Research Natural Area Research Articles

Effects of Bison Grazing, Fire, and Topography on Floristic Diversity in Tallgrass Prairie

Grazed and ungrazed sites subjected to different fire frequencies were sampled on the Konza Prairie Research Natural Area in northeast Kansas after 4 years of bison grazing (1987-1991). The objective was to study effects of bison grazing on plant species composition and diversity components (plant species richness, equitability, and spatial heterogeneity) in sites of contrasting fire frequency. Cover and frequency of cool-season graminoids (e.g. Poa pratensis L., Agropyron smithii Rydb., Carex spp.) and some fortes (e.g. Aster ericoides [A. Gray] Howell, and Oxalis stricta L.) were consistently higher in sites grazed by bison than in ungrazed exclosures, whereas the dominant warm-season grasses (Andropogon gerardii Vitman, Sorghastrum nutans [L.] Nash, Panicum virgatum L., Schizachyrium scoparium [Michx.] Nash) and other forbs (e.g. Solidago missouriensis Nutt.) decreased in response to bison. Plant species diversity (H') and spatial heterogeneity in all areas sampled were significantly increased by bison. Increased heterogeneity and mean species richness in grazed prairie (40 species per sample site) compared to ungrazed prairie (29 species per site) were likely a result of greater microsite diversity generated by bison, whereas preferential grazing of the dominant grasses and concomitant increases in subordinate species resulted in an increase in equitability of species abundances. Species/area relationships indicated greater effects of bison on plant species richness with increasing sample area. Increases in plant diversity components associated with bison grazing were generally greater in annually burned than in 4-year burned sites. Effects of ungulate grazers on floristic diversity have important implications given recent evidence that plant species diversity and the compositional and production stability of grassland plant communities are positively related.

The R. R. Reynolds Research Natural Area in Southeastern Arkansas

The R. R. Reynolds Research Natural Area is a 32-ha pine-hardwood forest in southeastern Arkansas, U.S.A., that originated from diameter-limit cutting of the virgin forest before 1915. In 1935, these 32 ha were reserved from timber management. Between 1937 and 1993, eight inventories were taken of all living trees > 9-cm DBH, using 2.5-cm DBH classes within three species groups: Pinus spp., Quercus spp., and other hardwoods. In 1994, all standing dead snags of pines and hardwoods > 9-cm DBH were inventoried by 2.5-cm DBH classes. During 56 years, thc overstory pine-hardwood ratio remained stable in terms of relative basal area, but pine density decreased with a commensurate increase in hardwood density. In 1993, pines represented 63% of basal area but only 23% of stem density. Just before the 1993 inventory, a pine bark-beetle infestation developed on the area, and within one year the pines lost about 2.5 m2/ha in basal area and had 180% more snags than were contributed by hardwoods. The overstory pine co...

Interannual variability in primary production in tallgrass prairie: climate, soil moisture, topographic position, and fire as determinants of aboveground biomass

From 1975 to 1993, aboveground net primary production (NPP) at the Konza Prairie Research Natural Area in NE Kansas varied from 179 g/m to 756 g/m. Across a variety of sites, NPP was significantly related to precipitation (r = 0.37), but much variability was unexplained. Thus, we evaluated the relationship between NPP with meteorological variables and soil moisture measurements in tallgrass prairie sites that varied in fire frequency and topographic position. Annually burned lowland sites had significantly higher NPP than either annually burned upland or unbumed sites. NPP in burned sites was more strongly related to meteorological variables and soil moisture when compared to unbumed sites. The lack of significant correlation between soil moisture with NPP on unbumed sites suggests that factors other than water availability limit production in these sites. When NPP data were analyzed separately by life forms, interannual variability in forb NPP was not correlated with any meteorological variables, but was negatively correlated with grass NPP (r = ‐0.49). The inability of a single factor, such as precipitation to explain a large portion of the interannual variability in NPP is consistent with the concept that patterns of NPP in tallgrass prairie are a product of spatial and temporal variability in light, water, and nutrients, driven by a combination of topography, fire history, and climate.

Effects of Urine Deposition on Small‐Scale Patch Structure in Prairie Vegetation

Large grazing mammals contribute to the species diversity of grasslands via direct and indirect effects of defoliation and urine deposition. We examined the influence of one and two applications of simulated bovine urine on vegetation structure on (1) tallgrass prairie burned either every 2 or 4 yr at Konza Prairie Research Natural Area (KPRNA), Kansas, and (2) unburned sandhills prairie at the Niobrara Valley Preserve, Nebraska. We also examined the influence of urine and the clipping of graminoids on an annually burned site at KPRNA. Plant abundance in general increased on urine patches but the response appeared dependent on litter accumulation. C4 grasses increased at the annual burn and Niobrara sites where litter levels were low. C3 forbs increased at the 2— and 4—yr burn sites where litter levels were high. Urine treatment significantly affected community composition at all but the 2—yr burn site. Alpha—diversity decreased on urine patches at the annual and 4—yr burn sites but increased on urine patches at the Niobrara site. Beta—diversity increased on urine patches at the annual burn and Niobrara site but decreased on urine patches at the 4—yr burn site. The clipping of graminoids at the annual burn site reduced both α— and β—diversity and graminoid abundance while forb abundance was not affected.

Local Distribution of Prairie Voles (Microtus ochrogaster) on Konza Prairie: Effect of Topographic Position

Prairie voles (Microtus ochrogaster) were studied in ungrazed tallgrass prairie on the Konza Prairie Research Natural Area near Manhattan, Kansas in 1984-1987. Prairie voles on our topographically diverse site demonstrated a nonrandom association with topography as upland prairie was preferred over lowland and slope prairie. Preference for upland was highly consistent among male and female residents and nonresidents, however, absolute strengths of the preference for upland did differ among years and among sex-residency classes. For example, female residents were associated more strongly with upland prairie than were male residents, whereas male residents were more likely to be captured in lowland prairie than were female residents. Patterns of use of the three topographic positions also were more variable between years and between males and females for nonresidents than residents. The latter difference was consistent with nonresidents, in part, being dispersing voles that were moving through less than optimal habitat in their search for permanent home sites.

Temporal Variation in Abundance of Peromyscus leucopus in Wooded Habitats of Eastern Kansas

-White-footed mice (Peromyscus leucopus) were sampled during spring, summer and autumn on the Konza Prairie Research Natural Area, Kansas, from autumn 1981 to spring 1988. Abundance was greater in gallery forest than in wooded outcrop habitat, although temporal patterns of abundance were similar with highs in 1981-1982 and 19851986. Standard deviations (SD) of the common logarithm of abundance were ' 0.32 for each of four sites in spring, summer and autumn, except for one forest site in spring. Our SD values were similar to those reported for other populations of P leucopus. Abundance of P leucopus was related to its abundance in the previous season (spring abundance vs. that in previous autumn, summer abundance vs. that in previous spring and autumn abundance vs. that in previous summer), seed production by woody vegetation and precipitation, but the factors that had a major influence on abundance of P leucopus varied among spring, summer and autumn populations. Ambient temperature was unrelated to abundance of P leucopus during each of the 3 seasons studied.

Benefits of protective fencing to plant and rodent communities of the western Mojave Desert, California

Human disturbance in the western Mojave Desert takes many forms. The most pervasive are livestock grazing and off-highway vehicle use. Over the past few decades several areas within this region have been fenced to preclude human disturbance. These areas provide opportunities to study the impact of human activities in a desert ecosystem. This paper documents the response of plant and small mammal populations to fencing constructed between 1978 and 1979 at the Desert Tortoise Research Natural Area, Kern County, California. Aboveground live annual plant biomass was generally greater inside than outside the fenced plots during April 1990, 1991, and 1992. The alien grassSchismus barbatus was a notable exception, producing more biomass in the unprotected area. Forb biomass was greater than that of alien annual grasses inside the fence during all three years of the study. Outside the fence, forb biomass was significantly higher than that of alien grasses only during spring 1992. Percent cover of perennial shrubs was higher inside the fence than outside, while no significant trend was detected in density. There was als more seed biomass inside the fence; this may have contributed to the greater diversity and density of Merriam's kangaroo rats (Dipodomys merriami), long-tailed pocket mice (Chaetodipus formosus), and southern grasshopper mice (Onychomys torridus) in the protected area. These results show that protection from human disturbance has many benefits, including greater overall community biomass and diversity. The significance and generality of these results can be further tested by studying other exclosures of varying age and configurations in different desert regions of the southwestern United States.

Leaf-level responses to light and temperature in two co-occurring Quercus (Fagaceae) species: implications for tree distribution patterns

Leaf-level responses to light environment (sun vs. shade) and high temperature were compared in two gallery forest oaks, bur oak ( Quercus macrocarpa) and chinquapin oak ( Quercus muehlenbergii). Our goal was to determine if species-specific differences in response to light or temperature could explain the distribution of these oaks in tallgrass prairie gallery forests. On the Konza Prairie Research Natural Area in NE Kansas (USA), bur oak is more abundant than chinquapin oak along lower reaches of streams where the forests are most productive and canopy closure is greatest. We hypothesized that bur oak is better able to acclimate physiologically to reduced light availability than chinquapin oak. Leaf level acclimation to low light in both oaks occurred through reduced photosynthetic light compensation points (PLC) (bur oak, 60.77 ± 3.02 μmol photons m −2 s −1 in sun leaves vs. 29.25 ± 0.29 photons m −2 s −1 in shade leaves; chinquapin oak, 49.47 ± 2.82 photons m −2 s −1 in sun leaves vs. 26.48 ± 1.74 photons m −2s −1 in shade leaves), resulting from nearly 50% reductions in dark respiration and specific leaf mass in both species. Apparent quantum requirement ( Q req; mol photons mol −1 O 2) did not differ between sun and shade leaves in either species, and only bur oak showed differences in sun and shade leaf shape. We also hypothesized that chinquapin oak, which is more abundant in exposed, upper stream reaches, has greater photosynthetic tolerance to the high temperatures characteristics of this region. The maximum temperature tolerances ( T max) of these oaks were compared by measuring increases in chlorophyll fluorescence. For most of the growing season, T max in chinquapin oak was 2.0°C higher than in bur oak. Field measurements of leaf vs. air temperatures in seedlings and adult trees indicated that T max was more likely to be exceeded in bur than in chinquapin oak, particularly in seedlings exposed to high solar radiation loads. We concluded that differences in thermal tolerance, in addition to previously documented differences in water relations are important determinants of distributional patterns of these oaks within gallery forests.

Mycorrhizal Dependence of Andropogon gerardii and Schizachyrium scoparium in Two Prairie Soils

-Previous research in tallgrass prairie in Kansas indicated that warm-season, C4, grasses are obligate mycotrophs and do not grow normally in the absence of mycorrhizal symbiosis. However, the degree to which such grasses depend on mycorrhizae in other prairie soils has not been examined. Growth and mycorrhizal colonization of roots of Andropogon gerardii and Schizachyrium scoparium were compared in soil collected from Konza Prairie Research Natural Area (KPRNA), Riley County, Kansas and from Sand Ridge State Forest (SRSF), Mason County, Illinois. Plants of both species were grown in the two soils and were inoculated with Glomus etunicatum spores originally collected from KPRNA or colonized root pieces from S. scoparium plants collected from SRSF. Glomus etunicatum inoculum resulted in significantly greater root colonization and biomass of both plant species in steamed KPRNA soil than did root piece inoculum. There was no benefit from inoculation in nonsterile soil which contained indigenous mycorrhizal fungi. In SRSF soil, there was no response to inoculation with mycorrhizal fungi from either source. The lack of mycorrhizal response in SRSF soil is attributed to the greater plant-available P level of this soil. For S. scoparium grown in SRSF soil, plants grown in steamed soil produced more biomass than plants grown in steamed soil amended with nonsterile soil sievings (containing soil organisms other than mycorrhizal fungi), or in nonsterile soil. These differences could be due to competition for inorganic nutrients between soil microbes and the plant or antagonistic relationships between the plant or the mycorrhizal association and the soil microbes. Thus, the mycorrhizal dependence of these plant species is related to both soil and inoculum type or species.

Dynamics and Pattern of a Managed Coniferous Forest Landscape in Oregon

We examined the process of fragmentation in a managed forest landscape by comparing rates and patterns of disturbance (primarily clear—cutting) and regrowth between 1972 and 1988 using Landsat imagery. A 2589—km2 managed forest landscape in western Oregon was classified into two forest types, closed—canopy conifer forest (CF) (typically, > 60% conifer cover) and other forest and nonforest types (OT) (typically, < 40 yr old or deciduous forest) The percentage of CF declined from 71 to 58% between 1972 and 1988. Declines were greatest on private land, least in wilderness, and intermediate in public nonwilderness. High elevations (> 914 m) maintained a greater percentage of CF than lower elevations (< 914 m). The percentage of the area at the edge of the two cover types increased on all ownerships and in both elevational zones, whereas the amount of interior habitat (defined as CF at least 100 m from OT) decreased on all ownerships and elevational zones. By 1988 public lands contained °45% interior habitat while private lands had 12% interior habitat. Mean interior patch area declined from 160 to 62 ha. The annual rate of disturbance (primarily clear—cutting) for the entire area including the wilderness was 1.19%, which corresponds to a cutting rotation of 84 yr. The forest landscape was not in a steady state or regulated condition which is not projected to occur for at least 40 yr under current forest plans. Variability in cutting rates within ownerships was higher on private land than on nonreserve public land. However, despite the use of dispersed cutting patterns on public land, spatial patterns of cutting and remnant forest patches were nonuniform across the entire public ownership. Large remaining patches (< 5000 ha) of contiguous interior forest were restricted to public lands designated for uses other than timber production such as wilderness areas and research natural areas.

Odor as a Factor in Nut Discovery by Fox Squirrels

Odor as a Factor in Nut Discovery by Fox Squirrels

Water Relations of Juniperus virginiana and Andropogon Gerardii in an Unburned Tallgrass Prairie Watershed

Water Relations of Juniperus virginiana and Andropogon Gerardii in an Unburned Tallgrass Prairie Watershed

Gall Wasp (Hymenoptera: Cynipidae) Mortality in a Spring Tallgrass Prairie Fire

The life history of the cynipid gall wasp, Antistrophus silphii Gillette , leaves it vulnerable to mortality in spring fires in the habitat of its host plant, Silphium integrifolium var. laeve Torrey and Gray. This article examines the mortality of gall wasp larvae during a prescribed bum at the Konza Prairie Research Natural Area in the tallgrass prairie of northeast Kansas. The three goals of the study were to determine (1) if A: silphii galls are most frequently found on the tallest S. integrifolium shoots, (2) if there is a relationship between growing season gall height and overwinter lodging, and (3) if gall wasps can survive fire at the heights at which they are found after overwinter lodging. A 1988 survey found that galls were relatively rare on short shoots ( 140 cm), suggesting that gall wasps avoided short shoots but that gall placement did not maximize gall height. A 1992 survey found that growing season gall height was not related to overwinter lodging of galled shoots, indicating that any gall wasp shoot selection patterns affecting later gall height were disrupted by winter lodging. Gall wasps placed at post-overwinter lodging heights were unable to survive an experimental fire, but a few gall wasps survived when placed above the range of naturally-occurring gall heights. A. silphii and other species with similar life histories must re-establish after fire by immigration, with subsequent population levels affected by indirect effects of fire on host-plant quality.

Detecting change in grasslands using measures of spatial dependence with landsat TM data

Spatial dependence is a fundamental structure of spatial data that can be readily measured. Change in landscapes can be monitored effectively using measures of spatial dependence. Scale of fluctuation analysis estimates the dimensional extent to which data are significantly autocorrelated by observing the behavior of sample variance under extended local averaging. Algorithms to estimate the one-dimensional scale of fluctuation (correlation length) and two-dimensional scale fluctuation (correlation area) in image data are described. The approach is demonstrated with an analysis of a 9-year TM image series of a tallgrass prairie preserve, Konza Prairie Research Natural Area, to assess the impact of bison on the spatial patterning of vegetation.

Water Relations and Growth of Tallgrass Prairie Forbs in Response to Selective Grass Herbivory by Bison

The indirect effects of selective grass herbivory by bison (Bison bison) on the xylem pressure potentials (ψ) and primary production of ungrazed tallgrass prairie forbs were assessed during the 1990 growing season on the Konza Prairie Research Natural Area (KPRNA) in northeast Kansas. Responses in three forb species, Vernonia baldwinii, Ambrosia psilostachya, and Aster ericoides, were measured in grazed and adjacent ungrazed patches. Seasonal predawn and midday ψ responses in these forbs were also compared with responses in Andropogon gerardii, the dominant grass in tallgrass prairie. Reductions in grass leaf area by herbivory may result in greater availability of soil water to the remaining forbs, if the forbs and grasses utilize similar soil water resources. Overall, selective herbivory of the grasses had an inconsistent effect on leaf ψ in the remaining forbs. When ψ were relatively high during the growing season, no significant differences in water status were detected between A. gerardii and the forbs. However, when ψ were low, predawn and midday ψ were higher in V. baldwinii (as much as 1.0 MPa), and midday ψ were higher in A. psilostachya (0.5 MPa) compared with A. gerardii. In contrast, ψ were not significantly different between A. ericoides and A. gerardii. Grazing by bison substantially increased light availability to ungrazed forbs (> 40%) in grazed relative to ungrazed areas. Compared with ungrazed areas, end-of-season aboveground biomass in grazed patches was 40% greater for individuals of A. psilostachya and 33% and 26% greater for reproductive biomass and floret numbers, respectively, in V. baldwinii. We conclude that the potential benefits of selective grass herbivory on forb water relations may be reduced or offset by other factors such as significant alterations in the microclimate of grazed patches. Nonetheless, increased productivity in grazed patches in two of the three forbs studied indicates that selective consumption of grasses by bison may benefit some forb species.

Fire Frequency and Community Heterogeneity in Tallgrass Prairie Vegetation

Few studies have directly addressed the effects of disturbance on spatial and temporal heterogeneity. Spatial heterogeneity. Spatial heterogeneity is the degree of dissimilarity in species composition from one point to another into a community, whereas temporal heterogeneity is compositional change within a site over time. The purposes of this study were to determine (1) if a quadratic relationship exists between within—site heterogeneity and disturbance frequency as predicted by the intermediate disturbance hypothesis (IDH), (2) if disturbed and undisturbed sites have similar heterogeneity as implied by the disturbance heterogeneity hypothesis (DHM), and whether or not these results differed with scale, and (3) if there is a relationship between spatial and temporal heterogeneity as implied by the DHM. Analyses were based on plant species composition data collected over 9 yr in quadrats permanently located in experimental management units subjected to differ burning frequencies at Konza Prairie Research Natural Area, Kansas, USA. The relationship between disturbance frequency and within—site heterogeneity was opposite that predicted by the IDH. Heterogeneity was lowest at intermediate disturbance frequencies. Heterogeneity in annually burned prairie was lower than in unburned prairie and prairies burned once every 4 yr in contrast to predictions of the DHM. However, this relationship did not hold at larger spatial scales. There was a positive relationship between within—site spatial and temporal heterogeneity on annual burned sites, sites burned once every 4 yr, and nearly so on sites burned every other year. Within—site heterogeneity was negatively correlated with cover of Andropogon gerardii, and positively correlated with total richness and species diversity. Studies of variation, in addition to averages, will increase our ability to predict patterns of species distribution and abundance within and between communities in response to disturbance.

Disturbances in tallgrass prairie: local and regional effects on community heterogeneity

Community heterogeneity in tallgrass prairie was analyzed at regional and local levels to assess the effects of disturbances on community structure at different spatial scales. The sites were part of NASA's First ISLSCP Field Experiment (FIFE) in Kansas, and were located on grassland treatments that were undisturbed, and burned-only on Konze Prairie Research Natural Area, and grazed-only and grazed + burned on adjacent ranch land. Sites in grazed-only or grazed + burned treatments were less similar to each other, on a regional scale (15 × 15 km), than were burned-only or undisturbed sites. Grazing reduced the cover of dominant species, making space available for the establishment of immigrants from the region. Each site was different because of establishment by different species from the large regional species pool. At the local scale (0.1 ha), the most homogeneous treatments were those that were most heterogeneous at the regional scale. Undisturbed treatments at the local scale were the most heterogeneous compared to sites under other treatments. Therefore, regional responses to disturbances were more variable than local responses, and were not predictable from within-site analyses.

Surface energy fluxes on four slope sites during FIFE 1988

Four slopes (facing north, south, east, and west) in the Konza Prairie Research Natural Area were selected to study the effect of topography on surface energy balance and other micrometeorological variables. Energy fluxes, air temperature, and vapor pressure were measured on the slopes throughout the 1988 growing season. Net radiation was highest on the south facing slope and lowest on the north facing slope, and the difference was more than 150 W m−2 (20–30%) at solar noon. For daily averages the difference was about 25 W m−2 (15%) early in the season and increased to about 60 W m−2 (30–50%) in September. Soil heat fluxes were similar for all the slopes. The absolute values of sensible heat flux (H) were consistently lower on the north facing slope compared with other slopes. The south facing slope had the greatest day‐to‐day fluctuation in latent heat flux as a result of the interaction of net radiation, soil moisture, and green leaf area. Differences were found in the partitioning of the available energy among the slopes, and the north facing slope had a higher percentage of energy dissipated into latent heat flux. The north facing slope had higher air temperatures during the day and higher vapor pressures both during the day and at night when the wind was from the south.

LEAF GAS EXCHANGE IN QUERCUS MACROCARPA (FAGACEAE): RAPID STOMATAL RESPONSES TO VARIABILITY IN SUNLIGHT IN A TREE GROWTH FORM

Responses in net photosynthesis (A), stomatal conductance to water vapor (g), and leaf xylem pressure potential (ψ) were measured in the deciduous tree Quercus macrocarpa during alternating periods of sun (photosynthetic photon flux, PPF > 1,500 μmol m‐2 sec‐1) and shade (ca. 350 μmol m‐2 sec‐1 simulating cloud cover). Measurements were made on trees growing at the gallery forest‐prairie edge on the Konza Prairie Research Natural Area in northeast Kansas. The region is near the westernmost extension of the range of Q. macrocarpa where this species experiences significant seasonal water stress (minimum ψ < ‐2.9 MPa). Quercus macrocarpa was chosen for study because it has relatively high A (15 μmol m‐2 sec‐1) and g (300 mmol m‐2 sec‐1) in contrast to the deciduous and evergreen subalpine trees previously studied. Both trees and large saplings of Q. macrocarpa responded to alternating several minute periods of sun and shade with relatively rapid changes in A and g. Reductions in g (110 mmol m‐2 sec‐1) during shade periods lowered transpirational water losses (E) by 13% (and reduced A by 5%) relative to estimates of A and E made assuming g remained constant. Partial stomatal closure during shade was correlated with moderate enhancement in ψ (0.31 MPa) in Q. macrocarpa. However, greater increases in ψ were measured in adjacent prairie grasses exposed to similar periods of shade (0.72 MPa in Andropogon gerardii, 0.61 MPa in Sorghastrum nutans). Reduced variability in ψ in tree growth forms may reflect greater buffering of water relations associated with the large size of trees, the amount of tissue devoted to water storage, and differences in hydraulic resistance relative to herbs. Nonetheless, the gas exchange and water relations responses in Q. macrocarpa were much more similar to those previously measured in herbaceous subalpine and grassland species than to those documented for subalpine trees. Thus, rapid gas exchange responses to variable PPF may also occur in tree growth forms.

Soil, Nitrogen, Phosphorus, and Organic Matter Processing by Earthworms in Tallgrass Prairie

Earthworms represent the dominant fraction of soil macroinvertebrate biomass in Kansas tallgrass prairie and thus may have a large impact on nutrient budgets. To evaluate this impact I estimated cast production and rates of nutrient processing by the native genus Diplocardia and by exotic Lumbricidae in permanent plots on two soil types at Konza Prairie Research Natural Area. Total cast production was estimated from field population estimates, soil climate data, and cast production—temperature relationships obtained in the laboratory. The organic matter and mineral N and P contents of surface casts by month were used to estimate total processing of these substances in the field. Earthworms present in the study sites were Diplocardia longiseta, D. kansensis, D. rugosa, D. singularis, D. smithii, D. verrucosa, and the European species Aporrectodea caliginosa and Octolasion cyaneum. Estimated total cast production and nutrient processing by Diplocardia spp. exceeded that of exotics in both soil types, in spite of higher populations of Lumbricidae in one soil. Total annual soil consumption by all earthworms was 4—10% of the A—horizon, depending on soil type. Organic matter equivalent to 10% of total soil organic matter in the top 15 cm or to 100—300% of plant annual belowground production appears to pass through the earthworms each year. Mineral N processed was °10—12% of annual plant N uptake, comparable to half of the input from precipitation, while the P processed was equivalent to 50% of annual uptake. In contrast to other grassland systems studied, the introduction of Lumbricidae had a negative effect on soil turnover and nutrient mineralization due to the lower throughput of Lumbricidae and their relative intolerance of summer soil temperatures.