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 ODonnell, 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 NumberStand Age Tree CoverLitter/OMineral 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, ,231 Cleared and 23 to >100 yr Poplar, Alder, White Spruce, Black spruce mix = 9 ~ Silt Loam Stratified Sand/Peat Moderately Drained (category 4) Note: Litter/O may change over time Exact depths to be re-measured at probe sites

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 Date Xylem Flux Initiation, Julian Date K.C.McDonald in prep 2007 Thaw of Litter Intermittent Continuous StartFinish Xylem Flux Continuous Thaw at 10cm Intermittent Start Finish Xylem Flux X=Y line is Best Proxy for onset of growing season (litter, 10 cm important)

Start Finish Thaw of Litter Intermittent Continuous StartFinish Xylem Flux Continuous Thaw at 10cm Intermittent Xylem Flux K.C.McDonald, in prep X=Y line is Best proxy for onset of growing season (only litter important) Xylem Flux Initiation, Julian Date Soil Thaw, Julian Date 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)

Note – phase changes are evident in flat lines that directly precede thaw or freeze Determination of thaw and freeze dates

All sites Floodplain weather station, mowed area Trends in Air Temperature: Summer & Spring

Floodplain weather station, mowed area All sites Trends in Air Temperature: Winter & Fall

Decreases in mean daily minimum soil temperatures: Early winter Uplands Mean daily minimum (ºC) Floodplains

Mean daily minimum (ºC) Significant decreases in mean daily minimum soil temperatures: Mid-winter

Significant decreases in mean daily minimum soil temperatures: Late winter Uplands Floodplains Mean daily minimum (ºC)

Decreases in snow depth Snow depth (cm) Early winter Mid-winterLate winter

Early winter Mid-winter Late winter Mean daily minimum soil temp. (ºC, 5 cm depth) 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 UplandsFloodplains

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

Number of days of unfrozen ground** 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. For 10 and 20 cm depths: R 2 = 0.3, p < 0.05

Number of days of unfrozen ground** **Obtained by subtracting the Julian day of freeze from the Julian day of thaw. Floodplain sites: (no trend at 10 and 20 cm depth)

Uplands Floodplains Mean day of soil thaw between cm depth10 cm depth20 cm depth Julian day

Mean day of soil freeze between cm depth10 cm depth20 cm depth Uplands Floodplains Julian day

Mean number of days of unfrozen ground between cm depth10 cm depth20 cm depth Uplands Floodplains

Revisiting our Questions I. Trends over time 1. Less snow in early and mid winter 2. 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 years Impact 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 Questions II 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 are not well instrumented Weighted Drainage Class Dry (Gravelly) Wet (Fens,Lakes) (LTER 100s) (LTER 200s) (LTER 250s) Revisiting our Questions III. 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

O O O (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 Drained Moderately Drained Probe Issues

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