Wayne T. Swank

Fluxes of Dissolved Organic Nutrients and Humic Substances in a Deciduous Forest

We evaluated the importance of dissolved organic matter as a vehicle for the movement of N and P from the canopy and the forest floor into the mineral soil of a deciduous forest. We also examined the origin and nature of dissolved organic matter from the forest floor to see whether it was simply soluble plant material or highly humified matter. The average annual output from the forest floor in the form of dissolved organic matter was 18, 28, and 14% of the input in solid litterfall for C, N, and P, respectively. In throughfall, about half of the dissolved N and P was organic. But, in solution percolating from the forest floor, 94% of the N and 64% of the P was organic. Leaching from the forest floor was not a source of inorganic N and P for the mineral soil. Instead, the forest floor was a sink for the removal of these inorganic nutrients delivered in throughfall. Microbial immobilization was the most likely explanation for much of the inorganic nutrient removal. In contrast, the forest floor was an abundant contributor of N and P to the mineral soil in the form of dissolved, and possibly particulate, organic matter. Much of the dissolved organic matter entering the A horizon originated from the upper (Oa and Oe horizon) forest floor, but it was modified in several respects compared to the original soluble material. The solution percolating from the forest floor over most of the year was much richer in nitrogen, contained a much larger proportion of hydrophilic acids, and contained a much smaller proportion of carbohydrate-rich hydrophilic neutrals, than did the original water- extractable material in autumn litter. However, the fresh autumn litter did contain a similar proportion of soluble hydrophobic acids that resembled dissolved humic substances in several respects. Most of the flux of nitrogen from the forest floor to the A horizon was carried by humic substances and highly colored hydrophilic acids.

Long-term hydrologic and water quality responses following commercial clearcutting of mixed hardwoods on a southern Appalachian catchment

Long-term changes (20 years) in water yield, the storm hydrograph, stream inorganic chemistry, and sediment yield were analyzed for a 59 ha mixed hardwood covered catchment (Watershed 7) in the southern Appalachian mountains (USA) following clearcutting and cable logging. The first year after cutting, streamflow increased 26 cm or 28% above the flow expected if the forest had not been cut. In subsequent years, discharge increases declined at a rate of 5‐7 cm per year until the fifth year when changes in flow returned to baseline values. Later in forest succession, between ages 15 and 18 years, both significant increases and decreases in annual water yield were observed; these discharge dynamics are discussed in relation to vegetation regrowth dynamics. Flow responses predicted from an empirical regional scale model were within 17% of experimental values during the first 4 years of regrowth. Intra-annual analysis showed that proportionally larger increases (48%) in flow occurred in the low flow months of August‐October. Storm hydrograph analysis showed that, on an average, initial flow rate and peakflow rates increased 14‐15% and stormflow volume increased 10%. Analyses of stream solute concentrations and catchment nutrient fluxes showed small increases in nutrient losses following clearcutting and logging. Responses were largest the third year after treatment with annual values of 1.3, 2.4, 2.7, 3.2, 1.4, 0.39, and 2.1 kg ha ˇ1 for NO3-N, K, Na, Ca, Mg, S, and Cl, respectively. Explanations for the retention of nutrients and high ecosystem resistance and resilience are discussed in relation to internal biogeochemical cycles based on long-term process level studies on the catchment. A second, sustained pulse of NO3 ˇ to the stream (exceeding post-harvest values) observed later in succession is also discussed in the context of ecosystem processes. Large increases in sediment yield were measured immediately after road construction due to two major storm events. Subsequently, during logging, sediment yield from roads was greatly reduced and insignificant when logging activities were completed. In contrast, cumulative increases in sediment yield were observed downstream over the next 15 years which illustrate the lag between pulsed sediment inputs to a stream and the routing of sediments through a stream system. The relevance of sedimentation to stream sustainability is discussed in the context of long-term responses in the benthic invertebrate community structure and productivity measured on WS7. Published by Elsevier Science B.V.

Early Regeneration of a Clear-Cut Southern Appalachian Forest

The components of hardwood forest regeneration on a southern Appalachian watershed were assessed during the 1st yr following clear—cutting. First—year net primary production (NPP) on the clear—cut was 1955 kg/ha, representing 22% of the NPP of a nearby undisturbed hardwood forest. First—year nutrient pools in NPP for N, P, K, Mg, and Ca were estimated at 29—44% of those in the NPP of the control. The greatest NPP and nutrient pools were represented in descending order by hardwood sprouts, herbs, vines, and seedlings. Woody successional species (Robinia pseudo—acacia, Liriodendron tulipifera, and Vitis aestivalis var. argentifolia) and herbs (Aster spp., Solidago spp., and Erechtites hieracifolia) were important in revegetation due to competitive advantages in growth rates, growth forms, and propagative capacities. The woody successional species had higher tissue concentrations of N and P than most other woody species. Herbs as a group had significantly higher foliar concentrations of K than woody species. Woody successional and herbaceous species collectively had higher biomass and elemental pools than other woody species. Following forest disturbance, these fast—growing species conserve substantial pools of nutrients in their biomass and initiate a rapid recovery of forest elemental cycling processes.

Stability of Stream Ecosystems

The ability of ecosystems to recover from external disturbances, that is, their stability, is a fundamental property of these systems. Quantification of the ability for various ecosystems to recover and understanding of the mechanisms behind stability are currently areas of major ecological research. In this paper we present an overview of how the stability concept has been used in ecology and a more specific discussion of the application of these ideas to stream ecosystems. This is followed by a case study in which we have been observing the stability of small streams in response to watershed logging and comparing stream stability to stability of the adjacent forest ecosystem.

Sources, fates, and impacts of nitrogen inputs to terrestrial ecosystems: review and synthesis

The relative importance of nitrogen inputs from atmospheric deposition and biological fixation is reviewed in a number of diverse, non-agricultural terrestrial ecosystems. Bulk precipitation inputs of N (l–l2 kg N ha−1 yr−1) are the same order of magnitude as, or frequently larger than, the usual range of inputs from nonsymbiotic fixation (< 1 – 5 kg N ha−1 yr−1), especially in areas influenced by industrial activity. Bulk precipitation measurements may underestimate total atmospheric deposition by 30–40% because they generally do not include all forms of wet and dry deposition. Symbiotic fixation generally ranges from ≅ 10–160 kg N ha−1 yr−1) in ecosystems where N-fixing species are present during early successional stages, and may exceed the range under unusual conditions.Rates of both symbiotic and nonsymbiotic fixation appear to be greater during early successional stages of forest development, where they have major impacts on nitrogen dynamics and ecosystem productivity. Fates and impacts of these nitrogen inputs are important considerations that are inadequately understood. These input processes are highly variable in space and time, and few sites have adequate comparative information on both nitrogen deposition and fixation.- more intensive studies of total atmospheric deposition, especially of dry deposition, are needed over a wide range of ecosystems;- additional studies of symbiotic fixation are needed that carefully quantify variation over space and time, examine more factors regulating fixation, and focus upon the availability of N and its effects upon productivity and other nutrient cycling processes;- process-level studies of associative N-fixation should be conducted over a range of ecosystems to determine the universal importance of rhizosphere fixation;- further examination of the role of free-living fixation in wood decomposition and soil organic matter genesis is needed, with attention upon spatial and temporal variation; and- investigations of long-term biogeochemical impacts of these inputs must be integrated with process-level studies using modern modelling techniques.

Studies of Cation Budgets in the Southern Appalachians on Four Experimental Watersheds with Contrasting Vegetation

Nutrient fluxes within and through watershed ecosystems at the Coweeta Hydrologic Laboratory are under study. This paper describes the annual budgets and seasonal fluctuations for selected cations. Concentrations and flux of cations moving through a hardwood forest stand, a weed to forest succession, a hardwood coppice stand and an eastern white pine stand on steep mountain topography are compared. Stream discharge was greater by 6% for the successional weed stand, and 10% for the second hardwood coppice, but 15% less for the young pine stand in contrast to pretreatment levels. Although concentrations for Ca(++), Mg(++), K+ and Na+ combined were usually less than 3.5 ppm, over 98% of the loss of each cation was in dissolved form on all four watersheds. Regression analysis showed that 50 to 60% of the variation in monthly weighted average concentration was accounted for by monthly discharge amounts. Annual losses of the four cations from the mature hardwood stand were in the amounts of approximately 7, 3, 5, and 10 kg/ha respectively for the Ca(++), Mg(++), K+ and Na+. Annual budgets showed net changes to be —0.8, —2.0, and —4.3 kg/ha, respectively, for this mature hardwood ecosystem. In contrast, the weed stand lost significantly greater amounts, and the young pine and hardwood coppice watersheds showed a net gain in Ca(++) and significantly lower losses than the mature ecosystem for the other three ions. These budgets show that major alterations to these forest ecosystems are not now producing a substantial out—flux for these cations.

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.

Regeneration Patterns in Canopy Gaps of Mixed-Oak Forests of the Southern Appalachians: Influences of Topographic Position and Evergreen Understory

-Canopy gaps in southern Appalachian mixed-oak forests were assessed for the effects of topographic, gap and stand variables on density of wood seedlings. Seedling density was significantly correlated with percent slope and positively with gap age (1-5 yr). Density varied substantially among topographic positions and increased with gap size. Species richness decreased over time and increased with gap size. Regeneration was dominated by Acer rubrum L. Other important species included Quercus coccinea Muench., Q. rubra L., Q. velutina Lamarck, Liriodendron tulipifera L. and Cornus florida L. Some known gap species increased in density with increasing gap size. Competitive inhibition effects of the evergreen understory (Rhododendron maximum L. and Kalmia latifolia L.) were also examined. Gaps containing over 50% cover of R. maximum had significantly lower densities than all other gaps, including gaps with >50% K latifolia cover. Height distributions of major regenerating species were skewed away from small (<15 cm) height classes. Species establishment was a function of gap area, gap age, topographic position and cover of R. maximum. In addition, species of varying degrees of tolerance of understory conditions are capable of establishment in small to medium size canopy openings in the absence of an evergreen shrub understory.

Long-term nitrogen dynamics of Coweeta Forested Watersheds in the southeastern United States of America

We analyzed long-term (23 years) data of inorganic N deposition and loss for an extensive network of mature mixed hardwood covered watersheds in the southern Appalachians of North Carolina to assess trends and dynamics of N in baseline ecosystems. We also assessed watershed N saturation in the context of altered N cycles and stream inorganic N responses associated with management practices (cutting prescriptions, species replacement, and prescribed burning) and with natural disturbances (drought and wet years, insect infestations, hurricane damage, and ozone events) on reference watersheds. Reference watersheds were characterized as highly conservative of inorganic N with deposition < 9.0 kg ha -1 yr -1 and stream water exports below 0.25 kg ha -1 yr -1 . However, reference watersheds appeared to be in a transition phase between stage 0 and stage 1 of watershed N saturation as evidenced by significant time trend increases in annual flow-weighted concentrations of NO 3 - in stream water and increases in the seasonal amplitude and duration of NO 3 concentrations during 1972-1994. These stream water chemistry trends were partially attributed to significant increases in NO 3 - and NH 4 + concentrations in bulk precipitation over the same period and/or reduced biological demand due to forest maturation. Levels and annual patterns of stream NO 3 - concentrations and intra-annual seasonal patterns characteristic of latter phases of stages 1 and 2 of watershed N saturation were found for low-elevation and high-elevation clear-cut watersheds, respectively, and were related to the dynamics of microbial transformations of N and vegetation uptake. Evidence for stage 3 of N saturation, where the watershed is a net source of N rather than a N sink, was found for the most distributed watershed at Coweeta (hardwood converted to glass, fertilized, limed, treated with herbicide, and subsequently characterized by successional vegetation). Compared to other intensive management practices, prescribed burning had little effect on stream water NO 3 - concentrations, and stream NO 3 - losses associated with natural disturbances are small and short-lived.

Seasonal changes of leaf area index (LAI) in a tropical deciduous forest in west Mexico

Light canopy transmittance and the Beer-Lambert equation were utilized to assess monthly leaf area index (LAI) of a tropical deciduous forest ecosystem on the west coast of Mexico. The light transmittance coefficient (k) was obtained by analyzing vertical leaf and light distribution in the forest canopy. An independent LAI estimate was obtained using litterfall data. The calculated k value was 0.610 ± 0.035 (standard error). Average maximum LAI obtained with litterfall data was 4.2 ± 0.4 m2m−2. There was a significant correlation (P < 0.001, r = 0.98) between litter-LAI estimations and those obtained with the Beer-Lambert equation. The regression explained 95% of the variation; however, light-LAI overestimated litter-LAI by a constant of 0.87 ± 0.12 m2 m−2 (the slope was 1.03 and Y intercept was 0.87). The discrepancy is partially attributed to leaf retention of the few evergreen species, and perhaps leaf retention of a few deciduous species beyond the end of the litterfall collection. Maximum annual LAI was similar in both study years (4.5 ± 0.3 m2 m−2 in 1990 and 4.9 ± 0.4 in 1991). Minimum LAI showed considerable variation between years with similar values in the dry seasons of 1990 and 1991 (1.0 ± 0.1 m2 and 0.9 ± 0.1 m2 m−2, respectively), but much higher values in 1992 (2.7 ± 0.2 m2 m−2). The difference is probably attributed to an atypical rainfall event in January 1992 (644 mm), which retarded leaf abscission.