Sugar storage is prioritized over growth in pine trees at a dry
site and explains auto-correlation in tree-ring width
Georg von Arx, Alberto Arzac, Patrick Fonti, David Frank, Roman Zweifel, Andreas
Rigling, Lucia Galiano, Arthur Gessler and José Miguel Olano
Carbohydrate reserves play a crucial role
in tree resistance and resilience to
drought. Ample reserves may buffer
temporary periods when plants reduce or
stop carbon uptake to prevent excessive
water loss during photosynthesis. But
prolonged drought might deplete
carbohydrate reserves. In addition, the
allocation of absorbed carbon to
carbohydrate reserves competes with
growth. This poses interesting questions
about carbon allocation priority during
periods of limited carbon uptake. For
example, the availability of older
carbohydrate reserves to support future
growth would alleviate trade-offs
between carbohydrate reserves and
growth. Larger carbohydrate reserves
may require larger storage capacity,
which in mature conifer trees is to a
large part provided by storage cells (ray
parenchyma) in the stem sapwood.
However, a sound understanding of how
carbohydrate reserves, parenchyma
abundance and growth rates are
interrelated, and how they respond to
changing water availability, is still
missing.
In this study, we investigated these
questions using 40 mature Scots pine
(Pinus sylvestris L.) trees from a 10-year
irrigation experiment conducted at a dry
site in Switzerland. From each tree, we
extracted several wood cores from bark
to pith and quantified carbohydrate
reserves, parenchyma abundance and
ring width along radial sapwood
segments. Results show that
The stem sapwood, visible here as the
translucent part of a wood core, is an important
storage container for carbohydrates. The
background shows the experimental forest stand
used in this study. Photo credit: Georg von Arx.
parenchyma abundance varied more
than twofold among trees, but only very
little from year to year within trees. In
addition, parenchyma abundance slightly
increased in irrigated trees, with a lag of
several years, but was unrelated to the
amount of carbohydrate reserves. This
means we did not find evidence for
parenchyma abundance limiting
carbohydrate storage. While wider tree
rings contained a lower concentration of
stored carbohydrates, they still contained
a larger absolute amount of
carbohydrate reserves. We also found
that rings with more carbohydrate
reserves were usually followed by a
wider ring.
Our results indicate a prioritization of
carbon allocation to storage instead of
ring growth, which we interpret as a
mechanism to ensure long-term survival.
In addition, the absolute amount of
carbohydrate reserves proved to be a
cause of the auto-correlation in tree-ring
growth from one year to the next.