Download methods - Proceedings of the Royal Society B

METHODS
Qualitative trait measurements
Trait measurements and scores were based on methods and results of Bond et al
(2004) for New Zealand plants that resisted ratite browsing. The resistance of stems to
the tugging action of bird browsing was simulated by applying a force of ~10 kg to
five or more lignified lateral branches (i.e. new green shoots were avoided) usually on
five or more individuals of a species. This measures the breaking load of the branch
structure, rather than tensile strength of the stem material (force per unit area required
to break a material), since branches often broke at a node. Branches with a breaking
load < 10 kg were shown to be vulnerable to loss from the tugging action of bird
browsers (Bond et al. 2004). Plants with low breakages (<20% of branches tested)
were assigned a value of 1 for ‘tensile strength’ vs. 0 for >80% breakages. We
measured the spring-like extension of branches when a browser tugs as lateral
displacement of a branch when pulled with a force of ~10 kg. Branches that extended
50 cm or more from their resting position were scored as 1 indicating high lateral
displacement vs. 0 for <15 cm. Wide-angled branching can thwart bird browsers by
contributing to the springiness of a shoot and, when a shoot is detached, by exceeding
gape width preventing ingestion. A wide angle was defined as >70o from a lower
order branch. A score of 1 was assigned where most (>50%) of the side branches on a
terminal 50 cm shoot were wide-angled.). Leaf, rather than leaflet, dimensions were
measured for compound leaves since browsers would be likely to pluck off whole
leaves than individual leaflets. Leaf size (0.67 x length x breadth) was ranked as
follows: 0-60 mm2 = 1, 61-120 = 0.7, 121-240 = 0.5, 241-480 = 0.3, >480 mm2 = 0.
All traits listed in Table 1 were scored from 0 to 1 where 1 represents trait values
most similar to New Zealand divaricates and/or the ‘anti-ratite’ syndrome described
by Bond et al. (2004). Intermediate values (usually 0.5) were assigned in cases where
trait scores fell mid-way between definitions for 0 and 1.
Quantitative trait measurements on heteroblastic samples
Juvenile and adult branches, each 50 cm long, were collected from the same plant. We
measured the total length of all stems on the sample shoot together with the total
length of all shoots < 3 mm in diameter. The angle of branching of five side shoots
from the main stem was measured with a protractor (Atkinson 1992; Kelly 1994). We
measured the breaking load of side shoots by clamping a vice grip attached to a
dynamometer onto a lignified shoot (avoiding current year's growth). The equipment
was pulled slowly until structural failure or until the maximum capacity of the
dynamometer (10 kg) had been reached. We discarded trials where twigs broke at the
clamp. Failures were often structural, at the branch joints, rather than in the shoot
material of the internodes. The stem diameter at the break was measured. Leaf length
and breadth were measured on at least five fully expanded leaves and leaf area was
taken as 0.67 x length x breadth. Leaf, rather than leaflet dimensions were measured
for compound-leaved plants.