@article {KNZ002034, title = {Is a drought a drought in grasslands? Productivity responses to different types of drought}, journal = {Oecologia}, year = {2021}, keywords = {LTER-KNZ}, author = {Carroll, C.J.W. and Slette, I.J. and Griffin-Nolan, R.J. and Baur, L.E. and Hoffman, A.M. and Denton, E.M. and Gray, J.E. and Post, A.K. and Johnston, M.K. and Yu, Q. and Collins, S.L. and Luo, Y. and Smith, M.D. and Knapp, A.K.} } @article {KNZ001798, title = {Quantifying global soil carbon losses in response to warming}, journal = {Nature}, volume = {540}, year = {2016}, pages = {104 - 108}, abstract = {

The majority of the Earth\’s terrestrial carbon is stored in the soil. If anthropogenic warming stimulates the loss of this carbon to the atmosphere, it could drive further planetary warming, Despite evidence that warming enhances carbon fluxes to and from the soil, the net global balance between these responses remains uncertain. Here we present a comprehensive analysis of warming-induced changes in soil carbon stocks by assembling data from 49 field experiments located across North America, Europe and Asia. We find that the effects of warming are contingent on the size of the initial soil carbon stock, with considerable losses occurring in high-latitude areas. By extrapolating this empirical relationship to the global scale, we provide estimates of soil carbon sensitivity to warming that may help to constrain Earth system model projections. Our empirical relationship suggests that global soil carbon stocks in the upper soil horizons will fall by 30\ \±\ 30 petagrams of carbon to 203\ \±\ 161 petagrams of carbon under one degree of warming, depending on the rate at which the effects of warming are realized. Under the conservative assumption that the response of soil carbon to warming occurs within a year, a business-as-usual climate scenario would drive the loss of 55\ \±\ 50 petagrams of carbon from the upper soil horizons by 2050. This value is around 12\–17 per cent of the expected anthropogenic emissions over this period. Despite the considerable uncertainty in our estimates, the direction of the global soil carbon response is consistent across all scenarios. This provides strong empirical support for the idea that rising temperatures will stimulate the net loss of soil carbon to the atmosphere, driving a positive land carbon\–climate feedback that could accelerate climate change.

}, keywords = {LTER-KNZ, Biogeochemistry, carbon cycle}, doi = {10.1038/nature20150}, url = {https://www.nature.com/articles/nature20150}, author = {Crowther, T. W. and Todd-Brown, K. E. O. and Rowe, C. W. and Wieder, W. R. and Carey, J. C. and Machmuller, M. B. and Snoek, B. L. and Fang, S. and Zhou, G. and Allison, S. D. and John M. Blair and Bridgham, S. D. and Burton, A. J. and Carrillo, Y. and Reich, P. B. and Clark, J. S. and Classen, A. T. and Dijkstra, F. A. and Elberling, B. and Emmett, B. A. and Estiarte, M. and Frey, S. D. and Guo, J. and Harte, J. and Jiang, L. and Johnson, B. R. and Kr{\"o}el-Dulay, G. and Larsen, K. S. and Laudon, H. and Lavallee, J. M. and Luo, Y. and Lupascu, M. and Ma, L. N. and Marhan, S. and Michelsen, A. and Mohan, J. and Niu, S. and Pendall, E. and {\~n}uelas, J. and Pfeifer-Meister, L. and Poll, C. and Reinsch, S. and Reynolds, L. L. and Schmidt, I. K. and Sistla, S. and Sokol, N. W. and Templer, P. H. and Treseder, K. K. and Welker, J. M. and Bradford, M. A.} } @article {KNZ001684, title = {Characterizing differences in precipitation regimes of extreme wet and dry years: Implications for climate change experiments}, journal = {Global Change Biology}, volume = {21}, year = {2015}, pages = {2624 -2633}, abstract = {

Climate change is intensifying the hydrologic cycle and is expected to increase the frequency of extreme wet and dry years. Beyond precipitation amount, extreme wet and dry years may differ in other ways, such as the number of precipitation events, event size, and the time between events. We assessed 1614 long-term (100 year) precipitation records from around the world to identify key attributes of precipitation regimes, besides amount, that distinguish statistically extreme wet from extreme dry years. In general, in regions where mean annual precipitation (MAP) exceeded 1000 mm, precipitation amounts in extreme wet and dry years differed from average years by ~40\% and 30\%, respectively. The magnitude of these deviations increased to \>60\% for dry years and to \>150\% for wet years in arid regions (MAP\<500 mm). Extreme wet years were primarily distinguished from average and extreme dry years by the presence of multiple extreme (large) daily precipitation events (events \>99th percentile of all events); these occurred twice as often in extreme wet years compared to average years. In contrast, these large precipitation events were rare in extreme dry years. Less important for distinguishing extreme wet from dry years were mean event size and frequency, or the number of dry days between events. However, extreme dry years were distinguished from average years by an increase in the number of dry days between events. These precipitation regime attributes consistently differed between extreme wet and dry years across 12 major terrestrial ecoregions from around the world, from deserts to the tropics. Thus, we recommend that climate change experiments and model simulations incorporate these differences in key precipitation regime attributes, as well as amount into treatments. This will allow experiments to more realistically simulate extreme precipitation years and more accurately assess the ecological consequences.

}, keywords = {LTER-KNZ, rainfall patterns}, doi = {10.1111/gcb.12888}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.12888}, author = {Alan K. Knapp and D.L. Hoover and K.R. Wilcox and M.L. Avolio and Koerner, S.E. and Kimberly J. La Pierre and Loik, M.E. and Luo, Y. and Sala, O.E. and M.D. Smith} } @article {KNZ001633, title = {Differential effects of extreme drought on production and respiration: Synthesis and modeling analysis}, journal = {Biogeosciences}, volume = {11}, year = {2014}, pages = {621 -633}, abstract = {

Extremes in climate may severely impact ecosystem structure and function, with both the magnitude and rate of response differing among ecosystem types and processes. We conducted a modeling analysis of the effects of extreme drought on two key ecosystem processes, production and respiration, and, to provide a broader context, we complemented this with a synthesis of published results that cover a wide variety of ecosystems. The synthesis indicated that across a broad range of biomes, gross primary production (GPP) was generally more sensitive to extreme drought (defined as proportional reduction relative to average rainfall periods) than was ecosystem respiration (ER). Furthermore, this differential sensitivity between production and respiration increased as drought severity increased; it occurred only in grassland ecosystems, and not in evergreen needle-leaf and broad-leaf forests or woody savannahs. The modeling analysis was designed to enable a better understanding of the mechanisms underlying this pattern, and focused on four grassland sites arrayed across the Great Plains, USA. Model results consistently showed that net primary productivity (NPP) was reduced more than heterotrophic respiration (Rh) by extreme drought (i.e., 67\% reduction in annual ambient rainfall) at all four study sites. The sensitivity of NPP to drought was directly attributable to rainfall amount, whereas the sensitivity of Rh to drought was driven by soil drying, reduced carbon (C) input and a drought-induced reduction in soil C content \– a much slower process. However, differences in reductions in NPP and Rh diminished as extreme drought continued, due to a gradual decline in the soil C pool leading to further reductions in Rh. We also varied the way in which drought was imposed in the modeling analysis; it was either imposed by simulating reductions in rainfall event size (ESR) or by reducing rainfall event number (REN). Modeled NPP and Rh decreased more by ESR than REN at the two relatively mesic sites but less so at the two xeric sites. Our findings suggest that responses of production and respiration differ in magnitude, occur on different timescales, and are affected by different mechanisms under extreme, prolonged drought.

}, keywords = {LTER-KNZ}, doi = {10.5194/bg-11-621-2014}, url = {https://www.biogeosciences.net/11/621/2014/}, author = {Shi, Z. and Thomey, M.L. and Mowll, M. and Litvak, M.E. and N. Brunsell and Scott. L. Collins and Pockman, W.T. and M.D. Smith and Alan K. Knapp and Luo, Y.} } @article {KNZ001538, title = {Coordinated distributed experiments: an emerging tool for testing global hypotheses in ecology and environmental science}, journal = {Frontiers in Ecology and the Environment}, volume = {11}, year = {2013}, pages = {147 -155}, abstract = {

There is a growing realization among scientists and policy makers that an increased understanding of today\&$\#$39;s environmental issues requires international collaboration and data synthesis. Meta-analyses have served this role in ecology for more than a decade, but the different experimental methodologies researchers use can limit the strength of the meta-analytic approach. Considering the global nature of many environmental issues, a new collaborative approach, which we call coordinated distributed experiments (CDEs), is needed that will control for both spatial and temporal scale, and that encompasses large geographic ranges. Ecological CDEs, involving standardized, controlled protocols, have the potential to advance our understanding of general principles in ecology and environmental science.

}, keywords = {LTER-KNZ}, doi = {10.1890/110279}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/110279}, author = {Fraser, L.H. and Henry, H.A. and Carlyle, C.N. and White, S.R. and Beierkuhnlein, C. and Cahill, J.F. and Casper, B.B. and Cleland, E.E. and Scott. L. Collins and Dukes, J.S. and Alan K. Knapp and Lind, E. and Long, R. and Luo, Y. and P.B. Reich and M.D. Smith and Sternberg, M. and Turkington, R.} } @article {KNZ001187, title = {Consequences of more extreme precipitation regimes for terrestrial ecosystems}, journal = {BioScience}, volume = {58}, year = {2008}, pages = {811 -821}, abstract = {

mplification of the hydrological cycle as a consequence of global warming is forecast to lead to more extreme intra-annual precipitation regimes characterized by larger rainfall events and longer intervals between events. We present a conceptual framework, based on past investigations and ecological theory, for predicting the consequences of this underappreciated aspect of climate change. We consider a broad range of terrestrial ecosystems that vary in their overall water balance. More extreme rainfall regimes are expected to increase the duration and severity of soil water stress in mesic ecosystems as intervals between rainfall events increase. In contrast, xeric ecosystems may exhibit the opposite response to extreme events. Larger but less frequent rainfall events may result in proportional reductions in evaporative losses in xeric systems, and thus may lead to greater soil water availability. Hydric (wetland) ecosystems are predicted to experience reduced periods of anoxia in response to prolonged intervals between rainfall events. Understanding these contingent effects of ecosystem water balance is necessary for predicting how more extreme precipitation regimes will modify ecosystem processes and alter interactions with related global change drivers.

}, keywords = {LTER-KNZ, Climate change, Drought, Ecosystems, Precipitation, soil water}, doi = {10.1641/B580908}, url = {https://academic.oup.com/bioscience/article/58/9/811/250853}, author = {Alan K. Knapp and Beier, C. and Briske, D.D. and Classen, A.T. and Luo, Y. and Reichstein, M. and M.D. Smith and Smith, S.D. and Bell, J.E. and Fay, P.A. and Heisler, J.L. and Leavitt, S.W and Sherry, R. and Smith, B. and Weng, E.} } @article {KNZ001184, title = {Modeled interactive effects of precipitation, temperature, and CO2 on ecosystem carbon and water dynamics in different climatic zones}, journal = {Global Change Biology}, volume = {14}, year = {2008}, pages = {1986 -1999}, abstract = {

Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modeling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts [doubled (DP) and halved (HP)] and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO2] (C) on net primary production (NPP), heterotrophic respiration (Rh), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones. The modeling analysis showed that none of the three-way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two-way interactive effects on NPP, Rh, and NEP were generally positive (i.e. amplification of one factor\&$\#$39;s effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor\&$\#$39;s effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two-way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, Rh, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modeling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for the further improvement of ecosystem models in order to adequately simulate complex interactive processes.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2486.2008.01629.x}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2008.01629.x}, author = {Luo, Y. and Gerten, D. and Le Maire, G. and Parton, W.J. and Weng, E. and Zhou, X. and Keough, C. and Beier, C. and Ciais, P. and Cramer, W. and Dukes, J.S. and Emmett, B. and Hanson, P.J. and Alan K. Knapp and Linder, S. and Nepstad, D. and Rustad, L.} } @article {KNZ001183, title = {Modelled effects of precipitation on ecosystem carbon and water dynamics in different climatic zones}, journal = {Global Change Biology}, volume = {14}, year = {2008}, pages = {1 -15}, abstract = {

The ongoing changes in the global climate expose the world\&$\#$39;s ecosystems not only to increasing CO2 concentrations and temperatures but also to altered precipitation (P) regimes. Using four well-established process-based ecosystem models (LPJ, DayCent, ORCHIDEE, TECO), we explored effects of potential P changes on water limitation and net primary production (NPP) in seven terrestrial ecosystems with distinctive vegetation types in different hydroclimatic zones. We found that NPP responses to P changes differed not only among sites but also within a year at a given site. The magnitudes of NPP change were basically determined by the degree of ecosystem water limitation, which was quantified here using the ratio between atmospheric transpirational demand and soil water supply. Humid sites and/or periods were least responsive to any change in P as compared with moderately humid or dry sites/periods. We also found that NPP responded more strongly to doubling or halving of P amount and a seasonal shift in P occurrence than that to altered P frequency and intensity at constant annual amounts. The findings were highly robust across the four models especially in terms of the direction of changes and largely consistent with earlier P manipulation experiments and modelling results. Overall, this study underscores the widespread importance of P as a driver of change in ecosystems, although the ultimate response of a particular site will depend on the detailed nature and seasonal timing of P change.

}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2486.2008.01651.x}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2008.01651.x}, author = {Gerten, D. and Luo, Y. and Le Maire, G. and Parton, W.J. and Keough, C. and Weng, E. and Beier, C. and Ciais, P. and Cramer, W. and Dukes, J.S. and Emmett, B. and Hanson, P.J. and Alan K. Knapp and Linder, S. and Nepstad, D. and Rustad, L. and Sowerby, A.} }