Shifting environmental controls on CH4 fluxes in a sub-boreal peatland

We monitored CO2 and CH4 fluxes using eddy covariance from 19 May to 27 September 2011 in a poor fen located in northern Michigan. The objectives of this paper are to: (1) quantify the flux of CH4 from a sub-boreal peatland, and (2) determine which abiotic and biotic factors were the most correlated to the flux of CH4 over the measurement period. Net daily CH4 fluxes increased from 70 mg CH4 m−2 d−1 to 220 mg CH4 m−2 d−1 from mid May to mid July. After July, CH4 losses steadily declined to approximately 50 mg CH4 m−2 d−1 in late September. During the study period, the peatland lost 17.4 g CH4 m−2. Both abiotic and biotic variables were correlated with CH4 fluxes. When the different variables were analyzed together, the preferred model included mean daily soil temperature at 20 cm, daily net ecosystem exchange (NEE) and the interaction between mean daily soil temperature at 20 cm and NEE (R2 = 0.47, p value \u3c 0.001). The interaction was important because the relationship between daily NEE and mean daily soil temperature with CH4 flux changed when NEE was negative (CO2 uptake from the atmosphere) or positive (CO2 losses to the atmosphere). On days when daily NEE was negative, 25% of the CH4 flux could be explained by correlations with NEE, however on days when daily NEE was positive, there was no correlation between daily NEE and the CH4 flux. In contrast, daily mean soil temperature at 20 cm was poorly correlated to changes in CH4 when NEE was negative (17%), but the correlation increased to 34% when NEE was positive. The interaction between daily NEE and mean daily soil temperature at 20 cm indicates shifting environmental controls on the CH4 flux throughout the growing season

Can carbon isotopes be used to predict watershed-scale transpiration?

The Penman‐Monteith equation is often used to estimate transpiration, but an important limitation to this approach, especially for mountainous forested sites, is an accurate estimate of canopy conductance averaged over the area of interest (Gs). We propose a method for estimating watershed‐scale transpiration using estimates of Gs derived from measurements of stable isotopes. To estimate Gs, we first determined the isotopic composition of ecosystem respiration (δ¹³CER) as derived from the ¹²C:¹³C ratio of respired CO₂ entrained within nocturnal cold air drainage flows exiting the base of the watershed. An isotope‐derived estimate of recent canopy conductance over the entire watershed (Gs‐I) was derived using biophysical models. To estimate daily average transpiration, we applied Gs‐I and other measured environmental variables to the Penman‐Monteith equation. The results were compared with an independent measure of transpiration using the heat dissipation method at four locations within the watershed. Considering the large number of assumptions required for both estimates of transpiration, the two estimates were remarkably similar. The relationship between the values derived by the two techniques was statistically significant (p value 0.1), but the standard error was large (SE = 0.48). The results demonstrate that this technique holds promise, but the effects of potential limitations require further attention. The future research necessary to fully demonstrate the validity of this potentially promising method is discussed

Monthly gridded data product of northern wetland methane emissions based on upscaling eddy covariance observations

Natural wetlands constitute the largest and most uncertain source of methane (CH4) to the atmosphere and a large fraction of them are found in the northern latitudes. These emissions are typically estimated using process (“bottom-up”) or inversion (“top-down”) models. However, estimates from these two types of models are not independent of each other since the top-down estimates usually rely on the a priori estimation of these emissions obtained with process models. Hence, independent spatially explicit validation data are needed. Here we utilize a random forest (RF) machine-learning technique to upscale CH4 eddy covariance flux measurements from 25 sites to estimate CH4 wetland emissions from the northern latitudes (north of 45∘ N). Eddy covariance data from 2005 to 2016 are used for model development. The model is then used to predict emissions during 2013 and 2014. The predictive performance of the RF model is evaluated using a leave-one-site-out cross-validation scheme. The performance (Nash–Sutcliffe model efficiency =0.47) is comparable to previous studies upscaling net ecosystem exchange of carbon dioxide and studies comparing process model output against site-level CH4 emission data. The global distribution of wetlands is one major source of uncertainty for upscaling CH4. Thus, three wetland distribution maps are utilized in the upscaling. Depending on the wetland distribution map, the annual emissions for the northern wetlands yield 32 (22.3–41.2, 95 % confidence interval calculated from a RF model ensemble), 31 (21.4–39.9) or 38 (25.9–49.5) Tg(CH4) yr−1. To further evaluate the uncertainties of the upscaled CH4 flux data products we also compared them against output from two process models (LPX-Bern and WetCHARTs), and methodological issues related to CH4 flux upscaling are discussed. The monthly upscaled CH4 flux data products are available at https://doi.org/10.5281/zenodo.2560163 (Peltola et al., 2019)

A Review and Evaluation of Forest Canopy Epiphyte Roles in the Partitioning and Chemical Alteration of Precipitation

Interactions between precipitation and forest canopy elements (bark, leaves, and epiphytes) control the quantity, spatiotemporal patterning, and the chemical concentration, character and constituency of precipitation to soils. Canopy epiphytes are an element that exerts a range of storm-related hydrological and biogeochemical effects due to their diversity of morphological traits and nutrient acquisition mechanisms. We reviewed and evaluated the state of knowledge regarding epiphyte interactions with precipitation partitioning (into interception loss, throughfall, and stemflow) and the chemical alteration of net precipitation fluxes (throughfall and stemflow). As epiphyte species are quite diverse, this review categorized findings by common paraphyletic groups: lichens, bryophytes, and vascular epiphytes. Of these groups, vascular epiphytes have received the least attention and lichens the most. In general, epiphytes decrease throughfall and stemflow and increase interception loss. Epiphytes alter the spatiotemporal pattern of throughfall and increase the overall latent heat fluxes from the canopy. Epiphytes alter biogeochemical processes by impacting the transfer of solutes through the canopy; however, the change in solute concentration varies with epiphyte type and chemical species. We discuss several important knowledge gaps across all epiphyte groups. We also explore innovative methods that currently exist to confront these knowledge gaps and past techniques applied to gain our current understanding. Future research addressing the listed deficiencies will improve our knowledge of epiphyte roles in water and biogeochemical processes coupled within forest canopies—processes crucial to supporting microbe, plant, vertebrate and invertebrate communities within individual epiphytes/epiphyte assemblages, host trees, and even the forest ecosystem as a whole

A Review and Evaluation of Forest Canopy Epiphyte Roles in the Partitioning and Chemical Alteration of Precipitation

Material and energy exchange at the Earth\u27s surface is drastically altered by the presence of forest canopy cover. Canopy structures that control this exchange (leaves, branches, bark, epiphytes, etc.) differentially alter the amount, spatiotemporal patterning, and solute concentration of precipitation reaching the surface (Levia et al., 2011; Pypker et al., 2011). Precipitation over forest canopies is either intercepted and evaporated, or reaches the surface below via gaps and drips (throughfall, Levia and Frost, 2006) and concentrated flow down the tree trunk (stemflow, Levia and Frost, 2003). The sum of precipitation that reached the ground (as throughfall and stemflow) is called net precipitation. Amount, type, spatiotemporal configuration, and composition of canopy elements control the proportion of precipitation partitioned into interception or net precipitation, as well as the solute concentration and spatiotemporal patterning of net precipitation at the forest floor (Pypker et al., 2011). Of the various canopy surface types, epiphyte cover arguably has received less attention from the precipitation partitioning research community (Levia and Frost, 2003, 2006). This is surprising as epiphytes are ubiquitous across forest types (e.g., Hölscher et al., 2004; Husk et al., 2004; Zotz, 2005; Pypker et al., 2006a; Hauck, 2009; Lundström et al., 2013; Van Stan et al., 2015), and their coverage, patterns, and forms can significantly alter canopy structural attributes, by (1) closing canopy gaps and connecting edges across, and branches within, individual trees (Fig. 1a), (2) filling voids in branch crotches and tree holes (Fig. 1b), (3) increasing area of “stable” above-ground biomass structures (Fig. 1c), and (4) ultimately changing the vertical biomass distribution of the host forest (Fig. 1d). This, in conjunction with epiphytes\u27 diversity and uniqueness of nutrient acquisition mechanisms (e.g., Pittendrigh, 1948; Madison, 1977; Martin, 1994), illustrates the need for work to evaluate epiphyte impacts on canopy precipitation partitioning and its solute dynamics

Impacts of one time biosolids and fertilizer application on long-term metal and nutrient concentrations on two tailings ponds in the BC Southern Interior

Previous research has demonstrated that the use of organic amendments, specifically biosolids, can address limitations to initial vegetation establishment on mine tailings. It is less understood how these systems will function in the long term. In 2015, a field study at Teck Highland Valley Copper in the BC Southern Interior was conducted to determine the long term effects of fertilizer and biosolids on nutrients and elemental concentrations in two tailings ponds. Seventeen years prior, biosolids were applied in a randomized complete block design at rates 50, 100, 150, 200, and 250 Mg haˉ¹. The biosolids treatments continue to demonstrate a very clear increase to the nutrient status of the tailings, while the fertilizer treatment does not statistically differ from the control treatments. There are also still elevated levels of metals within biosolids treated plots, but results vary by metal with many showing a plateau, where additional biosolids do not increase their concertation. With site specific planning, metal concentrations can be controlled below levels of concern, while at the same time promote nutrient cycling. In conclusion, it appears a one-time biosolids application can assist reclamation in a trajectory towards a self-sustaining state. Further research is also being done examining the plant community and soil development.Non UBCUnreviewedOthe

Effect of long-term water table manipulation on peatland evapotranspiration

Continuous measurements of ecosystem scale evapotranspiration (ET) were obtained using the eddy covariance method over the 2010 and 2011 growing seasons (May-September) at three adjacent peatlands that have undergone long-term water table manipulation. The three (wet, dry and intermediate) sites represent peatlands along a hydrological gradient, with different average depths to water table (WTD) and different resulting vegetation and microform assemblages. The 2010 growing season was warmer and wetter than normal, while 2011 conditions were near normal. The difference in maximum daily ET values (95th percentiles) between sites were greater in 2010 (3.14mmd-1-4.17mmd-1) compared to 2011 (3.68mmd-1-3.95mmd-1), yielding cumulative growing season ET that followed the wet to dry gradient in both 2010 and 2011. Synoptic weather conditions (i.e. air temperature, vapour pressure deficit, and incoming solar radiation, etc.) could not explain differences in ET between sites due to their proximity to one another. Peat surface wetness was more spatially homogeneous at the wet site due to small average microtopographic variations (0.15m) compared to the intermediate (0.30m) and dry (0.41m) sites. Although average Bowen ratios were less than one at all three sites, greater surface wetness and heating at the wettest site lead to differences in energy partitioning, with higher average Bowen ratios at the sites with a shallow average WTD. No significant relation between normalized ET and WTD was found at any of the sites that were consistent across both study years. In addition, the lack of a relation between ET and near-surface moisture suggests that the unsaturated hydraulic conductivity and the boundary layer resistance created by the vascular canopy combined with low surface roughness limits evaporative losses from the peat surface. This study suggests that the low ET of a dry site compared to a wet site may be due to the impact of a long-term change in WTD on leaf area and the relative distribution of plant functional groups. © 2013 Elsevier B.V

Chronological Changes in Canopy Hydrometeorological Dynamics May Aid Invasion of a Globally Invasive Species (Ailanthus Altissima Mill. Tree of Heaven)

We examined the effect of a globally-invasive species, Ailanthus altissima, on canopy hydrometeorological processes. Throughfall (TF), stemflow (SF) and interception loss (I) were measured in a chronosequence of three A. altissima stands (planted 1975, 1985, 1995). Canopy structural and ecohydrological parameters varied with age: woody area index (WAI), ratio of wet canopy evaporation and rainfall rates, and stem drainage coefficient increased; while leaf area index (LAI), canopy water storage, and gap fraction declined. This corresponded to increased SF and decreased TF across annual, seasonal, and inter-storm scales. Changes in canopy hydrologic flow paths (TF v. SF) may be advantageous to invasive species as the promotion of SF with canopy age may increase water supply to the roots and help distribute allelopathic chemicals through the soil. Further research is needed on the correlation between canopy architecture of A. altissima invasion and the distribution of water and chemicals to soils

The Absorption and Evaporation of Water Vapor by Epiphytes in an Old-growth Douglas-fir Forest During the Seasonal Summer Dry Season: Implications for the Canopy Energy Budget

Our goal was to determine how epiphytic lichens and bryophytes affect canopy latent heat fluxes in an old-growth Douglas-fir forest when the canopy was dry. The epiphyte water content (WCe expressed as a percent of dry weight) of representative epiphytic foliose lichens, fruticose lichens, and bryophytes was measured in the laboratory after 1 to 12 hr of exposure at five different values of vapor pressure deficit (VPD). After 12 hr of exposure, WCe increased fivefold to sixfold as VPD decreased from 1849 to 132 Pa. In addition, we measured WCe in the field using strain gauges. These field measurements were used to calibrate the models described below. Two models were created to estimate the potential latent heat flux from epiphytes at the canopy scale (LEe). The first model combined measured total biomass of epiphytes with a model that estimated the laboratory determined VPD-dependent changes in WCe of the lichens/bryophytes (VPD method). The second model estimated LEe by scaling the change in WCe of epiphyte-laden branches that were continuously monitored in situ in the canopy by a strain gauge (SG method). Both methods showed a strong diurnal trend in LEe when VPD was less than 645 Pa. Prior to sunrise, the epiphytes absorbed water, corresponding to a latent heat flux of 5 to 15 W/m2 per unit ground area, whereas after sunrise, the epiphytes lost water at a rate of −10 to −20 W/m2. For short periods, epiphytes may contribute a significant portion of the latent heat flux from Douglas-fir forests

The Importance of Considering Rainfall Partitioning in Afforestation Initiatives in Semiarid Climates: a Comparison of Common Planted Tree Species in Tehran, Iran

As plantations become increasingly important sources of wood and fiber in arid/semiarid places, they have also become increasingly criticized for their hydrological impacts. An examination and comparison of gross rainfall (GR) partitioning across commonly-planted tree species (Pinus eldarica, Cupressus arizonica, Robinia pseudoacacia, and Fraxinus rotundifolia) in semiarid regions has great value for watershed and forest managers interested in managing canopy hydrological processes for societal benefit. Therefore, we performed a field study examining GR partitioning into throughfall (TF), stemflow (SF), and rainfall interception (I) for these species in the semiarid Chitgar Forest Park, Tehran, Iran. An advantage to our study is that we explore the effects of forest structural differences in plantation forests experiencing similar climatic factors and storm conditions. As such, variability in GR partitioning due to different meteorological conditions is minimized, allowing comparison of structural attributes across plantations. Our results show that commonly-selected afforestation species experiencing the same climate produced differing stand structures that differentially partition GR into TF, SF, and I. P. eldarica might be the best of the four species to plant if the primary goal of afforestation is to limit erosion and stormwater runoff as it intercepted more rainfall than other species. However, the high SF generation from F. rotundifolia, and low GR necessary to initiate SF, could maximize retention of water in the soils since SF has been shown to infiltrate along root pathways and access groundwater. A consideration of GR partitioning should be considered when selecting a species for afforestation/reforestation in water-limited ecosystems