Prolonged drought duration, not intensity, reduces growth recovery and prevents compensatory growth of oak trees

Highlights

• Drought reduced oak growth but no difference existed among mild to extreme droughts.

• Drought intensity increased recovery, reduced resistance, and increased resilience.

• Prolonged drought duration impaired the recovery and prevented compensatory growth.

Abstract

Global forests suffer from increasing drought pressure, as drought are more intense, and last for longer, and occur more frequently, but how drought intensity and duration affect post-drought growth recovery and compensatory growth is still poorly known. Understanding the immediate and subsequent impacts of drought on tree growth is key to predict future forest dynamics. Here, we used a network of tree-ring data of 735 Quercus mongolica trees from 27 sites of North China representing a large portion of its distribution to study the effects of drought intensity and duration on radial growth. We analyzed growth resistance, recovery, and resilience of Q. mongolica by linking it to drought intensity and duration, and further quantified the post-drought growth trajectories. Our results confirmed that current summer drought is a major influence on radial growth of Q. mongolica across its distribution. While drought occurrence was related to decreased growth even for incipient droughts, we found no differences in growth decline with drought severity. Although drought intensity did not alter growth decline in dry years, drought intensity was positively related to recovery and resilience indices. The damage of recovery is due to the extension of drought duration rather than drought intensity. However, post-drought growth was even more strongly related to drought duration and not intensity. Q. mongolica showed compensatory growth in the year following single-year droughts before returning to pre-drought growth-levels, while multi-year drought led to little to no compensatory growth and prolonged growth reductions. Hence, multi-year drought is the key to predict post-drought recovery of Q. mongolica including compensatory growth in an increasingly arid climate.

Introduction

Droughts caused widespread tree growth decline and/or mortality across all forest types (Keen et al., 2022; Powers et al., 2020; Choat et al., 2018). The influences of droughts on tree growth can vary with drought intensity, duration, and frequency. Tree radial growth during droughts decreased significantly with increasing drought intensity (Zhang et al., 2021a; Gazol et al., 2017) . Increased drought duration has also been shown to result in cumulative effects amplifying impacts on growth and reducing the drought resistance of multiple tree species (Kannenberg et al., 2019; Gao et al., 2018). In the long term, drought may overstrain trees’ acclimation potential and make them less resistant to future droughts (Bose et al., 2020). Frequent drought conditions led to serious growth declines for Pinus ponderosa (Szejner et al., 2020). As species-specific adaptations to drought determine their growth performance during drought periods, these adaptions can lead to shifts in species composition of forests (Zhang et al., 2022; Werner et al., 2021; Senf et al., 2020; Zhang et al., 2019a). Given that the frequency and severity of droughts will continue to increase in the future (Pokhrel et al., 2021; Ault,2020; DeSoto et al., 2020; Yuan et al., 2019), major uncertainties remain with regard to the impact of different drought regimes on tree growth and forest restoration (Liu et al., 2022; D'Orangeville et al., 2018; Panetta et al., 2018). While it has been reported that growth of Quercus mongolica is generally related to site aridity throughout a large part of its distribution (Zhang et al., 2021b), the exact effects of drought characteristics on growth during the drought are still unknown.

Growth recovery from drought is affected by both drought intensity and duration. Picea abies showed lower recovery and resilience to mild drought, but higher recovery to severe drought (Bottero et al., 2021). Drought intensity had greater influence on recovery of Picea glauca than drought timing (Mood et al., 2021). However, recovery and resilience of Pinus halepensis, P. nigra and P. sylvestris were lower after multiple droughts (Serra-Maluquer et al., 2018), indicating that recovery dynamics can change with multiple drought exposure. In fact, multi-year drought was a better predictor of post-drought tree growth of P. halepensis than drought intensity (Helluy et al., 2020). Long-term drought has also been reported to delay and limit post-drought recovery for P. sylvestris (Dang et al., 2019). Important differences in post-drought recovery seem to exist between differing wood types with ring-porous species tending towards a quicker recovery than diffuse-porous species (Kannenberg et al., 2019). Overall, recovery dynamics vary by species and there is less work on angiosperm species, such as Q. mongolica, making it very difficult to anticipate their reaction to future droughts.

Some species can even grow more after drought than before, presumably to compensate for reduced growth during the drought (Li et al., 2021; Alfaro-Sánchez et al., 2019). Such rapid recovery associated with compensatory growth has been observed in several tree species (Ovenden et al., 2021; Seidel et al., 2019; Minucci et al., 2017). The compensatory mechanisms has been hypothesized to mitigate some of the otherwise lasting effects of drought on the tree physiology (Balducci et al., 2016). Q. petraea and Q. robur recovered quickly to the pre-drought growth level after summer drought (Bose et al., 2021), while P. sylvestris showed rapid growth in the next year after spring and summer drought, which weakened the overall negative impact of drought (Seidel et al., 2019). However, increased drought frequency caused the compensatory effect on growth to fade for Larix sibirica, L. gmelinii, and P. sylvestris (Xu et al., 2022), suggesting that compensatory growth is limited to isolated drought events. Whether this effect of multi-year drought on compensatory growth is also evident in oaks remains an open question.

Oak species are widely distributed in Asia, Europe and North America (Sun et al., 2020; Cavender‐Bares, 2019; Mölder et al., 2019). Most oak species can adapt to a certain degree of drought (Suseela et al., 2020; Kunz et al., 2018). Q. mongolica, an oak species that is widely distributed and a dominant species of forests in northern China, had been predicted to expand its distribution under intensified drought regimes in model simulations (Leng et al., 2008). In contrast, analysis of tree-rings suggested that the expansion of Q. mongolica would be highly restricted by increasing drought, especially in the drier portion of its distribution (Zhang et al., 2021b). Growth declines caused by drought have been reported for other oak species (Gea-Izquierdo et al., 2021; Colangelo et al., 2018) and the vulnerability of oak species to extreme drought can increase (Camarero et al., 2021; Móricz et al., 2021). Yet, few studies have reported how drought intensity and duration affect the post-drought growth recovery for oak species (Gonzalez de Andres et al., 2021; Ouyang et al., 2021).

Here, we use a network of tree-ring collections from 27 stands covering a large part of the distribution of Q. mongolica in northern China, to investigate the influences of multi-year drought on post-drought growth recovery. Our main objectives were to (1) determine whether drought was a primary factor that influences radial growth of Q. mongolica; (2) investigate the growth changes of Q. mongolica with different drought intensity; (3) assess how drought intensity and duration affect variations of post-drought recovery of Q. mongolica; and (4) reveal how compensatory growth varies with drought duration and intensity. Accordingly, we hypothesized that (1) drought is a primary factor that influences radial growth of Q. mongolica; (2) drought intensity was positively related to growth reductions during the drought year; (3) multi-year drought is a strong indicator of recovery; and (4) that both severity and duration are negatively related to post-drought compensatory growth.