Murray K. Clayton

HUMAN INFLUENCE ON CALIFORNIA FIRE REGIMES

Periodic wildfire maintains the integrity and species composition of many ecosystems, including the mediterranean-climate shrublands of California. However, human activities alter natural fire regimes, which can lead to cascading ecological effects. Increased human ignitions at the wildland-urban interface (WUI) have recently gained attention, but fire activity and risk are typically estimated using only biophysical variables. Our goal was to determine how humans influence fire in California and to examine whether this influence was linear, by relating contemporary (2000) and historic (1960-2000) fire data to both human and biophysical variables. Data for the human variables included fine-resolution maps of the WUI produced using housing density and land cover data. Interface WUI, where development abuts wildland vegetation, was differentiated from intermix WUI, where development intermingles with wildland vegetation. Additional explanatory variables included distance to WUI, population density, road density, vegetation type, and ecoregion. All data were summarized at the county level and analyzed using bivariate and multiple regression methods. We found highly significant relationships between humans and fire on the contemporary landscape, and our models explained fire frequency (R2 = 0.72) better than area burned (R2 = 0.50). Population density, intermix WUI, and distance to WUI explained the most variability in fire frequency, suggesting that the spatial pattern of development may be an important variable to consider when estimating fire risk. We found nonlinear effects such that fire frequency and area burned were highest at intermediate levels of human activity, but declined beyond certain thresholds. Human activities also explained change in fire frequency and area burned (1960-2000), but our models had greater explanatory power during the years 1960-1980, when there was more dramatic change in fire frequency. Understanding wildfire as a function of the spatial arrangement of ignitions and fuels on the landscape, in addition to nonlinear relationships, will be important to fire managers and conservation planners because fire risk may be related to specific levels of housing density that can be accounted for in land use planning. With more fires occurring in close proximity to human infrastructure, there may also be devastating ecological impacts if development continues to grow farther into wildland vegetation.

Predicting spatial patterns of fire on a southern California landscape

Humans influence the frequency and spatial pattern of fire and contribute to altered fire regimes, but fuel loading is often the only factor considered when planning management activities to reduce fire hazard. Understanding both the human and biophysical landscape characteristics that explain how fire patterns vary should help to identify where fire is most likely to threaten values at risk. We used human and biophysical explanatory variables to model and map the spatial patterns of both fire ignitions and fire frequency in the Santa Monica Mountains, a human-dominated southern California landscape. Most fires in the study area are caused by humans, and our results showed that fire ignition patterns were strongly influenced by human variables. In particular, ignitions were most likely to occur close to roads, trails, and housing development but were also related to vegetation type. In contrast, biophysical variables related to climate and terrain (January temperature, transformed aspect, elevation, and slope) explained most of the variation in fire frequency. Although most ignitions occur close to human infrastructure, fires were more likely to spread when located farther from urban development. How far fires spread was ultimately related to biophysical variables, and the largest fires in southern California occurred as a function of wind speed, topography, and vegetation type. Overlaying predictive maps of fire ignitions and fire frequency may be useful for identifying high-risk areas that can be targeted for fire management actions.

Terrestrial, benthic, and pelagic resource use in lakes: results from a three-isotope Bayesian mixing model

Fluxes of organic matter across habitat boundaries are common in food webs. These fluxes may strongly influence community dynamics, depending on the extent to which they are used by consumers. Yet understanding of basal resource use by consumers is limited, because describing trophic pathways in complex food webs is difficult. We quantified resource use for zooplankton, zoobenthos, and fishes in four low-productivity lakes, using a Bayesian mixing model and measurements of hydrogen, carbon, and nitrogen stable isotope ratios. Multiple sources of uncertainty were explicitly incorporated into the model. As a result, posterior estimates of resource use were often broad distributions; nevertheless, clear patterns were evident. Zooplankton relied on terrestrial and pelagic primary production, while zoobenthos and fishes relied on terrestrial and benthic primary production. Across all consumer groups terrestrial reliance tended to be higher, and benthic reliance lower, in lakes where light penetration was low due to inputs of terrestrial dissolved organic carbon. These results support and refine an emerging consensus that terrestrial and benthic support of lake food webs can be substantial, and they imply that changes in the relative availability of basal resources drive the strength of cross-habitat trophic connections.

Stable Isotope Turnover and Half-Life in Animal Tissues: A Literature Synthesis

Stable isotopes of carbon, nitrogen, and sulfur are used as ecological tracers for a variety of applications, such as studies of animal migrations, energy sources, and food web pathways. Yet uncertainty relating to the time period integrated by isotopic measurement of animal tissues can confound the interpretation of isotopic data. There have been a large number of experimental isotopic diet shift studies aimed at quantifying animal tissue isotopic turnover rate λ (%·day-1, often expressed as isotopic half-life, ln(2)/λ, days). Yet no studies have evaluated or summarized the many individual half-life estimates in an effort to both seek broad-scale patterns and characterize the degree of variability. Here, we collect previously published half-life estimates, examine how half-life is related to body size, and test for tissue- and taxa-varying allometric relationships. Half-life generally increases with animal body mass, and is longer in muscle and blood compared to plasma and internal organs. Half-life was longest in ecotherms, followed by mammals, and finally birds. For ectotherms, different taxa-tissue combinations had similar allometric slopes that generally matched predictions of metabolic theory. Half-life for ectotherms can be approximated as: ln (half-life) = 0.22*ln (body mass) + group-specific intercept; n = 261, p<0.0001, r2 = 0.63. For endothermic groups, relationships with body mass were weak and model slopes and intercepts were heterogeneous. While isotopic half-life can be approximated using simple allometric relationships for some taxa and tissue types, there is also a high degree of unexplained variation in our models. Our study highlights several strong and general patterns, though accurate prediction of isotopic half-life from readily available variables such as animal body mass remains elusive.

Historical forest baselines reveal potential for continued carbon sequestration

One-third of net CO2 emissions to the atmosphere since 1850 are the result of land-use change, primarily from the clearing of forests for timber and agriculture, but quantifying these changes is complicated by the lack of historical data on both former ecosystem conditions and the extent and spatial configuration of subsequent land use. Using fine-resolution historical survey records, we reconstruct pre-EuroAmerican settlement (1850s) forest carbon in the state of Wisconsin, examine changes in carbon after logging and agricultural conversion, and assess the potential for future sequestration through forest recovery. Results suggest that total above-ground live forest carbon (AGC) fell from 434 TgC before settlement to 120 TgC at the peak of agricultural clearing in the 1930s and has since recovered to approximately 276 TgC. The spatial distribution of AGC, however, has shifted significantly. Former savanna ecosystems in the south now store more AGC because of fire suppression and forest ingrowth, despite the fact that most of the region remains in agriculture, whereas northern forests still store much less carbon than before settlement. Across the state, continued sequestration in existing forests has the potential to contribute an additional 69 TgC. Reforestation of agricultural lands, in particular, the formerly high C-density forests in the north-central region that are now agricultural lands less optimal than those in the south, could contribute 150 TgC. Restoring historical carbon stocks across the landscape will therefore require reassessing overall land-use choices, but a range of options can be ranked and considered under changing needs for ecosystem services.

Glucose and phosphate toxicity in hamster preimplantation embryos involves disruption of cellular organization, including distribution of active mitochondria

While perinuclear clustering of active mitochondria, as revealed by Rhodamine 123 staining and confocal microscopy, is part of normal hamster embryo development, it is not known whether this reorganization is necessary for development. To determine if disruption of mitochondrial organization occurs in developmentally compromised embryos, the intensity of Rhodamine 123 staining was quantitated using NIH Image Software in different regions of cultured hamster 2‐cell embryos exposed to either blocking (contains glucose and phosphate) or non‐blocking culture conditions. Three regions within each blastomere were defined based on the organization of freshly collected embryos: cortical (ring beneath plasma membrane), perinuclear, and intermediate regions. While there was no treatment effect on the total staining intensity, glucose and phosphate treated embryos had significantly higher Rhodamine 123 staining in the intermediate region, with corresponding reduced intensity in the perinuclear region, implicating glucose and phosphate in the redistribution of mitochondria. Glucose and phosphate treatment also selectively reduced the FITC Phalloidin staining of actin microfilaments in the interior of the embryo. Neither cytochalasin D nor colchicine, at doses that blocked the second cleavage, caused redistribution of mitochondria like that seen with glucose and phosphate treatment. Additionally, cytochalasin D was unable to disrupt actin microfilaments in the perinuclear region, although it induced a “clumpy” appearance in both the mitochondria and microfilaments. This report not only offers a more mechanistic explanation of the embryo 2‐cell block (translocation of mitochondria involved in glucose and phosphate inhibition) but suggests that appropriate cellular organization, including the spatial positioning of the mitochondria, may be a prerequisite for normal development and that the physical organization of the embryo is susceptible to damage by exposure to culture conditions. Mol. Reprod. Dev. 48:227–237, 1997.

Effects of an Introduced Bacterium on Bacterial Communities on Roots

The objective of this work was to determine whether introduction of a bac- terium altered microbial communities associated with roots. We conducted experiments in the field and in a growth chamber to determine whether coating soybean seeds with marked strains derived from the biological control agent Bacillus cereus UW85 affected the bacterial community in the rhizosphere of soybeans. We characterized 2651 individual isolates of bacteria from root-free soil and from soybean seedlings based on 43-50 phys- iological attributes. Discriminant analysis of the bacterial communities according to these attributes showed that the communities of rhizosphere bacteria that developed on non- treated plants and on plants grown from seeds coated with a single strain of bacteria were sometimes dramatically different. This occurred even when the introduced strain did not persist as a common member of the community. In two of four experiments we could as easily differentiate between bacterial communities on roots of UW85-treated and non- treated seedlings as between the communities in the rhizosphere and root-free soil. In the other two experiments we could differentiate only between communities in root-free soil and on roots. In the comparison of bacteria from root-free soil and from the soybean rhizosphere, we found that bacteria from root-free soil were more likely to degrade complex carbon sources than were rhizosphere bacteria, whereas bacteria from roots were resistant to more antimicrobial substances and were able to grow on a wider range of simple carbon sources than were bacteria isolated from root-free soil.

Legacies of historical land use on regional forest composition and structure in Wisconsin, USA (mid-1800s-1930s-2000s)

Historical land use can influence forest species composition and structure for centuries after direct use has ceased. In Wisconsin, USA, Euro-American settlement in the mid- to late 1800s was accompanied by widespread logging, agricultural conversion, and fire suppression. To determine the maximum magnitude of change in forest ecosystems at the height of the agricultural period and the degree of recovery since that time, we assessed changes in forest species composition and structure among the (1) mid-1800s, at the onset of Euro-American settlement; (2) 1930s, at the height of the agricultural period; and (3) 2000s, following forest regrowth. Data sources included the original U.S. Public Land Survey records (mid-1800s), the Wisconsin Land Economic Inventory (1930s), and U.S. Forest Service Forest Inventory and Analysis data (2000s). We derived maps of relative species dominance and tree diameters for the three dates and assessed change using spatial error models, nonmetric multidimensional scaling ordination, and Sørenson distance measures. Our results suggest that since the mid-1800s, hemlock and white pine have declined in absolute area from 22% to 1%, and the proportion of medium (25-<50 cm) and large-diameter (> or = 50 cm) trees of all species has decreased from 71% to 27% across the entire state. Early-successional aspen-birch is three times more common than in the mid-1800s (9% vs. 3%), and maple and other shade-tolerant species are increasing in southern areas formerly dominated by oak forests and savannas. Since the peak agricultural extent in the 1930s, species composition and tree size in northern forests have shown some recovery, while southern forests appear to be on a novel trajectory of change. There is evidence of regional homogenization, but the broad north-south environmental gradient in Wisconsin constrains overall species composition. Although the nature of the future forests will be determined in part by climate change and other exogenous variables, land use is likely to remain the driving factor.

Road Density and Landscape Pattern in Relation to Housing Density, and Ownership, Land Cover, and Soils

Roads are conspicuous components of landscapes and play a substantial role in defining landscape pattern. Previous studies have demonstrated the link between roads and their effects on ecological processes and landscape patterns. Less understood is the placement of roads, and hence the patterns imposed by roads on the landscape in relation to factors describing land use, land cover, and environmental heterogeneity. Our hypothesis was that variation in road density and landscape patterns created by roads can be explained in relation to variables describing land use, land cover, and environmental factors. We examined both road density and landscape patterns created by roads in relation to suitability of soil substrate as road subgrade, land cover, lake area and perimeter, land ownership, and housing density across 19 predominantly forested counties in northern Wisconsin, USA. Generalized least squares regression models showed that housing density and soils with excellent suitability for road subgrade were positively related to road density while wetland area was negatively related. These relationships were consistent across models for different road types. Landscape indices showed greater fragmentation by roads in areas with higher housing density, and agriculture, grassland, and coniferous forest area, but less fragmentation with higher deciduous forest, mixed forest, wetland, and lake area. These relationships provide insight into the complex relationships among social, institutional, and environmental factors that influence where roads occur on the landscape. Our results are important for understanding the impacts of roads on ecosystems and planning for their protection in the face of continued development.

OLD-GROWTH NORTHERN HARDWOOD FORESTS: SPATIAL AUTOCORRELATION AND PATTERNS OF UNDERSTORY VEGETATION

The goal of this research was to examine spatial patterns of forest understory vegetation at a fine resolution for future work on underlying processes. We used a cyclic, two-dimensional sampling design, sampling plants in 2100 quadrats (0.25 m2), placing one 62 × 29.5 m grid within each of four old-growth northern hardwood (Acer saccharum–Betula alleghaniensis) stands. The specific plan used was designed to maximize spatial information and sampling efficiency. The study was done in the Sylvania Wilderness Area, Ottawa National Forest, Upper Michigan, USA. Spatial patterns of ground-layer species vary with the environment, species ecological characteristics, and their interactions. Competition with maple saplings may be a strong determinant of understory spatial patterns of these forests. Based on 95% confidence intervals, spatial analysis showed that most ground-layer species were positively autocorrelated to distances of <2.5 m in stands with high sapling density, while many of these same species were autocorrelated at up to 21 m in stands with low sapling density. Most ground-layer species also had distributions indistinguishable from random at three other resolutions (9.0 × 10.5 m to 9.0 × 31.5 m blocks) in stands with high sapling density, but aggregated distributions in stands with low sapling density. Logistic regression analysis yielded direct and indirect negative correlations between ground-layer species and maple saplings. Plant species temporal guilds were autocorrelated according to the following rank distances: spring ephemerals > evergreen ≥ early summer > late summer > dimorphic. More species were autocorrelated to greater distances on loamy soils than on sandy soils. Plant species dispersal guilds were autocorrelated according to the following rank distances: ballistic ≥ spores ≥ ant > ingested > adhesive. Thus there is a general inverse relationship between autocorrelation distance and migration rates of dispersal guilds. The spatial distribution of microtopography and decayed, coarse woody debris appears to be important for the maintenance of plant diversity and heterogeneity in old-growth stands. Forest managers can help maintain biological diversity by giving preferential management to those species that are rare and, if once locally extirpated, have poor re-colonization ability. This requires maintaining regional landscape diversity, as well as within-stand microhabitats.