. Similarly, the basal area at HJP75 is about 70 m2 ha1, compared to only 14 m2 ha1 at F77. We do not have a sequential record of stem densities at these sites to

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Table 4 – Ecosystem respiration at reference temperature of 10 8C (R10), temperature coefficient of ecosystem respiration (Q10), parameters (A and B) for the first-order exponential relationship (Eq. (1)) between night-time ecosystem respiration and soil temperature at the 2-cm depth, standard error of estimate (S.E.), and coefficient of determination (R2) for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005. All relationships are significant ( p < 0.002). Year

R10 (mmol m2 s1)

Site

Q10

Parameters A  S.E.

B  S.E.

S.E.

R2

2004

F77 F89 F98 HJP75 HJP94 HJP02 OJP

5.60 4.82 2.01 2.31 1.53 1.05 2.81

2.78 5.53 3.43 2.56 3.44 2.73 3.08

2.01  0.35 0.87  0.17 0.59  0.20 0.90  0.08 0.44  0.08 0.39  0.05 0.91  0.18

0.10  0.02 0.17  0.01 0.12  0.03 0.09  0.01 0.12  0.01 0.10  0.01 0.11  0.02

0.70 0.63 0.54 0.15 0.21 0.12 0.45

0.91 0.98 0.84 0.98 0.95 0.95 0.92

2005

F77 F89 F98 HJP75 HJP94 HJP02 OJP

5.79 4.53 2.30 1.91 1.60 0.80 2.35

4.27 5.19 2.92 3.53 3.44 4.17 3.59

1.17  0.20 0.44  0.32 0.71  0.18 0.56  0.08 0.64  0.10 0.55  0.03 0.17  0.08

0.15  0.01 0.19  0.05 0.11  0.02 0.13  0.01 0.09  0.01 0.12  0.01 0.14  0.01

0.65 1.59 0.43 0.22 0.21 0.10 0.22

0.97 0.83 0.88 0.98 0.92 0.90 0.97

evaluate whether they had a similar initial density, or whether any of the differences can be solely attributed to the differences between successional trajectories of fire and harvesting. However, it is likely that nutrients, and potentially water, could be more limiting at HJP75 with much greater competition among trees and sandier soil.

3.7.

Additional factors affecting C dynamics

Besides forest age, disturbance and climatic conditions, a range of other factors, including soil nutrients, particularly

Table 5 – Parameters (C and D) for relationship (Eq. (2)) between gross ecosystem production (GEP) and photosynthetic active radiation (PAR), standard error of estimate (S.E.), coefficient of determination (R2) and probability ( p) values for three post-fire sites (F77, F89, F98), three post-harvest sites (HJP75, HJP94, HJP02) and one mature site (OJP) in 2004 and 2005. All relationships are significant ( p = 0.0001). Year

2004

2005

Site

Parameters

S.E.

R2

C  S.E.

D  S.E.

F77 F89 F98 HJP75 HJP94 HJP02 OJP

15.9  0.53 23.7  0.83 10.0  0.67 10.8  0.69 7.2  0.38 1.7  0.10 11.6  0.57

372.7  41.6 485.1  49.9 547.9  101.1 436.8  83.6 434.9  71.9 193.4  50.5 426.1  63.8

0.77 0.99 0.71 0.83 0.50 0.19 0.70

0.94 0.96 0.90 0.86 0.90 0.77 0.92

F77 F89 F98 HJP75 HJP94 HJP02 OJP

18.1  0.60 23.8  1.22 13.9  0.90 13.1  0.68 7.4  0.40 2.2  0.16 11.9  0.59

392.5  42.9 486.8  72.9 770.4  116.4 490.3  74.3 411.3  70.7 239.1  69.4 352.5  60.6

0.85 1.45 0.70 0.80 0.54 0.26 0.91

0.95 0.92 0.94 0.92 0.80 0.77 0.89

nitrogen (N) and water availability, play a major role in determining the magnitudes of CO2 fluxes in boreal forests. Nitrogen availability is a primary limiting factor for tree productivity in the boreal forest (Turkington et al., 1998; Melnychuk and Krebs, 2005; De Vries et al., 2006; Hyvonen et al., 2007; DeLuca et al., 2008). Total soil N measured in 2003 was significantly ( p < 0.05) higher at the three youngest fire sites (F77, F89, F98) compared to the harvested sites and OJP (Table 1). However, leaf N concentrations measured as an average of sun and shade leaves were often higher at the harvested sites. Hence, the N content is not an obvious reason for site differences. Similarly, volumetric soil water content during both 2004 and 2005 was much higher at the three youngest fire sites compared to the harvested sites and OJP (Fig. 4). This may have contributed to the higher GEP at the burned sites, particularly at F77 and F89. Increased water stress from drought during the growing season reduces GEP (Barr et al., 2002). We therefore suspect that the differences in CO2 fluxes between the fire and harvested sites are also partly explained by the differences in soil water. However, the vegetation also partially dictates the water and nutrient regime, which complicates the reasons for the differences among sites. As pointed out earlier, the challenges to comparing the two processes of fire and harvesting are related to establishing treatments that have similar climates, soil conditions, and tree species.

4.

Conclusions

This study has shown that the fire sites had generally higher GEP, Re and ET than the harvested sites, which we believe is largely a result of greater species diversity at the fire sites coupled with higher soil water content. Regardless of disturbance history, NEP was generally negative for the younger sites, indicating that recently disturbed sites are C sources. This is consistent with previous studies of forest

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chronosequences that used carbon inventories (e.g., Wirth et al., 2002; Rothstein et al., 2004; Gough et al., 2007). The F77 site was a significant C source, most likely a result of decaying woody material at this site. Water use efficiency was lowest for the youngest sites (HJP02, HJP94 and F98), most likely because of greater relative surface evaporation at these sites. At all sites, Re could be predicted reasonably well as an exponential function of soil temperature at the 2-cm depth, with the F89 and F77 sites having the strongest temperature response. Similarly, F89 and F77 had the greatest responses of GEP to PAR. The loss of carbon from the Canadian boreal forest will likely increase in the future because the estimated increases in disturbance will overcome potential growth increases in a warming climate (Kurz et al., 2008a). Fire (Flannigan et al., 2005) and insect effects (Logan et al., 2003) are likely to increase, even if harvesting is reduced. The post-disturbance effects of insects are not well studied, with few flux towers established. Further, flux tower measurements of ecosystem carbon loss by the F77 site, three decades after disturbance, suggest a more dynamic trend that does not develop into a smooth approach to a steady asymptote. Even though the sites compared in this study were in relatively close proximity, differences in the site characteristics have complicated the comparisons. This makes it difficult to attribute the flux differences solely to the disturbance history. However, the three harvested sites and the three youngest fire sites form individual chronosequences that are internally comparable. This indicates that there is a need for a better characterisation of the relationship between the carbon balance and forest development following disturbance, which we have not yet captured fully.

Acknowledgements We gratefully acknowledge the help and support from many individuals: C. Day, D. Flanders, B. Mottus, R. Hurdle, and T. Varem-Sanders of the Canadian Forest Service; R. Ketler and A. Sauter of the University of British Columbia; J. Eley, C. Hrynkiw, C. Smith, B. Cole, and S. Enns of the Meteorological Service of Canada; D. Zuiker, S. McQueen, and D. Finch of Queen’s University; and J. Weir and D. Wieder of Parks Canada. Funding was provided by the Climate Research Branch of the Meteorological Service of Canada, the Canadian Forest Service, the Natural Sciences and Engineering Research Council of Canada (NSERC), the FLUXNET-Canada Network (NSERC, the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS), and BIOCAP Canada), the Canadian Carbon Program (CFCAS), Parks Canada, the Program of Energy Research and Development (PERD), and Action Plan 2000. The HJP94 site was established by H. Iwashita, S. Murayama, and N. Saigusa of the National Institute of Advanced Industrial Science and Technology, Japan, funded by the Special International Joint Research Program of the Agency of Industrial Science and Technology and the Institute for Environmental Management Technology of AIST. We also recognize support from Weyerhaeuser Canada for access to the HJP94 and SOJP sites, and permission from Saskatchewan Environment to work at F89 and F77.

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