Drought limits: hydraulic adaptation in woody plants
Several members of WP2 of Forgenius co-authored this recent publication, which synthetises several decades of research examining the plasticity of the hydraulic systems of woody plants. As it is well known, many parts of the world have experienced longer and more severe droughts in recent decades. These conditions, combined with high temperatures, have led to substantial tree decline and death in different regions of the world. To inform the debate existing over the causes of these events, we compiled >3,000 observations contained in studies that looked at time courses of tree hydraulic changes at single sites (wet versus dry seasons), comparison of common garden sites (moister versus drier gardens), and experimental manipulations of water availability (control versus drought treatments), all of which allow to estimate plasticity. In addition, we also compiled observational field studies comparing populations of trees of the same species living under different conditions of water availability (relatively moister versus relatively drier sites). This type of studies allow to estimate phenotypic variability, resulting from a combination of plasticity and adaptive evolutionary responses. For each pair of observations, we then quantified indices of response to water stress relative to controls, periods of high-water availability or locations with moister conditions.
No matter whether we looked at plasticity or phenotypic variability, we found that plants could indeed adjust their hydraulic system to better resist water scarcity, consistent with adaptive responses to stress (acclimation). However, the levels of these adjustments were limited, and generally not sufficient to cope with the level of water stress established in the experiments or experienced in the field. This suggests that responses to ongoing climate change are equally limited compared to the increased levels of stress, helping to explain the episodes of dieback mentioned above.
In other words, while the acclimation of the hydraulic system to drought is an evolutionary feature allowing plants to cope with lack of water, overall, the adjustment is incomplete. Plants experience a declining safety margin from critical vulnerability thresholds when faced with intense droughts.
In addition to examining mechanisms by which woody plants adjust to drought, we also examined the responses of the hydraulic system to other environmental variables related to climate change, such as changes in solar radiation (linked to phenomena such as dimming and brightening), increases in temperature, increases in the concentration of CO2 in the atmosphere and changes in soil nutrient availability (linked to soil eutrophication or oligotrophication). Specifically, we show that responses of the hydraulic system of woody plants to atmospheric CO2 concentration do not ease the threat caused by drought, albeit they do not aggravate it. Conversely, lack of nutrients, often a side consequence of high CO2 concentration, interact with droughts to make woody plants even more sensitive to water stress conditions.
Maurizio Mencuccini on behalf of all co-authors