- The study examined variation in hydraulic traits like xylem vessel diameter and specific leaf area (SLA) across populations of Protea repens (King protea) plants from different climate regions grown in a common garden experiment. - Traits differed significantly among populations in the common garden, showing low phenotypic plasticity, with populations from drier areas having smaller vessel diameters and slower growth. - All populations employed similar stomatal control and hydraulic strategies irrespective of source, regulating stress to avoid hydraulic failure during drought while maintaining photosynthesis and transpiration. - This strategy is vulnerable to carbon starvation during prolonged drought, so populations with lower water stress thresholds may be more vulnerable to climate change impacts on water availability.

1. Gradient sampling and the common garden
6 P. repens individuals from 12 populations across
the CFR sampled in the field and in a common garden.
Trait sampling:
Quantify Xylem vessel diameters & frequency
Hydraulic diameter (DH) calculated as (Σd4/n)0.25
Specific Leaf Area (SLA) and Leaf Specific Hydraulic Diameter (DHL)
SLA = Leaf Area/Dry mass
DHL = DH/AL
Physiological rates:
Photosynthesis, transpiration, water use efficiency and plant water stress.
Models:
Linear mixed effect models with population as nested random effect
(Zuur et al. 2009)
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Kobus Kellermann; Co-Authors: A/Prof Edmund February and Dr Jasper Slingsby
• P. repens traits differ significantly
among populations planted in the
common garden, showing low
phenotypic plasticity (Figure 3).
• Differences in hydraulic traits should
affect water stress thresholds among
populations.
• Populations in more arid areas have
smaller xylem vessel diameters and
slower growth rates when grown under
the same conditions in a common
garden (Figure 3)
• Protea repens regulates hydraulic stress
through stomatal control, to avoid
hydraulic failure during extremely hot
and dry periods (late February).
• Individuals employ similar hydraulic
strategy irrespective of their source
populations and thus photosynthetic
and transpiration rates do not differ
significantly between populations
during times of drought stress.
• This hydraulic strategy is vulnerable to
carbon starvation during prolonged
drought and we expect that P. repens
populations with lower water stress
thresholds should be more vulnerable to
carbon starvation.
• GOAL 1: Explore hydraulic trait variation across climatic
gradients of the Cape Floristic Region in the widespread species
Protea repens.
• GOAL 2: Test whether variation is limited by genotype or
environmental conditions by comparing field measurements
with those taken from an experimental common garden.
INTRODUCTION
OBJECTIVES
METHODS
RESULTS
• Anthropogenic climate change may affect plant available water,
increasing plant water stress.
• Increased atmospheric CO2 may alleviate water stress through
decreasing transpiration rates (Donohue et al 2013).
• An ability to respond to these changes will affect both species
distribution as well as vegetation composition.
• Hydraulic traits such as leaf area and xylem vessel
diameter affect water use efficiency and drought
tolerance (Jacobsen et al., 2006 & Thuiller et al., 2004).
• Examining these traits across climate gradients
allows development of models to predict vegetation
change over time.
LITERATURE CITED AKNOWLEDGEMENTS
I am grateful for the financial support received from
the Applied Centre for Climate and Earth System
Services (ACCESS), through the South African
Environmental Observation Network (SAEON). I
would not have been able to complete my thesis
without the immeasurable guidanceand support of
my supervisors, A/Prof February and Dr Slingsby.
Figure 2: Examples of 20µm cross section of Protea repens stem. Radial
diameters were measured for the 30 biggest xylem vessels of each
individual, visible as white circular shapes.
Hydraulic Trait Variation in Protea repens with Change
in Climate and Atmospheric CO2
DISCUSSION
Figure 1: Sampled Protea repens populationsdistributedacross the Cape
Floristic Region.
Figure 3: Variation in Hydraulic Diameter (DH), Specific Leaf Area (SLA) and Leaf
specific hydraulic diameter (DHL) between Protea repens populations from wild
populations(blue)and a common garden in Kirstenbosch (green). Hydraulic
diameter varies significantly between populations(p<0.001) and treatment (field
vs. garden; p<0.001). SLA varies significantly between wild samples and the
common garden (p<0.001) but do not differ significantly between populations
(p=0.111).