A Drought By Any Other Name

What is a drought? I know I don’t know — I live in the temperate northeastern United States and my field site is frequently wrapped in fog — but I get the feeling that I am not alone. According to a paper born from a Colorado State University graduate student seminar on ecology and drought, we should all be asking ourselves this question.

Drought seems to have lost its meaning for ecologists, and not in the semantic satiation way, where if you say a word over and over again it become aural nonsense. Ingrid J. Slette and her co-authors published ‘How ecologists define drought, and why we should do better’ in Global Change Biology this summer. As Slette tells it, “This project grew out of discussions during a grad student seminar course about ecology and drought. Everyone in the class approached drought from a different perspective, and when we looked to the literature to find a definition of drought that we could all agree on as a starting point for the class, we couldn't find one.” The class decided they needed to take a step back, and they shifted from synthesizing the impacts of drought to simply defining it. This might seem like a trivial point of semantics, but as they write in their paper, “Failure to define or characterize drought conditions in the published literature challenges out ability to advance ecological understanding.” You can’t compare studies, or compile a meta-analysis without understanding the idiosyncratic environmental conditions hidden under the catch-all term ‘drought.’

Perhaps we should not be surprised that ecologists can’t agree on drought; as I discovered while reading Slette’s paper, meteorologists and climatologists also struggle to define drought. But, the sticking point is not that we don’t have a clear definition of drought, it’s that ecologists use the term ‘drought’ in the literature as if we do. When Slette and her team surveyed 564 publications from the last fifty years of drought research, less than a third of the papers explicitly defined drought or cited a definition of drought. In addition, they report: “ecologists most often use the term drought as a synonym for generally dry conditions (~30% of papers). In other words, authors state they are studying drought without quantifying and/or contextualizing how dry conditions are relative to normal (e.g., by reporting stardardized index values, or some measure of deviation from average conditions).”

But wait, it gets even juicer — it turns out that hand-waving about drought may be distracting ecologists from noticing the actual climatic conditions at their study sites.Slette and her coauthors selected a subset of studies from their review that were (a) bad at defining drought, but (b) good at providing details about their geographic location. They pulled the location coordinates and timeframes of these studies to calculate Standardized Precipitation Evapotranspiration Index (SPEI) values using the Global SPEI database. Only half of the droughts in this subset were characterized by especially dry SPEI values, outside the range of normal climate variability for their ecosystem. They found that 87% of the drought studies took place during times that were drier than average for the study site, but 13% of these “drought” studies were from periods that were slightly wetter than average based on estimated SPEI values. And while there may have been extremely local conditions that were truly dry at some of these "wet-droughts", we don't know because the authors did not report on them or place them within the context of the local long-term climate records. 

I asked Slette about the review process for this paper. I had seen on twitter that it was her first publication as a lead author, and I wondered if journal editors had recognized the importance of this topic. I assumed that Slette may have faced the same challenges as authors of ‘advice papers’ who struggled to find the right home for their work. Both this paper and Dyson et al’s advice for urban ecologists working on private property had origin stories in graduate students creating the resources that they were searching for early in their careers. Slette and her seminar wanted a straightforward ecological definition on drought and couldn’t find it. Slette wrote, “I anticipated that it would be quite difficult to get this paper published, but I was actually pleasantly surprised by how well the editors and reviewers received it. Choosing to submit this paper as an Opinion was an important decision in terms of finding a good home for it, I think that turned out to be a better fit for it than as a primary research article.” Then, I asked her about her own research, aside from writing sharp reviews of ecological literature. I wanted to know what definition of drought she used and how it had changed since writing her definition paper. Slette is a PhD candidate at CSU, and she answered, “I study how changing precipitation amounts and variability affect plant production. Specifically, I have been studying how experimentally-imposed extreme droughts affect plant root production and aboveground vs. belowground resource allocation in Central U.S. grasslands. For these experiments, drought was defined as a reduction in precipitation similar to what this area experienced during the Dust Bowl, about a 2/3 reduction from average. After writing this review paper, I am much more cognizant of all drought definitions, including my own. In every paper that I write from now on, I am definitely going to include more detail about the conditions of the drought itself, not just about its impacts.”

Finally, I asked her if the process of mining hundreds of papers for definitions of drought has made her a tougher reviewer or raised her standards for precise language from other ecologists. “I will definitely become a tougher reviewer now! I'm going to evaluate for precise wording and ask for lots of information about study design and justification.” I think that anyone who reads Slette’s paper will walk away with similar raised standards. And those of use who work in wet ecosystems should think about this too — we need to evaluate how we define our own work and what assumptions are hidden in our terms and jargon. As Slette notes, “I hope that the positive feedback and acceptance of this paper signals increased interest in (re)evaluating how ecologists define their work.”   

References:

Slette, I.J., Post, A.K., Awad, M., Even, T., Punzalan, A., Williams, S., Smith, M.D. and Knapp, A.K., 2019. How ecologists define drought, and why we should do better. Global Change Biology. 25(10), pp.3193-3200.

Common Gardens For All Your Climate Change Needs

A guest post from PLOS Ecology Reporting Fellows, Caitlin McDonough MacKenzie & Daniel E. Winkler, on research from the Ecological Society of America Scientific Meeting in Ft. Lauderdale, Florida, August 7-11, 2016. 

Experimental gardens are an old-school methodology. In perhaps the best known example in the 1930s and 1940s Clausen, Keck, and Hiesey transplanted Potentilla glandulosa across their range in the Sierras to explore the roles of environment and genetics played in determining growth form. Clausen, Keck, and Hiesey’s classic methodology of reciprocal transplanting has a contemporary application in climate change studies, whereby researchers relocate a plant (or seed) from its home and current climate to a transplant garden and new (and perhaps future) climate. Seven decades later, the Ecological Society of America’s 2016 Annual Meeting features experimental gardens that include species ranging from alpine forbs to douglas fir trees to a dune-loving annual—collected along latitudinal, elevation, and habitat gradients. 

Nicole Rafferty opened the Climate Change: Ranges & Phenology I session presenting her research on patterns of bumblebee visitation at the Rocky Mountain Biological Laboratory. As a part of this project, she installed a reciprocal transplant experiment with seeds from three elevations planted at 12 plots per elevation site. She wanted to test how alpine plant-pollinator relationships might change as plant communities experience new microclimates (for example, if a species is transplanted to a warmer site at a lower elevation). Unfortunately, the first year of this study coincided with a dry summer and low germination rates — as a result, in 2016 she switched to seedlings. In her 2015 seed study, the glacier lily seeds from mid-elevation had the lowest success in the transplants, suggesting that mid-elevation might be a barrier to plant migrations upslope for this species.  

Range shifts and phenological are also on the minds of researchers at the U.S. Forest Service. This time with an applied focus aimed at aiding land managers who will likely need to develop strategies to make Forest Service lands more resilient to climate change impacts. Sheel Bansal at the U.S. Forest Service’s Pacific Northwest Research Station and colleagues carried out a large-scale common garden study aptly named the Douglas-fire Seed-Source Movement trial. Their experiment used seeds from 60 sources throughout the species range in Washington, Oregon, and California and grew trees from each of the sources in 9 climatically-divergent field sites and also used artificial freeze experiments to test the impacts of changing environmental queues on Douglas fir cold hardiness and associated genetic linkages. They found strong differences in cold hardiness, with minimum winter temperatures and fall frosts as major predictors of cold hardiness based on seed source. Their results have important implications for the ability of species to shift their ranges by tracking climate envelopes, and further extend to land management efforts to maintain healthy forests experiencing future climates.

In the Great Lakes region, Elizabeth LaRue from the Emery Lab at the University of Colorado Boulder used a common garden to explore dispersal traits in American sea rocket (Cakile edentula var. lacustris). She knew that dispersal traits like pericarp, or seed wall, thickness and wet mass varied across the Cakile edentula range, but it was unclear if the variability was caused by environmental or genetic differences. Collecting seeds from across the range, and growing them together in a common garden isolated the role of genetic differences and revealed lower dispersal traits at the range edges. This data was used to inform species distribution models with different scenarios for starting dispersal genetics for Cakile edentula under climate change.

Kennedy Rubert-Nason in the Department of Entomology at the University of Wisconsin-Madison and his colleagues looked at the role of vernal freezes in determining aspen phenology and growth. They planted 6 aspen genotypes into common gardens at varying temperatures and examined a number of biological responses.  The number of days it took aspen to break bud accelerated in trees that experienced freeze-damage. Freeze-damaged trees were also stunted in their second year of growth when they experienced a freeze event during their first year. Defense compounds were also dramatically impacted, potentially indicating the negative effects of freeze events and the associated ability of the trees to defend against herbivores during their most vulnerable life stage. Their study nicely highlights the importance of the timing of environmental queues in dictating species susceptibility to a changing climate. 

Caitlin McDonough MacKenzie is a PhD candidate in the Primack Lab in the Biology Department at Boston University. She spends her field seasons in Acadia National Park, Maine studying leaf out and flowering phenology and patterns of historical species loss across plant communities. Her field methods include three ridge transects that are conveniently located adjacent to beautiful running trails and carriage roads. Away from Acadia’s granite ridges, she’s interested in underutilized sources of historical ecology data including herbarium specimens, field notebooks, photographs, and old floras; the potential for citizen science in phenology research; and the intersection of science and policy.  (Follow Caitlin on Twitter @CaitlinInMaine) 

Daniel Winkler is a PhD candidate at the University of California, Irvine and a recent National Park Service Young Leader in Climate Change. Daniel is a plant ecophysiologist interested in invasive species, evolutionary ecology, and climate change impacts on native communities in “extreme” environments. His field sites include much of the desert southwest, alpine regions of Colorado, the subalpine forests of Baja California, and the tundra of northern Japan. All of Daniel’s research focuses on climate change impacts on native systems, with an emphasis on parks and protected areas. You can follow him on Twitter @DanielEWinkler, his research on Facebook at www.facebook.com/GeoMustard/, or find more information on his website at www.winklerde.com.