Simulation of the effects of forest harvesting under changing climate to inform long-term sustainable forest management using a biogeochemical model.

Process ecosystem models are useful tools to provide insight on complex, dynamic ecological systems, and their response to disturbances. The biogeochemical model PnET-BGC was modified and tested using field observations from an experimentally whole-tree harvested northern hardwood watershed (W5) at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. In this study, the confirmed model was used as a heuristic tool to investigate long-term changes in hydrology, biomass accumulation, and soil solution and stream water chemistry for three different watershed cutting intensities (40%, 60%, 80%) and three rotation lengths (30, 60, 90 years) under both constant (current climate) and changing (MIROC5-RCP4.5) future climate scenarios and atmospheric CO2 through the year 2200. For the no future cutting scenario, total ecosystem stored carbon (i.e., sum of aboveground biomass, woody debris and soil) reached a maximum value of 207 t C ha-1 under constant climate but increased to 452 t C ha-1 under changing climate in 2200 due to a CO2 fertilization effect. Harvesting of trees decreased total ecosystem stored carbon between 7 and 36% for constant climate and 7-60% under changing climate, respectively, with greater reductions for shorter logging rotation lengths and greater watershed cutting intensities. Harvesting under climate change resulted in noticeable losses of soil organic matter (12-56%) coinciding with loss of soil nutrients primarily due to higher rates of soil mineralization associated with increases in temperature, compared with constant climate conditions (3-22%). Cumulative stream leaching of nitrate under climate change (181-513 kg N ha-1) exceeded constant climate values (139-391 kg N ha-1) for the various cutting regimes. Under both climate conditions the model projected greater sensitivity to varying the length of cutting period than cutting intensities. Hypothetical model simulations highlight future challenges in maintaining long-term productivity of managed forests under changing climate due to a potential for a deterioration of soil fertility.

The application of an integrated biogeochemical model to simulate dynamics of vegetation, hydrology and nutrients in soil and streamwater following a whole-tree harvest of a northern hardwood forest.

Understanding the impacts of clear-cutting is critical to inform sustainable forest management associated with net primary productivity and nutrient availability over the long-term. Few studies have rigorously tested model simulations against field measurements which would provide more confidence in efforts to quantify logging impacts over the long-term. The biogeochemical model, PnET-BGC has been used to simulate forest production and stream chemistry at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. Previous versions of PnET-BGC could accurately simulate the longer-term biogeochemical response to harvesting, but were unable to reproduce the marked changes in stream NO3- immediately after clear-cutting which is an important impact of forest harvesting. Moreover, the dynamics of nutrients in major pools including mineralization and plant uptake were poorly predicted. In this study, the model was modified and parametrized allowing for a lower decomposition rate during the earlier years after the clear-cut and increased NH4+ plant uptake with the regrowth of new vegetation to adequately reproduce hydrology, aboveground forest biomass, and soil solution and stream water chemistry in response to a whole-tree harvest of a northern hardwood forest watershed (W5) at the HBEF. Modeled soil solution and stream water chemistry successfully captured the rapid recovery of leaching nutrients to pre-cut levels within four years after the treatment. The model simulated a substantial increase in aboveground net primary productivity (NPP) from around 36% to 97% of pre-cut aboveground values within years 2 to 4 of the cut, which closely reproduced the measured values. The projected accumulation of aboveground biomass 70 years following the harvest was almost 190 t ha-1, which is close to the pre-cut measured value. A first-order sensitivity analysis showed greater sensitivity of projections of the model outputs for the mature forest than the strongly aggrading forest.

Methods of Soil Resampling to Monitor Changes in the Chemical Concentrations of Forest Soils.

Recent soils research has shown that important chemical soil characteristics can change in less than a decade, often the result of broad environmental changes. Repeated sampling to monitor these changes in forest soils is a relatively new practice that is not well documented in the literature and has only recently been broadly embraced by the scientific community. The objective of this protocol is therefore to synthesize the latest information on methods of soil resampling in a format that can be used to design and implement a soil monitoring program. Successful monitoring of forest soils requires that a study unit be defined within an area of forested land that can be characterized with replicate sampling locations. A resampling interval of 5 years is recommended, but if monitoring is done to evaluate a specific environmental driver, the rate of change expected in that driver should be taken into consideration. Here, we show that the sampling of the profile can be done by horizon where boundaries can be clearly identified and horizons are sufficiently thick to remove soil without contamination from horizons above or below. Otherwise, sampling can be done by depth interval. Archiving of sample for future reanalysis is a key step in avoiding analytical bias and providing the opportunity for additional analyses as new questions arise.

Silica uptake and release in live and decaying biomass in a northern hardwood forest.

In terrestrial ecosystems, a large portion (20-80%) of the dissolved Si (DSi) in soil solution has passed through vegetation. While the importance of this "terrestrial Si filter" is generally accepted, few data exist on the pools and fluxes of Si in forest vegetation and the rate of release of Si from decomposing plant tissues. We quantified the pools and fluxes of Si through vegetation and coarse woody debris (CWD) in a northern hardwood forest ecosystem (Watershed 6, W6) at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA. Previous work suggested that the decomposition of CWD may have significantly contributed to an excess of DSi reported in stream-waters following experimental deforestation of Watershed 2 (W2) at the HBEF. We found that woody biomass (wood + bark) and foliage account for approximately 65% and 31%, respectively, of the total Si in biomass at the HBEF. During the decay of American beech (Fagus grandifolia) boles, Si loss tracked the whole-bole mass loss, while yellow birch (Betula alleghaniensis) and sugar maple (Acer saccharum) decomposition resulted in a preferential Si retention of up to 30% after 16 yr. A power-law model for the changes in wood and bark Si concentrations during decomposition, in combination with an exponential model for whole-bole mass loss, successfully reproduced Si dynamics in decaying boles. Our data suggest that a minimum of 50% of the DSi annually produced in the soil of a biogeochemical reference watershed (W6) derives from biogenic Si (BSi) dissolution. The major source is fresh litter, whereas only ~2% comes from the decay of CWD. Decay of tree boles could only account for 9% of the excess DSi release observed following the experimental deforestation of W2. Therefore, elevated DSi concentrations after forest disturbance are largely derived from other sources (e.g., dissolution of BSi from forest floor soils and/or mineral weathering).

Natural and anthropogenic drivers of calcium depletion in a northern forest during the last millennium.

The pace and degree of nutrient limitation are among the most critical uncertainties in predicting terrestrial ecosystem responses to global change. In the northeastern United States, forest growth has recently declined along with decreased soil calcium (Ca) availability, suggesting that acid rain has depleted soil Ca to the point where it may be a limiting nutrient. However, it is unknown whether the past 60 y of changes in Ca availability are strictly anthropogenic or partly a natural consequence of long-term ecosystem development. Here, we report a high-resolution millennial-scale record of Ca and 16 other elements from the sediments of Mirror Lake, a 15-ha lake in the White Mountains of New Hampshire surrounded by northern hardwood forest. We found that sedimentary Ca concentrations had been declining steadily for 900 y before regional Euro-American settlement. This Ca decline was not a result of serial episodic disturbances but instead the gradual weathering of soils and soil Ca availability. As Ca availability was declining, nitrogen availability concurrently was increasing. These data indicate that nutrient availability on base-poor, parent materials is sensitive to acidifying processes on millennial timescales. Forest harvesting and acid rain in the postsettlement period mobilized significant amounts of Ca from watershed soils, but these effects were exacerbated by the long-term pattern. Shifting nutrient limitation can potentially occur within 10,000 y of ecosystem development, which alters our assessments of the speed and trajectory of nutrient limitation in forests, and could require reformulation of global models of forest productivity.

Soil mercury and its response to atmospheric mercury deposition across the northeastern United States.

Terrestrial soil is a large reservoir of atmospherically deposited mercury (Hg). However, few studies have evaluated the accumulation of Hg in terrestrial ecosystems in the northeastern United States, a region which is sensitive to atmospheric Hg deposition. We characterized Hg and organic matter in soil profiles from 139 sampling sites for five subregions across the northeastern United States and estimated atmospheric Hg deposition to these sites by combining numerical modeling with experimental data from the literature. We did not observe any significant relationships between current net atmospheric Hg deposition and soil Hg concentrations or pools, even though soils are a net sink for Hg inputs. Soil Hg appears to be preserved relative to organic carbon (OC) and/or nitrogen (N) in the soil matrix, as a significant negative relationship was observed between the ratios of Hg/OC and OC/N (r = 0.54, P < 0.0001) that shapes the horizonal distribution patterns. We estimated that atmospheric Hg deposition since 1850 (3.97 mg/m2) accounts for 102% of the Hg pool in the organic horizons (3.88 mg/m2) and 19% of the total soil Hg pool (21.32 mg/m2), except for the southern New England (SNE) subregion. The mean residence time for soil Hg was estimated to be 1800 years, except SNE which was 800 years. These patterns suggest that in addition to atmospheric deposition, the accumulation of soil Hg is linked to the mineral diagenetic and soil development processes in the region.

Simulating effects of changing climate and CO(2) emissions on soil carbon pools at the Hubbard Brook experimental forest.

Carbon (C) sequestration in forest biomass and soils may help decrease regional C footprints and mitigate future climate change. The efficacy of these practices must be verified by monitoring and by approved calculation methods (i.e., models) to be credible in C markets. Two widely used soil organic matter models - CENTURY and RothC - were used to project changes in SOC pools after clear-cutting disturbance, as well as under a range of future climate and atmospheric carbon dioxide (CO(2) ) scenarios. Data from the temperate, predominantly deciduous Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA, were used to parameterize and validate the models. Clear-cutting simulations demonstrated that both models can effectively simulate soil C dynamics in the northern hardwood forest when adequately parameterized. The minimum postharvest SOC predicted by RothC occurred in postharvest year 14 and was within 1.5% of the observed minimum, which occurred in year 8. CENTURY predicted the postharvest minimum SOC to occur in year 45, at a value 6.9% greater than the observed minimum; the slow response of both models to disturbance suggests that they may overestimate the time required to reach new steady-state conditions. Four climate change scenarios were used to simulate future changes in SOC pools. Climate-change simulations predicted increases in SOC by as much as 7% at the end of this century, partially offsetting future CO(2) emissions. This sequestration was the product of enhanced forest productivity, and associated litter input to the soil, due to increased temperature, precipitation and CO(2) . The simulations also suggested that considerable losses of SOC (8-30%) could occur if forest vegetation at HBEF does not respond to changes in climate and CO(2) levels. Therefore, the source/sink behavior of temperate forest soils likely depends on the degree to which forest growth is stimulated by new climate and CO(2) conditions.

Chemical properties of upland forest soils in the Catskills region.

The Catskills forest provides a valuable array of ecosystem services for local and regional populations, including the provision of forest products, wildlife habitats, and high-quality water. These services depend on chemical and biological processes that occur in forest soils. In 2011, we sampled soils in 25 headwater catchments in the Catskills region to quantify the pools of soil nutrients and examine the variation in soil properties in the region. The average soil depth in the 50 excavated pits was 56.6 cm. Average soil mass was 205 kg/m(2). The pools of soil carbon and nitrogen averaged 58.5 and 3.95 Mg/ha, respectively. The thin organic horizons accounted for less than 1% of soil mass, but included 14% of the soil carbon and 11% of soil nitrogen. Catskills forest soils are highly acidic, with mean pH ranging between 3.9 and 4.75. Base saturation was high (>60%) in organic horizons and low (12-31%) in mineral soils. The pool of exchangeable calcium is approximately equivalent to 20 years of calcium export from headwater streams, raising concerns regarding the ability of these catchments to maintain current stream calcium concentrations. The data and samples collected in this study provide a baseline for future soil monitoring in the region.

Long-term changes in aluminum fractions of drainage waters in two forest catchments with contrasting lithology.

Aluminum (Al) chemistry was studied in soils and waters of two catchments covered by spruce (Picea abies) monocultures in the Czech Republic that represent geochemical end-members of terrestrial and aquatic sensitivity to acidic deposition. The acid-sensitive Lysina catchment, underlain by granite, was compared to the acid-resistant Pluhuv Bor catchment on serpentine. Organically-bound Al was the largest pool of reactive soil Al at both sites. Very high median total Al (Alt) concentrations (40 micromol L(-1)) and inorganic monomeric Al (Ali) concentrations (27 micromol L(-1)) were observed in acidic (pH 4.0) stream water at Lysina in the 1990s and these concentrations decreased to 32 micromol L(-1) (Alt) and 13 micromol L(-1) (Ali) in the 2000s. The potentially toxic Ali fraction decreased in response to long-term decreases in acidic deposition, but Ali remained the largest fraction. However, the organic monomeric (Alo) and particulate (Alp) fractions increased in the 2000s at Lysina. In contrast to Lysina, marked increases of Alt concentrations in circum-neutral waters at Pluhuv Bor were observed in the 2000s in comparison with the 1990s. These increases were entirely due to the Alp fraction, which increased more than 3-fold in stream water and up to 8-fold in soil water in the A horizon. Increase of Alp coincided with dissolved organic carbon (DOC) increases. Acidification recovery may have increased the content of colloidal Al though the coagulation of monomeric Al.

Changes in aluminum concentrations and speciation in lakes across the northeastern U.S. following reductions in acidic deposition.

We surveyed 113 lakes in the northeastern U.S. in 2001 that had previously been sampled in 1986 to evaluate the effects of reductions in acidic deposition on the concentrations and speciation of aluminum (Al). We found ubiquitous decreases in the concentrations of total Al and inorganic monomeric aluminum (Ali) across the region. Median total Al decreased from 1.45 to 1.01 micromol L(-1) across the region, with the largest decrease in the Adirondacks (4.60 micromol L(-1) to 2.59 micromol L(-1)). Organic monomeric aluminum (Alo) also decreased region-wide and in all the subregions except the Adirondacks. The speciation of Ali shifted from largely Al-F complexes in 1986 to largely Al-OH complexes in 2001 in ponds whose concentrations were above the detection limit (>0.7 micromol L(-1)). In 2001, only seven lakes studied, representing a population of 130 lakes in the region, had Al1 concentrations above a toxic limit of 2 micromol L(-1) compared with 20 sample lakes, representing 449 lakes, in 1986. Thus, we estimate that more than 300 lakes in the northeastern United States no longer have summer Ali concentrations at levels considered harmful to aquatic biota.

Chemical recovery of surface waters across the northeastern united states from reduced inputs of acidic deposition: 1984-2001.

Changes in lake water chemistry between 1984 and 2001 at 130 stratified random sites across the northeastern United States were studied to evaluate the population-level effects of decreases in acidic deposition. Surface-water S04(2-) concentrations decreased across the region at a median rate of -1.53 microequiv L(-1) year(-1). Calcium concentrations also decreased, with a median rate of -1.73 microequiv L(-1) year(-1). This decrease in Ca2+ retarded the recovery of surface water acid neutralizing capacity (Gran ANC), which increased at a median rate of 0.66 microequiv L(-1) year(-1). There were small increases in pH in all subregions except central New England and Maine, where the changes were not statistically significant. Median NO3- trends were not significant except in the Adirondacks, where NO3- concentrations increased at a rate of 0.53 microequiv L(-1) year(-1). A regionwide decrease in the concentration of total Al, especially in ponds with low ANC values (ANC < 25 microequiv L(-1)), was observed in the Adirondack subregion. These changes in Al were consistent with the general pattern of increasing pH and ANC. Despite the general pattern of chemical recovery, many ponds remain chronically acidic or are susceptible to episodic acidification. The continued chemical and biological recovery at sites in the northeastern United States will depend on further controls on S and N emissions.

Mycorrhizal weathering of apatite as an important calcium source in base-poor forest ecosystems.

The depletion of calcium in forest ecosystems of the northeastern USA is thought to be a consequence of acidic deposition and to be at present restricting the recovery of forest and aquatic systems now that acidic deposition itself is declining. This depletion of calcium has been inferred from studies showing that sources of calcium in forest ecosystems namely, atmospheric deposition and mineral weathering of silicate rocks such as plagioclase, a calcium-sodium silicate do not match calcium outputs observed in forest streams. It is therefore thought that calcium is being lost from exchangeable and organically bound calcium in forest soils. Here we investigate the sources of calcium in the Hubbard Brook experimental forest, through analysis of calcium and strontium abundances and strontium isotope ratios within various soil, vegetation and hydrological pools. We show that the dissolution of apatite (calcium phosphate) represents a source of calcium that is comparable in size to known inputs from atmospheric sources and silicate weathering. Moreover, apatite-derived calcium was utilized largely by ectomycorrhizal tree species, suggesting that mycorrhizae may weather apatite and absorb the released ions directly, without the ions entering the exchangeable soil pool. Therefore, it seems that apatite weathering can compensate for some of the calcium lost from base-poor ecosystems, and should be considered when estimating soil acidification impacts and calcium cycling.