Stephanie Schmiege: leaf morphology impacts respiration in conifers

In this new post, Stephanie C. Schmiege from Columbia University (New York) presents her work on the physiological differences between flat and needle-leaved conifers, how temperature influences plants respiration mechanisms and the big opportunity she had working in tropical forests.

Stephanie Schmiege. Photo by Kevin Griffin.
Stephanie Schmiege. Photo by Kevin Griffin.

I can think of nothing more inspiring than a grove of majestic pines or hemlocks dancing in the wind.  For as long as I can remember, I have been fascinated by the complexity and beauty of the natural world I could discover on a tromp through the woods. In college, I was drawn immediately to ecology because it married scientific questioning and curiosity with observation skills cultivated through visual arts.  Over the years this interest has crystalized and evolved to focus on how trees, specifically the conifers of forests around the globe, respond to environmental stressors such as those imposed by climate change.

Conifers (such as pines, hemlocks and spruces) are found throughout the northern hemisphere. They are some of the longest-living plants on earth and tolerate some of the harshest conditions imaginable.  Yet, despite this, they are surprisingly rare in the highly productive and densely canopied tropical forests of the world.  What about the physiology and the morphology of these stalwart survivors makes them poor competitors in these tropical environments?  Of the conifers that do grow in the tropics, the majority seem to differ in their leaf morphology from those we commonly recognize in the northern hemisphere – they are flat-leaved in contrast to the typical conifer needle.  It is possible that this flattened leaf morphology may be indicative of a shade tolerance strategy.  Through my research, I have aimed to gain a better understanding of the physiological mechanisms and differences that might explain the success of flat-leaved conifers in tropical forests.

Marveling at the massive size of this Pinus krempfii individual. P. krempfii is a canopy emergent conifer that is endemic to the tropical forests of the Central Highlands of Vietnam. Photo by Kevin Griffin.
Marvelling at the massive size of this Pinus krempfii individual. P. krempfii is a canopy emergent conifer that is endemic to the tropical forests of the Central Highlands of Vietnam. Photo by Kevin Griffin.

To tackle this question, my colleagues and I have focused on measuring the gas exchange, that is the photosynthesis and respiration, of conifer species in a unique tropical location – the Central Highlands of Vietnam.  This location is home to a surprising diversity of conifer species, especially for tropical areas.  For example, it boasts an unusual endemic pine that is the only-known flat-leaved pine in the world – Pinus krempfii.  This species was first discovered during a scientific expedition in the 1920s by a French botanist, Paul Henri Lecomte.  It’s leaves are so similar to many coexisting species of predominantly flat-leaved conifers from the family Podocarpaceae that it has actually been misidentified in the past!  Our goal was to measure the physiology of this species and many other coexisting conifers in order to assess whether the physiology, in addition to the morphology, might be driving the differing success of flat versus needle-leaved conifers in the tropics.

In our recent paper published in Functional Ecology, we focus specifically on increasing our understanding of respiration in these conifer species.  Respiration is an important physiological process that is critical to plant function because it provides the energy for plant growth and for the maintenance of plant tissues.  However, any energy usage through respiration cannot exceed the energy gained through photosynthesis.  If a plant is functioning in a low light environment where light capture and thus photosynthesis might be limited, we deduce that a plant might maximize the leaf area for light capture and photosynthesis by flattening its leaves while minimizing the energy expenditure through respiration.  This is exactly what we have found in comparing flat- and needle-leaved conifers in Vietnam.  Flat-leaved species have significantly lower respiration than needle-leaved species.  We suggest that the extra energy expenditure of needle-leaved species may be detrimental to their survival in low light tropical environments, leading to their absence from the majority of tropical forests.

Pinus krempfii is one of the only-known flat-leaved pines in the world. Photo by Stephanie Schmiege.
Pinus krempfii is one of the only-known flat-leaved pines in the world. Photo by Stephanie Schmiege.

This study provides insight into the mechanistic drivers of conifer survival in tropical forests, providing key information on the respiration of these understudied species.  Additionally, we also gained insight into the short-term response of respiration to temperature.  This is incredibly important as we try to assess the effects of climate change on this highly diverse ecosystem.  Understanding the future carbon balance of these species under conditions of climate change will allow us to predict the future survival of these species and the impacts of climate change on carbon cycling in these unique tropical forests.

Conducting research in the Central Highlands of Vietnam has been one of the highlights of my graduate school career.  Collecting the data was not always easy, but always an adventure.  Plant physiological ecology requires a wonderful combination of field collections and the use of, and ability to tinker with, complex equipment.  In order to take the measurements presented in our paper, we hauled a 75 pound suitcase from the United States to our remote field location in Vietnam!  Our days were filled with sample collections, where we hiked through dense forest in search of adult conifers.  We even hired a local tree climber to help us collect samples from the tops of the sunlit canopies.  After traveling back with our samples tied to the back of our motorcycle, we would select leaves to measure using a complex instrument called a li-cor6400 that can measure the flux of carbon in and out of the leaves.  By retrofitting our instrument with a large leaf chamber attached to a heating block, we were able to measure both the respiration and the response of respiration to rising temperature.

Ecophysiologists love the field, but they also love tinkering with equipment. Here the authors have added a custom leaf chamber attached to a temperature block in order to assess respiration as temperature is slowly increased. Photo by Stephanie Schmiege.
Ecophysiologists love the field, but they also love tinkering with equipment. Here the authors have added a custom leaf chamber attached to a temperature block in order to assess respiration as temperature is slowly increased. Photo by Stephanie Schmiege.

In addition to researching this eccentric flat-leaved pine, I have had the extraordinary opportunity to meet forest rangers, friends, colleagues and researchers in Vietnam who have taught me some basic Vietnamese (putting up with my terrible pronunciation), introduced me to the food and culture of Vietnam and shared their profound knowledge of the flora of Vietnam.  To me, this combination of scientific exploration and cultural immersion is one of the most exciting aspects of being a field ecologist.  Ecology is inherently interdisciplinary because the questions asked span multiple scientific disciplines.  These cross-disciplinary studies also create the fascinating opportunity to explore languages and cultures, and to learn about different environmental practices in diverse areas of the globe.  I believe that the more we embrace this interdisciplinary studies and experiences, the more impactful our science will become.

Read the paper in full here.

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