Heather D. Alexander

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release w ...

Freshwater inundation effects on emergent vegetation of a hypersaline salt marsh.

The coastal marshlands of the Nueces estuary, Texas depend upon periodic freshwater inundation to support current community structure and promote further establishment and expansion of emergent halophytes. Decades of watershed modifications have dramatically decreased freshwater discharge into the upper estuary resulting in hypersaline and dry conditions. In an attempt to partially restore inflow, the U.S. Bureau of Reclamation excavated two overflow channels re-connecting the Nueces River to the marshlands. Freshwater-mediated (precipitation and inflow) changes in tidal creek and porewater salinity and emergent marsh vegetation were examined over a 5-yr period at three stations in the upper Nueces Marsh with the aid of a Geographical Information System (GIS). Two stations were potentially subjected to freshwater inflow through the channels, while one station experienced only precipitation. Decreased tidal creek and porewater salinity were significantly correlated with increased freshwater at all stations (R2=0.37 to 0.56), although porewater salinities remained hypersaline. GIS analyses indicated the most considerable vegetation change following freshwater inundation was increased cover of the annual succulentSalicornia bigelovii. Fall inundation allowed seed germination and rapid expansion of this species into previously bare areas during the subsequent winter and following spring. The station affected by both inflow and precipitation exhibited greaterS. bigelovii cover than the station affected solely by precipitation in both spring 1999 (58. 7% compared to 27.9%) and 2000 (48.6% compared to 1.9%). Percent cover of the perennialBatis maritima temporarily increased after periods of consistent rainfall. The response was short term, and cover quickly returned to pre-inundation conditions within 3 mo. Prolonged inundation led to longterm (>2yr) decreases in percent cover ofB. maritima. Our results suggest that the timing and quantity of freshwater inundation strongly dictate halophyte response to precipitation and inflow. Brief periods of freshwater inundation that occur at specific times of year alleviate stress and promote seed germination and growth, but extended soil saturation can act as a disturbance that has a negative impact on species adapted to hypersaline conditions.

Implications of a predicted shift from upland oaks to red maple on forest hydrology and nutrient availability

Fire suppression has facilitated the spread of red maple (Acer rubrum L.), a fire-sensitive, yet highly adaptable species, in historically oak-dominated forests of the eastern United States. Here, we address whether a shift from upland oaks to red maple could influence forest hydrology and nutrient availability because of species-specific effects on precipitation distribution and inorganic nitrogen (N) cycling. In eastern Kentucky, we measured seasonal variations in red maple, chestnut oak (Quercus montana Willd.), and scarlet oak (Quercus coccinea Munchh.) throughfall and stemflow quantity and quality following discrete precipitation events, and we assessed net N mineralization rates in underlying soils over a 2-year period (2006–2008). Throughfall was 3%–9% lower underneath red maple than both oaks, but red maple generated 2–3× more stemflow. Consequently, NH4+ throughfall deposition was less under red maple than chestnut oak, whereas stemflow-derived nutrient inputs were substantially larger for red ma...

Implications of increased deciduous cover on stand structure and aboveground carbon pools of Alaskan boreal forests

Fire activity in boreal forests has increased recently with climate warming, altering stand structure and composition in many areas. Changes in stand dynamics have the potential to alter C cycling and biophysical processes, with feedbacks to global and regional climate. Here, we assess the interactions between fire, stand structure, and aboveground C accumulation and storage within boreal forests of interior Alaska, where increased fire severity is predicted to shift forest composition from predominantly black spruce (Picea mariana) to greater deciduous cover. We measured aboveground biomass and net primary productivity (ANPP) of trees and large shrubs, snags, and downed woody debris across 44 mid-successional (20–59 years since fire) stands of varying deciduous importance value (IV), determined by relative density, basal area, and frequency of deciduous trees and large shrubs within each stand. Aboveground biomass, ANPP, and deciduous snag biomass increased significantly with increased deciduous IV and years since fire. Deciduous IV had little influence on evergreen snag biomass and downed woody debris, but both C pools decreased with years since fire. Forest type also affected stand structure and C pools. Black spruce stands had shorter trees with less basal area and aboveground biomass and slower rates of biomass accumulation and ANPP compared to those dominated by trembling aspen (Populus tremuloides) or Alaska birch (Betula neoalaskana). These parameters in black spruce stands were similar to mixed stands of black spruce and aspen but were often lower than mixed stands of black spruce and Alaska birch. Much of the biomass accumulation in deciduous stands was attributed to higher tree-level ANPP, allowing individual stems of deciduous species to accumulate more stemwood/bark faster than black spruce trees. If increased fire activity shifts stand composition from black spruce to increased deciduous cover, ANPP, aboveground tree/large shrub biomass, and deciduous snag biomass will increase, leading to increased aboveground C pools in mid-successional forest stands of interior Alaska. While species dominance shifts like these will impact aboveground patterns of landscape-level C cycling in boreal forests, variations in soil C pools and forest properties like albedo must also be assessed to accurately determine implications for global and regional climate.

Treated Wastewater Effluent as an Alternative Freshwater Source in a Hypersaline Salt Marsh: Impacts on Salinity, Inorganic Nitrogen, and Emergent Vegetation

Abstract Reservoir construction and river diversions have dramatically reduced freshwater inflow to coastal salt marshes of the Nueces Estuary, Texas, facilitating hypersaline conditions. To moderate salinities and enhance habitat quality, ∼ 7570 m3 d−1 of treated wastewater effluent was used as a nutrient-rich freshwater source in the lower estuary. For ∼1.5 years prior to and ∼3.5 years following wastewater diversion, we quantified salinity, inorganic nitrogen, and emergent vegetation changes at four stations various distances downstream from the diversion point. Wastewater prevented hypersaline conditions at station 72, closest to the diversion, but not downstream. Increased tidal creek NH4+ and NO3− + NO2− concentrations were detectable 1200 m downstream, but concentrations were 50%–80% of those measured at station 72 within 325 m downstream. Emergent vegetation responded at station 72 only. Composition rapidly shifted (over a period of ≤ 1 y) from ∼80% monospecific cover of a highly salt-tolerant succulent, Salicornia virginica, to ∼50% cover of a less salt-tolerant shrub, Borrichia frutescens, intermixed with several other species. Percent cover ordination plots on each sampling date verified distinct pre- and postdiversion plant communities at this station. Mean porewater nutrient concentrations were not different among stations postdiversion, but lower C : N ratios and increased δ15N signatures of B. frutescens confirmed assimilation of wastewater-derived nitrogen at station 72. A variety of plants colonized bare areas near the diversion, creating about seven hectares of newly vegetated salt marsh. Wastewater diversions lowered salinity, increased nutrients, and increased cover of less salt-tolerant vegetation species near the diversion. However, these changes were restricted to a limited area, suggesting that increased diversions are necessary to produce substantial downstream effects.

A Canopy Shift in Interior Alaskan Boreal Forests: Consequences for Above- and Belowground Carbon and Nitrogen Pools during Post-fire Succession

AbstractGlobal change models predict that high-latitude boreal forests will become increasingly susceptible to fire activity as climate warms, possibly causing a positive feedback to warming through fire-driven emissions of CO2 into the atmosphere. However, fire-climate feedbacks depend on forest regrowth and carbon (C) accumulation over the post-fire successional interval, which is influenced by nitrogen (N) availability. To improve our understanding of post-fire C and N accumulation patterns in boreal forests, we evaluated above- and belowground C and N pools within 70 stands throughout interior Alaska, a region predicted to undergo a shift in canopy dominance as fire severity increases. Stands represented gradients in age and successional trajectory, from black spruce (Picea mariana) self-replacement to species replacement by deciduous species of trembling aspen (Populus tremuloides) and Alaska paper birch (Betula neoalaskana). Stands undergoing deciduous trajectories stored proportionally more of their C and N in aboveground stemwood and had 5–7 times faster rates of aboveground net primary productivity of trees compared to stands undergoing a black spruce trajectory, which stored more of their C and N in the soil organic layer (SOL), a thick layer of mostly undecomposed mosses. Thus, as successional trajectories shift, total C and N pool sizes will remain relatively unchanged, but there will be a trade-off in pool location and a potential increase in C and N longevity due to decreased flammability and decomposition rates of deciduous stemwood. Despite often warmer, drier conditions in deciduous compared to black spruce stands, deciduous stemwood has a C:N around 10 times higher than the black spruce SOL and often remains standing for many years with reduced exposure to fungal decomposers. Thus, a fire-driven shift in successional trajectories could cause a negative feedback to climate warming because of increased pool longevity in deciduous trajectories.

Effects of pulsed riverine versus non-pulsed wastewater inputs of freshwater on plant community structure in a semi-arid salt marsh

The use of treated wastewater to restore freshwater inputs to arid and semi-arid wetlands is a relatively new concept, and the long-term effects of such practices on plant community structure are largely unknown. We compared vegetation composition, pore water salinity, and soil moisture along permanent transects at a restoration site receiving wastewater effluent since October 1998 to three nearby downstream sites subjected only to freshwater inputs via precipitation and river flooding. Local climate during this period was highly variable and included two droughts and a wet period that began with a series of large floods. Significantly lower pore water salinities were observed at the wastewater site compared to downstream sites, particularly during droughts, when salinities were 20%.–40%. lower at the wastewater site. Between July 1997 and July 2002, cover of the clonal stress-tolerator, Salicornia virginica, decreased from 87% to 33% at the wastewater site, while cover of the clonal dominant, Borrichia frutescens, increased from 5% to 55%. In contrast, S. virginica cover increased at two downstream sites during the same period, while cover of B. frutescens remained relatively stable. Following large floods in summer 2002, which marked the beginning of a three year-wet period, B. frutescens cover increased at all sites. We concluded that constant wastewater additions and climate-driven wet periods affected plant community structure similarly by promoting expansion of the clonal dominant B. frutescens and inhibiting expansion of the stress-tolerant species S. virginica. We propose distinct management strategies for using wastewater to 1) increase plant cover, 2) promote endemic plant assemblages, and 3) maximize species richness.

Variability in above- and belowground carbon stocks in a Siberian larch watershed

Permafrost soils store between 1330 and 1580 Pg carbon (C), which is 3 times the amount of C in global vegetation, almost twice the amount of C in the atmosphere, and half of the global soil organic C pool. Despite the massive amount of C in permafrost, estimates of soil C storage in the high-latitude permafrost region are highly uncertain, primarily due to undersampling at all spatial scales; circumpolar soil C estimates lack sufficient continental spatial diversity, regional intensity, and replication at the field-site level. Siberian forests are particularly undersampled, yet the larch forests that dominate this region may store more than twice as much soil C as all other boreal forest types in the continuous permafrost zone combined. Here we present aboveand belowground C stocks from 20 sites representing a gradient of stand age and structure in a larch watershed of the Kolyma River, near Chersky, Sakha Republic, Russia. We found that the majority of C stored in the top 1 m of the watershed was stored belowground (92 %), with 19 % in the top 10 cm of soil and 40 % in the top 30 cm. Carbon was more variable in surface soils (10 cm; coefficient of variation (CV) = 0.35 between stands) than in the top 30 cm (CV= 0.14) or soil profile to 1 m (CV= 0.20). Combined active-layer and deep frozen deposits (surface – 15 m) contained 205 kg C m−2 (yedoma, non-ice wedge) and 331 kg C m−2 (alas), which, even when accounting for landscape-level ice content, is an order of magnitude more C than that stored in the top meter of soil and 2 orders of magnitude more C than in aboveground biomass. Aboveground biomass was composed of primarily larch (53 %) but also included understory vegetation (30 %), woody debris (11 %) and snag (6 %) biomass. While aboveground biomass contained relatively little (8 %) of the C stocks in the watershed, aboveground processes were linked to thaw depth and belowground C storage. Thaw depth was negatively related to stand age, and soil C density (top 10 cm) was positively related to soil moisture and negatively related to moss and lichen cover. These results suggest that, as the climate warms, changes in stand age and structure may be as important as direct climate effects on belowground environmental conditions and permafrost C vulnerability. Published by Copernicus Publications on behalf of the European Geosciences Union. 4280 E. E. Webb et al.: Variability in aboveand belowground carbon stocks

Environmental constraints on transpiration and stomatal conductance in a Siberian Arctic boreal forest

Boreal forest ecosystems are experiencing changes in plant productivity that are likely to continue with ongoing climate change. Transpiration (T) and canopy stomatal conductance (gc) are a key influence on plant productivity, and a better understanding of drivers and limitations of T and gc is necessary for constraining estimates of boreal ecosystem change. We describe patterns in T and gc of a deciduous conifer, Larix cajanderi, in an arctic boreal forest in northeastern Russia across three growing seasons from 2013 to 2015. We examine the influence of environmental drivers on gc using a phenomenological model. T was highly variable across days and varied between 0.03 and 0.75 L m−2 d−1. T and gc largely covaried with daily fluctuations in air temperature and vapor pressure deficit. gc was highly suppressed on days when the vapor pressure deficits exceeded 0.75 kPa with an average daily gc of 37.55 mmol m−2 s−1, and the average daily gc was almost double (71.25 mmol m−2 s−1) when vapor pressure deficits stayed below 0.75 kPa. Daily variation in gc was significantly related to air temperature, permafrost thaw depth, and past precipitation. The influence of past precipitation and permafrost thaw depth on gc indicates that belowground conditions relating to soil moisture status are a key limitation for T. Such limitations on gc and T suggest that soil water and plant water stress play an important role in plant productivity and water relations in far northeastern Siberia.

Spatially explicit estimation of aboveground boreal forest biomass in the Yukon River Basin, Alaska

Quantification of aboveground biomass (AGB) in Alaska’s boreal forest is essential to the accurate evaluation of terrestrial carbon stocks and dynamics in northern high-latitude ecosystems. Our goal was to map AGB at 30 m resolution for the boreal forest in the Yukon River Basin of Alaska using Landsat data and ground measurements. We acquired Landsat images to generate a 3-year (2008–2010) composite of top-of-atmosphere reflectance for six bands as well as the brightness temperature (BT). We constructed a multiple regression model using field-observed AGB and Landsat-derived reflectance, BT, and vegetation indices. A basin-wide boreal forest AGB map at 30 m resolution was generated by applying the regression model to the Landsat composite. The fivefold cross-validation with field measurements had a mean absolute error (MAE) of 25.7 Mg ha−1 (relative MAE 47.5%) and a mean bias error (MBE) of 4.3 Mg ha−1 (relative MBE 7.9%). The boreal forest AGB product was compared with lidar-based vegetation height data; the comparison indicated that there was a significant correlation between the two data sets.