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Unexpected ecological advances made possible by long‐term data: A Coweeta example

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C. Rhett Jackson, Jackson R. Webster, Jennifer D. Knoepp, Katherine J. Elliott, Ryan E. Emanuel, Peter V. Caldwell, Chelcy F. Miniat

Published Online: Jan 09 2018

DOI: 10.1002/wat2.1273

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In the 1970s, Forest Service and academic researchers clearcut the forest in Watershed 7 in the Coweeta Basin to observe how far the perturbation would move the ecosystem and how quickly the ecosystem would return to its predisturbance state. Our long‐term observations demonstrated that this view of resistance and resilience was too simplistic. Forest disturbance triggered a chain of ecological dynamics that are still evolving after 40 years. Short‐term pulses in dissolved inorganic nitrogen (DIN) (3 years) and streamflows (4 years) were followed by several years in which the system appeared to be returning to predisturbance conditions. Then however, changes in forest composition triggered a regime change in DIN dynamics from biological to hydrological control as well as persistent high stream DIN levels mediated by climatic conditions. These forest composition changes also led to later reductions in streamflow. These long‐term observations of streamflows, stream DIN concentrations, stream DIN exports, and stand composition have substantially advanced our understanding of forest ecosystem dynamics; and they demonstrate the value of long‐term observational data in revealing ecosystem complexities and surprises, generating new hypotheses, and motivating mechanistic research. Shorter observational records from this experiment would have produced incomplete or erroneous inference. WIREs Water 2018, 5:e1273. doi: 10.1002/wat2.1273 This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality Water and Life > Nature of Freshwater Ecosystems

Percent aboveground biomass for the whole watershed in WS 7 through time (Boring, Elliott, & Swank, ). Species code are ACRU, Acer rubrum; BELE, Betula lenta; CARY, Carya spp.; LITU, Liriodendron tulipifera; QUCO, Quercus coccinea; QUPR, Quercus prinus; QURU, Quercus rubra; QUVE, Quercus velutina; and ROPS, Robinia pseudoacacia. Robinia became dominant early in succession then diminished substantially but remained well above preharvest levels 30 years after harvest

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Changes in annual water yield (Q) from WS7 between water years 1967 and 2016 (bars), and the prediction intervals evaluated at α = .05 (lines). Changes in Q that lie outside prediction intervals are significant. Predicted WS7 yield in the absence of treatment was estimated by fitting a linear model of treatment to reference watershed Q during the calibration period and using this model to predict what would have been WS7 Q in the posttreatment period. Data are previously published (Swank et al., ) but updated to include 2013–2016

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Map of WS7 nitrate‐N concentrations (μg/L) across the stream network in 2008. Numbers in parentheses are tributary concentrations, and numbers not in parentheses are concentration in the main stream. Patterns were basically the same in sampling that occurred in post treatment sampling in 1977–1978, 1999, 2004, and 2008. The increase in nitrate at the headwater spring was first observed in April 1977, just after logging in January–February. The 2004 samples were taken as part of the LINX project(LINX Colaborators, )

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Progression of annual DIN and discharge deviation between Coweeta Watersheds 7 (treatment) and 2 (reference) from 1973 to 2013. The discharge deviation is the difference between the observed WS7 flow and that predicted from WS2 based on the pretreatment model. The DIN deviation is WS7 DIN–WS2 DIN. Each point is marked by the last two digits of the year. Symbol colors denote separate clusters identified by k‐means clustering by year. Green points encompass the initial watershed response (1977–1979) featuring large increases in water yield and moderate nitrogen release after harvest. Red points encompass the preharvest period (1972–1976), but also a period when the watershed seemed to be returning to preharvest conditions (1980–1988) after the initial response. Light blue points encompass the period of high DIN concentrations (1990–1998) due to nitrogen fixation following the emergence of Robinia as a dominant forest tree. Dark blue points encompass the most recent period (1999–2013) featuring lower streamflows and DIN. These latter two periods suggest interactions between droughts and DIN. The data do not yet indicate a move back to the initial condition

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Mean annual disolved inorganic nitrogen (DIN) concentration in Coweeta watersheds 7 (WS 7, logged 1977) and 2 (WS 2, reference) and annual discharge from WS 7. The gray bars represent periods of drought.(Reprinted with permission from Webster, Knoepp et al. ()

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