Vegetation anomalies caused by antecedent precipitation in most of the world

Quantifying environmental controls on vegetation is critical to predict the net effect of climate change on global ecosystems and the subsequent feedback on climate. Following a non-linear Granger causality framework based on a random forest predictive model, we exploit the current wealth of multi-decadal satellite data records to uncover the main drivers of monthly vegetation variability at the global scale. Results indicate that water availability is the most dominant factor driving vegetation globally: about 61% of the vegetated surface was primarily water-limited during 1981-2010. This included semiarid climates but also transitional ecoregions. Intraannually, temperature controls Northern Hemisphere deciduous forests during the growing season, while antecedent precipitation largely dominates vegetation dynamics during the senescence period. The uncovered dependency of global vegetation on water availability is substantially larger than previously reported. This is owed to the ability of the framework to (1) disentangle the co-linearities between radiation/temperature and precipitation, and (2) quantify non-linear impacts of climate on vegetation. Our results reveal a prolonged effect of precipitation anomalies in dry regions: due to the long memory of soil moisture and the cumulative, nonlinear, response of vegetation, water-limited regions show sensitivity to the values of precipitation occurring three months earlier. Meanwhile, the impacts of temperature and radiation anomalies are more immediate and dissipate shortly, pointing to a higher resilience of vegetation to these anomalies. Despite being infrequent by definition, hydro-climatic extremes are responsible for up to 10% of the vegetation variability during the 1981-2010 period in certain areas, particularly in water-limited ecosystems. Our approach is a first step towards a quantitative comparison of the resistance and resilience signature of different ecosystems, and can be used to benchmark Earth system models in their representations of past vegetation sensitivity to changes in climate

Impact of droughts on the carbon cycle in European vegetation : a probabilistic risk analysis using six vegetation models

Peer reviewedPublisher PD

Hydroclimate variability from western Iberia (Portugal) during the Holocene: insights from a composite stalagmite isotope record

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Thatcher, D. L., Wanamaker, A. D., Denniston, R. F., Asmerom, Y., Polyak, V. J., Fullick, D., Ummenhofer, C. C., Gillikin, D. P., & Haws, J. A. Hydroclimate variability from western Iberia (Portugal) during the Holocene: insights from a composite stalagmite isotope record. Holocene, (2020): 095968362090864, doi:10.1177/0959683620908648.Iberia is predicted under future warming scenarios to be increasingly impacted by drought. While it is known that this region has experienced multiple intervals of enhanced aridity over the Holocene, additional hydroclimate-sensitive records from Iberia are necessary to place current and future drying into a broader perspective. Toward that end, we present a multi-proxy composite record from six well-dated and overlapping speleothems from Buraca Gloriosa (BG) cave, located in western Portugal. The coherence between the six stalagmites in this composite stalagmite record illustrates that climate (not in-cave processes) impacts speleothem isotopic values. This record provides the first high-resolution, precisely dated, terrestrial record of Holocene hydroclimate from west-central Iberia. The BG record reveals that aridity in western Portugal increased secularly from 9.0 ka BP to present, as evidenced by rising values of both carbon (δ13C) and oxygen (δ18O) stable isotope values. This trend tracks the decrease in Northern Hemisphere summer insolation and parallels Iberian margin sea surface temperatures (SST). The increased aridity over the Holocene is consistent with changes in Hadley Circulation and a southward migration of the Intertropical Convergence Zone (ITCZ). Centennial-scale shifts in hydroclimate are coincident with changes in total solar irradiance (TSI) after 4 ka BP. Several major drying events are evident, the most prominent of which was centered around 4.2 ka BP, a feature also noted in other Iberian climate records and coinciding with well-documented regional cultural shifts. Substantially, wetter conditions occurred from 0.8 ka BP to 0.15 ka BP, including much of the ‘Little Ice Age’. This was followed by increasing aridity toward present day. This composite stalagmite proxy record complements oceanic records from coastal Iberia, lacustrine records from inland Iberia, and speleothem records from both northern and southern Spain and depicts the spatial and temporal variability in hydroclimate in Iberia.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported, in part, by the US National Science Foundation (Grants: #1804528 to ADW; #1804635 to RD; #1804132 to CCU; #1806025 to YA and VP; #1805163 to DPG; BCS-0455145, BCS-0612923, and BCS-1118155 to JAH)

A non-linear Granger-causality framework to investigate climate-vegetation dynamics

Satellite Earth observation has led to the creation of global climate data records of many important environmental and climatic variables. These come in the form of multivariate time series with different spatial and temporal resolutions. Data of this kind provide new means to further unravel the influence of climate on vegetation dynamics. However, as advocated in this article, commonly used statistical methods are often too simplistic to represent complex climate-vegetation relationships due to linearity assumptions. Therefore, as an extension of linear Granger-causality analysis, we present a novel non-linear framework consisting of several components, such as data collection from various databases, time series decomposition techniques, feature construction methods, and predictive modelling by means of random forests. Experimental results on global data sets indicate that, with this framework, it is possible to detect non-linear patterns that are much less visible with traditional Granger-causality methods. In addition, we discuss extensive experimental results that highlight the importance of considering non-linear aspects of climate-vegetation dynamics

Soil respiration in a northeastern US temperate forest: a 22‐year synthesis

To better understand how forest management, phenology, vegetation type, and actual and simulated climatic change affect seasonal and inter‐annual variations in soil respiration (Rs), we analyzed more than 100,000 individual measurements of soil respiration from 23 studies conducted over 22 years at the Harvard Forest in Petersham, Massachusetts, USA. We also used 24 site‐years of eddy‐covariance measurements from two Harvard Forest sites to examine the relationship between soil and ecosystem respiration (Re). Rs was highly variable at all spatial (respiration collar to forest stand) and temporal (minutes to years) scales of measurement. The response of Rs to experimental manipulations mimicking aspects of global change or aimed at partitioning Rs into component fluxes ranged from −70% to +52%. The response appears to arise from variations in substrate availability induced by changes in the size of soil C pools and of belowground C fluxes or in environmental conditions. In some cases (e.g., logging, warming), the effect of experimental manipulations on Rs was transient, but in other cases the time series were not long enough to rule out long‐term changes in respiration rates. Inter‐annual variations in weather and phenology induced variation among annual Rs estimates of a magnitude similar to that of other drivers of global change (i.e., invasive insects, forest management practices, N deposition). At both eddy‐covariance sites, aboveground respiration dominated Re early in the growing season, whereas belowground respiration dominated later. Unusual aboveground respiration patterns—high apparent rates of respiration during winter and very low rates in mid‐to‐late summer—at the Environmental Measurement Site suggest either bias in Rs and Re estimates caused by differences in the spatial scale of processes influencing fluxes, or that additional research on the hard‐to‐measure fluxes (e.g., wintertime Rs, unaccounted losses of CO2 from eddy covariance sites), daytime and nighttime canopy respiration and its impacts on estimates of Re, and independent measurements of flux partitioning (e.g., aboveground plant respiration, isotopic partitioning) may yield insight into the unusually high and low fluxes. Overall, however, this data‐rich analysis identifies important seasonal and experimental variations in Rs and Re and in the partitioning of Re above‐ vs. belowground

Beyond a warming fingerprint: individualistic biogeographic responses to heterogeneous climate change in California.

Understanding recent biogeographic responses to climate change is fundamental for improving our predictions of likely future responses and guiding conservation planning at both local and global scales. Studies of observed biogeographic responses to 20th century climate change have principally examined effects related to ubiquitous increases in temperature - collectively termed a warming fingerprint. Although the importance of changes in other aspects of climate - particularly precipitation and water availability - is widely acknowledged from a theoretical standpoint and supported by paleontological evidence, we lack a practical understanding of how these changes interact with temperature to drive biogeographic responses. Further complicating matters, differences in life history and ecological attributes may lead species to respond differently to the same changes in climate. Here, we examine whether recent biogeographic patterns across California are consistent with a warming fingerprint. We describe how various components of climate have changed regionally in California during the 20th century and review empirical evidence of biogeographic responses to these changes, particularly elevational range shifts. Many responses to climate change do not appear to be consistent with a warming fingerprint, with downslope shifts in elevation being as common as upslope shifts across a number of taxa and many demographic and community responses being inconsistent with upslope shifts. We identify a number of potential direct and indirect mechanisms for these responses, including the influence of aspects of climate change other than temperature (e.g., the shifting seasonal balance of energy and water availability), differences in each taxon's sensitivity to climate change, trophic interactions, and land-use change. Finally, we highlight the need to move beyond a warming fingerprint in studies of biogeographic responses by considering a more multifaceted view of climate, emphasizing local-scale effects, and including a priori knowledge of relevant natural history for the taxa and regions under study

The response of global terrestrial photosynthesis to rising CO2

In this dissertation I examine how photosynthesis on land has re- sponded to rising CO2 concentration in recent decades, and how we can use this knowledge to better predict the evolution of the climate- carbon system throughout the 21st century. More than three decades of satellite data reveal widespread and per- sistent changes in Earth’s ecosystems. The drivers underlying these changes and the implications for the terrestrial sink of anthropogenic carbon emissions are controversial. In the first part of this thesis, I examine a long-term satellite record of global leaf area observations (1981–2017) and identify clusters of significant change on the biome level. Using process-based models and a framework relying on causal theory, I disentangle and attribute vegetation changes to CO2-induced climatic changes and the CO2 fer- tilization effect. I show that 40% of Earth’s naturally vegetated surface is greening, predominately in the extratropics, and 14% is browning, mostly in the tropics. Although previous studies attributed the green- ing to CO2 fertilization, I show that only some biomes show a marked response to this effect, whereas many biomes bear the signature of climatic changes, i.e. warming and rainfall anomalies. The leaf area loss in the tropical forests due to increased droughts and long-term drying could be an early indicator of a slow-down in the terrestrial carbon sink. In the second part, I examine if the observed vegetation response to rising CO2 can be used to reduce uncertainty in the evolution of the carbon cycle. Using an approach called Emergent Constraint (EC), I combine satellite observations and multi-model simulations to de- rive an estimate for the increase in photosynthetic carbon fixation of northern ecosystems for 2×CO2 (3.4 ± 0.2 Pg C yr−1). Three compara- ble independent estimates from CO2 measurements and atmospheric inversions corroborate this result. The EC estimate is considerably larger than most model projections which suggests that the effect of rising CO2 concentration on photosynthesis in northern terrestrial ecosystems is underestimated. In the third part, I investigate the applicability of the EC method in a broader context of Earth system sciences. More and more EC estimates are being reported, however their robustness is controversial. By means of a thought experiment and analyses of a multi-model ensemble, I address the main caveats and highlight limitations as well as potential sources of uncertainty in the application of the EC method. All parts in this thesis highlight how the variety of observational data and the strength of process-based models in conjunction with new methods advance the understanding of the terrestrial biosphere.In dieser Dissertation untersuche ich, wie die terrestrische Photosyn- these auf die steigende CO2-Konzentration der letzten Jahrzehnte reagiert hat und wie wir dieses Wissen nutzen können, um die Ent- wicklung des Klima-Kohlenstoff-Systems im 21. Jahrhundert besser vorherzusagen. Satellitendaten aus mehr als drei Jahrzehnten zeigen, dass sich die Ökosysteme der Erde in großem Maßstab verändert haben. Die Treiber, die diesen Veränderungen zugrunde liegen und die damit verbunde- nen Auswirkungen auf die Landsenke von anthropogenen Kohlenstof- femissionen, sind umstritten. Im ersten Teil dieser Arbeit untersuche ich Langzeit-Satellitenbeo- bachtungen der globalen Vegetation (Blattfläche, 1981–2017). Unter Berücksichtigung verschiedener Biome identifiziere ich Regionen, die sich signifikant verändert haben. Mit Hilfe von prozessbasierten Mo- dellen und Kausaltheorie trenne ich die Auswirkungen des CO2- verursachten Düngungs- und Treibhauseffekts auf die Vegetation auf. Ich zeige, dass 40% der natürlich bewachsenen Erdoberfläche ergrünt, vornehmlich in den Extratropen, und 14% erbraunen, hauptsächlich in den Tropen. Frühere Studien haben den CO2-Düngungseffekt als Haupttreiber der Ergrünung identifiziert. Meine Analyse zeigt, dass nur einige Biome eine deutliche Reaktion auf diesen Effekt zeigen und dass Klimaveränderungen (Erwärmung und Niederschlagsanomalien) in vielen Ökosystemen eine stärkere Auswirkung haben. Der Verlust von Blattfläche in den Tropenwäldern durch häufiger auftretender Dürren und/oder stetigen Niederschlagsrückgang könnte ein Vorbote einer Abschwächung der terrestrischen Kohlenstoffsenke sein. Im zweiten Teil untersuche ich, ob die beobachteten Veränderungen in der Vegetation als Folge der steigenden CO2 Konzentration herange- zogen werden können, um die Entwicklung des globalen Kohlenstoff- kreislaufs besser abschätzen zu können. Unter der Zuhilfenahme von Emergent Constraints (EC) kombiniere ich Satellitenbeobachtungen und Multi-Modell-Simulationen, um die Zunahme der photosynthe- tischen Kohlenstoffaufname der Ökosysteme in den hohen Breiten für 2×CO2 vorherzusagen (3.4 ± 0.2 Pg C yr−1). Drei unabhängi- ge, vergleichbare Schätzungen, die auf CO2-Messungen und atmo- sphärischen Inversionen basieren, untermauern dieses Ergebnis. Die EC-basierte Schätzung ist wesentlich höher als die meisten Modell- vorhersagen, was darauf hindeutet, dass der Einfluss der steigenden CO2-Konzentration auf die Photosynthese der nördlichen terrestri- schen Ökosysteme unterschätzt werden könnte. Im dritten Teil untersuche ich die Anwendbarkeit der EC-Methode im allgemeinen Kontext der Erdsystemwissenschaften. Es werden mehr und mehr EC-Studien durchgeführt, die Ergebnisse sind jedoch kon- trovers. Anhand eines Gedankenexperiments und der Analyse eines Multi-Modell-Ensembles untersuche ich die wichtigsten Kritikpunkte und zeige mögliche Unsicherheitsquellen in der Anwendung der EC- Methode auf. Die vorliegende Dissertation zeigt, wie die Vielzahl an Beobachtungs- datensätzen und prozessbasierten Modellen mit neuen Methoden zum besseren Verständnis der terrestrischen Biosphäre verknüpft werden kann