Large‐scale catastrophic disturbance regimes can mask climate change impacts on vegetation – a reply to Pederson et al. (2014)

The Pederson et al. (2014) letter deals ostensibly with their opinion that we (Nowacki & Abrams, 2014) failed to consider the lessening of drought after the 1930s as a major driver of post-European forest dynamics in the eastern U.S. We are befuddled by this because our paper includes long-term precipitation data for four study regions and we report a slight increase (2–8% change) from 1895 to the present. We report a similar range of increase in temperature for that time period. As such, we accounted for these factors in the context of our board-scale study. Our paper discusses major land-use history changes following European settlement, which include the clear-cut and catastrophic fire era from the 1800s to the early 1900s, the chestnut blight starting in the early 1900s, and post-1940 fire suppression. In response to these factors, we report a large decline in conifer species due to their inability to sprout as hardwoods often do, the large expansion of aggressive, pioneer, mostly drought-tolerant, aspen species on highly disturbed sites, the loss of chestnut from the blight, and the general stability of drought-tolerant oak populations from being well adapted to disturbance (Abrams, 1990). It is difficult to assign any of these major forest changes predominantly to the relatively slight increases in precipitation or temperature. We also reported that fire-sensitive maples increased during the post-1940 fire suppression era and that their increase may have been facilitated by increasing precipitation (citing the McEwan et al., 2010 paper). Nevertheless, red maple, a major species increasing in the east, is reasonably drought tolerant (Abrams, 1998) and would have increased in most fire-suppressed forests with or without the realized precipitation increase. As stated above, we also report increases in drought-tolerant aspen, general stability of drought-tolerant oaks, but a decrease in mesophytic white oak (Abrams, 2003), which run counter to the lessening drought hypothesis. Taken as a whole, the Pederson et al. letter does not change our belief that land-use history, rather than climate change, has been the major driver of post-European forest dynamics in the eastern U.S. We are not denying the fact that climate has impacted vegetation dynamics at certain times and places during the Holocene. However, we posit that trying to prove that climate was the major driver after European settlement, particularly after 1850, in the eastern U.S. is problematic and/or confounded from the profound, over-riding influences of a series and reversals of catastrophic disturbances. Indeed, this is the central thesis of our paper. We appreciate the idea that drought conditions have changed in the eastern U.S. during the late Holocene; however, the ecological importance of this idea over large geographic scales still needs to be proven. The climate–disturbance debate is almost as old as the field of ecology. Researchers focusing more on climatic mechanism side, including the cosigners of the Pederson letter, do a disservice by decoupling climate impacts from other disturbance and land-history artifacts and processes. Ironically, the enormity of European disturbances on forest composition and structure has been reported by many of the cosigners of the letter (Iverson, 1988; Mladenoff et al., 1993; Foster et al., 1998, 2003; Schulte et al., 2007). Thus, in a sense, they are effectively arguing against their own past research and findings. Pollen records indicate that strong climate–vegetation relations existed prior to European arrival in certain segments of North America (Shuman et al., 2004). However, climatic controls of vegetation seemingly dissipated during the onset of European settlement, with humans (disturbance) becoming the primarily driver of compositional change and leading to regional vegetation homogenization (Foster et al., 1998; Fuller et al., 1998). Our work corroborates this post-European dissipation of climate–vegetation relationships, whereby compositional changes have been attributed to changes in land-use history rather than climate (Nowacki & Abrams, 2014). In other words, large-scale catastrophic disturbances (e.g., The Great Cutover) and disturbance regime alterations (e.g., the removal of fire from formerly pyrogenic landscapes) can mask climate change impacts on vegetation. In closing, we think our data show that disturbance events can be ubiquitous and still the dominate drivers of vegetation composition and dynamics over large regions, consistent with the previous observations of Dale (1997) and Rhemtulla et al. (2009). We believe that is exactly what happened for the time period and geographic area included in our study. Efforts to better integrate and synthesize these research foci are desperately needed as resource managers go forward in managing for resilience in environments that are subject to a variety of change and perturbations.