Sugar transport together with environmental conditions controls time lags between xylem and stem diameter change.

In the present study the seasonal patterns of time lags
between diurnal xylem and whole stem diameter variations
at the top and at the base of two Scots pine trees (
Pinus
sylvestris
L.) were compared. The diameter variations were
measured during the summers of 2001 and 2002. Time lags
were determined using the cross-correlation method. The
lags were found to vary in time according to the different
stages of growth. At the top the xylem lagged behind the
whole stem between the beginning of stem growth and the
end of shoot growth in both years. In 2001 the time lags at
the base showed a similar behaviour during stem growth.
That kind of seasonal pattern of the time lags would result
from the changes in the sink strength due to changing
growth rate at different parts of the tree and the differences
in the annual rhythm of growth and water availability in the
soil (based on precipitation measurements) between the
years 2001 and 2002 were reflected in the patterns. The time
lags of shrinking and swelling periods during high and low
photosynthetic activity (measured using a shoot chamber)
were also compared. It was found, for example, that in 2001
in the middle of the growing season at the top of the tree
the whole stem lagged on average 15 min more behind the
xylem on the days of high photosynthetic activity than on
the days of low or moderate. These results show for the first
time that the transportation of carbohydrates and variable
sink activity could be detected during the growing season
in field conditions using stem and xylem diameter variation
measurements. Furthermore, these results provide evidence
of the pressure gradient-driven flow also in the
phloem of gymnosperms.