Year-round productivity in a mid-montane mixed conifer forest in the Sierra Nevada
Anne E. Kelly, Michael L. Goulden
Department of Earth System Science, University of California Irvine
(2020 m)
Figure 2. Study site in the
Kings River watershed, 60
km NE of Fresno, CA. Landsat Geocover image.
mmol CO 2 m
40
20
12
8
4
0
0
(c) VPD
(g) daily ET
6
mm day
−1
1.5
1
0.5
0
0.3
2
0
92.5
(d) Soil VWC
0.25
0.2
0.15
0.1
0.05
Oct
4
(h) Tree growth
92
91.5
91
Dec
Mar
Jun
Sep
Oct
Dec
Mar
Jun
Sep
Figure 3. Weather and ecosystem measurements for WY 2010. a) Daily high and low air
temperature, b) daily total PPFD, c) daily high and low VPD, d) soil VWC at 30 cm (blue)
and 90 cm (dashed red), e) cummulative GEE, f) mean daily NEE rates during sunny
periods, g) daily evapotrainspiration, h) stem biomass.
What is the growing season length?
During sunny periods (PAR > 200 µmol m-2 s-1), daily mean net
ecosystem exchange (NEE) rates were positive throughout the
year (Fig 3f). Positive NEE values indicate ecosystem uptake.
Stormy winter days experienced the lowest mean uptake rates.
Highest mean rates occured in late spring and summer. Minor decline was seen in late fall, and the forest did not experience winter
dormancy. About 1/3 of annual gross ecosystem exchange (GEE)
occured from October through March (Fig 3e).
s )
−2 −1
2
NEE ( µmol CO m
0
−5
Light/day length/storms
500
1000
1500
PAR (µmol γ m−2 s−1)
2000
Figure 5. Half-hourly NEE fit to incoming PAR for
PAR > 50. NEE data were binned inPAR incre2
ments of 50 before fitting, R = 0.36.
15
spring
autumn
10
5
−5
−10
−15
0
200
400
600
800
1000
−2 −1
PPFD (µmol γ m
s )
1200
1400
1600
Figure 6. Relationship between residual NEE
and PAR in spring and autumn. (Mean annual
light curve subtracted.)
30
spring
autumn
25
20
Temperature effects
15
are more substantially
10
different between
spring and autumn
5
(Fig 7). Springtime
0
shows higher produc−5
tivity at all tempera−10
−10
−5
0
5
10
15
20
25
tures, but highest proAir T (°C)
ductivity difference at Figure 7. Relationship between NEE (half hourly averages) and air temperature in summer and autumn.
the midrange. In
autumn, production is
depressed overall but shows no obviously different trends from
springtime temperature response.
Evapotranspiration rates are low in winter and spring (~1 mm
day-1) and highest in
summer (~3 mm day-1; Fig
400
8). Water use efficiency is
300
highest in winter and de200
clines by about half in
100
summer. Summer WUE
0
shows the most consistency
mean
5
2009
from year to year, indicating
2010
4
2011
that there is probably a
3
physiological limit to WUE
despite interannual variabil2
ity in environmental condi1
autumn
winter
spring
summer
tions such as temperature,
Figure 8. Seasonal evapotraspiration (top)
soil water content, and VPD.
ET (mm)
Tower
Light is the primary control on NEE (Fig 5). Winter light is the
primary cause of lower rates of winter production, rather than cold
temperatures as might be expected. Heavy cloud cover during
winter storms, shorter days, and low solar angle all contribute to
lower rates seen in winter. When light levels are high, winter NEE
rates match summer rates.
Temperature
0
What is the relationship between
productivity and water use?
Experimental design
Our experimental design focuses on two
years of eddy covariance measurements
which provide GEE, NEE, and evapotranspiration (ET) as well as climate and energy
balance data (Figure 2). Measurements of
aboveground NPP (ANPP) include tree diameter and increment measurements in 1
ha around tower footprint. Soil moisture
data come from SSCZO sites nearby.
Additional measurements will be used in
subsequent analysis, including foliar production, sap flux, and a vertical canopy air
temperature profile.
5
−15
0
2
s
−2
d
−2
mol PARm
(f) Mean net ecosystem uptake
16
−1
(b) PPFD
10
−10
Residual NEE (µmol CO m−2 s−1)
−1
tC ha
°C
−1
0
15
O−1)
Fresno
−10
20
Autumn production is lower at identical temperatures when compared to spring production (Fig 7). When light response is removed from the analysis, springtime production is greater only
within the temperature range from ~5 - 15 °C. There is no hardening off of the foliage as is found in other snow-dominated ecosystems: the trees remain able to photosynthesize at high rates in
mid-winter, as long as light is available and temperatures are
above freezing. It is also important to note rates of production are
high when air temperatures are below 5 °C. The temperature
threshold for photosynthesis shutdown is much lower in this forest
than almost every other forest type.
Drought
Soil water availability can be described as biomodal in this forest.
Soils never freeze in the root zone, so while snow is on the
ground, soils are nearly saturated. Less than 10% of annual precipitation falls between mid-April and October: after the snowpack melts, the forest must sustain itself on available soil water.
By late June, soils above 90 cm depth are depleted of water (Fig
3d), yet evapotranspiration rates remain very high through late
summer (Figs 3 and 8). These forests are likely tapping deep
sources of soil water, and thus access to deep soils is essential to
the survival of this forest type.
Acknowledgements
We would like to thank Roger Bales and the Southern
Sierra Critical Zone Observatory (NSF EAR-0725097).
We also thank Greg Winston, Matt Meadows, Scot
Parker, Paige Austin, Glen Cajar, and Ryan Johnson for
assistance in the field.
2
The study site is located in the central Sierra Nevada mountains of California, east of Fresno. Measurements are focused on a plot at 2020 m elevation within the Southern
Sierra Critical Zone Observatory. The site is dominated by
white fir (A. concolor) with some pines, a few oaks, and incense cedars. The site receives ~1100 mm precipitation each
year, with a mean summer
temperature
(JJA) of 18°C
and a mean
winter temperx
ture of 1°C.
Study site
5
What are the limits on forest
growth?
The controls on forest productivity explored here are light, temperature, and drought stress. Despite long, dry summers and cold,
snowy winters, this forest stays productive year-round. Early
summer production is highest, but winter NEE rates often reach
summer production rates. About a third of annual net production
takes place during the six coldest months.
WUE (g C kg
H
Study site - Southern Sierra
CZO
0
10
Half-hourly NEE was fit
to incoming PAR (Fig 5).
The residual of the light
response curve of NEE
shows little difference
between autumn and
spring (Fig 6). Autumn
shows a depression NEE
rates of less than 3 µmol
m-2 s-1 as compared to
spring.
Seasonal differences in
VPD response are similar; autumn NEE rates
are only slightly lower
than spring at VPD > 1
kPa (not shown). This
means that despite
nearly complete depletion of shallow soil
water in autumn months
(Fig 2d), the forest is
not experiencing
enough drought stress to
severely dampen net
CO2 uptake.
25
NEE ( µmol m−2 s−1)
and topography to the study site.
10
60
(e) cummulative GEE
15
−1
• How large and productive is this
forest?
• What is the growing
season of this forest?
• How do summer
drought and winter
cold limit productivity in this forest?
• What is the relationship between productivity and water Figure 1. The Sugarbowl Grove of giant seuse in this forest?
quoias, a nearby site with similar climatology
20
20
(a) Air T
tC ha
Research questions
How do summer drought and winter
cold limit productivity?
This forest has an aboveground stem biomass of about 92 tC ha-1,
comparable to the biomass of tropical forests (Fig 2h). Stem biomass accumulated about 1.2 tC ha-1 yr-1 during the study period,
with most growth during the summer.
kPa
The mid-montane forest of the Sierra Nevada supports the most
massive trees in the world, despite snowy winters and long, dry
summers. This region receives most of its precipitation as snow,
which serves as a seasonal reservoir of winter precipitation in its
Mediterranean climate. Understanding the feedbacks between
this forest and the water cycle is critical to predicting the vulnerability of this ecosystem and California’s water resources to climate change.
The climatology of these forests predicts savanna and not
dense forests of gigantic trees (Figure 1). The growing season is
predicted to be only a few months, with drought limitation in
summer and cold limitation in winter. The Miami model predicts an aboveground NPP of just 300 gC m-2 yr-1. How does
the climatology of the
mid-montane Sierra sustain such a large standing
biomass?
What is the biomass and productivity of the Sierra forest?
cm3/cm3
Introduction
Figure 4. Study site and eddy
covariance tower. (Courtesy M. Meadows)
and water use efficiency (bottom) for WY
2009-2011.
Contact
Anne Kelly: a.kelly@uci.edu; Michael Goulden:
mgoulden@uci.edu; Croul Hall, Department of Earth
System Science, University of California, Irvine, CA
92697-3100