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Second-growth forest establishment over abandoned agricultural lands has been envisaged as a nature-based solution to reduce atmospheric CO2 through carbon sequestration. However, we still have a very limited knowledge about the climate sensitivity of these second-growth forests in comparison to forests with a more continuous land-use history. Here, we compare the climate sensitivity of recently established forests (post-1956) and long-existing forests (pre-1956), by analyzing their responses to climate variability at 456 inventory plots in NE Spain. We analyzed remotely sensed estimates of leaf area index (LAI) and normalized difference vegetation index (NDVI) from 2000 to 2022, relating yearly variation patterns with annual meteorological drought intensity. Then, we used process-based simulations of forest functioning to explore whether different responses of both forest types to climate variability are consistent with differences in root allocation patterns modulated by land-use legacies. Although recent- and long-existing forests had a very similar structure (that is, similar LAI), NDVI variation in recent-established forests was more strongly related to meteorological drought indices, recording steeper NDVI decreases during drier periods, but also higher NDVI increases during rainy years. This pattern is consistent with differences in root allocation between the two forest types, as simulated and remotely sensed forest responses to climate exhibited higher correlation when we inputted recent-established forest with shallower root systems or lower fine root density in comparison with long-existing forests. A higher climate sensitivity of second-growth forests during drier but also wetter years entails the potential for compensating drought impacts, but it also indicates a long-term higher vulnerability to climatic disturbances. [ABSTRACT FROM AUTHOR]

The stability of estuarine channel-shoal systems is important for port utilization, navigation maintenance, habitat protection and ecosystem service functions. This paper uses the South Channel of the Changjiang (Yangtze River) Estuary as a typical example to investigate the channel-shoal adjustment mechanism and its future trend. The combined approaches of bathymetric data analysis and process-based modeling (Delft3D) are applied. Quantitative analysis of morphological changes indicates that the South Channel experienced remarkable channel-shoal adjustment during 1958–2018. Periodic evolution was identified, including shoal migration, incision and emergence under natural conditions before the mid-1980s. Since then, fluvial sediment decline and local human intervention have interrupted the periodic processes. After 1986, as river sediment discharge started to decline, the South Channel converted to net erosion, and both the mid-channel shoal at the bifurcation node and the tail of the Ruifeng Shoal showed significant scour. Process-based hydrodynamic simulations revealed that the northern rotation of the mainstream downstream of Wusong triggered the erosion of the Ruifeng Shoal, while unordered sand mining at the shoal tail in approximately 2002 enhanced shoal shrinkage. In addition, the self-adjustment of the transverse section shape resulted in abnormal accretion in 2002–2007. Afterward, the South Channel underwent overall erosion as sediment discharge decreased to a low level (<150 Mt/a). Five stages of channel-shoal pattern adjustment and accretion/erosion status during the past 60 years were defined, i.e., the accretion stage (1958–1965), remarkable channel-shoal adjustment stage (1978–1986), slow erosion stage (1986–1997), shoal scour and shrinkage stage (1997–2007) and overall channel-shoal erosion stage (2007–2018). Model prediction of the evolutionary trend indicates that overall erosion within the South Channel is most likely to continue in 2015–2050. Further adjustment of the South Channel under extremely low sediment discharge may threaten the riverbed stability and the sustainable development of this large-scale estuary. Future work on adaptive strategies for varying conditions is recommended. [ABSTRACT FROM AUTHOR]

Alpine ecosystems are, similar to arctic ecosystems, characterized by a very long snow season. Previous studies investigating arctic or alpine ecosystems have shown that winter CO2 effluxes can dominate the annual balance and that the timing and duration of the snow cover plays a crucial role for plant growth and phenology and might also influence the growing season ecosystem CO2 strength and dynamics. The objective of this study was to analyze seasonal and annual CO2 balances of a grassland site at an elevation of 2440 m a.s.l in the Swiss central Alps. We continuously measured the NEP using the eddy covariance method from June 2013 to October 2014, covering two growing seasons and one winter. We analyzed the influence of snow melt date on the CO2 exchange dynamics at this site, because snow melt differed about 24 days between the 2 years. To this end, we employed a process-based ecosystem carbon cycling model to disentangle the co-occurring effects of growing season length, environmental conditions during the growing season, and physiological/structural properties of the canopy on the ecosystem carbon balance. During the measurement period, the site was a net sink for CO2 although winter efflux contributed significantly to the total balance. The cumulative growing season NEP as well as mean and maximum daily CO2 uptake rates was lower during the year with the later snow melt, and the results indicated that the differences were mainly due to differing growing season lengths. [ABSTRACT FROM AUTHOR]

The objective of this paper is to assess how much carbon (C) is currently stored in a forest district in Thuringia, Germany, and how the carbon stocks will develop up to the year 2099 with a changing climate and under various management regimes (including no management), with different assumptions about carbon dioxide (CO) fertilization effects. We applied the process-based model 4C and a wood product model to a forest district in Germany and evaluated both models for the period from 2002 to 2010, based on forest inventory data for the stands in the district. Then, we simulated the growth of the stands in the forest district under three different realizations of a climate change scenario, combined with different management regimes. Our simulations show that in 2099, between 630 and 1149 t C ha will be stored in this district. The simulations also showed that climate change affects carbon sequestration. The no management strategy sequestered the highest amount of carbon (8.7 t C ha year), which was greater than the management regimes. In the model, the possible fertilization effect of CO is an important factor. However, forest management remains the determining factor in this forest district. [ABSTRACT FROM AUTHOR]

The aim of this study was to simulate impacts of regional climate change in the 2070s on carbon (C) cycle of a Mediterranean watershed combining field measurements, Envisat MERIS and IKONOS data, and the Carnegie Ames Stanford Approach model. Simulation results indicated that the present total C sink status (1.36 Mt C year) of Mediterranean evergreen needleleaf forest, grassland and cropland ecosystems is expected to weaken by 7.6 % in response to the climate change in the 2070s (Mt = 10 g). This decreasing trend was mirrored in soil respiration ( R), aboveground and belowground net primary production (NPP), NEP, and net biome production (NBP). The decrease in NEP in the 2070s was the highest (21.9 %) for mixed forest where the smallest present C sink of 0.03 Mt C year was estimated. The average present net ecosystem production (NEP) values were estimated at 110 ± 15, 75 ± 19, and 41 ± 25 g C m years in forest, grassland, and cropland, respectively, with a watershed-scale mean of 95 ± 30 g C m years. The largest present C sink was in grassland, with a total C pool of 0.55 Mt C year, through its greater spatial extent. [ABSTRACT FROM AUTHOR]

Quantifications of Earth surface topography are essential for modeling the connections between physical and chemical processes of erosion and the shape of the landscape. Enormous investments are made in developing and testing process-based landscape evolution models. These models may never be applied to real topography because of the difficulties in obtaining high-resolution (1–2 m) topographic data in the form of digital elevation models (DEMs). Here we present a simple methodology to extract the high-resolution three-dimensional topographic surface from photographs taken with a hand-held camera with no constraints imposed on the camera positions or field survey. This technique requires only the selection of corresponding points in three or more photographs. From these corresponding points the unknown camera positions and surface topography are simultaneously estimated. We compare results from surface reconstructions estimated from high-resolution survey data from field sites in the Oregon Coast Range and northern California to verify our technique. Our most rigorous test of the algorithms presented here is from the soil-mantled hillslopes of the Santa Cruz marine terrace sequence. Results from three unconstrained photographs yield an estimated surface, with errors on the order of 1 m, that compares well with high-resolution GPS survey data and can be used as an input DEM in process-based landscape evolution modeling. [ABSTRACT FROM AUTHOR]