Longleaf Pine Woodlands Research Articles

Fire exclusion alters forest evapotranspiration: A comprehensive water budget analysis in longleaf pine woodlands

AbstractForests are critical to water resources, but high evapotranspiration (ET) can reduce water yield. Thinning and prescribed fire reduce forest density and often reduce ET, promoting higher water yield. However, results from such treatments have been inconsistent, possibly because of unknown interactions among individual ET components. We compare water budget components of longleaf pine (Pinus palustris Mill.) woodlands with frequent prescribed fire to the water budget components of fire‐excluded stands. We hypothesized that fire exclusion would result in higher ET due to increased midstory transpiration (Et) and interception (Ei), and higher evaporation from litter (Ilitter). Reference plots were burned every two years while treatment plots had fire excluded for 15–20 years. Fire treatments were repeated in two sites representing a soil moisture gradient, noted as mesic and xeric. We measured woody Et using sap flux, and we modeled groundcover Et using physiological models. We measured Ei of canopy and groundcover layers, modeled Es litter biomass, and constructed a total component‐based water budget for each site and treatment. Compared with reference plots, midstory Et was 300%–800% higher in fire exclusion plots. Groundcover Et was ~80% less than reference treatments, countering the effects of midstory growth on total ET. Stand Ei followed similar trends, with groundcover Ei in reference plots countering the effects of midstory and litter Ei in fire exclusion plots. As expected, total ET in the xeric site was 18% higher in fire exclusion plots. However, ET in the mesic site was 16% lower in the fire exclusion plots due to high groundcover Et and Ei in reference plots. Thus, our results show that fire exclusion changes total forest ET, but the size and direction of the effect vary depending on the balance between midstory and groundcover transpiration and interception. These results highlight the importance of groundcover in ecosystem function in low‐density forests and may help explain inconsistent results from studies of water yields following thinning and fire. While prescribed fire is a valuable tool in forest management, we suggest that the effects of fire on ET are complex and require careful accounting of all water fluxes within a forest ecosystem.

Embedded Complexity of Evolutionary Sequences.

Multiple pathways and outcomes are common in evolutionary sequences for biological and other environmental systems due to nonlinear complexity, historical contingency, and disturbances. From any starting point, multiple evolutionary pathways are possible. From an endpoint or observed state, multiple possibilities exist for the sequence of events that created it. However, for any observed historical sequence-e.g., ecological or soil chronosequences, stratigraphic records, or lineages-only one historical sequence actually occurred. Here, a measure of the embedded complexity of historical sequences based on algebraic graph theory is introduced. Sequences are represented as system states S(t), such that S(t - 1) ≠ S(t) ≠ S(t + 1). Each sequence of N states contains nested subgraph sequences of length 2, 3, …, N - 1. The embedded complexity index (which can also be interpreted in terms of embedded information) compares the complexity (based on the spectral radius λ1) of the entire sequence to the cumulative complexity of the constituent subsequences. The spectral radius is closely linked to graph entropy, so the index also reflects information in the sequence. The analysis is also applied to ecological state-and-transition models (STM), which represent observed transitions, along with information on their causes or triggers. As historical sequences are lengthened (by the passage of time and additional transitions or by improved resolutions or new observations of historical changes), the overall complexity asymptotically approaches λ1 = 2, while the embedded complexity increases as N2.6. Four case studies are presented, representing coastal benthic community shifts determined from biostratigraphy, ecological succession on glacial forelands, vegetation community changes in longleaf pine woodlands, and habitat changes in a delta.

Differentiating Historical Open Forests and Current Closed Forests of the Coastal Plain, Southeastern USA

The southeastern United States was historically characterized by open forests featuring fire-adapted species before land-use change. We compared tree composition and densities of historical tree surveys (1802 to 1841) to contemporary tree surveys, with the application of a similarity metric, in the Coastal Plain ecological province of Mississippi, southeastern USA. We detected the boundary between historical pine and oak-pine open forests and differentiated historical and current forests. In the Coastal Plain, historical open forests converted from fire-tolerant longleaf pine (Pinus palustris) dominance, with pines comprising 88% of all trees, to loblolly (Pinus taeda) and slash (P. elliottii) pines within monocultures (45% of all trees). Wetland and successional tree species increased to 33% of all trees. Contemporary forests have greater tree densities, transitioning from closed woodlands (range of 168 to 268 trees ha−1) to closed forests (336 trees ha−1). In the ecotonal boundary of the northern Coastal Plain between historical pine and pine-oak woodlands, the pine component shifted over space from 88% to 34% of all trees due to a greater oak component. Fire-tolerant shortleaf pine and oak dominance converted to planted loblolly pine (52% of all trees), while successional tree species increased (20% of all trees). Historical tree densities represented woodlands (range of 144 to 204 trees ha−1) but developed into closed forests (400 trees ha−1). Historical Coastal Plain longleaf pine woodlands differed more from historical ecotonal oak-pine woodlands than contemporary forests differed from each other, demonstrating unique historical ecosystems and landscape-scale homogenization of ecosystems through forestation.

The effects of a moderate severity hurricane on gap characteristics in a longleaf pine (Pinus palustris Mill.) woodland

Modern forestry research emphasizes infusing management practices with an understanding of natural disturbance regimes—often called ecological forestry. Forestry practices that emulate aspects of natural disturbance regimes are considered an effective approach to balance silvicultural and ecological objectives. Silvicultural research is often available to guide successful regeneration in many forest types, but little information is available about gap patterns from common disturbances in the eastern U.S. like hurricanes. We examined the size, shape, and spatial distribution of canopy gaps formed in a longleaf pine woodland by Hurricane Michael across multiple landscape factors including stand size, composition, and soil types. We found high variation in many gap characteristics but no significant differences in gap size or shape among landscape factors. However, spatial distribution of gaps differed among landscape types in nuanced ways. We also found that stand size complexity may prevent the formation of very large gaps that can disrupt fire continuity in systems managed with frequent fire. The results highlight the ecological importance of hurricane events and provide insight into hurricane gap formation at the landscape scale. The implementation of silviculture practices that emulate a large, rapid, single disturbance event may be more practically applied than management based on disturbances such as lightning or insects which occur over longer timeframes.

Plant growth forms influence sandhill longleaf pine regeneration

Longleaf pine researchers have quantified many aspects of regeneration, growth and yield, but have focused on productive sites such as pine flatwoods and upland pine. Sandhill longleaf pine woodlands represent >40 % of remnant stands of longleaf pine ecosystems; most have no history of agriculture due to poor site productivity, yet many have been degraded by resin collection, grazing and fire exclusion. Restoration of structure and function of sandhill longleaf pine depends on understanding early stand-development processes, which are governed by fire, propagule availability, and competition from a range of overstory tree species not limited to longleaf pine. We analyzed a 16-year monitoring dataset from Eglin Air Force Base in northwest Florida, USA, focusing on how four tree growth forms influenced longleaf pine juvenile stages of seedling and sapling numbers and ingrowth to the subadult size class. Analyses were done with generalized linear mixed models using Poisson, normal, or negative binomial error distributions according to the response variable.Restoration activities were effective at reducing numbers of sapling evergreen hardwoods, and basal area of overstory sand pine and other non-longleaf conifers was reduced by 35 % over the 16-year study. We observed threshold values for adult longleaf pine effects on seedlings and ingrowth; seedling numbers were maximized at 7.8 m2/ha, and ingrowth was maximum at 5.1 m2/ha. Seedling density was strongly governed by mast year. Sand pine, an invasive native pine, was detrimental to longleaf pine at all juvenile life stages even at low basal area. Deciduous oaks, which have been observed to facilitate longleaf pine seedling establishment, instead facilitated ingrowth to the subadult size class. Evergreen hardwoods, which may dampen fire behavior, only decreased longleaf pine density at the sapling stage. Ongoing silvicultural restoration activities should continue to focus on frequent fire and sand pine removal; deciduous oaks should be retained for their facilitative value; and removal of evergreen hardwoods should not be a high priority as long as prescribed fire can propagate in these lower productivity sites.

At a fine scale, hardwood patches support wildlife diversity in longleaf pine woodlands

AbstractRestoring and maintaining biodiversity in a changing world is increasingly challenging due to the competing needs of species for suitable space and resources. One ecosystem that has seen considerable anthropogenic changes in extent and structure is the longleaf pine (Pinus palustris) ecosystem. Understanding how wildlife responds to restoration is important to informing forest restoration and conservation. We monitored game birds and mid‐large‐sized mammal occupancy in and around hardwood patches embedded within a longleaf pine woodland at The Jones Center at Ichauway in Newton, GA. We found that 11 species use the transition zone between the longleaf pine and hardwood hammocks. Gray squirrels (Sciurus carolinensis), Virginia opossums (Didelphis virginiana), and nine‐banded armadillos (Dasypus novemcinctus) occupancy increased along the gradient while fox squirrel (Sciurus niger) declined. Our results suggest that oak patches and transitional zones are important to maintaining biodiversity within the longleaf pine ecosystem.

Land-use history, fire regime, and large-mammal herbivory affect deer-preferred plant diversity in longleaf pine woodlands

Since ecological systems often experience multiple disturbances, understanding changes in important ecological interactions, such as plant-herbivore interactions, may require studies capable of disentangling the unique and interactive effects of multiple forms of disturbance. For example, understanding how mammalian herbivores affect plant communities may require understanding how widespread past disturbances, such as agricultural land use, interact with contemporary disturbances, such as prescribed fire. We tested if past agricultural land use and contemporary fire regime modified the effects of mammalian herbivory on focal plant communities by measuring the richness of plant species preferred by deer at 26 longleaf pine woodlands containing paired open and large-mammal exclusion plots. Land-use history significantly affected the community composition of deer-preferred species, and focal species were 8.19 times more likely to be present in post-agricultural sites than non-agricultural sites. Large-mammal herbivory only affected plant species richness in woodlands with low fire frequencies wherein focal species were on average 2.23 times more likely to be present in exclusion plots than open plots. These results suggest that past and present disturbances can mediate contemporary plant-animal interactions and may explain spatial patterns in the intensity of large-mammal herbivory. Our findings also suggest that common management practices, such as use of prescribed burns, may indirectly promote plant species richness by reducing deer herbivory.

Hardwoods influence effect of climate and intraspecific competition on growth of woodland longleaf pine trees

AbstractLongleaf pine woodlands of the North American Coastal Plain are proposed to be resilient to climate change impacts, but little is known about changes in limiting factors to longleaf pine growth as climate has changed in late 20th century and early 21st century. Moreover, the role that neighborhood trees play in the context of climate change remains largely unexplored. We used static and moving window tree ring and climatic analyses to measure the effects of climate on longleaf pine growth at a site in southwest Georgia, USA. We then performed maximum likelihood analysis to examine the influence of neighboring hardwoods on the response of longleaf pine growth to the joint effects of competition and climate. Analysis of climate data from local stations in southwest Georgia over six decades indicated that mean air temperature decreased until the late 20th century then began to rise, and that the variability of spring and summer precipitation has increased. Tree ring and climate analyses indicated longleaf pine radial growth is sensitive to precipitation and air temperature, and that the strength of correlation of longleaf pine growth to summer air temperature and summer precipitation increased since the 1950s. Likelihood models, which were applied over a shorter (23‐year) period and explicitly incorporated competition, did not support a link between summer temperature and growth but did indicate summer precipitation increased growth. Furthermore, basal area (BA) of neighboring hardwoods was correlated with greater pine growth per millimeter of precipitation. BA of neighboring longleaf pine negatively affected growth of conspecific trees; the presence of hardwoods increased the competitive effect when BA of neighboring pine trees was low (<10 m2 ha−1) but decreased the competitive effect when BA of neighboring pine trees was high (≥10 m2 ha−1). These results suggest that retention or recruitment of hardwood trees when restoring longleaf pine woodlands may contribute to increased ability to withstand dry summers and may help to allay concerns of managers that retention of hardwoods will unduly affect growth of residual mature longleaf pines.

Xerophytic hardwood retention promotes competition over facilitation in longleaf pine woodlands in the absence of fire

Recent reports of xerophytic hardwood facilitation of longleaf pine seedlings challenge the traditional paradigm that co-occurring hardwood tree species constrain longleaf pine (Pinus palustris Mill.) regeneration. To better understand this dynamic, we established an experiment examining the effects of hardwood retention duration (no retention, one-year retention, four-year retention), overstory basal area (2-to-41 m2 ha−1), and understory vegetation cover on microclimatic conditions and longleaf pine seedling survival, development, physiological performance, and brownspot needle blight (Mycosphaerella dearnessii) infection in the Sandhills Ecoregion of North Carolina, USA. Light availability (μmol m−2 s−1) at the forest floor (∼20 cm) was reduced by 27% in the four-year retention treatment which significantly exceeded reductions found in the one-year (<1%) and no retention (<1%) treatments, respectively, post midstory removal. However, midstory treatment had little effect on soil moisture and temperature at 15 and 30 cm depth. Longleaf pine seedling survival (85%), aboveground biomass (7.51 g dry weight), and root collar size (1.53 cm) were significantly highest in the no retention treatment compared to the four-year retention treatment. Between treatments with midstory retention (one-year vs. four-year retention), longleaf pine seedlings were statistically larger in the one-year retention treatment (5.37 g dry weight) compared to the four-year retention treatment (4.84 g dry weight). Seedling brownspot infection did not statistically vary among midstory retention treatments. Across all midstory treatments, overstory basal area had a stronger negative effect on longleaf pine seedling aboveground biomass development, root collar development and brownspot infection compared to midstory hardwood stem density or understory vegetation cover. Moreover, for most measured metrics, overstory basal area had a stronger negative effect on longleaf pine physiological performance compared to midstory hardwood stem density. Collectively, these results indicate that hardwood retention did not facilitate longleaf pine survival and development in the grass stage and that asymmetric competition from the overstory represents a stronger impediment to longleaf pine sapling recruitment than midstory hardwoods.

Effects of hurricane canopy gaps on longleaf pine and upland oak sapling growth

Longleaf pine forests and woodlands are an ecologically important ecosystem that once dominated much of the southeastern U.S. Frequent fire is well-known to be a driver of forest dynamics in these systems, but far less is known about how wind disturbance may reorganize plant communities. To gain a better understanding of how hurricane impacts may alter dynamics in a second-growth longleaf pine woodland, we investigated growth response of saplings from three competing upland species (Pinus palustris, Quercus falcata, Q. margaretta) following Hurricane Michael (2018) in southwest Georgia, United States. We used maximum likelihood to model height and radial growth of saplings as species-specific Michaelis-Menten functions of light availability and examined the effect of sapling size, soil water holding capacity, and annual precipitation using a model comparison approach. The best model for both height and radial growth included species-specific growth rates, a power function for sapling size, and an effect of 2-year lagged precipitation. Overall, we found that height growth rate increased with irradiance for all species, but asymptotic height growth rate for all species was approached at low irradiance. Rates of radial growth also increased with irradiance and were more strongly differentiated among species. Radial growth in longleaf pine and southern red oak responded most strongly to increasing irradiance. Our findings suggest that increases in light availability from hurricanes has direct effects on altering growth dynamics of saplings. Further, because stem radius is a strong predictor of survival from surface fires, hurricane damage may also have an indirect effect on sapling dynamics, by altering survival in future fires. Our results suggest that hurricanes can reinforce the dominance of longleaf pine and potentially play a role in stabilizing community structure. Understanding the effects of common disturbances like wind and fire—together and individually—can reveal important mechanisms that shape the structure and function of fire-prone ecosystems.

Improving generalized models of forest structure in complex forest types using area- and voxel-based approaches from lidar

Modeling forest attributes using lidar data has been a useful tool for forest management but the need to correlate lidar to ground-based measurements creates challenges to modeling in diverse forest landscapes. Many lidar models have been based on metrics derived from summarizations of individual lidar returns over sample plot areas, but more recently, metrics based on summarization by volumetric pixel (voxel) have shown promise to better characterize forest structure and distinguish between diverse forest types. Voxel-based metrics may improve characterization of leaf area distribution and horizontal forest structure, which could help create general models of forest attributes applicable in complex landscapes composed of many distinct forest types. We modeled wood volume in longleaf pine woodlands and associated forests to compare how area- and voxel- based lidar metrics predicted wood volume in forest type specific and general predictive models. We created four area-based and six voxel-based metrics to fit models of wood volume using a multiplicative power function. We selected models and compared metric importance using AIC and evaluated model performance using cross-validated mean prediction error. We found that one area-based metric and four voxel-based metrics consistently improved model predictions We suggest that area-based metrics alone may have limitations for characterizing complex forest structure. Area-based summarizes of lidar returns are more heavily influenced by upper canopy returns because lidar returns attenuate below the canopy. By contrast, summarizing lidar returns into a single value per voxel prior to summarization over plots homogenizes point density, giving added weight to sub-canopy returns. Thus voxel-based metrics may be more sensitive to structural variation that may not be adequately captured by area-based metrics alone. This study highlights the potential of voxel-based metrics for characterizing complex forest structure and model generalization capable of accurate forest attribute prediction across diverse forest types.

Stand-level structural characteristics dictate hurricane resistance and resilience more than silvicultural regime in longleaf pine woodlands

Hurricanes frequently impact coastal forests throughout the southeastern United States and are expected to increase in frequency and intensity in the future. Given this projection, interest has grown in adopting forest management practices that can reduce the risk of hurricane damage by increasing stand resistance and resilience. Empirical evidence suggests that uneven-aged stands are more resistant and resilient to hurricanes than even-aged stands, due to their heterogeneous stand structure and multiple age classes. Nevertheless, opportunities to compare the impact of hurricanes on stands managed under different silvicultural regimes at the same location are uncommon. Moreover, no research has detailed how different stocking regulation methods influence hurricane risk in uneven-aged stands. An opportunity to conduct a natural experiment presented itself in 2004 when Hurricane Ivan passed 24 km west of the Escambia Experimental Forest in southern Alabama, USA. At the time of the storm, the experimental forest contained long-term experiments examining the performance of longleaf pine (Pinus palustris) managed under different silvicultural systems. Here, we report on the resistance and resilience of three even-aged stands compared to six uneven-aged stands that were managed under BDq (two stands), Volume Control Guiding Diameter Limit (VC-GDL) (two stands), and Diameter Limit Cutting (DLC) (two stands) stocking control methods.Average stand resistance (proportion of basal area remaining in 2005) was not statistically different among even-aged (0.866)and uneven-aged stands (all variants pooled) (0.858) p > 0.05). Among uneven-aged variants, resistance was highest in the VC-GDL stands (0.876) and lowest in the DLC stands (0.840). Across all stands, resistance was positively related to stand density (p = 0.03) and negatively associated with increases in quadratic mean diameter (p = 0.03) and size class diversity (p = 0.01). Post-hurricane (2005) departure indices found left shifts (shift in density towards smaller size classes) in the diameter distribution of 33 % of even-aged stands and 83 % of pooled uneven-aged stands. Among uneven-aged variants, significant left shifts in diameter distribution were greatest in stands managed with VC-GDL and lowest with BDq. Average post-hurricane resilience (post hurricane basal area recovery) was statistically similar in the pooled uneven-aged stands (0.42) and even-aged stands (0.37). Yet, stands with greater tree size class diversity demonstrated increased resilience. Regardless of management regime, stands carrying a high density of smaller diameter trees are more resistant to hurricane damage than stands managed at a lower density with a higher proportion of larger diameter trees. Moreover, while not statistically significant, our findings suggest that uneven-aged stands possess a greater potential for rapid hurricane recovery than even-aged stands. Therefore, land managers seeking to foster hurricane resistance and resilience should consider modifying stand prescriptions to carry a higher proportion of stocking in smaller size classes.

Interspecific variation in the timing and magnitude of hydraulic redistribution in a forest with distinct water sources

AimsTrees regulate water availability among their rooting strata through a nocturnal, passive transference of water known as hydraulic redistribution (HR). This study investigates differences in HR and groundwater use among common canopy species in longleaf pine (Pinus palustris Mill., Pinaceae) woodlands and explores environmental factors influencing HR.MethodsHR was estimated by sap flux of lateral roots and main stems of three mature canopy species (P. palustris, Quercus laevis Walter., Fagaceae and Quercus margarettae Ashe., Fagaceae). We used δ18O and δD of xylem water, soil water, and groundwater to determine water source. Finally, we related HR to environmental factors (Temperature, VWC, VPD) to better understand controls of HR dynamics.ResultsPinus palustris had higher water use than either Quercus species, and also redistributed significantly more water as a nocturnal subsidy. HR fluxes were inversely related with mean nightly temperature and independent of shallow soil moisture. Stable isotope mixing models, based on δ18O and δD, indicated that all species have access to groundwater, but utilized shallow soil water in differing amounts when available.ConclusionsIn systems with strong water potential gradients among soil strata, any species with access to a groundwater source is likely capable of HR; however, the magnitude of HR varies significantly by species, even among closely related taxa.

Long-Term Recovery of Selected Indicator Species Following Soil Disturbance and Compaction in an Old-Growth Longleaf Pine (Pinus palustris) Woodland

Reference sites with relatively unaltered plant communities are used regularly to establish targets for ecological restoration, yet few are free from anthropogenic disturbances. In this study, we sought to understand the capacity for native community indicators and other plant species to recover from abandoned woods roads and tilled soils within reference sites. We mapped disturbances in a 34 ha portion of the Wade Tract, an old-growth longleaf pine-wiregrass (Pinus palustris-Aristida stricta) woodland using GPS and LiDAR and counted stems of 25 native species (20 woody and 5 herbaceous) in plots within disturbances (n = 32; disturbed), 3 m adjacent to disturbances (n = 32; adjacent), and >5 m from any disturbance (n = 30; reference). Ten species were considered indicator species of intact longleaf pine woodlands. We also quantified soil conditions within the disturbed, adjacent, and reference sites. Disturbed sites had more compacted surface soils than adjacent or reference sites and, in a subset of the sites, soil bulk density and soil strength were greater in disturbed than in adjacent. Indicator species were less frequent in disturbed sites than in adjacent or reference sites (except for Quercus pumila). Three oaks (Q. incana, Q. margaretta, and Q. laevis) and 3 other woody species (Rhus copallinum, Sassafras albidum, and Morella cerifera) were significantly less common in the disturbed sites than in adjacent or reference sites. The near absence of some oaks and other woody species on disturbed sites warrants including these species as indicators of intact regional ground cover for longleaf pine woodlands. Legacies of small-scale soil disturbances in relatively undisturbed perennial-dominated ground cover flora reveals the potential for cumulative degradation over decades and underscores the need to avoid such disturbances in remnant longleaf pine woodlands.

Documenting at‐risk status of terrestrial ecosystems in temperate and tropical North America

AbstractThe International Union for Conservation of Nature (IUCN) Red List of Ecosystems (RLE) is an emerging global standard for ecosystem risk assessment that integrates data and knowledge to document the relative risk status of ecosystem types. Here, we summarize initial findings from applying four IUCN RLE criteria to 655 terrestrial ecosystems in temperate and tropical North America, or 8.5% of the global land surface. A series of indicators are measured for each criterion to address trends in ecosystem extent (A), the relative restricted nature of its distribution (B), and the extent and relative severity of environmental degradation (C), and the extent and relative severity of disruption of biotic processes (D); all to gauge the probability of range wide “collapse.” Ecosystems are listed as collapsed, critically endangered, endangered, vulnerable, near threatened, least concern, data deficient, or not evaluated. Taking uncertainty into account, 219 (33%) of terrestrial ecosystem types were listed as threatened (i.e., either critically endangered, [7%], endangered [14%], or vulnerable [13%]). Examples include tallgrass prairies, oak savannas, longleaf pine woodlands, floodplain forests, mesic hardwood forests, and dry tropical forests. Historically, these threatened ecosystems occurred across about 45% of the continental study area, and today account for about 30%. The RLE provides one important focus for prioritizing conservation effort.

Effects of silvicultural selection treatments on spatial pattern and dynamics in a Pinus palustris Mill. woodland

Natural disturbance-based silviculture emphasizes harvest methods that emulate the timing and structural changes of natural disturbances. Longleaf pine woodlands are ecologically important ecosystems of the southeastern U.S. that support high biodiversity. Options for multi-aged silviculture include individual tree and group selection methods to promote regeneration—the latter method may be modified by retention of reserve trees. To explore the extent to which selection methods are congruent with natural disturbance regimes, we evaluated how treatments in mature second-growth longleaf pine woodlands affected overstory structure, pattern, and dynamics, and we made comparisons to an old-growth longleaf stand. In 2010, stands were harvested using individual tree selection, group selection, or group selection with reserves. We compared treatment effects on spatial pattern of residual trees, recruitment of trees into the 10 cm diameter class (hereafter “recruitment”), and tree mortality 8 years after harvest. Basal area and residual volume were similar among treatments, but the individual tree selection treatment had lower density and a unimodal rather than bimodal diameter distribution compared to other treatments. Group selection and group selection with reserves increased spatial aggregation, compared to individual tree selection which reduced aggregation. Recruitment was similar across treatments but usually occurred near existing trees in the group selection treatments and was further from existing trees in the individual tree selection treatment. Tree mortality primarily occurred as single trees rather than tree groups for all treatments. These results indicate that natural disturbance-based treatments vary in their effects on overstory spatial pattern and alter forest dynamics. In longleaf pine woodlands, the rationale for individual tree selection emphasizes maintaining a continuous input of needles as fine fuels for control of resprouting hardwoods with prescribed fire. This method, may simplify overstory spatial structure and alter forest dynamics after initial harvest. Maintenance of vertical and horizontal complexity is a central tenet of natural disturbance-based management, thus attention to spatial pattern must be given when individual tree selection methods are used. In longleaf pine woodlands, natural disturbance-based techniques such as group selection with reserves may better mimic spatial patterns seen in old-growth stands while preserving continuity of fine fuels.

Hurricane Michael Altered the Structure and Function of Longleaf Pine Woodlands

AbstractTropical cyclones can physically alter ecosystems, causing immediate and potentially long‐lasting effects on carbon dynamics. In 2018, Hurricane Michael hit the southeastern United States with category 5 winds at landfall and category 2 winds reaching over 100 miles inland, resulting in extensive damage. Longleaf pine woodlands in the path of the hurricane were damaged, but severity varied based on the storm track. We used a combination of eddy covariance measurements, airborne LiDAR, and forest inventory data to determine whether hurricane affects structure, function, and recovery of two longleaf pine woodlands at the ends of an edaphic gradient. We found that the carbon sink potentials in both sites were diminished following the storm, with reductions in net ecosystem exchange (NEE) primarily due to lower rates of photosynthesis, as respiration only increased marginally. The xeric site carbon losses and physiological reductions were smaller following the disturbance, which led to the recovery of ecosystem physiological activity to prestorm rates before that of the mesic site, as indicated by maximum ecosystem CO2 uptake rates. Two years following the hurricane both stands continued to have reduced NEE, which signaled altered function. We expect both locations to recover their lost carbon stocks in ∼10–35 years; however, long‐term studies are needed to examine how longleaf woodlands respond to compounding disturbances, such as drought, fire, or other wind storms, which vary significantly across the ecosystem's range. Additionally, hurricanes are intensifying due to climate change, potentially amplifying the degree to which they will alter this ecosystem in the future.

Past agricultural land use affects multiple facets of ungulate antipredator behavior

Abstract Antipredator behavior affects prey fitness, prey demography, and the strength of ecological interactions. Although predator-prey interactions increasingly occur in habitats that experience multiple forms of human-generated disturbance, it is unclear how different forms of disturbance might affect antipredator behavior. Fire is a contemporary disturbance that has dramatic effects on terrestrial habitats. Such habitats may have also experienced past disturbances, like agricultural land use, that leave lasting legacies on habitat structure (e.g., overstory and understory composition). It is unclear how these past and present disturbances affect the use of different antipredator behaviors, like temporal avoidance and vigilance. We examined whether variation in disturbance regimes generates differences in ungulate antipredator behavior by using cameras to measure white-tailed deer vigilance and activity time across 24 longleaf pine woodlands that vary in past land use and contemporary fire regime. Regardless of land-use history, woodlands with high fire frequencies had 4 times less vegetation cover than low-fire woodlands, generating riskier habitats for deer; however, deer responded to fire with different antipredator strategies depending on land-use history. In nonagricultural woodlands, fire affected deer activity time such that activity was nocturnal in low-fire woodlands and crepuscular in high-fire woodlands. In post-agricultural woodlands, fire affected vigilance and not activity time such that deer were more vigilant in high-fire woodlands than in low-fire woodlands. These results suggest that ungulate antipredator behavior may vary spatially depending on past land use and contemporary fire regime, and such disturbances may generate “landscapes of fear” that persist for decades after agricultural use.

Initial plant community responses to hardwood control treatments in restoration of remnant longleaf pine (Pinus palustris) woodlands

Changes in land use over the past century have contributed to substantial losses of longleaf pine (Pinus palustris) woodlands in the southeastern USA and replacement with higher density, mixed pine and hardwood stands that suppress understory development and limit application of prescribed fire. To increase understanding of limiting factors and identify potential approaches for restoring longleaf pine woodlands, we studied initial availability of light and soil water and 2-year (2018–2019) plant community responses after controlling overstory hardwoods in five remnant longleaf pine woodlands having no evidence of previous agriculture at the Savannah River Site near Aiken, SC, USA. Seven hardwood control treatments and a non-treated check were compared in a randomized compete block experiment: cutting, cutting + shredding of logging residues, stem injection with imazapyr herbicide, cutting + basal spray with imazapyr herbicide, cutting + basal spray with triclopyr herbicide, cutting + directed foliar spray with a mixture of glyphosate and imazapyr herbicides, and cutting + broadcast foliar spray with the same herbicide mixture. In the year prior to hardwood cutting (2016), understory light availability averaged 23% of full sun. Throughout the year of treatment installation (2017), average soil water content (SWC) was below permanent wilting point (5.5% SWC) where overstory hardwoods were retained; whereas, it was above wilting point (7.5%) where they had been cut. Thus, combined effects of shade and root competition from overstory hardwoods probably limited cover of herbaceous species in the non-treated check. In the 2 years following treatment, hardwood survival averaged > 99%, 62%, 42% and < 1% for the non-treated check, cutting, shredding, and herbicide treatments, respectively. Relative to the non-treated check, herbaceous species richness was increased by shredding, stem injection, basal spray, or directed foliar treatments; herbaceous cover was increased by basal spray or directed foliar treatments; and woody cover was decreased by directed or broadcast foliar treatments. The directed foliar spray was the most effective treatment for achieving many of the desired understory characteristics of a longleaf pine woodland, including a diverse understory dominated by herbaceous vegetation capable of supporting periodic prescribed fires. Although the imazapyr basal spray treatment increased cover of remnant woodland indicator species, none of the treatments increased richness of this species group, suggesting that restoration of native species composition will be expedited by enrichment seedings or plantings.

Hurricane effects on climate-adaptive silviculture treatments to longleaf pine woodland in southwestern Georgia, USA

Abstract The Adaptive Silviculture for Climate Change (ASCC) network tests silvicultural treatments to promote ‘resistance’ or ‘resilience’ to climate change or speed ‘transition’ to new forest types. Based on projected increases in air temperatures and within-season dry periods in southeastern USA, we installed resistance, resilience and transition treatments involving species selection and varied intensities of density reduction, plus an untreated control, in mixed longleaf pine-hardwood woodland in southwest Georgia USA. Within a year of treatment a tropical cyclone, Hurricane Michael, exposed the site to the unforeseen climatic stress of &amp;gt;44-m s−1 winds. We measured inventory plots post-cyclone and compared the data to pre-storm and pre-treatment values. We analysed stand density index (metric SDI, species maximum value = 1000), stand complexity index (SCI), composition and individual tree characteristics. The ASCC treatments decreased both SDI (from 220 to 124 in the transition treatment) and SCI. The cyclone did not greatly decrease SDI (mean decrease 4.5 per cent) and decreased SCI only in the Controls. Xeric hardwoods were more prone to damage than other functional groups, and ordination showed that the cyclone shifted species composition to greater longleaf pine dominance. Taller trees were more likely to be damaged, except in the resilience treatment, which had a relatively large representation of shorter, more easily damaged xeric hardwoods. The open canopy of the longleaf-hardwood woodland, only 22 per cent of maximum SDI before treatment, evidently fostered wind-firmness, thereby limiting the destructive effect of the cyclone. The sensitivity of xeric hardwoods to hurricane damage suggests that there may be a trade-off between wind tolerance and drought tolerance among functional groups. Maintaining a mixture of drought and wind-resistant species, as in the resilience treatments, may provide broader insurance against multiple climate change impacts in longleaf pine and other forested systems dominated by a single foundation species.