Compounding human stressors cause major regeneration debt in over half of eastern US forests

Abstract

1. The future of temperate forests in the face of global change and anthropogenic stressors remains uncertain. The regeneration stage, which is a critical bottleneck for many organisms, is a key indicator of forest health, future canopy composition and forest adaptive capacity.2. In trees, seemingly healthy forests can be at long-term risk due to insufficient juveniles to replace them (regeneration failure), or compositional differences between juveniles and adults (regeneration mismatch). We propose 'regeneration debt' to collectively describe regeneration failure and mismatch in analogy to extinction debt. To demonstrate this concept, we conducted a macroecological analysis of regeneration debt and anthropogenic stressors in eastern US forests.3. Using U.S. Forest Service-Forest Inventory and Analysis data, we quantified regeneration debt in 18 states from Maine to South Carolina, and evaluated the influence of site, anthropogenic stressors and climate drivers in the most affected regions.4. We identified three distinct regions, with little debt in the north, moderate debt in the south and severe regeneration debt in the central, mid-Atlantic region. In this region, multiple anthropogenic stressors (invasive plants, deer overabundance and land use) were associated with both low-regeneration abundance and the prevalence of disease-prone and/or suboptimal species. Synthesis and applications.Without management intervention, the severe regeneration debt in the mid-Atlantic region will likely lead to long-term declines in forest cover, with cascading negative effects on forest-dependent taxa and ecosystem services. Moreover, the location of the regeneration debt, which is at the northern edge of and involves many of the tree species that are predicted to gain suitable habitat in the Northeastern US, has consequences that extend far beyond its current geographic extent. In fact, this regeneration debt may already be functioning as a barrier to poleward tree migration. Our results demonstrate the value of regeneration debt as an indicator of ecosystem health and forest adaptive capacity. K E Y W O R D Santhropogenic stressors, forest adaptive capacity, forest inventory and analysis, mesophication, regeneration debt, temperate forests, tree migration, tree regeneration

Red oak seedlings as indicators of deer browse pressure: Gauging the outcome of different white‐tailed deer management approaches

“…Despite differences among locations and years, our study demonstrated that deer browse was the overwhelming threat to growth of unprotected Q. rubra seedlings, with rodents and other factors relatively unimportant (Figure ), confirming our second hypothesis. These results align well with results of regional studies (Kelly, ; Miller & McGill, ) and the demographic model for Q. lobata in California (Davis et al, ), all indicating that after successful germination, seedlings are unable grow and transition to larger saplings under high deer browse pressure. This browse (and rodent attack) occurred rapidly in spring and early summer, and we would expect the same to occur for naturally germinating oaks.…”

“…Despite differences among locations and years, our study demonstrated that deer browse was the overwhelming threat to growth of unprotected Q. rubra seedlings, with rodents and other factors relatively unimportant ( Figure 6), confirming our second hypothesis. These results align well with results of regional studies (Kelly, 2019;Miller & McGill, 2019) Note: Initial vegetation height was not significant and dropped from best models. The null model was the best model predicting unknown mortality (for 2010 and 2011) and rodent attack (2011).…”

“…Specifically, recreational hunting was unable to decrease deer densities sufficiently to protect growth of the majority of Q. rubra seedlings, as reported elsewhere (Bengsen & Sparkes, 2016;Blossey et al, 2017;Simard, Dussault, Huot, & Cote, 2013;Williams et al, 2013). This inability of woody species to transition from seedlings to saplings over much of the eastern US, and not just of palatable species (Kelly, 2019;Miller & McGill, 2019), occurs in a region where (Kuznetsova, Brockhoff, & Christensen, 2017). (Brown, Decker, & Kelley, 1984;Decker, Stedman, Larson, & Siemer, 2015).…”

“…Despite repeated calls to adopt accountability and good governance principles in more holistic stewardship and wildlife management (Decker et al, ; Hare & Blossey, ; Leopold et al, ), agencies continue to focus largely on interests of stakeholders who buy hunting and fishing licenses. Our own experience and the overwhelming scientific evidence for the primary role of deer in the deterioration of ecological, economic, and health of our landscapes in the presence of recreational hunting (Côté et al, ; Kelly, ; Kilpatrick et al, ; Miller & McGill, ; Nuttle et al, ; Raizman et al, ) does not bode well for the future, unless major changes are implemented.…”

“…Specifically, recreational hunting was unable to decrease deer densities sufficiently to protect growth of the majority of Q. rubra seedlings, as reported elsewhere (Bengsen & Sparkes, ; Blossey et al, ; Simard, Dussault, Huot, & Cote, ; Williams et al, ). This inability of woody species to transition from seedlings to saplings over much of the eastern US, and not just of palatable species (Kelly, ; Miller & McGill, ), occurs in a region where recreational hunting is widespread, ubiquitous, and accepted by the vast majority of citizens (Brown, Decker, & Kelley, ; Decker, Stedman, Larson, & Siemer, ). Some authors claim that hunting can reduce deer browse pressure on herbaceous and woody species, but browse reductions were either small (Hothorn & Müller, ), or we lack information about differences in hunting pressure in reference areas that also saw improvements in woody and herbaceous plant performance (Jenkins, Jenkins, Webster, Zollner, & Shields, ; Jenkins, Murray, Jenkins, & Webster, ).…”