The Full Annual Carbon Balance of Boreal Forests Is Highly Sensitive

Jun 17, 2014 - Our results call for future analysis of this dual effect of precipitation on NEE and LCE. Methodology ... Decoupling of carbon dioxide ...
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Letter pubs.acs.org/journal/estlcu

The Full Annual Carbon Balance of Boreal Forests Is Highly Sensitive to Precipitation M. G. Ö quist,*,† K. Bishop,‡,§ A. Grelle,∥ L. Klemedtsson,⊥ S. J. Köhler,‡ H. Laudon,† A. Lindroth,# M. Ottosson Löfvenius,† M. B. Wallin,@ and M. B. Nilsson† †

Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83 Umeå, Sweden Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences (SLU), SE-75 007 Uppsala, Sweden § Department of Earth Sciences, Uppsala University, SE-752 36 Uppsala, Sweden ∥ Department of Ecology, Swedish University of Agricultural Sciences (SLU), SE-75 007 Uppsala, Sweden ⊥ Department of Earth Sciences, Gothenburg University, SE-405 30 Gothenburg, Sweden # Department of Physical Geography and Ecosystem Analysis, Lund University, SE-221 00 Lund, Sweden @ Department of Ecology and Genetics/Limnology, Uppsala University, SE-752 36 Uppsala, Sweden ‡

S Supporting Information *

ABSTRACT: The boreal forest carbon balance is predicted to be particularly sensitive to climate change. Carbon balance estimates of these biomes stem mainly from eddy-covariance measurements of net ecosystem exchange (NEE). However, a full net ecosystem carbon balance (NECB) must include the lateral carbon export (LCE) through discharge. We show that annual LCE at a boreal forest site ranged from 4 to 28%, averaging 11% (standard deviation of 8%), of annual NEE over 13 years. Annual LCE and NEE are strongly anticorrelated; years with weak NEE coincide with high LCE. The decreased NEE in response to increased precipitation is caused by a reduction in the amount of incoming radiation caused by clouds. If our finding is also valid for other sites, it implies that increased precipitation at high latitudes may shift forest NECB in large areas of the boreal biome. Our results call for future analysis of this dual effect of precipitation on NEE and LCE.

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INTRODUCTION The northern hemisphere’s vast boreal forest ecosystems are major contributors to the global terrestrial C sink1 and predicted to be particularly sensitive to climate change.2 The current understanding of their carbon balance stems mainly from eddy-covariance net ecosystem exchange (NEE) measurements.3 However, NEE includes only the vertical CO2 exchange driven by photosynthesis and ecosystem respiration, and estimates of full net ecosystem carbon balance (NECB)4 require inclusion of lateral carbon export (LCE) through catchment discharge. In boreal forests, NEE is not generally limited by water availability,5 although correlations (both positive and negative) between precipitation and both NEE and gross primary production (GPP) have been detected.6 Under climatic conditions imposing little or no water limitation on plants, the most important consequence of precipitation will be that overcast skies reduce incoming photosynthetic photon flux density (PPFD), resulting in a decreased GPP and C sink strength of the system.7,8 Cloud cover has complex effects on tree photosynthesis, and in some cases, shading by clouds may increase GPP because it increases relative levels of diffuse radiation in the canopy.9 However, this may occur only under thin or scattered clouds, which substantially increase the © 2014 American Chemical Society

proportion of diffuse light with minor reductions in the total amount of incoming radiation.8 Thicker clouds, typically associated with precipitation, have negative overall effects on GPP because they significantly decrease the total amount of radiation (direct and diffuse).7,8 Inland waters are important for global terrestrial C cycling. The most recent estimate of global C export from terrestrial systems via the “aquatic conduit”, including both dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC), is ∼3 Gt of C year−1.10−12 This LCE, which includes emissions of CO2 from water surfaces,13 approximately equals the estimated net terrestrial C sink.1,14,15 Thus, even small changes in either NEE or LCE in the boreal region could greatly affect the NECB, with global implications. The NEE of boreal forest ecosystems varies substantially interannually, typically within 3−5-fold ranges.15−17 The LCE is controlled by both the discharge rate and concentrations of dissolved C species in the groundwater. The discharge rate has been identified as the main driver for annual LCE (including Received: Revised: Accepted: Published: 315

May 30, 2014 June 16, 2014 June 17, 2014 June 17, 2014 dx.doi.org/10.1021/ez500169j | Environ. Sci. Technol. Lett. 2014, 1, 315−319

Environmental Science & Technology Letters

Letter

Table 1. Annual Precipitation, Estimated Lateral C Export of DIC and DOC across the Soil−Stream Interface, Measured NEE, and Fraction of NEE Lost Annually by Lateral Export (LCE/NEE) from 1997 to 2009 (values in parentheses are SDs) year

precipitationa (mm)

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

515 848 550 828 824 469 598 643 581 628 561 659 667

DIC export (g of C m−2) 2.5 5.9 2.9 6.9 6.2 2.3 2.3 3.5 2.6 3.1 2.4 4.2 4.6

DOC export (g of C m−2)

(0.5) (1.1) (0.6) (1.3) (1.2) (0.4) (0.4) (0.7) (0.5) (0.6) (0.5) (0.8) (0.9)

3.5 14.2 4.0 11.7 12.8 2.7 2.8 6.0 3.9 4.9 3.3 7.5 8.8

(0.5) (1.8) (0.5) (1.5) (1.7) (0.4) (0.4) (0.8) (0.5) (0.6) (0.4) (1.0) (1.1)

LCE (g of C m−2) 6.0 20.1 6.9 18.5 19.5 5.1 5.1 9.5 6.6 8.0 5.7 11.7 13.3

(0.7) (2.2) (0.8) (2.0) (2.0) (0.6) (0.6) (1.0) (0.7) (0.9) (0.6) (1.3) (1.4)

NEE (g of C m−2) −143 (14) −71 (7) −134 (13) −81 (8) −92 (9) −139 (14) −116 (12) −80 (8) −88 (9) −159 (16) −137 (14) −106 (11) −87 (9)

LCE/NEE 0.04 0.28 0.05 0.23 0.21 0.04 0.04 0.12 0.07 0.05 0.04 0.11 0.15

(0.01) (0.06) (0.01) (0.05) (0.04) (0.01) (0.01) (0.02) (0.02) (0.01) (0.01) (0.02) (0.03)

a Total annual precipitation [Ottosson-Löfvenius, M. Klimatdata (http://www.slu.se/sv/fakulteter/s/om-fakulteten/institutioner/enheten-forskoglig-faltforskning/miljoanalys/arsrapporter/)].

both DOC18 and DIC19) by boreal headwater streams because discharges vary more interannually than dissolved C concentrations (typically within 5-fold18 and 1.5−2-fold ranges,18−20 respectively). Variations in annual discharge in the systems are largely controlled by variations in annual precipitation; hence, the LCE is highly sensitive to changes in precipitation patterns.21 Given the potential variations in annual LCE due to changes in annual discharge and observed variations in annual forest NEE, the interannual covariation of these processes could strongly influence the boreal NECB. Thus, the presented investigation assessed the importance of LCE mediated by surface waters for total boreal forest C budgets and the sensitivity of NECB to changes in precipitation and runoff. We hypothesized that LCE significantly increases during wetter years as compared to drier years, making precipitation a key environmental factor for the boreal forest NECB. As a model system, we used a 12 ha forested catchment draining into a small first-order stream and estimated the LCE from forest soils to surface waters over 13 years. To assess the relative importance of LCE for NECB, we then compared the estimated rates with NEE rates during the same time frame in a forest 20 km away with similar stand characteristics. In addition to LCE, a full NECB, by definition,4 includes removals by, e.g., harvest, fires, and herbivory. However, no harvest or fire occurred during the study period, and C losses from herbivory were neglected.

groundwater DIC concentrations taken at three replicate sites over the course of a year.19 The predicted DIC export rates did not significantly differ from an additional 70 independent groundwater measurements taken over 18 months, validating the model (see Figure S1 of the Supporting Information and Figure S2 for more detail). Annual NEE values were derived from EC measurements16,25 at a forested site in the same area as the catchment used for the LCE study.26 EC data were treated according to the EUROFLUX methodology.27 Negative and positive values for C flux terms indicate C sinks and sources, respectively. To identify other boreal forest areas with similar water balances, a humidity index (H-index) was defined on the basis of the Koeppen climate classification method,28,29 which requires only monthly mean temperature and precipitation data. We used high-resolution gridded observation data, mean values from 1961 to 1990 (http://www.ipcc-data.org) for Eurasia and North America. The H-index indicates the amount of water (in millimeters), calculated as the difference between precipitation and evapotranspiration, potentially available for discharge at the spatial scale represented by the surface grids. Thus, the H-index should not be used as a measure of actual discharge, because the biophysical processes determining local and site specific water balances are far more complex.30 However, the H-index is useful for scaling climate conditions to the boreal zone and identifying broad regions where water balance properties are probably similar to those in the investigated area. See the Supporting Information for further details about the methods and uncertainty estimates of C flux terms.

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METHODS LCE was investigated in a small catchment (∼12 ha; 64°14′N, 19°46′E) with 100% forest cover dominated by Norway spruce [Picea abies (L.) Karst], with pine (Pinus sylvestris) present in elevated areas.22 Daily stream discharges over the 13 years (1997−2009) were calculated from continuous stage height measurements at a 90° V notch weir at the stream outlet.23 DOC export was estimated from weekly to biweekly sampling of streamwater (>500 samples in total). Daily values of DOC concentrations were estimated by linear interpolation between sampling points.24 Estimates of daily lateral DIC export from soils, including that which was subsequently emitted vertically from the stream surface to the atmosphere, were obtained using a relationship between discharge and DIC export. This relationship is modeled from a total of 192 samples of

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RESULTS AND DISCUSSION The annual LCE (DIC + DOC) over the 13 year period ranged from ∼5 to ∼20 g of C m−2 year−1 (CV = 0.54), and the NEE of the forest ranged from −159 to −71 g of C m−2 year−1 (CV = 0.27) [negative values indicate ecosystem uptake (Table 1)]. Thus, annual LCE varied much more than annual NEE. The two C fluxes correlated strongly (r = 0.76; p = 0.003), so years with weak negative NEE coincided with years of high LCE. Consequently, the fraction of NEE lost annually through LCE varied markedly from ∼4 to ∼28% {mean of 11 ± 8% [standard deviation (SD)]; n = 13 (Table 1)}. Clearly, investigations limited to a few years are insufficient to estimate the 316

dx.doi.org/10.1021/ez500169j | Environ. Sci. Technol. Lett. 2014, 1, 315−319

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importance of LCE for terrestrial C balances, as they may either over- or underestimate this exchange relative to NEE. One of the longest and most complete time series of DOC and DIC records for any boreal site is available for the catchment used for estimating LCE,23 which is regarded as being representative of headwater catchments in the region.22−24,31 The mean and range of annual LCE (10.5 and ∼5−20 g of C m−2 year−1, respectively) agree well with reported LCE values in the area, based on measurements at 16 catchments ranging in size from 10 to 7000 ha,20 and other parts of the boreal region (3.5−19.3 g of C m−2 year−1).32,33 Further, both DOC and DIC exports appear to be only weakly related to stand age (inferred from stem volume per unit area) and tree species composition.20,24,31 We therefore assume the LCE data to be valid also for the site used to acquire NEE data. While the LCE term in our measurements represents a fairly robust estimate of C export from a substantial area, the NEE estimates are derived from a more limited area representing C exchange under specific stand conditions. Multiyear averages of NEE in Fennoscandian boreal forest ecosystems reportedly average approximately −77 ± 108 g of C m−2 year−1 (five sites, 38 site years), ranging from approximately −200 to 50 g of C m−2 year−1.34 Thus, the site used in this study, with an average NEE of −111 (66 SD) g of C m−2 year−1, appears to be fairly representative for boreal conditions. Nonetheless, the spatial variability in forest NEE suggests some caution with respect to the absolute contribution of NEE and LCE for the NECB and to the extent the values may be generalized. However, this does not affect the observed anticorrelation of NEE and LCE that exacerbates the sensitivity of NECB to precipitation-related environmental conditions during the growing season. The δ13C−CO2 values of soil and streamwater DIC range from −21 to −26‰, indicating a biogenic source of DIC and a direct relationship with the contemporary C balance.35,36 Annual precipitation was strongly correlated with LCE (r = 0.97; p < 0.001), in accordance with previous studies of LCE in boreal areas.18,37 It was also correlated with NEE (r = 0.74; p = 0.004), and the C sink was smaller during wet years. Further, the deviation in annual precipitation from the 28 year average (1980−2008) explained 90% of the variation observed in the fraction of newly sequestered C lost annually through LCE, and a predicted 10−20% increase in annual precipitation in the boreal region1 would approximately double the fraction of NEE lost annually through LCE from the terrestrial system to surface waters (Figure 1). This would significantly alter the current NECB properties of northern forest ecosystems. In major boreal forest areas, precipitation exceeds evapotranspiration and plant growth is not generally limited by water availability.38 However, clouds strongly affect net C uptake by reducing the amount of incoming radiation and concomitant photosynthesis.7,8 The strong negative correlation we observed between precipitation and the average amount of incoming radiation [r = 0.77; p = 0.002 (Figure 2)] supports this conclusion in concert with the strong influence of precipitation on NEE. NEE represents the sum of GPP and ecosystem respiration, and the correlation between NEE and precipitation may also be caused by effects on respiratory components. During drier years, heterotrophic respiration in the O layer could be water-limited,39 and increased precipitation would increase soil respiration, further reducing net C uptake by the system. However, the reduction in the amount of incoming radiation could also reduce soil temperatures and thus soil respiration. Therefore, most of the strong correlation between

Figure 1. Relative annual LCE as a function of the deviation of annual precipitation from the 30 year (1980−2009) average. “Relative” is defined as the quotient between cumulated LCE and cumulated NEE. The solid line represents a function including all data (n = 13), and the dashed line excludes the three years with the highest annual precipitation (n = 10).

Figure 2. PCA loading plot of the correlation structure of LCE, NEE, and measured environmental variables. The annual precipitation (P), stream discharge (Q), length of the growing season (GS), temperature sum during the growing season (Tsum), and radiation sum during the growing season (Rsum) were obtained from http://www.slu.se/sv/ fakulteter/s/om-fakulteten/institutioner/enheten-for-skogligfaltforskning/miljoanalys/arsrapporter/. The start and end of the growing season were defined as the dates when the average daily air temperature 1.7 m above ground level was above and below 5 °C, respectively. The average daily temperature (Tav) and radiation (Rav) during the vegetation period were obtained by dividing Tsum and Rsum by GS (days). The C flux is expressed relative to the atmosphere, with negative values representing ecosystem C uptake.

precipitation and NEE we observed is probably due to effects of precipitation on GPP (Figure 2). Our findings may be relevant for other forest ecosystems where NEE is not water-limited, so a reduction in PPFD by overcast skies is the main precipitation-associated effect on NEE; excess water is available, allowing lateral export of dissolved C, and groundwater discharge is insufficiently high to 317

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typically occurs during the growing season. This is corroborated by a strong correlation between the spatial distribution of precipitation during the growing season (May to September) in the data set used to calculate discharges and estimated Hindices (Figure 3 and Figure S3 of the Supporting Information). Thus, most forested areas of the northern hemisphere appear to meet criteria required for the generality of our findings. In conclusion, our recorded effect of precipitation on NEE and LCE implies that both observed,38 and predicted,1 increases in annual precipitation may change NECB in large boreal forest areas. Increases in precipitation during the growing season reduce the level of C sequestration (due to cloud-induced reductions in PPFD and associated reductions in GPP) in areas where photosynthesis is not limited by water supplies, further reducing the strength of the C sink. More comprehensive long-term investigations of NEE and LCE at diverse boreal forest sites are needed to corroborate our conclusions. In addition, improvements are needed in spatial and temporal scaling of the C flux components and their environmental drivers (with rigorous validation of assumptions). Nonetheless, our results strongly suggest that effects of precipitation on both NEE and LCE should be considered in future analyses of the climate carbon cycle and associated feedbacks for relevance to ecosystem NECB.

exhaust soluble soil C supplies during the course of the year. Our results are consistent with findings of a previous analysis of relationships among NEE, cloud cover, and the amount of incoming radiation at 38 sites (spanning 97 site years), including 15 forested sites north of 50°N,7 indicating that the first requirement is generally met. The annual discharge at our LCE study site ranged from ∼150 to ∼500 mm year−1. Although annual discharge was tightly coupled to annual precipitation at the site, available runoff is the difference between evaporation (which is sensitive to radiation, vapor pressure, soil moisture, and vegetation properties) and precipitation. Thus, the prevailing discharge conditions [estimated from the H-index used as proxies for discharge (see Methods and the Supporting Information for a definition)] are apparently typical for most terrestrial areas between 50°N and 65°N (Figure 3). For areas north of 65°N

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ASSOCIATED CONTENT

S Supporting Information *

Methodology, including [DOC], [DIC], and discharge measurements, DIC export model validation, eddy-covariance measurements, estimation of uncertainty, H-index generation, and four figures containing information about DIC model validation, H-index generation, and a comparison between the H-index and measured discharge at the investigated site. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS This work was supported by grants from FORMAS, VR, Future Forest, and the Kempe Foundation.

Figure 3. Humidity index (H-index) distribution in the northern hemisphere. Potential annual runoff (millimeters) during the reference period (1961−1990) in (top) North America and (bottom) Eurasia. See Methods and the Supporting Information for further information. No calculations were made for polar climates, i.e., those where the temperature in the warmest month was