We extend the recently proposed heat-bath configuration interaction (HCI) method Holmes, Tubman, Umrigar, J. Chem. Theory Comput. 2016, 12, 3674, by introducing a semistochastic algorithm for ...performing multireference Epstein–Nesbet perturbation theory, in order to completely eliminate the severe memory bottleneck of the original method. The proposed algorithm has several attractive features. First, there is no sign problem that plagues several quantum Monte Carlo methods. Second, instead of using Metropolis–Hastings sampling, we use the Alias method to directly sample determinants from the reference wave function, thus avoiding correlations between consecutive samples. Third, in addition to removing the memory bottleneck, semistochastic HCI (SHCI) is faster than the deterministic variant for many systems if a stochastic error of 0.1 mHa is acceptable. Fourth, within the SHCI algorithm one can trade memory for a modest increase in computer time. Fifth, the perturbative calculation is embarrassingly parallel. The SHCI algorithm extends the range of applicability of the original algorithm, allowing us to calculate the correlation energy of very large active spaces. We demonstrate this by performing calculations on several first row dimers including F2 with an active space of (14e, 108o), Mn-Salen cluster with an active space of (28e, 22o), and Cr2 dimer with up to a quadruple-ζ basis set with an active space of (12e, 190o). For these systems we were able to obtain better than 1 mHa accuracy with a wall time of merely 55 s, 37 s, and 56 min on 1, 1, and 4 nodes, respectively.
Climate change is expected to increase wildfire activity in boreal ecosystems, thus threatening the carbon stocks of these forests, which are currently the largest terrestrial carbon sink in the ...world. Describing the ecological processes involved in fire regimes in terms of frequency, size, type (surface vs. crown) and severity (biomass burned) would allow better anticipation of the impact of climate change on these forests. In Fennoscandia, this objective is currently difficult to achieve due to the lack of knowledge of long‐term (centuries to millennia) relationships between climate, fire and vegetation.
We investigated the causes and consequences of changes in fire regimes during the Holocene (last ~11,000 years) on vegetation trajectories in the boreal forest of northern Finland. We reconstructed fire histories from sedimentary charcoal at three sites, as well as vegetation dynamics from pollen, moisture changes from Sphagnum spore abundance at two sites, and complemented these analyses with published regional chironomid‐inferred July temperature reconstructions.
Low‐frequency, large fires were recorded during the warm and dry mid‐Holocene period (8500–4500 cal. year BP), whereas high‐frequency, small fires were more characteristic of the cool and wet Neoglacial period (4500 cal. year BP onward). A higher proportion of charcoal particles with a woody aspect—characterizing crown fires—was recorded at one of the two sites at times of significant climatic and vegetational changes, when the abundance of Picea abies was higher.
Synthesis. Our results show both a direct and an indirect effect of climate on fire regimes in northern Fennoscandia. Warm and dry periods are conducive to large surface fires, whereas cool and moist periods are associated with small fires, either crown or surface. Climate‐induced shifts in forest composition also affect fire regimes. Climatic instability can alter vegetation composition and structure and lead to fuel accumulation favouring stand‐replacing crown fires. Considering the ongoing climate warming and the projected increase in extreme climatic events, Fennoscandian forests could experience a return to a regime of large surface fires, but stand‐replacing crown fires will likely remain a key ecosystem process in areas affected by climatic and/or vegetational instability.
Abstrakti
Ilmastonmuutoksen odotetaan lisäävän metsäpalojen aktiivisuutta boreaalisissa ekosysteemeissä. Boreaaliset metsät ovat yksi maailman suurimmista maanpäällisistä hiilivarastoista ja lisääntyvät metsäpalot uhkaavat vaikuttaa niiden kykyyn sitoa hiiltä. Metsäpaloihin liittyvien ekologisten prosessien kuvaaminen palojen esiintymistiheyden, koon, tyypin (pinta‐ vs. latvapalo) ja voimakkuuden (palanut biomassa) avulla parantaa mahdollisuuksiamme ennakoida ilmastonmuutoksen vaikutuksia boreaalisiin metsiin. Fennoskandiassa tätä tavoitetta on vaikea saavuttaa, koska ilmaston, metsäpalojen ja kasvillisuuden väliset pitkäaikaiset (vuosisatojen tai vuosituhansien mittaiset) vuorovaikutussuhteet tunnetaan heikosti.
Tutkimme holoseenin (viimeiset ~ 11 000 vuotta) aikaisia metsäpalojen esiintymisen muutosten syitä ja seurauksia Pohjois‐Suomen boreaalisissa metsissä. Rekonstruoimme metsäpalohistoriat kolmen järven pohjasedimenttien hiilipartikkeleiden avulla. Lisäksi tutkimme kasvillisuuden muutoksia siitepölyn avulla ja kosteusmuutoksia rahkasammaleiden itiöiden avulla. Täydensimme näitä analyyseja aiemmin julkaistuilla alueellisilla lämpötilarekonstruktioilla.
Harvoin esiintyviä suuria metsäpaloja todettiin lämpimän ja kuivan holoseenikauden puolivälin aikana (8500‐4500 vuotta sitten), kun taas useammin esiintyvät mutta pienemmät metsäpalot olivat tyypillisempiä viimeisen 4500 vuoden aikana, kun ilmasto oli viileä ja kostea.
Synteesi. Tuloksemme antavat tietoa ilmaston suorista ja epäsuorista vaikutuksista metsäpaloihin Pohjois‐Fennoskandiassa. Lämpimät ja kuivat jaksot edistävät suuria pintapaloja, kun taas pienet metsäpalot, jotka voivat olla pinta‐ tai latvapaloja, liittyvät viileisiin ja kosteisiin ilmastojaksoihin. Ilmaston aiheuttamat muutokset metsien koostumuksessa vaikuttavat myös metsäpalojen esiintymiseen. Ilmaston vaihtelu voi muuttaa kasvillisuuden koostumusta ja rakennetta, mikä johtaa palavan biomassan kertymiseen ja edesauttaa siten latvapalojen esiintymistä. Ilmaston lämpenemisen ja äärevöitymisen seurauksena Fennoskandian metsät saattavat palata suurten pintapalojen tilaan. Metsiä uudistavat latvapalot pysyvät todennäköisesti tärkeinä niillä alueilla, missä ilmaston ja kasvillisuuden muutosten vaikutukset ovat voimakkaimmat.
Climate change is expected to increase wildfire activity in boreal ecosystems. Considering the ongoing climate warming and the projected increase in extreme climatic events, Fennoscandian forests could experience a return to a regime of large surface fires, but stand‐replacing crown fires will likely remain a key ecosystem process in areas affected by climatic and/or vegetational instability.
Molecular profiling of tumors promises to advance the clinical management of cancer, but the benefits of integrating molecular data with traditional clinical variables have not been systematically ...studied. Here we retrospectively predict patient survival using diverse molecular data (somatic copy-number alteration, DNA methylation and mRNA, microRNA and protein expression) from 953 samples of four cancer types from The Cancer Genome Atlas project. We find that incorporating molecular data with clinical variables yields statistically significantly improved predictions (FDR < 0.05) for three cancers but those quantitative gains were limited (2.2-23.9%). Additional analyses revealed little predictive power across tumor types except for one case. In clinically relevant genes, we identified 10,281 somatic alterations across 12 cancer types in 2,928 of 3,277 patients (89.4%), many of which would not be revealed in single-tumor analyses. Our study provides a starting point and resources, including an open-access model evaluation platform, for building reliable prognostic and therapeutic strategies that incorporate molecular data.
Forest fires are an important disturbance in the boreal forest. They are influenced by climate, weather, topography, vegetation, surface deposits, and human activities. In return, forest fires affect ...the climate through emission of gases and aerosols, and changes in surface albedo, soil processes, and vegetation dynamics. The net effect of these factors is not yet well established but seems to have caused a negative feedback on climate during the 20th century. However, an increase in boreal forest fires is predicted by the end of the 21st century, possibly changing the effect of fires on climate change to a positive feedback that would exacerbate global warming. This review presents (1) an overview of fire regimes and vegetation succession in boreal forests; (2) the effects on climate of combustion emissions and post-fire changes in ecosystem functioning; (3) the effects of fire regime variations on climate, especially on carbon stock and surface albedo; (4) an integrative approach of fire effects on climate dynamics; and (5) the implications of increased fire activity on global warming by calculating the radiative forcing of several factors by 2100 in the boreal region, before discussing the results and exposing the limits of the data at hand. Generally, losses of carbon from forest fires in the boreal region will increase in the future and their effect on the carbon stock (0.37 W/m2/decade) will be greater than the effect of fire on surface albedo (−0.09 W/m2/decade). The net effect of aerosol emissions from boreal fires will likely cause a positive feedback on global warming. This review emphasizes the importance of feedbacks between fires and climate in the boreal forest. It presents limitations and uncertainties to be addressed in future studies, particularly with regards to the effect of CO2fertilization on forest productivity, which could offset or mitigate the effect of fire.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Aim
Documenting past vegetation dynamics and fire‐vegetation relationships at a regional scale is necessary to understand the mechanisms that control the functioning of the boreal forest, which is ...particularly sensitive to climate change. The objective of this study is to document these interactions in the Komi Republic during the Holocene.
Location
Yaksha, Vychegda river basin, Republic of Komi, Russia.
Taxon
Plantae, gymnosperms, angiosperms.
Methods
Two palaeoecological approaches are combined, based (1) on pollen (this study) and charcoal analysis (recomputed from our previous analysis) applied to cores from two peatlands and (2) on a REVEALS model (a part of the Landscape Reconstruction Algorithm “LRA”) applied to six regional pollen cores in order to obtain a regional estimate of vegetation cover during the Holocene.
Results
The pollen diagram produced locally from Yaksha was compared with the regional vegetation cover determined by REVEALS. Taxa such as Abies sp. and Pinus spp. showed differences between the two approaches, but vegetation signals remain qualitatively consistent. From 10,000 to 6,000 cal. yr BP, the forest was mainly a light taiga (composed of Pinus sylvestris and Betula spp.) and low fire activity was recorded. From 6,000 to 3,500 cal. yr BP, a dark taiga (composed of Picea spp., Abies sibirica and Pinus sibirica) was established due to favourable climatic conditions, despite higher fire activity. From 3,500 cal. yr BP onwards, the continuous increase in fire activity allowed for a gradual return of light taiga, Betula spp., likely reinforced by human activities. The dynamics of Picea spp. and Abies sp. were asynchronous between the sites. For Picea spp., the hypothesis of local inter‐site expansion distributed along stream corridors is supported by the data. For Abies sp., a bias in REVEALS, and in climate cooling may explain disparities between sites.
Main conclusions
We found evidence that in the early and mid‐Holocene, vegetation dynamics were probably more influenced by climate, as fire activity was low. During the late Holocene, fire activity and geomorphology, eventually augmented by human activities, increased in influence on vegetation dynamics and led to the predominance of the light taiga forest up to the present.
In this paper, an inertial S-iteration iterative process for approximating a common fixed point of a finite family of quasi-Bregman nonexpansive mappings is introduced and studied in a reflexive ...Banach space. A strong convergence theorem is proved. Some applications of the theorem are presented. The results presented here improve, extend, and generalize some recent results in the literature.
Although lacustrine sedimentary charcoal has long been used to infer paleofires, their quantitative reconstructions require improvements of the calibration of their links with fire regimes (i.e. ...occurrence, area, and severity) and the taphonomic processes that affect charcoal particles between the production and the deposition in lake sediments. Charcoal particles >150 µm were monitored yearly from 2011 to 2016 using traps submerged in seven head lakes situated in flat-to-rolling boreal forest landscapes in eastern Canada. The burned area was measured, and the above-ground fire severity was assessed using the differentiated normalized burn ratio (dNBR) index, derived from LANDSAT images, and measurements taken within zones radiating 3, 15, and 30 km from the lakes. In order to evaluate potential lag effects in the charcoal record, fire metrics were assessed for the year of recorded charcoal recording (lag 0) and up to 5 years before charcoal deposition (lag 5). A total of 92 variables were generated and sorted using a Random Forest-based methodology. The most explanatory variables for annual charcoal particle presence, expressed as the median surface area, were selected. Results show that, temporally, sedimentary charcoal accurately recorded fire events without a temporal lag; spatially, fires were recorded up to 30 km from the lakes. Selected variables highlighted the importance of burned area and fire severity in explaining lacustrine charcoal. The charcoal influx was thus driven by fire area and severity during the production process. The dispersion process of particles resulted mostly of wind transportation within the regional (<30 km) source area. Overall, charcoal median surface area represents a reliable proxy for reconstructing past burned areas and fire severities.
Strategic introduction of less flammable broadleaf vegetation into landscapes was suggested as a management strategy for decreasing the risk of boreal wildfires projected under climatic change. ...However, the realization and strength of this offsetting effect in an actual environment remain to be demonstrated.
Here we combined paleoecological data, global climate models and wildfire modelling to assess regional fire frequency (RegFF, i.e. the number of fires through time) in boreal forests as it relates to tree species composition and climate over millennial time-scales.
Lacustrine charcoals from northern landscapes of eastern boreal Canada indicate that RegFF during the mid-Holocene (6000–3000 yr ago) was significantly higher than pre-industrial RegFF (ad c. 1750). In southern landscapes, RegFF was not significantly higher than the preindustrial RegFF in spite of the declining drought severity. The modelling experiment indicates that the high fire risk brought about by a warmer and drier climate in the south during the mid-Holocene was offset by a higher broadleaf component.
Our data highlight an important function for broadleaf vegetation in determining boreal RegFF in a warmer climate. We estimate that its feedback may be large enough to offset the projected climate change impacts on drought conditions.
Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire ...regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire–climate–vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca . 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.
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