Joint optimization of land carbon uptake and albedo can help achieve moderate instantaneous and long-term cooling effects

Both carbon dioxide uptake and albedo of the land surface affect global climate. However, climate change mitigation by increasing carbon uptake can cause a warming trade-off by decreasing albedo, with most research focusing on afforestation and its interaction with snow. Here, we present carbon uptake and albedo observations from 176 globally distributed flux stations. We demonstrate a gradual decline in maximum achievable annual albedo as carbon uptake increases, even within subgroups of non-forest and snow-free ecosystems. Based on a paired-site permutation approach, we quantify the likely impact of land use on carbon uptake and albedo. Shifting to the maximum attainable carbon uptake at each site would likely cause moderate net global warming for the first approximately 20 years, followed by a strong cooling effect. A balanced policy co-optimizing carbon uptake and albedo is possible that avoids warming on any timescale, but results in a weaker long-term cooling effect.

Networking the forest infrastructure towards near real-time monitoring - A white paper

Forests account for nearly 90 % of the world's terrestrial biomass in the form of carbon and they support 80 % of the global biodiversity. To understand the underlying forest dynamics, we need a long-term but also relatively high-frequency, networked monitoring system, as traditionally used in meteorology or hydrology. While there are numerous existing forest monitoring sites, particularly in temperate regions, the resulting data streams are rarely connected and do not provide information promptly, which hampers real-time assessments of forest responses to extreme climate events. The technology to build a better global forest monitoring network now exists. This white paper addresses the key structural components needed to achieve a novel meta-network. We propose to complement - rather than replace or unify - the existing heterogeneous infrastructure with standardized, quality-assured linking methods and interacting data processing centers to create an integrated forest monitoring network. These automated (research topic-dependent) linking methods in atmosphere, biosphere, and pedosphere play a key role in scaling site-specific results and processing them in a timely manner. To ensure broad participation from existing monitoring sites and to establish new sites, these linking methods must be as informative, reliable, affordable, and maintainable as possible, and should be supplemented by near real-time remote sensing data. The proposed novel meta-network will enable the detection of emergent patterns that would not be visible from isolated analyses of individual sites. In addition, the near real-time availability of data will facilitate predictions of current forest conditions (nowcasts), which are urgently needed for research and decision making in the face of rapid climate change. We call for international and interdisciplinary efforts in this direction.

Correction to: The Tree Drought Emission MONitor (Tree DEMON), an innovative system for assessing biogenic volatile organic compounds emission from plants

[This corrects the article DOI: 10.1186/s13007-017-0166-6.].

Quantification of monoterpene emission sources of a conifer species in response to experimental drought

Monoterpene (MT) emissions of conifer tree species, emitted from de novo synthesis and storage pools, play an important role in plant ecology and physiology. During drought stress both emission sources are affected differently and with conventional measuring techniques they are difficult to separate. We investigated 13C labelled MT emission of eight 3-year-old Scots pine seedlings in a drought stress experiment using a dynamic gas exchange chamber system (Tree DEMON). Monoterpene, water vapour and CO2 gas exchange were measured for a 2-day normal watering, a 11-day treatment and a 3-day re-watering period. In each period all trees were 13C labelled once for 5 h. Results showed the expected decrease of MT, water vapour and CO2 gas exchange with decreasing soil water content. However, during re-watering water vapour and CO2 gas exchange recovered fast to pre-drought levels, whereas MT increased to a lower level compared to the initial non-stressed phase. The 13C labelling showed highly variable %13C values for different MTs, which ranged compound-specific from 0.5 to 95 % for unstressed trees. Overall, around 36 ± 5 % of the total emission rate originated from de novo synthesized MTs during the 2-day prior to stress period. During full drought, the de novo fraction was reduced to 3 %. For the re-watering phase de novo emissions recovered only partly to 20 %, while pool emissions reached pre-drought conditions. Thus, emissions of de novo synthesized MTs of Scots pine are down-regulated by soil drought rather than MT emissions from pools.

The Tree Drought Emission MONitor (Tree DEMON), an innovative system for assessing biogenic volatile organic compounds emission from plants

BACKGROUND

Biogenic volatile organic compounds (BVOC) emitted by plants play an important role for ecological and physiological processes, for example as response to stressors. These emitted compounds are involved in chemical processes within the atmosphere and contribute to the formation of aerosols and ozone. Direct measurement of BVOC emissions requires a specialized sample system in order to obtain repeatable and comparable results. These systems need to be constructed carefully since BVOC measurements may be disturbed by several side effects, e.g., due to wrong material selection and lacking system stability.

RESULTS

In order to assess BVOC emission rates, a four plant chamber system was constructed, implemented and throughout evaluated by synthetic tests and in two case studies on 3-year-old sweet chestnut seedlings. Synthetic system test showed a stable sampling with good repeatability and low memory effects. The first case study demonstrated the capability of the system to screen multiple trees within a few days and revealed three different emission patterns of sweet chestnut trees. The second case study comprised an application of drought stress on two seedlings compared to two in parallel assessed seedlings of a control. Here, a clear reduction of BVOC emissions during drought stress was observed.

CONCLUSION

The developed system allows assessing BVOC as well as CO₂ and water vapor gas exchange of four tree specimens automatically and in parallel with repeatable results. A canopy volume of 30 l can be investigated, which constitutes in case of tree seedlings the whole canopy. Longer lasting experiments of e.g., 1-3 weeks can be performed easily without any significant plant interference.

Impact of summer drought on isoprenoid emissions and carbon sink of three Scots pine provenances

Scots pine (Pinus sylvestris L.) provenances cover broad ecological amplitudes. In a greenhouse study, we investigated the impact of drought stress and rewetting on gas exchange for three provenances (Italy: Emilia Romagna; Spain: Alto Ebro; Germany: East-German lowlands) of 2-year old Scots pine seedlings. CO₂, water vapour and isoprenoid exchange of stressed and control trees were quantified with a four-chamber dynamic-enclosure system in the controlled environment of a climate chamber. The three provenances showed distinct isoprenoid emission patterns and were classified into a non-Δ³-carene, with either high α-/β-pinene or β-myrcene fraction, and a Δ³-carene dominated type. Isoprenoid emission rates, net-photosynthesis and transpiration were reduced during summer drought stress and significantly recovered after rewetting. A seasonal increase of isoprenoid emission rates towards autumn was observed for all control groups. Compared with the German provenance, the Spanish and Italian provenances revealed higher isoprenoid emission rates and more plastic responses to drought stress and seasonal development, which points to a local adaptation to climate. As a result of drought, net carbon uptake and transpiration of trees was reduced, but recovered after rewetting. We conclude from our study that Scots pine isoprenoid emission is more variable than expected and sensitive to drought periods, likely impacting regional air chemistry. Thus, a provenance-specific emission assessment accounting for reduced emission during prolonged (summer) drought is recommend for setting up biogenic volatile organic compound emission inventories used in air quality models.

Climate sensitivity and variation in first flowering of 26 Narcissus cultivars

The first flowering dates of 26 Narcissus cultivars were recorded over a 27-year period on the island of Guernsey, within the English Channel. We analysed flowering trends over time and relationships with climate variables. The study revealed that earlier flowering cultivars advanced most and were more variable than later flowering Narcissus. We furthermore discovered a strong relationship between flowering and climate variables, with temperature appearing to be the main driver. Whilst the first flowering date averaged over all cultivars did show a significant advance, this was significant for only one of the individual cultivars. This is likely because temperatures from December to March had not significantly increased in Guernsey during the study period (1985 to 2011).

Changes in first flowering dates and flowering duration of 232 plant species on the island of Guernsey

Climate change has affected plant phenology; increasing temperatures are associated with advancing first flowering dates. The impact on flowering duration, however, has rarely been studied. In this study, we analysed first flowering dates and flowering durations from a 27 year dataset of weekly flower observations on 232 plant species from the island of Guernsey in the English Channel. The aim of this study was to explore variation in trends and relationships between first flowering dates, flowering duration and temperature. We specifically looked for evidence that traits, such as life forms and phylogenetic groups, explained variation in sensitivity of first flowering and flowering duration among species. Overall trends revealed significantly earlier flowering over time, by an average of 5.2 days decade(-1) since 1985. A highly significant shortening of flowering duration was observed by an average of 10 days decade(-1) . Correlations between first flowering, flowering duration and year varied between different species, traits and flowering periods. Significant differences among traits were observed for first flowering and to a lesser degree for flowering duration. Overall, in comparison to first flowering, more species had significant trends in flowering duration. Temperature relationships revealed large differences in strength and direction of response. 55% of the species revealed a significant negative relationship of first flowering dates and temperature. In contrast, only 19% of flowering durations had a significant negative temperature relationship. The advance in first flowering date together with a shortening of flowering duration suggests potentially serious impacts on pollinators, which might pose a major threat to biodiversity, agriculture and horticulture. Human health, in terms of pollen allergies, however, might benefit from a shortening of specific plant pollen seasons.

Nitrogen deposition along differently exposed slopes in the Bavarian Alps

The Alps are affected by high nitrogen deposition, particularly in the fringe of the Northern and Southern Alps. In the framework of a two-year monitoring study performed in 2010 and 2011, we investigated the ammonia and nitrogen dioxide air concentration and ammonium and nitrate deposition at different altitudes between 700 and 1,600 ma.s.l. in the Garmisch-Partenkirchen district in the Upper Bavaria region (Germany). Four-weekly measurements of deposition collected with bulk open field samplers and under-crown were performed in a profile perpendicular to the axis of the Loisach valley; measurements were conducted at eight sites. Whereas open field deposition ranged from 5 to 11 kg ha(-1)a(-1), nitrogen throughfall has reached up to 21 kg ha(-1)a(-1). Data from the valley and the slopes were compared with measurements performed on the platform of the Environmental Research Station Schneefernerhaus (Zugspitze) at an altitude of 2,650 ma.s.l. For the rough estimation of the total yearly deposition rate of nitrogen, the canopy uptake model was applied. By regarding nitrogen uptake by the trees, total deposition can exceed the throughfall in all sites by up to 50%. Additionally, we estimated the total deposition from the sum of wet and dry deposition. On the one side, the wet deposition could be extrapolated from the open field deposition. On the other side, we used the inferential method to calculate the dry deposition on the basis of NH3 and NO2 air concentrations and their literature based deposition velocities. Since fixed deposition velocities are inappropriate particularly in complex orography, we tried to find correction factors based upon terrain characteristics and meteorological considerations. Temperature monitoring at the eight sites and wind measurements at two sites provided some evidence for the semi-empirical parameterization. Due to numerous imponderabilities, the results of the two methods were not consistent for all sites.

Enhanced ozone strongly reduces carbon sink strength of adult beech (Fagus sylvatica)--resume from the free-air fumigation study at Kranzberg Forest

Ground-level ozone (O(3)) has gained awareness as an agent of climate change. In this respect, key results are comprehended from a unique 8-year free-air O(3)-fumigation experiment, conducted on adult beech (Fagus sylvatica) at Kranzberg Forest (Germany). A novel canopy O(3) exposure methodology was employed that allowed whole-tree assessment in situ under twice-ambient O(3) levels. Elevated O(3) significantly weakened the C sink strength of the tree-soil system as evidenced by lowered photosynthesis and 44% reduction in whole-stem growth, but increased soil respiration. Associated effects in leaves and roots at the gene, cell and organ level varied from year to year, with drought being a crucial determinant of O(3) responsiveness. Regarding adult individuals of a late-successional tree species, empirical proof is provided first time in relation to recent modelling predictions that enhanced ground-level O(3) can substantially mitigate the C sequestration of forests in view of climate change.

Simulating stand climate, phenology, and photosynthesis of a forest stand with a process-based growth model

In the face of climate change and accompanying risks, forest management in Europe is becoming increasingly important. Model simulations can help to understand the reactions and feedbacks of a changing environment on tree growth. In order to simulate forest growth based on future climate change scenarios, we tested the basic processes underlying the growth model BALANCE, simulating stand climate (air temperature, photosynthetically active radiation (PAR) and precipitation), tree phenology, and photosynthesis. A mixed stand of 53- to 60-year-old Norway spruce (Picea abies) and European beech (Fagus sylvatica) in Southern Germany was used as a reference. The results show that BALANCE is able to realistically simulate air temperature gradients in a forest stand using air temperature measurements above the canopy and PAR regimes at different heights for single trees inside the canopy. Interception as a central variable for water balance of a forest stand was also estimated. Tree phenology, i.e. bud burst and leaf coloring, could be reproduced convincingly. Simulated photosynthesis rates were in accordance with measured values for beech both in the sun and the shade crown. For spruce, however, some discrepancies in the rates were obvious, probably due to changed environmental conditions after bud break. Overall, BALANCE has shown to respond to scenario simulations of a changing environment (e.g., climate change, change of forest stand structure).

Ozone fumigation (twice ambient) reduces leaf infestation following natural and artificial inoculation by the endophytic fungus Apiognomonia errabunda of adult European beech trees

In 2006, a controlled infection study was performed in the 'Kranzberger Forst' to address the following questions: (1) Will massive artificial inoculation with Apiognomonia errabunda override the previously observed inhibitory effect of chronic ozone? (2) Can biochemical or molecular markers be detected to account for the action of ozone? To this end six adult beech trees were chosen, three ozone fumigated (2x ozone) and three control trees (ambient = 1x ozone). Spore-sprayed branches of sun and shade crown positions of each of the trees, and uninoculated control branches, were enclosed in 100-L plastic bags for one night to facilitate infection initiation. Samples were taken within a five-week period after inoculation. A. errabunda infestation levels quantified by real-time PCR increased in leaves that were not fumigated with additional ozone. Cell wall components and ACC (ethylene precursor 1-amino cyclopropane-1-carboxylic acid) increased upon ozone fumigation and may in part lead to the repression of fungal infection.

Spectral multichannel monitoring of radiation within a mature mixed forest

A multi-sensor system is described based on fiber optic technology and a diode array spectrometer for near-simultaneous measurement of spectral photon fluence rates (PFR) in the range of 360 nm to 1020 nm with a resolution of 0.8 nm, within a mature Norway spruce ( Picea abies [L.] Karst.) - European beech ( Fagus sylvatica L.) stand. 126 space-integrating spherical sensors, deployed in a regular grid above and within the canopy and on the forest floor, are sequentially connected to the spectrometer by means of fiber optics. About 1 s per sensor is needed to collect spectral data, store them on hard disk and move the channel multiplexer to the next fiber optic position. Data thus obtained serve to determine vertical profiles of wavelength-dependent photon extinction, especially for spectral ratios and wavebands, characterization of phenological stages, analyses of time series, and meteorological influences such as solar altitude and cloud cover. First measurements during leaf fall 2004 show a non-linear relation of the red/far-red ratio (R/FR) with relative photosynthetic PFR (PPFR (rel)). An analysis of relative PFR (PFR (rel)) quantifies the frequency of penumbral sunfleck occurrence and the fraction of incoming radiation on the forest floor. In-canopy measurements of daily means of PPFR (rel) and R/FR indicate that leaf unfolding and leaf fall can be described by a single sensor, independent of its vertical location within the canopy.

Comparison of ozone uptake and sensitivity between a phytotron study with young beech and a field experiment with adult beech (Fagus sylvatica)

Chamber experiments on juvenile trees have resulted in severe injury and accelerated loss of leaves along with reduced biomass production under chronically enhanced O3 levels. In contrast, the few studies conducted on adult forest trees in the field have reported low O3 sensitivity. In the present study, young beech in phytotrons was more sensitive to O3 than adult beech in the field, although employed O3 regimes were similar. The hypotheses tested were that: (1) differences in O3 uptake were caused by the ontogenetically higher stomatal conductance of young compared to adult trees, (2) the experimental settings in the phytotrons enhanced O3 uptake compared to field conditions, and (3) a low detoxification capacity contributes to the higher O3 sensitivity of the young trees. The higher O3 sensitivity of juvenile beech in the phytotrons is demonstrated to relate to both the experimental conditions and the physiological responsiveness inherent to tree age.