Leaf drip tips are one of those features of tropical rain forests that always draws the eye. Walking through the forest during a hard rain, it just seems so obvious that drip tips- long narrow tips on the end of the leaves -must be associated with....drip. 
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In a new paper published in New Phytologist, my collaborators and I explore drip tips in the larger context of traits associated with leaf wettability. Plants in tropical rain forests frequently get wet. Wet leaf surfaces are considered bad for plant function. For instance, wet leaves have long been associated with increasing pathogen establishment and growth, decreasing rates of photosynthesis, and leaching nutrients out of the leaf. Drip tips are thought to increase the rate at which leaves dry by funneling water off of the leaf surface. 

The problem with this idea is that no one can really find any evidence that it works.

We demonstrate that drip tips do not vary with rainfall, but rather with temperature. The warmer the forest, the higher the proportion of species with drip tips. In fact, we also demonstrate that leaf water repellency, a trait that describes the hydrophobicity of the leaf surface, also does not vary with rainfall. The most hydrophobic leaves appear to occur in cold and dry environments, rather than warm wet environments where it would be beneficial to be hydrophobic.

What does all this mean? One possibility is that wet environments simply do not impair plant function as much as we might imagine. A second possibility is that we are measuring the wrong traits. 

As far as drip tips are concerned, the best evidence I can find suggests that they may simply be a function of leaf development - the formation of a long central vein followed by expansion of the remainder of the leaf.

Maybe it's time to stop calling them drip tips...

Goldsmith, G.R., L.P. Bentley, A. Shenkin, N. Salinas-Revilla, B. Blonder, R.E. Martin, R. Castro-Ccossco, P. Chambi-Porroa, S. Diaz, B.J. Enquist, G.P. Asner, & Y. Malhi. In Press Variation in leaf wettability traits along a tropical montane elevation gradient. New Phytologist DOI: 10.1111/nph.14121​

I am a co-author on a new paper that is in press at Global Change Biology. The paper explores how landscape level patterns in precipitation and soil drive differences in the carbon cycle of lowland Amazon forests. I'll write more on this soon, but in the interim, you can check out the layperson's abstract and access the full text through the link at the bottom: 

Understanding the relationship between photosynthesis, net primary productivity and growth in forest ecosystems is key to understanding how these ecosystems will respond to global anthropogenic change, yet the linkages among these components are rarely explored in detail. We provide the first comprehensive description of the productivity, respiration and carbon allocation of contrasting lowland Amazonian forests spanning gradients in seasonal water deficit and soil fertility. Using the largest dataset assembled to date, ten sites in three countries all studied with a standardized methodology, we find that (i) gross primary productivity (GPP) has a simple relationship with seasonal water deficit, but that (ii) site-to-site variations in GPP have little power in explaining site-to-site spatial variations in net primary productivity (NPP) or growth because of concomitant changes in carbon use efficiency (CUE), and conversely, the woody growth rate of a tropical forest is a very poor proxy for its productivity. Instead, (iii) spatial patterns of biomass are much more driven by patterns of residence times (i.e. tree mortality rates) than by spatial variation in productivity or tree growth. Current theory and models of tropical forest carbon cycling under projected scenarios of global atmospheric change can benefit from advancing beyond a focus on GPP. By improving our understanding of poorly understood processes such as CUE, NPP allocation and biomass turnover times, we can provide more complete and mechanistic approaches to linking climate and tropical forest carbon cycling.


Malhi, Y. C.E. Doughty, G.R. Goldsmith, D.B. Metcalfe, C.A.J. Girardin, T.R. Marthews, J. del Aguila-Pasquel, L.E.O.C. Aragão, A. Araujo-Murakami, P. Brando, A.C.L. da Costa, J.E. Silva-Espejo, F.F. Amézquita, D.R. Galbraith, C.A. Quesada, W. Rocha, N. Salinas-Revilla, D. Silvério, P. Meir & O.L. Phillips. 2015. The linkages between photosynthesis, productivity, growth and biomass in lowland Amazonian forests. Global Change Biology DOI: 10.1111/gcb.12859