Leaf Shedding and Non-Stomatal Limitations of Photosynthesis Mitigate Hydraulic Conductance Losses in Scots Pine Saplings During Severe Drought Stress

Abstract

During drought, trees reduce water loss and hydraulic failure by closing their stomata, which also limits photosynthesis. Under severe drought stress, other acclimation mechanisms are trigged to further reduce transpiration to prevent irreversible conductance loss. Here, we investigate two of them: the reversible impacts on the photosynthetic apparatus, lumped as non-stomatal limitations (NSL) of photosynthesis, and the irreversible effect of premature leaf shedding. We integrate NSL and leaf shedding with a state-of-the-art tree hydraulic simulation model (SOX+) and parameterize them with example field measurements to demonstrate the stress-mitigating impact of these processes. We measured xylem vulnerability, transpiration, and leaf litter fall dynamics in Pinus sylvestris (L.) saplings grown for 54 days under severe dry-down. The observations showed that, once transpiration stopped, the rate of leaf shedding strongly increased until about 30% of leaf area was lost on average. We trained the SOX+ model with the observations and simulated changes in root-to-canopy conductance with and without including NSL and leaf shedding. Accounting for NSL improved model representation of transpiration, while model projections about root-to-canopy conductance loss were reduced by an overall 6%. Together, NSL and observed leaf shedding reduced projected losses in conductance by about 13%. In summary, the results highlight the importance of other than purely stomatal conductance-driven adjustments of drought resistance in Scots pine. Accounting for acclimation responses to drought, such as morphological (leaf shedding) and physiological (NSL) adjustments, has the potential to improve tree hydraulic simulation models, particularly when applied in predicting drought-induced tree mortality.

Predicting resilience through the lens of competing adjustments to vegetation function

“…Foliage growth was sufficient in the relatively wetter years to buffer carbon losses associated with lower LAI during the drier years (2013–2016/2017 vs. 2017–2020; see Figure 6). These findings concur with Eucalyptus canopy dynamics observed in the field (Battaglia et al, 1998; Pook, 1985; Whitehead & Beadle, 2004), but explicit linkage of observed hydraulic status, LAI, and carbon storage/biomass remain rare (but see Atwell et al, 2007; Nadal‐Sala et al, 2021; Poyatos et al, 2013). Future work that examines this link across species and ecosystems will be critical.…”

Predicting resilience through the lens of competing adjustments to vegetation function

“…Foliage growth was sufficient in the relatively wetter years to buffer carbon losses associated with lower LAI during the drier years (2013-2016/2017 vs. 2017-2020; see Figure 6). These findings concur with Eucalyptus canopy dynamics observed in the field (Battaglia et al, 1998;Pook, 1985;Whitehead & Beadle, 2004), but explicit linkage of observed hydraulic status, LAI, and carbon storage/ biomass remain rare (but see Atwell et al, 2007;Nadal-Sala et al, 2021;Poyatos et al, 2013). work that examines this link across species and ecosystems will be critical.…”

The responses of three dominant species to increased rainfall under different grazing systems in a desert steppe

“…In all three types of plants, the leaf‐tissue thickness correlated negatively with increased rainfall. In arid and semi‐arid regions, the leaf‐tissue thickness is often considered an indicator of drought resistance (Ahrens et al, 2019; Amitrano et al, 2021; Nadal‐Sala et al, 2021). Thick leaves can store larger amounts of water, which reduces sunburn and increases the plant's tolerance to drought conditions (Scoffoni et al, 2013; Velikova et al, 2020).…”