@article {JRN54668, title = {Pushing precipitation to the extremes in distributed experiments: recommendations for simulating wet and dry years}, journal = {Global Change Biology}, volume = {23}, year = {2017}, pages = {1774-1782}, abstract = {

Intensification of the global hydrological cycle, ranging from larger individual precipitation events to more extreme multiyear droughts, has the potential to cause widespread alterations in ecosystem structure and function. With evidence that the incidence of extreme precipitation years (defined statistically from historical precipitation records) is increasing, there is a clear need to identify ecosystems that are most vulnerable to these changes and understand why some ecosystems are more sensitive to extremes than others. To date, opportunistic studies of naturally occurring extreme precipitation years, combined with results from a relatively small number of experiments, have provided limited mechanistic understanding of differences in ecosystem sensitivity, suggesting that new approaches are needed. Coordinated distributed experiments (CDEs) arrayed across multiple ecosystem types and focused on water can enhance our understanding of differential ecosystem sensitivity to precipitation extremes, but there are many design challenges to overcome (e.g., cost, comparability, standardization). Here, we evaluate contemporary experimental approaches for manipulating precipitation under field conditions to inform the design of {\textquoteleft}Drought-Net{\textquoteright}, a relatively low-cost CDE that simulates extreme precipitation years. A common method for imposing both dry and wet years is to alter each ambient precipitation event. We endorse this approach for imposing extreme precipitation years because it simultaneously alters other precipitation characteristics (i.e., event size) consistent with natural precipitation patterns. However, we do not advocate applying identical treatment levels at all sites {\textendash} a common approach to standardization in CDEs. This is because precipitation variability varies \>fivefold globally resulting in a wide range of ecosystem-specific thresholds for defining extreme precipitation years. For CDEs focused on precipitation extremes, treatments should be based on each site{\textquoteright}s past climatic characteristics. This approach, though not often used by ecologists, allows ecological responses to be directly compared across disparate ecosystems and climates, facilitating process-level understanding of ecosystem sensitivity to precipitation extremes.

}, keywords = {climate extremes, drought, field experiments, precipitation regimes, wet years}, doi = {10.1111/gcb.13504}, url = {files/bibliography/JRN00660.pdf}, author = {A. K. Knapp and Avolio, Meghan L. and Claus Beier and Carroll, Charles J. W. and S.L. Collins and Dukes, Jeffrey S. and Fraser, Lauchlan H. and Griffin-Nolan, Robert J. and David L. Hoover and Jentsch, Anke and Loik, Michael E. and Richard P. Phillips and Post, Alison K. and O. E. Sala and Slette, Ingrid J. and Laura Yahdjian and M. D. Smith} } @article {JRN54225, 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 = {climate change experiments, climate extremes, drought, ecoregions, global patterns, precipitation, rainfall patterns}, doi = {10.1111/gcb.12888}, url = {files/bibliography/JRN00634.pdf}, author = {A. K. Knapp and David L. Hoover and Wilcox, Kevin R. and Avolio, Meghan L. and Koerner, Sally E. and La Pierre, Kimberly J. and Loik, Michael E. and Luo, Yiqi and O. E. Sala and M. D. Smith} } @inbook {JRN52665, title = {Patterns of net primary production across sites}, booktitle = {Long-Term Trends in Ecological Systems: A Basis for Understanding Responses to Global Change}, year = {2013}, pages = {42-45}, publisher = {National Technical Information Services}, organization = {National Technical Information Services}, chapter = {5}, address = {Springfield, Virginia}, abstract = {

Net primary production (NPP) is a fundamentally important and commonly measured ecosystem process that provides an integrative estimate of energy capture and flow into systems, and consequently the energy available for use by other trophic levels. A wide range of productivity levels occurs globally with high temporal dynamics among sites. This chapter discusses approaches to estimating NPP, highlights site-specific trends in productivity, and provides examples of past synthetic analyses across space and time. In terrestrial ecosystems, aboveground NPP are commonly measured or estimated. In many sites, spatial variation in NPP among locations within a site can overwhelm temporal variation.\  In terms temporal patterns, at some sites clear temporal trends exist: NPP either increases or decreases with time; at other sites, strong interannual variations exist.\  Past synthetic analyses have provided key insights into the relative roles of biotic versus abiotic drivers of dynamics as well as elucidating where and when biogeochemical versus climatic factors underlie patterns of NPP across biomes.\  The chapter proposes four questions that can be addressed as long-term data sets at more sites become available.

}, keywords = {atmospheric chemistry, climate change, cross-site comparisons, disturbance, ecological response, ecology, ecosystem, EcoTrends, experimental forests, global change, human demography, human population growth, Long Term Ecological Research (LTER), long-term datasets, precipitation, rangeland, rangeland research stations, surface water chemistry}, url = {/files/bibliography/13-029-5.pdf}, author = {A. K. Knapp and M. D. Smith and Debra C. Peters and S.L. Collins} } @article {49953, title = {Patterns of net primary production across sites. Cross-site comparisons of state change dynamics}, year = {2011}, abstract = {

Net primary production (NPP) is a fundamentally important and commonly measured ecosystem process that provides an integrative estimate of energy capture and flow into systems and consequently of the energy available for use by other trophic levels. A wide range of productivity levels occurs globally (figure 5-1) with high temporal dynamics among sites (chapter 14). In this brief overview, we discuss approaches to estimating NPP, highlight site-specific trends in productivity, and provide examples of past synthetic analyses across space and time. We focus on aboveground components of NPP for reasons explained below.

}, keywords = {cross-site comparisons, global change, net primary production, NPP}, url = {files/bibliography/JRN00580.pdf}, author = {A. K. Knapp and M. D. Smith and Debra C. Peters and S.L. Collins} } @article {50230, title = {Shrub encroachment in North American grasslands: Shifts in growth form dominance rapidly alters control of ecosystem carbon inputs}, journal = {Global Change Biology}, volume = {14}, year = {2008}, month = {January 30, 2008}, pages = {615-623}, publisher = {Blackwell Publishing}, abstract = {

Shrub encroachment into grass-dominated biomes is occurring globally due to a variety of anthropogenic activities, but the consequences for carbon (C) inputs, storage and cycling remain unclear. We studied eight North American graminoid-dominated ecosystems invaded by shrubs, from arctic tundra to Atlantic coastal dunes, to quantify patterns and controls of C inputs via aboveground net primary production (ANPP). Across a 4-fold range in mean annual precipitation (MAP), a key regulator of ecosystem C input at the continental scale, shrub invasion decreased ANPP in xeric sites, but dramatically increased ANPP (\>1000 g/m2) at high MAP, where shrub patches maintained extraordinarily high leaf area. Concurrently, the relationship between MAP and ANPP shifted from being non-linear in grasslands to linear in shrublands. Thus, relatively abrupt (\<50 yrs) shifts in growth form dominance, without changes in resource quantity, can fundamentally alter continental-scale pattern of C inputs and control by MAP in ways that exceed the direct effects of climate change alone.

}, keywords = {aboveground net primary production, carbon, climate change, encroachment, grass-dominated biomes, grasslands, growth form, LAI, MAP, North American, shrub, shrublands}, doi = {10.1111/j.1365-2486.2007.01512.x }, url = {/files/bibliography/08-005.pdf}, author = {A. K. Knapp and J. M. Briggs and S.L. Collins and Steven R. Archer and M.S. Bret-Harte and Ewers, B. and Debra C. Peters and Young, D. and Shaver, G. and Cleary, M.B.} } @inbook {50351, title = {Estimating aboveground net primary production in grassland and herbaceous dominated ecosystems}, booktitle = {Principles and standards for measuring primary production}, year = {2007}, note = {//OTHER//LTER-V//entered 08/15/2007; updated 06/02/2008 per DEPProCite field[6]: In}, month = {2007}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {Oxford}, abstract = {

Although grass- and herb-dominated ecosystems may be easier to sample for aboveground primary production than most others, a variety of challenges for standardizing approaches in different settings and circumstances must be overcome. This chapter describes the most common harvest approaches for grassland production measurement and a suite of site-specific criteria that affect the choice of approach (e.g., grazing, decomposition, phenology). Errors leading to either underestimation or overestimation are examined, and a case study describing approaches for evaluating sample adequacy is provided.

}, keywords = {ANPP, book, chapter, grassland, ANPP method, method, aboveground net primary production, NPP, plant, ANPP, report, technique, aboveground NPP}, doi = {10.1093/acprof:oso/9780195168662.003.0003}, author = {A. K. Knapp and J. M. Briggs and Daniel L. Childers and O. E. Sala}, editor = {Fahey, Timothy J. and A. K. Knapp} }