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​

​As part of a series of plant gas exchange measurements I have been making in collaboration with Rolf Siegwolf and Lucas Cernusak, we have estimated boundary layer conductance in the Walz gas exchange system that we are using. As a contribution for the common good, I thought I would make those estimates available.
 
The estimates are made for a Walz 3010GWK gas exchange chamber with and without their new elbow flange. We measured conductance to water vapor as the evaporative flux from a saturated piece of filter paper cut into the shape of a poplar leaf with different fan speeds. To do so, we cut two pieces of filter paper, made a thin slit in one, and inserted the thermocouple between the two pieces in the leaf cuvette.
 
The results show that the new flange makes an overwhelming difference in removing the boundary layer and increasing conductance. There are modest differences in leaf size, but this is largely due to the difficulty of keeping the smaller leaf saturated at high fan speeds. Overall, these estimates may be lower than those made in other systems, particularly in comparison to smaller leaf cuvettes, but the result is still a very low resistance.
 
If you have questions or would like access to the raw data, please do not hesitate to contact me. 

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

Peel off the bark of your nearest pine tree, fasten it to a source of dirty water, and wait for the clean water to come out the other side. 

Providing a simple, cheap, and sustainable method for filtering dirty water is that simple, according to a new study published this week in PLoS ONE. The study demonstrates that a 2 cm long piece of branch with a 1 cm diameter can filter 99.9% of the E. coli out of a water sample and that each segment could be effective for a few liters a day. Not surprisingly, it needs to be fresh branch wood, not dried.

The plant's xylem, and more specifically the torus-margo pit membrane,  is responsible for all of the action. Xylem is the series of tubes that water travels through in a branch --- torus-margo pits are tiny closable valves between tubes. The interesting thing about the torus-margo pits is that they have the appearance of a spiderweb, with a pancake in the middle (the best description I can come up with...), and that spiderweb is where all of the E. coli is captured. Torus-margo pits serve a vital function in the plants, when there is not enough water the spiderweb flexes so that the pancake presses against the connection between the two tubes, effectively isolating one (empty) tube from another (full tube). It's very cool to consider another possible function, albeit totally unintended, for this anatomical feature. 

Boutilier et al. 2014. Water filtration using plant xylem. PLoS ONE 9(2): e89934.