Matt D. Busse

Lethal soil temperatures during burning of masticated forest residues

Mastication of woody shrubs is used increasingly as a management option to reduce fire risk at the wildland-urban interface. Whether the resulting mulch layer leads to extreme soil heating, if burned, is unknown. We measured temperature profiles in a clay loam soil during burning of Arctostaphylos residues. Four mulch depths were burned (0, 2.5, 7.5 and 12.5 cm), spanning typical conditions at forested sites in northern California with dense pre-mastication shrub cover. Two soil moisture contents were compared at each fuel depth to simulate spring prescribed burning (moist soil) and late-season wildfire (dry soil). Maximum temperatures reached 600 ◦ C on the surface of dry soils and were 100-200 ◦ C lower for moist soil. Heating was extensive in dry soil for the two deepest mulch depths, exceeding the lethal threshold for plants (60 ◦ C) for a minimum of 7 h throughout the 10-cm soil profile. Minimal heat pulse was found with less mulch. Moist soil also dampened heat penetration; peak temperatures exceeded 60 ◦ C only to 2.5 cm in the soil profile for all but the deepest mulch layer. No adverse effects of burning on water repellency were found in dry or moist soil. The potential for biological damage from soil heating during fire exists following mastication, particularly in dry soil with a mulch depth of 7.5 cm or greater. Field projections indicate that up to one-fourth of treated areas with dense pre-mastication vegetation would surpass lethal soil temperatures during a surface wildfire.

Suitability and use of the 15N-isotope dilution method to estimate nitrogen fixation by actinorhizal shrubs.

Nitrogen fixation rates were estimated by the 15 N-isotope dilution method for Ceanothus velutinus and Purshia tridentata in the understory of central Oregon ponderosa pine forests. Field rates were measured in small pole-, large pole-, and sawtimbersized pine stands using two shrubs (Arctostaphylos patula, Ribes cereum) and one graminoid (Carex rossii) as non-fixing reference species. Shrub cover ranged from 23 to 59% for Ceanothus and from 9 to 15% for Purshia. Foliage samples were collected monthly following two applications of 15 N ammonium sulfate (5 kg N ha ˇ1 at 10 atom% 15 N) to compare N uptake patterns and determine the fraction of N derived from fixation (Ndff). Several violations of the underlying assumption of the isotope dilution method that N fixing and reference plants have access to similar pools of soil 15 N were identified. These included non-uniform distribution of 15 N in the soil profile and dissimilarities in rooting volumes, N uptake patterns, and plant ages between the reference and N fixing plants. Despite these compromises, there were only minor differences in percentage of Ndff when calculated independently using each reference species. Low 15 N uptake by Ceanothus and Purshia evidently negated the requirement that N fixing and reference plants have comparable spatial and temporal access to soil 15 N. Both Ceanothus and Purshia were highly effective, obtaining more than 80% of their N from fixation regardless of pine stand, shrub cover, or shrub age. Annual N fixation by Ceanothus ranged from 4 to 15 kg ha ˇ1 year ˇ1 . This rate is lower than previously reported for Ceanothus in other ecosystems in the western United States, yet is sufficient to replace N losses from disturbances such as prescribed fire. Purshia fixed considerably less N, ca. 1 kg ha ˇ1 year ˇ1 , roughly equivalent to the rate of atmospheric

Ectomycorrhizal Formation in Herbicide-Treated Soils of Differing Clay and Organic Matter Content

Herbicides are commonly used on private timberlands in the western United States for site preparation and control of competing vegetation. How non-target soil biota respond to herbicide applications, however, is not thoroughly understood. We tested theeffects of triclorpyr, imazapyr, and sulfometuron methyl on ectomycorrhizal formation in a greenhouse study. Ponderosa pine, Douglas-fir, and white fir seedlings were grown in four forest soils ranging in clay content from 9 to 33% and organic matter content from 3 to 17%, and treated with commercial formulations of each herbicide at 0, 1.0, and 2.0 times the recommended field rate. Many of the possible herbicide-soil combinations resulted in reduced seedling growth. Root development was particularly sensitive to the three herbicides, with an average of 51% fewer root tips compared to the control treatment. The ability of mycorrhizal fungi to infectthe remaining root tips, however, was uninhibited. Mycorrhizal formation was high, averaging 91% of all root tips, regardless of herbicide, application rate, soil type, or conifer species. In agreement, soil microbial biomass and respiratory activity were unaffected by the herbicide treatments. The results show that these herbicides do not alter the capability of mycorrhizal fungi to infect roots, even at concentrations detrimental to seedling growth.

The Response of Soil CO 2 Fluxes to Progressively Excluding Vertebrate and Invertebrate Herbivores Depends on Ecosystem Type

Grasslands support large populations of herbivores and store up to 30% of the world’s soil carbon (C). Thus, herbivores likely play an important role in the global C cycle. However, most studies on how herbivory impacts the largest source of C released from grassland soils—soil carbon dioxide (CO2) emissions—only considered the role of large ungulates. This ignores all other vertebrate and invertebrate herbivores and their collective effects on ecosystem properties. We progressively excluded large, medium, and small vertebrates and invertebrates from two subalpine grasslands (productive, heavily grazed short-grass; less productive, lightly grazed tall-grass) using size-selective fences, assessed the impact on soil CO2 emissions and related biotic and abiotic variables. Exclusion resulted in significant changes in soil CO2 emissions in both vegetation types. Short-grass soil CO2 emissions progressively increased when large and medium mammals were excluded. However, no difference was detected among plots were all or no herbivores grazed. In contrast, tall-grass soil CO2 emissions were not affected by mammal exclusion, but excluding all herbivores lead to reduced emissions. Soil micro-climatic parameters best predicted the patterns of soil CO2 emissions in short-grass vegetation, whereas root biomass was the best predictor of CO2 release in tall-grass vegetation. Our results showed that diverse herbivore communities affect soil respiration differently than assumed from previous studies that only excluded large ungulates. Such information is important if we are to understand how changes in herbivore species composition—as could happen through altered management practices, extinction or invasion—impact grassland C storage and release.

Do changes in soil properties after rooting by wild boars (Sus scrofa) affect understory vegetation in Swiss hardwood forests

Recovering from small fragmented populations, wild boars (Sus scrofa L.) have considerably increased their num- bers and their habitat range in many European countries during the past two decades. Although several studies have focused on the impact of wild boar rooting on selected vegetation properties, little is known about effects on entire forest ecosys- tems. The main goal of our study was to assess how rooting by boars alters soil and vegetation properties. We measured soil chemical and biological properties (C and N concentrations, N availability, and microbial biomass C) as well as several vegetation characteristics (total plant cover, plant species diversity, and number and height of saplings) on paired rooted and non-rooted plots in six hardwood forests in Switzerland. We found that rooting by wild boars led to significant increases in mineral soil C and N concentrations and microbial biomass C, which could lead to improved growth conditions for plants. However, total plant cover and sapling counts were reduced on rooted plots, possibly due to mechanical disturbance or due to reduced plant available N (measured as supply rate in contrast with the observed increase in total stocks of mineral soil N). In view of these results, simple characterizations of wild boar rooting as beneficial or detrimental to forest ecosystems should be handled with care.

Linkages between grazing history and herbivore exclusion on decomposition rates in mineral soils of subalpine grasslands

Background & aimsHerbivore-driven changes to soil properties can influence the decomposition rate of organic material and therefore soil carbon cycling within grassland ecosystems. We investigated how aboveground foraging mammalian and invertebrate herbivores affect mineral soil decomposition rates and associated soil properties in two subalpine vegetation types (short-grass and tall-grass) with different grazing histories.MethodsUsing exclosures with differing mesh sizes, we progressively excluded large, medium and small mammals and invertebrates from the two vegetation types in the Swiss National Park (SNP). Mineral soil decomposition rates were assessed using the cotton cloth (standard substrate) method between May and September 2010.ResultsDecomposition displayed strong spatio-temporal variability, best explained by soil temperature. Exclusion of large mammals increased decomposition rates, but further exclusion reduced decomposition rates again in the lightly grazed (tall-grass) vegetation. No difference among treatments was found in the heavily grazed (short-grass) vegetation. Heavily grazed areas had higher decomposition rates than the lightly grazed areas because of higher soil temperatures. Microbial biomass carbon and soil C:N ratio were also linked to spatio-temporal decomposition patterns, but not to grazing history.ConclusionsDespite altering some of the environmental controls of decomposition, cellulose decomposition rates in the SNP’s subalpine grasslands appear to be mostly resistant to short-term herbivore exclusion.

Soil heating during burning of forest slash piles and wood piles

Pile burning of conifer slash is a common fuel reduction practice in forests of the western United States that has a direct, yet poorly quantified effect on soil heating. To address this knowledge gap, we measured the heat pulse beneath hand-built piles ranging widely in fuel composition and pile size in sandy-textured soils of the Lake Tahoe Basin. The soil heat pulse depended primarily on fuel composition, not on pile size. Burn piles dominated by large wood produced extreme temperatures in soil profile, with lethal heating lasting up to 3 days. In contrast, the heat pulse was moderate beneath piles containing a mixture of fuel sizes. Considerable spatial variability was noted, as soil temperatures were generally greatest near pile centres and decline sharply toward the pile edges. Also, saturating pile burns with water 8 h after ignition (‘mopping up’) effectively quenched the soil heat pulse while allowing near-complete fuel consumption. The findings suggest that burning of hand piles will not result in extreme or extensive soil heating except for uncommon conditions when piles are dominated by large wood and occupy a high percentage of the ground surface.

Aboveground vertebrate and invertebrate herbivore impact on net N mineralization in subalpine grasslands

Aboveground herbivores have strong effects on grassland nitrogen (N) cycling. They can accelerate or slow down soil net N mineralization depending on ecosystem productivity and grazing intensity. Yet, most studies only consider either ungulates or invertebrate herbivores, but not the combined effect of several functionally different vertebrate and invertebrate herbivore species or guilds. We assessed how a diverse herbivore community affects net N mineralization in subalpine grasslands. By using size-selective fences, we progressively excluded large, medium, and small mammals, as well as invertebrates from two vegetation types, and assessed how the exclosure types (ET) affected net N mineralization. The two vegetation types differed in long-term management (centuries), forage quality, and grazing history and intensity. To gain a more mechanistic understanding of how herbivores affect net N mineralization, we linked mineralization to soil abiotic (temperature; moisture; NO3-, NH4+, and total inorganic N concentrations/pools; C, N, P concentrations; pH; bulk density), soil biotic (microbial biomass; abundance of collembolans, mites, and nematodes) and plant (shoot and root biomass; consumption; plant C, N, and fiber content; plant N pool) properties. Net N mineralization differed between ET, but not between vegetation types. Thus, short-term changes in herbivore community composition and, therefore, in grazing intensity had a stronger effect on net N mineralization than long-term management and grazing history. We found highest N mineralization values when only invertebrates were present, suggesting that mammals had a negative effect on net N mineralization. Of the variables included in our analyses, only mite abundance and aboveground plant biomass explained variation in net N mineralization among ET. Abundances of both mites and leaf-sucking invertebrates were positively correlated with aboveground plant biomass, and biomass increased with progressive exclusion. The negative impact of mammals on net N mineralization may be related partially to (1) differences in the amount of plant material (litter) returned to the belowground subsystem, which induced a positive bottom-up effect on mite abundance, and (2) alterations in the amount and/or distribution of dung, urine, and food waste. Thus, our results clearly show that short-term alterations of the aboveground herbivore community can strongly impact nutrient cycling within ecosystems independent of long-term management and grazing history.

Biogeography and organic matter removal shape long-term effects of timber harvesting on forest soil microbial communities

The growing demand for renewable, carbon-neutral materials and energy is leading to intensified forest land-use. The long-term ecological challenges associated with maintaining soil fertility in managed forests are not yet known, in part due to the complexity of soil microbial communities and the heterogeneity of forest soils. This study determined the long-term effects of timber harvesting, accompanied by varied organic matter (OM) removal, on bacterial and fungal soil populations in 11- to 17-year-old reforested coniferous plantations at 18 sites across North America. Analysis of highly replicated 16 S rRNA gene and ITS region pyrotag libraries and shotgun metagenomes demonstrated consistent changes in microbial communities in harvested plots that included the expansion of desiccation- and heat-tolerant organisms and decline in diversity of ectomycorrhizal fungi. However, the majority of taxa, including the most abundant and cosmopolitan groups, were unaffected by harvesting. Shifts in microbial populations that corresponded to increased temperature and soil dryness were moderated by OM retention, which also selected for sub-populations of fungal decomposers. Biogeographical differences in the distribution of taxa as well as local edaphic and environmental conditions produced substantial variation in the effects of harvesting. This extensive molecular-based investigation of forest soil advances our understanding of forest disturbance and lays the foundation for monitoring long-term impacts of timber harvesting.

Does the Aboveground Herbivore Assemblage Influence Soil Bacterial Community Composition and Richness in Subalpine Grasslands

Grassland ecosystems support large communities of aboveground herbivores that are known to directly and indirectly affect belowground properties such as the microbial community composition, richness, or biomass. Even though multiple species of functionally different herbivores coexist in grassland ecosystems, most studies have only considered the impact of a single group, i.e., large ungulates (mostly domestic livestock) on microbial communities. Thus, we investigated how the exclusion of four groups of functionally different herbivores affects bacterial community composition, richness, and biomass in two vegetation types with different grazing histories. We progressively excluded large, medium, and small mammals as well as invertebrate herbivores using exclosures at 18 subalpine grassland sites (9 per vegetation type). We assessed the bacterial community composition using terminal restriction fragment length polymorphism (T-RFLP) at each site and exclosure type during three consecutive growing seasons (2009–2011) for rhizosphere and mineral soil separately. In addition, we determined microbial biomass carbon (MBC), root biomass, plant carbon:nitrogen ratio, soil temperature, and soil moisture. Even though several of these variables were affected by herbivore exclusion and vegetation type, against our expectations, bacterial community composition, richness, or MBC were not. Yet, bacterial communities strongly differed between the three growing seasons as well as to some extent between our study sites. Thus, our study indicates that the spatiotemporal variability in soil microclimate has much stronger effects on the soil bacterial communities than the grazing regime or the composition of the vegetation in this high-elevation ecosystem.