Oak Savanna Research Articles

Responses of oak seedlings to increased herbivory and drought: a possible trade-off?

Anthropogenic disturbances are causing a co-occurring increase in biotic (ungulate herbivory) and abiotic (drought) stressors, threatening plant reproduction in oak-dominated ecosystems. However, we wonder whether herbivory could compensate for the adverse impact of drought by reducing evapotranspiration. Thus, we investigate the isolated and joint effects of herbivory and drought on oak seedlings of two contrasting Mediterranean species that differ in leaf habit and drought resistance. California oak seedlings from the evergreen, and more drought-resistant, Quercus agrifolia and the deciduous Q. lobata (n=387) were assigned to a fully crossed factorial design with herbivory and drought as stress factors. Seedlings were assigned in a greenhouse to 3-4 clipping levels simulating herbivory and 3-4 watering levels, depending on the species. We measured survival, growth, and leaf attributes (chlorophyll, secondary metabolites, leaf area and weight) once a month (May-Sep) and harvested above- and below-ground biomass at the end of the growing season. For both oak species, simulated herbivory enhanced seedling survival during severe drought or delayed its adverse effects, probably due to reduced transpiration resulting from herbivory-induced leaf area reduction and compensatory root growth. Seedlings from the deciduous, and less drought-resistant species, benefitted from herbivory at lower levels of water stress, suggesting different response across species. We also found complex interactions between herbivory and drought on their impact on leaf attributes. In contrast to chlorophyll content which was not affected by herbivory, anthocyanins increased with herbivory - although water stress reduced differences in anthocyanins due to herbivory. Herbivory seems to facilitate Mediterranean oak seedlings to withstand summer drought, potentially alleviating a key bottleneck in the oak recruitment process. Our study highlights the need to consider ontogenetic stages and species-specific traits in understanding complex relationships between herbivory and drought stressors for the persistence and restoration of multi-species oak savannas.

Impact of water stress on Mediterranean oak savanna grasslands productivity: Implications for on-farm grazing management

This study analyzes the productivity of grasslands in a Mediterranean oak-savanna ecosystem, focusing on its linkages with water availability. In these water-controlled ecosystems, grassland environmental preservation and sustainable management depend on quantitatively understanding these links. Productivity and water stress were modeled in southern Spain (2001–2018), integrating meteorological information and MODIS sensor data into a light-use efficiency model and a surface energy balance. The results provided valuable insights into how grasslands behaved during droughts at different spatiotemporal scales. During the most significant droughts, 2004/2005 and 2011/2012, aerial biomass production was reduced by 42 % and 67 %, respectively. The spatial analysis identified the central east side of the region, with low slopes and moderate tree cover, as the most productive area. The biomass production time series classification identified four distinct trends, all showing shifted relationships with similar slopes between production and anomalies of relative evapotranspiration. The seasonal analysis highlighted the importance of autumn, accounting for nearly 30 % of the annual biomass production, which was essential in years with spring water deficits. The proposed methodology provides on-farm grassland production curves depending on water availability (max-mean-min range with a mean error of 15.5 %). Together with weather forecast data, this could help farmers decide on the optimal level of management intensification and stocking rate. Although the regional specificity may limit the study’s direct applicability, this scheme offers valuable metrics that could be adapted to other areas under water scarcity conditions.

Crown fires remove a fire-sensitive canopy dominant from oak-juniper woodlands: results from long-term monitoring of wildfires

BackgroundIn central Texas, re-sprouting oaks (Quercus spp.) co-occur with non-resprouting Ashe juniper (Juniperus ashei) in a mosaic of fire-dependent (oak savanna) and fire-sensitive (oak-juniper woodland) habitats. The region’s mature woodlands are the only nesting habitat for the endangered golden-cheeked warbler (Setophaga chrysoparia). We studied long-term recovery of woodland structure and species composition after single and repeated crown fires on three soil types (mesa, slope, and deep savanna soils).ResultsOn once-burned sites, density and basal area of non-juniper trees (all woody species except juniper) reached or exceeded unburned levels after 14–24 years, indicating successful recruitment (24 years vs unburned, mesa: 481 ± 254 vs 155 ± 137 stems ha−1, 2 ± 1 vs 1 ± 2 m2 ha−1; slope: 910 ± 330 vs 251 ± 103 stems ha−1, 5 ± 2 vs 3 ± 2 m2 ha−1). Ashe juniper, however, remained mostly absent from burned woodlands (juniper tree density, 24 years vs unburned, mesa: 6 ± 10 vs 691 ± 410 stems ha−1; slope: 20 ± 17 vs 731 ± 183 stems ha−1) and total basal area was 47–87% lower than in unburned areas. In formerly fire-suppressed savannas, non-juniper tree density exceeded unburned levels and juniper density recovered the most (24 years vs unburned, non-juniper: 679 ± 250 vs 251 ± 103 stems ha−1; juniper: deep: 50 ± 71 vs 317 ± 297 stems ha−1). Juniper trees were still absent from twice-burned sites in year 11 and understory density was recovering more slowly, at least on slopes (one fire: 224 ± 206 stems ha−1; two fires: 26 ± 47 stems ha−1). Juniper recovery was correlated with distance to the wildfire perimeter, suggesting that regeneration is limited in part by dispersal.ConclusionsWe found successful recruitment of resprouting hardwood species after one and two crown fires, likely due to the low deer densities at Fort Cavazos. In fire-suppressed oak savannas, a single crown fire did not restore savanna structure and Ashe juniper is slowly re-establishing. Long-term restoration of a savanna on these soils will require additional treatments, like repeated prescribed fire. In oak-juniper woodlands, crown fires removed the fire-sensitive Ashe juniper from canopy co-dominance for decades, making the woodlands unsuitable as habitat for golden-cheeked warblers. Given the long-term consequences of crown fires for golden-cheeked warbler habitat, existing mature oak-juniper woodlands should be protected from crown fire.

Effects of prescribed fire on planted oak growth and survival in restored savannas

Most oak savannas in the Midwestern United States have been lost to agriculture and habitat degradation. Because of their rarity and high plant and animal diversity, savannas are often a target for restoration, which frequently relies on the direct planting of oak seedlings to establish the necessary canopy. Returning fire to the system is critical to the herbaceous component, but with planted seedlings, managers risk damaging or killing trees if burning is introduced too soon. We studied the growth and physiological responses of three oak species (Quercus alba, Quercus macrocarpa, and Quercus velutina) to prescribed fire to determine impacts on planted trees. This study utilized two restored oak savanna units that were planted in 1995 and 1998, each with burned and unburned areas. We tracked trees ranging from 0.9 to 29.8 cm in diameter at breast height (DBH) to determine the size threshold above which top kill is unlikely and documented differences in leaf structure and extension growth between the burned and unburned areas. There was no mortality observed. Moreover, no trees larger than 4 cm DBH were top killed by the fire. Fire responses in leaf mass per unit area and chlorophyll content were small and inconsistent across species. However, all oak species grew more in burned areas than trees in adjacent unburned areas. Therefore, the addition of low‐intensity prescribed fire to an oak savanna planting may increase the growth rate of planted trees with minimal risk of mortality once trees have reached sufficient size.

Effects of livestock on arthropod biodiversity in Iberian holm oak savannas revealed by metabarcoding

Increasing food production while avoiding negative impacts on biodiversity constitutes one of the main challenges of our time. Traditional silvopastoral systems like Iberian oak savannas (“dehesas”) set an example, where free-range livestock has been reared for centuries while preserving a high natural value. Nevertheless, factors decreasing productivity need to be addressed, one being acorn losses provoked by pest insects. An increased and focalized grazing by livestock on infested acorns would kill the larvae inside and decrease pest numbers, but increased livestock densities could have undesired side effects on ground arthropod communities as a whole. We designed an experimental setup including areas under trees with livestock exclosures of different ages (short-term: 1-year exclusion, long-term: 10-year exclusion), along with controls (continuous grazing), using DNA metabarcoding (mitochondrial markers COI and 16S) to rapidly assess arthropod communities’ composition. Livestock removal quickly increased grass cover and arthropod taxonomic richness and diversity, which was already higher in short-term (1-year exclosures) than beneath the canopies of control trees. Interestingly, arthropod diversity was not highest at long-term exclosures (≥10 years), although their community composition was the most distinct. Also, regardless of treatment, we found that functional diversity strongly correlated with the vegetation structure, being higher at trees beneath which there was higher grass cover and taller herbs. Overall, the taxonomic diversity peak at short term exclosures would support the intermediate disturbance hypothesis, which relates it with the higher microhabitat heterogeneity at moderately disturbed areas. Thus, we propose a rotatory livestock management in dehesas: plots with increased grazing should co-exist with temporal short-term exclosures. Ideally, a few long-term excluded areas should be also kept for the singularity of their arthropod communities. This strategy would make possible the combination of biological pest control and arthropod conservation in Iberian dehesas.

Aspect Differences in Vegetation Type Drive Higher Evapotranspiration on a Pole‐Facing Slope in a California Oak Savanna

AbstractQuantifying evapotranspiration (ET) is critical to accurately predict vegetation health, groundwater recharge, and streamflow generation. Hillslope aspect, the direction a hillslope faces, results in variable incoming solar radiation and subsequent vegetation water use that drive ET. Previous work in watersheds with a single dominant vegetation type (e.g., trees) have shown that equator‐facing slopes (EFS) have higher ET compared to pole‐facing slopes (PFS) due to higher evaporative demand. However, it remains unclear how differences in vegetation type (i.e., grasses and trees) influence ET and water partitioning between hillslopes with opposing aspects. Here, we quantified ET and root‐zone water storage deficits between a PFS and EFS with contrasting vegetation types within central coastal California. Our results suggest that the cooler PFS with oak trees has higher ET than the warmer EFS with grasses, which is counter to previous work in landscapes with a singule dominant vegetation type. Our root‐zone water storage deficit calculations indicate that the PFS has a higher subsurface storage deficit and a larger seasonal dry down than the EFS. This aspect difference in subsurface water storage deficits may influence the subsequent replenishment of dynamic water storage, groundwater recharge and streamflow generation. In addition, larger subsurface water deficits on PFS may reduce their ability to serve as hydrologic refugia for oaks during multi‐year droughts. This research provides a novel integration of field‐based and remotely‐sensed estimates of ET required to properly quantify hillslope‐scale water balances. These findings emphasize the importance of resolving hillslope‐scale vegetation structure within Earth system models, especially in landscapes with diverse vegetation types.

Inclusion of bedrock vadose zone in dynamic global vegetation models is key for simulating vegetation structure and function

Abstract. Across many upland environments, soils are thin and plant roots extend into fractured and weathered bedrock where moisture and nutrients can be obtained. Root water extraction from unsaturated weathered bedrock is widespread and, in many environments, can explain gradients in vegetation community composition, transpiration, and plant sensitivity to climate. Despite increasing recognition of its importance, the “rock moisture” reservoir is rarely incorporated into vegetation and Earth system models. Here, we address this weakness in a widely used dynamic global vegetation model (DGVM; LPJ-GUESS). First, we use a water flux-tracking deficit approach to more accurately parameterize plant-accessible water storage capacity across the contiguous United States, which critically includes the water in bedrock below depths typically prescribed by soil databases. Secondly, we exploit field-based knowledge of contrasting plant-available water storage capacity in weathered bedrock across two bedrock types in the Northern California Coast Ranges as a detailed case study. For the case study in Northern California, climate and soil water storage capacity are similar at the two study areas, but the site with thick weathered bedrock and ample rock moisture supports a temperate mixed broadleaf–needleleaf evergreen forest, whereas the site with thin weathered bedrock and limited rock moisture supports an oak savanna. The distinct biomes, seasonality and magnitude of transpiration and primary productivity, and baseflow magnitudes only emerge from the DGVM when a new and simple subsurface storage structure and hydrology scheme is parameterized with storage capacities extending beyond the soil into the bedrock. Across the contiguous United States, the updated hydrology and subsurface storage improve annual evapotranspiration estimates as compared to satellite-derived products, particularly in seasonally dry regions. Specifically, the updated hydrology and subsurface storage allow for enhanced evapotranspiration through the dry season that better matches actual evapotranspiration patterns. While we made changes to both the subsurface water storage capacity and the hydrology, the most important impacts on model performance derive from changes to the subsurface water storage capacity. Our findings highlight the importance of rock moisture in explaining and predicting vegetation structure and function, particularly in seasonally dry climates. These findings motivate efforts to better incorporate the rock moisture reservoir into vegetation, climate, and landscape evolution models.

Alternating Conditional Expectations: Introducing a Non‐Parametric Statistical Method to Interpret Long‐Term Greenhouse Gas Flux Measurements Over Semi‐Arid and Wetland Ecosystems

AbstractWe explore the potential of using a non‐parametric statistical method called Alternating Conditional Expectations, ACE, to quantify functional relationships in biogeosciences. Here, ACE is used to quantify the non‐linear and multi‐faceted responses of greenhouse gas fluxes to a set of biophysical forcings, when the shapes of those response surfaces are unknown. We evaluated the statistical method over two contrasting ecosystems and two contrasting time steps. One case involved quantifying the biophysical controls of water vapor and carbon dioxide (CO2) fluxes over a semi‐arid oak savanna using daily integrated fluxes. The other case evaluated the responses of CO2 and methane (CH4) flux measurements to a set of biophysical forcings at a restored tidal wetland using thirty‐minute averages. The statistical model, based on 4 independent variables, explained up over 90% of the variation in daily integrated flux densities of water vapor and net carbon dioxide exchange at the savanna site. This fit was defined by distinct non‐linear responses to such drivers as gross primary production, photosynthetically active radiation, air temperature, vapor pressure deficit and soil moisture. At the tidal wetland site, we evaluated net carbon dioxide and methane fluxes with short‐term measurements to capture the influence of rising and falling tides and seasonality in biological activity. The statistical model defined the shape of the forcing of fluxes due to the roles of carbon exudates, water table depth, oxygen level in the water column, temperature and vegetation status. The statistical fits of the greenhouse gas fluxes were less precise than the savanna case. The fetch varies on a run‐to‐run basis as it is comprised of a heterogeneous mosaic of open water and vegetation. Furthermore, it is difficult to monitor the environmental conditions of the archaea and bacteria in the sediments that produce methane and carbon dioxide.

X-ray chemical imaging for assessing redox microsites within soils and sediments

Redox reactions underlie several biogeochemical processes and are typically spatiotemporally heterogeneous in soils and sediments. However, redox heterogeneity has yet to be incorporated into mainstream conceptualizations and modeling of soil biogeochemistry. Anoxic microsites, a defining feature of soil redox heterogeneity, are non-majority oxygen depleted zones in otherwise oxic environments. Neglecting to account for anoxic microsites can generate major uncertainties in quantitative assessments of greenhouse gas emissions, C sequestration, as well as nutrient and contaminant cycling at the ecosystem to global scales. However, only a few studies have observed/characterized anoxic microsites in undisturbed soils, primarily, because soil is opaque and microsites require µm-cm scale resolution over cm-m scales. Consequently, our current understanding of microsite characteristics does not support model parameterization. To resolve this knowledge gap, we demonstrate through this proof-of-concept study that X-ray fluorescence (XRF) 2D mapping can reliably detect, quantify, and provide basic redox characterization of anoxic microsites using solid phase “forensic” evidence. First, we tested and developed a systematic data processing approach to eliminate false positive redox microsites, i.e., artefacts, detected from synchrotron-based multiple-energy XRF 2D mapping of Fe (as a proxy of redox-sensitive elements) in Fe-“rich” sediment cores with artificially injected microsites. Then, spatial distribution of FeII and FeIII species from full, natural soil core slices (over cm-m lengths/widths) were mapped at 1–100 µm resolution. These investigations revealed direct evidence of anoxic microsites in predominantly oxic soils such as from an oak savanna and toeslope soil of a mountainous watershed, where anaerobicity would typically not be expected. We also revealed preferential spatial distribution of redox microsites inside aggregates from oak savanna soils. We anticipate that this approach will advance our understanding of soil biogeochemistry and help resolve “anomalous” occurrences of reduced products in nominally oxic soils.

Soil carbon, nitrogen, and phosphorus storage in juniper–oak savanna: role of vegetation and geology

Abstract. Woody-plant encroachment into grasslands and savannas has been globally widespread during the past century, likely driven by interactions between grazing, fire suppression, rising atmospheric CO2, and climate change. In the southernmost US Great Plains, Ashe juniper and live oak have increased in abundance. To evaluate potential interactions between this vegetation change and the underlying soil parent material on ecosystem biogeochemistry, we quantified soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and δ13C of SOC in soils obtained from trenches passing through grassland, juniper, and oak patches on soils lying atop the respective Edwards and Buda limestone formations in central Texas. Soils on the Edwards formation are more shallow and have more rock outcropping than those on Buda. The δ13C values of SOC under grasslands was −19 ‰, whereas those under woody patches were −21 ‰ to −24 ‰, indicating that wooded areas were relatively recent components of the landscape. Compared with grasslands, areas now dominated by juniper or oak had elevated SOC, TN, and TP storage in soils lying atop Edwards limestone. In Buda soils, only oak patches had increased SOC, TN, and TP storage compared with grasslands. Woody encroachment effects on soil nutrients were higher in soils on the Edwards formation, perhaps because root and litter inputs were more concentrated in the relatively shallow layer of soil atop the Edwards bedrock. Our findings suggest that geological factors should be considered when predicting nutrient store responses in savannas following vegetation change. Given that woody encroachment is occurring globally, our results have important implications for the management and conservation of these ecosystems. The potential interactive effects between vegetation change and soil parent material on C, N, and P storage warrant attention in future studies aimed at understanding and modeling the global consequences of woody encroachment.

Fungal symbionts associate with holm oak tree health in declining oak savannas of the southwest of the Iberian Peninsula

Despite being well adapted to the long seasonal droughts of the Mediterranean Basin, the holm oak (Quercus ilex L. subsp. ballota (Desf.) Samp.) has undergone a gradual decline during the last decades. The reasons of this decline are not fully understood, but evidence suggests that the quality and quantity of the interactions of holm oaks with the soil microbiota are playing a central role in the resilience of this species. With the aim to test the previous statement as our hypothesis, 9 oak savannah sites (called dehesas from here on) were sampled, in which asymptomatic (healthy) and symptomatic (early stage of decline) trees were interspersed. Using high throughput amplicon sequencing, soil bacterial and fungal biota (called soil microbiota from here on) were sequenced. Our results showed that soil fungal community composition differed between healthy and declining trees whereas bacterial communities did not show significant differences. Specifically, the degree of holm-oak defoliation was negatively correlated to the relative abundance of ectomycorrhizal (ECM) fungi and fungal endophytes, suggesting a relation between trees' health and soil fungal symbionts. Although our observational study cannot give a final answer to the directionality of the relationships observed, our results support the idea that small changes in symbiont abundance might be causing holm oaks to lose their ability to withstand the strong environmental pressures.

Nurturing Hope Through Reciprocity

Climate change has become a mainstream concern, with proposed solutions focusing on mitigating the impact of environmental degradation on human lives. I use Toronto’s High Park as a case study to explore why an essential aspect of achieving profound and enduring environmental restoration involves recognizing the deep interconnectedness between human beings and what we commonly refer to as “nature”. High Park’s black oak savannah was managed for thousands of years by Indigenous peoples, who used fire to maintain the savannah’s open canopy and activate seeds. European colonization halted these burns, leading to most of the savannah being lost to closed-canopy forests and invasive plants; globally, less than one percent of oak savannah ecosystems remain. Amid despair, we rekindle our hope through prescribed burns, stewardship programs, and the planting of grasses and wildflowers with long-stand relationships with the land; we know that hope is a discipline where collective commitment and action are essential. Indigenous knowledge and storytelling remind us of our responsibility of reciprocity to the land and all our relations. We sow these seeds of hope and trust they will flourish and generate new life, much like ancestral seeds activated by fire.

Changes in Groundlayer Communities with Variation in Trees, Sapling Layers, and Fires during 34 Years of Oak Savanna Restoration

Changes in Groundlayer Communities with Variation in Trees, Sapling Layers, and Fires during 34 Years of Oak Savanna Restoration

Ecosystem groundwater use enhances carbon assimilation and tree growth in a semi-arid Oak Savanna

Ecosystem reliance on groundwater, defined here as water stored in the saturated zone deeper than one meter beneath the surface, has been documented in many semi-arid, arid, and seasonally-dry regions around the world. In California, groundwater sustains ecosystems and mitigates mortality during drought. However, the effect of groundwater on carbon cycling still remains largely unresolved. Here we use 20 years of eddy covariance, groundwater, and tree growth measurements to isolate the impact of groundwater on carbon cycling in a semi-arid Mediterranean system in California during the summer dry season. We show that daily ecosystem groundwater use increases under positive groundwater anomalies and is associated with increased carbon assimilation and evapotranspiration rates. Negative groundwater anomalies result in significantly reduced ecosystem groundwater uptake, gross primary productivity, and evapotranspiration, with a simultaneous increase in water use efficiency. Three machine learning algorithms better predict gross primary productivity and tree growth anomalies when trained using groundwater data. These models suggest that groundwater has a unique effect on carbon assimilation and allocation to woody growth. After controlling for the effect of soil moisture, which is often decoupled from groundwater dynamics at the site, wet groundwater anomalies increase canopy carbon assimilation by 179.4 ± 25.7 g C m−2 (17 % of annual gross primary productivity) over the course of the summer season relative to dry groundwater anomalies. Similarly, annual tree growth increases by 0.175 ± 0.035 mm (17.7 % of annual growth) between dry and wet groundwater anomalies, independent of soil moisture dynamics. Our results demonstrate the importance of deep subsurface water resources to carbon assimilation and woody growth in dryland systems, as well as the benefits of collocated, long-term eddy covariance and ancillary datasets to improve understanding of complex ecosystem dynamics.

Acorn woodpecker movements and social networks change with wildfire smoke

Climate change has contributed to increased wildfires1,2. Wildfire smoke exposes wildlife to hazards and mortality from particulate matter on a scale larger than the area impacted by fire3,4. Using automated radiotelemetry, we illustrate how smoky conditions are associated with changes in behavior of acorn woodpeckers (Melanerpes formicivorus), a flagship species of oak (Quercus spp.) savannas of western North America. On smoky days, birds spent more time at their home territory and reduced visitation to others, especially to distant territories. Associations between birds decreased, and individuals were less assorted by group in co-visitation networks, suggesting less inter-individual coordination on smoky days. We show that between 2016 and 2020, ∼14% of the acorn woodpecker population in the US experienced fire, potentially exposing on average 89.42% of the range to atmospheric smoke annually. These findings highlight how potential effects of smoke on animal behavior may be widespread and exacerbate negative impacts of increasingly common "megafires", even in fire-adapted ecosystems.

Deciphering the role of fungi in post-fire succession in Canada's Great Lakes–St. Lawrence forest region

Fungi are important drivers of forest health, as well as culturally important for many Indigenous communities. In Canada, increasingly intense and frequent wildfires affect fungal substrates and habitats, environmental conditions, and physiological responses of hosts with which fungi may interact. In Ontario, aggressive fire suppression raises the question of how fungal regeneration, and taxonomic compositions, may be affected when fires do appear on the landscape. This study investigates the fungal regrowth post-fire above and below-ground and the role fungal species play in post fire ecological succession in Southeastern Ontario. The study sites are two black oak savannas in restoration, previously prescribed a burn (Northumberland County, 2022 and 2023), and a recent wildfire burned mixed deciduous forest (Centennial Lake, 2023). We collected fruiting body and soil samples for fungal identification via observational and DNA methods. Above ground vegetation assays were made at 9 burned plots within these three sites and another 8 adjacent unburned (control) plots. We extracted soil DNA and will amplify fungal ITS2 amplicons by PCR. Then, we will sequence the amplified DNA using MiSeq to compare burned and unburned soil fungi. Lastly, we will use statistical methods to differentiate effects from fire and soil texture and type in burned and control plots. Thus far, we found that Pyropyxis rubra, a pyrophilous ascomycete fungus, is extremely abundant just weeks after the lightning-induced fire at Centennial Lake. Our combined observational and quantitative descriptions of above-and below ground soil fungi provide a deeper understanding of how fire affects species composition, fungal reproduction, and population dynamics in Ontario. This research may help inform other successional processes, and fungal harvesting of culturally important species.

Diversification deep in the heart of Texas: seven new grasshopper species and establishment of the Melanoplusdiscolor species group (Orthoptera, Acrididae, Melanoplinae).

Melanoplusdiscolor and Melanopluskendalli were previously placed in the texanus species group. Here seven new species are described from central Texas and the combined nine species placed into the discolor group based on emergence time and shape of the male terminalia and genital structures. Six of these new species are from the Edwards Plateau, a known area of high endemism. Species of the discolor group are inhabitants of shortgrass or mixed-grass prairies, Ashe juniper or oak savannas.

Sipping on Nectar: Butterflies and Flowers in Oak Savannas on Vancouver Island, British Columbia

Photo 1. View of a Garry oak savanna field site in Camrose Park, Victoria, BC, Canada. Oak savannas in the region are characterized by rocky outcroppings, a dense understory of forbs, and an open canopy of mostly Garry oak trees (Quercus garryana) mixed in with Douglas Firs (Pseudotsuga menziesii) and Arbutus (Arbutus menziesii). In front of the treeline, the nectar-less yellow flowers of invasive Scotch Broom (Cytisus scoparius) can be seen. Photo credit: Stephanie A. Rivest. Photo 2. Examples of flowering plants recorded in Garry oak savannas around Victoria, BC. Western Buttercup (top left panel: Ranunculus occidentalis) and Sea Blush (top right panel: Plectritis congesta) are native to the region, while Himalayan Blackberry (bottom right panel: Rubus armeniacus) and Cornflower (bottom right panel: Centaurea cyanus) are non-native. Photo credit: Stephanie A. Rivest. Photo 3. A native Propertius Duskywing (Erynnis propertius) butterfly foraging for nectar on a native White Brodiaea (Triteleia hyacinthine) flower in an oak savanna in Victoria, BC. Photo credit: Stephanie A. Rivest. Photo 4. A native Painted Lady (Vanessa cardui) butterfly foraging for nectar on a native Oceanspray (Holodiscus discolor) flower in an oak savanna in Victoria, BC. Photo credit: Stephanie A. Rivest. Photo 5. A native Lorquin's Admiral (Limenitis lorquini) perched on a researcher's hand during a field survey of an oak savanna in Victoria, BC. The butterfly's proboscis explores the researcher's skin, hoping to collect the sodium from sweat. Photo credit: Stephanie A. Rivest. Source paper: These photographs illustrate the article “Flowering phenology influences butterfly nectar foraging on non-native plants in an oak savanna” by Stephanie A. Rivest, E.M. Wolkovich, and Heather M. Kharouba published in Ecology. https://doi.org/10.1002/ecy.4004

Oak savanna vegetation response to layered restoration approaches: Thinning, burning, and grazing

Temperate savannas are unique, biodiverse ecosystems that have undergone extensive habitat conversion globally. In the midwestern United States, 99% of historic oak savanna area has been lost. Most remaining patches of savanna are degraded due to woody encroachment following the removal of both fire and large herbivore disturbances from the landscape. Restoring degraded savanna remnants is challenging because we lack an understanding of how to best apply contemporary restoration tools to mimic historic disturbance dynamics. To that end, we evaluated the outcomes of ongoing oak savanna restorations that have received a gradient of restoration actions: 1) no management, 2) tree thinning, 3) thinning + burning, and 4) thinning + burning + cattle grazing. We assessed several metrics of restoration success including canopy, shrub, herbaceous, and non-native cover, herbaceous diversity, and plant community composition. We found that layering restoration approaches achieved certain, but not all, structural vegetation goals. Compared to no management, thinning and fire successfully increased canopy openness, herbaceous cover, and herbaceous diversity, but had the unwanted effect of increased shrub cover. The addition of low-intensity cattle grazing did not improve structural outcomes. We also found that each restoration treatment left a unique signature on understory plant community composition. Unmanaged and thin-only treatments were characterized by tree saplings and woodland herbs, while burned and grazed treatments were defined by shrubs and savanna-associate species. We conclude that reintroducing multiple disturbances does not guarantee the successful restoration of disturbance-dependent ecosystems such as oak savannas. Restoration outcomes are not dictated by how many management approaches are applied, but rather, the nuances of how they are applied such as burn season and livestock density.

Flowering phenology influences butterfly nectar foraging on non-native plants in an oak savanna.

The negative impacts of non-native species have been well documented, but some non-natives can play a positive role in native ecosystems. One way that non-native plants can positively interact with native butterflies is by provisioning nectar. Relatively little is known about the role of phenology in determining native butterfly visitation to non-native plants for nectar, yet flowering time directly controls nectar availability. Here we investigate the phenological patterns of flowering by native and non-native plants and nectar foraging by native butterflies in an oak savanna on Vancouver Island, British Columbia, Canada. We also test whether native butterflies select nectar sources in proportion to their availability. We found that non-native plants were well integrated into butterfly nectar diets (83% of foraging observations) and that visitation to non-natives increased later in the season when native plants were no longer flowering. We also found that butterflies selected non-native flowers more often than expected based on their availability, suggesting that these plants represent a potentially valuable resource. Our study shows that non-native species have the potential to drive key species interactions in seasonal ecosystems. Management regimes focused on eradicating non-native species may need to reconsider their aims and evaluate resources that non-natives provide.