Bear Brook Watershed In Maine Research Articles

Landscape acidification has trophic-mediated effects on Ovenbirds (Seiurus aurocapilla)

Acid deposition from fossil fuel combustion caused calcium depletion in eastern North America during the 20th century, and calcium is a critical resource for birds during both the egg laying and nesting periods. The effects of acid deposition on animals have largely acted through trophic relationships, with reduced calcium availability in prey possibly being a limiting factor for forest songbirds. Following 21 years of experimental whole-watershed acidification at the Bear Brook Watershed in Maine, we investigated Ovenbird (Seiurus aurocapilla) territory size and potential prey (leaf litter arthropod) abundance, assemblage, and calcium concentration during one breeding season. We delineated and estimated the area of 8 Ovenbird territories in an acidified and a control watershed, but found that territory size did not differ between watersheds. However, while calcium concentrations of potential prey also did not differ between watersheds, territory size decreased as calcium concentrations of potential prey increased, regardless of watershed. Our results suggest that calcium limitation can be a determinant of territory size in some landscapes, and this provides further support to importance of the role of calcium in the lives of passerines.

The Bear Brook Watershed in Maine: Multi‐decadal whole‐watershed experimental acidification

AbstractThe Bear Brook Watershed in Maine (BBWM) is a long‐term research site established to study the response of forest ecosystem function to environmental disturbances of chronic acidic deposition and ecosystem nitrogen enrichment. Starting in 1989, the West Bear (treated) watershed received bimonthly applications of ammonium sulfate [(NH4)2SO4] fertilizer from above the canopy, whereas East Bear (reference) received ambient deposition. The treatments were stopped in 2016, marking the beginning of the recovery phase. Research at the site has focused on soils, streams, and vegetation. Here, we describe data collected over three decades at the BBWM—input and stream output nutrient fluxes, quantitative soil pits and soil chemistry, and soil temperature and moisture.

Differences in microbial community response to nitrogen fertilization result in unique enzyme shifts between arbuscular and ectomycorrhizal-dominated soils.

While the effect of nitrogen (N) deposition on belowground carbon (C) cycling varies, emerging evidence shows that forest soils dominated by trees that associate with ectomycorrhizal fungi (ECM) store more C than soils dominated by trees that associate with arbuscular mycorrhizae (AM) with increasing N deposition. We hypothesized that this is due to unique nutrient cycling responses to N between AM and ECM-dominated soils. ECM trees primarily obtain N through fungal mining of soil organic matter subsidized by root-C. As such, we expected the largest N-induced responses of C and N cycling to occur in ECM rhizospheres and be driven by fungi. Conversely, as AM trees rely on bacterial scavengers in bulk soils to cycle N, we predicted the largest AM responses to be driven by shifts in bacteria and occur in bulk soils. To test this hypothesis, we measured microbial community composition, metatranscriptome profiles, and extracellular enzyme activity in bulk, rhizosphere, and organic horizon (OH) soils in AM and ECM-dominated soils at Bear Brook Watershed in Maine, USA. After 27years of N fertilization, fungal community composition shifted across ECM soils, but bacterial communities shifted across AM soils. These shifts were mirrored by enhanced C relative to N mining enzyme activities in both mycorrhizal types, but this occurred in different soil fractions. In ECM stands these shifts occurred in rhizosphere soils, but in AM stands they occurred in bulk soils. Additionally, ECM OH soils exhibited the opposite response with declines in C relative to N mining. As rhizosphere soils account for only a small portion of total soil volume relative to bulk soils, coupled with declines in C to N enzyme activity in ECM OH soils, we posit that this may partly explain why ECM soils store more C than AM soils as N inputs increase.

Snowmelt periods as hot moments for soil N dynamics: a case study in Maine, USA.

The vernal transition represents the seasonal transition to spring, occurring as temperatures rise at the end of winter. With rapid snowmelt, microbial community turnover, and accelerated nutrient cycling, this is a critical but relatively under-studied period of ecosystem function. We conducted a study over two consecutive winters (2015-2016) at the Bear Brook Watershed in Maine to examine how changing winter conditions (warming winters, reduced snow accumulation) altered soil nitrogen availability and stream N export during winter and the vernal transition, and how these patterns were influenced by ecosystem N status (N-enriched vs. N-limited). Of the two study years, 2016 had a warmer winter with substantially less snow accumulation and a discontinuous snowpack-and as a result, had a longer vernal transition and a snowpack that thawed before the vernal transition began. Across both years, snowmelt triggered a transition, signaled by increased ammonium concentrations in soil, decreased soil nitrate concentrations due to flushing by meltwater, and increased stream nitrate exports. Despite the contrasting winter conditions, both years showed similar patterns in N availability and export, differing only in the timing of these transitions. The vernal transition has conventionally been considered a critical period for biogeochemical cycling, because the associated snowmelt event triggers physicochemical and biochemical changes in soil systems. This was consistent with our results in 2015, but our data for 2016 show that this may not always hold true, and instead, that warmer, low-snow winters may demonstrate a temporal asynchrony between snowmelt and the vernal transition. We also show that ecosystem N status is a strong driver of the seasonal N pattern, and the interaction of N status and changing climate must be further investigated to understand ecosystem function under our current predicted trajectory of warming winters, declining snowfall, and winter thaw events.

Contrasting stream nitrate and sulfate response to recovery from experimental watershed acidification

Improvements in air quality have led to ecosystem recovery from acidic deposition, but the mechanisms and trajectories of this recovery are not fully understood. Here, we present long-term stream response and recovery data for paired watersheds at the Bear Brook Watershed in Maine (BBWM) during declining ambient SO4 and NO3 in precipitation. East Bear (EB) received ambient deposition from 1989 to 2018; West Bear (WB) received artificially elevated N + S from 1989 to 2016. The WB treatment was discontinued after 2016, the beginning of the recovery from both the experimental N + S and ambient decline. Stream SO4 in WB gradually declined after the treatment ended, from ~147 μeq L−1 in 2010–16 to ~126 μeq L−1 in 2017–18. The declining S inputs induced desorption of SO4 from soil phase surfaces, with stream loss far exceeding precipitation input. At the current rate of recovery, it will be many decades before the WB stream returns to pre-treatment SO4 concentrations. In contrast, NO3 is only weakly adsorbed in soil, and WB stream NO3 concentrations rapidly declined from ~39 μeq L−1 in 2010–16 to ~5 μeq L−1 in 2017–18, comparable to the N-limited EB stream. The acid anions are strongly coupled to base cation chemistry in streams, and there was a distinct hysteretic response of Ca and Mg to the chronic acidification, as (Ca + Mg) increased rapidly during the initial years, followed by declining values due to depletion of the soil exchange complex. This 30-year monitoring study (1989–2019) provides insights into recovery mechanisms from acidic deposition and highlights the role of abiotic processes in soil that mediate nutrient cycling and retention. Documenting the rapid response of N alongside the slower recovery for S identifies the temporal resolution necessary for other whole-watershed recovery studies.

Phosphorus Limits Nitrogen Dynamics in the O Horizon of a Forested Watershed in Maine, USA

Anthropogenic emissions have altered the N status of forests in the northeastern United States, highlighting the importance of our imperfect understanding of the interaction between nitrogen (N) and phosphorus (P) in response to atmospheric N deposition. We investigated soil N and P dynamics at the Bear Brook Watershed in Maine (BBWM), a long‐term paired watershed experiment established to study ecosystem response to experimental N enrichment. One watershed (West Bear [WB]) was treated bimonthly for 25 yr with ammonium sulfate, and the reference East Bear (EB) received only ambient N deposition. We conducted field and laboratory studies to examine the interaction between N and P in O horizon soils in both EB and WB. In the “Field Addition” study, an addition of 100 kg P ha−1 resulted in an overall 38% decrease in extractable ammonium (NH4+–N) due to increased immobilization. Even under ambient N deposition rates, EB displayed parallel, although lower, evidence of P limitation compared with WB. In contrast, in the “Laboratory Addition” study, soils incubated in the laboratory at rates of 0 to 200 kg P ha−1 showed increases in potential net N mineralization (PNNM) and NH4+–N concentrations in response to P additions. There was no effect of N status in the Laboratory Addition study, but hardwood soils showed a faster response and greater rates of PNNM compared with softwood soils. Despite contrasting responses, both the field and laboratory studies suggested that N dynamics at BBWM were P limited and that N status and forest type influenced the N and P interaction.Core Ideas N transformations in Maine's forest soils are P limited. P limitation is greater in N‐enriched soils. Hardwood soils show greater response to P additions than softwood soils.

Forest N Dynamics after 25 years of Whole Watershed N Enrichment: The Bear Brook Watershed in Maine

Core Ideas Chronic N enrichment increased N exports, although ecosystem N retention was still high. Soil was the largest ecosystem N pool, but it was not significantly altered by chronic N enrichment. Chronic N enrichment increased biomass N accumulation in hardwood but not in softwood stands. We examine the temporal trend of input–output N fluxes and net ecosystem N retention, and estimate a mass balance for ecosystem soil and vegetation pools, after 25 yr of chemical manipulation at the Bear Brook Watershed in Maine (BBWM). The BBWM is a paired whole watershed manipulation experiment designed to study the effects of elevated N and S deposition on forest ecosystem function. Starting in 1989, 25.2 kg N ha−1 was added annually to the West Bear (treated) watershed. The N additions in West Bear stimulated N loss through stream exports, and West Bear retained 81% of the annual N inputs, compared to 94% retention in the reference, East Bear. After 25 yr of N additions, the West Bear watershed had accumulated ∼700 kg ha−1 more N than East Bear in soils and vegetation, with ∼10% of the accumulated N stored in forest biomass. The treatment increased recent biomass N accumulation rates in the hardwood stands, but not in the softwoods. Soils did not show detectable differences in total N content between watersheds, although the organic soils had greater N in West Bear. This paper presents a unique set of findings from one of the few long‐term whole forest ecosystem N enrichment studies in the world. While N dynamics were clearly altered in West Bear, with evidence of accelerated N cycling, the treated watershed did not attain an advanced stage of N saturation during the study period, based on the evidence from forest growth and stream N exports.

Dynamic species-specific metabolic changes in the trees exposed to chronic N+S additions at the Bear Brook Watershed in Maine, USA

Red spruce (Picea rubens Sarg.) and American beech (Fagus grandifolia Ehrh.) nutritional imbalances observed during 1998–2000 in response to nitrogen additions beginning in 1989 at Bear Brook Watershed in Maine, USA, were reversed by 2013. However, nitrogen-containing metabolites continued to accumulate to detoxify ammonia. While sugar maple (Acer saccharum Marsh.) was N-limited and benefitted from N+S additions, spruce and birch established new homeostatic status via adjusting cellular metabolism. Increased deposition of atmospheric N leads to changes in forest productivity. Effects of added N+S on changes in cellular metabolism will yield information on species-specific sensitivity to N+S. To evaluate foliar metabolic changes in American beech (Fagus grandifolia Ehrh.), sugar maple (Acer saccharum Marsh.), and red spruce (Picea rubens Sarg.) that were exposed to ammonium sulfate [(NH4)2SO4); ~ 28.8 kg S ha−1 yr−1 and 25.2 kg N ha−1 yr−1] additions at West Bear Watershed (WBW) starting in 1989 until the end of this experiment, while East Bear Watershed served as a reference. Foliage was collected in 1998–2000 and 2013. Sapwood plugs were also collected in 2013. All were analyzed for ions and metabolites using HPLC and ICP. During 1998–2000, only N+S-treated beech and spruce foliage had a reduction in Ca and Mg. All species had significantly higher content of N-rich metabolites. In 2013, ammonia detoxification continued in the absence of nutrient deficiencies. Significant changes in growth promoting metabolites occurred only in maple throughout this study. Metabolic changes indicated that sugar maple at this site was and still is N-limited, whereas red spruce and American beech had to make metabolic adjustments in order to survive under chronic N+S inputs. We conclude that even in the absence of knowledge about individual species tolerance limits for nutrients and critical N load for the site, monitoring with a suite of metabolites that are centrally connected to both C and N pathways could be a very useful tool in assessing stress from nutrient imbalance in various tree species.

Nitrogen mineralization in O horizon soils during 27years of nitrogen enrichment at the Bear Brook Watershed in Maine, USA.

Chronic elevated nitrogen (N) deposition has altered the N status of temperate forests, with significant implications for ecosystem function. The Bear Brook Watershed in Maine (BBWM) is a whole paired watershed manipulation experiment established to study the effects of N and sulfur (S) deposition on ecosystem function. N was added bimonthly as (NH4)2SO4 to one watershed from 1989 to 2016, and research at the site has studied the evolution of ecosystem response to the treatment through time. Here, we synthesize results from 27years of research at the site and describe the temporal trend of N availability and N mineralization at BBWM in response to chronic N deposition. Our findings suggest that there was a delayed response in soil N dynamics, since labile soil N concentrations did not show increases in the treated watershed (West Bear, WB) compared to the reference watershed (East Bear, EB) until after the first 4years of treatment. Labile N became increasingly available in WB through time, and after 25years of manipulations, treated soils had 10× more extractable ammonium than EB soils. The WB soils had 200× more extractable nitrate than EB soils, driven by both, high nitrate concentrations in WB and low nitrate concentrations in EB. Nitrification rates increased in WB soils and accounted for ~ 50% of net N mineralization, compared to ~ 5% in EB soils. The study provides evidence of the decadal evolution in soil function at BBWM and illustrates the importance of long-term data to capture ecosystem response to chronic disturbance.

Fifteen\u2013year record of soil temperature at the Bear Brook Watershed in Maine

This paper describes a record of air and soil temperature collected from 2001 to 2016 in temperate forests at the Bear Brook Watershed in Maine (BBWM). BBWM is a long-term research site established to study the response of forest ecosystem function to various environmental disturbances, including chronic acidic deposition. Replicate HOBO data loggers were deployed in BBWM’s two forest types (coniferous and deciduous), to record temperatures at four positions: (1) air temperature, 100 cm above the forest floor; (2) surface organic soil, 2 cm below the forest floor surface; (3) mineral soil, 10 cm below the organic–mineral horizon interface; and (4) mineral soil, 25 cm below the organic–mineral horizon interface. Data were recorded every three hours, and these raw data were used to compute daily maximum, daily minimum, daily average, and monthly average values. This fifteen–year record represents one of the few readily–available soil temperature datasets in the region, and provides information on long-term changes in climatology, and seasonal and episodic weather patterns.

Long-Term Experimental Acidification Drives Watershed Scale Shift in Dissolved Organic Matter Composition and Flux.

Over the last several decades dissolved organic carbon concentrations (DOC) in surface waters have increased throughout much of the northern hemisphere. Several hypotheses have been proposed regarding the drivers of this phenomenon including decreased sulfur (S) deposition working via an acidity- change mechanism. Using fluorescence spectroscopy and data from two long-term (24+ years at completion of this study) whole watershed acidification experiments, that is, the Bear Brook Watershed in Maine (BBWM) and Fernow Experimental Forest in West Virginia (FEF) allowed us to control for factors other than the acidity-change mechanism (e.g., differing vegetation, shifting climate), resulting in the first study we are aware of where the acidity change mechanism could be experimentally isolated at the whole ecosystem and decadal scales as the driver of shifts in DOM dynamics. The multidecadal record of stream chemistry at BBWM demonstrates a significantly lower DOC concentration in the treated compared to the reference watershed. Additionally, at both BBWM and FEF we found significant and sustained differences in stream fluorescence index (FI) between the treated and reference watersheds, with the reference watersheds demonstrating a stronger terrestrial DOM signature. These data, coupled with evidence of pH shifts in upper soil horizons support the hypotheses that declines in S deposition are driving changes in the solubility of soil organic matter and increased flux of terrestrial DOC to water bodies.

Nitrogen and phosphorus regulation of soil enzyme activities in acid forest soils

The activities of soil extracellular enzymes are important in governing rates of organic matter decomposition and nutrient cycling in forest ecosystems. Measurements of soil enzyme activities can provide insights on microbial function, in terms of how much energy microbial communities are investing to acquire particular nutrients from OM substrates. In ecosystems enriched with nitrogen (N), phosphorus (P) supply may play an important role in regulating microbial activity, enzyme production, and organic matter decomposition. The response of extracellular enzyme activity to chronic N-enrichment was assessed at a long-term paired watershed N-enrichment experiment, the Bear Brook Watershed in Maine (BBWM) in hardwood and softwood forest types. Specifically, we measured the ambient (extant) activity of C hydrolyzing β-glucosidase (BG) and xylosidase (XYLO), N hydrolyzing N-acetylglucosaminidase (NAG), and P hydrolyzing acid phosphatase (AP) in the watershed subjected to chronic N-enrichment, and in the reference watershed. Secondly, in a series of soil incubations, we characterized the extent to which microbial C and P acquisition were regulated by N and P availability. In these incubations, we measured BG and AP activity response to acute (high-dose fertilizer) nutrient amendments. We hypothesized that soil enzyme activities would respond more to enhanced P availability than N, particularly in the N-enriched watershed. Our results from extant enzyme activity measurements suggest that chronic N-enrichment inhibited rather than stimulated extant soil hydrolytic enzyme activities, which could reflect suppression of microbial biomass and activity. In the acute nutrient amendment incubations, our data indicate that inorganic P was more important than N in regulating soil microbial C and P acquisition in soils from both the N-enriched and reference watersheds. Our results also indicate that the extent to which P availability regulated microbial acquisition of P in O horizon soils was greatest in softwood soils subjected to chronic N-enrichment. Findings from this study suggest that both forest type and soil inorganic P availability could be more important in influencing soil biological response to N pollution than previously recognized.

Microbial enzyme activity at the watershed scale: response to chronic nitrogen deposition and acute phosphorus enrichment

Microbial enzymes play a critical role in organic matter decomposition and enzyme activity can dynamically respond to shifts in inorganic nutrient and substrate availability, reflecting the nutrient and energy limitation of the microbial community. We character- ized microbial enzyme response to shifting nitrogen (N) and phosphorus (P) availability across terrestrial and aquatic environments at the Bear Brook Watershed in Maine, the site of a whole-watershed N enrichment experiment. We compared activity of b-1,4-glucosi- dase (BG); b-1,4-N-acetylglucosaminidase (NAG); acid phosphatase (AP) in soil, leaf litter in terrestrial and stream habitats and stream biofilms in a reference and N enriched watershed, representing whole-eco- system response to chronic N enrichment. In addition, we used shorter, experimental P enrichments to address potential P limitation under ambient and elevated N availability. We found that BG and NAG activity were not affected by the long-term N enrichment in either habitat. Enhanced P limitation due to N enrichment was evident only in the aquatic habitats with 5- and 8-fold higher treated watershed AP activity in stream biofilms and stream litter, respectively. Acute P additions reduced AP activity and increased BG activity and these effects were also most pronounced in the streams. The stoichiometry of enzyme activity was constrained across ecosystem compartments with regression slopes for lnBG:lnNAG, lnBG:lnAP, and lnNAG:lnAP close to 1, ranging 1.142-1.241. We found that microbial enzyme response to shifting N and P availability varied among watershed compartments, typically with stronger effects in aquatic habitats. This suggests that understanding the response of ecosystem function to disturbance at the watershed scale requires simultaneous consideration of all compartments.

Soil Drainage Class Influences on Soil Carbon in a New England Forested Watershed

Imperfectly drained soils under forests can occupy extensive areas of the landscape and understanding how they might differ from upland forest soils can be critical for understanding soil organic carbon (SOC) across forested landscapes. Research on forest SOC across drainage gradients has been limited. This research was a case study at the Bear Brook Watershed in Maine (BBWM), a long‐term ecological research site, that focused on SOC pools and fluxes across three soil drainage classes (moderately well drained‐MWD, somewhat poorly drained‐SWPD, poorly drained‐PD) and two forest types (coniferous‐CF, broadleaved deciduous‐BLD). Soil respiration (RS), soil temperature, soil moisture content, and extractable soluble C were measured from May through November to assess Rsas an indicator of biological activity. The MWD mineral soil had significantly higher C concentrations(0.71 g kg−1) compared to SWPD (0.30 g kg−1) and PD (0.34 g kg−1). The MWD soils in CF had significantly greater C content (433.3 Mg C ha−1) compared to SWPD (113.95 Mg C ha−1) and PD (103.3 Mg C ha−1). Soil C concentration and content was not significantly different between SWPD and PD soils. Mean mineral soil C fraction concentrations (active, stable, passive) expressed as a percent of the total soil mass were significantly different among soil drainage classes. Results from this case study suggest that pedogenesis, above and belowground plant productivity, and moisture availability across the growing season play important roles in determining SOC dynamics in these forests.

Chronic catchment nitrogen enrichment and stoichiometric constraints on the bioavailability of dissolved organic matter from leaf leachate

Summary1. Chronic nitrogen (N) deposition may alter the bioavailability of dissolved organic matter (DOM) in streams by multiple pathways. Elevated N deposition may alter the nutrient stoichiometry of DOM as well as nutrient availability in stream water.2. We evaluated the influence of a decadal‐scale experimental N enrichment on the relative importance of DOM nutrient content and inorganic nutrient availability on the bioavailability of DOM. We measured the consumption of dissolved organic carbon (DOC) and changes in nutrient concentration, DOM components and enzyme activity in a bottle incubation assay with different DOM and nutrient treatments. To evaluate the effect of DOM stoichiometry, we used leaf leachates of different carbon/N/phosphorus (C : N :P) ratio, made from leaf litter sourced in the reference and N‐enriched catchments at the Bear Brook Watershed in Maine (BBWM). We also manipulated the concentration of inorganic N and P to compare the effect of nutrient enrichment with DOM stoichiometry.3. DOC from the N‐enriched catchment was consumed 14% faster than that from the reference catchment. However, mean DOC consumption for both leachates was more than doubled by the simultaneous addition of N and P, compared to controls, while the addition of N or P alone increased consumption by 42 and 23%, respectively. The effect of N and/or P enrichment consistently had a greater effect than DOM source for all response variables considered.4. We subsequently conducted DOC uptake measurements using leaf leachate addition under ambient and elevated N and P in the streams draining the reference and N‐enriched catchments at BBWM. In both streams, DOC uptake lengths were shorter when N and P were elevated.5. Although both DOM stoichiometry and inorganic nutrient availability affect DOM bioavailability, N and P co‐limitation appears to be the dominant driver of reach‐scale processing of DOM.

Soil Solution Chemical Response to Two Decades of Experimental Acidification at the Bear Brook Watershed in Maine

We examined long-term changes in soil solution chemistry associated with experimental, whole watershed-acidification at the Bear Brook Watershed in Maine (BBWM). At BBWM, the West Bear (WB) watershed has been treated with bimonthly additions of ((NH4)2 SO4) since 1989. The adjacent East Bear (EB) watershed serves as a biogeochemical reference. Soil solution chemistry in the EB watershed was relatively stable from 1989–2007, with the exception of declining SO4–S concentrations associated with a progressive decline in SO4–S deposition during this period. Soil solution chemistry in WB reflected a progressive change in acid-neutralization mechanisms from base cation buffering to Al buffering associated with treatment during this period. Total dissolved Al concentrations progressively increased over time and were ~4× higher in 2007 than in 1989. Treatment of WB was also associated with long-term increases in soil solution H+, SO4–S, and NO3–N, whereas soil solution dissolved organic carbon (DOC) was unresponsive to treatment. For solutes such as Ca, H+, and SO4–S, changes in stream chemistry were generally parallel to changes in soil solution chemistry, indicating a close coupling of terrestrial and aquatic processes that regulate the chemistry of solutions in this first-order stream watershed. For other solutes such as Al and DOC, solute concentrations were higher in soil solutions compared with streams, suggesting that sorption and transformation processes along hydrologic flow-paths were important in regulating the chemistry of solutions and the transport of these solutes.

Phenology at the Bear Brook Watershed in Maine, USA: Foliar Chemistry and Morphology

Annual developmental events in biological systems are dependent, in part, on environmental conditions and can be valuable bio-indicators of environmental change. Many studies have been done on the effects of temperature and photoperiod on phenophases, but fewer have explored the consequences of nutrient availability in terrestrial ecosystems on forest phenology. Here we examined phenological phenomena at a long-term experimental forested watershed subjected to decadal-scale ecosystem acidification and nitrogen (N) enrichment. Phenophases of Acer rubrum, Acer saccharum, and Picea rubens in both watersheds were observed throughout the 2010 growing season and included bud burst, flowering (A. rubrum), leaf or needle emergence and unfolding, leaf senescence (Acer spp.), and leaf fall (Acer spp). Clear species-specific phenological patterns were observed, but no treatment effects were evident. Chemical phenology of canopy tree foliage was also examined on a monthly basis from May through October 2010. Nitrogen was the only element that was significantly higher in the WB watershed for all species, although not all months showed significant differences. Other treatment differences in elemental composition of foliage are discussed. Foliar N and P concentrations decreased in all species throughout the growing season, while foliar Ca, K, and Al concentrations increased or were constant. This study found clear species-specific patterns of morphological and chemical phenology with time, but did not show evidence for visible alterations in seasonal development as a result of ecosystem acidification and N enrichment. Treatment effects on chemical phenology, as applied here, showed some responses and warrant further consideration for application to coupled chemical-biological indicators of a changing chemical and physical climate.

Streamflow variability and hydroclimatic change at the Bear Brook Watershed in Maine (BBWM), USA

Seasonal variations in streamflow and the associated hydrologic extremes impart significant temporal structure to watershed-scale chemical fluxes. Consequently, a careful characterization of the episodic-to-seasonal and longer-term streamflow variations is a first step toward developing a comprehensive view of the temporal dynamics of watershed processes in a changing climate. Here we analyze a nearly two-decade-long streamflow record for the East Bear subwatershed within the Bear Brook Watershed in Maine (BBWM) (USA) to understand the envelope of streamflow variability by season, with a particular focus on the high flow events that have a disproportionately large impact on the biogeochemical processes and fluxes. Interannual and longer-term variations in a number of derived statistical metrics of hydrologic variability are examined. Our analysis shows substantial interannual and longer-term variability in seasonal flow volumes and peak flows. Furthermore, a long, unimpaired streamflow record for the Narraguagus River (a proximate watershed to the BBWM) is examined with a view to understand the relative coherence in hydrologic variability, as well as quantifying the decadal and longer-term hydrologic variations in this region. We find that the streamflow variability in the two watersheds shows similarity in all seasons. A moving window analysis to assess the changing flood potential over time indicates upward trends in the recent decades. Spring season (March-May) flood estimates show a near-monotonic trend over the 1949-2008 record. Finally, empirical relationships between streamflow and large-scale atmospheric circulation patterns highlight the regional and global climatic drivers of hydrologic extremes in this region, including impacts from remnants of Atlantic hurricanes.

Twenty-year inter-annual trends and seasonal variations in precipitation and stream water chemistry at the Bear Brook Watershed in Maine, USA

Mean annual concentration of SO4(-2) in wet-only deposition has decreased between 1988 and 2006 at the paired watershed study at Bear Brook Watershed in Maine, USA (BBWM) due to substantially decreased emissions of SO(2). Emissions of NO(x) have not changed substantially, but deposition has declined slightly at BBWM. Base cations, NH4+, and Cl(-) concentrations were largely unchanged, with small irregular changes of <1 μeq L(-1) per year from 1988 to 2006. Precipitation chemistry, hydrology, vegetation, and temperature drive seasonal stream chemistry. Low flow periods were typical in June-October, with relatively greater contributions of deeper flow solutions with higher pH; higher concentrations of acid-neutralizing capacity, Si, and non-marine Na; and low concentrations of inorganic Al. High flow periods during November-May were typically dominated by solutions following shallow flow paths, which were characterized by lower pH and higher Al and DOC concentrations. Biological activity strongly controlled NO3- and K(+). They were depressed during the growing season and elevated in the fall. Since 1987, East Bear Brook (EB), the reference stream, has been slowly responding to reduced but still elevated acid deposition. Calcium and Mg have declined fairly steadily and faster than SO4(-2), with consequent acidification (lower pH and higher inorganic Al). Eighteen years of experimental treatment with (NH(4))(2)SO(4) enhanced acidification of West Bear Brook's (WB) watershed. Despite the manipulation, NH4+ concentration remained below detection limits at WB, while leaching of NO3- increased. The seasonal pattern for NO3- concentrations in WB, however, remained similar to EB. Mean monthly concentrations of SO4(-2) have increased in WB since 1989, initially only during periods of high flow, but gradually also during base flow. Increases in mean monthly concentrations of Ca(2+), Mg(2+), and K(+) due to the manipulation occurred from 1989 until about 1995, during the depletion of base cations in shallow flow paths in WB. Progressive depletion of Ca and Mg at greater soil depth occurred, causing stream concentrations to decline to pre-manipulation values. Mean monthly Si concentrations did not change in EB or WB, suggesting that the manipulation had no effect on mineral weathering rates. DOC concentrations in both streams did not exhibit inter- or intra-annual trends.

Soil chemical and physical properties at the Bear Brook Watershed in Maine, USA

Acidic deposition leads to the acidification of waters and accelerated leaching and depletion of soil base cations. The Bear Brook Watershed in Maine has used whole-watershed chemical manipulations to study the effects of elevated N and S on forest ecosystem function on a decadal time scale. The objectives of this study were to define the chemical and physical characteristics of soils in both the reference and treated watersheds after 17 years of treatment and assess evidence of change in soil chemistry by comparing soil studies in 1998 and 2006. Results from 1998 confirmed depletion of soil base cation pools and decreased pH due to elevated N and S within the treated watershed. However, between 1998 and 2006, during a period of declining SO4(2-) deposition and continued whole-watershed experimental acidification on the treated watershed, there was little evidence of continued soil exchangeable base cation concentration depletion or recovery. The addition of a pulse of litterfall and accelerating mineralization from a severe ice storm in 1998 may have had significant effects on forest floor nutrient pools and cycling between 1998 and 2006. Our findings suggest that mineralization of additional litter inputs from the ice storm may have obscured temporal trends in soil chemistry. The physical data presented also demonstrate the importance of coarse fragments in the architecture of these soils. This study underscores the importance of long-term, quantitative soil monitoring in determining the trajectories of change in forest soils and ecosystem processes over time.