Variation in Bark Allocation and Rugosity Across Seven Co-occurring Southeastern US Tree Species

Bark is a complex multifunctional structure of woody plants that varies widely among species. Thick bark is a primary trait that can protect trees from heat generated in surface fires. Outer bark on species that allocate resources to thick bark also tends to be rugose, with bark being thickest at the ridges and thinnest in the furrows. Tree diameter or wood diameter is often used as a predictor for bark thickness but little attention has been made on other factors that might affect bark development and allocation. Here we test multiple mixed effect models to evaluate additional factors (height growth rate, measure height) that correlate with bark allocation and present a method to quantify bark rugosity. We focused on seven co-occurring native tree species in the Tallahatchie Experimental Forest in north Mississippi. Approximately ten saplings of Carya tomentosa, Nyssa sylvatica, Prunus serotina, Pinus echinata, Pinus taeda, Quercus marilandica, and Quercus falcata were destructively sampled for stem analyses. Outer bark thickness (OBT) ranged from 0.01 to 0.77 cm with the thickest maximum outer bark occurring on P. taeda (0.77 cm) and the thinnest maximum outer bark occurring on P. serotina (0.17 cm). Our outer bark allocation models suggest that some individuals with rapid height growth allocate less to outer bark in C. tomentosa, N. sylvatica, P. taeda, and P. serotina, but not for P. echinata or either oak species. All species except for C. tomentosa and N. sylvatica showed evidence for outer bark taper, allocating more outer bark at the base of the bole. Inner bark also was tapered in Carya and the oaks. Bark rugosity varied among species from 0.00 (very smooth) to 0.17 (very rugose) with P. Serotina and C. tomentosa having the smoothest bark. OBT was the best fixed effect for all species. Aside from providing data for several important yet understudied species, our rugosity measures offer promise for incorporating into fluid dynamics fire behavior models.

Transition from Fire-Dependent Open Forests: Alternative Ecosystem States in the Southeastern United States

Land use and fire exclusion have influenced ecosystems worldwide, resulting in alternative ecosystem states. Here, I provide two examples from the southeastern United States of fire-dependent open pine and pine-oak forest loss and examine dynamics of the replacement forests, given continued long-term declines in foundation longleaf (Pinus palustris) and shortleaf (Pinus echinata) pines and recent increases in commercial loblolly (Pinus taeda) and slash (Pinus elliottii var. elliottii) pines. Shortleaf pine-oak forest historically may have been dominant on about 32 to 38 million ha, a provisional estimate based on historical composition of 75% of all trees, and has decreased to about 2.5 million ha currently; shortleaf pine now is 3% of all trees in the northern province. Longleaf pine forest decreased from about 30 million ha, totaling 75% of all trees, to 1.3 million ha and 3% of all trees in contemporary forests of the southern province. The initial transition from open pine ecosystems to closed forests, primarily comprised of broadleaf species, was countered by conversion to loblolly and slash pine plantations. Loblolly pine now accounts for 37% of all trees. Loss of fire-dependent ecosystems and their foundation tree species affect associated biodiversity, or the species that succeed under fire disturbance.

Evaluation of Long-Term Shortleaf Pine Progeny Tests in the Ouachita and Ozark National Forests, USA

Between the late 1970s and the early 1990s, the USDA Forest Service installed 155 shortleaf pine (Pinus echinata Mill.) progeny tests in national forests across the Southern Region of the United States. Using control-pollinated crosses from the Mount Ida Seed Orchard, 84 of these progeny tests were established in the Ouachita and Ozark-St. Francis National Forests in Arkansas and Oklahoma. Each of these 84 test locations had, on average, 33 full-sibling families representing three local geographic seed sources (East Ouachita, West Ouachita, and Ozark). Though largely abandoned years ago, the progeny tests that remain provided an opportunity to determine if significant genetic and genetic × environment variance exists for performance traits (d.b.h., tree height, and survival) decades after installation. In 2018 and 2019, we remeasured d.b.h. and height and determined survival in 15 fully stocked progeny tests. Family variances were significant (p < 0.01) for both d.b.h. and height but not for survival (p > 0.05). Seed sources differed significantly (p < 0.05) for d.b.h., with more pronounced latitudinal differences. Additionally, we determined that individual tree and full-sibling family mean heritabilities were moderate (0.15 and 0.72, respectively, for d.b.h and 0.09 and 0.41, for height), suggesting relatively high genetic to environmental variation and good potential for genetic improvement. We also found that shortleaf pine families were broadly adapted in this region since family-by-test variances were non-significant (p > 0.05).

Competition Effects on Growth and Crown Dimensions of Shortleaf and Loblolly Pine in Mature, Natural-Origin, Pine–Hardwood Mixtures of the Upper West Gulf Coastal Plain of Arkansas, USA: A Neighborhood Analysis

The need for a comprehensive and mechanistic understanding of competition has never been more important as plants adapt to a changing environment and as forest management evolves to focus on maintaining and enhancing complexity. With the recent decline in shortleaf pine (Pinus echinata Mill.) land area, it is critical to determine the effects of competition on shortleaf pine and its performance against loblolly pine (Pinus taeda L.), the preferred planted replacement. We evaluate differences in shortleaf and loblolly pine 10 year mean basal area increment (BAI) and crown dimensions across a gradient of neighborhoods. Linear mixed-effects regression models were developed using BAI and several crown metrics as responses and crowding, competitor species abundance and identity, and initial size and species identity of focal tree as predictors. Crowding of focal trees negatively impacted BAI and crown size (p < 0.001, respectively). Although loblolly pine had three times higher BAI as compared to shortleaf pine within similar neighborhoods, BAI was variable, and the crowding effect did not differ between shortleaf and loblolly pine (p ranged from 0.51–0.99). Competitive impacts on focal trees did not differ by competitor identity (p ranged from 0.07–0.70). Distance-independent competition indices better explained the variation in BAI and horizontal crown metrics, while distance-dependent size ratios were more effective at evaluating vertical crown metrics. These findings highlight shortleaf pine competitive potential in mature, natural-origin stands and provide support for the restoration of pine–hardwood and hardwood–pine stratified mixtures as well as management of shortleaf pine at long rotations.

Frequent Prescribed Fire Sustains Old Field Loblolly Pine–Shortleaf Pine Woodland Communities: Results of a 53-Year Study

Frequently burned old field shortleaf pine (Pinus echinata)–loblolly pine (Pinus taeda) woodlands in the southeastern US provide important wildlife habitat and multiple ecosystem services. Because these communities differ in composition of dominant plant species and have different land use legacies than native pine savannas, the ability to prevent encroachment by off-site broadleaf woody tree species using fire alone is in question. We use a long-term fire experiment to demonstrate that old field pine communities have been prevented from transitioning to hardwood forests for over 50 years through judicious application of prescribed fire applied at 1–2 year intervals, whereas communities with three-year fire intervals show signs of transitioning to hardwood forest. We emphasize tailoring fire regimes to particular contexts of land use history to achieve the most historic and sustainable ecosystem structure and function possible for conservation of native flora and fauna. Study Implications: Demonstrating the ability to maintain natural forest structure of old field loblolly pine–shortleaf pine communities in the southeastern US using frequent prescribed fire has implications for the future sustainability of hundreds of thousands of hectares of such land used to provide critical habitat for many species of imperiled and culturally valued wildlife. It also provides insight into restoration of longleaf pine communities on postagricultural land as promoted by multiple highly funded government initiatives. Frequently burned pine savannas and woodlands are resilient to wildfire and sustain natural hydrological cycles, both important for mitigating the effects of global climate change.

Mapping and Monitoring the Canopy Cover and Greenness of Southern Yellow Pines (Loblolly, Shortleaf, and Virginia Pines) in Central-Eastern Tennessee Using Multi-Temporal Landsat Satellite Data

Southern yellow pines such as loblolly, Virginia and shortleaf pines constitute forest products and contribute significantly to the economy of the United States (U.S.). However, little is understood about the temporal change in canopy cover and greenness of southern yellow pines, especially in Tennessee where they are used for timber and pulpwood. This study aims to map and monitor the canopy cover and greenness of southern yellow pines i.e., loblolly (Pinus taeda), shortleaf (Pinus echinata), and Virginia (Pinus Virginiana) pines in the years 1988, 1999 and 2016 in central-eastern Tennessee. Landsat time-series satellite data acquired in December 1988, November 1999 and February 2016 were used to map and monitor the canopy cover and greenness of loblolly, shortleaf and Virginia pines. The classification and mapping of the canopy cover of southern yellow pines were performed using a machine-learning random forest classification algorithm. Normalized Difference Vegetation Index (NDVI) was used to monitor the temporal variation in canopy greenness. In total, the canopy cover of southern yellow pines decreased by about 35% between December 1988 and February 2016. This information could be used by foresters and forest managers to support forest inventory and management.

Overstory Retention and Stock Type Impact Survival and Growth of Underplanted Shortleaf Pine Beneath a Hardwood Canopy

This study was established to evaluate underplanting as a method of reestablishing a shortleaf pine (Pinus echinata Mill.) component to a dry upland hardwood stand in the Piedmont region of the southeastern United States. Replicated treatment plots were harvested to retain four levels (approximately 0, 3, 7, and 10 m2 of basal area per hectare) of residual overstory density. One-year-old containerized seedlings with both smaller (93.4 cm3) and larger (113.1 cm3) plugs and bareroot seedlings were underplanted beneath the residual overstory treatments. After five growing seasons, seedling survival averaged 61% and was not meaningfully affected by residual overstory density. Seedling height growth ranged from 1.42 m to 2.61 m and was inversely related to residual overstory density. Containerized seedlings with larger plugs had the highest survival (77.4%) and best height growth (2.11 m), followed by containerized seedlings with smaller plugs (64.3%, 1.76 m) and bareroot seedlings (40.2%, 1.85 m). The results of this study indicated that underplanting containerized seedlings, particularly those with higher plug volume and greater plug depth, was a suitable option for reestablishing shortleaf pine on drier, hardwood dominated upland sites in the Piedmont. However, even low levels of overstory retention suppressed seedling height growth after a few years. Study Implications The study was conducted on a dry upland site typical of the North Carolina Piedmont. Retaining up to 10 m2 ha–1 of oak and hickory overstory basal area did not strongly affect survival among underplanted shortleaf pine seedlings after five growing seasons. However, overstory cover as low as 3 m2 ha–1 had negative effects on height growth of underplanted seedlings over the same time period. Height growth declined as overstory density increased. Containerized seedlings had better survival than bareroot seedlings. Further improvements in survival and height growth were realized by planting containerized seedlings with higher plug volume and greater plug depth.

Correction to: Revealing historical fire regimes of the Cumberland Plateau, USA, through remnant fire-scarred shortleaf pines (Pinus echinata Mill.)

An amendment to this paper has been published and can be accessed via the original article.

Revealing historical fire regimes of the Cumberland Plateau, USA, through remnant fire-scarred shortleaf pines (Pinus echinata Mill.)

Background Vegetation of the Cumberland Plateau (USA) has undergone dramatic transitions since the last glaciation and particularly since the onset of widespread logging and twentieth century fire exclusion. Shortleaf pine (Pinus echinata Mill.), one of the most fire-dependent conifers in the US, occurs throughout the Cumberland Plateau, but its abundance has declined dramatically since Euro-American settlement and continues to decline. To better understand the historical ecology of fire within the natural range of shortleaf pine, we reconstructed fire regimes at three new sites throughout the central and southern Cumberland Plateau region based on fire scars on shortleaf pine trees. Results Fire event chronologies extended back to the seventeenth century and revealed historical fire regimes that were frequent and dominated by dormant-season and low-severity events. Fires occurred on average every 4.4 to 5.3 years at the study sites before widespread Euro-American settlement, and were more frequent (2.3 to 3.8 years) following settlement. Cumberland Plateau fires may be linked to adjacent ecoregions such as the Eastern Highland Rim to the west. Among all sites, we found that long-term trends in fire activity were similar and fit into a regional waveform pattern of fire activity likely driven by humans (i.e., Native American depopulation, European settlement, and twentieth century fire exclusion). Conclusions The decline in shortleaf pine and other fire-dependent ecosystems across the Cumberland Plateau is due to multiple interacting factors and, based on these data, frequent fire should be considered a historically important ecological driver of these systems.