Jatropha curcas recently is highly promoted to become an important source for biofuel production ... more Jatropha curcas recently is highly promoted to become an important source for biofuel production in tropical regions. The often mentioned advantage of the species is it´s assumed adaptation to droughtstress and the possibility to cultivate plantations on marginal land under arid climate conditions. Little is known, however, on the productivity under these conditions and the mechanisms of drought tolerance. To test the response of photosynthesis of different Jatropha species to water limitations as a measure of adaptation and productivity we combined a green house experiment with measurements under natural conditions. In all cases we estimated photosynthetic rates and stomatal conductance from light response curves in relation to soil moisture. The green house experiment was performed in G¨ottingen with Jatropha curcas and the field trials on a farm in the vicinity of Patos, Paraiba in the north-east of Brazil with Jatropha mollissima as a natural element of the regional dry forests....
Jatropha curcas recently is highly promoted to become an important source for biofuel production ... more Jatropha curcas recently is highly promoted to become an important source for biofuel production in tropical regions. The often mentioned advantage of the species is it´s assumed adaptation to droughtstress and the possibility to cultivate plantations on marginal land under arid climate conditions. Little is known, however, on the productivity under these conditions and the mechanisms of drought tolerance.
Background/Question/Methods Despite the widespread recognition that root hydraulic conductivity (... more Background/Question/Methods Despite the widespread recognition that root hydraulic conductivity (Lp) is a major determining factor of whole plant water loss and/or status, there is very little information on how it responds to rising levels of atmospheric CO2. Studies of plant water status responses to high CO2 often use stomatal conductance (gs) as a critical mechanism but it is currently unknown if Lp is coordinated or decoupled from shoot conductances. Therefore, in this study we examined Lp of four contrasting conifers to CO2 enrichment. Because Lp has been shown to be highly sensitive to availability and/or N-form we also examined if the potential CO2 effects are mediated by nitrate-N or ammonium-N nutrition. One year-old seedlings of Colorado spruce (Picea pungens), Douglas-fir (Pseudotsuga menziesii), Frasier-fir (Abies fraseri) and Norway spruce (Picea abies) were grown at near ambient (400 ppm) or elevated (600 or 800 ppm) CO2 concentrations under identical environmental re...
Forest transpiration estimates are frequently based on xylem sap flux measurements in the outer s... more Forest transpiration estimates are frequently based on xylem sap flux measurements in the outer sections of the hydro-active stem sapwood. We used Granier's constantheating technique with heating probes at various xylem depths to analyze radial patterns of sap flux density in the sapwood of seven broad-leaved tree species differing in wood density and xylem structure. Study aims were to (1) compare radial sap flux density profiles between diffuse-and ring-porous trees and (2) analyze the relationship between hydro-active sapwood area and stem diameter. In all investigated species except the diffuse-porous beech (Fagus sylvatica L.) and ring-porous ash (Fraxinus excelsior L.), sap flux density peaked at a depth of 1 to 4 cm beneath the cambium, revealing a hump-shaped curve with species-specific slopes. Beech and ash reached maximum sap flux densities immediately beneath the cambium in the youngest annual growth rings. Experiments with dyes showed that the hydro-active sapwood occupied 70 to 90% of the stem cross-sectional area in mature trees of diffuse-porous species, whereas it occupied only about 21% in ring-porous ash. Dendrochronological analyses indicated that vessels in the older sapwood may remain functional for 100 years or more in diffuse-porous species and for up to 27 years in ring-porous ash. We conclude that radial sap flux density patterns are largely dependent on tree species, which may introduce serious bias in sap-flux-derived forest transpiration estimates, if non-specific sap flux profiles are assumed.
The importance of tree species identity and diversity for biogeochemical cycles in forests is not... more The importance of tree species identity and diversity for biogeochemical cycles in forests is not well understood. In the past, forestry has widely converted mixed forests to pure stands while contemporary forest policy often prefers mixed stands again. However, the hydrological consequences of these changes remain unclear. We tested the hypotheses (i) that significant differences in water use per ground area exist among the tree species of temperate mixed forests and that these differences are more relevant for the amount of stand-level canopy transpiration (E c ) than putative complementarity effects of tree water use, and (ii) that the seasonal patterns of E c in mixed stands are significantly influenced by the identity of the present tree species. We measured xylem sap flux during 2005 (average precipitation) and 2006 (relatively dry) synchronously in three nearby old-growth forest stands on similar soil differing in the abundance of European beech (pure beech stand, 3-species stand with 70% beech, 5-species stand with <10% beech). In summer 2005 with average rainfall, E c was 50% higher in the beech-poor 5-species stand than in the two stands with moderate to high beech presence (158 vs. 97 and 101 mm yr À1 ); in the dry summer 2006, all stands converged toward similar E c totals (128-139 mm yr À1 ). Species differences in E c were large on a sapwood area basis, reflecting a considerable variation in hydraulic architecture and leaf conductance regulation among the co-existing species. Moreover, transpiration per crown projection area (E CA ) also differed up to 5-fold among the different species in the mixed stands, probably reflecting contrasting sapwood/crown area ratios. We conclude that E c is not principally higher in mixed forests than in pure beech stands. However, tree species-specific traits have an important influence on the height of E c and affect its seasonal variation. Species with a relatively high E CA (notably Tilia) may exhaust soil water reserves early in summer, thereby increasing drought stress in dry years and possibly reducing ecosystem stability in mixed forests.
We examined root hydraulic conductivity (L p ) responses of one-year-old seedlings of four conife... more We examined root hydraulic conductivity (L p ) responses of one-year-old seedlings of four conifers to the combined effects of elevated CO 2 and inorganic nitrogen (N) sources. We found marked interspecific differences in L p responses to high CO 2 ranging from a 37% increase in P. abies to a 27% decrease in P. menziesii, but these effects depended on N source. The results indicate that CO 2 effects on root water transport may be coupled to leaf area responses under nitrate (NO 3 À ), but not ammonium (NH 4 þ ) dominated soils. To our knowledge, this is the first study that highlights the role of inorganic N source and species identity as critical factors that determine plant hydraulic responses to rising atmospheric CO 2 levels. The results have important implications for understanding root biology in a changing climate and for models designed to predict feedbacks between rising atmospheric CO 2 , N deposition, and ecohydrology.
Elevated CO 2 significantly affected k S but this effect was highly species dependent. For exampl... more Elevated CO 2 significantly affected k S but this effect was highly species dependent. For example, in Colorado spruce, increasing CO 2 levels from 400 to 800 ppm, increased k s by an average of 23 % across all N treatments but decreased k s in Douglas fir by 24 %. ...
& Introduction Sight-based field measurements of tree crown projection area and canopy height are... more & Introduction Sight-based field measurements of tree crown projection area and canopy height are common praxis in forest science but difficult to validate. We quantified their measurement errors based on the virtual representation of an 11-species old-growth forest provided by high-resolution terrestrial LIDAR (light detection and ranging) measurements. & Objectives Based on the expectations (a) that violations of the triangulation theory are the main error source of height measurements, and (b) that approximations of tree crowns with fixed angles are not flexible enough for irregular crown shapes in natural stands, we investigated the relative accuracies of triangulation measurements of height of crown base (B T ) vs. tree height (H T ) and of different crown projection methods. B T (±0.52 m) showed lower measurement errors than H T (±2.4 m). & Results and conclusions Larger deviations between fieldmeasured and virtually executed crown projections could partly be attributed to structural differences of the crowns that were two-dimensionally quantified as space capture index (SCI). The largest deviations between both methods occurred on suppressed tree crowns and tall Quercus robur trees in the stand. Because of the method-inherent underestimation of crown projections with fixed angular grid, we propose the use of flexible angles by trained operators.
Keywords: leaf conductance / leaf water potential / drought sensitivity / mixed forest / xylem sa... more Keywords: leaf conductance / leaf water potential / drought sensitivity / mixed forest / xylem sap flux Abstract • Five temperate broad-leaved tree species were compared with respect to their water consumption strategies under ample and restricted water supply. We measured synchronously leaf conductance (g L ) in the sun canopy, xylem sap flux (J s ) and leaf water potential (predawn, Ψ pd and noon, Ψ noon ) in adult trees in a mixed stand and related them to the fluctuations in vapor pressure deficit (D) and soil moisture.
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Papers by Tobias Gebauer