Tuesday, September 30, 2014
Fall is in the air. Here in North Carolina that means drastic temperature swings that cause me to dress incorrectly on any given day. It also means the arrival of fall colors. Indeed, fall colors are incredibly beautiful, but biologically speaking, you are watching death happen. This autumn splendor got me to thinking about these colors a little closer, specifically the phenology of trees.
Phenology is the study of the annual timing of recurring life cycle events. The timing of these events is typically influenced by seasonal environmental changes. In the case of trees, specifically hardwood forests, this is the leafing-out (flushing) and dropping-off (senescence) of leaves. But what actually triggers a plant to leaf-out? This can vary a bit by species or even individual, but there are a couple of general categories you can look to. The first is changes in air temperature, the chilling in the winter and warming in the spring. The other is photoperiod, or the day length, which often interacts with temperature, allowing plants to quickly respond to changing conditions.
Considering that these events are triggered by environmental changes, it is logical to assume that global climate change can force changes in the phenology of many species and communities. This is another think-about-the-plants moment. How plants respond to climate change has huge consequences for world ecosystems – growing seasons, species ranges, carbon and water cycling, interactions with animals, etc.
A paper published earlier this year in PNAS took a look at variations in leaf flushing and senescence dates in relation to warming. Many phonological studies focus on specific phenophases (like leaf-out in the spring), but this study is unique in that it looks at subsequent phenological events. The authors aimed to see if effects of warming lasted longer than the current growing season. To do this, in December 2009 they took seventy 3-4 year old cloned oak and beech trees and put them in growth chambers where they could very carefully control the winter environmental conditions. They manipulated the temperatures of the growth chambers to create treatment groups of winter-spring warming, winter-only warming, and spring-only warming. Then, in spring of 2010 when the flushing was complete, they moved the trees out of the chambers and into a field. The trees stayed outside and were measured until the following spring of 2011. Leaf-out rates were determined using a scale that went from undeveloped bud to unfolded leaf, and leaf senescence was recorded as the date at which half of the leaves were colored or dropped. These measurements allowed for a quantification of growing season length. Additional measurements of numbers of leaf per tree, specific leaf area, total leaf area per tree, number of buds, dry weights of various parts of the trees, carbohydrate content, and carbon and nitrogen content were taken. They also combined their data with that of the European phenology network to get both a larger sample size and a wider geographic area.
The researchers found both leaf flushing and senescence in both species to be advanced 15-18 days by winter-spring warming. In the long-term, the timing of autumn leaf senescence was found to be positively correlated with spring leaf flushing dates, and advanced leaf flushing lead to earlier leaf flushing the following year. This suggests that the physiological impacts of a warmer winter last longer than just one growing season. Advanced leaf flushing in this winter-spring-warming treatment was also associated with some physiological and morphological changes, particularly in the oaks. These included higher leaf number, higher leaf area per tree, and higher starch accumulation.
The trends of the experiment were also observed in the mature trees in the long-term field-based phenology observations of the European phenology network. The underlying cause in both cases is likely that the plants never really fulfill the winter chilling requirements necessary for them to enter dormancy. Currently, the most widely accepted mechanism for leaf senescence is the environmental control hypothesis, which proposes that leaf senescence is triggered with the unfavorable autumn season comes (changes in photoperiod, temperature, or both). This study shows that perhaps that isn’t all that’s going on.
*sigh* nothing is ever simple is it?
Fu, Y., Campioli, M., Vitasse, Y., De Boeck, H., Van den Berge, J., AbdElgawad, H., Asard, H., Piao, S., Deckmyn, G., & Janssens, I. (2014). Variation in leaf flushing date influences autumnal senescence and next year's flushing date in two temperate tree species Proceedings of the National Academy of Sciences, 111 (20), 7355-7360 DOI: 10.1073/pnas.1321727111
For lots of really great info on the science of leaf-out, I recommend this review article:
Polgar, C., & Primack, R. (2011). Leaf-out phenology of temperate woody plants: from trees to ecosystems New Phytologist, 191 (4), 926-941 DOI: 10.1111/j.1469-8137.2011.03803.x
And you can contribute to leaf phenology research through Project Budburst!
Tuesday, September 23, 2014
Monday, September 22, 2014
Wednesday, September 17, 2014
|Figure 1: (A) "Dryophyllum" subfalcatum, (B) unknown nonmonocot, |
(C) "Ficus" planicostata, (D) "Populus" nebrascensis
As of now, it is widely accepted that an epic asteroid collision ended the 135 million year reign of the dinosaurs. The Cretaceous-Paleogene boundary (KPB) extinction event is marked by the Chicxulub (CHEEK-sheh-loob) impact on the Yucatán Peninsula in Mexico. This asteroid or comet is estimated to have been about 6 miles (10 km), releasing as much energy as 100 trillion tons of TNT that caused a crater more can 110 miles (180 km) across! This impact coincides with a mass extinction event that includes the dinosaurs. Dramatic climate swings caused by the dust kicked up into the atmosphere were likely the culprit behind many of these extinctions. Before we go further, take a second to think about what you know about this extinction event. You probably think of the mass die-off of the dinosaurs and the subsequent rise of the mammals, right? But, as I have in the past, I’ll now pose a question: What about the plants?
A new paper published yesterday in PLOS Biology asks just that question. We know that in temperate North America the Chicxulub impact resulted in the extinction of over 50 percent of the plant species. From an evolutionary and ecological stand-point, that’s a lot of competitors that were taken out of the game. However, the environment was dramatically altered as well, changing to a cold and dark “impact winter.” Combined, these factors created a unique selection scenario for certain ecological strategies. The new paper takes a close look at the functional traits associated with these strategies.
The researchers measured fossil leaf assemblages spanning a 2.2 million year interval across the KPB, assessing four differing selection scenarios for functional traits. First, wrap your head around the concept of “functional traits.” These are characteristics that define species in terms of their ecological roles. In the case of leaves, these include leaf mass per area (LMA; Do you make a big, expensive leaf or a light, cheap one?) and leaf minor vein density (VD; Do you have more veins to transport lots of water?), among many others. Because leaves are the food producers, these traits are linked to plant growth and fitness. Next, you can relate these traits to the “leaf economic spectrum” (LES) that contrasts species with inexpensive short-lived leaves with fast returns on carbon and nutrients (deciduous, angiosperm, broadleaf) to costly long-lived leaves with slow returns (coniferous, gymnosperm, evergreen). The former is typically selected for in a less resource variable environment and vice versa. From this, you can get a more global perspective on changes in species composition.
The researchers measured LMA and VD for fossil leaf assemblages spanning the KPB. To do this they digitally photographed specimens that could be measured and confidently reconstructed. Then they used Photoshop to digitally separate the leaf from its rock matrix. For LMA they used ImageJ to calculate leaf area and petiole width, and then ran these numbers through empirical scaling functions (a.k.a. equations). For VD, they used a MATLAB line-counting program to isolate the veins and then manually counted the number of vein-line intersections, computing the mean distance between veins as the sum of all line counts divided by the sum of all distances (a.k.a. a slightly less complicated equation). They ran a few scenarios to account for site and region plant specificity as well.
They found LMA to decrease and VD to increase across this time period. Even changes just these two traits reflect large physiological and biological shifts in plant functioning over a relatively short period of time. According to their data, the Chicxulub impact led to the selective extinction of species with slow strategies. This caused a directional selection away from evergreen species along with a stabilizing selection of deciduous angiosperms. The authors pose a few hypotheses in their discussion that are worth mentioning. The higher observed VD in angiosperms, and their ensuing selection, could have been driven by declining atmospheric carbon dioxide (CO2), which selects for higher hydraulic capacity. This CO2 hypothesis would, of course, not really hold water (no pun intended) for nonangiosperms and shade species, but the authors suggest that the observed increase in VD is more likely to be a direct consequence of the impact selecting for specific leaf economic strategies rather than ongoing-longer term climate change.
In this case, slow and steady did not win the race.
Blonder B, Royer DL, Johnson KR, Miller I, & Enquist BJ (2014). Plant Ecological Strategies Shift Across the Cretaceous-Paleogene Boundary. PLoS biology, 12 (9) PMID: 25225914
(image via above citation)