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

I had a great experience recently collaborating with my post-doctoral advisor, Yadvinder Malhi, as well as Toby Gardner, Miles Silman and Przemek Zelazowski on a new review paper considering the effects of the Anthropocene on tropical forests. I will not go into the details of the review, other than to say that the primary focus is in identifying and exploring what human activities we expect to be the most critical for the future of tropical forest functioning. The paper is now available at the Annual Review of Environment and Resources.  

As part of the paper, we consider what research has been done on anthropogenic impacts in tropical forests and where that research has been done. In fact, we studied a subset of the entire peer-reviewed literature carried out in tropical forests for the last five years. Here's what we came up with: 

The size of the circle indicates the number of studies carried out at the location. The big circle with all those studies is Smithsonian's Barro Colorado Island Research Station in Panama; the big embedded circle within it is the Organization for Tropical Studies' La Selva Biological Station in Costa Rica. The number of research studies carried out there dwarfs any other place in the Tropics. 

On the whole, the takeaway message is that there is a disproportionate amount of research in Mesoamerica relative to its size.

 While many of us may be cognizant of this geographic bias, there has been no systematic effort (that I am aware of) to correct it. The advantages of research station infrastructure are obvious, as are the disadvantages of a knowledge base focused on such a small proportion of the Tropics. As a community, it may be time for a strategic plan for the future of tropical science. In the absence of such an approach, I fear that we'll never truly understand the impacts of the Anthropocene. 

A high resolution version of the figure, the figure legend, and the underlying data are freely available. Of course, do read the paper for all the interpretation....and please send me an email if you have thoughts. 

I am privileged to be visiting with Nalini Nadkarni this week at the University of Utah. Nalini has an incredibly distinguished career in both science and science communication. One need look no further than this pile of field research notebooks to understand that she has truly dedicated her life to understanding the ecology of tropical montane cloud forests. For me, opening these old books has brought nothing but new ideas for research. That we are doing new science- based on data collected over 25 years ago -speaks to just how meticulous Nalini was in the field. If I have learned nothing from this week, it is how incredibly valuable it can be to spend just a few extra minutes at the end of the day writing those seemingly obvious details down...you never know who will be reading it in the future. 

“A total of 20 terapixels of data were processed using one million CPU-core hours on 10,000 computers….”

A brief summary of the computational requirements for what I consider to be the most groundbreaking scientific paper that I read in 2013. The result of this number crunching is the first truly comparable and global map of forest cover, loss, and gain. All at the staggering scale of only 30 m. The paper, published by Hansen et al. in Science in November, processed satellite images of the earth’s surface using Google’s Earth Engine cloud platform between 2000 and 2012. The data will shortly be made publicly available, but for the time being they can be viewed on an absolutely captivating and easy-to-use website. When you open a browser to look at this map, be prepared to spend some time exploring.

Below, I have plotted the loss of tree cover in tropical countries. For me, the noticeable features are 1) the rates of deforestation in Brazil (there as been a ~50% reduction in ~7 years, the direct result of effective governance) and 2) the striking need for improved conservation in SE Asia (probably an underestimate, as the results do not accurately account for expansion of oil palm monocultures). 

In the past, different entities with different underlying data, different analysis and different motivations have produced different maps of deforestation at different spatial scales. Each map was….different. This is the first time we have ever had a global map applied with the same methodology. The results are not without their problems; however, I have every expectation that the quality will improve with time. That this map is online for anyone from governments to grandmas to use is going to fundamentally transform our capacity for effective conservation. 

Citation: M.C. Hansen et al. 2013. High-resolution maps of 21st-century forest cover change. Science 342: 850-853. 

There are some advantages to having blue leaves. If you were even aware that plants could have blue leaves to begin with...

The story from a new paper by Simon Queenborough, Margaret Metz and colleagues demonstrates that a number of different plant species in tropical forests demonstrate what is known as delayed greening. This is a phenomenon whereby plants growing new leaves delay the addition of green pigmentation (i.e. the chlorophyll for photosynthesis) until the leaves are more mature. This can lead to slightly less green leaves, white leaves, red leaves....and even blue leaves! The idea behind this, most extensively explored by Lissy Coley and Tom Kursar at the University of Utah, is that delayed greening serves as a defense against insects trying to eat the leaves (known as herbivores). Chlorophyll has a lot of nitrogen, which is a valuable nutrient for animals. 

The question is whether or not delayed greening ultimately benefits the plant. A leaf without green pigmentation can't be used effectively to build new sugars through photosynthesis. But this new paper, in a special issue of the American Journal of Botany, demonstrates that seedlings with delayed greening have lower rates of mortality. At the same time, it demonstrates that saplings with delayed greening have lower rates of growth. In other words, there is a tradeoff: delayed greening appears to improve your chances of survival, but decrease the rate at which you can grow. 

A few outstanding questions based on my read of the paper: 

Is delayed greening hard-wired into a species throughout its ontogeny (all life history stages)? 
Is delayed greening solely a function of herbivory?
How does delayed greening vary with phylogeny (e.g. the species location on the tree of life)?

This new research demonstrates that from an evolutionary perspective, delayed greening is more than a novelty....it's a phenomenon that could lead us to a much better understanding of the trade-offs involved in growth and defense over the course of a plant's lifetime. I am looking forward to seeing what new doors fly open as a result...

Queenborough SA et al. Demographic consequences of chromatic leaf defence in tropical tree communities: do red young leaves increase growth and survival? Annals of Botany 112: 677-684.