Oak Savanna Research Articles

The removal of woody encroachment restores biophysical gradients in Midwestern oak savannas

Summary Savannas throughout the world are characterized by spatial gradients of resources created by scattered overstorey trees. These gradients are important for maintenance of understorey biodiversity; however, they may be disrupted by woody encroachment, resulting in alterations to understorey vegetation. Little is known about the status of understorey gradients in encroached savannas, or whether they redevelop during restoration and if so, by what mechanism. We used a large‐scale restoration experiment with Midwestern oak savannas (USA) to address these issues. We established understorey transects radiating from overstorey Quercus alba L. trees to inter‐canopy gaps in four control and four restoration treatment sites. Along each transect, we sampled understorey vegetation over three field seasons and we recorded physical factors in the final year of study. Restoration produced a strong relationship between visible sky (e.g. light penetrating to the understorey) and distance from overstorey trees, while none existed in control sites. Restored sites had greater variability in soil moisture due to higher levels immediately after rain at all distances from trees, coupled with greater drying rates, particularly at farther distances from overstorey trees. With restoration, a positive relationship between cover by understorey vegetation and distance from overstorey trees developed and strengthened over time, whereas no relationship existed in control sites. Cover by each of the major functional groups, forbs, graminoids, and woody species, contributed to this pattern. Furthermore, after restoration, species richness increased with distance from overstorey trees in the final year of study. Nonmetric multidimensional scaling (NMS) showed that common understorey species were correlated with gradients of canopy cover and soil moisture, which were associated with restoration plots, and gradients of soil texture and N, which were associated with both restoration and control plots. Furthermore, restoration strengthened correlations between NMS plot scores and distance from overstorey trees. Synthesis and applications. Re‐establishing overstorey structure was an important step during restoration of the oak savannas in this study. While encroached savannas contained a homogenized understorey, restored sites contained understorey patterning typical of intact savannas. The timeframe for re‐establishment – within years of restoration, after decades of degradation – demonstrates high resiliency and suggests restorability of other highly degraded sites; however, we recognize the importance of prescribed fire for maintaining open savanna structure and, probably, promoting further understorey pattern development.

Consumers indirectly increase infection risk in grassland food webs

Most pathogens exist within complicated food webs of interacting hosts, vectors, competitors, and predators. Although theory has demonstrated a variety of mechanisms by which predation and competition in food webs can indirectly control infection risk in hosts, there have until now been no experimental tests of this theory. We sampled the effect of long-term exclusion of large vertebrate herbivores on the prevalence of infection by a group of aphid-vectored viruses that infect grasses (barley and cereal yellow dwarf viruses) in an oak savannah in central California. We found that pathogen prevalence was approximately 4-fold higher in the presence of consumers than in areas where they were excluded. Vertebrate consumers did not directly alter infection rates by this aphid-vectored pathogen group, but rather increased infection risk by increasing the relative abundance of highly-competent hosts in the grassland community. This large-scale experiment, measuring changes in host abundance and infection risk in response to altered consumption rates, confirms theoretical predictions that consumers can indirectly increase infection risk by altering the composition of whole communities. Most importantly, these results demonstrate that, even in complex natural communities, alterations to food web composition such as consumer invasion or extinction can lead to significant impacts that cascade throughout entire communities, including changes in infection risk.

Dynamics and determinants of Quercus alba seedling success following savanna encroachment and restoration

The scattered tree layer that defines savannas is important for structuring the understory community and determining patterns of overstory recruitment. However, encroachment by woody plants has altered overstory tree densities and regeneration dynamics. We characterized seedling success of the savanna-forming species Quercus alba within Midwestern (USA) oak savannas that had been degraded by encroachment (control; n = 4) or experimentally restored by removal of encroaching woody vegetation (treatment; n = 4). In early 2004, 981 seedlings were transplanted along transects radiating from tree boles of overstory Q. alba trees to inter-canopy gaps and monitored for three growing seasons. Seedlings in restored sites had greater survival (>2×), height growth (by >50%), and basal diameter growth (by >20%). In general, seedling survival and growth parameters increased with distance from overstory trees and were greatest in inter-canopy gaps of restored sites. By the final growing season (2006), the seedling survival-by-distance from tree correlation was stronger in control ( r 2 = 0.25) than treatment sites ( r 2 = 0.18), due to relatively uniform (and greater) survival at all distances from trees in treatment sites. In 2006, growth parameters (seedling height, diameter, Δ height, Δ diameter, and # leaves) were significantly (and more strongly) positively correlated with distance from trees in treatment sites. However, seedling herbivory was also greater after treatment and increased with distance from overstory trees. To understand seedling/microenvironment relationships, we created logistic (survival) and linear regression models (Δ height, Δ basal diameter, # leaves in 2006). Control seedling models had consistently greater predictive power and included more variables, suggesting that savanna restoration may decouple seedlings from their microenvironments, potentially by decreasing competition for limiting resources. Encroachment of the savannas in this study is limiting regeneration of Q. alba, suggesting substantially altered regeneration dynamics from those under which these savannas originally formed. Initial responses from our test of restoration, however, were promising and mechanical encroachment removal may be a means to promote overstory regeneration of this species. Finally, the savannas in this study appear inherently unstable and a scattered canopy tree configuration is unlikely to persist without regular disturbance, even in the restoration sites. Repeated mechanical thinning treatments with selected retention of recruiting Q. alba individuals or reintroduction of understory fire or grazing animals may be potential mechanisms for promoting long-term persistence of savannas at these sites.

Behavior as a tool for assessing a managed landscape: a case study of the Karner blue butterfly

In an increasingly human-dominated landscape, effective management of disturbance-maintained ecosystems, such as grasslands and savannas, is critical to the conservation of biodiversity. Yet, the response of individual organisms to landscapes created by disturbances and management is rarely studied. In this study, we examined the endangered Karner blue butterfly, Lycaeides melissa samuelis, in a heterogeneous oak savanna. Our objective was to quantify the butterfly’s habitat use and behavior to assess the effects of prescribed burning. The oak savanna management in Ohio, USA divides each Karner blue site (n = 4) into three units. Each one-third unit is then burned, mowed, or unmanaged in an annual rotation within each site, and the result is a fire return interval of ~3 years. Our surveys measured habitat use, while behavior observations quantified reproduction and foraging for the two annual broods. Our habitat use results showed burned treatments were recolonized quickly, but there was not a clear selection for burned treatments. Foraging rates were similar in all treatments; however, females oviposited significantly less in unmanaged treatments (only 5 of 127 ovipositions). This oviposition preference was likely due to habitat degradation and the availability of recently burned, early successional habitat. Since Karner blues avoided reproduction in units unburned for ≥4 years, these units could be burned to create high quality early successional habitat. These results demonstrate how behavioral decisions can be pivotal forces driving spatial population dynamics. Our case study demonstrates how a fine-scale landscape perspective combined with measurements of behavioral processes can assist with management decision-making.

Effects of fire frequency on oak litter decomposition and nitrogen dynamics

Fire can alter a multitude of ecosystem properties that have the potential to affect rates of litter decomposition and nitrogen dynamics. In this study, we examined the effect of long-term variation in fire frequency in Minnesota oak savanna on rates of litter mass loss of a common tree species (Quercus ellipsoidalis) to determine how site and intraspecific litter characteristics impacted by variation in fire frequency affect rates of decomposition, litter N dynamics, and litter microbial biomass. Although an increase in fire frequency resulted in higher litter temperatures, lower litter moisture, and decreased soil N and P availability, site characteristics had no net effect on rates of mass loss. Rather, litter C:N ratio, which increased with increased fire frequency, was the dominant predictor of rates of decomposition and litter N dynamics. Increased litter C:N led to decreased rates of decomposition and N immobilization, regardless of the characteristics of the site of decomposition. Therefore, it is the indirect effects of long-term variation in fire frequency on litter characteristics rather than fire's direct effects on site characteristics that determine fire effects on decomposition and N dynamics in this system. Slower rates of decomposition and increased N immobilization of litter produced in frequently burned sites may enhance fire-induced N losses, further decelerating rates of N cycling in frequently burned sites.

Factors Affecting Host-plant Quality and Nectar Use for the Karner Blue Butterfly: Implications for Oak Savanna Restoration

In the Midwestern United States, more than 99.99% of pre-settlement oak (Quercus) savanna has been lost due to agriculture and fire suppression. Thus, the restoration of this ecosystem is imperative to secure the biodiversity, which depends on oak savanna. In this study, we characterized factors affecting the host-plant quality and nectar use of the endangered Karner blue butterfly (Lycaeides melissa samuelis Nabokov) in Ohio. Past research has shown butterfly abundance to be correlated with host-plant quantity, habitat area, and nectar plant abundance. However, there is growing recognition that host-plant quality is important at small spatial scales. We measured host-plant quality by quantifying leaf nitrogen content for the first larval brood and a PCA analysis of nitrogen and water content for the second larval brood. Additionally, observations quantified adult female foraging rates. Our results for the first brood larval stage found no significant difference in leaf nitrogen between burned, mowed, and unmanaged treatments. We used Akaike's Information Criteria (AIC) to determine that host-plant quality for the second brood was primarily explained by herbaceous vegetation density followed by canopy cover and aspect. Greater herbaceous vegetation density, greater canopy cover, and flat/north aspects were associated with higher quality host-plants. Lower host-plant nitrogen for the second brood was accompanied by a greater adult foraging rate. Management of Karner blue habitats should include restoring areas with a compatible herbaceous structure and increasing historically abundant forbs, which provide nectar to second brood Karner blues. This ecosystem-based management should positively impact many species in this rare oak savanna community.

Patterns in bird community structure related to restoration of Minnesota dry oak savannas and across a prairie to oak woodland ecological gradient

There is limited understanding of the influence of fire and vegetation structure on bird communities in dry oak (Quercus spp.) savannas of the Upper Midwest and whether bird communities in restored savanna habitats are similar to those in remnant savannas. During the 2001 and 2002 breeding seasons, we examined the relationship between bird communities and environmental variables, including vegetation characteristics and site prescribed-burn frequencies, across a habitat gradient in dry oak savannas in central Minnesota. The habitat gradient we studied went from: (1) prairie to (2) remnant oak savanna to (3) oak woodland undergoing savanna restoration via fire or mechanical removal of woody vegetation to (4) oak woodland. We conducted fixed-radius point counts (n = 120) within habitats with either prairie groundcover or predominately oak canopy. We described canopy and groundcover characteristics at a sub-sample (n = 28) of non-prairie points, and collected canopy and woody species richness data and prescribed-burn frequencies over the past 20 years for all points. Observed bird communities were most strongly correlated with canopy cover and burn frequency and, to a lesser extent, attributes of the shrub component. Most savanna points had bird communities that were distinct from those found at oak woodland or oak woodland points undergoing restoration via burning. Savanna points similar to oak woodland points were in areas managed by periodic cutting rather than burning. Remnant savanna bird communities were more strongly associated with prescribed burning than those in other habitat types, but it appeared that most oak woodlands that had undergone ≥ 20 years of prescribed burning remained ecologically distinct from remnant savannas. This suggests that some savannas that have converted to oak woodlands may exist in an alternative, or stable, ecological state even following extended restoration efforts.

Modest enhancement of nitrogen conservation via retranslocation in response to gradients in N supply and leaf N status

Plant nutrient resorption, a ubiquitous mechanism of nutrient conservation, has often been proposed to be more pronounced in infertile than fertile habitats, and in species common to infertile compared to fertile habitats, because of the presumed advantage when nutrients are scarce. However, previous studies provide weak and inconsistent empirical support for these hypotheses, although few have examined intraspecific variation across well-quantified resource gradients. This study addresses intraspecific patterns of nutrient resorption for eight species across two N availability gradients on similar soils in an N-limited oak savanna ecosystem: a long-term fire frequency gradient with a negatively correlated N fertility gradient and a long-term N fertilization gradient. We hypothesized that both resorption proficiency (the minimum nutrient level retained in a senesced leaf) and efficiency (the proportional change in leaf nutrient concentration) would decrease with increasing soil N availability and plant N status. For the seven non-N fixers, either resorption proficiency or efficiency decreased modestly in treatments with higher N availability. In contrast, the legume Amorpha canescens Pursh had higher N levels in green and senesced leaves, and resorbed N much more weakly than the non-fixers, and did not respond in terms of proficiency or efficiency to soil N availability. Across all species and sites in each N fertility gradient, a scaling analysis showed greater resorption efficiency in plants with lower N concentrations. Our data suggest that species can have modest resorption responses reflective of soil nutrient availability and differences in resorption related to their N economy that represent mechanisms of nutrient conservation in nutrient-limited soils.

Environmentally biased fragmentation of oak savanna habitat on southeastern Vancouver Island, Canada

Quantifying the degree to which natural or protected areas are representative of a specified baseline provides critical information to conservation prioritization schemes. We report results on southeastern Vancouver Island, Canada, where we compared environmental conditions represented across the entire landscape, in oak savanna habitats prior to European settlement (<1850), and in both protected and unprotected oak savannas in the present-day. In this region, oak savannas represent a rare habitat type, harboring many threatened species. Before European settlement, oak savannas occurred in a distinctly different subset of environmental conditions than they do today. Compared to the entire landscape, oak savannas were historically found predominantly in warm, dry, flat, and low-lying areas, but habitat destruction has left oak savannas in largely the exact opposite set of conditions at present. Thus, the range of conditions in both protected and unprotected oak savannas at present are highly unrepresentative of historical conditions. It appears that fire management by indigenous peoples maintained oak savannas historically across large areas of flat low-lying conditions with deep soils, where succession otherwise produces closed coniferous forest. These areas have since been almost entirely converted to agricultural and urban areas, leaving remnant oak savannas largely on steep, rocky hilltops, where the habitat is maintained by shallow soils. Our results provide quantitative guidance for setting conservation priorities for oak savannas in this region, while highlighting the important general issue of the major role traditional land-use practices can play in shaping landscapes, and therefore in influencing the baselines used to set conservation priorities.

A comparison of seed banks across a sand dune successional gradient at Lake Michigan dunes (Indiana, USA)

In habitats where disturbance is frequent, seed banks are important for the regeneration of vegetation. Sand dune systems are dynamic habitats in which sand movement provides intermittent disturbance. As succession proceeds from bare sand to forest, the disturbance decreases. At Indiana Dunes National Lakeshore, we examined the seed banks of three habitat types across a successional gradient: foredunes, secondary dunes, and oak savanna. There were differences among the types of species that germinated from each of the habitats. The mean seed bank density increased across the successional gradient by habitat, from 376 to 433 to 968 seeds m−2, but with foredune and secondary dune seed bank densities being significantly lower than the savanna seed bank density. The number of seeds germinated was significantly correlated with soil organic carbon, demonstrating for this primary successional sequence that seed density increases with stage and age. The seed bank had much lower species richness than that of the aboveground vegetation across all habitats. Among sites within a habitat type, the similarity of species germinated from the seed banks was very low, illustrating the variability of the seed bank even in similar habitat types. These results suggest that restoration of these habitats cannot rely on seed banks alone.

Gradient analysis of an eastern sand savanna's woody vegetation, and its long-term responses to restored fire processes

Little information is available comparing historic and modern sand savannas, and how remnants respond to restored fire. We compared short- and long-term effects of restored fire on the Tefft Savanna, a 197 ha eastern sand savanna in northwest Indiana that had undergone three decades of fire protection. U.S. Public Land Survey data from Tefft in 1833 indicate black and white oak barrens, and pin oak savanna, with trees averaging 50 stems/ha and 4 m 2/ha basal area. We used ordination and a digital elevation model to assess topographic distribution of tree species in 1986. In 1986, we also compared initial effects of high- and low-intensity dormant season fire on woody vegetation among nine blocks containing black oak, white oak, and pin oak stands. Twenty years later, we compared the same blocks, all of which had been burned three times per decade with low-intensity fires. In 1986, black oak, white oak and pin oak occurred across a gradient of decreasing elevation and slope. At that time, unburned black oak and white oak stands averaged >400 stems/ha and about 10 m 2/ha basal area, and their smaller size classes contained non-oak woody vegetation that apparently had invaded with fire exclusion. After initial burns, black oak and white oak stands receiving high-intensity fire averaged <200 stems/ha and had significantly lower oak canopy cover and basal area than unburned stands. Stands receiving low-intensity fire had intermediate oak canopy cover, with basal area similar to unburned stands. Pin oak stands were more fire-resistant, apparently because spring flooding often reduced fire effects. Density, cover and basal area of non-oak tree species were much lower than oaks, and were not reduced by initial burning. Repeated low-intensity burning over 20 years tended to maintain structure caused by initial fires. However, it reduced lower size class stem densities, promoted post-fire sprouting into the shrub layer, and allowed oak basal area to increase in larger size classes. Time since fire regulated shrub layer structure on a 4-year cycle. Density and cover of trees and shrubs returned to pre-burn conditions by the second and fourth growing seasons after fire, respectively, with non-oak tree species exceeding pre-burn cover and density by the fourth season. These results suggest that high-intensity fire is more important than repeated low-intensity burning in structuring and restoring eastern sand savanna, and that non-oak tree species, once established, may be resistant to low-intensity fire.

Observations and stochastic modeling of soil moisture control on evapotranspiration in a Californian oak savanna

The study of water exchange between soil, plants, and the atmosphere in response to seasonal or periodic droughts is critical to modeling the hydrologic cycle and biogeochemical processes in water‐controlled ecosystems. An essential step in such studies is to characterize changes in evaporation and transpiration under water stress. The objectives of this study are to investigate how soil moisture controls the evapotranspiration in a Californian oak savanna that experiences seasonal droughts, using multiyear field observations at the daily and stand scale, and to model these controls stochastically. The influence of soil moisture on evapotranspiration at the stand scale is studied using correlations between tower‐based evapotranspiration measurements and representative soil moisture obtained by aggregating point measurements. The observed pattern of this effect is found in agreement with an existing model that features a linear reduction of the evapotranspiration when soil moisture falls below a critical value. The model parameters are inferred using a Bayesian framework, and they are found to vary from year to year because of climate variability. The comparison between various aggregations of soil moisture at the stand scale from point measurements demonstrates that the spatial variability of the soil moisture and the water uptake capacity limited by the root biomass need be taken into account to produce a model that is most resistant to interannual variability. Finally, the parameterized model is used to predict the actual evapotranspiration with uncertainty estimates determined using the joint distribution of the parameters derived from the Bayesian framework. The satisfactory agreement between the predicted and measured evapotranspiration suggests that the calibrated model can be incorporated into water balance studies in the future.

Woody Encroachment Removal from Midwestern Oak Savannas Alters Understory Diversity across Space and Time

Recovering biodiversity is a common goal during restoration; however, for many ecosystems, it is not well understood how restoration influences species diversity across space and time. I examined understory species diversity and composition after woody encroachment removal in a large-scale savanna restoration experiment in central Iowa, United States. Over a 4-year time series, restoration had profound effects across space and time, increasing richness at local and site-level scales. Restoration sites had increased α (within sample) Simpson’s diversity and α and γ (site level) species richness relative to control sites, although γ and β (among sample) Simpson’s diversity, β richness, and α species evenness were not affected. Changes in richness were driven by graminoids at the α and γ scales and woody species (and some evidence for forbs) at the α scale. Interestingly, indicator species analysis revealed that at least some species from all functional groups were promoted by restoration, although no species were significant indicators of pre-treatment or control sites. Both savanna and nonsavanna species were indicators of restored sites. Restoration promoted exotic species at both scales, although species with spring phenologies were unaffected. Woody encroachment removal may be a means to promote species establishment in savannas; however, in this study, it resulted in establishment and proliferation of native and exotic and savanna and nonsavanna species. Future work might consider reintroduction of key savanna species to supplement those that have established. Work like this demonstrates the utility of restoration experiments for conducting research on large- and multiscale processes, such as species diversity.

Drought during canopy development has lasting effect on annual carbon balance in a deciduous temperate forest

* Climate change projections predict an intensifying hydrologic cycle and an increasing frequency of droughts, yet quantitative understanding of the effects on ecosystem carbon exchange remains limited. * Here, the effect of contrasting precipitation and soil moisture dynamics were evaluated on forest carbon exchange using 2 yr of eddy covariance and microclimate data from a 50-yr-old mixed oak woodland in northern Ohio, USA. * The stand accumulated 40% less carbon in a year with drought between bud-break and full leaf expansion (354 +/- 81 g C m(-2) yr(-1) in 2004 and 252 +/- 45 g C m(-2) yr(-1) in 2005). This was caused by greater suppression of gross ecosystem productivity (GEP; 16% = 200 g) than of ecosystem respiration (ER; 11% = 100 g) by drought. Suppressed GEP was traced to lower leaf area, lower apparent quantum yield and lower canopy conductance. The moisture sensitivity of ER may have been mediated by GEP. * The results highlight the vulnerability of the ecosystem to even a moderate drought, when it affects a critical aspect of development. Although the drought was preceded by rain, the storage capacity of the soil seemed limited to 1-2 wk, and therefore droughts longer than this are likely to impair productivity in the region.

Tree Establishment During Dry Spells at an Oak Savanna in Minnesota

Recent research has challenged the long-standing hypothesis that forests in the Upper Midwest of the United States developed during wetter periods and retreated during dry periods. We explored this debate by examining patterns of tree establishment on an oak savanna in east-central Minnesota within the context of variable moisture availability and fire suppression. We used superposed epoch analyses (SEA) to evaluate the mean moisture conditions for a 21-year window surrounding tree establishment dates. Before effective fire suppression (1809–1939), 24 of 42 trees with pith dates (62%) grew to 30-cm height during dry years (Palmer Drought Severity Index < −1), versus only 5 of 42 (12%) that established in wet years (PDSI > 1). Significantly more trees established during dry periods (negative PDSI values) than would be expected with the proportion of wet-to-dry years (χ2 = 10.738, df = 1, p-value = 0.001). Twenty of the complete sample of 74 trees with pith dates (27%) established during drought in the 1930s. We hypothesize that dry conditions limited plant productivity, which in turn decreased competition between grasses and tree seedlings and reduced rates of accumulation of fine fuels, enabling seedlings to grow tall enough to resist subsequent fires. We recommend SEA as a methodological approach to compare historical climate conditions with the timing of regeneration success in other regions of forest expansion.

Anthropogenic disturbance and the formation of oak savanna in central Kentucky, USA

AbstractAim To deepen understanding of the factors that influenced the formation of oak savanna in central Kentucky, USA. Particular attention was focused on the link between historical disturbance and the formation of savanna ecosystem structure.Location Central Kentucky, USA.Methods We used dendrochronological analysis of tree‐ring samples to understand the historical growth environment of remnant savanna stems. We used release detection and branch‐establishment dates to evaluate changes in tree growth and the establishment of savanna physiognomy. We contrasted our growth chronology with reference chronologies for regional tree growth, climate and human population dynamics.Results Trees growing in Kentucky Inner Bluegrass Region (IBR) savanna remnants exhibited a period of suppression, extending from the establishment date of the tree to release events that occurred c. 1800. This release resulted in a tripling of the annual radial growth rate from levels typical of oaks suppressed under a forest canopy (&lt; 1 mm year−1) to levels typical of open‐grown stems (3 mm year−1). The growth releases in savanna trees coincided with low branch establishment. Over the release period, climatic conditions remained relatively constant and growth in regional forest trees was even; however, the growth increase in savanna stems was strongly correlated with a marked increase in Euro‐American population density in the region.Main conclusions Our data suggest that trees growing in savanna remnants originated in the understorey of a closed canopy forest. We hypothesize that Euro‐American land clearing to create pasturelands released these trees from light competition and resulted in the savanna physiognomy that is apparent in remnant stands in the IBR. Although our data suggest that savanna trees originated in a forest understorey, this system structure itself may have been a result of an unprecedented lack of Native American activity in the region due to population loss associated with pandemics brought to North America by Euro‐Americans. We present a hypothetical model that links human population dynamics, land‐use activities and ecosystem structure. Our model focuses on the following three land‐use eras: Native American habitation/utilization; land abandonment; and Euro‐American land clearance. Ecological understanding of historical dynamics in other ecosystems of eastern North America may be enhanced through recognition of these eras.

Ancient Savannah Roots of the Carbohydrate Revolution in South-Central North America

Along the ecotone between North America’s southern Plains and southeast Woodlands, the oldest known earth ovens with rock heating elements date to 8,000–9,000 years ago. Through the millennia, the number and diversity of earth ovens increased markedly, but they typically functioned as plant-baking facilities, especially for geophytes, plants with underground storage organs. Among the important early Holocene geophytes in oak savannah regions were onions (Allium spp.) and camas (Camassia spp.). In the drier savannahs, including Texas’ Edwards Plateau, agave (Agave lechuguilla) and sotol (Dasylirion spp.), plants with above-ground storage organs, dominated oven cookery. Rock-filled earth ovens were common in comparatively wet regions with sandy soils, including Texas’ Post Oak Savannah, but floral preservation conditions are poor. Ethnobotanically and ecologically derived expectations, however, suggest false garlic (Northoscordum bivalve), onions, and spring beauty (Claytonia virginica) as potential plant food staples. Various plant foods were undoubtedly eaten throughout the pre-Columbian era but geophytes appear to have sparked and sustained an ancient carbohydrate revolution that led to significant early Holocene land-use intensification. Such a profound dietary shift may yet be shown to have played an important role in changes in Native American cranial morphology during the early Holocene.

Grazing removal decreases the magnitude of methane and the variability of nitrous oxide emissions from spring-fed wetlands of a California oak savanna

Spring-fed wetlands are embedded within Californian oak savannas whose understory is dominated by annual grasslands that are grazed by livestock. Because there is mounting pressure to remove livestock from riparian areas in the western U.S., we excluded livestock from one-half of three spring-fed wetlands and monitored greenhouse gas (CH4 and N2O) fluxes in 2000 and 2002. In 2003, we also measured several ecosystem characteristics to help understand treatment differences in gas fluxes. Bootstrapped estimates of mean CH4 and N2O fluxes over the study period showed that these wetlands were sources of CH4 and N2O to the atmosphere; we compare the magnitude of these fluxes to estimates from other wetland studies. Grazing removal decreased the magnitude of CH4 emissions and their variability during our study period. A regression tree analysis showed lower soil temperature and higher soil water content to be the best predictors of lower CH4 emissions, both of which were observed under grazing removal. The magnitude of N2O emissions was not influenced by grazing removal, but fluxes from ungrazed plots were less variable. Grazing exclusion during hot summer months in California should reduce CH4 emissions from spring-fed wetlands, but have little effect on the magnitude of N2O loss to the atmosphere. Implications of climate change for these processes are discussed.

Patterns of oak regeneration in a Midwestern savanna restoration experiment

Savannas’ defining feature is a scattered tree layer. However, woody plants have encroached savannas throughout the world, altering tree densities and potentially modifying regeneration dynamics. We used a replicated large-scale restoration experiment with Midwestern oak savannas (USA) to understand spatial patterns of regeneration by the dominant overstory species, Quercus alba. Q. alba is not only of interest as a savanna-forming species, but also a species of concern due to widespread low rates of regeneration. In this experiment, four sites received a restoration treatment, whereby all encroaching mesophytic trees and large shrubs were mechanically removed, and four sites operated as encroached controls. Within each site, we monitored naturally occurring Q. alba seedlings in plots under overstory trees and in plots in inter-canopy gaps for 4 years, including 1 year before and 3 years after treatment. Seedlings were ∼5× more abundant below overstory Q. alba trees, which we attribute to seed-rain. There was no effect of restoration on seedling densities. Year-to-year survival and survival of seedlings tracked over the course of the study were greater in sites undergoing restoration. Four-year survival rates were 31% in treatment sites and 10% in control sites. Across all sites, seedlings survived equally as well under canopy trees as in inter-canopy gaps. Mean seedling height, basal diameter, and number of leaves were greater in sites undergoing restoration, relative to control sites, and in inter-canopy gaps, relative to under canopy trees. Furthermore, there was an interaction between treatment and plot type, whereby mean seedling attributes were greatest in inter-canopy gaps of treatment sites. Seedlings tracked over the course of this study displayed greater survival, Δ (change in) height, Δ basal diameter, and Δ number of leaves in treatment sites, relative to control sites, and greater survival, Δ basal diameter, and Δ number of leaves in inter-canopy gaps, relative to under canopy trees. Individual seedling survival, Δ basal diameter, and Δ number of leaves were greatest in inter-canopy gaps of treatment sites. Our study suggests that woody encroachment is limiting regeneration of Q. alba in savannas; however, this may be ameliorated by removal of encroaching woody vegetation. Thus, the act of restoring these savannas may hold promise for promoting regeneration of Q. alba. Finally, the restored savannas in this study appear inherently unstable, as seedlings performed best in inter-canopy gaps. Further work with prescribed fire and/or grazing may elucidate stable tree–herbaceous understory coexistence in Midwestern oak savannas.

The Need for Savanna Restoration in the Cross Timbers

Along the prairie/forest transition zone oak savannas have been severely degraded by logging, clearing for agriculture, fire suppression, invasion of exotic plants, and excessive livestock grazing. Savanna shares equal billing with tallgrass prairie as the most threatened plant community in the Midwest. As such, there is increasing interest in restoring these communities. Conservation criteria have not been developed for the post oak (Querces stellata) and blackjack oak (Querces marilandica) savanna of the Cross Timbers. Oak savanna was arguably an important component of the historical Cross Timbers region. Following settlement, overgrazing in conjunction with a decrease in fire frequency and/or intensity has increased the density of oak stands to the point where they resemble closed-canopy forests rather than savanna. This is a threat to the biodiversity of the Cross Timbers. Proactive land management practices are recommended for restoring savanna communities. Such efforts may require thinning-out areas of degraded oak savanna to help re-establish the herbaceous understory. Fire is recommended to restore ecological processes that limit woody plant encroachment and promote biodiversity. Further research should investigate the ecological dynamics and functions of oak savannas, as well as provide further guidelines for its conservation.