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1. Assessing the factors influencing natural regeneration patterns in the diverse, multi‐cohort, and managed forests of Maine, USA

   https://doi.org/10.1111/jvs.12433  

   Bose, Arun K. ; Weiskittel, Aaron ; Wagner, Robert G. ; Kuehne, Christian ; Michalet, ed., Richard ( June 2016 , Journal of Vegetation Science)

   What are the primary biotic and abiotic factors driving composition and abundance of naturally regenerated tree seedlings across forest landscapes of Maine? Do seedling species richness (SR) and density (SD) decrease with improved growing conditions (climate and soil), but increase with increased diversity of overstorey composition and structure? Does partial harvesting disproportionately favour relative dominance of shade‐intolerant hardwoods (PIHD) over shade‐tolerant softwoods (PTSD)?

   Forest landscapes across the diverse eco‐regions and forest types of Maine,USA.

   This study usedUSDAForest Service Forest Inventory Analysis permanent plots (n = 10 842), measured every 5 yr since 1999. The best models for each response variable (SR,SD,PIHDandPTSD) were developed based onAICand biological interpretability, while considering 35 potential explanatory variables incorporating climate, soil, site productivity, overstorey structure and composition, and past harvesting.

   Mean annual temperature was the most important abiotic factor, whereas overstorey tree size diversity was the most important biotic factor forSRandSD. Both mean annual temperature and overstorey tree size diversity had a curvilinear relationship withSRandSD. Average overstorey shade tolerance and percentage tolerant softwood basal area in the overstorey were the top predictor variables ofPIHDandPTSD,respectively. Partial harvesting favouredPIHDbut notPTSD.

   This is one of the first studies to comprehensively evaluate a number of factors influencing naturally established tree seedlings at a broad landscape scale in the Northern Forest region of the easternUSAand Canada. Despite limitations associated with relatively small plot size, large seedling size class and lack of direct measurements of light, water and nutrients, this study documents the influence of these factors amid high variability associated with patterns of natural regeneration. The curvilinear relationship between mean annual temperature withSRandSDsupports the argument that species richness and abundance usually have unimodal relationships with productivity indicators, whereas the curvilinear relationship between overstorey tree size diversity andSRandSDsuggest that moderate overstorey diversity incorporates multiple species as well as higher seedling individuals.

    

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2. Species origin affects the rate of response to inter‐annual growing season precipitation and nutrient addition in four Australian native grasslands

   https://doi.org/10.1111/jvs.12450  

   Morgan, John W. ; Dwyer, John M. ; Price, Jodi N. ; Prober, Suzanne M. ; Power, Sally A. ; Firn, Jennifer ; Moore, Joslin L. ; Wardle, Glenda M. ; Seabloom, Eric W. ; Borer, Elizabeth T. ; et al ( August 2016 , Journal of Vegetation Science)

   Predicted increases in temperature and changes to precipitation are expected to alter the amount of plant available nutrients, in turn, altering rates of primary production and exotic plant invasions. However, it remains unclear whether increased responses occur in wetter than average years, even in low fertility and low rainfall regions.

   Four Australian grasslands, including sites in arid Western Australia, semi‐arid Victoria, alpine Victoria and sub‐tropical Queensland.

   Using identical nutrient addition experiments, we use 6‐years of biomass, cover and species richness data to examine how rates of biomass production and native and exotic cover and richness are affected by growing season precipitation [proportion of yearly growing season precipitation (GSP) to long‐term meanGSP] and nutrient (N, P, K and micronutrients) addition.

   Rates of grassland productivity strongly increased with increasingGSP.GSPincreased rates of native cover but not native or exotic richness, nor rates of exotic cover change. We detected no significantNPKeffect on rates of grassland productivity, exotic cover or exotic richness change. In contrast,NPKaddition decreased rates of native cover change and fertilized plots had significantly fewer native species. We did not detect a significant interaction betweenNPKandGSP_.

   Grassland productivity was more strongly predicted by variation in growing season precipitation than by nutrient addition, suggesting it will vary with future changes in rainfall. Response to nutrients, however, depend on species origin, suggesting that increasing soil nutrient availability due to anthropogenic activities is likely to lead to negative effects on native species richness and cover.

    

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3. Biome Transition Along Elevational Gradients in New Mexico (SEON) Study: Flux Tower Net Primary Productivity (NPP) Quadrat Study at the Sevilleta National Wildlife Refuge, New Mexico

   https://doi.org/10.6073/pasta/3e38dae78feeb17cbe5b8666ace0f1ab  

   Baur, Lauren ; Collins, Scott ; Muldavin, Esteban ; Rudgers, Jennifer A ; Pockman, William T. ( January 2021 , Environmental Data Initiative)

   The varied topography and large elevation gradients that characterize the arid and semi-arid Southwest create a wide range of climatic conditions - and associated biomes - within relatively short distances. This creates an ideal experimental system in which to study the effects of climate on ecosystems. Such studies are critical given that the Southwestern U.S. has already experienced changes in climate that have altered precipitation patterns (Mote et al. 2005), and stands to experience dramatic climate change in the coming decades (Seager et al. 2007; Ting et al. 2007). Climate models currently predict an imminent transition to a warmer, more arid climate in the Southwest (Seager et al. 2007; Ting et al. 2007). Thus, high elevation ecosystems, which currently experience relatively cool and mesic climates, will likely resemble their lower elevation counterparts, which experience a hotter and drier climate. In order to predict regional changes in carbon storage, hydrologic partitioning and water resources in response to these potential shifts, it is critical to understand how both temperature and soil moisture affect processes such as evaportranspiration (ET), total carbon uptake through gross primary production (GPP), ecosystem respiration (Reco), and net ecosystem exchange of carbon, water and energy across elevational gradients. We are using a sequence of six widespread biomes along an elevational gradient in New Mexico -- ranging from hot, arid ecosystems at low elevations to cool, mesic ecosystems at high elevation to test specific hypotheses related to how climatic controls over ecosystem processes change across this gradient. We have an eddy covariance tower and associated meteorological instruments in each biome which we are using to directly measure the exchange of carbon, water and energy between the ecosystem and the atmosphere. This gradient offers us a unique opportunity to test the interactive effects of temperature and soil moisture on ecosystem processes, as temperature decreases and soil moisture increases markedly along the gradient and varies through time within sites. This dataset examines how different stages of burn affects above-ground biomass production (ANPP) in a mixed desert-grassland. Net primary production is a fundamental ecological variable that quantifies rates of carbon consumption and fixation. Estimates of NPP are important in understanding energy flow at a community level as well as spatial and temporal responses to a range of ecological processes. Above-ground net primary production is the change in plant biomass, represented by stems, flowers, fruit and foliage, over time and incorporates growth as well as loss to death and decomposition. To measure this change the vegetation variables in this dataset, including species composition and the cover and height of individuals, are sampled twice yearly (spring and fall) at permanent 1m x 1m plots. The data from these plots is used to build regressions correlating biomass and volume via weights of select harvested species obtained in SEV157, "Net Primary Productivity (NPP) Weight Data." This biomass data is included in SEV292, "Flux Tower Seasonal Biomass and Seasonal and Annual NPP Data." 

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4. Asymmetric responses of primary productivity to precipitation extremes: A synthesis of grassland precipitation manipulation experiments

   https://doi.org/10.1111/gcb.13706  

   Wilcox, Kevin R. ; Shi, Zheng ; Gherardi, Laureano A. ; Lemoine, Nathan P. ; Koerner, Sally E. ; Hoover, David L. ; Bork, Edward ; Byrne, Kerry M. ; Cahill, Jr., James ; Collins, Scott L. ; et al ( May 2017 , Global Change Biology)

   Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes inNPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations betweenNPPand precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta‐analytical techniques to search for generalities and asymmetries of abovegroundNPP(ANPP) and belowgroundNPP(BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) ofBNPPwas similar under precipitation additions and reductions, butANPPwas more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven byANPPresponses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider howANPPandBNPPresponses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.

    

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5. Semiarid ecosystem sensitivity to precipitation extremes: weak evidence for vegetation constraints

   https://doi.org/10.1002/ecy.2572  

   Felton, Andrew J. ; Zavislan‐Pullaro, Sam ; Smith, Melinda D. ( January 2019 , Ecology)

   In semiarid regions, vegetation constraints on plant growth responses to precipitation (PPT) are hypothesized to place an upper limit on net primary productivity (NPP), leading to predictions of future shifts from currently defined linear to saturatingNPP–PPTrelationships as increases in both dry and wetPPTextremes occur. We experimentally tested this prediction by imposing a replicated gradient of growing seasonPPT(GSP,n = 11 levels,n = 4 replicates), ranging from the driest to wettest conditions in the 75‐yr climate record, within a semiarid grassland. We focused on responses of two key ecosystem processes: abovegroundNPP(ANPP) and soil respiration (R_s).ANPPandR_sboth exhibited greater relative responses to wet vs. dryGSPextremes, with a linear relationship consistently best explaining the response of both processes toGSP. However, this responsiveness toGSPpeaked at moderate levels of extremity for both processes, and declined at the most extremeGSPlevels, suggesting that greater sensitivity ofANPPandR_sto wet vs. dry conditions may diminish under increased magnitudes ofGSPextremes. Underlying these responses was rapid plant compositional change driven by increased forb production and cover asGSPtransitioned to extreme wet conditions. This compositional shift increased the magnitude ofANPPresponses to wetGSPextremes, as well as the slope and variability explained in theANPP–GSPrelationship. Our findings suggest that rapid plant compositional change may act as a mediator of semiarid ecosystem responses to predicted changes inGSPextremes.

    

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