Browsing by Author "Zimmermann, Niklaus E."

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Biomes as evolutionary arenas: Convergence and conservatism in the trans-continental succulent biome
(2020-07) Ringelberg, Jens J.; Zimmermann, Niklaus E.; Weeks, Andrea; Lavin, Matthew; Hughes, Colin E.
Aim: Historically, biomes have been defined based on their structurally and functionally similar vegetation, but there is debate about whether these similarities are superficial, and about how biomes are defined and mapped. We propose that combined assessment of evolutionary convergence of plant functional traits and phylogenetic biome conservatism provides a useful approach for characterizing biomes. We focus on the little-known succulent biome, a trans-continentally distributed assemblage of succulent-rich, drought-deciduous, fire-free forest, thicket and scrub vegetation as a useful exemplar biome to gain insights into these questions. Location: Global lowland (sub)tropics. Time period: Present. Major taxa studied: Angiosperms. Methods: We use a model ensemble approach to model the distribution of 884 species of stem succulents, a plant functional group representing a striking example of evolutionary convergence. Using this model, phylogenies, and species occurrence data, we quantify phylogenetic succulent biome conservatism for 10 non-succulent trans-continental plant clades including prominent elements of the succulent biome, representing over 800 species. Results: The geographical and climatic distributions of stem succulents provide an objective and quantitative proxy for mapping the distribution of the succulent biome. High fractions of succulent biome occupancy across continents suggest all 10 non-succulent study clades are phylogenetically conserved within the succulent biome. Main conclusions: The trans-continental succulent and savanna biomes both show evolutionary convergence in key biome-related plant functional traits. However, in contrast to the savanna biome, which was apparently assembled via repeated local recruitment of lineages via biome shifts from adjacent biomes within continents, the succulent biome forms a coherent trans-continental evolutionary arena for drought-adapted tropical biome conserved lineages. Recognizing the important functional differences between the succulent-rich, grass-poor, fire-free succulent biome and the grass-dominated, succulent-poor, fire-prone savanna biome, and defining them as distinct seasonally dry tropical biomes, occupying essentially non-overlapping distributions, provides critical insights into tropical biodiversity and the extent of biome stasis versus biome shifting.
Climate change and European forests: What do we know, what are the uncertainties, and what are the implications for forest management?
(Elsevier BV, 2014-12) Lindner, Marcus; Fitzgerald, Joanne B.; Zimmermann, Niklaus E.; Reyer, Christopher; Delzon, Sylvain; van der Maaten, Ernst; Schelhaas, Mart-Jan; Lasch, Petra; Eggers, Jeanette; van der Maaten-Theunissen, Marieke; Suckow, Felicitas; Psomas, Achilleas; Poulter, Benjamin; Hanewinkel, Marc
The knowledge about potential climate change impacts on forests is continuously expanding and some changes in growth, drought induced mortality and species distribution have been observed. However despite a signiﬁcant body of research, a knowledge and communication gap exists between scientists and non-scientists as to how climate change impact scenarios can be interpreted and what they imply for European forests. It is still challenging to advise forest decision makers on how best to plan for climate change as many uncertainties and unknowns remain and it is difﬁcult to communicate these to practitioners and other decision makers while retaining emphasis on the importance of planning for adaptation. In this paper, recent developments in climate change observations and projections, observed and projected impacts on European forests and the associated uncertainties are reviewed and synthesised with a view to understanding the implications for forest management. Current impact assessments with simulation models contain several simpliﬁcations, which explain the discrepancy between results of many simulation studies and the rapidly increasing body of evidence about already observed changes in forest productivity and species distribution. In simulation models uncertainties tend to cascade onto one another; from estimating what future societies will be like and general circulation models (GCMs) at the global level, down to forest models and forest management at the local level. Individual climate change impact studies should not be uncritically used for decision-making without reﬂection on possible shortcomings in system understanding, model accuracy and other assumptions made. It is important for decision makers in forest management to realise that they have to take long-lasting management decisions while uncertainty about climate change impacts are still large. We discuss how to communicate about uncertainty e which is imperative for decision making e without diluting the overall message. Considering the range of possible trends and uncertainties in adaptive forest management requires expert knowledge and enhanced efforts for providing science-based decision support.
Emerging role of wetland methane emissions in driving 21st century climate change
(2017-09) Zhang, Zhen; Zimmermann, Niklaus E.; Stenke, Andrea; Li, Xin; Hodson, Elke L.; Zhu, Gaofeng; Huang, Chunlin; Poulter, Benjamin
Wetland methane (CH4) emissions are the largest natural source in the global CH4 budget, contributing to roughly one third of total natural and anthropogenic emissions. As the second most important anthropogenic greenhouse gas in the atmosphere after CO2, CH4 is strongly associated with climate feedbacks. However, due to the paucity of data, wetland CH4 feedbacks were not fully assessed in the Intergovernmental Panel on Climate Change Fifth Assessment Report. The degree to which future expansion of wetlands and CH4 emissions will evolve and consequently drive climate feedbacks is thus a question of major concern. Here we present an ensemble estimate of wetland CH4 emissions driven by 38 general circulation models for the 21st century. We find that climate change-induced increases in boreal wetland extent and temperature-driven increases in tropical CH4 emissions will dominate anthropogenic CH4 emissions by 38 to 56% toward the end of the 21st century under the Representative Concentration Pathway (RCP2.6). Depending on scenarios, wetland CH4 feedbacks translate to an increase in additional global mean radiative forcing of 0.04W.m(-2) to 0.19W.m(-2) by the end of the 21st century. Under the \worst-case\" RCP8.5 scenario, with no climate mitigation, boreal CH4 emissions are enhanced by 18.05 Tg to 41.69 Tg, due to thawing of inundated areas during the cold season (December to May) and rising temperature, while tropical CH4 emissions accelerate with a total increment of 48.36 Tg to 87.37 Tg by 2099. Our results suggest that climate mitigation policies must consider mitigation of wetland CH4 feedbacks to maintain average global warming below 2 degrees C.
The geography of climate and the global patterns of species diversity
(Springer Science and Business Media LLC, 2023-09) Coelho, Marco Túlio P.; Barreto, Marco Túlio P.; Barreto, Elisa; Rangel, Thiago F.; Diniz-Filho, José Alexandre F.; Wüest, Rafael O.; Bach, Wilhelmine; Skeels, Alexander; McFadden, Ian R.; Roberts, David W.; Pellissier, Loïc; Zimmermann, Niklaus E.; Graham, Catherine H.
Climate’s effect on global biodiversity is typically viewed through the lens of temperature, humidity and resulting ecosystem productivity1,2,3,4,5,6. However, it is not known whether biodiversity depends solely on these climate conditions, or whether the size and fragmentation of these climates are also crucial. Here we shift the common perspective in global biodiversity studies, transitioning from geographic space to a climate-defined multidimensional space. Our findings suggest that larger and more isolated climate conditions tend to harbour higher diversity and species turnover among terrestrial tetrapods, encompassing more than 30,000 species. By considering both the characteristics of climate itself and its geographic attributes, we can explain almost 90% of the variation in global species richness. Half of the explanatory power (45%) may be attributed either to climate itself or to the geography of climate, suggesting a nuanced interplay between them. Our work evolves the conventional idea that larger climate regions, such as the tropics, host more species primarily because of their size7,8. Instead, we underscore the integral roles of both the geographic extent and degree of isolation of climates. This refined understanding presents a more intricate picture of biodiversity distribution, which can guide our approach to biodiversity conservation in an ever-changing world.
Sensitivity of global terrestrial carbon cycle dynamics to variability in satellite-observed burned area
(2015-02) Poulter, Benjamin; Cadule, Patricia; Chelney, Audrey; Ciais, Philippe; Hodson, Elke L.; Peylin, Phili; Plummer, Stephen; Spessa, Allan; Saatchi, Sassan S.; Yue, Chao; Zimmermann, Niklaus E.
Fire plays an important role in terrestrial ecosystems by regulating biogeochemistry, biogeography, and energy budgets, yet despite the importance of fire as an integral ecosystem process, significant advances remain to improve its prognostic representation in carbon cycle models. To recommend and to help prioritize model improvements, this study investigates the sensitivity of a coupled global biogeography and biogeochemistry model, LPJ, to observed burned area measured by three independent satellite-derived products, GFED v3.1, L3JRC, and GlobCarbon. Model variables are compared with benchmarks that include pantropical aboveground biomass, global tree cover, and CO2 and CO trace gas concentrations. Depending on prescribed burned area product, global aboveground carbon stocks varied by 300 Pg C, and woody cover ranged from 50 to 73 Mkm2. Tree cover and biomass were both reduced linearly with increasing burned area, i.e., at regional scales, a 10% reduction in tree cover per 1000 km2, and 0.04-to-0.40 Mg C reduction per 1000 km2. In boreal regions, satellite burned area improved simulated tree cover and biomass distributions, but in savanna regions, model-data correlations decreased. Global net biome production was relatively insensitive to burned area, and the long-term land carbon sink was robust, ~2.5 Pg C yr−1, suggesting that feedbacks from ecosystem respiration compensated for reductions in fuel consumption via fire. CO2 transport provided further evidence that heterotrophic respiration compensated any emission reductions in the absence of fire, with minor differences in modeled CO2 fluxes among burned area products. CO was a more sensitive indicator for evaluating fire emissions, with MODIS-GFED burned area producing CO concentrations largely in agreement with independent observations in high latitudes. This study illustrates how ensembles of burned area data sets can be used to diagnose model structures and parameters for further improvement and also highlights the importance in considering uncertainties and variability in observed burned area data products for model applications.
Water-use efficiency & transpiration across European forests during the Anthropocene
(2015-05) Frank, D.C.; Poulter, Benjamin; Saurer, M.; Esper, J.; Huntingford, C.; Helle, G.; Treydte, K.; Zimmermann, Niklaus E.; Schleser, G.H.; Ahlstrom, A.; Ciais, Philippe
The Earth’s carbon and hydrologic cycles are intimately coupled by gas exchange through plant stomata1, 2, 3. However, uncertainties in the magnitude4, 5, 6 and consequences7, 8 of the physiological responses9, 10 of plants to elevated CO2 in natural environments hinders modelling of terrestrial water cycling and carbon storage11. Here we use annually resolved long-term δ13C tree-ring measurements across a European forest network to reconstruct the physiologically driven response of intercellular CO2 (Ci) caused by atmospheric CO2 (Ca) trends. When removing meteorological signals from the δ13C measurements, we find that trees across Europe regulated gas exchange so that for one ppmv atmospheric CO2 increase, Ci increased by ~0.76 ppmv, most consistent with moderate control towards a constant Ci/Ca ratio. This response corresponds to twentieth-century intrinsic water-use efficiency (iWUE) increases of 14 ± 10 and 22 ± 6% at broadleaf and coniferous sites, respectively. An ensemble of process-based global vegetation models shows similar CO2 effects on iWUE trends. Yet, when operating these models with climate drivers reintroduced, despite decreased stomatal opening, 5% increases in European forest transpiration are calculated over the twentieth century. This counterintuitive result arises from lengthened growing seasons, enhanced evaporative demand in a warming climate, and increased leaf area, which together oppose effects of CO2-induced stomatal closure. Our study questions changes to the hydrological cycle, such as reductions in transpiration and air humidity, hypothesized to result from plant responses to anthropogenic emissions.