Figure 1: Avoid the
avoiders for paved and
highly restricted sites. This
birch (Betula utilis subsp.
jacquemontii), growing in a
highly restricted soil volume,
has lost almost all its leaves
in an attempt to avoid
deleterious water deficits. In
this image, other trees with
better growing environments
still have plenty of leaves.
Stress-avoiding species
are often poor choices for
challenging urban sites.
Arguably, the characteristic that is most helpful to
a tree in a paved environment is drought tolerance,
or more precisely, tolerance to water deficits. Even
in cool, humid regions, trees in paved sites are vul-
nerable to water deficits because the impermeable
surfaces effectively decouple soil moisture availabil-
ity from precipitation: rainfall often gets diverted
away from the root-zone or evaporated before it
can recharge the soil water within the tree pit.
In situations where soil volumes are small, water
deficits in trees can develop rapidly, especially
if the tree has a large crown. Therefore, trees
often face acute challenges in water acquisi-
tion in paved environments. These challenges
are also likely to come about with greater fre-
quency and severity than other potential stresses,
such as nutrient deficiencies or soil salinity.
In natural environments, trees tend to cope with
water deficits either by avoiding them or by toler-
ating them (Hirons and Thomas 2018). Deep rooting
(to access moisture deep in the soil profile), stoma-
tal closure at an early stage of the drying cycle (to
increase resistance to water loss from their leaves),
and leaf abscission (to reduce the water demand
from the crown) are all strategies that help trees
avoid water deficits. In contrast, some trees have
an ability to tolerate low (more negative) water
potentials as water availability declines. They have a
lower leaf turgor loss point, tend to keep their sto-
mata open, and can maintain hydraulic conductivity
in their stems for longer during the drying cycle.
In paved urban sites with highly constrained root-
ing environments, species that avoid water deficits
tend to be thwarted by the lack of exploitable >>
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