Precipitation gradients, plant biogeography, and the incidence of drip-tips in Cerrado plant species

Examining how both climate and species distribution patterns
correlate with leaf morphology can give insights into the ecological and
evolutionary patterns that drive adaptive selection of leaf form and
function. Drip-tips are a common feature of leaves in rain forest tree
species; they are thought to be an adaptation that aids leaf drying and
maximizes photosynthesis in areas with high-rainfall climates. We tested
whether this macroecological pattern holds true across the precipitation
gradients in a non-rain forest region – the woodland savannas of Brazil
known as the Cerrado – and compared our results with previous studies from
Amazonia. Drip-tips were, as expected, less common overall in the drier
Cerrado than in Amazonia. In addition, within the Cerrado, drip-tips were
more prevalent in areas with higher rainfall as well as in Cerrado sites
that were closer to Amazonia. Moreover, species that occurred across both
the Cerrado and Amazonia had drip-tips more often than species that were
found only in the Cerrado. These findings support the hypothesis that
drip-tips are adaptive and that either the cost of retaining drip-tips is
low or that in drier regions they have other benefits.
Methods
Data on leaf characteristics were collected using online herbarium specimens from Kew (K), New York Botanic Garden (NY), the Field Museum (F), and Missouri Botanical Garden (MO), Flora Neotropica, and Gottsberger & Gottsberger (2006). Most specimens were collected in Brazil; we obtained 1–17 specimens per species (median = 2). Excluding two leafless tree-like Cactaceae, we found leaf size and tip data for 845 of the remaining 846 species. We were unable to locate specimens of Mezilaurus mattogrossensis (Lauraceae).

Leaf tip characteristics vary continuously across species (Gentry 1969). Furthermore, intra-specific variation in leaf form is common in some species but not others. To ensure consistency with previous data from Amazonia, we scored leaf tip type for each species as one of five ordinal categories: retuse, rounded, acute, small tip, and drip-tip, following Malhado et al. (2012) (Fig. 1 and SI Fig. 1) using the most abundant tip type observed in the specimens. To reduce variability that could be associated with observer bias, all images were assessed by the lead author (MKS). Because leaf tip can vary with leaf size, we classified leaf size into one of seven size categories using Webb’s (1959) modification of Raunkiaer’s (1934) method (sensu Malhado et al., 2009).