Tree mortality response to drought‐density interactions suggests opportunities to enhance drought resistance

Abstract

1. The future of dry forests around the world is uncertain given predictions that rising temperatures and enhanced aridity will increase drought-induced tree mortality. Using forest management and ecological restoration to reduce density and competition for water offers one of the few pathways that forests managers can potentially minimize drought-induced tree mortality. Competition for water during drought leads to elevated tree mortality in dense stands, although the influence of density on heat-induced stress and the durations of hot or dry conditions that most impact mortality remain unclear.2. Understanding how competition interacts with hot-drought stress is essential to recognize how, where and how much reducing density can help sustain dry forests in a rapidly changing world. Here, we integrated repeat measurements of 28,881 ponderosa pine trees across the western US (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) with soil moisture estimates from a water balance model to examine how annual mortality responds to competition, temperature and soil moisture conditions.3. Tree mortality responded most strongly to basal area, and was elevated in places with high mean temperatures, unusually hot 7-year high temperature anomalies, and unusually dry 8-year low soil moisture anomalies. Mortality was also lower in places that experienced unusually wet 3-year soil moisture anomalies between measurements. Importantly, we found that basal area interacts with temperature and soil moisture, exacerbating mortality during times of stress imposed by high temperature or low moisture.4. Synthesis and applications. Our results imply that a 50% reduction in forest basal area could reduce drought-driven tree mortality by 20%-80%. The largest impacts of density reduction are seen in areas with high current basal area and places that experience high temperatures and/or severe multiyear droughts. These interactions between competition and drought are critical to understand past and future patterns of tree mortality in the context of climate change, and provide information for resource managers seeking to enhance dry forest drought resistance.

Sixth Oregon climate assessment

“…Challenges to such predictions in part reflect that climate change interacts with land-use change, other human-caused and natural environmental changes, and interactions among species (Biel and Hacker 2021, Carlson et al 2021, Cinoglu et al 2021, Reed et al 2021. In both aquatic and terrestrial systems, the duration of a climate anomaly, such as a heat wave, also may affect stress, mortality, and population persistence (Bradford et al 2022). Furthermore, responses can vary among events; for example, two apparently similar marine heat waves may not induce the same changes in distribution, abundance, or reproduction of a given species ).…”

Denser forests across the USA experience more damage from insects and pathogens

“…Foremost, the results reinforce the importance of long-standing prescriptions for effective forest management, such as maintaining moderate stand densities through selective thinning, crop tree release, culling of inferior, diseased or damaged trees, and prescribed burning [12,23,27,28]. Forest cover across the USA is relatively stable, while most areas are growing more wood volume than they harvest [46], leading to overly dense forests that are more susceptible to drought and temperature stress associated with a changing climate [9,20,23,38]. Chronic and worsening bark beetle outbreaks and increasing frequency and intensity of wild res are a poignant reminder of this, particularly in conifer forests of the western USA [9,38], while hardwood forests in the eastern USA are increasingly composed of aging tree cohorts of poor quality that are under threat of eventual replacement by less desirable tree species [46].…”

“…While it is logical at local scales to associate the risk and severity of a particular insect or pathogen with the basal area of its susceptible host trees, we propose that total basal area, irrespective of the host status of the trees comprising that total, is a useful predictor of conditions conducive to disturbance in general. The total basal area of a forest stand is an indicator of its vulnerability to tree stress due to competition; in short, a reduction in total basal area decreases competition among the stand's trees for key resources (e.g., water, nutrients, light), decreasing stress and, by extension, the rate of tree mortality [20]. This concept has been explored in the context of facilitating forest adaptation to increased drought under a warming climate [20,21].…”

“…The total basal area of a forest stand is an indicator of its vulnerability to tree stress due to competition; in short, a reduction in total basal area decreases competition among the stand's trees for key resources (e.g., water, nutrients, light), decreasing stress and, by extension, the rate of tree mortality [20]. This concept has been explored in the context of facilitating forest adaptation to increased drought under a warming climate [20,21]. We assert, as have others [22,23], that it has similar implications for forest insect and pathogen disturbances.…”

Short-term severe drought influences root volatile biosynthesis in eastern white pine (Pinus strobus L)

“…It has been speculated that the expected environmental changes will strongly affect growth, reproduction, defense, and communication processes of plants ( Zhang et al., 2020 ; Ullah et al., 2021 ). Drought is a multifaceted environmental restraint that can elicit tree responses from the molecular to the forest level ( Li et al., 2021 ; Bradford et al., 2022 ). It affects soil nutrient availability for plants and the uptake capacities of plant roots, and consequently influences the nutrient status of trees ( Bista et al., 2020 ; Tan et al., 2021 ).…”