[Show abstract][Hide abstract] ABSTRACT: The Yatir forest in Israel is a solitary forest at the dry timberline that is surrounded by semi-arid shrubland. Due to its low albedo and its high surface roughness, the forest has a strong impact on the surface energy budget, and is supposed to induce a secondary circulation, which was assessed using eddy-covariance (EC) and Doppler lidar measurements and large-eddy simulation (LES). The buoyancy fluxes were 220-290Wm-2 higher above the forest, and the scale of the forest relative to the boundary-layer height is ideal for generating a secondary circulation, as confirmed by a LES run without background wind. However, usually a relatively high background wind (6ms-1) prevails at the site. Thus, with the Doppler lidar a persistent updraft above the forest was detected only on 5 of the 16 measurement days. Nevertheless, the secondary circulation and convective coherent structures caused low-frequency flux contributions in the mixed-layer w spectra, the surface layer u and w spectra and in the surface-layer momentum fluxes. According to the ogive functions from the tower data and a control volume approach using the LES, such low-frequency contributions with timescales >30min were a major reason the non-closure of the energy balance at the desert site. In the roughness sublayer above the forest, these large structures were broken up into smaller eddies and the energy balance was closed.
Full-text · Article · Oct 2015 · Agricultural and Forest Meteorology
[Show abstract][Hide abstract] ABSTRACT: Methane (CH4) and carbon dioxide (CO2) exchange were investigated over 15 months above a natural bog-pine site in the pre-alpine region of southern Germany. The measurements indicate annual methane emissions of +5.3 ± 0.34 g C m−2 a−1 and an annual CO2 uptake of −62 ± 20 g C m−2 a−1, resulting in a global warming potential balance of −50 ± 74 g [CO2 eq.] m−2 a−1. Air temperature was identified as the environmental parameter showing the highest correlation with methane production, except for periods with low water table (<−0.12 m). Furthermore, we compared three different methane flux gap-filling methods: the mean daily variation approach (MDV), a look up table (LUT) with various control parameters and an exponential regression function between methane flux and air temperature (NLR). It turns out that the LUT provides the best result for the gap-filling of half-hourly CH4 fluxes for the present data. By increasing the number of parameters in the LUT, the CH4 flux prediction could be considerably improved. Except for dry periods, day to day variations could be reproduced very well by the NLR method, but results for sub-daily fluctuations were poor. The choice of gap-filling method affects the annual methane budget estimate by at most ±0.5 g C m−2 a−1, or about 10% of the annual flux.
This study presents one of the first eddy covariance based annual methane- and CO2-exchange estimates over a natural bog-pine ecosystem outside the boreal zone.
No preview · Article · Nov 2014 · Agricultural and Forest Meteorology
[Show abstract][Hide abstract] ABSTRACT: Mesoscale transport of energy and matter between the surface and the atmosphere often occurs in the form of non-propagating turbulent organized structures or thermally-induced circulations. Spatially resolving measurements are required to capture such fluxes and, thus far, airborne measurements are the only means to accomplish this. In contrast, tower-based eddy-covariance measurements are conducted at one point and therefore inherently cannot capture the total atmospheric exchange, which is recognized as a major contributor to the energy balance closure problem. As long as there are mean vertical thermal and humidity gradients in the Atmospheric Boundary-Layer, with higher potential temperatures and specific humidities in the surface layer as compared with the outer-layer, such organized structures will lead to a systematic underestimation of turbulent energy fluxes from eddy-towers. Firstly, we address the question of how deep such meso-γ scale motions penetrate into the surface layer. We present indications from Doppler-LiDAR, airborne and tower-based measurements, which show that mesoscale motions can indeed be found quite close to the surface, but the mesoscale effect vanishes when measurements are actually conducted within the roughness sublayer and when shear stress is sufficiently large to break up mesoscale contributions into smaller eddies. This is illustrated by observations from Germany and Israel. Secondly, we investigate whether the common practice of adjusting the measured eddy tower fluxes for energy balance closure by conserving the Bowen ratio is supported by experimental evidence. Mesoscale and small-scale turbulent fluxes from four different flight campaigns are presented, which were carried out on board of the Canadian Twin Otter (National Research Council of Canada) and the German Polar 5 (Alfred-Wegener Institute) research aircraft over different landscapes in Canada and Alaska.
[Show abstract][Hide abstract] ABSTRACT: Net ecosystem exchange (NEE) was measured in a wind-throw-disturbed upland spruce forest in the Bavarian Forest National Park (Germany) continuously over four years from 2009 to 2013 by the eddy-covariance method. Estimated annual NEE (positive values stand for a net carbon source) of the non-cleared wind-throw resulted in 347 ±104, 255 ±77, 221 ±66, 240 ±52, and 167 ±50 gC m˗2. However, two to six years after the storm event (windstorm Kyrill, January 2007) GEP was already strong, increasing from 393 (2009) to 649 gC m˗2yr˗1 (2013). Ecosystem respiration showed a high inter-annual variability during the measurement period, ranging from 656 to 816 gC m−2. Carbon dioxide (CO2) fluxes during snow-covered periods averaged about 0.8 µmol m˗2s˗1 with only little variation.
The contributions of spruces and grasses to the overall carbon exchange, and the differentiation into autotrophic and heterotrophic respiration have been estimated by modeling using the biogeochemical model LandscapeDNDC (formerly MoBiLE). Comparisons with observations indicate that the model represents gross primary productivity very well, but underestimates ecosystem respiration during early spring and late autumn, and thus tends to diverge from measurements over multi-year simulation periods.
These results show that 1) low productive mountainous forest sites may switch from a carbon source to a carbon sink within relatively few years after disturbance, and 2) model uncertainties are particularly related to soil respiration, decomposition of coarse woody debris, and succession of ground cover species.
Full-text · Article · Oct 2014 · Agricultural and Forest Meteorology
[Show abstract][Hide abstract] ABSTRACT: The timing of phenological events exerts a strong control over ecosystem function and leads to multiple feedbacks to the climate system1. Phenology is inherently sensitive to temperature (although the exact sensitivity is disputed2) and recent warming is reported to have led to earlier spring, later autumn3,4 and increased vegetation activity5,6. Such greening could be expected to enhance ecosystem carbon uptake7,8, although reports also suggest decreased uptake for boreal forests4,9. Here we assess changes in phenology of temperate forests over the eastern US during the past two decades, and quantify the resulting changes in forest carbon storage. We combine long-term ground observations of phenology, satellite indices, and ecosystem-scale carbon dioxide flux measurements, along with 18 terrestrial biosphere models. We observe a strong trend of earlier spring and later autumn. In contrast to previous suggestions4,9 we show that carbon uptake through photosynthesis increased considerably more than carbon release through respiration for both an earlier spring and later autumn. The terrestrial biosphere models tested misrepresent the temperature sensitivity of phenology, and thus the e�ect on carbon uptake. Our analysis of the temperature–phenology–carbon coupling suggests a current and possible future enhancement of forest carbon uptake due to changes in phenology. This constitutes a negative feedback to climate change, and is serving to slow the rate of warming.
[Show abstract][Hide abstract] ABSTRACT: Changes in climatic conditions and their effects on phenology and carbon cycling of terrestrial ecosystems are well documented at local to global scales. While there is a general consensus on the existence of such links between warming trends, changes in phenology (e.g. vegetative season length) and the effectiveness of ecosystems as carbon sinks, the impacts of short-term climatic extreme events (e.g., droughts or frosts) on carbon cycling, in combination with long-term trends, are less certain. Here, we argue that the timing of extreme events relative to phenology determines whether, and how, net ecosystem exchange of carbon (NEE) is affected. Further, we hypothesize that long-term trends in phenology and climate make forest ecosystems more susceptible to some extreme events, and less so to others.
In a recent study, Dragoni et al. (2010, Global Change Biology 17, 886-897) reported on long-term trends in carbon exchange at the Morgan Monroe State Forest in Indiana (MMSF, USA). From 1998 to 2008, an overall increase in late-summer temperatures led to an extension of the active vegetative season progressively later into the fall and was shown to drive significantly increased net ecosystem productivity (NEP) at MMSF (enhanced carbon uptake responsible for an increase of NEP by about 50 gC m-2 over a decade). However, in later years (2009 to 2012), a series of extreme short-term climate events, ranging from early to late season droughts, or colder-than-normal springs, have led to very low annual productivity (on average 25% lower than the 1998-2008 mean). Such extreme climate events impacted phenology, carbon exchange and allocation, and nutrient cycling. In part due to the fall extension of the carbon uptake season, the 2012 drought started already in the fully active uptake period and led to strong declines in NEE that canceled out the effects of an early start to the season. Late summer droughts in earlier years started close to or after the onset of senescence and had considerably smaller effects on annual NEP. Thus, depending on their timing, extreme climate events have the potential to outweigh positive trends in NEP that are induced by gradual shifts in climate and phenology. Given that extreme short-term climate events are expected to increase in intensity and frequency in a warming world, our results highlight the need to investigate the role of short-term climatic stressors in the acclimation of ecosystems to gradual shifts in climate.
[Show abstract][Hide abstract] ABSTRACT: Forests, especially in mid-latitudes are generally designated as large carbon sinks. However, stand-replacing disturbance events like fires, insect-infestations, or severe wind-storms can shift an ecosystem from carbon sink to carbon source within short time and keep it as this for a long time. In Addition, extreme weather situations which promote the occurrence of ecosystem disturbances are likely to increase in the future due to climate change. The development and competition of different vegetation types (spruce vs. grass) as well as soil organic matter (SOM), and their contribution to the net ecosystem exchange (NEE), in such disturbed forest ecosystems are largely unknown.
In a large wind-throw area (ca. 600 m diameter, due to cyclone Kyrill in January 2007) within a mature upland spruce forest, where dead-wood has not been removed, in the Bavarian Forest National Park (Lackenberg, 1308 m a.s.l., Bavaria, Germany), fluxes of CO2, water vapor and energy have been measured with the Eddy Covariance (EC) method since 2009. Model simulations (MoBiLE) were used to estimate the GPP components from trees and grassland as well as to differentiate between soil and plant respiration, and to get an idea about the long term behavior of the ecosystems carbon exchange.
For 2009, 2010, 2011, 2012, and 2013 estimates of annual Net Ecosystem Exchange (NEE) showed that the windthrow was a marked carbon source. However, the few remaining trees and newly emerging vegetation (grass, sparse young spruce, etc.) lead to an already strong Gross Ecosystem Production (GEP). Model simulations conformed well with the measurements.
To our knowledge, we present the worldwide first long-term measurements of NEE within a non-cleared windthrow-disturbed forest ecosystem.
[Show abstract][Hide abstract] ABSTRACT: A general lack of energy balance closure indicates that tower-based eddy-covariance (EC) measurements underestimate turbulent heat fluxes, which calls for robust correction schemes. Two parametrization approaches that can be found in the literature were tested using data from the Canadian Twin Otter research aircraft and from tower-based measurements of the German Terrestrial Environmental Observatories (TERENO) programme. Our analysis shows that the approach of Huang et al. (Boundary-Layer Meteorol 127:273–292, 2008), based on large-eddy simulation, is not applicable to typical near-surface flux measurements because it was developed for heights above the surface layer and over homogeneous terrain. The biggest shortcoming of this parametrization is that the grid resolution of the model was too coarse so that the surface layer, where EC measurements are usually made, is not properly resolved. The empirical approach of Panin and Bernhofer (Izvestiya Atmos Oceanic Phys 44:701–716, 2008) considers landscape-level roughness heterogeneities that induce secondary circulations and at least gives a qualitative estimate of the energy balance closure. However, it does not consider any feature of landscape-scale heterogeneity other than surface roughness, such as surface temperature, surface moisture or topography. The failures of both approaches might indicate that the influence of mesoscale structures is not a sufficient explanation for the energy balance closure problem. However, our analysis of different wind-direction sectors shows that the upwind landscape-scale heterogeneity indeed influences the energy balance closure determined from tower flux data. We also analyzed the aircraft measurements with respect to the partitioning of the “missing energy” between sensible and latent heat fluxes and we could confirm the assumption of scalar similarity only for Bowen ratios
Full-text · Article · May 2014 · Boundary-Layer Meteorology
[Show abstract][Hide abstract] ABSTRACT: This study compares the CO2 exchange of a natural bog forest, and of a bog drained for forestry in the pre-alpine region of southern Germany. The sites are separated by only ten kilometers, they share the same formation history and are exposed to the same climate and weather conditions. In contrast, they differ in land use history: at the Schechenfilz site a natural bog-pine forest (Pinus mugo rotundata) grows on an undisturbed, about 5 m thick peat layer; at Mooseurach a planted spruce forest (Picea abies) grows on drained and degraded peat (3.4 m). The net ecosystem exchange of CO2 (NEE) at both sites has been investigated for two years (July 2010 to June 2012), using the eddy covariance technique. Our results indicate that the drained, forested bog at Mooseurach is a much stronger carbon dioxide sink (-130 ± 31 and -300 ± 66 g C m-2 a-1 in the first and second year respectively) than the natural bog forest at Schechenfilz (-53 ± 28 and -73±38 g C m-2 a-1). The strong net CO2 uptake can be explained by the high gross primary productivity of the spruces that over-compensates the two times stronger ecosystem respiration at the drained site. The larger productivity of the spruces can be clearly attributed to the larger LAI of the spruce site. However, even though current flux measurements indicate strong CO2 uptake of the drained spruce forest, the site is a strong net CO2 source, if the whole life-cycle, since forest planting is considered. We determined the difference between carbon fixation by the spruces and the carbon loss from the peat due to drainage since forest planting. The estimate resulted in a strong carbon release of +156 t C ha-1 within the last 44 yr, means the spruces would need to grow for another 100 yr, at the current rate, to compensate the peat loss of the former years. In contrast, the natural bog-pine ecosystem has likely been a small but consistent carbon sink for decades, which our results suggest is very robust regarding short-term changes of environmental factors.
[Show abstract][Hide abstract] ABSTRACT: Predicted decreases in water availability across the temperate forest biome have the potential to offset gains in carbon (C) uptake from phenology trends, rising atmospheric CO2 , and nitrogen deposition. While it is well-established that severe droughts reduce the C sink of forests by inducing tree mortality, the impacts of mild but chronic water stress on forest phenology and physiology are largely unknown. We quantified the C consequences of chronic water stress using a 13-year record of tree growth (n = 200 trees), soil moisture, and ecosystem C balance at the Morgan-Monroe State Forest (MMSF) in Indiana, and a regional 11-year record of tree growth (n >300,000 trees) and water availability for the 20 most dominant deciduous broadleaf tree species across the Eastern and Midwestern USA. We show that despite ~26 more days of C assimilation by trees at the MMSF, increasing water stress decreased the number of days of wood production by ~42 days over the same period, reducing the annual accrual of C in woody biomass by 41%. Across the deciduous forest region, water stress induced similar declines in tree growth, particularly for water-demanding "mesophytic" tree species. Given the current replacement of water-stress adapted "xerophytic" tree species by mesophytic tree species, we estimate that chronic water stress has the potential to decrease the C sink of deciduous forests by up to 17% (0.04 Pg C yr(-1) ) in the coming decades. This reduction in the C sink due to mesophication and chronic water stress is equivalent to an additional 1 to 3 days of global C emissions from fossil fuel burning each year. Collectively, our results indicate that regional declines in water availability may offset the growth-enhancing effects of other global changes and reduce the extent to which forests ameliorate climate warming. This article is protected by copyright. All rights reserved.
Full-text · Article · Jan 2014 · Global Change Biology
[Show abstract][Hide abstract] ABSTRACT: Multicompartment and multiscale long-term observation and research are important prerequisites to tackling the scientific challenges resulting from climate and global change. Long-term monitoring programs are cost intensive and require high analytical standards, however, and the gain of knowledge often requires longer observation times. Nevertheless, several environmental research networks have been established in recent years, focusing on the impact of climate and land use change on terrestrial ecosystems. From 2008 onward, a network of Terrestrial Environmental Observatories (TERENO) has been established in Germany as an interdisciplinary research program that aims to observe and explore the
long-term ecological, social, and economic impacts of global change at the regional level. State-of-the-art methods from the field of environmental monitoring, geophysics, and remote sensing will be used to record and analyze states and fluxes for different environmental
compartments from groundwater compartments from groundwater through the vadose zone, surface water, and
biosphere, up to the lower atmosphere.
Full-text · Article · Dec 2013 · Vadose Zone Journal
[Show abstract][Hide abstract] ABSTRACT: Montane grasslands of Central Europe are expected to be exposed to strong warming and to altered precipitation patterns, suggesting that biosphere–atmosphere–hydrosphere exchange of carbon (C) and nitrogen (N) compounds may be vulnerable to future climatic conditions. By transferring small lysimeters along an altitudinal gradient, we assessed the impact of climate change conditions on soil–atmosphere exchange of methane (CH4) and nitrous oxide (N2O) as well as on ammonium (NH4+) and nitrate (NO3−) in soil water in extensively managed montane grassland in southern Germany. Lysimeter transfer to lower altitude increased air and soil temperatures by more than 2°C and reduced summer precipitation as well as soil moisture throughout the year compared with a control transfer within the high altitude site. This simulation of climate change conditions almost doubled the CH4 sink strength from −0.11 to −0.19 g C m−2 year−1, which appeared to be mainly related to improved gas diffusion after reduced soil moisture. Mean NH4+ and NO3− concentrations in soil water (0.05 mg NH4+–N l−1 and 0.08 mg NO3−–N l−1) and N2O emissions (approximately 0.03 g N m−2 year−1) remained small and unaffected by climate change simulation. Our study suggests that expected climate change conditions will have positive effects on the non‐CO2 greenhouse gas balance in extensively managed montane grassland because of increased net CH4 uptake in soil. For N2O emission, we conclude that potential effects of management changes may override the small effects of simulated climate change on N2O emissions observed in this study.
No preview · Article · Oct 2013 · European Journal of Soil Science
[Show abstract][Hide abstract] ABSTRACT: Terrestrial plants remove CO2 from the atmosphere through photosynthesis, a process that is accompanied by the loss of water vapour from leaves. The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is central to the global cycles of water, energy and carbon. Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange. We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO2 fertilization effect. The results suggest a partial closure of stomata-small pores on the leaf surface that regulate gas exchange-to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in the carbon- and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation-climate models.