Heatwaves, which are several consecutive days of extremely high temperatures, are likely to increase in frequency and intensity with climate change, which may impair tree function and forest C uptake. How do trees respond to heatwaves? John Drake, along with 19 collaborators across 9 institutions, exposed trees to an extreme experimental heatwave of four days exceeding 43 degrees Celsius (~110 degrees F). The team expected to document the process by which trees physiologically fail and begin to die. However, the trees surprised us. They were fine! You can read the paper in Global Change Biology here. The trees coped with this extreme heatwave through two mechanisms. First, they accessed deep soil water and continued to transpire, which cooled their leaves through the process of latent heat exchange. This is similar to how sweating on a hot day helps keep humans cool. This continued transpiration cooled the leaves by an average of 3 degrees C. Secondly, the trees increased the thermotolerance of their leaves. That is, after just one day, the leaves were able to tolerate a much higher temperature before physiological failure. We are not sure what biochemical process was responsible for this increased thermotolerance, but it likely involved volatile organic C emissions, the expression of heat shock proteins, and/or alterations to the lipids in the cellular membranes. We conclude that this tree species was remarkably capable of tolerating an extreme heatwave via mechanisms that have implications for future heatwave intensity and forest resilience in a warmer world.
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A new paper by Sebastian Pfaustch was just published in Annals of Botany, which included John Drake as the third author. Get the paper here. There is currently a controversy regarding the anatomical and physiological characteristics of trees that allow them to efficiently transport water from the soil to their leaves, while also minimizing the chance of hydraulic failure from xylem cavitation. A portion of this controversy stems from the fact that most hydraulic research is done at the scales of cells, tissues, or organs, often separately. Very few studies have taken a whole-organismal approach with large trees and studied how hydraulic traits co-vary along the stems of large trees. Pfaustch et al. heavily instrumented three Eucalyptus grandis trees with sapflow probes and stem psychrometers and collected physiological data for a month, and then felled the trees and measured a wide range of anatomical traits with light and electron microscopy. Pfaustch et al. may have set a record with the number of sapflow probes installed on an individual tree, with 52 heat-ratio method probes. The study revealed complex and synchronized covariation among traits and tradeoff with increasing height in these trees. In particular, the anatomical traits of xylem vessels and pit membranes co-varied to increase efficiency and apical dominance in water transport in ways that are likely to minimize cavitation. That is, coherent trait variation may minimize the strength of the trade-off between efficient water transport and resistance to xylem caviation. Global Change Biology just published our new article here. We tested whether a widely-distributed species of Eucalypt consisted of several genetically divergent populations, or if the species consisted of trees with a broad climate suitability. Forest red gum (Eucalyptus tereticornis) is distributed along the eastern coast of Australia, spanning 2,200 km and a 13 degree Celcius difference in mean annual temperature. Despite this gradient, we found that trees of this species had convergent temperature-dependencies of photosynthesis, respiration, and growth. That is, this species consists of trees with broad climatic suitability and a common thermal niche. This was unexpected, as many Northern Hemispheric trees that are widely distributed consist of populations with divergent climate responses. We speculate that long-distance pollen dispersal by migrating pollinators (bats and birds) enable long-distance gene flow in this species, preventing strong adaptation to local climate. A common thermal niche means that the effects of climate warming will vary across the range of this species. Warming is likely to increase growth for trees of this species in cool home climates, but decrease the growth of trees in warm climates. We published a new article in Australasian Plant Conservation summarizing our work on forest red gum (Eucalyptus tereticornis). Forest red gum is a widely distributed species, with a native range spanning 2,500 kilometers across eastern Australia.
Revegetation efforts commonly use local seed sources of tree species, as local sources are assumed to be well-adapted to the environment. With climate warming, local sources may not be well adapted to future conditions, and assisted migration may be used to plant trees that are "climate-ready". Using a series of controlled greenhouse warming experiments, we show that seed sources of forest red gum were not strongly locally adapted. We found no evidence of the "local is best" paradigm for this species. This suggests that planting local seed sources or seed sources from warmer climates would have equivalent results. Here is a pdf. John Drake just published a new article in Agricultural and Forest Meteorology- find it here. John and his many colleagues exposed four tree species to a series of controlled droughts using large rain exclusion shelters in Australia. They use a combination of mathematical models and measurements of leaf-level gas exchange and water potential to document and predict tree responses to drought.
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Drake lab
Tree ecophysiology at SUNY-ESF Archives
October 2022
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