Plant Hydraulic Stress Explained Tree Mortality and Tree Size Explained Beetle Attack in a Mixed Conifer Forest

Abstract

Drought predisposes conifer forests to bark beetle attacks and mortality. Although plant hydraulic stress mechanistically links to tree mortality, its capacity to predict trees' susceptibility to beetle attacks has not been evaluated. Further, both tree size and water supply could influence plant hydraulic stress, but their relative importance remained unknown. In this study, we modeled plant hydraulic stress of individual trees in a mixed forest of Lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and Subalpine fir (Abies lasiocarpa) in southern Wyoming, using an integrated model of plant hydraulics and hydrology, ground surveys of tree size as well as physiological and geophysical measurements. Based on the established link between plant hydraulic stress and tree mortality, we found interspecific differences in the relative importance of water availability and tree size. Pine mortality was best explained by the combination of tree size and water supply, and fir mortality was best explained by variations in water supply. We next compared the prediction of beetle attack by modeled plant hydraulic stress versus tree size and found tree size best explained beetle attack consistently for all three species. Taken together, our results suggested beetle attack was primarily influenced by beetle preference for large trees, potentially as food sources, rather than more hydraulically stressed trees. These findings highlighted the importance of integrated understanding of biotic/abiotic factors and their mechanistic pathways in order to accurately predict the sustainability of forests susceptible to drought and beetle outbreaks.

Hillslope Hydrology Influences the Spatial and Temporal Patterns of Remotely Sensed Ecosystem Productivity

“…This supported the hypothesis that plants in valleys receive higher water supply and experience less productivity decline when precipitation is low, through lateral flow or soil water storage (Z. Hoylman et al, 2019;Tai et al, 2018Tai et al, , 2019. Areas with higher GPP had higher sensitivity to precipitation (green boxes in Figure 6), consistent with the expectation that greater productivity accumulate and translate into higher biomass, leaf area, and higher water demand and result in faster depletion of water storage during drought (Jump et al, 2017;Tai et al, 2019).…”

“…When sensitivity increased with higher water demand (more positive or less negative β), we expected positive correlations between sensitivity and GPP(green bar in panel e). Tai et al, 2019), resulting in higher sensitivity to precipitation decline (green line in Figure 1c). When there is too much water, increased water supply might further reduce productivity (blue line in Figure 1d), whereas increased water demand might help release the anoxia stress and mediate productivity decline (green line in Figure 1d).…”

“…On one hand, plants in valleys may be more buffered from drought and have lower sensitivity to precipitation (blue line in Figure 1c), due to additional water supply through lateral hydrologic flow (Tai et al, 2018). On the other hand, plants in wet valleys tend to accumulate high biomass and/or leaf area from high productivity and have higher water demand and faster depletion of soil water during drought (Jump et al, 2017; Tai et al, 2019), resulting in higher sensitivity to precipitation decline (green line in Figure 1c). When there is too much water, increased water supply might further reduce productivity (blue line in Figure 1d), whereas increased water demand might help release the anoxia stress and mediate productivity decline (green line in Figure 1d).…”

Parasitism of trees by marmosets (Primates: Callitrichidae) enhance tree turnover

“…We showed that there is a larger proportion of dead trees among gouged trees, evidencing that gouging by marmosets increases the probability of death of the trees. This can be driven either by mechanical damage to the hydraulic system of the tree, interfering with the water supply (physiological stress), or by increasing the exposure of the tree to potential pathogens (Kautz, Meddens, Hall & Arneth, 2017; Tai et al, 2019). Furthermore, this effect is larger on smaller trees.…”

Tree‐gouging by marmosets (Primates: Callitrichidae) enhances tree turnover

“…We showed that there is a larger proportion of dead trees among gouged trees, indicating that gouging by marmosets increases the probability of tree mortality. This could be driven either by mechanical damage to the hydraulic system of the tree, interfering with the water supply (physiological stress), or by increasing the exposure of the tree to pathogens (Kautz, Meddens, Hall, & Arneth, 2017; Tai et al., 2019). Furthermore, mortality is higher for small trees.…”