Soulé, P.T. and
Knapp, P.A.
2019
Radial
growth rate
responses
of western juniper
(Juniperus occidentalis Hook.)
to atmospheric and
climatic changes:
A longitudinal study
from central Oregon,
USA.
Forests 10: 1127,
doi:10.3390/f10121127
NOTE:
Climate
conditions
have been
less favorable
for growth of
western juniper
in recent years.
The climate of
central Oregon
has become
"warmer and drier
during the
western juniper
growing season"
over the past
two decades
( 1998-2017 ).
The two researchers
wondered if the known
growth-enhancing
benefits of rising
atmospheric CO2
would benefit the
western juniper
in an otherwise
negative growing
environment.
SUMMARY:
"Despite a
trend toward
increasing aridity
manifested through
significant increases in
maximum temperatures
during the summer
growing season,
western juniper
radial growth
post-1960 exceeds
growth pre-1960,
especially during
drought years."
"Since the
observed
changes
in climate
have not been
conducive for
enhanced
radial growth,
a logical conclusion
is that some other
mechanism is allowing
the trees to grow better
during drought conditions
in the later portion
of the record."
Soulé and Knapp say
"in central Oregon,
the most logical
mechanism is that
CO2 enrichment
continues to
increase the
intrinsic
water-use
efficiency of
western juniper,
allowing the trees
to grow at
relatively faster
rates during
periods of drought
in recent decades
when compared
to growth experienced
during drought periods
of the early 20th century."
As the CO2
concentration
continues to rise,
trees and plants
are becoming
better equipped
to face the
growth-retarding
influences of aridity
( caused by a higher
temperature or drought ).
DETAILS:
Soulé and Knapp
examined
tree ring samples
from mature
western juniper
( Juniperus occidentalis )
trees at four sites
in central Oregon.
Those sites are free
of outside influences,
that include
fire suppression,
logging and grazing.
The resultant
chronology
covered
the years
1905-2017.
"Maximum temperatures
in May-June were above
the long-term (1905-2017)
average during
70% of the years,
averaging
+0.64 degrees C.
above average
during these decades."
"June values
of the Palmer Drought
Severity Index (PDSI),
which measures
soil moisture based on
precipitation and
temperature conditions
and is a primary
determinant
of radial growth
for western juniper,
averaged -0.92
(-1.0 is "abnormally dry")
for the 20-year period,
and 45% of those years
were in the
"moderate drought" range
(-2.0 to -2.9) or worse."