Legacies of drought in climate and tree-ring records

Previous research has indicated that trees experience long-term deficits in annual growth following major drought events. Acute drought stress can result in hydraulic damage, and loss of foliage, fine roots, and stored sugars all of which can take years to recover. These physiological “legacies” persist in tree growth for several years, even after a drought has ended. Studies across forest types indicate that drought legacies occur across both arid and mesic regions, and in both coniferous and broadleaf tree species. However, drought variables themselves often exhibit significant trends over similar timescales as the physiological legacies identified in physiological research. Because tree rings integrate climate and biological signals across entire growing seasons, and even across multiple growing seasons, “disentangling” the physiological carryover resulting from drought from ongoing climate variability is a challenge for drought legacy researchers. My dissertation research combines tree-ring data with large-scale climate and remote sensing datasets to track the responses of North American forests to drought stress.

Strength of year-by-year trends in drought variables. Analyzed are long-term (1900-2014) records of the Palmer Drought Severity Index (PDSI) (Dai 2013). Data sourced from the University of East Anglia Climate Research Unit (CRU) (Harris et al. 2014). Pictured are the degree of temporal autocorrelation in drought records on one to three-year timescales, measured across all grid cells by taking the Pearson’s correlation coefficient of each time series lagged by one (left), two (center) and three (right) years. Citations: 1) 1.Dai, A. Global Palmer Drought Severity Index (PDSI). (2017) doi:10.5065/D6QF8R93.. 2) Harris, I., Jones, P. D., Osborn, T. J. & Lister, D. H. Updated high-resolution grids of monthly climatic observations – the CRU TS3.10 Dataset. International Journal of Climatology 34, 623–642 (2014).

Boreal peatlands and global change

The northern arctic and boreal regions of the globe are warming at nearly twice the global average (IPCC 2013). This is resulting in widespread changes to ecosystems across the region. Northern ecosystems are incredibly important to the global carbon cycle, because they store large amounts of organic carbon in soils and plant biomass. This is particularly true of peatland ecosystems. Peatlands are characterized by thick organic soils that have accumulated since the retreat of the glaciers around 10,000 years ago. Over that timescale, they have built up vast stores of carbon. It has been estimated in a number of studies that peatlands store one-third of the terrestrial pool of soil organic carbon. Because of this, peatlands are very important in the context of climate change. As the climate changes, it is expected that peatland hydrology and ecology will shift in response to warming and drying. Prior research as shown that this can lead to a loss of carbon from soils. However, other research has indicated that warming leads to an increase in carbon sequestration through photosynthesis, which could potentially offset some of the losses of soil carbon. Through my MS research, I have worked in experiment that are attempting to determine how climate change will affect carbon cycling in peatlands.

Image of the boardwalks and experimental chambers at the Oak Ridge National Lab Spruce and Peatland Responses Under Changing Environments (SPRUCE) project located in northern Minnesota. I have conducted field research at SPRUCE since 2016, tracking the responses of understory plant communities to experimental warming and elevated carbon dioxide levels.

Remote sensing of community composition and biodiversity

An important component of my MS research involved using remote sensing to identify and track ecological processes such as community composition, diversity, and ecosystem productivity. Historically, remote sensing of Earth’s surface has predominantly involved collecting imagery at varying spatial resolution. Relatively low resolution satellite and aerial imagery made older remotely sensed data products less useful for ecologists, who have generally been more interested in fine-scale processes involving interactions between organisms and their environments. In recent years the technology has advanced such that now ecologists can employ near-earth and satellite-based remote sensing methods to address ecological questions. Remote sensing also offers ecologists the ability to integrate data across spatial scales from the plot-level to the landscape-scale. These new approaches allows ecologists to track changes in the composition and diversity of ecosystems around the globe.

Using historical maps and images to understand forest dynamics

In my research, I have always been interested in using large-scale remote sensing data to understand field-scale processes. For my undergraduate senior project, I used historic maps and aerial photos of the area around Bennington College, in southern Vermont to track the spread of forests after agricultural abandonment during the nineteenth century. I used this method to identify forest stands of different ages, and then formulated a series of ecological hypotheses regarding stand age. Link to project: http://wiki.bennington.edu/Mara_McPartland_Ecological_Theory_and_Research_2012

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An historic map of the Bennington area from 1835. Red outline indicates the AOI of my senior project.