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  1. Abstract

    The plant-specific family of WUSCHEL (WUS)-related homeobox (WOX) transcription factors is key regulators of embryogenesis, meristem maintenance, and lateral organ development in flowering plants. The modern/WUS clade transcriptional repressor STENOFOLIA/LAMINA1(LAM1), and the intermediate/WOX9 clade transcriptional activator MtWOX9/NsWOX9 antagonistically regulate leaf blade expansion, but the molecular mechanism is unknown. Using transcriptome profiling and biochemical methods, we determined that NsCKX3 is the common target of LAM1 and NsWOX9 in Nicotiana sylvestris. LAM1 and NsWOX9 directly recognize and bind to the same cis-elements in the NsCKX3 promoter to repress and activate its expression, respectively, thus controlling the levels of active cytokinins in vivo. Disruption of NsCKX3 in the lam1 background yielded a phenotype similar to the knockdown of NsWOX9 in lam1, while overexpressing NsCKX3 resulted in narrower and shorter lam1 leaf blades reminiscent of NsWOX9 overexpression in the lam1 mutant. Moreover, we established that LAM1 physically interacts with NsWOX9, and this interaction is required to regulate NsCKX3 transcription. Taken together, our results indicate that repressor and activator WOX members oppositely regulate a common downstream target to function in leaf blade outgrowth, offering a novel insight into the role of local cytokinins in balancing cell proliferation and differentiation during lateral organ development.

  2. Abstract. Frozen soil processes are of great importance incontrolling surface water and energy balances during the cold season and incold regions. Over recent decades, considerable frozen soil degradation andsurface soil warming have been reported over the Tibetan Plateau and NorthChina, but most land surface models have difficulty in capturing thefreeze–thaw cycle, and few validations focus on the effects of frozen soil processes on soil thermal characteristics in these regions. This paperaddresses these issues by introducing a physically more realistic andcomputationally more stable and efficient frozen soil module (FSM) into aland surface model – the third-generation Simplified Simple Biosphere Model (SSiB3-FSM). To overcome the difficulties in achieving stable numericalsolutions for frozen soil, a new semi-implicit scheme and a physics-basedfreezing–thawing scheme were applied to solve the governing equations. The performance of this model as well as the effects of frozen soil process onthe soil temperature profile and soil thermal characteristics were investigated over the Tibetan Plateau and North China using observationsites from the China Meteorological Administration and models from 1981 to 2005. Results show that the SSiB3 model with the FSM produces a more realistic soiltemperature profile and its seasonal variation than that without FSM duringthe freezing and thawing periods. The freezingmore »process in soil delays thewinter cooling, while the thawing process delays the summer warming. Thetime lag and amplitude damping of temperature become more pronounced withincreasing depth. These processes are well simulated in SSiB3-FSM. Thefreeze–thaw processes could increase the simulated phase lag days and land memory at different soil depths as well as the soil memory change with the soil thickness. Furthermore, compared with observations, SSiB3-FSM producesa realistic change in maximum frozen soil depth at decadal scales. This study shows that the soil thermal characteristics at seasonal to decadal scalesover frozen ground can be greatly improved in SSiB3-FSM, and SSiB3-FSM can be used as an effective model for TP and NC simulation during cold season. Overall, this study could help understand the vertical soil thermalcharacteristics over the frozen ground and provide an important scientificbasis for land–atmosphere interactions.« less
  3. Abstract Subseasonal-to-seasonal (S2S) precipitation prediction in boreal spring and summer months, which contains a significant number of high-signal events, is scientifically challenging and prediction skill has remained poor for years. Tibetan Plateau (TP) spring observed surface ­temperatures show a lag correlation with summer precipitation in several remote regions, but current global land–atmosphere coupled models are unable to represent this behavior due to significant errors in producing observed TP surface temperatures. To address these issues, the Global Energy and Water Exchanges (GEWEX) program launched the “Impact of Initialized Land Temperature and Snowpack on Subseasonal-to-Seasonal Prediction” (LS4P) initiative as a community effort to test the impact of land temperature in high-mountain regions on S2S prediction by climate models: more than 40 institutions worldwide are participating in this project. After using an innovative new land state initialization approach based on observed surface 2-m temperature over the TP in the LS4P experiment, results from a multimodel ensemble provide evidence for a causal relationship in the observed association between the Plateau spring land temperature and summer precipitation over several regions across the world through teleconnections. The influence is underscored by an out-of-phase oscillation between the TP and Rocky Mountain surface temperatures. This study reveals formore »the first time that high-mountain land temperature could be a substantial source of S2S precipitation predictability, and its effect is probably as large as ocean surface temperature over global “hotspot” regions identified here; the ensemble means in some “hotspots” produce more than 40% of the observed anomalies. This LS4P approach should stimulate more follow-on explorations.« less
    Free, publicly-accessible full text available December 1, 2023
  4. Abstract. Fire causes abrupt changes in vegetation properties and modifies fluxexchanges between land and atmosphere at subseasonal to seasonal scales. Yetthese short-term fire effects on vegetation dynamics and surface energybalance have not been comprehensively investigated in the fire-coupledvegetation model. This study applies the SSiB4/TRIFFID-Fire (the SimplifiedSimple Biosphere Model coupled with the Top-down Representation of InteractiveFoliage and Flora Including Dynamics with fire) model to studythe short-term fire impact in southern Africa. Specifically, we aim toquantify how large impacts fire exerts on surface energy throughdisturbances on vegetation dynamics, how fire effects evolve during the fireseason and the subsequent rainy season, and how surface-darkening effectsplay a role besides the vegetation change effects. We find fire causes an annual average reduction in grass cover by 4 %–8 %for widespread areas between 5–20∘ S and a tree cover reductionby 1 % at the southern periphery of tropical rainforests. The regionalfire effects accumulate during June–October and peak in November, thebeginning of the rainy season. After the fire season ends, the grass coverquickly returns to unburned conditions, while the tree fraction hardlyrecovers in one rainy season. The vegetation removal by fire has reduced theleaf area index (LAI) and gross primary productivity (GPP) by 3 %–5 % and5 %–7 % annually. The exposure of bare soilmore »enhances surface albedo andtherefore decreases the absorption of shortwave radiation. Annual meansensible heat has dropped by 1.4 W m−2, while the latent heat reductionis small (0.1 W m−2) due to the compensating effects between canopytranspiration and soil evaporation. Surface temperature is increased by asmuch as 0.33 K due to the decrease of sensible heat fluxes, and the warmingwould be enhanced when the surface-darkening effect is incorporated. Ourresults suggest that fire effects in grass-dominant areas diminish within1 year due to the high resilience of grasses after fire. Yet fire effectsin the periphery of tropical forests are irreversible within one growingseason and can cause large-scale deforestation if accumulated for hundredsof years.« less
  5. Abstract Land-use and land-cover change (LULCC) is one of the most important forcings affecting climate in the past century. This study evaluates the global and regional LULCC impacts in 1950–2015 by employing an annually updated LULCC map in a coupled land–atmosphere–ocean model. The difference between LULCC and control experiments shows an overall land surface temperature (LST) increase by 0.48 K in the LULCC regions and a widespread LST decrease by 0.18 K outside the LULCC regions. A decomposed temperature metric (DTM) is applied to quantify the relative contribution of surface processes to temperature changes. Furthermore, while precipitation in the LULCC areas is reduced in agreement with declined evaporation, LULCC causes a southward displacement of the intertropical convergence zone (ITCZ) with a narrowing by 0.5°, leading to a tripole anomalous precipitation pattern over the warm pool. The DTM shows that the temperature response in LULCC regions results from the competing effect between increased albedo (cooling) and reduced evaporation (warming). The reduced evaporation indicates less atmospheric latent heat release in convective processes and thus a drier and cooler troposphere, resulting in a reduction in surface cooling outside the LULCC regions. The southward shift of the ITCZ implies a northward cross-equatorial energy transportmore »anomaly in response to reduced latent/sensible heat of the atmosphere in the Northern Hemisphere, where LULCC is more intensive. Tropospheric cooling results in the equatorward shift of the upper-tropospheric westerly jet in both hemispheres, which, in turn, leads to an equatorward narrowing of the Hadley circulation and ITCZ.« less
  6. Abstract. Fire is one of the primary disturbances to the distribution and ecologicalproperties of the world's major biomes and can influence the surface fluxesand climate through vegetation–climate interactions. This study incorporatesa fire model of intermediate complexity to a biophysical model with dynamicvegetation, SSiB4/TRIFFID (The Simplified Simple Biosphere Model coupledwith the Top-down Representation of Interactive Foliage and Flora IncludingDynamics Model). This new model, SSiB4/TRIFFID-Fire, updating fire impact onthe terrestrial carbon cycle every 10 d, is then used to simulate theburned area during 1948–2014. The simulated global burned area in 2000–2014is 471.9 Mha yr−1, close to the estimate of 478.1 Mha yr−1 inGlobal Fire Emission Database v4s (GFED4s), with a spatial correlation of0.8. The SSiB4/TRIFFID-Fire reproduces temporal variations of the burnedarea at monthly to interannual scales. Specifically, it captures theobserved decline trend in northern African savanna fire and accuratelysimulates the fire seasonality in most major fire regions. The simulatedfire carbon emission is 2.19 Pg yr−1, slightly higher than the GFED4s(2.07 Pg yr−1). The SSiB4/TRIFFID-Fire is applied to assess the long-term fire impact onecosystem characteristics and surface energy budget by comparing model runswith and without fire (FIRE-ON minus FIRE-OFF). The FIRE-ON simulationreduces tree cover over 4.5 % of the global land surface, accompanied bya decrease in leaf area index and vegetation height bymore »0.10 m2 m−2and 1.24 m, respectively. The surface albedo and sensible heat are reducedthroughout the year, while latent heat flux decreases in the fire season butincreases in the rainy season. Fire results in an increase in surfacetemperature over most fire regions.« less
  7. It has been previously reported that a gaseous bubble trapped in a one-end-open tube oscillates in the presence of acoustic wave and generates strong microstreaming flows and thus a propulsion force. The propulsion highly depends on the frequency and the voltage of the external acoustic wave. This paper presents a new discovery that the direction of this propulsion is dependent on the relative location of the bubble interface. The oscillating bubble propels forward when its interface stays deep inside the tube. On the contrary, the bubble propels in a reverse direction when its interface is at the exit of the tube. Learning from this phenomenon, we developed and introduced physical structures (necks) to precisely control the location of the bubble interface. As a result, the length and interface position of the bubble is more controllable, and the bubble oscillation and propulsion becomes more predictable and consistent.