The legacy of episodic climatic events in shaping temperate, broadleaf forests

Abstract

In humid, broadleaf-dominated forests where gap dynamics and partial canopy mortality appears to dominate the disturbance regime at local scales, paleoecological evidence shows alteration at regional-scales associated with climatic change. Yet, little evidence of these broad-scale events exists in extant forests. To evaluate the potential for the occurrence of large-scale disturbance, we used 76 tree-ring collections spanning ;840 000 km 2 and 5327 tree recruitment dates spanning ;1.4 million km 2 across the humid eastern United States. Rotated principal component analysis indicated a common growth pattern of a simultaneous reduction in competition in 22 populations across 61 000 km 2 . Growth-release analysis of these populations reveals an intense and coherent canopy disturbance from 1775 to 1780, peaking in 1776. The resulting time series of canopy disturbance is so poorly described by a Gaussian distribution that it can be described as ''heavy tailed,'' with most of the years from 1775 to 1780 comprising the heavy-tail portion of the distribution. Historical documents provide no evidence that hurricanes or ice storms triggered the 1775-1780 event. Instead, we identify a significant relationship between prior drought and years with elevated rates of disturbance with an intense drought occurring from 1772 to 1775. We further find that years with high rates of canopy disturbance have a propensity to create larger canopy gaps indicating repeated opportunities for rapid change in species composition beyond the landscape scale. Evidence of elevated, regional-scale disturbance reveals how rare events can potentially alter system trajectory: a substantial portion of old-growth forests examined here originated or were substantially altered more than two centuries ago following events lasting just a few years. Our recruitment data, comprised of at least 21 species and several shade-intolerant species, document a pulse of tree recruitment at the subcontinental scale during the late-1600s suggesting that this event was severe enough to open large canopy gaps. These disturbances and their climatic drivers support the hypothesis that punctuated, episodic, climatic events impart a legacy in broadleaf-dominated forests centuries after their occurrence. Given projections of future drought, these results also reveal the potential for abrupt, meso-to largescale forest change in broadleaf-dominated forests over future decades.

Community ecology in a changing environment: Perspectives from the Quaternary

“…Those historical processes, although they involve both Eltonian and Grinnellian niche attributes, are effectively neutral because the ecological outcomes years, decades, or centuries later cannot be predicted from niche properties and environment alone. Recent paleoecological studies provide many examples (35,84,85).…”

Climatic history of the northeastern United States during the past 3000 years

“…Vegetation is controlled by many factors in addition to climate, however, and would ideally be reserved to evaluate the ecological responses to the climate history rather than used for both climate and ecological reconstruction, especially in a modeling context. Data reflecting hydroclimatic variability are more abundant in the NE US, coming from tree-rings (e.g., Cook andKrusic, 2004, 2008;Pederson et al, 2014), lake levels (e.g., Shuman et al, 2002), variations in lake chemistry (e.g., Li et al, 2007), shifts in diatoms (Boeff et al, 2016), and changes in testate amoeba composition from bog sediments (e.g., Booth et al, 2012). Such data show relatively wet conditions in the late versus the mid-Holocene (Digerfeldt et al, 1992;Almquist et al, 2001;Shuman et al, 2009) with increasing effective moisture towards present (Newby et al, 2014).…”

Long-term droughtiness and drought tolerance of eastern US forests over five decades

“…A major concern is if and when the water stress of future climates exceeds that observed over the previous 120 years, compositional shifts may occur rapidly, which is now apparent in the West (Allen and Breshears, 1998;Allen et al, 2010;Williams et al, 2013). Indeed, the North American Drought Atlas (Cook and Krusic, 2004) and further analysis (Pederson et al, 2014) indicate that 1950-2005 was one of the wettest periods since 1500 over much of the eastern US, suggesting that future drought may have relatively large impacts on eastern forests.…”

“…Cumulative drought severity index 1960-1986 1987-2013 Additionally, the relatively short durations of droughts in the East may have allowed tree species time to recover between prolonged periods of limited water availability (Pederson et al, 2014). However, both droughts and wet conditions have increased in recent decades, and these patterns of extreme climate variability are projected to increase.…”

“…The phenomenon of drought has been widely studied (Palmer, 1965;McKee et al, 1993;Paulo and Pereira, 2006), along with its impacts on forests (McKenzie et al, 2001;Breshears et al, 2005;Allen et al, 2010;Kardol et al, 2010;Pederson et al, 2014). Various studies have also sought to further our knowledge of drought tolerance levels (e.g., indications of stress and survival rates) among tree species (Niinemets and Valladares, 2006;McDowell et al, 2008;Williams et al, 2013).…”