Satellite remote sensing of vegetation dynamics in the context of climate change

Vegetation is a key component of the Earth's climate system. Understanding vegetation dynamics in a changing climate requires both in situ and remote sensing data. Satellite remote sensing is especially indispensible for continuous monitoring of vegetation over large areas. This dissertation is focused on investigation of vegetation dynamics in the broader context of climate change using satellite data over two critical regions: the arctic-boreal area in the northern high latitudes and Amazonia in South America. The northern high latitudes have experienced amplified warming. We found the response of the arctic-boreal vegetation to this warming to be different between North America and Eurasia during a 30-year period since 1982: the relationship between vegetation green-up and temperature rise was stable over Eurasia, but in North America, the amount of vegetation green-up per unit amount of warming has decreased since the beginning of 21st century. This could partly be explained by the unmatched northward movements of temperature and precipitation patterns in North America. The Amazonian rainforests have highly dense canopies of green leaves. In such dense media, reflection of solar radiation tends to saturate. Thus, the satellite measurements are weakly sensitive to vegetation changes. At the same time, the data are strongly influenced by changing sun-sensor geometry. This makes it difficult to discriminate between vegetation changes and sun-sensor geometry effects. We developed a new physically based approach to detect changes in dense forests. Analyses of several years of data from three sensors on two satellites under a range of sun-sensor geometries provide robust evidence for a sunlight driven seasonal cycle in structure and greenness of Amazonian rainforests. The 2005 and 2010 dry-season droughts decreased the photosynthetic activity of Amazonian rainforests. We demonstrate that satellite data capture such decreases. Furthermore, we show that in 2004 and 2007, when there was lower wet-season water abundance compared to normal years, the photosynthetic activity of Amazonian forests also decreased. Potentially frequent water deficits over Amazon in the future, irrespective of whether they occur in the dry or wet season, will decrease the photosynthetic activity of Amazonian forests, and provide a positive feedback to global warming

Changes in growing season duration and productivity of northern vegetation inferred from long-term remote sensing data

Monitoring and understanding climate-induced changes in the boreal and arctic vegetation is critical to aid in prognosticating their future. Weused a 33 year (1982-2014) long record of satellite observations to robustly assess changes in metrics of growing season (onset: SOS, end: EOS and length: LOS) and seasonal total gross primary productivity. Particular attention was paid to evaluating the accuracy of these metrics by comparing them to multiple independent direct and indirect growing season and productivity measures. These comparisons reveal that the derived metrics capture the spatio-temporal variations and trends with acceptable significance level (generally p < 0.05). We find that LOS has lengthened by 2.60 d dec(-1) (p < 0.05) due to an earlier onset of SOS (-1.61 d dec(-1), p < 0.05) and a delayed EOS (0.67 d dec(-1), p < 0.1) at the circumpolar scale over the past three decades. Relatively greater rates of changes in growing season were observed in Eurasia (EA) and in boreal regions than in North America (NA) and the arctic regions. However, this tendency of earlier SOS and delayed EOS was prominent only during the earlier part of the data record (1982-1999). During the later part (2000-2014), this tendency was reversed, i.e. delayed SOS and earlier EOS. As for seasonal total productivity, we find that 42.0% of northern vegetation shows a statistically significant (p < 0.1) greening trend over the last three decades. This greening translates to a 20.9% gain in productivity since 1982. In contrast, only 2.5% of northern vegetation shows browning, or a 1.2% loss of productivity. These trends in productivity were continuous through the period of record, unlike changes in growing season metrics. Similarly, we find relatively greater increasing rates of productivity in EA and in arctic regions than in NA and the boreal regions. These results highlight spatially and temporally varying vegetation dynamics and are reflective of biome-specific responses of northern vegetation during last three decades

Monitoring global vegetation dynamics with coarse and moderate resolution satellite data

Earth's annual average temperature has increased by about 0.6°C during the past three decades. This warming pulse has brought many changes in the climatic system. For example, the Amazon forests of South America experienced frequent droughts possibly from altered air-sea interaction patterns in the Pacific and Atlantic oceans. The response of vegetation to this unprecedented rate of warming is the subject of this dissertation. Vegetation greenness levels, a surrogate of vegetation photosynthetic activity, recorded by satellite-borne sensors offer repetitive synoptic views of the Earth's vegetation. This period of extraordinary warming coincided with the availability of multiple data sets of vegetation greenness levels from different satellites, thus providing an unique opportunity to assess the impact of warming on vegetation. The Amazon region has suffered two severe droughts during this decade - the so-called "once-in-a-century" drought in 2005 and an even stronger drought in 2010. Vegetation browning during the 2010 drought was four times greater than that in 2005 (2.4 million km^2). Notably, 51% of all drought-stricken forests showed browning in 2010 (1.68 million km^2) compared to only 14% in 2005 (0.32 million km^2). This large-scale decline in vegetation greenness denotes significant loss of photosynthetic capacity of Amazonian vegetation and thus a major perturbation to the global carbon cycle. In the northern latitudes (>50°N), vegetation seasonality (SV) is tightly coupled to temperature seasonality (ST). As ST diminished, so did SV. The observed declines of ST and SV are equivalent to 4 and 7° (5 and 6°) latitudinal shifts equatorward during the past 30 years in the Arctic (Boreal) region. Analysis of simulations from 17 state-of-the-art climate models indicates an additional ST diminishment equivalent to a 20&deg; equatorward shift this century. How SV will change in response to such large projected ST declines is not well understood. Hence there is a need for continued monitoring of northern lands as their seasonal temperature profiles evolve to resemble those further south. The results presented in this dissertation provide a better understanding of the impact of recent warming on three pristine ecosystems - the Amazonian forests, and the Arctic and Boreal ecosystems

Aspects of interannual climate variability

Interannual climate variability during the northern summer season has been investigated in this study. After Walker\u27s pioneering work (Walker and Bliss 1932, 1937), many previous studies have documented and discussed the structure and the dynamical/thermodynamical causes (e.g., Bjerkness 1969). However, relatively less attention have been devoted on the summer climate variability. Thus, two different aspects of the interannual climate variability during the northern summer season have been discussed. One is the interannual variation of the boreal-forest rainbelts, and the other is the interannual variation of the North American monsoon rainfall. Also, the summer climatological aspect of the boreal-forest rainbelts from a hydrological and dynamical perspectives was presented in prior to its interannual variability.;The boreal forests comprise one third of the global woodlands, while the warm-season runoff from the boreal-forest rainbelts provide a major amount of freshwater to the Arctic Ocean. It is shown the boreal-forest rainbelts are maintained by the convergence of water vapor by transients along these rainbelts and the interannual variation of these rainbelts is caused by the collective response of these rainbelts to the North Atlantic Oscillation and the East-Asian teleconnection monsoon pattern in Eurasia and the Nitta-like short-wave train and the North Atlantic Oscillation in the Alaska-Canadian subarctic region. It was hypothesized by our recent study that the North American Monsoon (including both the Mexican and the Southwest U.S. monsoon) is maintained by the east-west differential heating between the Western Tropical Atlantic heating and the Eastern Tropical Pacific cooling. Diagnostic analysis with NCEP/NCAR reanalysis data in this study substantiated this hypothesis. Our current study has primarily focused on diagnostic analysis of observations and reanalysis. Further analysis with the global climate model such as NCAR CAM or NASA NSIPP and the regional climate model is suggested to further substantiate our hypothesis proposed in this study

Toward a better understanding of changes in Northern vegetation using long-term remote sensing data

Cascading consequences of recent changes in the physical environment of northern lands associated with rapid warming have affected a broad range of ecosystem processes, particularly, changes in structure, composition, and functioning of vegetation. Incomplete understanding of underlying processes driving such changes is the primary motivation for this research. We report here the results of three studies that use long-term remote sensing data to advance our knowledge of spatiotemporal changes in growing season, greenness and productivity of northern vegetation. First, we improve the remote sensing-based detection of growing season by fusing vegetation greenness, snow and soil freeze/thaw condition. The satellite record reveals extensive lengthening trends of growing season and enhanced annual total greenness during the last three decades. Regionally varying seasonal responses are linked to local climate constraints and their relaxation. Second, we incorporate available land surface histories including disturbances and human land management practices to understand changes in remotely sensed vegetation greenness. This investigation indicates that multiple drivers including natural (wildfire) and anthropogenic (harvesting) disturbances, changing climate and agricultural activities govern the large-scale greening trends in northern lands. The timing and type of disturbances are important to fully comprehend spatially uneven vegetation changes in the boreal and temperate regions. In the final part, we question how photosynthetic seasonality evolved into its current state, and what role climatic constraints and their variability played in this process and ultimately in the carbon cycle. We take the ‘laws of minimum’ as a basis and introduce a new framework where the timing of peak photosynthetic activity (DOYPmax) acts as a proxy for plants adaptive state to climatic constraints on their growth. The result shows a widespread warming-induced advance in DOYPmax with an increase of total gross primary productivity across northern lands, which leads to an earlier phase shift in land-atmosphere carbon fluxes and an increase in their amplitude. The research presented in this dissertation suggests that understanding past, present and likely future changes in northern vegetation requires a multitude of approaches that consider linked climatic, social and ecological drivers and processes

Responses of boreal vegetation to recent climate change

The high northern latitudes have warmed faster than anywhere else in the globe during the past few decades. Boreal ecosystems are responding to this rapid climatic change in complex ways and some times contrary to expectations, with large implications for the global climate system. This thesis investigates how boreal vegetation has responded to recent climate change, particularly to the lengthening of the growing season and changes in drought severity with warming. The links between the timing of the growing season and the seasonal cycle of atmospheric CO2 are evaluated in detail to infer large-scale ecosystem responses to changing seasonality and extended period of plant growth. The influence of warming on summer drought severity is estimated at a regional scale for the first time using improved data. The results show that ecosystem responses to warming and lengthening of the growing season in autumn are opposite to those in spring. Earlier springs are associated with earlier onset of photosynthetic uptake of atmospheric CO2 by northern vegetation, whereas a delayed autumn, rather than being associated with prolonged photosynthetic uptake, is associated with earlier ecosystem carbon release to the atmosphere. Moreover, the photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn. Rapid warming since the late 1980s has increased evapotranspiration demand and consequently summer and autumn drought severity, offsetting the effect of increasing cold-season precipitation. This is consistent with ongoing amplification of the hydrological cycle and with model projections of summer drying at northern latitudes in response to anthropogenic warming. However, changes in snow dynamics (accumulation and melting) appear to be more important than increased evaporative demand in controlling changes in summer soil moisture availability and vegetation photosynthesis across extensive regions of the boreal zone, where vegetation growth is often assumed to be dominantly temperature-limited. Snow-mediated moisture controls of vegetation growth are particularly significant in northwestern North America. In this region, a non-linear growth response of white spruce growth to recent warming at high elevations was observed. Taken together, these results indicate that net observed responses of northern ecosystems to warming involve significant seasonal contrasts, can be non-linear and are mediated by moisture availability in about a third of the boreal zone

Northern Eurasia Future Initiative (NEFI): facing the challenges and pathways of global change in the twenty-first century

During the past several decades, the Earth system has changed significantly, especially across Northern Eurasia. Changes in the socio-economic conditions of the larger countries in the region have also resulted in a variety of regional environmental changes that can have global consequences. The Northern Eurasia Future Initiative (NEFI) has been designed as an essential continuation of the Northern Eurasia Earth Science Partnership Initiative (NEESPI), which was launched in 2004. NEESPI sought to elucidate all aspects of ongoing environmental change, to inform societies and, thus, to better prepare societies for future developments. A key principle of NEFI is that these developments must now be secured through science-based strategies co-designed with regional decision-makers to lead their societies to prosperity in the face of environmental and institutional challenges. NEESPI scientific research, data, and models have created a solid knowledge base to support the NEFI program. This paper presents the NEFI research vision consensus based on that knowledge. It provides the reader with samples of recent accomplishments in regional studies and formulates new NEFI science questions. To address these questions, nine research foci are identified and their selections are briefly justified. These foci include warming of the Arctic; changing frequency, pattern, and intensity of extreme and inclement environmental conditions; retreat of the cryosphere; changes in terrestrial water cycles; changes in the biosphere; pressures on land use; changes in infrastructure; societal actions in response to environmental change; and quantification of Northern Eurasia’s role in the global Earth system. Powerful feedbacks between the Earth and human systems in Northern Eurasia (e.g., mega-fires, droughts, depletion of the cryosphere essential for water supply, retreat of sea ice) result from past and current human activities (e.g., large-scale water withdrawals, land use, and governance change) and potentially restrict or provide new opportunities for future human activities. Therefore, we propose that integrated assessment models are needed as the final stage of global change assessment. The overarching goal of this NEFI modeling effort will enable evaluation of economic decisions in response to changing environmental conditions and justification of mitigation and adaptation efforts

Impact of changes in GRACE derived terrestrial water storage on vegetation growth in Eurasia

We use GRACE-derived terrestrial water storage (TWS) and ERA-interim air temperature, as proxy for available water and temperature constraints on vegetation productivity, inferred from MODIS satellite normalized difference vegetation index (NDVI), in Northern Eurasia during 2002–2011. We investigate how changes in TWS affect the correlation between NDVI and temperature during the non-frozen season. We find that vegetation growth exhibits significant spatial and temporal variability associated with varying trend in TWS and temperature. The largest NDVI gains occur over boreal forests associated with warming and wetting. The largest NDVI losses occur over grasslands in the Southwestern Ob associated with regional drying and cooling, with dominant constraint from TWS. Over grasslands and temperate forests in the Southeast Ob and South Yenisei, wetting and cooling lead to a dominant temperature constraint due to the relaxation of TWS constraints. Overall, we find significant monthly correlation of NDVI with TWS and temperature over 35% and 50% of the domain, respectively. These results indicate that water availability (TWS) plays a major role in modulating Eurasia vegetation response to temperature changes

Genetic legacies of past climate change on Arctic species: how past responses shape future impacts

Major ecosystem changes are under way in the rapidly warming Arctic region. Sea ice loss and tundra shrub expansion are leading to ecological impacts across multiple biological, spatial, and temporal scales. The distribution and population dynamics of reindeer/caribou (Rangifer tarandus L.) — the most numerous and widespread large herbivore in the Arctic — and the dwarf birches (Betula nana L. and Betula glandulosa Michx.) — dominant tundra shrubs — will be affected. Understanding how these species responded to rapid and large-scale climate and sea ice changes in the past will increase our understanding of the long-term ecological and evolutionary implications of anthropogenic climate change. The impacts of past climate change on species leave genetic imprints in their living descendants, which, in turn, influence their genetic variation and capacity to adapt to future changes. In this thesis, I aim to uncover how climate fluctuations in the Quaternary period (2.6 million years ago - present) shaped the population history of reindeer and the dwarf birches, to improve our understanding of ecological and evolutionary responses to past climate change. I analyse the contemporary genetic variation of these panarctic species in a phylogeographic approach. I model their population history and compare timings of inferred population events with those of climate and environmental changes using paleoenvironmental data. I then explore how the genetic legacies of past climate change impact responses to ongoing climate change in the dwarf birches, in the form of vegetation greening trends associated with the expansion of tundra shrubs. The thesis addresses this aim in three studies presented as research papers. The first paper, ‘Retracing the response of reindeer to postglacial climate change in Arctic islands’, compares reindeer population history and role of sea ice and ice sheet dynamics in postglacial island colonisation across two regions in the Arctic: the North American islands, and the Barents Sea islands. Using extant reindeer genetic variation, I modelled past population dynamics and tested hypotheses of glacial locations and postglacial dispersal. From the best supported models, I compared the timings of population isolation (genetic divergence) and connectivity (genetic admixture) with reconstructed and modelled changes in sea ice cover, glacial ice sheet dynamics, and other records of past environmental change. I found that the best supported model suggested postglacial dispersal onto deglaciated Arctic islands from continental glacial locations, with modelled divergence times broadly in agreement with fossil data. Sea ice changes often coincided with population events, with differing impacts in the two geographically different systems. The compiled evidence suggests that ice sheet retreat, sea ice concentration, and ocean currents appear to be important influences on postglacial reindeer history and genetic structure in Arctic islands. The second paper, ‘Molecular footprints of Quaternary climate fluctuations in the circumpolar tundra shrub dwarf birch’, uses similar phylogeographical methods to the first paper, but with a novel genome-wide genetic dataset compiled across the geographical range of the dwarf birches. I compared the timing of population divergence and admixture with the ice sheet configuration obtained from published reconstructions, and used published pollen, macrofossil, and sedimentary DNA (sedaDNA) records to externally evaluate the demographic events inferred from the dwarf birches’ present genetic configuration. The best supported model suggested a Mid-Quaternary origin of the dwarf birch species complex, likely in response to the global cooling and associated large climatic changes of the Mid-Pleistocene Transition. The results identified two distinct genetic groups in Betula glandulosa for the first time, which likely reflect glacial isolation north and south of the North American ice sheets. Population events were coeval with major climatic transitions, with interactions between ice sheet changes, climatic and environmental conditions in ice-free areas, and geographical constraints likely resulting in a complex population history. The results suggest that tundra shrubs such as the dwarf birches may have had more nuanced responses to past climatic changes than previously suggested, with implications for future eco-evolutionary responses to anthropogenic climate change. The third paper, ‘Arctic greening patterns reflect genetic legacies of glacial refugia and past climate change’ used the ongoing tundra shrub expansion trends, as measured by remotely sensed vegetation greening, as an opportunity to test whether the impacts of past climate change on the genetic structure of the dwarf birches may be influencing their response to contemporary climate change. I tested the association between population-level dwarf birch genetic diversity and genetic admixture, time since glacial ice sheet retreat, contemporary climate, and regional greening trends of the Arctic tundra and high latitude treeless areas. By modelling the relative importance of these factors, I was able to determine that landscape history and genetic diversity in the form of historical genetic admixture are important but previously neglected components of high latitude vegetation greening trends. The relationship between greening trends and genetic diversity suggests that Arctic shrub expansion may be an adaptive response to climate change, and that future evolutionary potential may therefore be modulated by responses to past climate change. Overall, this thesis demonstrates that postglacial climate change and glacial cycles influenced the evolution and population history of reindeer and the dwarf birches. Climatic fluctuations variously drove population isolation and connectivity. The impacts of these processes included driving allopatric speciation, generating diverse genetic lineages in different regions, enabling dispersal into deglaciated areas, and restoring connectivity between divergent lineages. Species responses were complex, with similar climatic processes resulting in different effects depending on geographical and temporal context. Finally, Arctic species responses to past climate change may impact their future population dynamics and evolution by influencing their contemporary genetic structure and adaptive potential, as illustrated by the link between the population history, genetic structure of the dwarf birches, and their response to ongoing climate change in the form of tundra vegetation greening. Reconstructing past species dynamics in relation to paleoclimatic changes is a useful aid for helping us understand the long-term ecological and evolutionary impacts of environmental change