Post on 30-Jun-2020
Columnar cacti: sensitive recorders
of CAM responses
to environmental change
David G. Williams
Departments of Botany and Ecosystem Science and Management,
University of Wyoming
Ensemble of GCMs, assuming moderate (A1B) emissions scenario, predict warming in the Southwest of North America of 4-5°C for summer and 3-4°C for winter by 2100 relative to average temperatures 1971-2000 (NOAA Earth System Research Laboratory )
“Mega-droughts” are common in paleoclimate record
Is the Sonoran Desert
losing its cool?
Weiss & Overpeck 2005
Increase in extreme
minimum temperatures
observed from 1960-2000.
Isotopes record source and process information
From West et al. 2006
Columnar cacti -
the ‘baleen whales’ of
the desert?
Do they record source
and process information?
Columnar cacti are‘foundation’ species
Carnegiea gigantea
saguaro
Cultural significance
Younger
Numerous areoles on a saguaro plant are produced
each year (4-8/yr along each outer rib)
Figure available at calib.qub.ac.uk/CALIBomb/
Data from Reimer, Brown and Reimer, 2004.
F1
4C
Year
Can we date the spines?
Post-bomb radiocarbon in spines
Fraction modern
saguaro #162
Observed growth and
demography Pierson and Turner (1998)
monitored growth of almost
3000 saguaro from 1963 to
1993.
Figure from E. Pierson and R. Turner 1998
English et al., 2007
Water uptake (two-end member mixing)
δtissue* = δrain (frain) + δtissue (1 – frain)
Transpired water
Cactus tissue water
δ1
8O
(‰
)
Fraction of water remaining (f, %)
δ = (1000+δi) f (α-1)-1000
Transpiration -
Rayleigh distillation?
Modeled and observed δ18O of stem water
English et al. 2007
Irrigation withheld
Oxygen isotope composition of spine tissue records
changes in water balance over time
English et al., 2007
4.4 -5.7
Stomatal closure or Increased carboxylation demand
Winter & Holtum 2002
Strong
CAM
Carbon isotope discrimination
in ‘strong’ CAM plants
English et al., 2010
D =
English et al., 2010
Minimum annual d13C is negatively correlated total annual
precipitation and average nighttime VPD
d18O and d13C of spine biomass are
positively correlated
On-line carbon isotope discrimination measurements
Modeled and observed D13C differ
Anet (mmol CO2 m-2 s-1)
Do
bs –
Di (‰)
Photosynthetic rate influences discrimination
Response to temperature
and drought stressors
Stem volume to
surface area ratio
(V:S) varies by over
an order of
magnitude in
columnar cacti Pachycereus schotii Pachycereus pringlei Cylindropuntia leptocaulis
Tradeoff between growth and storage?
36
38
40
42
44
46
48
50
52
54
56
Sp
ine d
18O
VS
MO
W
High V:S
Low V:SDry
Year
Wet
Year
0
5
10
15
20
25
30
35
0 20 40 60 80 100
water loss (%)
d1
8O
(V
SM
OW
)
High V:S
Low V:S
Low V:S
High V:S Rayleigh distillation curves
Approach
Examine trait correlations
across a broad range of
taxa that vary in V:S
> 40 columnar cacti spp
(+ Cylindropuntia)
Monitor growth and isotopic
composition of multiple columnar
cacti across a broad climate
gradient
Experiments
exposing plants
of different V:S
to carefully
controlled stress
conditions
Hydrogen isotope ratio of spine biomass is positively correlated to V:S
Conclusions Rapid and dramatic climate change has happened, is happening and will happen in regions supporting high CAM biodiversity Sub-annual to decadal variation in physiology and metabolism of columnar cacti are recorded in the stable isotope composition of spines We now are poised to examine responses of a range of columnar cacti to climate variation using, experiment, field monitoring and isotopic analysis Functional tradeoffs among key traits in CAM stem succulents may underlie responses to extreme mega-droughts anticipated for subtropical desert regions
Enrico Yepez, ITSON, Mexico
Darren Sandquist, Cal State Fullerton
Dustin Bronson, Wisconsin DNR
Kevin Hultine & Raul Puente-Martinez, Desert Botanical Garden, Phoenix
David Dettman, U. Arizona Nathan English, James Cook University