The Effects of Winter Moth Defoliation on Forest Growth and Production Inferred from Satellite Imagery and Dendrochronology
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The effects of winter moth defoliation on forest growth and production inferred from satellite imagery and dendrochronology Erin Gleeson, Rhodes College 2000 North Parkway, TN 38112 Co-authors: Christopher Neill and Greg Fiske Woods Hole Research Center 149 Woods Hole Road, Falmouth, MA 02540 Gleeson 1 Abstract Forest disturbances are events that can cause change in the structure and composition of a forest ecosystem. Insect outbreaks may be a consequence of climate change, and could create unexpected dynamics in nature. Recently, and invasive pest from Europe, called the winter moth, has invaded Massachusetts and most of New England. The winter moth causes widespread defoliation by stripping the leaves off of all deciduous trees. I attempted to use remote sensing satellite data from MODIS, to classify sites that had experienced heavy defoliation and sites that had experienced little to no defoliation. Then, by looking at radial increment growth in tree cores, I created estimates of forest biomass growth, which I used as a model to try and study the effects of winter moth defoliation on greater areas of land. I found that defoliation from the winter moth has a large effect on tree ring width and overall tree growth because of how time period (pre and post winter moth) at each treatment affected ring width increment. The winter moth also reduced carbon storage in southeastern Massachusetts by around 51%. Clearly there are serious implications for an insect outbreak like this one, and measures, such as biological control by releasing natural parasitic enemies of the winter moth are being utilized. Climate change and a shift towards warmer global temperatures are predicted to increase the spread and severity of winter moth defoliation, and it is important that we think about future studies, and how we can control the reintroduction and spread of invasive insects that cause so much harm to ecosystem services and native ecosystem dynamics. Introduction Forest disturbances are widespread and cause pronounced changes in different ecosystems. Disturbances take many forms, and often have consequences on the ecosystem services that forests provide. Ecological disturbances may reduce tree growth, and in turn affect Gleeson 2 carbon storing capabilities of the forest. Typically, disturbance events result in a net reduction in ecosystem carbon stocks. They alter how fixed carbon is allocated, whether it is directly released into the atmosphere, or stored in woody biomass, and they influence species composition and ecosystem structure (Williams et al 2016). Carbon sequestration is a crucial ecosystem service that our forests provide. Forests are carbon sinks, and increases in woody biomass directly increases the amount of carbon that is sequestered by the forest. Nutrient cycling in these systems may also be impacted by defoliation from the winter moth. Lovett et al. (2006) suggests that insect infestations such as winter and gypsy moth defoliations produce an increase in stream water nitrate concentrations. This influx of nitrogen in highly defoliated areas most likely comes from increased water drainage and leaching from damaged foliage (Lovett et al. 2006). Healthy hardwood trees usually recover and produce new leaves after winter moth outbreaks, however, after multiple years of defoliation and added stress, like drought, the winter moth can contribute to widespread mortality of trees. Multiple factors including natural disturbances, like fire, as well as human caused insect introductions, can decrease the amount of biomass in a forest and turn it into an atmospheric carbon source (Jacquet et al. 2012). On a large scale, these growing carbon sources have the potential to perpetuate global warming and climate change by increasing carbon dioxide concentrations in the atmosphere. The recurrent introduction of invasive insects is one of the most serious ecological threats to United States forests. These invasive pests are an undesirable repercussion of international trade and travel, and they are responsible for widespread ecological and economic damage (Lovett et al. 2016). Global warming and insect outbreaks can create a positive feedback loop, where warmer average temperatures trigger large-scale outbreaks of insect defoliators, which in turn increases overall tree mortality (Jacquet et al. 2012). It is Gleeson 3 imperative that we examine the impact of defoliation on overall tree growth, so that we can address the potential long term effects of these invasive insect outbreaks on global climate change and overall forest health. Northeastern terrestrial forests have been host to many different invasive species. One of these exotic pests, the winter moth (Operophtera brumata), is native to Europe, and was identified in Massachusetts around 2003, however, there is speculation that the winter moth has been present since the 1990s (Simmons et al. 2014). Specifically, the winter moth invaded Falmouth, MA in 2007 (Hibbard and Elkinton 2015). The winter moth caterpillar is known for defoliating native deciduous trees such as oak, and maple in the early summer months between late May and mid-June. Their larvae feed on the expanding buds and later on the foliage for approximately six weeks, effectively removing the photosynthetic tissue that is critical for plant maintenance and growth (Hibbard and Elkinton 2015). Then, the adult moths emerge from the soil during the winter months of November and December for mating purposes. Other than Massachusetts, the winter moth has invaded most of New England, Nova Scotia, and even Washington state and Oregon. Dendrochronology, the analysis of annual tree rings, is an effective tool for studying invasive insect outbreaks. Trees that are defoliated by the winter moth have to expend more energy to re-grow the leaves that they lost, so the radial growth observed in a tree core should be smaller in years with high winter moth infestation. Defoliation from winter moths is damaging to these forests because it decreases radial growth and basal area of the trees, which can be used as a predictor for tree mortality, and tree production and carbon sequestration decline. Increases in tree mortality may also alter the species composition of deciduous forests by creating gaps in the canopy that allow early successional species like grasses and shrubs to establish and grow Gleeson 4 (Simmons et al. 2014). Measuring tree diameter increment is a proxy used for whole tree growth, and is self-scaling because while large trees produce more biomass than small trees, a specific diameter increment represents more absolute biomass in a large tree than in a small tree (Bowman et al. 2013). Reducing radial tree growth could cause a measurable reduction in carbon storage, because trees store carbon, and sequester it as they grow (Nowak et al. 2012). Kulman (1971), states that the effects of winter moth defoliation on tree growth can be detected with a “horizontal sequencing mode” of observing the differences in thickness of the tree rings in a tree core. Defoliation not only has an effect on radial tree growth, but also overall tree productivity. Outbreaks from defoliating insects are known as ephemeral forest disturbances that are short lived, and often allow the forest recovers quickly within the same growing season (De Beurs and Townsend 2008). Defoliation causes a decrease in leaf area index (LAI), which, in turn, decreases the amount of chlorophyll, the tree’s photosynthesizing machinery. Because changes in LAI and leaf chlorophyll together make up the measure of vegetation canopy “greenness,” that can be seen clearly from satellite imagery, and “greenness” is commonly used to monitor the Earth’s vegetation cover from space (Jiang et al. 2008). Remote sensing techniques are a useful way to map the occurrence of defoliation, and also the level of severity (De Beurs and Townsend 2008). MODIS, or moderate resolution imaging spectroradiometer, is an instrument aboard the Terra and Aqua satellites. The Terra and Aqua satellites where MODIS is present, view the Earth’s surface every one to two days. MODIS is used in research devoted to understanding global processes, and develop models to assist in predicting global (National Aeronautics and Space Administration 2016). MODIS data has course special resolution which is more effective in mapping large-scale vegetation changes, and the data is available and updated daily (De Beurs and Townsend 2008). Gleeson 5 . In this study I attempt to answer three questions: 1) Can we use remote sensing techniques to estimate areas of the forest have been severely affected by defoliation from the winter moth? 2) What can remote sensing and dendrochronology (tree ring dating) tell us about how defoliation from the winter moth affects woody biomass growth? 3) How does defoliation from the winter moth affect overall carbon storage in northeastern forests? I was interesting in using these techniques to investigate local insect forest disturbances in Massachusetts. In this study, I attempted to test MODIS satellite data by comparing tree growth from two different changes in the enhanced vegetation index (EVI) since 2000, to see if there was any evidence of winter moth defoliation, other than what was observed based on the changes in foliar biomass. Then I want to find the effects that winter moth defoliation would have on the total amount of carbon stored in large areas of land to get a better idea of the severity of the outbreak and what the implications are for future northeastern forests that could eventually be affected by the winter moth. Materials and Methods Sampling Sites I identified treatments using MODIS satellite imagery to determine areas of Barnstable county that had experienced heavy defoliation or little to no defoliation. Then, based on these two treatments, I found twelve different sampling sites, six of these sites had experienced heavy defoliation from the winter moth since 2000, while the other six had experienced little to no defoliation from the winter moth since 2000. I paired the appropriate treatment level to each of the twelve sites by using a Mann-Kendall trend statistic, which assigned a numerical value to Gleeson 6 each randomly generated point across Barnstable County.