Winter and shoulder season climate trends at the upland and floodplain sites

of the Bonanza Creek LTER

Bonanza Creek LTER monthly meeting: January 25, 2007

Eugénie Euskirchen, Jennifer Harden, Kyle McDonald, Jonathan O’Donnell, Joy Clein, Brian Riordan

Motivation I

Bonanza Creek LTER has data for >15 years regarding air temperature, soil temperature, and snowpack.

Recent studies at various spatial scales in high latitudes have found

• increases in air temperature, • decreases in snow cover, • an earlier date of soil thaw, • a later date of soil freeze, and • a longer growing season during the past several decades.

Do we see similar trends at Bonanza Creek?

Motivation II

Bonanza Creek LTER also has two classes of landscapes:

1. well-drained uplands 2. moderately to (somewhat) poorly-drained lowlands

Recent studies at in boreal forests have found that compared to poorly drained soils:

• well-drained soils have warmer surface temperatures during the growing season• well-drained mineral soils have earlier soil thaw dates

Do we see similar trends at Bonanza Creek?

Questions

I. Do we see trends in air temperature, snow cover, and soil temperature over the past ~15 years?

II. Are trends different between the uplands & the floodplains?

III. To what extent are these results representative of Interior Alaska?

Strategy (outline for rest of talk)

A . Examine trends over time in:Air Temp, Soil temp, snowpack, soil thaw, freeze-up

Specifically For:Early winter (Oct/Nov)

Mid-winter (Dec/Jan/Feb)Late winter (Mar/Apr)

B. Examine thaw, freeze-up dates and growing season length (e.g., period of unfrozen ground) among uplands versus floodplains

Site Selection: BC LTER

Site Number Stand Age

Tree Cover Litter/O Mineral Texture

Soil Drainage

Well Drained Uplands

100,111,

121,131

Cleared and

23 to >90 yrs

Aspen,Birch,

White Spruce

< 5 cm to ~9 cm

Silt to Silt Loam

Well Drained (category 3)

Moderately to Somewhat Poorly Drained Floodplain Lowlands

200,211

221,231

Cleared

and

23 to >100 yr

Poplar, Alder, White Spruce,

Black spruce mix

< 5cm to >= 9

~ Silt Loam

Stratified

Sand/Peat

Moderately

Drained (category 4)

Note: Litter/O may change over timeExact depths to be re-measured at probe sites

Well Drained Uplands: White Spruce, Mixed Deciduous

-20

-10

0

10

20

30

40

50

60

0 20 40 60 80Root Abundance Index

He

igh

t a

bo

ve

Min

era

l So

il, c

m

Mineral Soil

Organic Soil

Root abundance index (Harden, unpub.) from Ping et al, soil descriptions at BZ LTER

Root Abundance in Uplands and Floodplains

BC LTER White Spruce

Soil Thaw, Julian Date90 100 120 140 160

Xyl

em F

lux

Initi

atio

n, J

ulia

n D

ate

160

140

120

100

K.C.McDonald in prep 2007

Thaw of Litter

IntermittentContinuous

Start Finish

Xyl

em F

lux Continuous

Thaw at 10cm

Intermittent

Start FinishX

ylem

Flu

x

X=Y line isBest Proxy for

onset of growing season

(litter, 10 cm important)

Start Finish

Thaw of Litter

IntermittentContinuous

Start Finish

Xyl

em F

lux Continuous

Thaw at 10cm

IntermittentX

ylem

Flu

x

K.C.McDonald, in prep. 2007

X=Y line isBest proxy for

onset of growing season

(only litter important)

Xyl

em F

lux

Initi

atio

n, J

ulia

n D

ate

160

140

120

100

Soil Thaw, Julian Date90 100 120 140 160

BC LTER Black Spruce

Data Status and Data Quality

Temperature:

Screened data for outliers: checked that data were not out of range (e.g., that diurnal minimums and maximums were not drastically far apart, unrealistic values, etc.)

Snow:Determined that data were not out of range (if

collected continuously)

site w/ 25 cm o rgan ics

12 /6 /991 /25 /00

3 /15 /005 /4 /00

6 /23 /008 /12 /00

10 /1 /0011 /20 /00

1 /9 /012 /28 /01

da te

-6

-4

-2

0

2

4

6

8

10

valu

e

Scatte rp lo t (Shee t1 in Imported from C :\Documents and Se ttings\jha rden \My Documen ts\LT ER2006 \so iltemps bz\bnz_so il_ou t_211 .xls 7v*65535c)

12 /6 /991 /25 /00

3 /15 /005 /4 /00

6 /23 /008 /12 /00

10 /1 /0011 /20 /00

1 /9 /012 /28 /01

da te

-4

-2

0

2

4

6

8

10

12

14

16

valu

e Note – phase changes are evident in ‘flat lines’ that directly precede thaw or freeze

Determination of “thaw and freeze dates”

Summer (Jun, Jul, Aug)

Mea

n te

mpe

ratu

re

12

13

14

15

16

17

18

19

Spring (Mar, Apr, May)

1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

Mea

n te

mpe

ratu

re

-4

-2

0

2

4

6

r = -0.10p = 0.18n = 175

r = -0.30p = 0.71n = 152

All sites

Summer (Jun, Jul, Aug)

Mea

n te

mpe

ratu

re

13

14

15

16

17

18

19

Spring (Mar, Apr, May)

1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

Mea

n te

mpe

ratu

re

-4

-3

-2

-1

0

1

2

3

4

r =0.04p = 0.88n = 18

r = 0.06p = 0.85n = 17

Floodplain weather station, mowed area

Trends in Air Temperature: Summer & Spring

Winter (Dec, Jan, Feb)

Mea

n te

mpe

ratu

re

-35

-30

-25

-20

-15

-10

-5

Fall (Sept, Oct, Nov)

1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

Mea

n te

mpe

ratu

re

-14

-12

-10

-8

-6

-4

-2

0

2

4

r = 0.36p < 0.0001n = 155

r = 0.40p < 0.0001n = 173

Winter (Dec, Jan, Feb)

Mea

n te

mpe

ratu

re

-35

-30

-25

-20

-15

-10

-5

Fall (Sept, Oct, Nov)

1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

Mea

n te

mpe

ratu

re

-10

-8

-6

-4

-2

0

r = 0.50p = 0.04n = 17

r = 0.51p = 0.03n = 18

Floodplain weather station, mowed area

All sites

Trends in Air Temperature: Winter & Fall

Decreases in mean daily minimum soil temperatures:Early winter

Uplands

Mea

n da

ily m

inim

um (

ºC)

Floodplains

R2 = 0.2- 0.3, p < 0.05

-12

-10

-8

-6

-4

-2

0

2

1988 1992 1996 2000 2004

5 cm depth

10 cm depth

20 cm depth

For 5 cm depth:

R2 = 0.2, p < 0.05

-12

-10

-8

-6

-4

-2

0

2

1988 1992 1996 2000 2004

5 cm depth

10 cm depth

20 cm depth

R2 = 0.2 - 0.5, p < 0.05

-18

-14

-10

-6

-2

2

1988 1992 1996 2000 2004

5 cm depth

10 cm depth

20 cm depth

R2 = 0.3, p < 0.05

-18

-14

-10

-6

-2

2

1988 1992 1996 2000 2004

5 cm depth

10 cm depth

20 cm depth

Mea

n da

ily m

inim

um (

ºC)

Significant decreases in mean daily minimum soil temperatures:Mid-winter

Significant decreases in mean daily minimum soil temperatures:Late winter

R2 = 0.3, p < 0.05

-12

-10

-8

-6

-4

-2

0

1989 1991 1993 1995 1997 1999 2001 2003 2005

5 cm depth

10 cm depth

20 cm depth

R2 = 0.3 - 0.6, p < 0.05

-12

-10

-8

-6

-4

-2

0

2

1989 1991 1993 1995 1997 1999 2001 2003 2005

5 cm depth

10 cm depth

20 cm depth

Uplands Floodplains

Mea

n da

ily m

inim

um (

ºC)

Decreases in snow depth

R2 = 0.6, p < 0.0001

0

20

40

60

80

100

120

1988 1992 1996 2000 2004

Late winterfloodplains

Late winteruplands

R2 = 0.3 - 0.5, p < 0.05

0

20

40

60

80

100

120

140

1988 1992 1996 2000 2004

Mid-winterfloodplains

Mid-winteruplands

R2 = 0.2 to 0.3, p < 0.05

0

10

20

30

40

50

60

1988 1992 1996 2000 2004

Early winterfloodplains

Early winteruplands

Sno

w d

epth

(cm

)

Early winter Mid-winter Late winter

Early winter Mid-winter Late winter

R2 = 0.2 - 0.5, p < 0.05

-12

-10

-8

-6

-4

-2

0

2

0 20 40 60

R2 = 0.2 - 0.3, p < 0.05

-20

-16

-12

-8

-4

0

4

0 50 100 150

R2 = 0.4, p < 0.05

-12

-8

-4

0

4

0 50 100 150

Mea

n d

aily

min

imu

m s

oil

te

mp

. (º

C,

5 cm

dep

th)

Snow depth (cm)

Uplands Floodplains

Snow depth versus soil temperature

Mean day of thaw (various depths) No significant trends in any of the time series.

Day

of

thaw

Uplands Floodplains

3-Apr

13-Apr

23-Apr

3-May

13-May

23-May

2-Jun

12-Jun

1988 1992 1996 2000 2004

Day of thaw at 5 cm depthDay of thaw at 10 cm depth

Day of thaw at 20 cm depth3-Apr

13-Apr

23-Apr

3-May

13-May

23-May

2-Jun

12-Jun

1988 1992 1996 2000 2004

Day of thaw at 5 cm depthDay of thaw at 10 cm depthDay of thaw at 20 cm depth

Mean day of soil freeze (various depths)No significant trends in any of the time series.

Day

of

free

ze

30-Sep

10-Oct

20-Oct

30-Oct

9-Nov

19-Nov

29-Nov

9-Dec

19-Dec

1988 1992 1996 2000 2004

Day of Freeze at 5 cm depthDay of Freeze at 10 cm depthDay of Freeze at 20 cm depth

30-Sep

10-Oct

20-Oct

30-Oct

9-Nov

19-Nov

29-Nov

9-Dec

19-Dec

1988 1992 1996 2000 2004

Day of freeze 5 cm depthDay of freeze 10 cm depthDay of freeze 20 cm depth

Uplands Floodplains

Num

ber

of d

ays

of

unfr

ozen

gro

und*

*

Upland sites: Decrease in growing season length ???

Slope suggests a loss of about 2 days per decade (no trend at 5 cm depth)

**Obtained by subtracting the Julian day of freeze from the Julian day of thaw.

150

160

170

180

190

200

210

220

230

1989 1991 1993 1995 1997 1999 2001 2003 2005

5 cm depth

10 cm depth

20 cm depth

For 10 and 20 cm depths: R2 = 0.3, p < 0.05

Num

ber

of d

ays

of

unfr

ozen

gro

und*

*

**Obtained by subtracting the Julian day of freeze from the Julian day of thaw.

Floodplain sites: (no trend at 10 and 20 cm depth)

For 5 cm depth: R2 = 0.3, p < 0.05

150

160

170

180

190

200

210

220

1988 1990 1992 1994 1996 1998 2000 2002 2004

5 cm depth

10 cm depth

20 cm depth

100

110

120

130

140

150

Uplands Floodplains

Mean day of soil thaw between 1989 - 2005

5 cm depth 10 cm depth 20 cm depth

Jul ia

n da

y

290

300

310

320

330

340

Mean day of soil freeze between 1989 - 2005

5 cm depth 10 cm depth 20 cm depth

Uplands FloodplainsJu

lian

day

170

180

190

200

210

Jul ia

n da

y

Mean number of days of unfrozen ground between 1989 - 2005

5 cm depth 10 cm depth 20 cm depth

Uplands Floodplains

Revisiting our Questions

I. Trends over time 1. Less snow in early and mid winter2. Colder night-time soil temperatures (as a result of snow)3. Changes in # days of unfrozen soil

Role of snow#1-2 likely causative: Less snow has led to colder (nighttime) soil temperatures over the past 15-20 yearsImpact of thaw?There are no trends in the timing of soil thaw or freeze-up, however…Changes in the period of frozen ground?…. There were small enough trends in each such that the period of frozen ground may be changing

(interactive effect for sites – see next slide)

Revisiting our QuestionsII Differences between uplands and lowlands

-Both uplands and floodplains show:-similar significant decreases in soil temperatures & snow depth-no trends in soil thaw or soil freeze days individually

-Earlier soil thaw (only slightly) & earlier soil freeze in the floodplains compared to uplands

- Growing season trends: Shorter period of unfrozen ground in the uplands

(result of night-time winter temps cooling?) Longer period of unfrozen ground in the lowlands

(result of air temperature warming?)

-Soil Drainage Class: ~ 40% of Interior is class > 4

-BZ LTER 100, 200 are classes 2 – 4

-BZ LTER 250s arenot well instrumented

Weighted Drainage ClassDry (Gravelly)

Wet (Fens,Lakes)

(LTER 100s)

(LTER 200s)

(LTER 250s)

0.00-1.00

1.01-2.95

2.96-3.90

3.91-5.05

5.06-5.90

5.91-6.86

Revisiting our QuestionsIII. Representative of Interior Alaska?

Questions and Issues for BC LTER:

1. Black spruce with thick O horizons under-represented in database

2. Soil moisture important for timing of thaw, seasonal moisture balance

3. Depth of O horizons (with litter) need to be measured every 1-2 yrs at LTER sites where probes are located

4. Moisture and temperature probes needed in O horizons because there are so many roots there

OOO (2 – 5 cm thick)

(2 – 9 cm thick) (20 – 25 cm thick)

7 to 14 cm deep

7 to 10 cm deep 25 to 30 cm deep

Well Drained Poorly DrainedModerately Drained

Probe Issues

Thank you for listening