Ralph E. J. Boerner

Fire and Nutrient Cycling in Temperate Ecosystems

Wildfire is an integral component of many temperate ecosystems. The impact of wildfire on the nutrient dynamics of an ecosystem is dependent on the proportion of biomass and nutrients aboveground, which is therefore susceptible to combustion. Mechanisms for postfire nutrient conservation are most strongly developed in nutrient-poor (oligotrophic) ecosystems, in which most of the nutrients are found aboveground, and least well developed in nutrient-rich (eutrophic) ecosystems, whose nutrients are predominantly belowground. (Accepted for publication 21 October 1981)

The Effects of Forest Fuel-Reduction Treatments in the United States

The current conditions of many seasonally dry forests in the western and southern United States, especially those that once experienced low- to moderate-intensity fire regimes, leave them uncharacteristically susceptible to high-severity wildfire. Both prescribed fire and its mechanical surrogates are generally successful in meeting short-term fuel-reduction objectives such that treated stands are more resilient to high-intensity wildfire. Most available evidence suggests that these objectives are typically accomplished with few unintended consequences, since most ecosystem components (vegetation, soils, wildlife, bark beetles, carbon sequestration) exhibit very subtle effects or no measurable effects at all. Although mechanical treatments do not serve as complete surrogates for fire, their application can help mitigate costs and liability in some areas. Desired treatment effects on fire hazards are transient, which indicates that after fuel-reduction management starts, managers need to be persistent with repeated treatment, especially in the faster-growing forests in the southern United States.

Microsite variations in soil chemistry and nitrogen mineralization in a beech-maple forest

Though whole forest stand and ecosystem estimates of soil nitrogen dynamics are common, few studies have included measurements of within-stand variations in N mineralization and nitrification among microsites. We evaluated soil chemical status, net mineralization and nitrification potential during Autumn 1986 in A- and B-horizon soils taken from near the bases of Fagus grandifolia (American beech), Acer saccharum (sugar maple), and Fraxinus americana (white ash) trees and from sites > 2 m from trees using buried bag exposures. Fagus sites had significantly lower pH in both horizons than did other sites. Soils near Acer trees had higher organic C and NH4+ concentrations and Fraxinus soils had significantly higher PO43− availability. Moisture and NO3− concentrations were similar among microsites. Using stepwise discriminant analysis, we constructed functions which differentiated among site types at P < 0.01 on the basis of these chemical measures. A-horizon N mineralization rates were generally 50% greater near Acer trees than away from trees, with Fagus and Fraxinus sites intermediate. B-horizon mineralization rates were highest under Fraxinus, but only 2–36% of A-horizon rates. Relative nitrification was approached 100% in all site types despite soil pH < 4. Calculation of N mineralization on an area basis with and without accounting for the differences among microsite types indicates that whole-ecosystem estimates of N mineralization generated by sampling schemes which do not explicitly include unique microsites around tree bases may be biased low by 8–20%.

Prescribed burning effects on soil enzyme activity in a southern Ohio hardwood forest: a landscape-scale analysis

We assessed the effect of a single, dormant season prescribed fire on soil enzyme activity in oak-hickory (Quercus-Carya) forests in southern Ohio, USA. Four enzymes specific for different C sources were chosen for monitoring: acid phosphatase, β-glucosidase, chitinase and phenol oxidase. Postfire acid phosphatase activity was generally reduced by burning and decreased with increasing longterm soil water potential. Postfire β-glucosidase differed little between control and burned plots. Chitinase activity increased after fire in proportion to fire intensity. Phenol oxidase activity was highly variable and did not correlate well with either fire or soil water potential. Enzyme activities tended to vary more between samples taken upslope vs. downslope of a given tree than as the result of fire or landscape position. Overall enzymes whose activities are controlled by microclimatic or edaphic factors were affected more than those controlled primarily by substrate availability. Single, dormant season fires may consume a large proportion of the forest floor and change the apparent character of the surface organic matter complex without having major effects on soil enzyme activity.

Correlations of understory herb distribution patterns with microhabitats under different tree species in a mixed mesophytic forest

SummaryThis study examines the role of canopy trees in the formation and maintenance of different herb microhabitats in a mixed mesophytic forest stand. Herb abundance and reproductive success were recorded in 54 circular plots under seven species of canopy trees and in 15 circular control plots>2 m from any tree. Soil moisture, soil nutrient levels, litter depth, and light intensity were measured in a subset of these plots. Ordination of plots by both herb relative abundance and by reproductive success of common species indicated that herb assemblages under most canopy tree species were similar to those away from trees. However, herb assemblages under Fagus grandifolia trees differed moderately from the others while plots under Quercus alba trees supported significantly different herb assemblages. Analyses of variance revealed that several herb species occurred at significantly closer mean distance to the base of Q. alba or Fagus trees or at higher densities under these tree species. Soils around Q. alba trees had significantly higher concentrations of calcium and sulfate ions, and higher pH than plots under other tree species and control plots. This correlated closely with Q. alba stemflow which had higher concentrations of calcium and sulfate ions and lower concentrations of hydrogen ions than stemflow from other trees at this site. The slightly lower soil pH near the base of Fagus trees may have been related to the high volumes of stemflow produced by this species. Stepwise regression showed significant correlations between abundances of five common herb species and soil nutrient patterns. Maintenance of spatial heterogeneity in forest floor resources by the presence of different species of canopy trees may therefore be important in the maintenance of diversity in these understory herb communities.

Canopy disturbance patterns and regeneration of Quercus species in two Ohio old-growth forests

This study was conducted to determine the abundance of Quercus species, the spatial pattern of Quercus regeneration, the current canopy disturbance pattern, and their interrelationship in two old-growth deciduous forests in Ohio (Goll Woods and Sears-Carmean Woods). Acer saccharum and Fagus grandifolia had the greatest density and basal area in both forests, yet the largest trees (by basal area) present at each site were Quercus spp. Quercus spp. appeared to be decreasing in abundance in both sites. Though Quercus seedlings were common, few Quercus saplings or subcanopy trees were present. The current disturbance regimes were dominated by small canopy gaps created by death of 1–2 trees; canopy gaps ≥100 m2 in size were rare and only 2.5–2.8% of the forest area was covered by recognizable canopy gaps. No significant differences in the density of Quercus seedlings or saplings were found between gaps and non-gap areas at either site. Though no significant barrier to seedling establishment appeared to exist, the present disturbance regimes are not well suited for the growth of Quercus into the subcanopy size class or the recruitment of Quercus into the canopy. The most frequent gapmakers in Goll Woods were Tilia americana and Acer saccharum, and those in Sears-Carmean Woods were A. saccharum and F. grandifolia. The species most frequent as gap fillers were A. saccharum (in both sites) and F. grandifolia (in Goll Woods). These results suggest that A. saccharum will continue to increase in abundance, and Quercus decrease in abundance, in these two old-growth stands.

Spatial patterns of mycorrhizal infectiveness of soils long a successional chronosequence

Abstract This study quantified intersite variation and spatial pattern in arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) infectivity of soils among six sites constituting a successional chronosequence in southwestern Ohio, USA. The study sites included an active agricultural field (chronic disturbance), a site which had been stripped of its surface soil (pseudo-stripmine, acute disturbance), 5- and 10-year-old fields, a 25- to 30-year-old prairie restoration, and an undisturbed, mature forest. AM infectivity was lower in the agricultural field, successional fields, and prairie than in the mature forest, but there was no clear correlation between time since disturbance and the overall level of AM infectivity. Spatial structure in AM infectivity decreased with time since disturbance. In the pseudo-stripmine site and active soybean field, semivariance analysis attributed 44–50% of the total variance in AM infectivity among samples to spatial structure, whereas spatial dependancy accounted for only 18% of total variance in the mature forest. Kriging of AM infectiveness demonstrated small, isolated areas in the disturbed plots that were devoid of AM infectiveness, whereas the kriged AM maps of the other four sites showed AM infectiveness to become progressively more homogeneous. ECM infectiveness was lacking from 35–50% of the samples from the disturbed sites, and both overall ECM infectiveness and ECM diversity increased with time since disturbance. Approximately 44% of the variance in ECM infectiveness was related to spatial structure in the two disturbed sites, and large areas entirely devoid of ECM infectivity were present on the kriged ECM maps for these sites. There was less spatial structure in ECM in the old fields and prairie and very little in the mature forest. The results of this study emphasize the need to explicitly evaluate spatial heterogeneity in mycorrhizal infectivity in studies of the role of mycorrhizae in succession.

Effects of belowground grazing by collembola on growth, mycorrhizal infection, and P uptake of Geranium robertianum

We hypothesized that the grazing of vesicular-arbuscular mycorrhizal (VAM) hyphae by soil animals could be responsible for the lack of a direct relationship between mycorrhizal infection intensity and nutrient uptake under field conditions. To test this hypothesis, we determined the effect of a range of densities of the collembola, Folsomia candida, on growth, VAM infection, and P uptake in Geranium robertianum, a common forest herb, under greenhouse conditions. Total and aboveground growth were greater at low collembola density than either at higher collembola density or without collembola. These differences were greater when the plants were grown in a high organic content soil mix than when grown in sand. Root mass was not affected by collembola density. In the soil mix, root length decreased with increasing collembola density, but not in the sand. The percent of root length infected with VAM was lower at any collembola density than when collembola were absent. Total infected root length decreased linearly with increasing collembola density. Few significant differences in P uptake or tissue concentration were found. Thus, plant growth (but not P uptake) may be stimulated at low collembola density and inhibited at high. We discuss mechanisms which may be responsible for this non-linear response, and the implications of the pattern of response to studies of plant competition, nutrient turnover, and revegetation.

Forty-eight years of landscape change on two contiguous Ohio landscapes

This study analyzes the current and historic structure of two contiguous, rural landscapes covering approximately 242 km2 in central Ohio, USA: a till plain landscape with relatively homogeneous topography and soils, and a moraine landscape with greater geomorphological diversity and heterogeneity. These landscapes were chosen because they were both heavily dominated by agriculture during 1900–1940 and were both initially surveyed by the metes-and-bounds system. They differed, however, in the temporal pattern of settlement and development and in the inherent agricultural capability of their soils. We combined analysis of aerial photographs from 1940, 1957, 1971, and 1988 with historical archives and other available mapped data in a GIS data base to facilitate analysis of both spatial and temporal patterns of change. On the moraine, the agricultural matrix decreased over time as forest, urban/suburban areas, and industry increased. In contrast, on the till plain agricultural landcover increased through 1988, with concommitant decreases in upland forest and oak savanna. The moraine landscape exhibited greater diversity and equitability than the till plain on each date. The till plain had its greatest diversity and equitability in 1940, whereas the moraine increased in diversity and equitability during each time period. The undulating topography of the moraine encouraged landcover dynamism rather than stability, whereas the more homogeneous till plain exhibited considerable inertia. Patch and matrix shape remained constant and predominantly angular over the 48 year study period. Differences in the physical environment, especially topography and soil capability, and the socioeconomic environment, especially agricultural policies and patterns of urbanization, resulted in these two contiguous landscapes having different trajectories of change. It is clear from this study that socioeconomic factors must be combined with the physical setting to fully understand patterns of change in human-dominated landscapes.

Impacts of Fire and Fire Surrogate treatments on forest soil properties: a meta-analytical approach

The soils underlying the 12 Fire and Fire Surrogates Network include six soil orders and >50 named soil series. Across the network, pretreatment soils varied from 3.7 to 7.1 in pH, and exhibited ranges of twofold in bulk density, fourfold in soil organic C (SOC) content, 10-fold in total inorganic N (TIN), and 200-1000-fold in extractable Ca and K. Nonmetric multidimensional (NMS) ordination of pretreatment soil conditions arrayed the FFS sites along gradients of pH/base cation status, net N transformation rates, bulk density, and SOC. At the network scale, mineral soil exposure was significantly greater in fire-only (mean of 9.2%) and mechanical + fire (5.0%) treatments than in the controls (1.5%) during the first posttreatment year, and this persisted through the later sampling year (second through fourth year, depending on site) in the fire-only treatment (fire 4.1%, control 1.1%). Bulk density was not affected significantly at the network scale. TIN concentrations during the first posttreatment year increased after all three manipulative treatments, but this effect did not persist to the later sampling year. Neither SOC content nor soil C:N ratio was affected by any of the treatments at the network scale. At the individual site scale, the combined mechanical + fire treatment produced more significant site x treatment x year effects than did the fire-only or mechanical-only treatments, though in most cases even the statistically significant differences produced by the manipulative treatments were modest in magnitude. Ordination of first-year standardized effect sizes produced no discernable separation of the three manipulative treatments but did separate the three sites with the greatest fire severity (based on proportional fuel consumption) from the majority of the network sites, with changes in pH, TIN, SOC content, and soil C:N ratio correlating most strongly with this separation. Ordination of the effect sizes from the later sampling year produced somewhat clearer separation of treatments than did the first-year ordination, though fewer sites were represented in this second ordination. Overall, the network-wide effects of the FFS treatments on soil properties appear to have been modest and transient.