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