Spatial Upscaling of Forest Response to Drought

We have integrated the observational capability of satellite remote sensing with plot-scale tree ring data to up-scale the evaluation of forest responses to drought. Satellite data, such as Normalized Difference Vegetation Index (NDVI), can provide a spatially continuous measure with limited temporal coverage, while tree Ring Width Index (RWI) provides accurate assessment with much longer time series local scales. Here, we explored the relationship between RWI and NDVI of three dominant species in the Southwestern United States (SWUS), and predicted RWI spatial distribution from 2001 to 2017 based on MODIS 1-km resolution NDVI data with stringent quality control. We detected the optimum time windows (around June-August) during which RWI and NDVI were most closely correlated for each species, when the canopy growth had the greatest effect on growth of tree trunks. Then, using our upscaling algorithm of NDVI-based RWI, we were able to detect the significant impact of droughts in 2002 and in 2011–2014, which supported the validity of this algorithm in quantifying forest response to drought on a large scale

Quantifying terrestrial ecosystem carbon dynamics with mechanistically-based biogeochemistry models and in situ and remotely sensed data

Terrestrial ecosystem plays a critical role in the global carbon cycle and climate system. Therefore, it is important to accurately quantify the carbon dynamics of terrestrial ecosystem under future climatic change condition. This dissertation evaluates the regional carbon dynamics by using upscaling approach, mechanistically-based biogeochemistry models and in situ and remotely sensed data. The upscaling studies based on FLUXNET network has provided us the spatial and temporal pattern of the carbon fluxes but it fails to consider the atmospheric CO2 effect given its important physiological role in carbon assimilation. In the second chapter, we consider the effect of atmospheric CO2 using an artificial neural network (ANN) approach to upscale the AmeriFlux tower of net ecosystem exchange (NEE) and the derived gross primary productivity (GPP) to the conterminous United States. We found that atmospheric CO 2effect on GPP/NEE exhibited a great spatial and seasonal variability. Further analysis suggested that air temperature played an important role in determining the atmospheric CO2 effects on carbon fluxes. In addition, the simulation that did not consider atmospheric CO2 failed to detect ecosystem responses to droughts in part of the US in 2006. The study suggested that the spatially and temporally varied atmospheric CO2 concentrations should be factored into carbon quantification when scaling eddy flux data to a region. The process-based ecosystem models are useful tools to predicting future change in the terrestrial ecosystem. However, they suffer the great uncertainty induced by model structure and parameters. The carbon isotope (13C) discrimination by terrestrial plants, involves the biophysical and biogeochemistry processes and exhibits seasonal and spatial variations, which may provide additional constraints on model parameters. In the third chapter, we found that using foliar 13C composition data, model parameters were constrained to a relatively narrow space and the site-level model simulations were slightly better than that without the foliar 13C constraint. The model extrapolations with three stomatal schemes all showed that the estimation uncertainties of regional carbon fluxes were reduced by about 40%. In addition, tree ring data have great potentials in addressing the forest response to climatic changes compared with mechanistic model simulations, eddy flux measurement and manipulative experiments. In the fourth chapter, we collected the tree ring isotopic carbon data at 12 boreal forest sites to develop a linear regression model, and the model was extrapolated to the whole boreal region to obtain the water use efficiency (WUE) and GPP spatial and temporal variation from 1948 to 2010. Our results demonstrated that most of boreal regions except parts of Alaska showed a significant increasing WUE trend during the study period and the increasing magnitude was much higher than estimations from other land surface models. Our predicted GPP by the WUE definition algorithm was comparable with site observation, while for the revised light use efficiency algorithm, GPP estimation was higher than site observation as well as land surface model estimates. In addition, the increasing GPP trends estimated by two algorithms were similar with land surface model simulations

Evapotranspiration in a catchment dominated by eucalypt forest and woodland

There is on-going need for reliable estimates of evapotranspiration (ET) at catchment scales to support objective decision-making for managing water supplies, and enhancing understanding of processes and modelling. Without reliable estimates of ET, water supply and catchment management agencies are exposed to significant economic, social and even environmental risks. This thesis focuses on identifying possible methodologies for estimating ET in a catchment dominated by eucalypt forest and woodland. More specifically, this thesis tests the hypothesis that different methods of deriving daily, catchment ET for a headwater in Australia meet underlying assumptions and yield similar results. The hypothesis was tested by using three approaches to estimate catchment ET: soil moisture changes (point scale), satellite imagery of leaf area index (MODIS, hillslope scale), and discharge (streamflow) and the storage-discharge relationship (catchment scale). Data from Corin Catchment, an unregulated catchment vital to the Australian Capital Territory and the surrounding region, is the basis of this study. After the General Introduction (Chapter 1), methods for estimating ET in eucalypt communities throughout Australia at various temporal and spatial scales are systematically reviewed (Chapter 2). Of the 1614 original research papers investigated, 52 were included for further investigation. A clear outcome is that transpiration by the overstorey, measured using sap flow, is the most frequently measured component of ET, and that physiological studies dominate estimates of ET. Very few studies were conducted at the catchment scale. Further, scaling ET from tree to catchment scales was rarely attempted, and the effect of scaling for water resource management is mostly unquantified and requires attention. The first method used to calculate catchment ET is based on up-scaling of soil moisture changes on the basis of a digital soil map (Chapter 4). The data presented here rejects the hypothesis that ET (derived from soil moisture) and overstorey transpiration (derived from sap flow) are well correlated. Instead, the data suggest that soil moisture-derived ET and overstorey transpiration obtained water from different sources. The key findings of this chapter are that this approach is not suitable for estimating ET at catchment scales because it is restricted to drier periods, and because trees did not solely rely on the defined root-zone for water supply. The second method to calculate catchment scale ET (Chapter 5) tests if hillslope-scale satellite imagery (MODIS leaf area index) can be up-scaled to estimate catchment ET. An outcome of this work is that caution is needed when using MODIS leaf area index for water resource planning in evergreen forests across the globe, particularly for forests with significant understorey and a relatively open overstorey canopy at some periods of the year. This method is deemed not suitable for estimating ET over the study area. The third method to calculate catchment scale ET (Chapter 6) is based on integrating discharge using a single non-linear equation to characterise the study area. This method yielded catchment ET far greater (18 times larger) than the largest observed measure of potential ET. As with the method based on soil moisture changes, it was restricted to drier periods. This method was clearly unsuitable for estimating ET over the study area due to relatively quick recession, large range in hourly discharge and significant scattered recession at low discharge. Overall, this thesis rejects the hypothesis that different methods of deriving daily, catchment ET for a headwater in Australia meet underlying assumptions and yield similar results. An important limitation identified through this research is the ability to determine a ‘correct’ estimate of catchment ET. Further research should focus on enhancing understanding of scaling ET within and beyond Australia, generating more daily catchment ET from up-scaled soil moisture changes, further evaluating ET from up-scaled satellite imagery, and identifying catchment characteristics to allow ET to be derived from discharge. Water resource managers must be diligent when selecting and applying a method to estimate catchment ET

Spatio-temporal assessment of beech growth in relation to climate extremes in Slovenia – An integrated approach using remote sensing and tree-ring data

Climate change is predicted to affect tree growth due to increased frequency and intensity of extreme events such as ice storms, droughts and heatwaves. Yet, there is still a lot of uncertainty on how trees respond to an increase in frequency of extreme events. Use of both ground-based wood increment (i.e. ring width) and remotely sensed data (i.e. vegetation indices) can be used to scale-up ground measurements, where there is a link between the two, but this has only been demonstrated in a few studies. We used tree-ring data together with crown features derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) to assess the effect of extreme climate events on the growth of beech (Fagus sylvatica L.) in Slovenia. We found evidence that years with climate extremes during the growing season (drought, high temperatures) had a lower ring width index (RWI) but we could not find such evidence for the remotely sensed EVI (Enhanced Vegetation Index). However, when assessing specific events where leaf burning or wilting has been reported (e.g. August 2011) we did see large EVI anomalies. This implies that the impact of drought or heatwave events cannot be captured by EVI anomalies until physical damage on the canopy is caused. This also means that upscaling the effect of climate extremes on RWI by using EVI anomalies is not straightforward. An exception is the 2014 ice storm that caused a large decline in both RWI and EVI. Extreme climatic parameters explained just a small part of the variation in both RWI and EVI by, which could indicate an effect of other climate variables (e.g. late frost) or biotic stressors such as insect outbreaks. Furthermore, we found that RWI was lower in the year after a climate extreme occurred in the late summer. Most likely due to the gradual increase in temperature and more frequent drought we found negative trends in RWI and EVI. EVI maps could indicate where beech is sensitive to climate changes and could be used for planning mitigation interventions. Logical next steps should focus on a tree-based understanding of the short -and long-term effects of climate extremes on tree growth and survival, taking into account differential carbon allocation to the crown (EVI) and to wood-based variables. This research highlights the value of an integrated approach for upscaling tree-based knowledge to the forest level

Spatio-temporal assessment of beech growth in relation to climate extremes in Slovenia – An integrated approach using remote sensing and tree-ring data

Climate change is predicted to affect tree growth due to increased frequency and intensity of extreme events such as ice storms, droughts and heatwaves. Yet, there is still a lot of uncertainty on how trees respond to an increase in frequency of extreme events. Use of both ground-based wood increment (i.e. ring width) and remotely sensed data (i.e. vegetation indices) can be used to scale-up ground measurements, where there is a link between the two, but this has only been demonstrated in a few studies. We used tree-ring data together with crown features derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) to assess the effect of extreme climate events on the growth of beech (Fagus sylvatica L.) in Slovenia. We found evidence that years with climate extremes during the growing season (drought, high temperatures) had a lower ring width index (RWI) but we could not find such evidence for the remotely sensed EVI (Enhanced Vegetation Index). However, when assessing specific events where leaf burning or wilting has been reported (e.g. August 2011) we did see large EVI anomalies. This implies that the impact of drought or heatwave events cannot be captured by EVI anomalies until physical damage on the canopy is caused. This also means that upscaling the effect of climate extremes on RWI by using EVI anomalies is not straightforward. An exception is the 2014 ice storm that caused a large decline in both RWI and EVI. Extreme climatic parameters explained just a small part of the variation in both RWI and EVI by, which could indicate an effect of other climate variables (e.g. late frost) or biotic stressors such as insect outbreaks. Furthermore, we found that RWI was lower in the year after a climate extreme occurred in the late summer. Most likely due to the gradual increase in temperature and more frequent drought we found negative trends in RWI and EVI. EVI maps could indicate where beech is sensitive to climate changes and could be used for planning mitigation interventions. Logical next steps should focus on a tree-based understanding of the short -and long-term effects of climate extremes on tree growth and survival, taking into account differential carbon allocation to the crown (EVI) and to wood-based variables. This research highlights the value of an integrated approach for upscaling tree-based knowledge to the forest level

Integrated approach for monitoring the vulnerability of Mediterranean forests affected by drought-induced dieback

Rising aridity, mostly driven by higher temperatures and reduced precipitation, will likely undermine the health status of forest ecosystems. Experiments and observations point to the likelihood that if climate changes proceed at its current rate, the resilience of many forests will be threated by altering their structure and functions and reducing their capability to provide ecosystem services. Such increasing drought conditions, coupled to other biotic and abiotic drivers, are synergistically leading to responses in tree morphology, physiology, growth, reproduction, and forest mortality in different areas of the Mediterranean Basin. However, our understanding of vegetation dynamics in response to climate changes is still lacking, as a robust comprehension relies on the need to obtain insights at multiple temporal and spatial scales. In this context, we sought to forecasting vegetation response to climate stressors, particularly during dieback episodes when tree vulnerability is exacerbated. The first section of this study focused on tree and shrub populations exhibiting recent dieback phenomena in Italy (Quercus pubescens, Quercus frainetto) and Spain (Pinus sylvestris, Juniperus phoenicea). The general aim was to investigate how remotely sensed measures of vegetation activity and radial growth (BAI, basal area increment) responded to climate extreme events. To this purpose, we compared trees and nearby stands showing different vigor, i.e., dieback vs non-dieback, assessed as growth decline, elevated canopy defoliation and rising tree mortality rate. To disentangle growth and NDVI responses to drought, we accounted for two components of drought, namely elevated vapor pressure deficit (VPD) and low soil moisture. As a whole, the response of the investigated species to VPD increase was characterized by growth reduction. In Scots pine, high VPD was linked to a loss of growth in declining individuals which did not respond to changes in soil moisture. Oaks responded mostly to soil moisture, whereas the juniper was the most negatively affected by higher VPD. Indeed, the different hydraulic strategies (anisohydric vs. isohydric species) could partially explain the contrasting growth responses to drought proxies. We also found that dieback stands exhibited lower NDVI values than non-dieback stands. In most cases, NDVI and BAI was positively correlated and such relation likely relied on specific time windows. In the second part of the thesis, the phenological behavior of Mediterranean oak forest stands (Quercus cerris, Quercus pubescens, and Quercus frainetto), showing evident decline symptoms, are investigated by using a satellite-based approach. We explored how a phenological (PPI, Plant Phenology Index) index would be capable to reflect the seasonal vegetative dynamics of forests affected by dieback phenomena. We found that dieback forest stands - characterized by a higher ratio of crown-defoliated trees - showed distinct phenological performance as compared to non-dieback stands. In detail, our results revealed that dieback stands lengthened the growing season by delaying autumn leaf senescence. Nevertheless, both seasonal amplitude and productivity were found to have higher values for non-dieback stands as compared to dieback stands. Furthermore, it was highlighted that non-dieback stands experienced either greening up or senescence periods more rapidly than dieback ones. Overall, our framework demonstrated that the effects of climate extremes on vegetation can be detected either in terms of canopy greenness or radial growth reductions, thus hinting at the opportunity to both employ remotely sensed data as a stand-level indicator of vegetation stress and to scaling up informations from tree to stand levels by using the maximum growing season NDVI and tree-ring width data taken at the individual scales. Our findings also highlighted how patterns of vegetation response to climate extremes may depend on both the water use strategies of trees and shrubs and site-specific climatic conditions. Hence, coupling proxies of forest productivity (NDVI, BAI) may be employed for retrospective modeling of the impact of drought stress on forest productivity and tree growth, enhancing our knowledge and forecast of drought-induced dieback phenomena in woody plant communities. Furthermore, the second part of the work revealed the phenological behaviour of Mediterranean forest populations showing clear symptoms of decline. We speculated that the lengthened growing season may be related to the dieback trees' effort to compensate for the reduction in whole-plant photosynthesis, associated to canopy decline. Increased photosynthesis during the season under higher temperatures and increased light availability, due to global warming, provided a possible explanation for the greater seasonal amplitude and productivity of healthier stands. Our findings may provide new insights on phenological response to climate change in semi-arid regions, highlighting how trees, showing clear symptoms of decline, may keep their vital activities by changing their phenological performance. What described above leads to a crucial question concerning the potential implications of observed phenological shifts on the global carbon and water balance of terrestrial ecosystems under future climate change. Hence, in the coming years, this study could provide a more comprehensive overview on climate-vegetation interactions, mainly in the Mediterranean Basin, where intensified global warming and aridification trends are expected. Nonetheless, more investigations on the interactive effects among different environmental factors, are needed to improve our understanding of the underlying mechanisms affecting vegetation response

Advances in the Assessment of Climate Change Impact on the Forest Landscape

Changing climates threaten the habitats and ecosystems at variable extents throughout the world. Forests are unique habitats and ecosystems that are vulnerable by the consequences of climate change. The climate change causes disturbances, alterations. and shifting on the forests that can be diagnosed at the tree and stand scales, as well as can be monitored and analyzed at the landscape scale. Furthermore, some recent researches concentrate on conveying the forest tree and stand-level shifting and disturbances to the forest landscape level by upscaling. In this study, the climate change impacts on the forest landscapes; principally, the disturbances including the drought-induced mortality, growth and productivity failures, and insect outbreaks are evaluated. Secondarily, climate change-induced alterations of the forest species distributions and forest landscape compositions, dynamics of the forest biodiversity, and tree migrations are discussed by focusing particularly on the relatively recent advances involving the modeling procedures. Ultimately, monitoring the climate change-driven shifting phenology of the forest landscape through the remote sensing techniques is referred in this study. Moreover, the study examples dependent upon the climate-ecological modeling and satellite data assessment of the forest landscapes throughout the world are also referenced. The landscape-scale assessment of the climate change impacts on the forest ecosystems provides integrated and comprehensive approach toward the proposal of sustainable mitigations and solutions to the phenomenon

Approaches for advancing scientific understanding of macrosystems

The emergence of macrosystems ecology (MSE), which focuses on regional- to continental-scale ecological patterns and processes, builds upon a history of long-term and broad-scale studies in ecology. Scientists face the difficulty of integrating the many elements that make up macrosystems, which consist of hierarchical processes at interacting spatial and temporal scales. Researchers must also identify the most relevant scales and variables to be considered, the required data resources, and the appropriate study design to provide the proper inferences. The large volumes of multi-thematic data often associated with macrosystem studies typically require validation, standardization, and assimilation. Finally, analytical approaches need to describe how cross-scale and hierarchical dynamics and interactions relate to macroscale phenomena. Here, we elaborate on some key methodological challenges of MSE research and discuss existing and novel approaches to meet them

Scarce population genetic differentiation but substantial spatiotemporal phenotypic variation of water‑use efficiency in Pinus sylvestris at its western distribution range

Water and carbon fluxes in forests are largely related to leaf gas exchange physiology varying across spatiotemporal scales and modulated by plant responses to environmental cues. We quantified the relevance of genetic and phenotypic variation of intrinsic water-use efficiency (WUEi, ratio of net photosynthesis to stomatal conductance of water) in Pinus sylvestris L. growing in the Iberian Peninsula as inferred from tree-ring carbon isotopes. Inter-population genetic variation, evaluated in a provenance trial comprising Spanish and German populations, was low and relevant only at continental scale. In contrast, phenotypic variation, evaluated in natural stands (at spatial level) and by tree-ring chronologies (at temporal inter-annual level), was important and ten- and threefold larger than the population genetic variance, respectively. These results points to preponderance of plastic responses dominating variability in WUEi for this species. Spatial phenotypic variation in WUEi correlated negatively with soil depth (r = − 0.66; p < 0.01), while temporal phenotypic variation was mainly driven by summer precipitation. At the spatial level, WUEi could be scaled-up to ecosystem-level WUE derived from remote sensing data by accounting for soil water-holding capacity (r = 0.63; p < 0.01). This outcome demonstrates a direct influence of the variation of leaf-level WUEi on ecosystem water and carbon balance differentiation. Our findings highlight the contrasting importance of genetic variation (negligible) and plastic responses in WUEi (large, with changes of up to 33% among sites) on determining carbon and water budgets at stand and ecosystem scales in a widespread conifer such as Pinus sylvestris.This work was supported by the Spanish Government [MINECO Grant Number AGL2015-68274-C3-3-R] and the Russian Science Foundation (Project Number 14-14-00219-P, mathematical approach). We acknowledge P. Sopeña and M.J. Pau for technical assistance and V. Muñoz, M. Sala and A. Teixidó for field sampling