David F. Grigal

Effects of extensive forest management on soil productivity

This paper focuses on the effects of extensive forest management on soil productivity, its capacity to produce plants. Forest productivity, the summation of the productivities of the individual landscape elements (stands) that comprise the forest, is the integration of soil productivity, climate, species composition and stocking, and stand history. Extensive forest management can be operationally defined by the monetary investment per unit area of land, or by the number of stand entries per rotation, or by a combination of those metrics. A stand entered once during a rotation, for harvest, is extensively managed while a stand that has been subjected to site preparation, planted with genetically improved stock, and thinned and fertilized is intensively managed. The distinction blurs between those extremes. Many reviews have summarized the effects of forest harvest, the major extensive management activity, on soil properties and hence on productivity. Rather than simply reiterating those reviews, I have framed the paper in a series of axioms (which all agree upon), corollaries (consequences to productivity that follow from the axioms and are also agreed upon), and postulates (proposed consequences that are subject to some uncertainty). It is axiomatic that forest management activities alter soil physical, chemical, and biological properties. Changes have been well-documented, although their intensity and duration varies among locations and associated soil and forest types. Consequences of the changes in soil physical properties are clearly corollaries, and include reduced productivity due to surface erosion, mass flow, soil compaction, and rutting and puddling. Although the negative consequences of roads and skid trails to stand-level productivity may be considered to be corollaries, extrapolations of those consequences to the landscape is less clear and should be considered to be postulates. Similarly, consequences of changes in soil chemical and biological properties due to forest management should be considered to be postulates; not fully tested. Although soil chemical and biological properties are changed by management, the duration of those changes and their influence on productivity are not clear. Forest ecosystems are dynamic and resilient. Assessment of the consequences of changes in properties should recognize that shifts in preferred species may not be equated with changes in soil productivity, and that short-term effects may not be indicative of longer-term effects. Both ethical and economic considerations demand good stewardship of our natural resources. Extensive forest management, if carried out with both wisdom and prudence, is not antithetical to good stewardship.

INFLUENCE OF LOGGING, FIRE, AND FOREST TYPE ON BIODIVERSITY AND PRODUCTIVITY IN SOUTHERN BOREAL FORESTS

The effects of logging on ecosystem sustainability are controversial. Sur- prisingly, existing data are inadequate to allow a comprehensive evaluation of logging effects on biodiversity, composition, and productivity since appropriate comparisons of stands of similar ages and differing disturbance histories are rare. We addressed this issue using a study of 2000 plots in 80 southern boreal forest stands in northern Minnesota, USA, wherein we contrasted naturally regenerated aspen (Populus tremuloides), jack pine (Pinus banksiana), and black spruce (Picea mariana) stands established following logging or the dominant natural disturbance, wildfire, for stands of two age classes (25-40 and 70-100 yr old). For young stands, those established postlogging had higher vascular plant diversity than those postwildfire. Otherwise, we found no evidence of differing species diversity (including canopy tree, shrub, herbaceous, and bryophyte species), composition, produc- tivity, or nitrogen cycling, in forest stands of comparable age and forest type that originated after logging compared to after wildfire. These variables, however, differed significantly among forest types, with aboveground net primary productivity and plant species diversity generally higher in aspen than jack pine stands, even when growing on comparable soils, and lowest in black spruce. Although there is evidence that logging has increased the proportional landscape dominance by aspen, a forest type with higher diversity, nutrient cycling, and productivity than other types, our evidence refutes the idea that disturbance by logging has diminished stand-scale productivity or plant diversity in comparison to the common natural disturbance, wildfire.

Nitrogen mineralization, nitrification and denitrification in upland and wetland ecosystems

SummaryNitrogen mineralization, nitrification, denitrification, and microbial biomass were evaluated in four representative ecosystems in east-central Minnesota. The study ecosystems included: old field, swamp forest, savanna, and upland pin oak forest. Due to a high regional water table and permeable soils, the upland and wetland ecosystems were separated by relatively short distances (2 to 5 m). Two randomly selected sites within each ecosystem were sampled for an entire growing season. Soil samples were collected at 5-week intervals to determine rates of N cycling processes and changes in microbial biomass. Mean daily N mineralization rates during five-week in situ soil incubations were significantly different among sampling dates and ecosystems. The highest annual rates were measured in the upland pin oak ecosystem (8.6 g N m−2 yr−1), and the lowest rates in the swamp forest (1.5 g N m−2 yr−1); nitrification followed an identical pattern. Denitrification was relatively high in the swamp forest during early spring (8040 μg N2O−N m−2 d−1) and late autumn (2525 μg N2O−N m−2 d−1); nitrification occurred at rates sufficient to sustain these losses. In the well-drained uplands, rates of denitrification were generally lower and equivalent to rates of atmospheric N inputs. Microbial C and N were consistently higher in the swamp forest than in the other ecosystems; both were positively correlated with average daily rates of N mineralization. In the subtle landscape of east-central Minnesota, rates of N cycling can differ by an order of magnitude across relatively short distances.

Mercury budget of an upland-peatland watershed.

Inputs, outputs, and pool sizes oftotal mercury (Hg) were measured in a forested 10 hawatershed consisting of a 7 ha hardwood-dominatedupland surrounding a 3 ha conifer-dominatedpeatland. Hydrologic inputs via throughfall andstemflow, 13±0.4 μg m−2 yr−1over the entire watershed, were about doubleprecipitation inputs in the open and weresignificantly higher in the peatland than in theupland (19.6 vs. 9.8 μg m−2 yr−1). Inputs of Hg via litterfall were 12.3±0.7μg m−2 yr−1, not different in thepeatland and upland (11.7 vs. 12.5 μg m−2yr−1). Hydrologic outputs via streamflow were2.8±0.3 μg m−2 yr−1 and thecontribution from the peatland was higher despiteits smaller area. The sum of Hg inputs were lessthan that in the overstory trees, 33±3 μgm−2 above-ground, and much less than eitherthat in the upland soil, 5250±520 μgm−2, or in the peat, 3900±100 μgm−2 in the upper 50 cm. The annual flux of Hgmeasured in streamflow and the calculated annualaccumulation in the peatland are consistent withvalues reported by others. A sink for Hg of about20 μg m−2 yr−1 apparently exists inthe upland, and could be due to either or bothstorage in the soil or volatilization.

Atmospheric Inputs of Mercury and Organic Carbon into a Forested Upland/Bog Watershed

Inputs of mercury (Hg) and dissolved organic carbon (DOC) in throughfall and stemflow waters were measured for an upland/bog watershed in northern Minnesota, and were compared to the deposition in a nearby opening to determine the influence of tree canopies on Hg and DOC deposition. Twice as much Hg and seven times as much DOC was deposited in the forested watershed compared to the opening. Mass balance studies that are based on wet-only deposition in openings severely underestimate atmospheric deposition of Hg in forests. Conifer canopies are more efficient filters of airborne particulates than are deciduous canopies as indicated by much higher Hg concentrations and total deposition in throughfall and stemflow waters under conifers. Significant positive relationships existed between Hg and DOC in both throughfall (36–57% of the variation) and stemflow waters (55–88% of the variation). Hg complexation by DOC appears to be related to the contact time between precipitation and carbon sources.

Sulfur cycling in a forested Sphagnum bog in northern Minnesota

The mass balance and internal cycle of sulfur within a small forested,Sphagnum bog in northern Minnesota are presented here based on a 4-year record of hydrologic inputs and outputs (precipitation, throughfall, streamflow, upland runoff) and a 3-year measurement of plant growth and sulfur uptake. Concentrations and accumulation rates of inorganic and organic sulfur species were measured in porewater. The bog is a large sink for sulfur, retaining 37% of the total sulfur input. Because of the relatively large export of organic S (21% of inputs), retention efficiency for total-S (organic S + SO4=; 37%) is less than that for SO4= (58%). There is a dynamic cycle of oxidation and reduction within the bog. Annual oxidation and recycling of S is equal to total inputs in the center of the bog. Plants receive 47% of their uptake requirement from atmospheric deposition, 5% from retranslocation from foliage, and the remainder from sulfur remineralized from peat. Mineralization is most intense in the aerobic zone above the water table. Inorganic sulfur species comprise <5% of the total sulfur burden within the peat.

Classification, Description, and Dynamics of Upland Plant Communities within a Minnesota Wilderness Area

A bstract. The major upland plant community types of the Boundary Waters Canoe Area (BWCA) of northeastern Minnesota, identified by multivariate analyses (clustering and canonical and discriminant analysis) of 68 stands disturbed by logging and 106 stands undisturbed by logging, include the following: lichen, jack pine-oak, red pine, jack pineblack spruce, jack pine-fir, black spruce-feather moss, maple-oak, aspen-birch, aspenbirch-white pine, maple-aspen-birch, maple-aspen-birch-fir, fir-birch, and white cedar. Each of these types is based on a complex of 53 common species, though the name may incorrectly imply that one or two dominant overstory species are indicative of the type. Other forest stands from the BWCA are quantitatively related to the regional vegetation through discriminant analysis. Succession on the uplands in the area, without disturbance, leads to fir-birch and ultimately to the white-cedar community type. Whitetail deer may have had an impact on restricting the occurrence and reproduction of the white cedar type.

Mercury Uptake by Trees: An Observational Experiment

We conducted a simple observational experiment to test whether differences in Hg in tissue of red pine (Pinus resinosa Ait.) were related to soil or to atmospheric sources of Hg. We sampled two plantations in each of three areas, and within each plantation sampled two sites with different levels of soil Hg. Woody tissue Hg concentration differed by area, and differences in foliar concentrations, though not statistically significant, were ranked in the same order. Total mass of Hg in forest floor and mineral soil also differed by area, but with ranking opposite that of tissue. On an individual-tree basis, concentrations of Hg in 1994 needles (2-year old) were about twice those in 1995 needles (1-year old) (r = 0.77). Neither woody tissue Hg nor any measure of Hg in soil or forest floor were closely related to foliar levels and some relationships were inverse. We interpret the data to indicate that Hg in plant tissue is derived directly from the atmosphere, not the soil. Tissue concentration by area was closely related to the respective growing season length (1994 needles, r = 0.88; 1995 needles, r = 0.97; wood, r = 0.97), as was total mass of Hg in forest floor and surface mineral soil (r = – 0.80). Other climatic measures, such as growing degree days and actual evapotranspiration, had similar relationships. These relationships imply that both foliar uptake of Hg0 from the atmosphere and efflux of Hg from the soil system depend on biological activity.

Changes in ecosystem carbon storage over 40 years on an old-field/forest landscape in east-central Minnesota

Organic carbon (C) storage in complex landscapes and its temporal change can be important in the global C budget. Change in C storage between 1938 and 1977 was estimated for a 2224 ha old-field/forest landscape in east-central Minnesota by coupling change in area of seven vegetation types (five forest and two non-forest) with vegetation-specific C densities (mg ha−1). Carbon densities were based on sampling carried out between 1974 and 1990. Areas of vegetation types in 1938 and 1977 were determined from aerial photographs. Carbon density was greatest in forest overstory (60–100 Mg ha−1) and organic and mineral soil (30–100 Mg ha−1 to 25 cm depth). Ecosystem C storage was approximately 212000 Mg in 1938 and 225000 Mg in 1977, an increase of ca. 13000 Mg across the study area. This was due largely to an increase in upland forest at the expense of non-forest area. The largest proportional increase in C storage was in trees (a 13% gain), while mineral soil gained 4% and herbs gained 6%. C storage in O horizon and shrubs remained constant. For the 20% of the landscape originally occupied by cultivated fields, an empirical model based on chronosequence studies indicated a 40% increase in C storage over 40 years; C increased in mineral soil, O horizon and trees as both herbaceous succession and forest encroachment occurred. Uncertainties of the estimates, based on propagation of standard errors, were 5% to 19% for C storage and 6% to 1000% for change in C storage. Uncertainty was due primarily to sample variability, but included uncertainty in biomass equations and GIS processing. This uncertainty demonstrates the difficulties associated with expanding from point sample data to landscape-scale estimates of C storage.

Forest soil mineral weathering rates: use of multiple approaches

Knowledge of rates of release of base cations from mineral dissolution (weathering) is essential to understand ecosystem elemental cycling. Although much studied, rates remain enigmatic. We compared the results of four methods to determine cation (Ca + Mg + K) release rates at five forested soils/sites in the northcentral U.S.A. Our premise was that multiple approaches, each with their own specific strengths and weaknesses, would yield a “best” overall estimate. We used (1) a cation input-output budget on a pedon scale; (2) trends in elemental and mineral depletion in silt-size particles; (3) a laboratory batch dissolution technique, with the results adjusted for field conditions; and (4) a steady-state soil chemistry model, PROFILE. The soils included a loamy sand Typic Udipsamment, a sandy loam Spodic Udipsamment, a fine sandy loam Typic Dystrochrept, a very fine sandy loam Glossic Eutroboralf, and a clayey Glossic Eutroboralf. Weathering rates varied among both soils and methods, and neither methods nor soils could easily be grouped; the data spanned a continuum with overlapping ranges of least significant differences. Although the assumptions necessary for some methods were better suited to specific soils, we rejected only one method-soil combination as being inappropriate (input-output budget for the clay). Mean release rates for the sum of cations ranged from 470 eq ha−1 yr−1 for the clayey soil, to 460 for the fine sandy loam soil, to 430 for the very fine sandy loam soil, to 375 for the sandy loam soil, to 195 for the loamy sand soil. These rates are lower than those reported for similar soils in the literature because most reported rates are based on watershed studies. Our low rates of cation release within soil pedons, the ultimate source of nutrient ions for plant growth, has implications for estimated nutrient budgets and long-term soil sustainability.