Early Triassic super-greenhouse climate driven by vegetation collapse

Domain: earth_science
Published: 2025-07-02
Editorial outcome: minor_revision

Abstract
The Permian–Triassic Mass Extinction (PTME), the most severe crisis of the Phanerozoic, has been attributed to intense global warming triggered by Siberian Traps volcanism. However, it remains unclear why super-greenhouse conditions persisted for around five million years after the volcanic episode, with one possibility being that the slow recovery of plants limited carbon sequestration. Here we use fossil occurrences and lithological indicators of climate to reconstruct spatio-temporal maps of plant productivity changes through the PTME and employ climate-biogeochemical modelling to investigate the Early Triassic super-greenhouse. Our reconstructions show that terrestrial vegetation loss during the PTME, especially in tropical regions, resulted in an Earth system with low levels of organic carbon sequestration and restricted chemical weathering, resulting in prolonged high CO2 levels. These results support the idea that thresholds exist in the climate-carbon system whereby warming can be amplified by vegetation collapse.

Introduction
The latest Permian to Early Triassic (~252–247 million years ago; Ma) was a period of intense environmental and biotic stress1,2. During the Permian–Triassic Mass Extinction (PTME) at ~252 Ma, around 81–94% of marine invertebrate species and 89% of terrestrial tetrapod genera became extinct2. It is generally agreed that the PTME was driven by volcanogenic carbon emissions from Siberian Traps volcanism, potentially coupled with additional thermogenic releases, resulting in intense greenhouse warming3–10. A major negative excursion in carbonate δ13C ratios, over a time interval of about 50–500 thousand years (kyrs), supports the notion of a major carbon cycle perturbation4–6. However, it is not well understood why the extreme hothouse climate persisted throughout the 5 million years (Myrs) of the Early Triassic. The precise time interval of Siberian Traps degassing is uncertain, although the main phase of volcanism occurred around the Permian-Triassic Boundary (PTB), possibly with a further pulse about two million years later, during the Smithian Substage of the Early Triassic8. Nevertheless, it would normally be expected that atmospheric CO2 and global surface temperature should have declined to pre-volcanism levels within ~100 kyr of the volcanic pulses, due to amplified global silicate weathering and/or increased burial of organic carbon2.

The unusual multimillion-year persistence of super-greenhouse conditions has sparked considerable debate, and it has been suggested that it may be linked to a change in the silicate weathering feedback, such that CO2 could not be efficiently removed from the surface system11. This could potentially have been due to reduced availability of weatherable material from erosion11, which would limit global silicate weathering rates12,13. Alternatively, continental weathering may have been rapid and accompanied by high rates of reverse weathering in a silica-rich ocean, removing silicate mineral-derived cations into clays instead of forming carbonate minerals, and thus limiting overall CO2 drawdown14,15. These are intriguing hypotheses, but it remains unclear why a severe reduction in global erosion, and/or an episode of high ocean silica levels, would necessarily persist for ~5 Myrs and then recover during the Middle Triassic. Although uncertain, existing compilations of sedimentation rates16,17 and the maintenance of sporadic siliceous rock records across and after the PTME18 (see Supplementary Fig. S1) are not clearly supportive of these timings, and suggest that while these processes likely contributed to climate regulation, our understanding of the timeframe of super-greenhouse conditions remains incomplete.

Here, we explore a further mechanism for elevated Early Triassic temperatures that is closely tied to the timeframe of extreme warmth. This approach is based on the concept of an ‘upper temperature steady state’, in which a change in the Earth system caused the climate-carbon cycle to stabilize at a much higher global temperature for millions of years19. Specifically, we investigate the hypothesis that the key driver of the transition to a super greenhouse Earth was the dramatic and prolonged reduction of low-latitude terrestrial biomass caused by the PTME20,21 and its delayed recovery22. Tropical peat-forming ecosystems are responsible for substantial drawdown of CO2, but these extensive biomes were lost at the end of the Permian20,23–25. Indeed, plant species richness and abundance dropped significantly during the Permian–Triassic transition, which is the only genuine mass extinction level event of land plants through the whole Phanzerzoic26, leaving a multimillion year “coalgap” in the Early to Middle Triassic where terrestrial plant materials did not build up as peat20,23,25. To test this hypothesis, we quantify the distribution of terrestrial plant productivity across the PTME and Early-to-Middle Triassic from the plant fossil record and use this information to guide a linked climate-biogeochemical model of the Early Triassic hothouse, testing whether these biotic changes may have resulted in a higher temperature steady state.

Results
Figure 4 shows the SCION model results through the latest Permian, and the Early and Middle Triassic, both with and without the inclusion of our palaeo-vegetation constraints. In the control run (dashed black line), the biogeographic NPP of each continental grid cell is kept constant at 420 g C/m2/yr to produce an overall productivity similar to our late Permian fossil-constrained vegetation map, and all changes in the model environment are driven by abiotic forcings, such as background tectonic degassing rates and Siberian Traps degassing. The major features of this default run are the spikes in CO2 concentration and temperature (Figs. 4E, 4G), and the accompanying δ13C excursion (Fig. 4D), driven by Siberian Traps degassing. The magnitude of the isotope excursion is consistent with the geological record and previous modeling5,60, and CO2 concentration rises from about 1,500 to 3,000 ppm, with a corresponding increase in equatorial surface temperature of about 2°C. The high background CO2 level and relatively small temperature increase are both features of the low climate sensitivity in the FOAM climate model61, which provides the steady state 3D climate emulator for SCION. Thus, we expect that a more complex model might allow for a more dramatic temperature increase and lower overall CO2 levels, as suggested by some proxy data7,10. However, no amount of climate model complexity can account for the data-model mismatch during the Early Triassic, where model temperatures decline immediately after the cessation of Siberian Traps emissions. Because SCION has a single-box ocean, it does not balance sub-million-year alkalinity and shallow sea carbonate deposition as accurately as multi-box models, in which CO2 levels decline even more rapidly60.

When included in the model, the loss of vegetation productivity from the end Permian through the Early Triassic, and the related effects on continental weathering, result in a sustained high atmospheric CO2 content7 and high Early Triassic temperatures10 (green line in Fig. 4C). In these model runs, the reduction in terrestrial organic carbon burial and nullification of silicate weathering result in CO2 levels stabilizing at around 7000 ppm, with maximum equatorial surface temperatures of up to 33–34°C over a ~5 Myr period, which is consistent with proxy inferences (Fig. 4)7,10. As before, low climate sensitivity in our model results in predictions of high CO2 concentrations, although the magnitude of predicted increase (~4 fold) is broadly equivalent to that suggested by proxies7. The modeled Early Triassic δ13C level (green line in Fig. 4D) is also around 2–3‰ lower than the control run (black dashed line in Fig. 4D), generally improving the fit to the geological record10 (blue solid line with dots in Fig. 4D). Two exceptions to the data-model fit are the Induan–Olenekian (Dienerian–Smithian) boundary and the late Olenekian (early Spathian), which are marked by transient positive carbon isotope excursions that may have been driven by increasing marine productivity, transgression, or marine anoxia8,10, none of which are considered in our stage level study. Model strontium isotope ratios, which are influenced by continental weathering fluxes and source lithologies, are not greatly affected by the inclusion of vegetation collapse, but show a slightly greater rise between the Changhsingian and Anisian as weathering migrates to higher latitudes and away from low latitude suture zones with low 87Sr/86Sr values62 (Fig. 4C, see Supplementary Fig. S4 for lithological map, and Methods 11). Our SCION model demonstrates how the prolonged hothouse environment could have been terminated by progressive terrestrial ecosystem recovery, starting in the Olenekian but accelerating in the Anisian, which is also consistent with the observed uptick in δ13C values across the Olenekian–Anisian boundary and the cooling which occurred during this time10. This dynamic fits with broader evidence for a more benign environment during the re-establishment of diverse ecosystems in Middle Triassic terrestrial settings20,35. A ‘CO2 fertilization’ effect occurs in the model in the early stages of warming, before atmospheric CO2 and land surface temperatures reach saturation point for C3 plant photosynthesis. However, this increased local productivity in some areas was insufficient to offset the global decline in biomass abundance.

Methods
Agedatingofplantrecords
ThegeologicaltimingsusedinthispaperarefromtheGeologicalTimeScale(GTS)2020.Selectedstudyareas(andsites)aretheKuznetskBasininSiberia,Junggar Basin (Dalongkou section) in Xinjiang, NW China,easternYunnanandwesternGuizhouinSWChina,UtahinUSA,Germanic Basin in western Europe, small Tethyan continents, now incorporated in southeastern Asia, Turkey in the Dead Sea area, Kashmir in NW India, Karoo Basin in South Africa, Sydney Basin in Australia, Prince Charles Mountains in Antarctica, and Argentina, coveringthepublishedplantfossilbearingareasfromvariouslatitudes(Fig.S2).Wereviewedthepublishedchronostratigraphiccorrelations between the floral records, other environmental events, and the lithologicalPermian-TriassicBoundaryineachareatodeterminetheglobal pattern of plant evolution. Chronostratigraphy of the non-marinestrataiscorrelatedbyfossilassemblagesincludinganimalsandplants,detrital zircon ages, and geologicalevents recorded by geochemicalproxies.ThedetailedrecordsandanalysisofeachlocationareprovidedintheSupplementaryInformationtext1.
From the evidence noted in the SI part 1, it is clear that end-PermianterrestrialcrisishappenedinthelateChanghsingianoveraninterval starting 750 kyrs before the PTB up to shortly after the boundary20,64–67.Inhighlatitudes,themacrofossilrecordsshowCordaitesinSiberiaandtheGlossopterisflorainGondwanadisappearedinthemidtolateChanghsingian68,69.Anabundantfloraofferns,seedfernpeltasperms,cycadophytesandconiferssurvivedthroughthePTBin thehighlatitudes20,68,69.Inouranalysis,plantfossiloccurrenceswerenotedatthestagelevelgivingtheimpressionthatalltheplantchangeswerenearthePTB.

Plantmacrofossilandpalynologydatanormalizationsteps
Plant macrofossils are typically fragmented into different parts (organs)priortofossilization,witheachpartoftennamedseparatelyusingLinneanbinomials29,70.Wenormalizedthedatasettocorrectforduplicationsinwhichdifferentpartsofthesameplantareincludedunderdifferentspeciesorgenusnames,andensuredthesametaxawithdifferentmorphologicalnamescouldbelinked.Innormalization,organssuchasspeciesorgeneraofseeds,trunks,rootsandleavesareremovedfromthedatasetifanotherorganfromthatplantgroupismorelikelytoreflectthewholeplanttaxonomy,sothateachwholeplantiscountedonlyonce29,70.Anexampleisthediversetrunkgroupoftreelycopods,wherespeciesofLepidodendronareusedastheyareabundantandsystematicallyinformative29,70,ratherthanotherorgansproducedbythesameplant,includingcones,sporophylls (fertile leaves)or roots (see ref. 20 for detail).For diverseleafgroups, for example,fernsandsphenophytes,leafspeciesorgeneraareused,asthesefossilstypicallylackmoredistinctiveorganswithsuitablepreservation.Indeterminatespeciesdenotedas“sp.”ofanexistinggenusareregardedaslikelytobepoorlypreservedexamplesoftheexistingspeciesofthatgenus,and are deleted. If the indeterminate species denotedas“sp.”istheonlyspeciesinthatgenus,theyarecountedasa single,unnamedspecies.NormalizedplantmacrofossilspeciesdataislistedinTableS4.Inadditiontothese,palynologicaloccurrencesarealsoconsidered.Mostofthepalynologicaldataarelinkedwithplantmacrofossilsatfamilylevel,withafewsporeandpollentaxapreservedin-situwithintheirparentplantforwhichthegenusandspeciesnamesoftheparentplantsareused.

Plantmacrofossilspeciesextinctionmagnitude
All the species occurrences presented arebased on the normalized data (Table S1). Longitude and latitude for each fossil location are listedinTableS2andS3.Thehighlatitudeareaisdefinedtobe>45 degreesnorthandsouthoftheequator,whilelow-middlelatitudearea is<45degreesnorthorsouth.Thisdefinitionisforthisstudyonlyand is not climate specific. The range of plant fossils in each stage was checkedandextendedforcalculatingtheextinctionmagnitudeovera global high-latitude and low–middle latitude area. The extinction magnitudeforeachstageistheextinctspeciesnumbercomparedtoa laterstage,minusthetotalnormalizedspeciesnumberofthisstage20. See the extinction magnitude results in Table S4. The extinction magnitudeoftheAnisianisnotestimated.

Plantlatitudinaldiversitycalculationandinfluenceofsamplingdensity
Toinvestigatetheinfluenceofplantfossilsamplingcompletenesson our estimates of diversity, squares and interpolation methods were appliedtoournormalizedplantmacrofossiloccurrencedata.Asfor therawdata,squaresandinterpolationwereappliedto15°latitudebinsfortheLatePermian(Changhsingian)toMiddleTriassic(Anisian). Coverage-basedinterpolationusestheabundancestructurepresentwithinsamples,toeithersubsampleorextrapolatediversityestimatestoparticularlevelsofsamplingcompleteness,knownasquorumlevels36,71,72.ThiswasappliedusingtheRpackageiNEXT36.Squaresisanextrapolatorbasedontheproportionofsingletonsinasampleandisthoughttobemorerobusttobiasesassociatedwithsmallsamplesizesandunevenabundancedistributions73,74.
Throughout the interval, the raw, squares and interpolated diversity estimates generally show similar latitudinal patterns, suggestingthatsamplingisnotastronginfluenceonourinferredlatitudinaldiversitygradients(Fig.2).However,manyofthepointsintheinterpolatedcurveswereremoved duetoover-extrapolation,whichindicates that many of the spatio-temporal bins may be undersampled. Our results indicate that during the Induan, the highest plant diversity was found in the high latitudes, particularly in the northernhemisphere.However,duringtheChanghsingian,Olenekian andAnisian,weseehigherdiversitylevelsattropicallatitudes,suggestingthatthelatitudinaldiversitygradienthadrevertedtoasituation similartothatofthepresentdayonetotwomillionyearsafterthePTME.

Plantfunctionaltraitevaluations
Plant functional traits are stable morphological, anatomical, and compositional characters that have evolved under specific climates andenvironments,linkingplantphysiologicalprocessestotheEarth’sbiogeochemistry and physical evolution31,32,75,76. The plant functional traitsoflatePermiantoMiddleTriassicfossilplantsarenotwellstudied.Here,weaimedtodeterminethehabitatofthefossilplants,theclimaticzoneinwhichtheylivedandtosemi-quantifythebiomassof the flora. We selected plant traits including plant growth form, reconstructedplantheight,whichindicatesthespatialstructureoftheflora,andleafsize,whichindicatesthepotentialbiomassoftheflora.Forwater,carbonandnutrientcyclingintheplant,leafshape,veinpattern and density, and cuticle thickness are considered, which determinestheplant’smoisturepreference,droughtresistance,andproductivity31,32. Cuticle thickness of present Ginkgo is positively related to productivity,although this is not further explored in our dataset due to the lack of experiments on these Permian-Triassic plant’srecentanalogs31.
LeafsizeandveindensityweremeasuredusingImageJ,andonly thelargestandmostcompleteleavesofeachtaxonarelistedinthe tableS5,withfossilplantdatacollectedfromreferencesinthesupplementarytablereferences.Othertraits,includingplantform,wholeplantheight,veintype,andcuticlethickness,aresemi-quantifiedandarebasedpartlyonthereconstructedfossilplant,includingLepidodendron, Triassic herbaceous lycopods, sphenophytes including Calamites,gigantopterids,glossopterids,andginkgophytes.Features ofplantsfromwhichreconstructionsareunknownareinferredfromreconstructed relatives in the same genus or family; these plants shouldbefurtherinvestigatedinthefuturetocharacterizethemmoreaccurately.
Fossils with measurable leaf size are listed in Table S5 which coverslowtohighlatitudes.Interpretationofthefloralclimatezone andvegetationlandscapeinformationarebasedonallthemicroandmacroplantfossils,includingthosewithoutmeasurableleafsize.The general concept is that plants with higher height, larger leaf size, higherveindensity,andmorecomplicatedveinsystemareofhigher biomassandreliedongreaterhumidityfortranspiration.Floraswitha higher proportion of these plants normally have higher species diversity and spatial structure complexity, which suggests higher productivity. Details of the plant trait relationships are outlined byref.31.

Floracharacterizationbyclusteringandmorphologicalgroup
ToanalysethecharacterofflorasfromthelatePermian(Changhsingian) to the Middle Triassic (Anisian) for comparison and matching betweenfloraswithcertainfunctionalanalysisandmissinginformation, family level clustering was used to group the floras with the normalized plant fossil data. The clustering result is based on the Euclideanmethod.Theplantsystematicinformationcomesfromthe listedliterature,withadditionsfromtheGlobalBiodiversityInformation Facility (GBIF) https://www.gbif.org/ database which were checkedagainsttheliteraturetoensuretheiraccuracy.Thetaxonomic affinityofmostsporeandpollentaxaareunknown,andsoonlyplantmacrofossildatawasusedinclustering,andthepalynologydatawasonlyusedinthemorphologicalgroupdiversityanalysis.Toshowtheuncertaintyoftheclusteringresults,welisttheplantspeciesnumberaftereachflorainFig.1B.Unsurprisingly,theclusteringresultsforflorawithfewertaxawerelessreliableandmorecrowdedtogether.Asanauxiliarymethodtoclustering,wecountedtheplantspeciesnumberineachmorphologicalgroup(see-thefourteenmorphologicalgroupclassificationsbelow),thencalculatedtheproportionofthespeciesnumberineachmorphologicalgroupwithinflorastodirectlyshowthecharacterandtoconstructarepresentativepiechartforeachflora.For floraswithfewertaxa,we adjustedthelocationofeachfloraintheclustering tree manually, according to the character shown by the morphologicalgroupdiversity.
Plants were divided into six habitats and fourteen groups, includingfourariduplandtypes:conifer,gymnosperm(forseedplantswheresystematicclass/groupisuncertain),peltaspermandseedfern; three humid upland types:cordaitalean, ginkgophyte, cycadophyte; onerainforesttype:gigantopterid;twohumidtypes:fernand‘fern2’ (for taxa thatcould be either ferns or seed fern); two marshtypes: sphenophyte and lycopod; one cold type: glossopterid – normally reportedinborealGondwana;andonearidlowlandtype:herbaceous lycopod.Thisclassificationisonlyfortheplantsincludedinthisstudy andmustbecarefullyappliedtoothertimeintervalsbycheckingthe habitatofthefossilplantsindetail.Floradominatedbyonehabitat groupwas classified intothe correspondingclimatezone,andflora withmorethanonehabitatgroupwasdefinedasamixture.Inthisstep, wealsotookpalynologydataintoaccount.Thegroupinformationof thein-situsporeandpollenproducingplantwerecounted77.Forflora withbothplantmacrofossilandpalynologydata,wechosethedataset whichcontainsmoreinformation.InFig.1B,florawithmorethan150 taxa,suchastheChanghsingianSouthChinaflora,havethebiggestpie chartarea,whileflorawithlessthan10taxa,liketheGermanflora,havethe smallest pie chart. After the plant function traits, habitat, and clusteringanalysis,thecharacteroftheflorafromtheEndPermianto theMiddleTriassicwassystematicallystudiedandclassifiedintoclimatezonesasshowninTableS5andS6.

Palaeogeographicreconstruction
Toreconstructthespatialvegetationmap,weassembledadatabaseof fossillocations,plantmacrofossil,palynology,andterrestrialtetrapoddataforourtimeperiods(TableS1–S3).Toaccountforplantrefugesor Mesozoic gymnosperm cradles that may not be represented in the fossildatabase,wefirstextendedtheplantmegafossilrangeineachbasin.Forexample,voltzialesandpeltaspermswerefoundinNorth ChinaandEuramericabeforethePTMEandreappearedintheMiddleTriassicbutwereabsentinEarlyTriassicstrata,soweextendedtheranges ofthosesurviving gymnosperms78–80. Plants thatwere dominantaftertheEarlyTriassicbuthadalreadyappearedinendPermian stratainArgentina,IndiaandNortheastChinahaveallhadtheirrangesextendedthroughtheEarlyTriassic69,81–83.Secondly,thepalynologicaldata tends to better record informationon upland floras,while the plantmegafossildatarecordsprimarilylowlandtaxa20,30.Wedetected hidden upland floras in South China, China Xinjiang, South Africa, Antarctica,andAustraliabasedongymnospermpollenevidenceafter thePTME21,84–91.Ouranalysisincludedalltheplantdatafrommacro andmicrofloralrecordsinallsedimentaryfacies,toavoidusingthe localinformationandtorepresentinformationfromthewholebasin. Thirdly,terrestrialtetrapoddatawasusedtoinfertheoccurrenceof plantsinregionswithoutaplantfossilrecord24,36.Generally,terrestrial tetrapodoccurrencesinourstudycoincidedwithoccurrencesinthe plantfossils,exceptfortheOlenekianrecordinAmericaandCanada. Therefore, vegetation type in those areas at this time was partly inferredfromthetetrapodinformationaloneonthepresumptionthat plantprimaryproducerswerenecessaryintheseregionstosupport vertebratecommunities.
The fossil locations were then reconstructed to their time of deposition using GPlates92. Because the older palaeogeographic reconstructionusedinSCION93hasnoavailablesetofrotationfiles,we used the reconstruction files of ref. 62, whose reconstruction at ~250Maisverysimilartothatofref.93Thisallowedustoplacefossil locationsinaninternallycorrectpositionat250Ma.However,minor manualmanipulationwasneededtothenmapsomeoftheselocations totheircorrectcorrespondingpositionsintheSCIONland-seamaps.

Vegetationproductivityreconstruction
Globalvegetationwasreconstructedbyextensionoffossilfloradata acrossappropriateclimatezonesindicatedbyasedimentaryclimatemap94.Inaridareas,plantfossilextrapolationisnotapplied95.Extrapolation was not carried out at the boundaries of humid and arid environmentsinplacesthatlackedsupportingmineralogicaldata.For example, the Early and Middle Triassic low-latitude inner Pangea continentisinferredtohavebeenaridsavannaorsteppe,basedonthe availablefossilrecordandlithologicalclimaticindicators.Fossilsfrom more productive biomes which are found nearer the coast are restricted to this setting and not extended far into the continental interiorwhereclimateisarid(Fig.3).
Threeprinciplesareusedforfunctionalcomparisonbetween ancient and recent floras to estimate palaeo-productivity: firstly, recentflorasmusthaveasimilarstructuretotheancientflorawe wish to imitate, so we compare the reconstructed size of the fossilplantsincludingheightandleafsizeoftheindividualplant and the spatial structure of the flora with the recent analog. For example, the end-Permian tropical South China floral canopy is dominated by tree lycopods Lepidodendron and sphenophytes, fern and gigantopterid understory, showing the highest complexity in forest spatial structure among all the floras from the late Permian to Middle Triassic. The dominant gigantopterids have giant leaves with typical rainforest drip-tip structure, complicated vein systems and high vein density, and thus recent rainforestischosenasananalogforthelatePermianSouthChina area,SoutheastAsia,ChinaXizang,andTurkeywhichsharedhigh similarityintaxacomposition33.Secondly,therecentandancient florasshouldbeinthesameclimatezone.Forexample,thelatest Permian South China tropical forest was a large, low-latitude island, and so present-day, large tropical islands like Indonesia and Thailand were chosen over (for example) continental Brazil. Each palaeo flora has suitable recent floras sharing similarity in plantfunction,climaticandgeographiczone.Thirdly,thechosen flora should fit in the global diversity and NPP gradient at a similar place to the ancient flora. For ancient floras with clear functionaltraitrecords,wecomparetheNPPbetweenflorasfrom thelatePermiantoMiddleTriassicbythetraitsmentionedabove. Wealsousepresent-daydatatoconfirmthehypothesisthatplant diversityhas a positive correlation with productivity, which generallyfitswithournormalized fossilresults96,97.Forinstance,the late Permian (Changhsingian) tropical South China flora is matched with a present day high-diversity and highest-productivity biome in the chosen recent non-continental tropical rainforest functional group range, that of present-day Thailand. The late PermiantoMiddleTriassicfossilplants’inferredhabitat,climate zoneandlandscapeareshowninTableS5.ThecalculatedNPPof each ancient flora is listed in Table S6 and details of correspondingrecentfloraareinTableS7.Thereconstructed NPPLis marked asNPPLfossil.
Althoughthisstudymakesprogressincomparingthephysiological and functional difference between palaeo- and recent plants, moredetailedstudiesarestillrequired.Forexample,theinfluenceof the interaction between plants and other organisms including mycorrhiza and insects are not considered, neither are soil texture differencesbetweenthePermian-Triassicandpresentday.Although physiologyinotherdeeptimeperiods,themethodsmentionedabove shouldnotbedirectlyappliedtoothertimeframes.

Taphonomicinfluenceonfossilpreservation
Inourinitialstepstoreconstructlandbiomesandproductivity,wedid notextendthedistributionoflandplantstoregionslackingclimaticor mineralrecordsofhospitableenvironments.Forinstance,thecentral areasofthePangeasupercontinent,characterizedbyevaporitesrather thancoaldeposits,wereidentifiedasdesertsorbarrenlandswhere plantgrowthwaspresumedunlikely.However,whiletheabsenceof fossilrecordsinsomeareasmayresultfromlocalbiomeextinctions,it couldalsobeduetopoorpreservationconditionscausedbyinsufficientwateravailability98.Toassesstheinfluenceoftaphonomicbiason ourreconstructions—essentially,whetherareaswithoutfossilrecords trulylackedlocalbiomesormerelylackedpreservationconditions— weemployedtwodifferentapproaches.
First of all, we assessed the sedimentary facies and strata thicknessofbasinsfromvariouslatitudesandlocations99(Fig.S3). For example, in low latitude South China, the fossil-plant-absence zoneoccursintheKayitouFormation,formedinashallowlakeor floodplain environment100. The central European basin recorded a warmseasonalhumidclimateintheEarlyTriassic,whichislikelya responsetoglobalwarmingafterthePTMEininnerPangea101.Inhigh latitude South Africa and Australia, sedimentation patterns and occurrences of green algae indicate widespread ponding environmentsthroughtheplant‘deadzone’85,102–104.Whilethesearestrictly localexamples,theyindicatethepresenceofwaterlogged,swampy environments suitable for plant preservation in the Early Triassic, suggestingthattaphonomyisnottheprimarycauseoftheabsence offossils,andthatthismayreflectagenuinereducedabundanceof plantsafterthePTMEintheseareas20.Additionally,frequentwildfires point to intensified seasonality or seasonal aridity, likely reducing lowland plant habitats and contributing to the sparse record of “hidden”uplandplants2,20,21,67,85.
Second,weexploredthepotentialexistenceofhidden“refuges” or“cradles”usingourclimatemodel,assumingplantscouldsurvivein gridcellswithlandsurfacetemperaturesbelow40°Candrunoffabove 0mm/yr, similar to the conditions required by most modern plants105,106.Theresultssuggestthatsuitableenvironmentsforplant survivalexistedinhigh-altitudeandcoastalareas,eveninsomelow-latituderegions,suchasSouthChina,NorthChina,Xinjiang,Europe, andCentralAsia.Thesefindingsareconsistentwithourreconstructionsbasedonfossilrecords.Forexample,theSouthChinaInduan plant macrofossil record is dominated by the herbaceous lycopod Tomiotrobus with a maximum height of 0.2 meters, while coeval palynologicaldatasuggestsahiddenuplandgymnospermflora20,21,107. Therefore,theInduanflorainSouthChinaiscomparedwithrecent Australian shrubs or the seasonal dry subtropical forest in China Yunnan Province which is herbaceous and shrub-dominated with sparsetreecover.

Climate-biogeochemicalmodeling
ToinvestigatetheeffectsofvegetationchangeonEarlyTriassicclimate, we ran the SCION Earth Evolution Model52. We removed the equationwhichcalculatesterrestrialvegetationbiomass(asasingle global number) and replaced this with values based on our reconstruction,mappedontothemodelcontinentalsurface.Tocalculate NPPL we multiplied the reconstructed NPPL by a factor biogeographic representingCO fertilization.ThisfollowstheMichaelis-MentonformulationusedintheGEOCARBbiogeochemicalmodels56,108:
Fert= (1) * RCO2^0.4 / (1+RCO2)
where RCO 2 is the atmospheric CO 2 concentration relative to the preindustrial280ppm.
Plantsplayacrucialroleinterrestrialweathering109.Present-day global-scale observations show that silicate weathering is linearly related to plant NPPL (Fig. S5)48,110,111. We altered the modelparameterf biota,whichrepresentsthebioticenhancement of continental weathering (again a single global average in SCION), to make this dependent on the local vegetation in the followingway:
f biota=0:002 * NPPL f+0:25
Functionally, this returns a value tending towards 0.25 when NPPL isverylow,andalinearscalingwithNPPL whenNPPL rises.The choiceof0.25relatestothefour-foldenhancementbetweensimple ground covers and higher plants used in first-generation long-term carbon cycle models like GEOCARB56,108, and based on field and laboratorystudies53.Wevarythis‘preplant’factorbetween0.15–1and modify the linear scaling to run sensitivity tests for various plant weatheringabilities(Fig.S6).Inallformulations,thescalingfactorfor NPPischosentoreturnpresentdayglobalweatheringratesforthe modelpresentdayintegration.

Strontiumisotope87Sr/86Srlithology
Thestrontiumisotopecompositionofriverwaterhasastronglocal lithologicalcontrol,sotosimulatetheSrisotoperecordweimposed basiclithologicalclassesonthemodelcontinentalsurfaceusingthe locations of continental arcs, LIPs and suture zones from the literature62,63,112 (Fig. S4). These zones were then prescribed representativeSrisotopicvalues52,57,113–115.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Early Triassic super-greenhouse climate driven by vegetation collapse\nZhen Xu et al\nXu and colleagues shed light on the unusually long persistence of “super” greenhouse conditions following the Permian-\nTriassic mass extinction. Previous hypotheses for why it appears CO2 could not be efficiently removed from the surface\nsystem include the reduced sediment flux of eroded material, which would limit silicate weathering, or contrastingly more\nrapid weathering regimes, both on the continents and in the oceans (reverse weathering). Neither satisfactorily explain why\nsuch conditions necessarily persist for 5 million years (approximately) after the event, and eventually recover during the\nmiddle Triassic. The authors counter this by proposing a model wherein the dramatic and prolonged diminishment of low-\nlatitude plants takes centre stage. In the absence of tropical, peat-forming ecosystems substantially less CO2 was likely to\nbe sequestered from the end Permian. In this absence, the authors propose that Earth’s climate-carbon cycle stabilized at a\nmuch higher average temperature than the perceived norm in the run up to the extinction. In this model, it would not be until\nthese plant biomes sufficiently recovered that sustained cooling got momentum. I generally like this idea, and think it is very\nworthwhile adding it to the mix of possibilities. By putting this modelling-led work out into the public domain, various\ngeochemical and petrological proxies might later be explored across a number of expanded sections, ultimately testing\nwhether or not this new hypothesis in genuine. I see this as an important step in this scientific process. Subject to minor line-\nby-line comments, I recommend publication in Nature Communications\nWilliam McMahon\nLine by line comments\nShould Andrew Merdith’s affiliation be updated?\nLine 63. I’m weary of such compilations of sedimentation rates honestly. How can one infer how much sedimentary stasis is\nlocked up in individual bedding planes, and how this varies throughout an expanded section? Not as easy as just dividing\nthickness by time.\nLine 94. A few tens of thousands of years.\nLine 128. How does one link tetrapod occurrences to local climate? I’m not even entirely certain whether most paleobotany-\nclimate links are so robust. Lithological indicators seem to be the key thing here (especially the three listed, in the absence\nof detailed accounts of sedimentary structures in other siliciclastic lithologies which may provide some clues). If we’re talking\ntetrapod trackway’s here, biases are more likely to pick out localized areas of greater accommodation/sediment aggradation\nrates, wherein the chances of reworking substrates where the trackways form decrease. So unless you have a huge amount\nof data to draw from, you’d maybe be picking out these preservational hot spots (or lack thereof), rather than anything\nmeaningful for global climate. Also, is local climate really what’s meant here. Or regional? Whilst I suspect the\npaleogeographic grid map is advanced, it’s surely compartmentalized on a “regional” rather than “local” level?\nLine 134. Reference in incorrect place.\nLine 134. “The ‘coal gap’ after the PTME was not associated with a significant loss of river or delta sediments.” Make it very\nclear how this is known. If it’s not from a global database of sedimentary rock volume of a particular age and lithotype (see\nbelow for examples) I’d advise removing.\n• Peters, S.E., Husson, J.M. and Czaplewski, J., 2018. Macrostrat: a platform for geological data integration and deep‐time\nEarth crust research. Geochemistry, Geophysics, Geosystems, 19(4), pp.1393-1409)\n• McMahon, W.J. and Davies, N.S., 2018. Evolution of alluvial mudrock forced by early land plants. Science, 359(6379),\npp.1022-1024.\n• Dunhill, A.M., Hannisdal, B. and Benton, M.J., 2014. Disentangling rock record bias and common-cause from redundancy\nin the British fossil record. Nature communications, 5(1), p.4818.\nI’d also suggest checking the references cited at this portion of the text (and maybe throughout) as a few seem out of place to\nme.\nLine 146. “The extinction zone of the land plants in the latitude ~45ºN–45ºS and death of the tetrapod in 30ºN–40ºS is likely\nto be the response of the extreme El Niño effect after the PTME spanning in the low to middle latitudes”. I realise this article\nis hot off the press and involves some authors here, but may be worth listing the other hypotheses of the trigger? By all\nmeans detail why you favour a particular mechanism. This may just be window dressing but worth taking the time to\ndedicate a couple of sentences to its interpretation and thought process behind it, rather than just throwing el nino’s in\nrandomly (which is how it comes across).\nLine 196. Should “Biogeographic productivity” have a loose definition?\nLine 246: Magnitude of predicted change?\nLine 267. Silicate weathering “rate”? Or intensity?\nLine 267: Would a better phrasing be: “than the direct effect of a decline in organic carbon burial”?\nLine 281. If these mechanisms are “previously suggested” shouldn’t there be some citations here?\nLine 287. Suggest change to “extremely hot by Phanerozoic standards”. Given this is quite an important sentence it could do\nwith being a bit shorter and snappier I think.\n288. Likewise “these conditions persisted for millions of years [can you place an estimate on this?], with cooling only\nachieved as plant productivity increased from the Middle Triassic”.\nLine 317 “cannot”\nLine 486. Discussion of evolution of plant functional traits in these papers (if you like):\n• Corenblit, D., Davies, N.S., Steiger, J., Gibling, M.R. and Bornette, G., 2015. Considering river structure and stability in the\nlight of evolution: feedbacks between riparian vegetation and hydrogeomorphology. Earth Surface Processes and\nLandforms, 40(2), pp.189-207.\n• Brückner, M.Z., Mcmahon, W.J. and Kleinhans, M.G., 2021. Muddying the waters: modeling the effects of early land plants\nin paleozoic estuaries. Palaios, 36(5), pp.173-181.\nLine 618. I’d be cautious about extending case study data which record local phenomena to an inferred widespread\nphenomena.\nFigure 1. Can the font size be increased in this? Even if it ends up going in as a single page spread as it is at present it’s\nquite hard to pick out the details. Perhaps uploads to the manuscript submission process is partly responsible.\nMethodology: Should there not be details of the tetrapod source data in the methodology somewhere?", "reviewer_2": "I think this is a great paper. I have been a part of the movement towards putting paleobotanical information “to work” via modeling and this stands as a prime example of such. The modeling is indepth and complicated but well described. Every question that popped into my head was answered in the methods. all of the problems I found are superficial. I recommend this paper for publication.\nLine 90–92: It took five tries to figure out what is being stated in this sentence. Please make more clear.\nThroughout paper: Macrofossils, not Macro fossils. Plus, it’s done both ways in the paper. Also, some paragraphs are\nindented, some not. There are little grammar mistakes throughout. I pointed some out, other I didn’t. Please proofread,\nLine 212: figure reference is not correct.\nLines 276 – 278 I was just thinking about the significant effect of T on weathering. I’m glad you mentioned it.\nLine 311: Tajikstan is mentioned twice.\nLine 358 – 360: I’m not sure what this means. Representative of what? Local or regionally representation? Globally?\nLine 522: There seems to be an unnecessary underscore symbol in this information.\nLine 536: Cordaites is a species and should be capitalized, unless you meant the generic word “cordaitalean” which is\nmostly used in the surrounding sentences.", "reviewer_3": "Summary\nThe manuscript \"Early Triassic super-greenhouse climate driven by vegetation collapse\" suggests a new hypothesis for the\n5-million-years long lasting greenhouse climate following the Permian-Triassic Extinction. To state this, the authors first use\nthe fossil plant record to reconstruct the vegetation distribution and plant productivity before and after the mass extinction.\nSecond, they use the reconstructed productivity for simulations with a climate-biogeochemical model to further investigate\nthe details of a reduced plant productivity on climate. They interpret the results as an example for a treshold in the climate\nsystem above which the system stabilizes at a very warm climate for million of years.\nGeneral remarks\nThe hypothesis and the way it is investigated are very interesting and this paper is a relevant contribution for the better\nunderstanding of the Permian-Triassic extinction, but more generalized also of mechanisms in the Earth system.\nHowever, I have several major remarks:\n1. My largest difficulty with the mansucript is its structure which makes it hard for the reader to read and understand (and also\nfor the reviewer!). The structure of the manuscript must be heavily revised. Please make clear where the description of what\nyou do ends, where you start describing results and where you discuss and interpret the results. It might need more\nsubheadings (if possible in the journal style?), or it might already work to make a section structure clearer by including blanc\nlines for paragraphs or even by choosing a clearer wording.\nOne linked remark to this is the Methods section in the end. In my opinion most of it goes into a supplementary material in its\ncurrent form. It needs substantial reduction and revision for a clear and short Methods part (see some ideas and details\nbelow).\n2. As usually for deep time paleo studies, the fossil occurences are very limited. This is the nature of exploring these time\nperiods and in my opinion it is justified to still do it although the amount of data is not enough for a statitic investigation. So\nmy critic is not that it is not enough data locations, but to discuss this problem more in depth and formulate conclusions\nbased on very few data more cautiously. In particular I would like the authors to adress or at least mention this issue for the\ntetrapod locations (for the Late Permian there is also not much data for the low latitudes, so please formulate your\nconclusions on the tetrapod distribution and in general the much highere low latitude extinction more carefully, l. 141-145).\nAnd for the extrapolation of the vegetation zones based on very few plant fossil locations. One particular question to discuss\nis the abundance of deserts in regions with low or no fossil occurrences. Why is it not just no preserved data? I think you\ntouch this very shortly, but this part is not clear and too short.\n3. Quality and selection of figures: the resolution of the figures is very low and together with a very small size this makes it\ndifficult to read. In addition, instead of figure 1 I would prefer a figure which gives a broader overview of the changes. I think it\ncould be reduced to less detailed information and diagram types which are intuitively understandble for an audience with a\nwider background. This is difficult for readers which do not have a botanical/paleobotanical background, but the topic and\nmethods of the paper is relevant to a broader audience. Please also make sure in figure 1 and 3 that the axes and lines are\nclearly labeled and are not only understandable with the very long captions.\nSome more detailed remarks:\nAbstract: the very strong last sentence is difficult to understand: what exactly would be the link between the treshold idea\nand the warming?\nIntroduction: The first sentence of the second part (l.80-84) would be helpful to have in the introduction, so one knows what\nwill be done.\nReconstructing plant productivity\nl.95: it is not clear to me with this sentence and the referenced figure 1b why this is robust. Can you give more details here or\nrefer to the Methods for this?\nl.103: I was confused why you only mention here the South China Chanhsingian? Is it an example or the only duplicates or\nthe highest?\nl.134-136: this seems to be a very important sentence, but I only partly understand it and I am not sure on which part of the\nMethods you refer - can you be more specific or more precise where to find the information in the Methods?\nl.145-147: I suggest to phrase this more carefully as I think it is very difficult and uncertain to establish a link like this\nl. 148-161: I am not sure if this is something you conclude from your results or bring together with other results (as you cite so\nmuch other work) or confirm other results (if yes: what is new about your results/method)? Please make this clearer\nl.189-192: something wrong with this sentence?\nModelling plant effects on long-term climate\nl.208-221: is this neccessary in this detail? Perhaps part of it could go to the Methods section\nl.248/249: please make clear that the exceptions relate to the difference between the geological record and the fossil-NPP\ndriven run. Would also be helpful to refer to figure 3 d and the blue and green line\nl.254: mention that Sr is a weathering proxy for the broader audience?\nl.255-257: is this shown somewhere? Like this I don't find this a helpful comparison\nl.266-268: something wrong in the formulation? I do not fully understand\nl.266-283: this seems to be a paragraph about the model limitations? I suggest to make it clear that you discuss the\nlimitations and to make this part more concise and maybe also shorter\nMaterials and Methods: I suggest the largest part can go to a SI. I suggest also to check the headings as I find them not\nalways fittingor at least nor summarizing the topic of the section. I guess the model part would be good to stay in the\nMaterials and Methods section.\nadditional remark on Figure 2: tetrapod dots should be smaller"}, "author_response": "Response to reviews for Early Triassic super-greenhouse climate driven by vegetation collapse\nZhen Xu et al.\nReviewers’ comments appear in black with our responses in blue. Modified text in the revised paper is\nalso colored blue, with line numbers for new text noted here.\nReviewer #1:\nComment (1): Xu and colleagues shed light on the unusually long persistence of “super” greenhouse\nconditions following the Permian-Triassic mass extinction. Previous hypotheses for why it appears\nCO2 could not be efficiently removed from the surface system include the reduced sediment flux of\neroded material, which would limit silicate weathering, or contrastingly more rapid weathering\nregimes, both on the continents and in the oceans (reverse weathering). Neither satisfactorily explain\nwhy such conditions necessarily persist for 5 million years (approximately) after the event, and\neventually recover during the middle Triassic. The authors counter this by proposing a model wherein\nthe dramatic and prolonged diminishment of low-latitude plants takes centre stage. In the absence of\ntropical, peat-forming ecosystems substantially less CO2 was likely to be sequestered from the end\nPermian. In this absence, the authors propose that Earth’s climate-carbon cycle stabilized at a much\nhigher average temperature than the perceived norm in the run up to the extinction. In this model, it\nwould not be until these plant biomes sufficiently recovered that sustained cooling got momentum. I\ngenerally like this idea, and think it is very worthwhile adding it to the mix of possibilities. By putting\nthis modelling-led work out into the public domain, various geochemical and petrological proxies\nmight later be explored across a number of expanded sections, ultimately testing whether or not this\nnew hypothesis in genuine. I see this as an important step in this scientific process. Subject to minor\nline-by-line comments, I recommend publication in Nature Communications\nWilliam McMahon\nResponse: We appreciate the reviewer’s positive and detailed comments and have revised the\nmanuscript to address them as outlined below.\nComment (2): Should Andrew Merdith’s affiliation be updated?\nResponse: It should! We have done this. [line 12]\nComment (3): Line 63. I’m weary of such compilations of sedimentation rates honestly. How can one\ninfer how much sedimentary stasis is locked up in individual bedding planes, and how this varies\nthroughout an expanded section? Not as easy as just dividing thickness by time.\nResponse: We agree that sedimentation rates are uncertain, and had intended this to be part of our\npoint here. We have revised the wording to specifically note that estimating sedimentation rates is\ndifficult. [line 67]\nComment (4): Line 94. A few tens of thousands of years.\nResponse: Thanks for the correction, this section has been reworded significantly and moved to\nsupplementary in line with other reviewer comments [line 1150]\nComment (5): Line 128. How does one link tetrapod occurrences to local climate? I’m not even\nentirely certain whether most paleobotany-climate links are so robust. Lithological indicators seem to\nbe the key thing here (especially the three listed, in the absence of detailed accounts of sedimentary\nstructures in other siliciclastic lithologies which may provide some clues). If we’re talking tetrapod\ntrackway’s here, biases are more likely to pick out localized areas of greater accommodation/sediment\naggradation rates, wherein the chances of reworking substrates where the trackways form decrease. So\nunless you have a huge amount of data to draw from, you’d maybe be picking out these preservational\nhot spots (or lack thereof), rather than anything meaningful for global climate. Also, is local climate\nreally what’s meant here. Or regional? Whilst I suspect the paleogeographic grid map is advanced,\nit’s surely compartmentalized on a “regional” rather than “local” level?\nResponse: We regret being unclear in this section. The tetrapod fossils are used as an additional\npositive indicator of grid cells being occupied by vegetation, rather than an indicator of climate. We\nhave made this clearer in the revision. We have also clarified the model grid size as both ‘local’ and\n‘regional’ do not have well defined spatial scales. [line 147]\nComment (6): Line 134. Reference in incorrect place.\nResponse: Thanks for the correction, we have fixed this in the rewritten section.\nComment (7): Line 134. “The ‘coal gap’ after the PTME was not associated with a significant loss of\nriver or delta sediments.” Make it very clear how this is known. If it’s not from a global database of\nsedimentary rock volume of a particular age and lithotype (see below for examples) I’d advise\nremoving.\n• Peters, S.E., Husson, J.M. and Czaplewski, J., 2018. Macrostrat: a platform for geological data\nintegration and deep‐time Earth crust research. Geochemistry, Geophysics, Geosystems, 19(4),\npp.1393-1409)\n• McMahon, W.J. and Davies, N.S., 2018. Evolution of alluvial mudrock forced by early land plants.\nScience, 359(6379), pp.1022-1024.\n• Dunhill, A.M., Hannisdal, B. and Benton, M.J., 2014. Disentangling rock record bias and common-\ncause from redundancy in the British fossil record. Nature communications, 5(1), p.4818.\nI’d also suggest checking the references cited at this portion of the text (and maybe throughout) as a\nfew seem out of place to me.\nResponse: We have rephrased this sentence to make it clear that we are discussing a series of local\ndatasets in which the coal gap is observed but river or delta sediments continue in these sedimentary\nsuccessions. We have made it clear that this is not a global database but we think it is worth including\nas it demonstrates how the coal gap is currently understood. The references for this in the main text\nand supplementary information have been checked and corrected where necessary. [line 155]\nComment (8): Line 146. “The extinction zone of the land plants in the latitude ~45ºN–45ºS and death\nof the tetrapod in 30ºN–40ºS is likely to be the response of the extreme El Niño effect after the PTME\nspanning in the low to middle latitudes”. I realise this article is hot off the press and involves some\nauthors here, but may be worth listing the other hypotheses of the trigger? By all means detail why\nyou favour a particular mechanism. This may just be window dressing but worth taking the time to\ndedicate a couple of sentences to its interpretation and thought process behind it, rather than just\nthrowing el nino’s in randomly (which is how it comes across).\nResponse: We have rewritten this section to expand on this idea and to add a more comprehensive\ndiscussion of other extinction mechanisms [line 178].\nComment (9): Line 196. Should “Biogeographic productivity” have a loose definition?\nResponse: We modified this sentence to make it clearer that the ‘biogeographic productivity’ is a\nfossil-based reconstruction which does not consider the effects of CO fertilization [line 244].\nResponse: We were referring to the ‘fold change’ which is a ~2-4 fold increase in both our model and\nthe proxy dataset. We have explicitly noted this now. [line 298]\nComment (11): Line 267. Silicate weathering “rate”? Or intensity?\nResponse: We have now noted that this is weathering intensity [line 320]\nComment (12): Line 267: Would a better phrasing be: “than the direct effect of a decline in organic\ncarbon burial”?\nResponse: We agree. We have modified this sentence as suggested [line 321].\nComment (13): Line 281. If these mechanisms are “previously suggested” shouldn’t there be some\ncitations here?\nResponse: We have added these citations [line 335].\nComment (14): Line 287. Suggest change to “extremely hot by Phanerozoic standards”. Given this is\nquite an important sentence it could do with being a bit shorter and snappier I think.\nResponse: We have made this change [line 343]\nComment (15): 288. Likewise “these conditions persisted for millions of years [can you place an\nestimate on this?], with cooling only achieved as plant productivity increased from the Middle\nTriassic”.\nResponse: We have now included an estimate here of nearly 5 million years [line 345]\nComment (16): Line 317 “cannot”\nResponse: We have modified this spelling mistake [line 367].\nComment (17): Line 486. Discussion of evolution of plant functional traits in these papers (if you\nlike):\n• Corenblit, D., Davies, N.S., Steiger, J., Gibling, M.R. and Bornette, G., 2015. Considering river\nstructure and stability in the light of evolution: feedbacks between riparian vegetation and\nhydrogeomorphology. Earth Surface Processes and Landforms, 40(2), pp.189-207.\n• Brückner, M.Z., Mcmahon, W.J. and Kleinhans, M.G., 2021. Muddying the waters: modeling the\neffects of early land plants in paleozoic estuaries. Palaios, 36(5), pp.173-181.\nResponse: Wehave now incorporated these references [line 517].\nComment (18): Line 618. I’d be cautious about extending case study data which record local\nphenomena to an inferred widespread phenomena.\nResponse: We have modified this section to be more circumspect [line 652]\nComment (19): Figure 1. Can the font size be increased in this? Even if it ends up going in as a single\npage spread as it is at present it’s quite hard to pick out the details. Perhaps uploads to the manuscript\nsubmission process is partly responsible.\nResponse: We have modified Figure 1 to make it easier to read and understand, and it is now supplied\nas a high-quality vector graphic.\nComment (20): Methodology: Should there not be details of the tetrapod source data in the\nmethodology somewhere?\nResponse: We have now added the sources of data in the Figure 3 caption.\nReviewer #2:\nComment (1): I think this is a great paper. I have been a part of the movement towards putting\npaleobotanical information “to work” via modeling and this stands as a prime example of such. The\nmodeling is in-depth and complicated but well described. Every question that popped into my head\nwas answered in the methods. all of the problems I found are superficial. I recommend this paper for\npublication.\nResponse: We sincerely thank the reviewer for their kind and encouraging feedback. We have\ncarefully addressed their detailed suggestions to improve the work.\nComment (2): Line 90–92: It took five tries to figure out what is being stated in this sentence. Please\nmake more clear.\nResponses: Our apologies. We have rewritten this part for clarity [line 100]\nComment (3): Throughout paper: Macrofossils, not Macro fossils. Plus, it’s done both ways in the\npaper. Also, some paragraphs are indented, some not. There are little grammar mistakes throughout. I\npointed some out, other I didn’t. Please proofread,\nResponses: We have modified this phrasing and have carefully proofread the revision.\nComment (4): Line 212: figure reference is not correct.\nResponses: We have corrected this, thanks for spotting it. This text is now in the methods [line 696].\nComment (5): Lines 276 – 278 I was just thinking about the significant effect of T on weathering.\nI’m glad you mentioned it.\nResponses: Thanks for the positive comment\nComment (6): Line 311: Tajikstan is mentioned twice.\nResponses: This was a mistake in our abbreviations, so in the revised manuscript we use the full\ncounty names and have increased the font size to make this more legible in Figure 1 and S2.\nComment (7): Line 358 – 360: I’m not sure what this means. Representative of what? Local or\nregionally representation? Globally?\nResponses: We have modified this wording to be clear that we have used all the published papers or\ndatasets we could locate in our analysis [line 446]\nComment (8): Line 522: There seems to be an unnecessary underscore symbol in this information.\nResponses: We have deleted this unnecessary symbol in the revision [line 550].\nComment (9): Line 536: Cordaites is a species and should be capitalized, unless you meant the\ngeneric word “cordaitalean” which is mostly used in the surrounding sentences.\nResponses: Thank you for pointing out this mistake. We have revised the text to use \"cordaitalean\" as\nit refers to more than just the genus Cordaites. In the sentences specifically referring to the genus,\nwe retained Cordaites and ensured it is italicized and capitalized.\nReviewer #3:\nSummary\nThe manuscript \"Early Triassic super-greenhouse climate driven by vegetation collapse\" suggests a\nnew hypothesis for the 5-million-years long lasting greenhouse climate following the Permian-\nTriassic Extinction. To state this, the authors first use the fossil plant record to reconstruct the\nvegetation distribution and plant productivity before and after the mass extinction. Second, they use\nthe reconstructed productivity for simulations with a climate-biogeochemical model to further\ninvestigate the details of a reduced plant productivity on climate. They interpret the results as an\nexample for a treshold in the climate system above which the system stabilizes at a very warm climate\nfor million of years.\nGeneral remarks\nThe hypothesis and the way it is investigated are very interesting and this paper is a relevant\ncontribution for the better understanding of the Permian-Triassic extinction, but more generalized also\nof mechanisms in the Earth system.\nHowever, I have several major remarks:\nResponse: We thank the reviewer for their thoughtful summary and constructive evaluation of our\nmanuscript. We appreciate their detailed feedback and have carefully considered the major remarks, as\noutlined below.\nComment (1): My largest difficulty with the mansucript is its structure which makes it hard for the\nreader to read and understand (and also for the reviewer!). The structure of the manuscript must be\nheavily revised. Please make clear where the description of what you do ends, where you start\ndescribing results and where you discuss and interpret the results. It might need more subheadings\n(if possible in the journal style?), or it might already work to make a section structure clearer by\nincluding blanc lines for paragraphs or even by choosing a clearer wording.\nOne linked remark to this is the Methods section in the end. In my opinion most of it goes into a\nsupplementary material in its current form. It needs substantial reduction and revision for a clear and\nshort Methods part (see some ideas and details below).\nResponse: Thank you for this constructive point. We have modified the manuscript structure as best\nas we can within the journal guidelines and considering the combined reviewer comments. We have\nrevised the headings to emphasize the main topics in each section, included clearer paragraph breaks,\nand have revised the Methods section for conciseness, with additional material now in the\nsupplement.\nComment (2): As usually for deep time paleo studies, the fossil occurences are very limited. This is\nthe nature of exploring these time periods and in my opinion it is justified to still do it although the\namount of data is not enough for a statitic investigation. So my critic is not that it is not enough data\nlocations, but to discuss this problem more in depth and formulate conclusions based on very few data\nmore cautiously. In particular I would like the authors to adress or at least mention this issue for the\ntetrapod locations (for the Late Permian there is also not much data for the low latitudes, so please\nformulate your conclusions on the tetrapod distribution and in general the much highere low latitude\nextinction more carefully, l. 141-145). And for the extrapolation of the vegetation zones based on very\nfew plant fossil locations. One particular question to discuss is the abundance of deserts in regions\nwith low or no fossil occurrences. Why is it not just no preserved data? I think you touch this very\nshortly, but this part is not clear and too short.\nResponse: We agree with the reviewers point that more discussion is beneficial here. We have added\na section to the paper noting the poor abundance of data, particularly in some locations and the need\nfor us to infer vegetation from combinations of fossil and climate-sensitive lithological data. We note\nspecifically that the tetrapod record is used here only to add gridcells where plants must have been\nabundant, rather than to restrict plant paleogeography where they are not found, and we have been\nclearer that the vegetation zones are based on climate proxies as well as fossils. Specifically, desert\nregions are denoted by lithological proxies indicating arid conditions, as well as climate model\noutputs that also suggest aridity [line 147, 167, 652]\nComment (3): Quality and selection of figures: the resolution of the figures is very low and together\nwith a very small size this makes it difficult to read. In addition, instead of figure 1 I would prefer a\nfigure which gives a broader overview of the changes. I think it could be reduced to less detailed\ninformation and diagram types which are intuitively understandble for an audience with a wider\nbackground. This is difficult for readers which do not have a botanical/paleobotanical background, but\nthe topic and methods of the paper is relevant to a broader audience. Please also make sure in figure 1\nand 3 that the axes and lines are clearly labeled and are not only understandable with the very long\ncaptions.\nResponse: We thank the reviewer for the constructive suggestions on the figures. We have expanded\nthe information in Figure 1 into two figures to allow us to use full names and clearer diagrams.\nFigures are now also supplied as high quality vector graphics.\nSome more detailed remarks:\nComment (4): Abstract: the very strong last sentence is difficult to understand: what exactly would\nbe the link between the treshold idea and the warming?\nResponse: We have added a sentence on this to the main text which explains how beyond a certain\ntemperature, vegetation die-back can result in further warming through removal of vegetation carbon\nsinks. We have also more clearly noted a reference to a recent numerical modelling paper specifically\non this mechanism [346].\nComment (5): Introduction: The first sentence of the second part (l.80-84) would be helpful to have\nin the introduction, so one knows what will be done.\nResponse: We moved the first sentence to the end of the introduction as suggested [line 87].\nComment (6): Reconstructing plant productivity\nl.95: it is not clear to me with this sentence and the referenced figure 1b why this is robust. Can you\ngive more details here or refer to the Methods for this?\nResponse: We now number the methods section and refer this sentence to the corresponding part of\nthe methods more clearly. We have also revised the wording here [line 105].\nComment (7): l.103: I was confused why you only mention here the South China Chanhsingian? Is it\nan example or the only duplicates or the highest?\nResponse: Yes, it is just an example to show the impact of normalization. We modified this sentence\nto explain this more clearly [line 114].\nComment (8): l.134-136: this seems to be a very important sentence, but I only partly understand it\nand I am not sure on which part of the Methods you refer - can you be more specific or more precise\nwhere to find the information in the Methods?\nResponse: We now make it clear that this refers to section 9 in the methods [line 155]\nComment (9): l.145-147: I suggest to phrase this more carefully as I think it is very difficult and\nuncertain to establish a link like this\nResponses: We have revised this section to be more balanced in line with this and other comments\n[line 178].\nComment (10): l. 148-161: I am not sure if this is something you conclude from your results or bring\ntogether with other results (as you cite so much other work) or confirm other results (if yes: what is\nnew about your results/method)? Please make this clearer\nResponse: We have modified this section to clarify that this is a new conclusion from our results, but\nis also consistent with previous studies that were either regional or were much more limited in scope\n[line 193].\nComment (11): l.189-192: something wrong with this sentence?\nResponse: We have revised the grammar here, this was a mistake incorporating our own revisions -\napologies for the confusion [line 237].\nComment (12): Modelling plant effects on long-term climate\nl.208-221: is this neccessary in this detail? Perhaps part of it could go to the Methods section\nResponse: We have removed as much of this as possible to the methods to provide a clearer overview\nfor the reader [methods section 10]\nComment (13): l.248/249: please make clear that the exceptions relate to the difference between the\ngeological record and the fossil-NPP driven run. Would also be helpful to refer to figure 3 d and the\nblue and green line\nResponse: We have made these modifications as suggested [line 299].\nComment (14): l.254: mention that Sr is a weathering proxy for the broader audience?\nResponse: We have made this useful addition [line 305].\nComment (15): l.255-257: is this shown somewhere? Like this I don't find this a helpful comparison\nResponse: We have decided to remove this line as showing this more clearly would require further\nwork to predict CIA directly from the model, which should be investigated in a separate study.\nComment (16): l.266-268: something wrong in the formulation? I do not fully understand\nResponse: We have revised this section for clarity [line 320]\nComment (17): l.266-283: this seems to be a paragraph about the model limitations? I suggest to\nmake it clear that you discuss the limitations and to make this part more concise and maybe also\nshorter\nResponse: We now state these are limitations and shorten the section as much as we can [line 322]\nComment (18): Materials and Methods: I suggest the largest part can go to a SI. I suggest also to\ncheck the headings as I find them not always fittingor at least nor summarizing the topic of the\nsection. I guess the model part would be good to stay in the Materials and Methods section.\nResponse: We have moved parts of the methods into the SI as requested.\nComment (19): additional remark on Figure 2: tetrapod dots should be smaller\nResponse: We have done this in the revision."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have done a great job responding to the reviewer comments.", "reviewer_2": "My concerns have been addressed and I suggest publication.", "reviewer_3": "I thank the authors for reading my review so carefully and for implementing many of my remarks. In particular, the paper is\nnow much easier to read and understand due to a clearer structure.\nI have only some minor remarks and recommend to accept it after these small changes (or at least the most relevant):\nl.169-173: I guess modeling with HadCM3 was done in reference 9? This is not clear to me from the wording used. Could\nyou make this clear by rephrasing it?\nl.232 and following: it is not clear to me whether you need temperature fields for initialising the model (as you don't give\nradiation I would not know how to get at least the start temperature). If this is the case, where are the temperatures coming\nfrom?\nl.250: add D for figure: (Fig. 4D)\nl.258: (Fig.4E, G)\nl.273: \"(green line in Fig. 4)\", please modify to \"...Fig. 4C)\"\nl.280, add again where the data from the gelogical record is coming from\nFigures:\nFigure 2: this is a kind of diagram I am not so familiar with - if it is standard to do it the way it is, tha's fine. I would prefer to\nhave information of what is shown on the x-axis and also add latitude [°] for the y-axis.\nFigure 3: it is cut off in the merged pdf version, but I guess the NPP maps are the same as in the first version of the\nmanuscript and this is fine with me. I only have one remark to what is called NPPL in Figure 2 and Figure 3 as well as the\ntext: I suggest to call it differently after CO2 fertilization is included when the variable is used, for example like NPPL_f or\nsimilar\nFigure 4: I find it confusing to use the same blue color (at least it looks like that) for the line in C and the line with points in D\nand G although the data in C is coming from a different data source. I suggest to change the color and also add a legend for\nthis color. I would find it helpful to have in this legend already included where the recorded data is coming from for both data\nsources, so something like \"recorded data from XX\".\nFigure S6: I expected that the 0.5 line should be the same as the green line in Figure 4G, but they seem to not agree with\neach other. Could you check this?"}, "author_response": "Reply: We thank both Reviewer 1 and 2 for their former suggestions on our work.\nReply: We thank the reviewer 3 for all their detailed suggestions, which indeed helped\nimprove this work. Updated line-by-line modifications in response to final comments are\nbelow.\nl.169-173: I guess modeling with HadCM3 was done in reference 9? This is not clear to me\nfrom the wording used. Could you make this clear by rephrasing it?\nReply [Line 171]: Yes, the HadCM3 simulation was done in reference 9, Yadong Sun’s 2024\nscience paper. We have added the reference 9 citation in this sentence for clarity.\nl.232 and following: it is not clear to me whether you need temperature fields for initialising\nthe model (as you don't give radiation I would not know how to get at least the start\ntemperature). If this is the case, where are the temperatures coming from?\nReply [Line 251]: We have added a line on the model initiation. The model is given a\nstarting CO concentration and calculates temperature based on this and the solar flux.\n2\nl.250: add D for figure: (Fig. 4D)\nReply [Line 251]: Modified.\nl.258: (Fig.4E, G)\nReply [Line 259]: Modified.\nl.273: \"(green line in Fig. 4)\", please modify to \"...Fig. 4C)\"\nReply [Line 275]: Modified.\nl.280, add again where the data from the gelogical record is coming from\nReply [Line 284]: Added citation\nFigures:\nFigure 2: this is a kind of diagram I am not so familiar with - if it is standard to do it the way\nit is, tha's fine. I would prefer to have information of what is shown on the x-axis and also\nadd latitude [°] for the y-axis.\nReply [Figure 2]: Both latitude and species number have now been added to Figure 2.\nFigure 3: it is cut off in the merged pdf version, but I guess the NPP maps are the same as in\nthe first version of the manuscript and this is fine with me. I only have one remark to what is\ncalled NPPL in Figure 2 and Figure 3 as well as the text: I suggest to call it differently after\nCO2 fertilization is included when the variable is used, for example like NPPL_f or similar\nReply [Throughout paper]: Good suggestion. We have modified the NPPL into NPPL in\nf\nthe main text. In the figure 2, we only show the original NPPL because the CO fertilization\n2\neffect is variable within the model run.\nFigure 4: I find it confusing to use the same blue color (at least it looks like that) for the line\nin C and the line with points in D\nand G although the data in C is coming from a different data source. I suggest to change the color and also add a legend for\nthis color. I would find it helpful to have in this legend already included where the recorded data is coming from for\nboth data sources, so something like \"recorded data from XX\".\nReply [Figure 4]: We prefer the consistency of having the model and geological record in\nconsistent colours throughout, which is how SCION papers have typically been displayed.\nWe have checked the caption to ensure all data sources are included.\nFigure S6: I expected that the 0.5 line should be the same as the green line in Figure 4G, but\nthey seem to not agree with each other. Could you check this?\nReply [Figure S6]: Thanks for spotting this. Figure S6 was from an older version of the\npaper and the lines were shifted slightly. We have updated this now. All text is unaffected.\nReviewer #3 (Remarks on code availability):\nI had a quick look at the code, but then realized it is only the general version of SCION. I\nguess the modified version is much more relevant (which they seem to provide as there is a\nsentence for this in the manuscript). In addition to the modified version, the authors need to\nmake sure to provide a readme with clear instructions how to reproduce their results and the\nnames of the variables they analyse and plot in their figures.\nReply [Lines 411–413]: We have deposited the modified version of the code and all the\nforcing/data files in Zhen Xu’s Github. Zhen will make it open access once the paper is\nonline. Here is the Github link: https://github.com/ZhenXuJane/SCION_Xu2025 A ReadMe\nfile is also listed there to show how to repeat the code."}]

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