Gregory J. Nowacki

The Demise of Fire and “Mesophication” of Forests in the Eastern United States

ABSTRACT A diverse array of fire-adapted plant communities once covered the eastern United States. European settlement greatly altered fire regimes, often increasing fire occurrence (e.g., in northern hardwoods) or substantially decreasing it (e.g., in tallgrass prairies). Notwithstanding these changes, fire suppression policies, beginning around the 1920s, greatly reduced fire throughout the East, with profound ecological consequences. Fire-maintained open lands converted to closed-canopy forests. As a result of shading, shade-tolerant, fire-sensitive plants began to replace heliophytic (sun-loving), fire-tolerant plants. A positive feedback cycle—which we term “mesophication”—ensued, whereby microenvironmental conditions (cool, damp, and shaded conditions; less flammable fuel beds) continually improve for shade-tolerant mesophytic species and deteriorate for shade-intolerant, fire-adapted species. Plant communities are undergoing rapid compositional and structural changes, some with no ecological antecedent. Stand-level species richness is declining, and will decline further, as numerous fire-adapted plants are replaced by a limited set of shade-tolerant, fire-sensitive species. As this process continues, the effort and cost required to restore fire-adapted ecosystems escalate rapidly.

Historical variation in fire, oak recruitment, and post-logging accelerated succession in central Pennsylvania

ABRAMS, M. D. AND G. J. NOWACKI (School of Forest Resources, Ferguson Building, Pennsylvania State University, University Park, PA 16802). Historical variation in fire, oak recruitment, and post-logging accelerated succession in central Pennsylvania. Bull. Torrey Bot. Club 119: 19-28. 1992.-Composition, structure and radial growth patterns were studied in relatively undisturbed, mature mixed-oak (Quercus), valley floor forests and in similar forests extensively logged between 1936-1946 in central Pennsylvania. These data were analyzed in relation to presettlement forest composition and historical fire records to investigate temporal variation in Quercus recruitment versus accelerated succession of more shade tolerant species following logging. Presettlement valley floor forests in the study area were dominated by Quercus alba and Pinus strobus. Recurring logging and fire between 1780-1900 associated with charcoal iron furnace activity increased Quercus and decreased Pinus dominance in second-growth forests established during that period. Between 1908-1989 the total area burned by wildfire throughout Pennsylvania decreased by >99% (from >400,000 ha to <3500 ha per year). The decreased influence of logging and fire this century facilitated recruitment of later successional Acer and Prunus species in Quercus forest understories. Logging of forests in this condition rapidly accelerated the rate of obtaining canopy dominance for A. rubrum, A. saccharum and P. serotina in area forests. This form of disturbance-mediated accelerated succession should be anticipated in a wide variety of forest types with an overstory dominated by early successional species and an understory comprised mainly of later successional species.

Is climate an important driver of post‐European vegetation change in the Eastern United States?

Many ecological phenomena combine to direct vegetation trends over time, with climate and disturbance playing prominent roles. To help decipher their relative importance during Euro-American times, we employed a unique approach whereby tree species/genera were partitioned into temperature, shade tolerance, and pyrogenicity classes and applied to comparative tree-census data. Our megadata analysis of 190 datasets determined the relative impacts of climate vs. altered disturbance regimes for various biomes across the eastern United States. As the Euro-American period (ca. 1500 to today) spans two major climatic periods, from Little Ice Age to the Anthropocene, vegetation changes consistent with warming were expected. In most cases, however, European disturbance overrode regional climate, but in a manner that varied across the Tension Zone Line. To the north, intensive and expansive early European disturbance resulted in the ubiquitous loss of conifers and large increases of Acer, Populus, and Quercus in northern hardwoods, whereas to the south, these disturbances perpetuated the dominance of Quercus in central hardwoods. Acer increases and associated mesophication in Quercus-Pinus systems were delayed until mid 20th century fire suppression. This led to significant warm to cool shifts in temperature class where cool-adapted Acer saccharum increased and temperature neutral changes where warm-adapted Acer rubrum increased. In both cases, these shifts were attributed to fire suppression rather than climate change. Because mesophication is ongoing, eastern US forests formed during the catastrophic disturbance era followed by fire suppression will remain in climate disequilibrium into the foreseeable future. Overall, the results of our study suggest that altered disturbance regimes rather than climate had the greatest influence on vegetation composition and dynamics in the eastern United States over multiple centuries. Land-use change often trumped or negated the impacts of warming climate, and needs greater recognition in climate change discussions, scenarios, and model interpretations.

Exploring the Early Anthropocene Burning Hypothesis and Climate-Fire Anomalies for the Eastern U.S.

This review explores the long-term role of climate versus human activity on vegetation and fire dynamics in the eastern U.S. Early Holocene warming resulted in a conversion of Picea (boreal) to temperate Quercus and Pinus forests when indigenous populations were sparse but charcoal abundances were relatively high, underscoring the importance of climate. Pyrogenic trees also dominated during the middle Holocene Thermal Maximum period, associated with increasing indigenous populations and high charcoal abundance on most sites. During Neoglacial Cooling (3300 to 150 BP) charcoal levels and pyrogenic trees remained high in the central and southern regions apparently due to Native American and early European burning trumping colder climate. In northern regions, oak-pine and charcoal abundance were distributed on intermittent dry and/or Native American sites. High levels of charcoal and pyrogenic species during the early Holocene and Neoglacial Cooling represent important anomalies in the ecological history of the eastern U.S. While the importance of warmer and drier climate is evident throughout, the Early Anthropocene burning hypothesis is plausible for the eastern U.S. where extensive lightning fires are rare, outside of the southeast coastal plain. The cessation of Native American burning and other early European disturbances were transformative to the ecology of the eastern U.S.

Rule-based mapping of fire-adapted vegetation and fire regimes for the Monongahela National Forest

A rule-based approach was employed in GIS to map fire-adapted vegetation and fire regimes within the proclamation boundary of the Monongahela National Forest. Spatial analyses and maps were generated using ArcMap 9.1. The resulting fireadaptation scores were then categorized into standard fire regime groups. Fire regime group V (200+ yrs) was the most common, assigned to more than 510,000 ha, primarily in the Allegheny Mountains Section. Fire regime group I (low & mixed severity, 0-35 years) and fire regime group III (low & mixed severity, 35-200 yrs) were assigned to almost 198,000 ha, primarily in the Ridge and Valley Section and one subsection of the Allegheny Mountains Section. These systems will likely require active silviculture using fire and/or fire surrogates for their maintenance or restoration. The transparent rule-based procedure can be easily modified and, as such, possesses the flexibility for universal application to other ecosystems with similar spatial databases.

An interdisciplinary approach to better assess global change impacts and drought vulnerability on forest dynamics

Studies of tree physiology within and outside the USA have historically focused on the impacts of environmental factors such as light, water and nutrients, and more recently on the global change factors of increasing CO2, ozone, and air and soil temperature, on various tree species ( Kramer and Kozlowski 1979, Norby et al. 1999, Hoeppner and Dukes 2012, Chung et al. 2013). One thing that has plagued tree physiology research is the limited scope and breadth of most individual studies. This normally involves measuring a few physiological parameters, in response to a few treatments or environmental factors, over a relatively short period of time in nonreplicated field or greenhouse settings with a small number of individuals and species, although these issues are certainly not limited to tree physiology ( Hurlbert 1984). Nevertheless, these studies have been invaluable in characterizing the ecophysiological attributes of many tree genera and species worldwide ( Abrams 1990, Burns and Honkala 1990, Prasad et al. 2007–ongoing, Peters et al. 2015). Fortunately, past limitations have been greatly improved upon over the last quarter century by network site initiatives like FACE, AmeriFlux and FluxNet, allowing global scaling of physiological and leaf data from many sites in various forest/vegetation types across several continents ( Schulze et al. 1994, Kattge et al. 2011, Norby and Zak 2011, Boden et al. 2013, Ali et al. 2015). In addition, analysis and synthesis of the numerous small-scale studies and the upscaling of these data have led to huge advances of our understanding of plant form and function at the global level ( Reich et al. 1997, Wright et al. 2004). Large data syntheses have produced more accurate predictions of global primary productivity and future climates, and the role of plant physiology and dynamic vegetation feedbacks in the climate response ( Kattge et al. 2009, Philippon-Berthier et al. 2010). Significant accelerations of learning and scientific advances often take place when science of one field is combined with another ( Linkov et al. 2014). In a recent study, we attempted to expand the knowledge of climate change, human impacts and land-use legacy by merging the fields of tree physiology and forest ecology ( Nowacki and Abrams 2015). This came forth from the realization that the distribution and dominance of each tree species corresponds to an ecophysiological expression and that long-term change in forest ecosystems is directly relatable to the underlying physiological attributes of component species. This analysis can provide a more robust assessment of the role and impacts of the most important drivers of forest dynamics, namely climate change and land-use history. This method also elucidates ecophysiological changes at the forest type and forest biome level by capitalizing on extensive and long-term forest survey records. The results of that study indicate (i) that vegetation changes since European settlement in the eastern USA are caused to a greater extent by anthropogenic alteration of disturbance regimes (e.g., clearing for agriculture, wood harvesting, introduction of nonnative pests and diseases, and fire suppression) than by climate change and (ii) that current vegetation reflects human influence and disturbance history more than the current environment. These findings are a very important counterpoint to the idea of steady-state response to environment that is embedded of many current vegetation models, including climate envelope models and Dynamic Global Vegetation Models ( Cramer et al. 2001, Iverson et al. 2008). The findings also contradict the conclusion of some that climate is the major driver of post-European vegetation change in our study area (cf. Pederson et al. 2015). This includes evidence that increases and decreases in moisture in the recent and more distant past affected forest composition and biomass ( Gustafson and Commentary

Altered Disturbance Regimes: the Demise of Fire in the Eastern United States

We generated a series of maps to help alert and educate people to the pervasiveness of fire regime changes across the eastern United States. Using geographic information systems (GIS), fire regimes were assigned to spatial vegetation databases to depict past and current conditions. Comparisons revealed substantial reductions in fire throughout the East. The most dramatic shifts took place in the former Midwestern grasslands and across a broad swath of southern and central States where pine and oak communities historically dominated. Land-use changes (e.g., agricultural and forest-type conversions) and recent fire suppression largely explain these shifts. Fire regime change was least in northern hardwood systems, in the mixed mesophytic region, and within the Mississippi Embayment. Negative ecological consequences of prolonged fire suppression are mounting while restoration opportunities are waning.