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-/ß-pinene or ß-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.

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.

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.

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.