Drought‐induced xylem cavitation and hydraulic deterioration: risk factors for urban trees under climate change?

Abstract

SummaryUrban trees help towns to cope with climate warming by cooling both air and surfaces. The challenges imposed by the urban environment, with special reference to low water availability due to the presence of extensive pavements, result in high rates of mortality of street trees, that can be increased by climatic extremes.We investigated the water relations and xylem hydraulic safety/efficiency of Quercus ilex trees growing at urban sites with different percentages of surrounding impervious pavements. Seasonal changes of plant water potential and gas exchange, vulnerability to cavitation and embolism level, and morpho-anatomical traits were measured.We found patterns of increasing water stress and vulnerability to drought at increasing percentages of impervious pavement cover, with a consequent reduction in gas exchange rates, decreased safety margins toward embolism development, and increased vulnerability to cavitation, suggesting the occurrence of stress-induced hydraulic deterioration.The amount of impermeable surface and chronic exposure to water stress influence the site-specific risk of drought-induced dieback of urban trees under extreme drought. Besides providing directions for management of green spaces in towns, our data suggest that xylem hydraulics is key to a full understanding of the responses of urban trees to global change.

X‐ray microtomography observations of xylem embolism in stems of Laurus nobilis are consistent with hydraulic measurements of percentage loss of conductance

“…Drought-induced xylem embolism is caused by air bubbles being aspirated into conduits filled with water under tension from nearby gas-filled compartments (Sperry & Tyree, 1988). This process of 'air seeding' occurs at the level of interconduit pits, and its output is the loss of hydraulic conductance of embolized conduits, with consequent impacts on whole-plant water transport and leaf gas exchange (Savi et al, 2015). If xylem embolism is severe enough to disrupt root-to-leaf water supply, plant desiccation and death might occur as a consequence of hydraulic failure (Barigah et al, 2013).…”

Variations in seasonal drought tolerance rankings

“…However, a combined high spring and summer tolerance is also desirable if a risk of spring drought is present. Drought stress is more likely to occur in all seasons in urban areas where limited rooting volumes and increased root injury are likely (Clark and Kjelgren 1990;Savi et al 2015). Data presented by Sjöman et al (2015) identify Acer monspessulanum as the most summer drought tolerant (π tlp ca.…”

Urbanization disrupts latitude‐size rule in 17‐year cicadas

“…Urbanization, whether through warming, landscape fragmentation, or pollution, may affect xylem quality and availability. Urban trees tend to have higher incidences of xylem cavitation due to urban warming (Bush et al., ; Litvak, McCarthy, & Pataki, ; Savi, Bertuzzi, Branca, Tretiach, & Nardini, ) and are of poorer quality compared to rural trees (McDonnell et al., ). Conversely, urban horticultural practices, such as the use of fertilizer and water supplementation, may have a positive impact on the quality of xylem fluid as cicadas associated with fertilized trees develop faster, have larger body sizes, and are found typically in higher densities (Karban, ; White & Lloyd, ; White, Lloyd, & Zar, ).…”

“…Periodical cicadas may be sensitive to a converse Bergmann's rule effect because they feed primarily on xylem fluid in tree roots, which consists of water and inorganic ions, and consequently grow very slowly (White & Strehl, 1978 cavitation due to urban warming (Bush et al, 2008;Litvak, McCarthy, & Pataki, 2012;Savi, Bertuzzi, Branca, Tretiach, & Nardini, 2015) and…”