Interactive effects of defoliation and water deficit on growth, water status, and mortality of black spruce (Picea mariana (Mill.) B.S.P.)

Abstract

& Key message Defoliation followed by water deficit showed time-dependent effects on plant water status and growth in black spruce (Picea mariana (Mill.) B.S.P.). Biotic stress negatively (during active defoliation by growing instars) and positively (after defoliation) affected plant water relations. However, water deficit, alone or combined with defoliation, prevails over defoliation-related stress for radial growth and sapling vitality. & Context Tree vitality is influenced by multiple factors such as insect damage, water deficit, and the timing of these stresses. Under drought, positive feedback via the reduction of leaf area may improve the water status of defoliated trees. However, the effect on tree mortality remains largely unknown. & Aims We investigated the effects of defoliation followed by a water deficit on tree growth, plant water status, and mortality in black spruce (Picea mariana (Mill.) B.S.P.) saplings. & Methods In a controlled greenhouse setting, saplings were submitted to combined treatments of defoliation and water stress. To assess the impact of these stresses and their interaction, we measured phenology, twig development, secondary growth of the stem, water potential, and mortality of the saplings. & Results Both defoliation and water deficits reduced growth; however, the effect was not additive. During active defoliation, we observed a higher evaporative demand and a lower midday leaf water potential Ψ md . We observed an opposite pattern of response post-stress. Drought alone increased sapling mortality immediately after the stress period, but after c.a. 20 days, mortality rates remained similar following combined drought and defoliation. & Conclusion Our results highlight two key periods during which defoliation affects plant water relations either negatively (during active defoliation) or positively (after defoliation). Mortality in defoliated saplings was reduced immediately following drought because available internal water increased in the stem.

The Threat of the Combined Effect of Biotic and Abiotic Stress Factors in Forestry Under a Changing Climate

“…Defoliation by pests during drought has been thought to reduce evapotranspiration and hence water deficit stress, thus avoiding damage due to the interaction between pests and water deficit stress (Bouzidi et al, 2019). In support of this, the interaction between climatic moisture index and defoliation by pests enhanced the growth of surviving Populus tremuloides despite the negative effect on their survival (Cortini and Comeau, 2020).…”

Variation of Stem Radius in Response to Defoliation in Boreal Conifers

“…A recent study showed that a 2-year defoliation increased internal water availability and reduced mortality in young trees (Bouzidi et al, 2019). Other researches indicated a significant reduction in tree-ring width only after 5-10 years of defoliation, although twig water content, supporting tree growth remained mainly stable with only a slight decrease (Candau et al, 1998;Deslauriers et al, 2015).…”

“…Seasonal change of stem dimensions in mature stands of boreal conifers has been observed during a 3-year drought (Belien et al, 2014). During a 1-year greenhouse studies in young trees repeated once, a 23-day controlled drought reduced radial growth more than a 20-days defoliation (Bouzidi et al, 2019;Deslauriers et al, 2019). To our knowledge, no attention has been paid to stem radius variation according to the level of defoliation in natural stands, which is essential given that a potential decrease in forest productivity is expected during an insect outbreak.…”

“…This could be due to temporal convergence between the appearance of current-year foliage (the optimal foraging source) and active insect feeding period, which determines the success of the insect establishment (Fuentealba et al, 2017b) at the cost of increased defoliation. Thus, mismatched host-insect phenology may reduce the negative effects of defoliation (Ren et al, 2020), potentially promoting physiological reactions in trees (Deslauriers et al, 2015(Deslauriers et al, , 2019Bouzidi et al, 2019). Some of these physiological reactions concern plant water status during and after the defoliation peak: leaf water potential decreases during active defoliation.…”

Linking radial growth patterns and moderate‐severity disturbance dynamics in boreal old‐growth forests driven by recurrent insect outbreaks: A tale of opportunities, successes, and failures

“…Forest management based on short‐rotation (70–100 years) clear‐cutting is the main cause of old‐growth forest loss because these stands are harvested first and at rate greater than that of primary disturbances (Barrette et al., 2018; Bergeron et al., 2017; Martin, Boucher, et al., 2020). In the boreal regions, climate change is also expected to increase the recurrence and severity of disturbances, in particular, spruce budworm outbreaks (Bouzidi et al., 2019; De Grandpré et al., 2018; Pureswaran et al., 2019). Hence, a better understanding of the resistance and resilience of boreal old‐growth forests in eastern Canada to recurrent moderate‐severity disturbances is vital to better evaluate the projected consequences of climate change and propose alternatives to clear‐cut‐based forest management.…”