Drought-related tree mortality occurs globally and may increase in the future, but we lack sufficient mechanistic understanding to accurately predict it. Here we present the first field assessment of the physiological mechanisms leading to mortality in an ecosystem-scale rainfall manipu- lation of a pinon-juniper (Pinus edulis-Juniperus mono- sperma) woodland. We measured transpiration (E) and modelled the transpiration rate initiating hydraulic failure (Ecrit). We predicted that isohydric pinon would experience mortality after prolonged periods of severely limited gas exchange as required to avoid hydraulic failure; anisohydric juniper would also avoid hydraulic failure, but sustain gas exchange due to its greater cavitation resistance. After 1 year of treatment, 67% of droughted mature pinon died with concomitant infestation by bark beetles (Ips confusus) and bluestain fungus (Ophiostoma spp.); no mortality occurred in juniper or in control pinon. As predicted, both species avoided hydraulic failure, but safety margins from Ecrit were much smaller in pinon, especially droughted pinon, which also experienced chronically low hydraulic conductance. The defining characteristic of trees that died was a 7 month period of near-zero gas exchange, versus 2 months for surviving pinon. Hydraulic limits to gas exchange, not hydraulic failure per se, promoted drought- related mortality in pinon pine.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2012.02512.x

Hydraulic limits preceding mortality in a piñon-juniper woodland under experimental drought.

Plant responses to drought and heat stress have been extensively studied, whereas post-stress recovery, which is fundamental to understanding stress resilience, has received much less attention. Here, we present a conceptual stress-recovery framework with respect to hydraulic and metabolic functioning in woody plants. We further synthesize results from controlled experimental studies following heat or drought events and highlight underlying mechanisms that drive post-stress recovery. We find that the pace of recovery differs among physiological processes. Leaf water potential and abscisic acid concentration typically recover within few days upon rewetting, while leaf gas exchange-related variables lag behind. Under increased drought severity as indicated by a loss in xylem hydraulic conductance, the time for stomatal conductance recovery increases markedly. Following heat stress release, a similar delay in leaf gas exchange recovery has been observed, but the reasons are most likely a slow reversal of photosynthetic impairment and other temperature-related leaf damages, which typically manifest at temperatures above 40 °C. Based thereon, we suggest that recovery of gas exchange is fast following mild stress, while recovery is slow and reliant on the efficiency of repair and regrowth when stress results in functional impairment and damage to critical plant processes. We further propose that increasing stress severity, particular after critical stress levels have been reached, increases the carbon cost involved in reestablishing functionality. This concept can guide future experimental research and provides a base for modeling post-stress recovery of carbon and water relations in trees.

/pdf/beyond-the-extreme-recovery-of-carbon-and-water-relations-in-2ngdw9j744.pdf

Beyond the extreme: recovery of carbon and water relations in woody plants following heat and drought stress

Heat waves that trigger severe droughts are predicted to increase globally; however, we lack an understanding of how trees respond to the combined change of extreme temperatures and water availability. Here, we studied the impacts of two consecutive heat waves as well as post-stress recovery in young Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) and Robinia pseudoacacia L. (black locust) growing under controlled conditions. Responses were compared under water supply close to the long-term average and under reduced irrigation to represent drought. Exposure to high temperatures (+10 °C above ambient) and vapour pressure deficit strongly affected the trees in terms of water relations, photosynthesis and growth. Douglas-fir used water resources conservatively, and transpiration decreased in response to mild soil water limitation. In black locust, heat stress led to pronounced tree water deficits (stem diameter shrinkage), accompanied by leaf shedding to alleviate stress on the hydraulic system. The importance of water availability during the heat waves became further apparent by a concurrent decline in photosynthesis and stomatal conductance with increasing leaf temperatures in both species, reaching the lowest rates in the heat-drought treatments. Stress severity determined both the speed and the amount of recovery. Upon release of stress, photosynthesis recovered rapidly in drought-treated black locust, while it remained below control rates in heat (t = -2.4, P < 0.05) and heat-drought stressed trees (t = 2.96, P < 0.05). In Douglas-fir, photosynthesis recovered quickly, while water-use efficiency increased in heat-drought trees because stomatal conductance remained reduced (t = -2.92, P < 0.05). Moreover, Douglas-fir was able to compensate for stem-growth reductions following heat (-40%) and heat-drought stress (-68%), but most likely at the expense of storage and other growth processes. Our results highlight the importance of studying heat waves alongside changes in water availability. They further suggest that we should look beyond the actual stress event to identify lagged effects and acclimation processes that may determine tree resilience in the long term.

/pdf/water-availability-as-dominant-control-of-heat-stress-2wz6b7k3fs.pdf

Water availability as dominant control of heat stress responses in two contrasting tree species.

Soil drought influences the C turnover as well as the fine-root system of tree saplings. Particularly during the period of establishment, the susceptibility to drought stress of saplings is increased because of incompletely developed root systems and reduced access to soil water. Here, we subjected beech saplings (Fagus sylvatica L.) to different levels of drought stress.

Beech saplings were planted in rhizotrons, which were installed in the soil of a Norway spruce forest before bud burst. Soil moisture was manipulated in the following year during May to September. We measured photosynthetic net CO2 uptake, volume production of fine roots, and rhizosphere respiration during the growing season. Biometric parameters of the fine-root system, biomass, and nonstructural carbohydrates were analyzed upon harvest in October.

Photosynthesis and rhizosphere respiration decreased with increasing drought-stress dose (cumulated soil water potential), and cumulative rhizosphere respiration was significantly negatively correlated with drought-stress dose. Fine-root length and volume production were highest at moderate soil drought, but decreased at severe soil drought. The proportion of fine-roots diameter < 0.2 mm and the root-to-shoot ratio increased whereas the live-to-dead ratio of fine roots decreased with increasing drought-stress dose.

We conclude that the belowground C allocation as well as the relative water-uptake efficiency of beech saplings is increased under drought.

Effects of drought stress on photosynthesis, rhizosphere respiration, and fine-root characteristics of beech saplings: A rhizotron field study

& Context Silver fir is declining and dying at its southern margin on the Mediterranean area, where climate is expected to be warmer and drier. At a regional scale, silver fir seems to be vulnerable to drought, and at a forest stand scale, tree death seems to be distributed according to soil water availability. & Aim To understand the vulnerability of silver fir to drought, factors involved in the regulation of transpiration were assessed with respect to soil properties in order to document the spatial distribution of death rates. & Methods Soil properties were characterized by electric resistivity measurements. Sap flow density and predawn needle water potential were recorded on sampled trees during several years, and crown specific transpiration was estimated. In addition, the vulnerability of coarse roots and branches to cavitation was quantified. & Results and conclusion Trees growing on soils with a large water storage capacity were the most vulnerable to drought induced soil water deficits. Transpiration was down-regulated as soon as predawn water potential decreased. The vulnerability to cavitation was low, which protected the trees from run-away xylem embolism. Severe soil water deficits led to a rapid decrease of transpiration, which was still visible the following year. The drop-off in transpiration was mainly due to inner sap flow that almost ceased after the drought on all monitored trees. Our results suggest that root dynamic and the ability of roots to take up water were modified by soil water deficit over several years. Such a regulation scheme needs to be better documented and included in models to address silver fir forest responses to drought.

https://hal.archives-ouvertes.fr/hal-01102695/document

Transpiration of silver Fir ( Abies alba mill.) during and after drought in relation to soil properties in a Mediterranean mountain area

17 weeks during the third year of the study, and post-treatment recovery was measured during the fourth year. Monthly fine root production was often reduced by low soil water content (SWC) during July–September in the pre-treatment years and by imposed drought. Irrigation resulted in higher summer fine root production than in pre-treatment years. In the recovery year, increased fine root production was observed in the previously droughted plots despite low SWC in August and September. Droughting decreased year-end fine root biomass in the treatment year, but biomass returned to pre-treatment levels during the recovery year. Droughting and irrigating did not affect foliage production during the treatment and recovery years. Our results suggest that for balsam fir, establishment and maintenance of a functional balance between foliage and fine root bio mass, with respect to moisture supply and demand, can depend on fine root dynamics occurring over more than one growing season. In addition, our findings provided insights into tree growth responses to interannual variation in moisture supply.

/pdf/effects-of-soil-moisture-manipulations-on-fine-root-dynamics-34qo6xmofk.pdf

Effects of soil moisture manipulations on fine root dynamics in a mature balsam fir (Abies balsamea L. Mill.) forest

Fine-root (≤2 mm) demographics change as forests age, but the direction and extent of change are unknown. Knowledge of the change and understanding of causes will improve predictions of climate change impacts. We used minirhizotrons at three young and three mature balsam fir (Abies balsamea (L.) Mill.) sites to measure median lifespan (MLS) for each site and for annual cohorts. We computed turnover rate from the inverse of MLS (Tinv) and calculated a second turnover rate (T) from annual mortality, annual production and previous year-end standing crop. Median lifespan at mature sites (436 days) was half that at young sites (872 days). Median lifespan of annual cohorts varied widely at all sites. Age-class distributions of fine roots seen by minirhizotrons changed with increasing years of observation, with older age classes accumulating more slowly at mature sites. Our findings highlight the need to determine whether the proportional contributions of absorbing and transporting fine roots to annual production and their median lifespans change during stand development. Due to its variation among annual cohorts, we believe robust estimates of MLS at our sites require 5-7 years of observation, and reliable estimates of Tinv are reached earlier than T.

/pdf/age-related-changes-in-survival-and-turnover-rates-of-balsam-26swthyz1j.pdf

Age-related changes in survival and turnover rates of balsam fir (Abies balsamea (L.) Mill.) fine roots.

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Effects of soil moisture manipulations on fine root dynamics in a mature balsam fir (Abies balsamea L. Mill.) forest

Age-related changes in survival and turnover rates of balsam fir (Abies balsamea (L.) Mill.) fine roots.