Drought response strategies define the relative contributions of hydraulic dysfunction and carbohydrate depletion during tree mortality

Abstract

SummaryPlant survival during drought requires adequate hydration in living tissues and carbohydrate reserves for maintenance and recovery. We hypothesized that tree growth and hydraulic strategy determines the intensity and duration of the 'physiological drought', thereby affecting the relative contributions of loss of hydraulic function and carbohydrate depletion during mortality.We compared patterns in growth rate, water relations, gas exchange and carbohydrate dynamics in three tree species subjected to prolonged drought.Two Eucalyptus species (E. globulus, E. smithii) exhibited high growth rates and water-use resulting in rapid declines in water status and hydraulic conductance. In contrast, conservative growth and water relations in Pinus radiata resulted in longer periods of negative carbon balance and significant depletion of stored carbohydrates in all organs. The ongoing demand for carbohydrates from sustained respiration highlighted the role that duration of drought plays in facilitating carbohydrate consumption.Two drought strategies were revealed, differentiated by plant regulation of water status: plants maximized gas exchange, but were exposed to low water potentials and rapid hydraulic dysfunction; and tight regulation of gas exchange at the cost of carbohydrate depletion. These findings provide evidence for a relationship between hydraulic regulation of water status and carbohydrate depletion during terminal drought.

Comparative Drought Responses of Quercus ilex L. and Pinus sylvestris L. in a Montane Forest Undergoing a Vegetation Shift

“…The more isohydric behavior in P. sylvestris, in terms of maintaining relatively high water potentials, was associated with greater reductions of stomatal conductance as soil water availability declined (Figure 4), and is consistent with a greater relative reduction in NSC during drought, compared to Q. ilex ( Figure 10) and with the low absolute levels of NSC concentration at the peak of summer drought (August). Similar NSC dynamics in responses to drought were found in a study comparing a conifer and a broadleaved species [68]. NSC reserves are known to become even more depleted in P. sylvestris affected by crown defoliation following chronic drought stress and they have been directly associated with drought-induced decline and mortality in the study population [30].…”

Drought effects on composition and yield for corn stover, mixed grasses, andMiscanthusas bioenergy feedstocks

“…This increase can be attributed in large part to the effects of drought rather than hybrid differences, as relative maturity ratings of the hybrids are similar and both hybrids are considered to have high total fermentable (HTF) characteristics [38]. The accumulation of osmolytes, such as soluble sugars and proline, is a common plant response to drought that has been reported previously [20,22,39,40]. These osmolytic solutes are included as components of the reported extractives values in this analysis.…”

“…As mentioned previously, increases in soluble carbohydrates and other extractable components and decreases in structural components, such as cellulose and lignin, have been linked to water stress. Starch, similar to cellulose, has been shown to decrease with prolonged drought [40]. In contrast to corn stover, mixed perennial grasses are typically harvested with seed, which contains more starch than other anatomical fractions of the plant; hence, both cellulose and starch contribute to the glucan value for mixed perennial grass samples.…”

Causes, consequences, and the future of forest mortality due to climate change

“…The processes of hydraulic failure and carbon starvation are nearly always observed to co-occur in both observational studies and during experimental drought-manipulations that induce mortality (Galiano et al 2011, Galvez et al 2011, Levanič et al 2011, Anderegg et al 2012b, Plaut et al 2012, Quirk et al 2013, Sevanto et al 2013, Hartmann et al 2013. However, the relative dominance of hydraulic failure versus carbon starvation in driving mortality appears to depend on the rate of stress onset in plants, with faster drought causing more hydraulic failure and vice versa (Mitchell et al 2013; consistent with the predictions of McDowell et al 2008). Despite these consistent results, there remains a large amount of variation in the degree of hydraulic failure and carbon starvation observed prior to mortality that is currently un-explained.…”