Increased water use efficiency leads to decreased precipitation sensitivity of tree growth, but is offset by high temperatures

Abstract

Both increases in temperature and changes in precipitation may limit future tree growth, but rising atmospheric CO2 could offset some of these stressors through increased plant Water Use Efficiency (WUE). The net balance between the negative impacts of climate change and positive effects of CO2 on tree growth is crucial for ecotones, where increased climate stress could drive mortality and shifts in range. Here, we quantify the effects of climate, stand structure, and rising CO2 on both annual tree-ring growth increment and intrinsic WUE (iWUE) at a savanna-forest boundary in the Upper Midwest United States. Taking a Bayesian hierarchical modelling approach, we find that plant iWUE increased by ~ 16–23% over the course of the twentieth century, but on average, tree-ring growth increments do not significantly increase. Consistent with higher iWUE under increased CO2 and recent wetting, we observe a decrease in sensitivity of tree growth to annual precipitation, leading to ~ 35–41% higher growth under dry conditions compared to trees of similar size in the past. However, an emerging interaction between summer maximum temperatures and annual precipitation diminishes the water-savings benefit under hot and dry conditions. This decrease in precipitation sensitivity, and the interaction between temperature and precipitation are strongest in open canopy microclimates, suggesting that stand structure may modulate response to future changes. Overall, while higher iWUE may provide some water savings benefits to growth under normal drought conditions, near-term future temperature increases combined with drought events could drive growth declines of about 50%.

Pinus resinosa Tree-Ring Latewood Response to Daily-Scale Precipitation Variability at Lake Itasca, Minnesota

“…The declining precipitation sensitivity in red pine latewood co-occurs with climatic changes at ISP that could hypothetically alter the conditions limiting to radial tree growth, including increases in precipitation (Supplementary Figure 1) and minimum temperature (Supplementary Figure 2), and a lengthening freeze-free season (Figure 1C). In addition, increases in atmospheric CO 2 (Heilman et al, 2021), and the local disturbance history at ISP could contribute to the fading strength of the rainfall signal in red pine at this site. Known disturbances at ISP include reduced fire activity and possible changes in understory composition and density since the park's establishment in 1891 (Frissell, 1973), wind events (Webb et al, 2001), and changes in nutrient status (Howard and McLauchlan, 2015).…”

“…As scientists increasingly recognize the prevalence of nonstationary relationships between tree growth and climate (e.g., D 'Arrigo et al, 2008), there has been a growing effort to critically examine the stability of climate-growth relationships over the full instrumental climate record (Wilmking et al, 2020). Some studies in the Great Lakes region demonstrate a decrease in sensitivity of radial tree growth to precipitation and drought (Maxwell et al, 2016;Heilman et al, 2021). Our results, although limited to a single site, are consistent with these studies and show what is, to our knowledge, the first evidence for declining precipitation sensitivity in red pine.…”

“…et al, 2020;Heilman et al, 2021). This "Fading Drought Signal" has been attributed to a lack of recent droughts in the region that could reduce radial growth (Maxwell et al, 2016), as well as higher CO 2 and plant intrinsic Water Use Efficiency (Heilman et al, 2021). If further replicated across space and tree species, a fading drought signal result would underscore the need to better understand the coupled systems linking climate, environment, and tree growth to ensure robust interpretation of tree rings as a proxy record of Great Lakes hydroclimate.…”

“…A possible weakening of the statistical relationship between tree rings and warm-season drought has been detected in recent decades in the Great Lakes region (Maxwell et al, 2016;Maxwell J.T. et al, 2020;Heilman et al, 2021). This "Fading Drought Signal" has been attributed to a lack of recent droughts in the region that could reduce radial growth (Maxwell et al, 2016), as well as higher CO 2 and plant intrinsic Water Use Efficiency (Heilman et al, 2021).…”

Preface: honoring the career of Russell K. Monson

“…It has long been known that vegetation response to environmental variability is revealed by tree rings (e.g., Monson et al 2018) and that a careful study of their anatomy provides critical insight into plant physiological responses to climate. Szejner et al (2021) assess stable isotopes in tree rings to reveal that trees near the arid range limit herald recent megadroughts in the southwestern U.S. Heilman et al (2021) use tree rings to find that plant water use efficiency has increased across the savanna-forest ecotone in the Midwestern U.S., but also show that growth has not and is now less sensitive to precipitation variability, especially in trees growing in open canopy microclimates. All these dynamics play out across large regions, and Fang et al (2021) note that Mediterranean Europe and North Africa have become shrubbier over the satellite record in part because shrubs are more resilient against recent extreme droughts than forests.…”