Gaby Deckmyn

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.

Poplar growth and yield in short rotation coppice: model simulations using the process model SECRETS

Abstract The process model SECRETS was adapted to simulate coppice growth of poplar. The effects of soil type, irrigation, nitrogen fertilization and rotation cycle on growth and yield were studied. Simulated average production on an agricultural soil was 12.4 t ha −1 year −1 . Poplar growth was strongly reduced on sandy soils ( 6 t ha −1 year −1 ). Irrigation increased yield on all soil types by 25% while fertilization increased yield by 26%. The highest yield was simulated for the combined irrigation/fertilization ( 22.45 t ha −1 year −1 ). Length of rotation cycle greatly influenced yield; optimal rotation cycle in terms of yield was 3 or 4 years. The results clearly indicate a useful role for process models in predicting effects of management strategies on poplar yield in short rotation coppice.

Variation in leaf flushing date influences autumnal senescence and next year’s flushing date in two temperate tree species

Significance Leaf phenology of temperate ecosystems is shifting in response to global warming. This affects surface albedo, ecosystem carbon balance, and evapotranspiration, and the response of leaf phenology to climatic drivers has therefore received particular interest. However, despite considerable effort, models have failed to accurately reproduce phenology patterns, likely because mechanistic understanding is incomplete. Here, we show that earlier leaf flushing in response to a warm winter translated into earlier leaf senescence and even earlier leaf flushing in the following year. This legacy effect of winter warming on leaf phenology has important implications for understanding and modelling leaf phenology and its impact on ecosystem functioning, especially in relation to global warming, and is likely to open new research lines. Recent temperature increases have elicited strong phenological shifts in temperate tree species, with subsequent effects on photosynthesis. Here, we assess the impact of advanced leaf flushing in a winter warming experiment on the current year’s senescence and next year’s leaf flushing dates in two common tree species: Quercus robur L. and Fagus sylvatica L. Results suggest that earlier leaf flushing translated into earlier senescence, thereby partially offsetting the lengthening of the growing season. Moreover, saplings that were warmed in winter–spring 2009–2010 still exhibited earlier leaf flushing in 2011, even though the saplings had been exposed to similar ambient conditions for almost 1 y. Interestingly, for both species similar trends were found in mature trees using a long-term series of phenological records gathered from various locations in Europe. We hypothesize that this long-term legacy effect is related to an advancement of the endormancy phase (chilling phase) in response to the earlier autumnal senescence. Given the importance of phenology in plant and ecosystem functioning, and the prediction of more frequent extremely warm winters, our observations and postulated underlying mechanisms should be tested in other species.

The Impact of Winter and Spring Temperatures on Temperate Tree Budburst Dates: Results from an Experimental Climate Manipulation

Budburst phenology is a key driver of ecosystem structure and functioning, and it is sensitive to global change. Both cold winter temperatures (chilling) and spring warming (forcing) are important for budburst. Future climate warming is expected to have a contrasting effect on chilling and forcing, and subsequently to have a non-linear effect on budburst timing. To clarify the different effects of warming during chilling and forcing phases of budburst phenology in deciduous trees, (i) we conducted a temperature manipulation experiment, with separate winter and spring warming treatments on well irrigated and fertilized saplings of beech, birch and oak, and (ii) we analyzed the observations with five temperature-based budburst models (Thermal Time model, Parallel model, Sequential model, Alternating model, and Unified model). The results show that both winter warming and spring warming significantly advanced budburst date, with the combination of winter plus spring warming accelerating budburst most. As expected, all three species were more sensitive to spring warming than to winter warming. Although the different chilling requirement, the warming sensitivity was not significantly different among the studied species. Model evaluation showed that both one- and two- phase models (without and with chilling, respectively) are able to accurately predict budburst. For beech, the Sequential model reproduced budburst dates best. For oak and birch, both Sequential model and the Thermal Time model yielded good fit with the data but the latter was slightly better in case of high parameter uncertainty. However, for late-flushing species, the Sequential model is likely be the most appropriate to predict budburst data in a future warmer climate.

The ratio UV-B/photosynthetically active radiation (PAR) determines the sensitivity of rye to increased UV-B radiation

Abstract To evaluate the effect of different naturally occurring irradiation conditions on the sensitivity of rye (Secale cereale L.) to increased UV-B levels, plants were grown under eight different light treatments. In the control series (at ambient levels of UV-B), UV-B and visible light were decreased in parallel, resulting in four different total irradiation treatments with the same UV-B/photosynthetically active radiation (PAR) ratios. A second series with a 30% increase in UV-B irradiation at each PAR level was used to investigate the effect of UV-B under the varying total irradiance levels. The different total irradiance levels resulted in significant differences in total dry weight, specific leaf weight, photosynthesis-light response and pigment concentrations. A 30% increase in UV-B resulted in equal reductions in total dry weight (from 20.0 to 28.6%) and effective photosynthesis for all light levels. The accumulation of protective pigments in the leaves was stimulated by PAR and even more by UV-B, except at the highest UV-B irradiation, where a small decrease was noted. These results indicate that rye plants are able to adapt to changes in the natural light environment as long as the ratio UV-B/PAR is constant.

Combined effects of enhanced UV-B radiation and nitrogen deficiency on the growth, composition and photosynthesis of rye (Secale cereale)

The interactive effects of N-deficiency and enhanced UV-B radiation on growth, photosynthesis and pigmentation of rye were studied. The plants were grown for 5 weeks in growth chambers with high (700 μmolm-2 s-2) irradiance levels. A 30% difference in UV-B at plant level was achieved by using different thicknesses of UV-B transparent Plexiglass. One half of the plants received optimal N nutrition, while the other received half of this dose. Both enhanced UV-B and N deficiency strongly decreased production (from 24–33%). The combined effect was additive (no interaction) on most parameters, including total dry weight production which was 52% lower than in the control series. Significant interaction was found on the root/shoot ratio. While reduced N supply induced an increase in the ratio at normal UV-B irradiation, under the increased UV-B, N deficiency had no effect on the root/shoot ratio. The reduced biomass due to UV-B was clearly correlated to a reduction in photosynthesis. At optimal N supply the plants increased the production of protective pigments in response to UV-B, but at reduced N supply this response was lacking. The increased N content of the high UV-B/high N plants could be a result of increased flavonoid production as well as changes in light penetration in the canopy.

Seasonal responses of six Poaceae to differential levels of solar UV-B radiation

Abstract The effects of changes in solar UV-B on the growth and pigmentation of six grass species from cold-temperate grasslands (Lolium perenne, Lolium multiflorum, Festuca arundinacea, Festuca rubra, Phleum pratense and Dactylis glomerata) in spring and summer were studied. The grasses were grown in greenhouses with different foils, resulting in three treatments: no UV-B, 80% of ambient and 90% of ambient UV-BBE.(biologically effective UV-B). The results indicated important effects of ambient UV-B levels on grass, but the different species reacted in very different ways. Both morphology and biomass production were influenced by UV-B in some species. However, changes in biomass production did not necessarily occur within the same species as changes in morphology. The grasses were more sensitive in summer. Overall, only F. rubra was positively influenced by UV-B under all circumstances. The biomass of D. glomerata and L. perenne was reduced by UV-B in spring and summer. Morphological changes included reduced height and increased tillering. The sensitivity of the different species was partially explained by their ability to reduce their specific leaf area in response to UV-B. Only the more sensitive species showed increased production of protective pigments. Overall, there were important differences between the effect of a low level of UV-B, and the further increase in UV-B, indicating that several mechanisms are operating at different light levels.

Correction to Multilayered Modeling of Particulate Matter Removal by a Growing Forest over Time, From Plant Surface Deposition to Washoff via Rainfall

Airborne fine particulate matter (PM) is responsible for the most severe health effects induced by air pollution in Europe. Vegetation, and forests in particular, can play a role in mitigating this pollution since they have a large surface area to filter PM out of the air. Many studies have solely focused on dry deposition of PM onto the tree surface, but deposited PM can be resuspended to the air or may be washed off by precipitation dripping from the plants to the soil. It is only the latter process that represents a net-removal from the atmosphere. To quantify this removal all these processes should be accounted for, which is the case in our modeling framework. Practically, a multilayered PM removal model for forest canopies is developed. In addition, the framework has been integrated into an existing forest growth model in order to account for changes in PM removal efficiency during forest growth. A case study was performed on a Scots pine stand in Belgium (Europe), resulting for 2010 in a dry deposit...

Reduced UV-B in greenhouses decreases white clover response to enhance CO2

Abstract The interaction between reduced UV-B radiation and elevated CO 2 on the growth, allocation and physiology of white clover ( Trifolium repens ) was studied in greenhouses of different UV-B transmittance (82 and 88% of ambient) with either ambient (371 ppm) or elevated (521 μmol mol −1 ) CO 2 concentration. The 88% UV-B levels in increased growth and flowering, indicating an important role for ambient UV-B levels in plant functioning and growth. High CO 2 increased growth significantly only under the 88% UV-B level. The interaction between UV-B and CO 2 effects on total biomass was significant. Allocation was affected significantly by both UV-B (increased root growth, increased flowering) and CO 2 (increased flowering, reduced stem weight). The results clearly indicate the importance of using UV-B transmittant greenhouse or open-top chamber when conducting CO 2 studies, or at least stating the reduction in UV-B within greenhouses.

UV-B and PAR in a grass (Lolium perenne L.) canopy

The penetration of natural and artificial UV-BBE (Biologically Effective UV-B, Caldwell 1971) and PAR (400–700 nm) in a grass canopy with increasing LAI was followed during 2 months. Overall, the transmission of UV-BBE sunlight is significantly higher than of PAR sunlight. This is mainly due to the higher proportion of diffuse light in the UV-B. Under cloudy conditions no difference between UV-BBE and PAR could be found. Sun angle and intensity of the radiation were less important in determining the penetration of light. Artificial light penetrates much more through the canopy, resulting in higher irradiation levels in the lower part of the canopy, but a lower UV-BBE/PAR ratio (since UV-B transmittance of the leaves is lower). The UV-BBE/PAR ratio reaching the leaves was influenced by LAI, sun angle, percent diffuse light and leaf angle. The large differences in UV-BBE/PAR ratio per unit leaf area under natural and artificial light conditions are important in understanding the influence of UV-B on plants.