Tree Physiology 32, 599–611 doi:10.1093/treephys/tps021 Research paper Size-mediated tree transpiration along soil drainage gradients in a boreal black spruce forest wildfire chronosequence J.L. Angstmann1,3, B.E. Ewers1 and H. Kwon2 Downloaded from 1Department of Botany, Program in Ecology, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82072, USA; 2Yonsei University, Seoul, Korea; 3Corresponding author ([email protected]) Received September 23, 2011; accepted February 24, 2012; published online April 25, 2012; handling Editor Nathan Phillips http://treephys.oxfordjournals.org/ Boreal forests are crucial to climate change predictions because of their large land area and ability to sequester and store carbon, which is controlled by water availability. Heterogeneity of these forests is predicted to increase with climate change through more frequent wildfires, warmer, longer growing seasons and potential drainage of forested wetlands. This study aims at quantifying controls over tree transpiration with drainage condition, stand age and species in a central Canadian black spruce boreal forest. Heat dissipation sensors were installed in 2007 and data were collected through 2008 on 118 trees (69 Picea mariana (Mill.) Britton, Sterns & Poggenb. (black spruce), 25 Populus tremuloides Michx. (trembling aspen), 19 Pinus banksiana Lamb. (jack pine), 3 Larix laricina (Du Roi) K. Koch (tamarack) and 2 Salix spp. (willow)) at four at University of Wyoming Libraries on June 27, 2016 stand ages (18, 43, 77 and 157 years old) each containing a well- and poorly-drained stand. Transpiration estimates from sap flux were expressed per unit xylem area, JS, per unit ground area, EC and per unit leaf area, EL, using sapwood (AS) and leaf (AL) area calculated from stand- and species-specific allometry. Soil drainage differences in transpiration were variable; only the 43- and 157-year-old poorly-drained stands had ~ 50% higher total stand EC than well-drained locations. Total stand EC tended to decrease with stand age after an initial increase between the 18- and 43-year-old stands. Soil drainage differences in transpiration were controlled primarily by short-term physiological drivers such as vapor pressure deficit and soil moisture whereas stand age differences were controlled by successional species shifts and changes in tree size (i.e., AS). Future pre- dictions of boreal climate change must include stand age, species and soil drainage heterogeneity to avoid biased estimates of forest water loss and latent energy exchanges. Keywords: chronosequence, Picea mariana, Pinus banksiana, Populus tremuloides, poorly drained, soil moisture. Introduction Bond-Lamberty et al. 2005, 2009). Poorly-drained soils result Ecosystem water fluxes have ecological and socioeconomic from the pooling of snow melt in lower-lying stands that have consequences including controls over carbon cycling, water Sphagnum-insulated permafrost, inhibiting water drainage into availability for plants and rivers, and subsequent agricultural deeper soil layers (Gorham 1991, Bond-Lamberty et al. and hydroelectric power production (Mimikou and Baltas 1997, 2004a). Paludification, or the buildup of deep organic soils as Jackson et al. 2005, Bonan 2008). Boreal forests are of par- a result of cold, anoxic conditions, results in dwarfed tree mor- ticular interest in climate change studies because of their large phology (Roy et al. 1999, Lavoie et al. 2005). While some spe- land area, location in northern latitudes (Manabe and Wetherald cies have shown adaptation and/or acclimation to flooded 1980, Goulden et al. 1998, Stocks et al. 1998), ability to conditions, such as lenticel and adventitious root (i.e., root sequester and store carbon in both aboveground plant matter structures originating from the stem) formation and symbiotic and belowground organic peat soils (Drushka 2003) and the relations with mycorrhizae, little is known about the influence presence of both soil water excess and drought (Grant 2004, of these structures on ecosystem fluxes of water and carbon in © The Author 2012. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected] 600 Angstmann et al. the boreal forest (LeBarron 1945, Vartapetian and Jackson because of unlimited root water access and subsequent high 1997, Fougnies et al. 2007). While the mechanisms behind flows of water through the transpirational stream C( ˇermák et al. paludification are well known, its impact on tree transpiration 1982, Roy et al. 1999, Herrera et al. 2008) while differences has not been investigated with changes in stand age and in EL and EC are primarily controlled by age-mediated (Dawson species. 1996, Alsheimer et al. 1998, Ryan et al. 2000, Ewers et al. Failure to account for drastic changes in topography-driven 2002, Moore et al. 2004, Meinzer et al. 2005) and soil drain- soil moisture in models quantifying carbon, water and energy age-mediated (Akeroyd et al. 1998, Kreuzwieser et al. 2002) fluxes may result in biased predictions. This is especially true differences in tree size. Ewers et al. (2005) hypothesized that with models quantifying parameters under various global early successional species in well-drained boreal stands would change scenarios because well- and poorly-drained stands will undergo increasing EL and EC with age, late-successional spe- be dichotomously affected by global change. Under a warmer, cies would experience decreasing EL and increasing EC, with drier climate with increased boreal fire frequency, well-drained the EL decrease attributed to a decrease in AS:AL found in coni- stands will likely become younger, more diverse and drought fers with a low AS:AL and long-lived leaves (McDowell et al. Downloaded from stressed, whereas poorly-drained stands will undergo soil 2002). Furthermore, trees in poorly-drained conditions are drainage through permafrost melt increasing production and expected to have lower EC and EL than well-drained trees due decomposition (Goulden et al. 1998, Stocks et al. 1998, to lower AS and AL (Oren et al. 1999, Roy et al. 1999, Islam and Schuur et al. 2008). Knowledge of the physiological and MacDonald 2004) but a similar pattern with age because AS:AL resource allocation controls over transpiration will refine pre- is constant regardless of soil drainage condition (e.g., no sig- http://treephys.oxfordjournals.org/ dictive models by accounting for divergent stand responses of nificant effect of soil drainage on allometric relationships; individual physiology and resources allocation patterns. Bond-Lamberty et al. 2002b). Short-term physiological controls over water and carbon We aim to explain the variability of tree transpiration across fluxes come from stomata which respond indirectly to vapor a soil drainage gradient by measuring sap flux of three boreal pressure deficit D( ) by plant regulation of minimum leaf water species (two early-successional and one late-successional) potential to avoid excessive cavitation during drought periods across four stand ages, each containing a well- and poorly- (Cowan and Farquhar 1977, Meinzer and Grantz 1991, Mott drained stand. We test the following predictions from data and Parkhurst 1991, Baldocchi 1997, Franks 2004). Extremes collected during the 2007 and 2008 growing seasons: at University of Wyoming Libraries on June 27, 2016 in soil moisture can also influence transpiration rates where (i) irrespective of stand age, well-drained stands will have both drought and flooded conditions inhibit root uptake of higher EL and EC than poorly-drained stands because reduc- water due to high negative soil water potentials or cold, anoxic tions in total AS and L will overwhelm higher JS in poorly- and/or nutrient-poor soil conditions, respectively (Gorham drained stands resulting from unlimited water availability and 1991, Wang et al. 2003a, Brodribb et al. 2005). Temporal (ii) younger stands will have higher EL and lower EC than older changes in transpiration are primarily driven by environmental stands because stand-level decreases in AS:AL with age are factors such as D and photosynthetic photon flux density Q( ) expected to occur in this ecosystem, which is dominated by where transpiration rates typically show a saturation response long-lived, low AS:AL coniferous black spruce trees. to increasing D and Q due to stomatal response to regulate minimum leaf water potential (Monteith 1995, Cowan and Farquhar 1997, Oren et al. 1999, Pataki et al. 2000, Franks Methods 2004, Ewers et al. 2005). Regulation of transpiration via sub- daily and daily plant physiological responses results in longer- Stand description term partitioning of resources that is highly dependent on the This study was conducted during the 2007 and 2008 air and soil environment. growing seasons in the central Canadian boreal forest in Consequently, long-term resource partitioning influences northern Manitoba, CA (55.8786°N–55.9137°N, transpiration rates and can be quantified by expressing transpi- 98.3818°W–98.9793°W), ~40 km west of Thompson, MB. ration at different scales within trees. For example, sap flux per Soils were sedimentary deposits from glacial Lake Agassiz unit xylem area (JS) rescaled to transpiration per unit ground with montmorillonite clay soil in the well-drained stands 3 −3 area, EC, or per unit leaf area, EL, by multiplication with sap- (volumetric soil moisture range 0.08–0.72 cm cm ) and wood to ground area (AS:AG) or sapwood to leaf area area overlain with a deep organic peat layer dependent on stand age (AS:AL) ratios, respectively, quantifies the influence of tree- and in the poorly-drained stands (volumetric soil moisture range forest-related parameters on transpiration rates (Whitehead 0.42–0.76 cm3 cm−3) (Bond-Lamberty et al. 2002b). Mean annual and Jarvis 1981, Vertessy et al. 1995, Ewers et al. 1999, precipitation and temperature were 439 mm and 0.8 °C, respec- Hatton and Wu 1995). JS in flood-tolerant species such as tively, with mean January and July air temperatures of −19.7 and black spruce is often increased under saturated soil conditions 16.5 °C (Bond-Lamberty 2002a, Environment Canada 2009).