The potential role of intrinsic processes in generating abrupt and quasi‐synchronous tree declines during the Holocene

Abstract

Multiple abrupt and sometimes near‐synchronous declines in tree populations have been detected in the temperate forests of eastern North America and Europe during the Holocene. Traditional approaches to understanding these declines focus on searching for climatic or other broad‐scale extrinsic drivers. These approaches include multi‐proxy studies that match reconstructed changes in tree abundance to reconstructed changes in precipitation or temperature. Although these correlative approaches are informative, they neglect the potential role of intrinsic processes, such as competition and dispersal, in shaping tree community dynamics. We developed a simple process‐based community model that includes competition among tree species, density‐dependent survival, and dispersal to investigate how these processes might generate abrupt changes in tree abundances even when extrinsic climatic factors do not themselves change abruptly. Specifically, a self‐reinforcing (i.e., positive) feedback between abundance and survival can produce abrupt changes in tree abundance in the absence of long‐term climatic changes. Furthermore, spatially correlated, short‐term environmental variation and seed dispersal can increase the synchrony of abrupt changes. Using the well‐studied, late‐Holocene crash of Tsuga canadensis (eastern hemlock) populations as an empirical case study, we find that our model generates abrupt and quasi‐synchronized crashes qualitatively similar to the observed hemlock patterns. Other tree taxa vary in the frequency and clustering of abrupt change and the proportion of increases and decreases. This complexity argues for caution in interpreting abrupt changes in species abundances as indicative of abrupt climatic changes. Nonetheless, some taxa show patterns that the model cannot produce: observed abrupt declines in hemlock abundance are more synchronized than abrupt increases, whereas the degree of synchronization is the same for abrupt decreases and increases in the model. Our results show that intrinsic processes can be significant contributing factors in abrupt tree population changes and highlight the diagnostic value of analyzing entire time series rather than single events when testing hypotheses about abrupt changes. Thus, intrinsic processes should be considered along with extrinsic drivers when seeking to explain rapid changes in community composition.

Using paleo-archives to safeguard biodiversity under climate change

“…For example, a rapid population increase of eastern hemlock (Tsuga canadensis) near its western range margin was not accompanied by geographic expansion(60), while its geographic distribution held steady during a dramatic range-wide population decline(61). The latter example represents a rapid ecosystem transformation, whereby a dominant conifer (hemlock) was replaced by deciduous trees and pines, forming forests with different structural and functional properties, in response to a contingent series of climatic and ecological processes operating at different temporal and spatial scales(61,63).…”

Climate change, ecosystems and abrupt change: science priorities

“…The palaeoecological record indicates that ACES have occurred repeatedly in many regions (e.g. [45][46][47] (Fagus grandifolia) over the last several thousand years may be due to combined effects of climate change, density-dependent competition among trees and pathogen outbreaks [52]. Over the past several millennia, a subalpine forest in Colorado (USA) abruptly shifted from closed canopy forest to a spatially patterned habitat with open meadows and narrow bands of closed forests [53,54].…”

Paleoecological Investigation of Vegetation, Climate and Fire History in, and Adjacent to, Kootenay National Park, Southeastern British Columbia, Canada

“…to 3,000 cal yr. B.P. years ago (Ramiadantsoa et al, 2019) likely supplied Dog and Marion lakes with a cool water supply from snowpacks melting long into the summer months. These wetter summers account for the low fire frequency and low carbon accumulation rates observed throughout this zone.…”