The rise of the mediocre forest: why chronically stressed trees may better survive extreme episodic climate variability

Abstract

Anthropogenic climate change is a relatively new phenomenon, largely occurring over the past 150 years, and much of the discussion on climate change impacts to forests has focused on long-term shifts in temperature and precipitation. However, individual trees respond to the much shorter impacts of climate variability. Historically, fast growing, fully canopied, non-chronically stressed (NCS) trees (e.g., those with minimal insect or disease damage, growing on high nutrient, moist soils), have been considered the model of individual health, while slower growing, broken crown, chronically stressed (CS) trees growing on nutrient poor, or dry soils, were considered to be unhealthy and prone to stress related mortality. Over the coming years and decades, climate variability (e.g., extreme precipitation events, drought, heat waves) is expected to increase. We hypothesize that as the duration, severity and interaction of stress becomes more pronounced under increasing climate variability, seemingly less healthy, CS trees could begin to survive environmental stress better than their traditionally considered healthy, NCS tree counterparts. We further hypothesize that this shift in survivorship will be attributed to tree response to increasingly extreme climate variability at levels not previously observed in recorded history. While NCS trees can better survive historic episodic stress, we hypothesize that CS trees may be better suited to survive the combination of chronic low-level stress, and extreme episodic stress associated with anthropogenic climate change and increasing climate variability. Specifically, morphological differences such as increased root and reduced leaf mass, and physiological differences such as more oleoresin production, could make slower growing, CS forests more resilient to catastrophic episodic mortality compared to more NCS stands under certain circumstances. General circulation models suggest that climate variability will increase in the future. Therefore, the occurrence of slow growth, CS forest stands out surviving faster growing NCS less stressed stands may become more common place, and eventually lead to “The Age of the Mediocre Forest”. This paper addresses the type of environmental conditions that could create such an inverse in forest health conditions, and the implications that this shift might have in forest genetics and management.