Willow response to changing
climate on Yellowstone’s
Northern Winter Range
Introduction
Beginning about 1998 willows that had been
surpressed by elk browsing for more than
50 years on Yellowstone National Park’s
northern winter range began to grow tall in
some places. This presentation explores
the possibility that climate change could
be the cause.
Willow patch near Blacktail Deer Cr. with released and suppressed willows. Note well browsed
willow in foreground including many broken branches and broken willow clump at the left side
of the larger clumps. Branches on the edge of the tall clumps are also browsed.
Hedged willow height and height at end of winter 1998-99 (0.50-0.60 m) is indicated for a
large patch on the edge of a ca. 2 ha stand of willows. Note old elk tracks in foreground and
feeding craters behind and to the right of the clump.
Willows grew about a meter during the summer of 1999 then were browsed to this height
during the winter of 1999-2000 (ca 1.30 m)
Stems were unbrowsed in winter of 2000-01 and grew only 0.2-0.4 m. Height of
willows at beginning of winter of 2001- 2002, (2.3-2.4 m).
Clump inside large stand. Note feeding crater and numerous elk tracks and an elk bed behind
and to right of willow clump. Hedged height at end of winter 1998-99 (0.50-0.60 m).
Clump inside large stand. Height of willows after browsing of winter of 1999-2000 (ca
1.30 m)
Clump inside large stand. Height of willows at beginning of winter of 2001- 2002, (2.3-2.4
m). Stems were unbrowsed in winter of 2000-01 and grew only 0.2-0.4 m.
Two unusual things happened
to produce tall willows.
• Phenomenal growth
• Effectively defended against large
herbivores
Why?? Could it be climatically induced?
Willow Physiology
Growth:
• Depends on carbohydrates available and
is regulated by plant hormones (auxins)
produced in buds and expanding leaves
• Auxin must be transported to sites of
growth in stems
Could change in precipitation be the cause?
Water relations affect growth through
stomatal control that regulates the
availability of CO2 for photosynthesis.
Willows grow in wet places and generally
have enough available water to keep
stomata open during the day all season as
shown in the next slide.
Seasonal Water Relations
S. wolfii
S. planifolia
Betula
Taken from Young et al. (1985)
Precipitation is not significantly different
before and after the change in growth habit
Precipitation at Northeast Entrance
Water Yr
May-Aug
Inches
40
30
20
10
0
1985
1990
1995
Year
2000
2005
Could change in temperature be the cause?
Rates of growth during control conditions (20ºC) and lowtemperature (10ºC) treatments show strong climate affect
Rate of growth in length (mm mm/ day -1)
Experiment 1
Control
First day
Final day (30)
% (of control)
Hordeum
1.92 ± 0.10
0.32 ± 0.03
0.75 ± 0.04
44a
Phalaris
1.22 ± 0.06
0.21 ± 0.06
0.13 ± 0.03
11b
Festuca
0.09 ± 0.02
0.00 ± 0.02
0.01 ± 0.00
9b
Betula
0.74 ± 0.07
0.04 ± 0.04
0.19 ± 0.03
28c
Salix
1.05 ± 0.07
0.09 ± 0.03
0.14 ± 0.03
15c
Taken from Hjelm and Ögren (2003)
Photosynthetic cold acclimation of fully developed
leaves allows for carbohydrate production in
willows transferred from 20ºC to 10ºC.
Taken from Hjelm and Ögren (2003)
Fig. 4. (A) Aspen trees subjected to short-day treatment in a climate chamber
Schrader, J. et al. (2003) Proc. Natl. Acad. Sci. USA 100, 10096-10101
Copyright ©2003 by the National Academy of Sciences
Auxin Levels after transfer from long to
short days
Taken from Olsen et al. (1995)
Willow Physiology
Defensive Chemicals:
• Depend on available carbohydrates
• Occur only after growth needs are satisfied
Glycoside content in the bark of Salix
alba vs. month
Taken from Thieme (1965)
Glycoside content in the bark of Salix
purpurea vs. month
Taken from Thieme (1965)
Glycoside content in the bark of Salix
cinerea vs. month
Taken from Thieme (1965)
Willow Physiology Summary
• Temperature has a definite affect on
carbohydrate production
• Willows are adapted to cooler
temperatures.
• Auxin production and transport is
temperature and day-length sensitive
• Defensive chemicals are produced during
cold period of the year
Effects of Climatic Change
Day length control means that growth occurs
during early half of warm season leaving
carbohydrates produced during last half
available for storage, defensive chemical
production and other processes.
• Longer periods of favorable conditions in the
spring provide longer stems
• Longer periods of favorable conditions in the fall
provide more defensive chemicals
Total degree-days (May-Oct) measured at Northeast
Entrance do not indicate that atmospheric heat was
different before and after the change in growth habit
Degree-Days Total
Degree Days
2000
1500
1000
500
0
1985
1990
1995
Year
2000
2005
Total days (May-Oct) with minimum temperature greater than
freezing at Northeast Entrance increased 25 days (29%)
May-Oct
130
120
Days
110
100
90
80
70
60
1985
1990
1995
Year
2000
2005
Early season days with minimum temperature greater than
freezing at Northeast Entrance increased 12 days (23%)
May-Jul
75
70
Days
65
60
55
50
45
40
1985
1990
1995
Year
2000
2005
Late season days with minimum temperature greater than
freezing at Northeast Entrance increased 14 days (37%)
Days
Aug-Oct
60
55
50
45
40
35
30
25
20
1985
1990
1995
Year
2000
2005
Variation in Willow Response in a Small Area could be caused by variation in
genetic factors and microsite conditions
Conclusion
Willow physiology indicates that the increase
in days with minimum temperature greater
than 0ºC could explain the recent change
in willow growth habit.
Literature cited:
Hjelm U, Oegren E. 2003. Is photosynthetic acclimation to low temperature controlled by
capacities for storage and growth at low temperatures? Results from comparative studies of
grasses and trees. Physiologia Plantarum 119113-20.
Schrader J, Baba K, May ST, Palme K, Bennett M, Bhalerao RP, Sandberg G. 2003. Polar
auxin transport in the wood-forming tissues of hybrid aspen is under simultaneous control of
developmental and environmental signals. Proceedings of the National Academy of
Science 100(17)10096-101.
Thieme H. 1965. Die Phenologlycoside der Salicaceen, 6 Mitteilung Untersuchen ueber die
Jahrzeitlich Veraenderungen der Glycosidkonzentrationen, ueber die Abhangigkeit des
Glycosidegehalts von der Tageszeit und vom Alter Pflanzenorgane. Pharmazie 20688-91.
Young DR, Burke IC, Knight DH. 1985. Water Relations of High-elevation Phreatophytes in
Wyoming. American Midland Naturalist 114(2)384-92.