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

Abstract

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.

Global change and terrestrial plant community dynamics

“…United States, synthesis of 190 tree census datasets from 50 studies comparing contemporary forests to those established before European settlement suggested that a regional increase in cool-adapted, shade-tolerant, fire-sensitive tree taxa, inconsistent with historic climate warming trends, may have resulted from fire suppression (40). Others argued, however, that high moisture during the same time period could explain the observed patterns (41).…”

Half-century evidence from western Canada shows forest dynamics are primarily driven by competition followed by climate

“…Although a large, global body of literature on climate change and forest dynamics has been developed during the last five decades (3,6,9,(11)(12)(13)(14), much of this literature provides insights on how trees respond to climate change without sufficient consideration of competition and other factors (43). Our study showed that accurately quantifying the relative contributions of both endogenous and exogenous forces simultaneously to tree demography is crucial not only for increasing our understanding of the underlying causes of the changes in forest dynamics, but also for adequately predicting the impacts of changing climates on forest dynamics.…”

Forest change in the Driftless Area of the Midwest: From a preferred to undesirable future

“…Climate change models also suggest an expansion in the habitat appropriate for some oak species within the white oak group (Prasad et al, 2007). Findings from a recent study by Nowacki and Abrams (2014), in which the authors evaluated the relative influence of climate and disturbance on forest composition in the eastern U.S, suggest the dominant role of a reduced disturbance regime. A continued decline in the dominance of white oak would decrease the potential for natural regeneration of future oak forests, thus questioning whether some oak species could indeed maintain or increase dominance despite favorable climate conditions, especially in the absence of adequate disturbance.…”