J. Morgan Varner

The Flammability of Forest and Woodland Litter: a Synthesis

Fire behavior and effects in forests and woodlands are influenced by surface fuels and senesced leaf litter in particular. We have known that species exhibit differential flammability for some time, but isolated efforts have often attributed differences to disparate mechanisms. Recent research has expanded the diversity of species evaluated, clarified patterns at the fuelbed level, and provided evidence that the physical and chemical traits of litter or fuelbeds drive flammability. To date, little effort has focused on uniting methods, clarifying the awkward terminology, or, perhaps most importantly, comparing laboratory findings to field observations of fire behavior. Here, we review recent literature and synthesize findings on what we know about the flammability of litter and propose future research directions.

Clarifying the role of fire in the deciduous forests of eastern North America: reply to Matlack

Fire is an important disturbance in ecosystems across the eastern deciduous forests of North America (Brose et al. 2014). Matlack (2013) provided an interpretation of historical and contemporary fire in this region. Although we applaud Matlack for correcting simplistic assumptions that fire was ubiquitous and all plant communities need to burn regularly to maintain biodiversity, we believe his interpretation of the role of fire is erroneous on several counts. Most problematic is his statement “ . . . it seems prudent to limit the use of prescribed burning east of the prairie-woodland transition zone.” Adherence to this overgeneralized advice would inevitably result in losses of native diversity across the eastern deciduous forest.

Flammability of the keystone savanna bunchgrass Aristida stricta

Understanding the flammability of species in fire-prone or fire-dependent ecosystems is necessary for modeling and predicting ecosystem dynamics. Wiregrass (Aristida stricta syn. A. beyrichiana), a keystone perennial bunchgrass, is a dominant groundcover species in southeastern United States pine savannas. Although wiregrass flammability as a driver of pine savanna fire regimes is a fundamental paradigm in pine savanna dynamics, no studies have quantified its fuel structure and flammability at the individual bunchgrass level. We studied wiregrass flammability at the Aiken Gopher Tortoise Heritage Preserve in Aiken County, South Carolina, USA. We linked tussock fuel structure characteristics (total biomass, live:dead biomass, mass of perched litter and pine needles, moisture content, and bulk density) to flammability (flaming duration, smoldering duration, and flame length). Flame length was strongly and positively related to wiregrass biomass. Pine needles and other litter fuels perched on wiregrass tussocks were not related to flame length, but increased the duration of flaming and smoldering. Within the ranges evaluated, neither fire weather (relative humidity, wind speed, and air temperature) nor fuel moisture significantly affected tussock flammability. Our results indicate that different fuel structural properties drive separate aspects of wiregrass flammability. Together with litter from pines and other groundcover shrubs and trees, wiregrass modifies fire behavior locally, potentially influencing ecosystem dynamics at larger scales. These results have strong implications for southeastern pine savannas and more broadly where grass-dominated vegetation influences fire regimes.

Defining Old-Growth Stand Characteristics in Fragmented Natural Landscapes: A Case Study of Old-Growth Pine in Florida (USA) State Parks

ABSTRACT: Despite historically dominating many landscapes of the southeastern United States, old-growth stands of southern pine (Pinus palustris, P. elliottii var. densa, P. elliottii Engelm. var. elliottii, P. echinata, and P. taeda) are extremely rare and have high ecological value, but may be underrepresented and mischaracterized due to small fragment size and restrictive definitions. Such misunderstanding is a barrier to future old-growth management and conservation. To quantify, characterize, and conserve these remnants, we conducted inventories for five species of old-growth southern pines on 16 sites across ∼44,110 forested ha managed by the Florida Park Service. We used characteristics of old-growth pines and forests from the literature to locate potential sites and assess status and threats. This inventory documented 4697.5 ha of old-growth southern pines in 16 parks throughout Florida. The median size stand we inventoried or included due to existing documentation is 121 ha, with stands as small as eight ha and as large as 2020 ha. Despite their small relative extent, these stands harbor rare, threatened, or endangered species and represent significant conservation areas in the region. A threat common to most of the old-growth stands in Florida State Parks is complications from reintroduction of prescribed fire following prolonged fire exclusion. Heavy fuel loading and invasion of off-site species has resulted in high post-fire mortality linked to both forest floor duff consumption and overstory crown consumption. Management measures are presented to improve conditions of old-growth southern pines in Florida State Parks, and have relevance to broader conservation efforts in the southeastern USA.

Prescribed fire and conifer removal promote positive understorey vegetation responses in oak woodlands

Summary Fire-prone woodlands and savannas world-wide face management challenges resulting from fire exclusion and subsequent encroachment of fire-sensitive trees. In the Pacific Northwest (USA), Quercus garryana oak woodlands and savannas are threatened by encroachment from the native conifer Pseudotsuga menziesii in the absence of fire. In the Bald Hills of Redwood National Park (California, USA), prescribed fire and conifer removal have been used to restore encroached woodlands. We examined the effects of encroachment and restoration on understorey vegetation, comparing four treatments: prescribed fire, prescribed fire and conifer removal, conifer removal, and encroached (control). Treatments including prescribed fire had the greatest native species richness. These two treatments also had the greatest non-native species richness, at both the site level and the treatment level. Woodlands treated with conifer removal and no prescribed fire were intermediate in species richness and diversity compared to burned treatments and encroached woodlands. Encroached woodlands had diminished richness and diversity compared to all restoration treatments. Non-metric multidimensional scaling (NMS) ordination demonstrated that conifer basal area, conifer litter and fine wood were associated with low species richness and diversity and that elevation and thatch were associated with higher species richness and diversity. Indicator species analysis identified that most native species and non-native species were associated with burned woodlands that were never encroached. Synthesis and applications. Our results suggest that both prescribed fire and conifer removal have benefits for understorey plant communities, increasing species richness, diversity and cover in oak woodlands and shifting understorey communities from forest-associated species to more woodland-associated species. Restoration of remnant Quercus garryana oak woodlands is complicated by the persistence and abundance of non-native herbaceous plants.