Root carbon and nutrient homeostasis determines downy oak sapling survival and recovery from drought

Abstract

The role of carbon (C) and nutrient uptake, allocation, storage and especially their interactions in survival and recovery of trees under increased frequencies and intensities of drought events is not well understood. A full factorial experiment with four soil water content regimes ranging from extreme drought to well-watered conditions and two fertilization levels was carried out. We aimed to investigate whether nutrient addition mitigates drought effects on downy oak (Quercus pubescens Willd.) and whether storage pools of non-structural carbohydrates (NSC) are modified to enhance survival after 2.5 years of drought and recovery after drought relief. Physiological traits, such as photosynthesis, predawn leaf water potential as well as tissue biomass together with pools and dynamics of NSC and nutrients at the whole-tree level were investigated. Our results showed that fertilization played a minor role in saplings’ physiological processes to cope with drought and drought relief, but reduced sapling mortality during extreme drought. Irrespective of nutrient supply, Q. pubescens showed increased soluble sugar concentration in all tissues with increasing drought intensity, mostly because of starch degradation. After 28 days of drought relief, tissue sugar concentrations decreased, reaching comparable values to those of well-watered plants. Only during the recovery process from extreme drought, root NSC concentration strongly declined, leading to an almost complete NSC depletion after 28 days of rewetting, simultaneously with new leaves flushing. These findings suggest that extreme drought can lead to root carbon exhaustion. After drought relief, the repair and regrowth of organs can even exacerbate the root carbon depletion. We concluded that under future climate conditions with repeated drought events, the insufficient and lagged carbon replenishment in roots might eventually lead to carbon starvation and further mortality.

Root Carbon Resources Determine Survival and Growth of Young Trees Under Long Drought in Combination With Fertilization

“…With increasing drought duration, tree mortality increased (Figure 6), leaf water potential and photosynthesis decreased for the two species (Figure 2), which is similar to those results found in drought intensity experiments with trees (Schönbeck et al, 2018;Lauder et al, 2019;Archambeau et al, 2020; et al, 2020a;Ouyang et al, 2021). For instance, extreme drought was found to significantly decrease predawn water potential and net photosynthetic rates and to increase the mortality for both Pinus sylvestris (Schönbeck et al, 2020a) and Quercus pubescens saplings (Ouyang et al, 2021). Drought, both severe drought intensity and long drought duration, decreases soil water availability and plant leaf water potential, and thus results in stomatal closure to prevent transpiration exceeding root water uptake capacity, which caused declined photosynthesis and CO 2 uptake (Li et al, 2020).…”

“…With increasing drought duration, tree mortality increased (Figure 6), leaf water potential and photosynthesis decreased for the two species (Figure 2), which is similar to those results found in drought intensity experiments with trees (Schönbeck et al, 2018;Lauder et al, 2019;Archambeau et al, 2020; et al, 2020a;Ouyang et al, 2021). For instance, extreme drought was found to significantly decrease predawn water potential and net photosynthetic rates and to increase the mortality for both Pinus sylvestris (Schönbeck et al, 2020a) and Quercus pubescens saplings (Ouyang et al, 2021).…”

“…Similar to results gained from most drought intensity experiments with trees (Li et al, 2013;Schönbeck et al, 2018Schönbeck et al, , 2020aZhang et al, 2020;Ouyang et al, 2021), the present study found that the longer drought duration treatments (D2, D3) did not decrease the end-season leaf NSC (pre-winter) levels (Figures 3A,F), although the D2 and D3 treatment significantly decreased leaf photosynthesis of the two species (Figures 2C-F). This might be explained by the osmoregulation strategy of plants suffering from drought stress on the one hand (O'Brien et al, 2014;Dickman et al, 2015), and on the other hand, it may be a result of basipetal carbon translocation failure (Rowland et al, 2015), if the phloem function becomes impaired and carbon translocation gets limited or stopped by hydraulic failure caused by severe or long drought stress (Griffin-Nolan et al, 2021).…”

“…However, in spite of decreased photosynthesis (Figures 2C-F) and NSC levels (Figures 3B,C,G,H) with increasing drought duration, the relative tree height increment of the two species did not differ among the drought treatments in our study (Figures 2G,H). Li et al (2013), Schönbeck et al (2018Schönbeck et al ( , 2020a, and Ouyang et al (2021) found that drought-stressed trees maintained relatively stable NSC levels at the expense of growth, implying an active process of NSC storage (Li et al, 2018a). For example, drought declined the growth but did not decrease tissue NSC level in Quercus faginea and Pinus halepensis (Sanz-Perez et al, 2009).…”

Antecedent Drought Condition Affects Responses of Plant Physiology and Growth to Drought and Post-drought Recovery

“…Consequently, an increase in non-structural carbohydrates (NSCs) is commonly observed in plants subjected to drought stress due to an imbalance in source-sink strength (Sala and Hoch, 2009;Sala et al, 2012;Granda and Camarero, 2017;Piper et al, 2017). Additionally, drought induced variation in tissue NSCs contents may occur in all organs, and the importance of sugar enriched organ to plant performance during water deficit and post-drought recovery can be markedly speciesspecific (Hagedorn et al, 2016;Joseph et al, 2020;Ouyang et al, 2021). Here, we primarily focus on leaves, the NSCs variation of which have been shown to predominantly regulates plant response to drought stress in some species (Martínez-Vilalta et al, 2016;Signori-Müller et al, 2021)., and dominates the variation of whole plant carbohydrates contents in Eucalyptus species (Duan et al, 2013).…”