Corporal Tree Hugging

(via Strange Matter)

Dealing with Drought: How Do Plants Cope?

Have you noticed how often drought has been in the news lately? You don’t have to be a scientist to know that drought is bad. But, if you’re a plant, how bad is bad? I mean, you’re a plant; it isn't like you can pick up your roots and go looking for the nearest water source. You must have ways to cope, strategies that will let you survive until water arrives. A new paper in Tree Physiology caught my eye today that examines how plants handle drought in our changing climate.

We know that drought and elevated temperatures have all sorts of effects on ecosystems worldwide. The ecosystem level is a bit broad so let's narrow the scope a bit, just looking at plants. The average person tends to think about plants as just that: plants. They are green, they grow. Okay, but we really need to think of them as different species that have their own strengths, weaknesses, and strategies to cope with hard times. Different species of plants react to drought and elevated temperatures differently, some are better able to cope than others. It is known that drought and temperature influence seedling and sapling establishment and survivorship. To anthropomorphize a bit, some infant and toddler plants survive to grow up into adults and others...well...don't. The results of this survivorship will profoundly affect the composition of the community (collectively, which species are there) and how that community changes over time. Additionally, drought and elevated temperatures may affect our efforts to restore habitats. If newly planted saplings die within the first few years of planting then the restoration fails. If we get even more doom-and-gloom with it, extreme drought will cause massive forest dieback, releasing huge amounts of stored carbon and exacerbating the problem.

Let's narrow the scope a bit more. The survivorship of plants is related to their physiology and productivity. In general, growth is the most sensitive to drought. This is followed by photosynthesis and then respiration. The timing and extent of these declines is governed by changes and coping methods for water balance (water supply vs. water use), carbon balance, and strategies to balance the water loss and carbon gain. Most climate models predict that with increased atmospheric carbon dioxide (CO2) and warming the intensity and timing of droughts will go up. Now, carbon dioxide is good for plants because they use it like we use oxygen, to make energy. Give plants more CO2 and you see increased leaf area, productivity, photosynthesis, and carbon storage (in this case we are talking about non-structural carbohydrates (TNC)). Warming under well-watered conditions isn't too bad either, but under drought, warming will decrease photosynthesis and carbon storage while increasing water loss. But increasing CO2, warming, and drought are not solitary factors; you need to look at them in combination. Elevated CO2 will lessen drought stress, but warming worsens it. Based on what we know of the individual factors, the effects of elevated CO2 plus warming plus drought may vary depending on the trade-offs a plant makes. This new study in Tree Physiology looks at how drought alone and in combination with CO2 and warming affects carbon dynamics (growth, photosynthesis, respiration, TNC).

To do this they raised Blue Gum (Eucalyptus globulus) seedlings under ambient CO2 and temperature conditions. After one month they transplanted and separated them into their various treatment groups. These treatment groups were put in whole-tree growth chambers where the conditions could be very carefully controlled. There were four CO2 and temperature combinations: two CO2 levels (400 and 640 μl l−1) and two temperatures (28/17 and 32/21°C day/night, a.k.a.ambient and ambient + 3°C). Within each of these treatments, seedlings were designated to a drought regime: well-watered/control, sustained drought, rewatered drought (watering after sustained drought), and progressive drought (on rewatered and sustained treatments). They were able to maintain the "sustained drought" condition by adding just enough water to maintain leaf stomatal conduction (a measure of the rate of CO2 entering and water vapor exiting the stomata/pores of the leaf).

The researchers took gas exchange measurements, specifically looking at leaf net photosynthesis and leaf respiration rates. They also randomly selected individuals at various times during the experiment’s duration to be harvested for dry mass measurements (leaf, stems and roots). TNC concentration was calculated as the sum of starch and soluble sugar concentrations.

The results showed effects of progressive drought to be similar in rewatered and sustained drought plants. These plants were limited in growth, photosynthesis and respiration. However, there was not a decrease in TNC, although the drought plants did convert quite a bit of their starch into soluble sugar. This means that the plants consumed less TNC, and the soluble sugars likely serve some other function apart from a respiratory carbon source during drought (perhaps osmotic adjustment and/or hydraulic transport). They also found that elevated CO2 ameliorated the stress of their plants in the moderate drought treatments. These plants showed increased photosynthesis and TNC reserves. This suggests that the plants have the capacity to withstand drought, having sugar available for osmotic adjustment (think: better water movement, which is good for growth and photosynthesis). Conversely, elevated temperature exacerbated moderate drought stress by reducing photosynthesis, increasing leaf respiration and decreasing TNC reserves, and reducing the plants’ capacity to withstand drought. The combined effect of elevated CO2 and increased temperature is a little more complex. This study found only moderate benefits to plants, with similar/lower carbon uptake and greater carbon loss during the moderate drought treatment. TNC was found to be higher which suggests that there may be some carbon storage going on. However, these benefits went away when the plants were subjected to extreme drought. I suppose that is to be expected. They don’t call it extreme for nothin’ right?

Perhaps we can find a silver lining from this story. Plants are resilient organisms that have the capacity to withstand more than we had thought even when they are hit with multiple stressors. They can deal with moderate droughts, but extreme is still extreme and nothing survives everything. I didn’t say it would be a thick silver lining, but we can potentially use this knowledge to help us mitigate some of the effects of climate change and work to better restore lost habitats.


Duan, Honglang, Jeffrey S. Amthor, Remko A. Duursma, Anthony P. O’Grady, Brendan Choat, & David T. Tissue (2013). Carbon dynamics of eucalypt seedlings exposed to progressive drought in elevated [CO2] and elevated temperature Tree Physiology, 33 (8), 779-792 DOI: 10.1093/treephys/tpt061


(image via TrendsUpdates)

Biological Beta Testing: Altitudinal Edition

Spatial patterns in biodiversity have always been a popular topic in ecology, and understanding these patterns helps us to address the looming threats to biodiversity.

Did you notice how I made the word ‘patterns’ plural? There isn’t a whole separate field of biogeography for nothin’. Biological diversity is difficult (some say impossible) to measure using a single metric. How do you count things? Simply by the number of species? How about rare versus common species? What about species turnover? What geographic scale are you using? How and/or do you define boundaries? Would you like me to keep going or do you get the point?

Lately I’ve noticed a surge in the number of studies that look at diversity gradients that occur along latitude and altitude. We visited the topic of latitude and diversity back in October of last year with the “Dinosaurs, Diversity, Distribution, and the LBG” post. Essentially, latitudinal gradients (LBG or LDG) occur when species richness (a simple count of species) is highest in the tropics and declines polewards (warm areas are more hospitable and produce more food). Altitudinal, or elevational, gradients are nearly as ubiquitous as latitudinal gradients, and they have many of the same characteristics. As you go up in elevation (as on a mountain) the temperature gets colder and habitat areas and the communities they support become smaller and more fragmented. Gradients such as these are overall patterns. Now mix them with diversity indices. In 1960, R.H. Whittaker described the terms alpha diversity (α-diversity), beta diversity (β-diversity), and gamma diversity (γ-diversity). Basically, the total species diversity of a geographic area (γ-diversity) is determined by the mean species diversity at the local, within-site or within-habitat scale (α-diversity) and extent of change in community composition between sites (β-diversity). These definitions have been much argued over (see Tuomisto 2010), and we won’t get in to that. We are going to focus on beta diversity (β-diversity), also called species turnover or differentiation diversity. It is a measure of how different sites are from each other and/or how far apart they are on a gradient of species composition. Factors that drive β-diversity are very important, very poorly understood, and very much argued over. Is it dispersal limitation? Habitat specialization? Environmental heterogeneity? The studies published so far seem to lean toward multiple processes operating at various scales. *sigh* Isn’t that always the way? Observational and experimental studies have shown that impacts on β-diversity vary with productivity. So it would make sense to test this in relation to altitudinal and elevational gradients.

A new study to be published in Global Ecology and Biogeography looks at how local community processes may create an altitudinal pattern of β-diversity. Their study site was the Shiretoko National Park in north-eastern Hokkaido, the northernmost island of Japan. In addition to being one of the richest temperate ecosystems in the world, the area is characterized by sharp altitudinal changes in forest structure and productivity as a result of strong winds on the western side of the mountains. The researchers chose this western side to measure the diversity of woody plants and mites (Oribatida) in mature stands where they set up seven plots (each containing 10 subplots) at different altitudes. For the woody plants they recorded the number of individuals taller than 0.5 m, measured the girth at breast height (GBH), estimated the total basal area (BA), recorded canopy height (CH) of the stand, measured the diameter and length of all coarse woody debris (CWD), measured understory light, took soil cores to extract the mites, removed roots from the soil samples, measured the thickness and dry mass of the soil surface litter (A0 layer), collected leaf litter, measured water content and pH of the litter, and the carbon-to-nitrogen (CN) ratios of the litter and soil. They then calculated the β-diversity of the two organism groups for each altitude. So as to take into account the dependence of β-diversity on gamma diversity (γ-diversity), they used null modeling. This type of modeling “randomly shuffles individuals among subplots while preserving γ-diversity, the relative abundance of each species per plot and the number of individuals per subplot” which enabled them to estimate how much observed β-diversity differed from expected β-diversity. They also calculated a β-deviation value for each altitude. This is equivalent to a standardized effect size, indicating the magnitude of the deviation from what you would expect of a random (stochastic or by chance) assembly process.

First we’ll hit the results for the oribatid mites. These results showed the altitudinal gradient of β-diversity to be less evident than it was for the plants, and β-deviation showed no altitudinal gradient. That said, they found β-deviation to always be greater than expected in all locations for both woody plants and oribatid mites. In woody plants, the magnitude of β-deviation increased with altitude, suggesting that deterministic processes dominate in low-productivity, high-altitude stands, the role of these processes increasing with decreasing productivity in plant communities. The authors conclude that their results support the hypothesis that “the mechanisms underlying community assembly (e.g. niche versus neutral) play an essential role in creating biogeographic patterns of β-diversity.” They found that niche-based processes (species correlate with environmental variables, the conditions in which the species can persist) govern high-altitude stands, particularly when they included the plants growing in lower layers in the analysis. They found similar results when they calculated β-deviation along a stand-structure gradient with basal area, not altitude, particularly when small understory plants were included. This suggests that “given the altitudinal changes in stand structure, the role of understory plants in deterministic assembly becomes more dominant with altitude.” So those little guys really make a difference! Why? Well, considering that light is a precious commodity, low-elevation, structurally well-developed stands have higher competition for available light, especially for those little understory guys. This fierce competition isn’t so fierce in the higher altitude sites where the upper canopy isn't there to usurp all of the light for itself. This favors a greater, more diverse understory in higher elevations. Again, the authors speculate that this greater high altitude diversity is due to fine-scale niche partitioning, which allows more individuals to exist together.

Okay, so that is a pretty good explanation for the high altitude communities. Now what is going on at low altitudes? Well, there you have to take the site’s history a little more into account. What made these stands so “structurally developed” and generated this stochasticity? Consider this: The canopy trees of these forests colonized before the understory individuals, negatively affecting understory species by reducing the availability of space and resources. You then see a relative dominance of these big-ole-bully trees (that’s a technical term, you know) and a one-sided competition scenario.

Did I scramble your brain and make you hate biogeography yet? Can you see any sort of overall conclusion here? Lemme help you out…

β-diversity is dependent on community processes and shaped by local factors within the landscape. The species in a place use what is in a place.

Do you agree?

Mori, A., Shiono, T., Koide, D., Kitagawa, R., Ota, A., & Mizumachi, E. (2013). Community assembly processes shape an altitudinal gradient of forest biodiversity Global Ecology and Biogeography DOI: 10.1111/geb.12058


(image via Japan-Guide.com)

I've Taken a Lichen to You

This year I've found quite a few sciency Valentine's Day e-cards. I figured I would start off a series a V-Day posts with a few created by a friend of mine, Jen Santoro, a forestry masters student and all-around awesome chica. She did a great job with these!

Burn Baby Burn: Fire, Forests, and Carbon

I know, I know. I've been a bit absent over the last few weeks. I'm going to call laying on the beach in South Florida at Christmas enough of an excuse. That and I was finishing up my Wildland Firefighter course. Yep, I'm now all certified to start and stop forest fires. Should you be afraid? Perhaps. But it got me to thinking about prescribed burning, an area of ecology that I know about but don't regularly keep up with the literature. So I decided to take a look at what has recently been published on the topic and found a nice paper in the journal Forest Ecology and Management about the balance of carbon sequestration and habitat conservation as they relate to fire.

Most people tend to think Fire = Bad. In many situations that's true, but in others fire is actually a natural, healthy process. In the past, various federal and state agencies have instituted wildfire suppression policies because they thought it was the best way to preserve the natural state of the land. All it really did was create a massive amount of fuel loading - lots and lots of fuel (dead logs, leaf litter, etc.) around to burn - which created even larger, more destructive fires. Now we know better, and the idea of a more natural fire regime is being used to control fuels and keep the balance of natural habitats.

Today's paper takes a look at woodlands, a subset of forests that are composed of low density, short trees  that typically have an open canopy with a ground layer made up of shrubs, grasses, and forbs. They can be economically important for lumber and livestock grazing, but they are also naturally important as significant carbon (C) sinks and as a biologically diverse habitat. Fire is known to be an important process in this ecosystem and is managed such that willdfires are allowed to burn unhindered, landscape level fuel management helps contain fire spread, and prescribed burning reduces fuels. This study investigates the effects of past fires on carbon storage, woody vegetation community composition, and habitat suitability. They do this by utilizing a "chronosequence" approach. This means that they grouped and compared stands of trees at different successional stages following fire events. These stands were located in the woodlands of Balcones Canyonlands National Wildlife Refuge (BCNWR), located northwest of Austin, Texas, USA. The dominant woody vegetation in this area is the evergreen Ashe's juniper (Juniperus ashei), with subdominant species including several oak (Quercus spp.) species and Escarpment Black Cherry (Prunus serotina var. eximia). These woodlands are also inhabited by the golden-cheeked warbler (Dendroica chrysoparia), an endangered bird species they used to assess habitat suitability. The mixed juniper-oak are critical for this warbler as they use the stringy, sloughed bark from the mature Ashe's juniper exclusively for nesting material and requre Lepidopteran (Geometridae) larvae that feed on broadleafed foliage during the spring breeding season.

The researchers established 60 plots next to trees that had been used for a previous dendrochronology (tree rings) study. That means that they had the history of the tree from the tree rings, this history including fire scars that could be pinned to exact years. The fires were low-intensity, surface burns that mostly consumed the ground layer, seedlings, and saplings. From this they were able to pick sites with single fire events and then assign their plots into three groups: recent-fire (less than 40 years), old fire (greater than 40 years), and no fire. Then, for each plot, they identified tree species and measured their diameter and heights, collected data on woody debris and leaf litter depths, and collected soil samples for carbon and nitrogen analysis.

The authors found significantly different total aboveground biomass C averages for the 60 study plots. Recent-fire showed 5.25 kg m-2, old fire averaged 6.86 kg m-2, and no fire had 9.18 kg m-2 of C. Previous studies have shown that low to moderate intensity fires have less effects on live biomass, stabilize carbon fast afterwards, and primarily affect trees in an age-specific and density-related way. This study found that "the impact of low-intensity fire on C storage increases for sites with higher proportions of biomass in small trees" (greater than 20cm in diameter) as these are more greatly impacted by surface fires.

 This study's results support that periodic surface fires maintain many forests by favoring fire-tolerant species, such as oaks, and decreasing the density of fire sensitive species, such as juniper. They found that sites with a single fire disturbance had a higher density of oak seedlings than juniper seedlings (though not significantly so), especially in years that had low precipitation in the summer. The densities of mature trees was found to be mainly influenced by differences in topography, herbivory, and historical climate.

I would like to say that all of this added up to a good conclusion about habitat suitability for the golden-cheeked warblers but, unfortunately, they found significant differences. Fire reduces mature tree density, which has a negative impact on habitat suitability, but high intensity fire is related to higher oak recruitment (food source for their food source). Managing these woodland for the golden-cheeked warbler may require both mechanical treatment and the use of fire. The critical core breeding habitat for this warbler includes old-growth stands composed of large, mature junipers and about a 40 percent oak species mix. These stands should be protected from intense fire through the mechanical thinning of the understory and the removal of heavy fuels.

Overall, the authors conclude that fire management can achieve multiple outcomes on a single site. This means that detailed ecological information about the species composition and population structure of an area are needed in order to assess conditions and projections on restoration.

Yao, J., Murray, D., Adhikari, A., & White, J. (2012). Fire in a sub-humid woodland: The balance of carbon sequestration and habitat conservation Forest Ecology and Management, 280, 40-51 DOI: 10.1016/j.foreco.2012.05.042

More on wildland fires and prescribed burning:
GeoSTAC's Wildland Fires page
University of Florida IFAS Extention's "Benefits of Prescribed Burning"
National Interagency Prescribed Fire Training Center


(image from Texas Department of Agriculture)

Lord of the Tree Rings

I've been analyzing a lot of tree cookies (cross sections of tree trunks) lately, measuring things like specific gravity, surface area, and tree rings. The latter is what makes me find videos like this one interesting, it is all about what you can find out from tree ring data. Also, I'm kind of digging these ScienceNation videos put out by NSF.