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

Drought legacies in mixed Mediterranean forests: Analysing the effects of structural overshoot, functional traits and site factors

Previous favorable climate conditions stimulate tree growth making some forests more vulnerable to hotter droughts. This so-called structural overshoot may contribute to forest dieback, but there is little evidence on its relative importance depending on site conditions and tree species because of limited field data. Here, we analyzed remote sensing (NDVI) and tree-ring width data to evaluate the impacts of the 2017 drought on canopy cover and growth in mixed Mediterranean forests (Fraxinus ornus, Quercus pubescens, Acer monspessulanum, Pinus pinaster) located in southern Italy. Legacy effects were assessed by calculating differences between observed and predicted basal area increment (BAI). Overall, the growth response of the study stands to the 2017 drought was contingent on site conditions and species characteristics. Most sites presented BAI and canopy cover reductions during the drought. Growth decline was followed by a quick recovery and positive legacy effects, particularly in the case of F. ornus. However, we found negative drought legacies in some species (e.g., Q. pubescens, A. monspessulanum) and sites. In those sites showing negative legacies, high growth rates prior to drought in response to previous wet winter-spring conditions may have predisposed trees to drought damage. Vice versa, the positive drought legacy found in some F. ornus site was linked to post-drought growth release due to Q. pubescens dieback and mortality. Therefore, we found evidences of structural drought overshoot, but it was restricted to specific sites and species. Our findings highlight the importance of considering site settings such as stand composition, pre-drought conditions and different tree species when studying structural overshoot. Droughts contribute to modify the composition and dynamics in mixed forests.

Drought resistance and resilience of rhizosphere communities in forest soils from the cellular to ecosystem scale - insights from 13C pulse labeling

The link between above- and belowground communities is a key uncertainty in drought and rewetting effects on forest carbon (C) cycle. In young beech model ecosystems and mature naturally dry pine forest exposed to 15-yr-long irrigation, we performed 13C pulse labeling experiments, one during drought and one 2 wk after rewetting, tracing tree assimilates into rhizosphere communities. The 13C pulses applied in tree crowns reached soil microbial communities of the young and mature forests one and 4 d later, respectively. Drought decreased the transfer of labeled assimilates relative to the irrigation treatment. The 13C label in phospholipid fatty acids (PLFAs) indicated greater drought reduction of assimilate incorporation by fungi (-85%) than by gram-positive (-43%) and gram-negative bacteria (-58%). 13C label incorporation was more strongly reduced for PLFAs (cell membrane) than for microbial cytoplasm extracted by chloroform. This suggests that fresh rhizodeposits are predominantly used for osmoregulation or storage under drought, at the expense of new cell formation. Two weeks after rewetting, 13C enrichment in PLFAs was greater in previously dry than in continuously moist soils. Drought and rewetting effects were greater in beech systems than in pine forest. Belowground C allocation and rhizosphere communities are highly resilient to drought.

The role of leaf superoxide dismutase and proline on intra-specific photosynthesis recovery of Schima superba following drought

Understanding the physiological and biochemical responses of tree seedlings under extreme drought stress, along with recovery during rewatering, and potential intra-species differences, will allow us to more accurately predict forest responses under future climate change. Here, we selected seedlings from four provenances (AH (Anhui), JX (Jiangxi), HN (Hunan) and GX (Guangxi)) of Schima superba and carried out a simulated drought-rewatering experiment in a field-based rain-out shelter. Seedlings were progressively dried until they reached 50% and 88% loss of xylem hydraulic conductivity (PLC) (i.e. P50 and P88), respectively, before they were rehydrated and maintained at field capacity for 30 days. Leaf photosynthesis (Asat), water status, activity of superoxide dismutase (SOD), and proline (Pro) concentration were monitored and their associations were determined. Increasing drought significantly reduced Asat, relative water content (RWC) and SOD activity in all provenances, and Pro concentration was increased to improve water retention; all four provenances exhibited similar response patterns, associated with similar leaf ultrastructure at pre-drought. Upon rewatering, physiological and biochemical traits were restored to well-watered control values in P50-stressed seedlings. In P88-stressed seedlings, Pro was restored to control values, while SOD was not fully recovered. The recovery pattern differed partially among provenances. There was a progression of recovery following watering, with RWC firstly recovered, followed by SOD and Pro, and then Asat, but with significant associations among these traits. Collectively, the intra-specific differences of S. superba seedlings in recovery of physiology and biochemistry following rewatering highlight the need to consider variations within a given tree species coping with future more frequent drought stress.

Photosynthetic and biochemical responses of four subtropical tree seedlings to reduced dry season and increased wet season precipitation and variable N deposition

Interspecific variations in phenotypic plasticity of trees that are affected by climate change may alter the ecosystem function of forests. Seedlings of four common tree species (Castanopsis fissa, Michelia macclurei, Dalbergia odorifera and Ormosia pinnata) in subtropical plantations of southern China were grown in the field under rainout shelters and subjected to changing precipitation (48 L of water every 4 days in the dry season, 83 L of water every 1 day in the wet season; 4 g m-2 year-1 of nitrogen (N)), low N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 8 g m-2 year-1 N), high N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 10 g m-2 year-1 N) and their interactive effects. We found that the changes in seasonal precipitation reduced the light-saturated photosynthetic rate (Asat) for C. fissa due to declining area-based foliar N concentrations (Na). However, we also found that the interactive effects of changing precipitation and N deposition enhanced Asat for C. fissa by increasing foliar Na concentrations, suggesting that N deposition could alleviate N limitations associated with changing precipitation. Altered precipitation and high N deposition reduced Asat for D. odorifera by decreasing the maximum electron transport rate for RuBP regeneration (Jmax) and maximum rate of carboxylation of Rubisco (Vcmax). Ormosia pinnata under high N deposition exhibited increasing Asat due to higher stomatal conductance and Vcmax. The growth of D. odorifera might be inhibited by changes in seasonal precipitation and N deposition, while O. pinnata may benefit from increasing N deposition in future climates. Our study provides an important insight into the selection of tree species with high capacity to tolerate changing precipitation and N deposition in subtropical plantations.

Nutritional changes in trees during drought-induced mortality: A comprehensive meta-analysis and a field study

Both macronutrients and micronutrients are essential for tree growth and development through participating in various ecophysiological processes. However, the impact of the nutritional status of trees on their ability to withstand drought-induced mortality remains inconclusive. We thus conducted a comprehensive meta-analysis, compiling data on 11 essential nutrients from 44 publications (493 independent observations). Additionally, a field study was conducted on Pinus sylvestris L. trees with varying drought-induced vitality loss in the "Visp" forest in southern Switzerland. No consistent decline in tree nutritional status was observed during tree mortality. The meta-analysis revealed significantly lower leaf potassium (K), iron (Fe), and copper (Cu) concentrations with tree mortality. However, the field study showed no causal relationships between nutritional levels and the vitality status of trees. This discrepancy is mainly attributed to the intrinsic differences in the two types of experimental designs and the ontogenetic stages of target trees. Nutrient reductions preceding tree mortality were predominantly observed in non-field conditions, where the study was conducted on seedlings and saplings with underdeveloped root systems. It limits the nutrient uptake capacity of these young trees during drought. Furthermore, tree nutritional responses are also influenced by many variables. Specifically, (a) leaf nutrients are more susceptible to drought stress than other organs; (b) reduced tree nutrient concentrations are more prevalent in evergreen species during drought-induced mortality; (c) of all biomes, Mediterranean forests are most vulnerable to drought-induced nutrient deficiencies; (d) soil types affect the direction and extent of tree nutritional responses. We identified factors that influence the relationship between tree nutritional status and drought survival, and proposed potential early-warning indicators of impending tree mortality, for example, decreased K concentrations with declining vitality. These findings contribute to our understanding of tree responses to drought and provide practical implications for forest management strategies in the context of global change.

The effects of previous summer drought and fertilization on winter non-structural carbon reserves and spring leaf development of downy oak saplings

It is still unknown whether the previous summer season drought and fertilization will affect the winter non-structural carbohydrate (NSC) reserves, spring leaf development, and mortality of trees in the next year. We, therefore, conducted an experiment with Quercus pubescens (downy oaks) saplings grown under four drought levels from field capacity (well-watered; ~25% volumetric water content) to wilting point (extreme drought; ~6%), in combination with two fertilizer treatments (0 vs. 50 kg/ha/year blended) for one growing season to answer this question. We measured the pre- and post-winter NSC, and calculated the over-winter NSC consumption in storage tissues (i.e. shoots and roots) following drought and fertilization treatment, and recorded the spring leaf phenology, leaf biomass, and mortality next year. The results showed that, irrespective of drought intensity, carbon reserves were abundant in storage tissues, especially in roots. Extreme drought did not significantly alter NSC levels in tissues, but delayed the spring leaf expansion and reduced the leaf biomass. Previous season fertilization promoted shoot NSC use in extreme drought-stressed saplings over winter (showing reduced carbon reserves in shoots after winter), but it also showed positive effects on survival next year. We conclude that: (1) drought-stressed downy oak saplings seem to be able to maintain sufficient mobile carbohydrates for survival, (2) fertilization can alleviate the negative effects of extreme drought on survival and recovery growth of tree saplings.

In situ 13CO2 labeling reveals that alpine treeline trees allocate less photoassimilates to roots compared with low-elevation trees

Carbon (C) allocation plays a crucial role for survival and growth of alpine treeline trees, however it is still poorly understood. Using in situ 13CO2 labeling, we investigated the leaf photosynthesis and the allocation of 13C labeled photoassimilates in various tissues (leaves, twigs and fine roots) in treeline trees and low-elevation trees. Non-structural carbohydrate concentrations were also determined. The alpine treeline trees (2000 m. a.s.l.), compared with low-elevation trees (1700 m a.s.l.), did not show any disadvantage in photosynthesis, but the former allocated proportionally less newly assimilated C belowground than the latter. Carbon residence time in leaves was longer in treeline trees (19 days) than that in low-elevation ones (10 days). We found an overall lower density of newly assimilated C in treeline trees. The alpine treeline trees may have a photosynthetic compensatory mechanism to counteract the negative effects of the harsh treeline environment (e.g., lower temperature and shorter growing season) on C gain. Lower temperature at treeline may limit the sink activity and C downward transport via phloem, and shorter treeline growing season may result in early cessation of root growth, decreases sink strength, which all together lead to lower density of new C in the sink tissues and finally limit the growth of the alpine treeline trees.

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

Current increases in not only the intensity and frequency but also the duration of drought events could affect the growth, physiology, and mortality of trees. We experimentally studied the effects of drought duration in combination with fertilization on leaf water potential, gas exchange, growth, tissue levels of non-structural carbohydrates (NSCs), tissue NSC consumption over-winter, and recovery after drought release in oak (Quercus petraea) and beech (Fagus sylvatica) saplings. Long drought duration (>1 month) decreased leaf water potential, photosynthesis, and NSC concentrations in both oak and beech saplings. Nitrogen fertilization did not mitigate the negative drought effects on both species. The photosynthesis and relative height increment recovered in the following rewetting year. Height growth in the rewetting year was significantly positively correlated with both pre- and post-winter root NSC levels. Root carbon reserve is critical for tree growth and survival under long-lasting drought. Our results indicate that beech is more sensitive to drought and fertilization than oak. The present study, in a physiological perspective, experimentally confirmed the view that the European beech, compared to oak, may be more strongly affected by future environmental changes.

Physiological and Biochemical Dynamics of Pinus massoniana Lamb. Seedlings under Extreme Drought Stress and during Recovery

In recent years, global forests have been facing an increase in tree mortality owing to increasing droughts. However, the capacity for plants to adjust their physiology and biochemistry during extreme drought and subsequent recovery is still unclear. Here, we used 1.5-year-old Pinus massoniana Lamb. seedlings and simulated drought conditions to achieve three target stress levels (50%, 85%, and 100% loss of stem hydraulic conductivity (PLC)), followed by rehydration. Needle water status, gas exchange, and biochemical parameters were assessed during drought and recovery. The results showed that drought had significantly negative impacts on needle water status and gas exchange parameters, with gas exchange declining to 0 after PLC85 was achieved. Soluble protein concentration (SPC), soluble sugar concentration (SSC), malondialdehyde (MDA) content, superoxide dismutase (SOD) activity, and needle water-use efficiency showed fluctuations. The activity of antioxidant enzymes and the values of osmotic regulators were then gradually decreased as the physiological and biochemical functions of seedlings were disturbed. Seedlings showed a stronger ability to recover from PLC50 than PLC85 and PLC100. We conclude that the physiological and biochemical recovery of P. massoniana seedlings is more likely to be inhibited when plants experience increasing drought stress that induces 85% and greater loss of hydraulic conductance.