Jennifer D. Knoepp

Loss of foundation species: consequences for the structure and dynamics of forested ecosystems

In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-harvesting. Through a series of case studies, we show that the loss of foundation tree species changes the local environment on which a variety of other species depend; how this disrupts fundamental ecosystem processes, including rates of decomposition, nutrient fluxes, carbon sequestration, and energy flow; and dramatically alters the dynamics of associated aquatic ecosystems. Forests in which dynamics are controlled by one or a few foundation species appear to be dominated by a small number of strong interactions and may be highly susceptible to alternating between stable states following even small perturbations. The ongoing decline of many foundation species provides a set of important, albeit unfortunate, opportunities to develop the research tools, models, and metrics needed to identify foundation species, anticipate the cascade of immediate, short- and long-term changes in ecosystem structure and function that will follow from their loss, and provide options for remedial conservation and management.

Biological indices of soil quality: an ecosystem case study of their use

Soil quality indices can help ensure that site productivity and soil function are maintained. Biological indices yield evidence of how a soil functions and interacts with the plants, animals and climate that comprise an ecosystem. Soil scientists can identify and quantify both chemical and biological soil-quality indicators for ecosystems with a single main function, such as agricultural lands and forest plantations. However, quantifying these indices in complex ecosystems — that have multiple uses or goals such as maintaining biodiversity, aesthetics, recreation, timber production and water quality — is much more difficult. In an ecosystem context all components — plants, animals and humans — interact with the soil differently, making soil quality indices variable. These interactions result in a combination of biological processes that make each ecosystem unique. We examined the soil and site quality of five forest stands (xeric oak-pine; two mixed hardwood, cove hardwood, northern

Effects of forest management on soil carbon: results of some long-term resampling studies.

The effects of harvest intensity (sawlog, SAW; whole tree, WTH; and complete tree, CTH) on biomass and soil C were studied in four forested sites in the southeastern US (mixed deciduous forests at Oak Ridge, TN and Coweeta, NC; Pinus taeda at Clemson, SC: and P. eliottii at Bradford, FL). In general, harvesting had no lasting effects on soil C. However, intensive temporal sampling at the NC and SC sites revealed short-term changes in soil C during the first few years after harvesting, and large, long-term increases in soil C were noted at the TN site in all treatments. Thus, changes in soil C were found even though lasting effects of harvest treatment were not. There were substantial differences in growth and biomass C responses to harvest treatments among sites. At the TN site, there were no differences in biomass at 15 years after harvest. At the SC site, greater biomass was found in the SAW than in the WTH treatment 16 years after harvest, and this effect is attributed to be due to both the N left on site in foliar residues and to the enhancement of soil physical and chemical properties by residues. At the FL site, greater biomass was found in the CTH than in the WTH treatment 15 years after harvest, and this effect is attributed to be due to differences in understory competition. Biomass data were not reported for NC. The effects of harvest treatment on ecosystem C are expected to magnify over time at the SC and FL sites as live biomass increases, whereas the current differences in ecosystem C at the TN site (which are due to the presence of undecomposed residues) are expected to lessen with time.

Rates of nitrogen mineralization across an elevation and vegetation gradient in the Southern Appalachians

We measured nitrogen (N) transformation rates for six years to examine temporal variation across the vegetation and elevation gradient that exists within the Coweeta Hydrologic Laboratory. Net N mineralization and nitrification rates were measured using 28-day in situ closed core incubations. Incubations were conducted at various intervals, ranging from monthly during the growing season, to seasonally based on vegetation phenology. Vegetation types included oak-pine, cove hardwoods, low elevation mixed oak, high elevation mixed oak, and northern hardwoods. Elevations ranged from 782 to 1347 m. Nitrogen transformation rates varied with vegetation type. Mineralization rates were lowest in the oak-pine and mixed oak sites averaging <1.2 mg N kg soil-1 28 day-1. Rates in the cove hardwood site were greater than all other low elevation sites with an annual average of 3.8 mg N kg soil-1 28 day-1. Nitrogen mineralization was greatest in the northern hardwood site averaging 13 mg N kg soil-1 28 day-1. Nitrification rates were typically low on four sites with rates <0.5 mg N kg soil-1 28 day-1. However, the annual average nitrification rate of the northern hardwood site was 6 mg N kg soil-1 28 days-1. Strong seasonal trends in N mineralization were observed. Highest rates occurred in spring and summer with negligible activity in winter. Seasonal trends in nitrification were statistically significant only in the northern hardwood site. Nitrogen mineralization was significantly different among sites on the vegetation and elevation gradient. While N mineralization rates were greatest at the high elevation site, vegetation type appears to be the controlling factor.

Using soil temperature and moisture to predict forest soil nitrogen mineralization

Abstract. Due to the importance of N in forest productivity ecosystem and nutrient cycling research often includes measurement of soil N transformation rates as indices of potential availability and ecosystem losses of N. We examined the feasibility of using soil temperature and moisture content to predict soil N mineralization rates (Nmin) at the Coweeta Hydrologic Laboratory in the southern Appalachians. We conducted seasonal laboratory incubations of A and AB horizon soils from three sites with mixed-oak vegetation using temperature and moisture levels characteristic of the season in which the soils were collected. The incubations showed that temperature and temperature-moisture interactions significantly affected net soil Nmin. We used the laboratory data to generate equations relating net Nmin to soil temperature and moisture data. Using field-collected temperature and moisture data we then calculated Nmin on similar forest sites and compared predicted rates with in situ, closed-core Nmin measurements. The comparison showed that the in situ Nmin was greater than rates predicted from laboratory generated equations (slope =3.22; r2=0.89). Our study suggests that while climatic factors have a significant effect on soil Nmin, other factors also influence rates measured in the laboratory and in situ.

Long-term effects of commercial sawlog harvest on soil cation concentrations

Abstract There is increasing concern about the effects of nutrient removal associated with various forest harvesting practices on long-term site productivity. We measured exchangeable soil cation concentration responses to a commercial clearcut sawlog harvest in mixed hardwoods on a 59-ha watershed in the southern Appalachians. Soils were sampled 17 months prior to, and periodically for 17 years after, harvest. Concentrations of Ca, Mg, and K, increased significantly in the 0–10-cm soil layer for 3 years following harvest compared to pre-treatment levels. Concentrations of Mg and K were still significantly above pre-treatment levels 17–20 years following harvest. Calcium concentrations did not change significantly at the 10–30 cm depth, but both Mg and K showed significantly higher concentrations in some post-treatment years. Soils in the adjacent reference watershed showed no significant changes in soil cation concentrations over the same 17-year period. Results indicate that sawlog harvest using cable-yarding techniques on these sites does not adversely impact soil cation concentrations.

Effects of prescribed fire in mixed oak forests of the southern Appalachians: forest floor, soil, and soil solution nitrogen responses

Abstract We examined nutrient cycling responses to prescribed fire on three sub-mesic, mixed-oak sites located in the Blue Ridge Physiographic province of the southern Appalachian Mountains: Alarka Laurel Branch (AL), Robin Branch (RB), and Roach Mill Branch (RM). Each study site was located within a sub-watershed that drained a first order stream. Our objective was to quantify the effects of prescribed burning on forest floor mass, nitrogen and carbon pools; and soil and soil water available nitrogen. Each site included a burned and unburned control area; both burned and control areas were sampled before and after burning. Within each plot, we sampled forest floor mass, carbon and nitrogen, soil and soil solution nitrate (NO3-N) and ammonium (NH4-N) concentrations before and after the prescribed burns. All prescribed fires were conducted in the dormant season and were low to moderate intensity. All sites lost a significant amount of forest floor mass due to burning; 82 to 91% of the Oi layer and 26 to 46% of the Oe + Oa layer. Soil NH4-N concentrations increased in surface soils (0–5 cm) only, immediately after burning, but return to pre-burn levels by mid-summer. Burning had no measurable effect on soil solution inorganic nitrogen concentrations. Low levels of solution NO3-N and NH4-N after burning and no change in stream NO3-N concentrations indicated that no inorganic nitrogen was lost from these sites.

Simulated effects of sulfur deposition on nutrient cycling in class I wilderness areas.

We predicted the effects of sulfate (SO(4)) deposition on wilderness areas designated as Class I air quality areas in western North Carolina using a nutrient cycling model (NuCM). We used three S deposition simulations: current, 50% decrease, and 100% increase. We measured vegetation, forest floor, and root biomass and collected soil, soil solution, and stream water samples for chemical analyses. We used the closest climate stations and atmospheric deposition stations to parameterize NuCM. The areas were: Joyce Kilmer (JK), Shining Rock (SR), and Linville Gorge (LG). They differ in soil acidity and nutrients, and soil solution and stream chemistry. Shining Rock and LG have lower soil solution base cation and higher acidic ion concentrations than JK. For SR and LG, the soil solution Ca/Al molar ratios are currently 0.3 in the rooting zone (A horizon), indicating Al toxicity. At SR, the simulated Ca/Al ratio increased to slightly above 1.5 after the 30-yr simulation regardless of S deposition reduction. At LG, Ca/Al ratios ranged from 1.6 to 2.4 toward the end of the simulation period, the 100% increase scenario had the lower value. Low Ca/Al ratios suggest that forests at SR and LG are significantly stressed under current conditions. Our results also suggest that SO(4) retention is low, perhaps contributing to their high degree of acidification. Their soils are acidic, low in weatherable minerals, and even with large reductions in SO(4) and associated acid deposition, it may take decades before these systems recover from depletion of exchangeable Ca, Mg, and K.

Interactive effects of disturbance and nitrogen availability on phosphorus dynamics of southern Appalachian forests

Understanding the main and interactive effects of chronically altered resource availability and disturbance on phosphorus (P) availability is increasingly important in light of the rapid pace at which human activities are altering these processes and potentially introducing P limitation. We measured P pools and fluxes in eighteen mixed forest stands at three elevations (low, mid, high) subjected to increasing atmospheric N deposition, where hemlock (Tsuga canadensis) was absent or declining due to infestation by the exotic hemlock woolly adelgid (Adelges tsugae). While total soil P was similar across the study area, phosphorus fractionation revealed distinct differences in the distribution of soil P fractions as elevation and N availability increased. Soils from high elevation plots where N availability was greatest had 139 % larger organic P pools and 55 % smaller residual and refractory P pools than soils from low elevation plots with less N availability, suggesting that increased N availability has driven the depletion of recalcitrant P pools by stimulating biotic demand and sequestration. These differences in P distribution among fractions influenced how tree mortality affected P dynamics. At high elevations, plots containing declining hemlocks had significantly greater foliar P concentrations and fluxes of P from the forest floor than reference plots at similar elevations, whereas at low and mid-elevations there were no consistent differences between plots. Across all elevation classes, hardwood foliar N:P ratios were lower in plots with declining hemlocks. Collectively, these results suggest that increased N availability enhances bioavailable P, which is sequestered in vegetation until disturbances liberate it.

Interacting effects of wildfire severity and liming on nutrient cycling in a southern Appalachian wilderness area

AimsWilderness and other natural areas are threatened by large-scale disturbances (e.g., wildfire), air pollution, climate change, exotic diseases or pests, and a combination of these stress factors (i.e., stress complexes). Linville Gorge Wilderness (LGW) is one example of a high elevation wilderness in the southern Appalachian region that has been subject to stress complexes including chronic acidic deposition and several wildfires, varying in intensity and extent. Soils in LGW are inherently acidic with low base cation concentrations and decades of acidic deposition have contributed to low pH, based saturation, and Ca:Al ratio. We hypothesized that wildfires that occurred in LGW followed by liming burned areas would accelerate the restoration of acidic, nutrient depleted soils. Because soils at LGW had extremely low concentrations of exchangeable Ca2+ and Mg2+ dolomitic lime was applied to further boost these cations. We evaluated the effectiveness of dolomitic lime application in restoring exchangeable Ca2+ and Mg2+ and subsequently increasing pH and Ca:Al ratio of soils and making Ca and Mg available to recovering vegetation.MethodsFive treatment areas were established: severely burned twice (2000 & 2007) with dolomitic lime application (2xSBL); moderately burned twice with lime application (2xMBL); severely burned twice, unlimed (2xSB); moderately burned once (2000), unlimed (1xMB); and a reference area (REF; unburned, unlimed). In 2008 and 2009, we measured overstory, understory, and ground-layer vegetation; forest floor mass and nutrients; and soil and soil solution chemistry within each treatment area.ResultsAll wildfire burned sites experienced substantial overstory mortality. However, understory biomass doubled between sample years on the most recently burned sites due to the rapid regrowth of ericaceous shrubs and prolific sprouting of deciduous trees. Burning followed by lime application (2xSBL and 2xMBL) significantly increased shallow soil solution NO3-N, but we found no soil solution NO3-N response to burning alone (2xSB and 1xMB). Surface soil base saturation and exchangeable Ca2+ were significantly affected by liming; Ca2+ concentrations were greater on 2xMBL and 2xSBL than 2xSB, 1xMB and REF. There was a smaller difference due to moderate burning along with greater soil Ca2+ on 1xMB compared to REF, but no difference between 2xSB and REF. Surface and subsurface soil exchangeable Al3+ were lower on 2xSBL than 2xSB, 2xMBL, 1xMB, and REF. Liming decreased soil acidity somewhat as surface soil pH was higher on the two burned sites with lime (pH = 3.8) compared to 2xSB without lime (pH = 3.6).ConclusionsLiming resulted in decreased soil Al3+ on 2xSBL coupled with increased soil Ca2+ on both 2xSBL and 2xMBL, which improved soil Ca/Al ratios. However, the soil Ca/Al ratio response was transitory, as exchangeable Al3+ increased and Ca/Al ratio decreased over time. Higher lime application rates may be necessary to obtain a substantial and longer-term improvement of cation-depleted soils at LGW.