No xylem phenotypic plasticity in mature Picea abies and Fagus sylvatica trees after five years of throughfall precipitation exclusion

Forest trees are experiencing increasing frequency and intensity of drought events with climate change. We investigated xylem and phloem traits from mature Fagus sylvatica and Picea abies trees after 5 years of complete exclusion of throughfall precipitation during the growing season. Xylem and phloem anatomy, leaf and branch biomass were analysed along top branches of ~1.5 m lenght in 5 throughfall precipitation excluded (TE) and 5 control (CO) trees of both beech and spruce. Xylem traits were analysed on wood cores extracted from the stem at breast height. In the top branches of both species, the lumen diameter (or area) of xylem and phloem conduits did not differ between TE and CO trees. At breast height, TE trees of both species produced narrower xylem rings and conduits. While allocation to branch (BM) and needle biomass (LM) did not change between TE and CO in P. abies, TE F. sylvatica trees allocated proportionally more biomass to leaves (LM) than BM compared to CO. Despite artificial drought increased the mortality in the TE plots, our results revealed no changes in both xylem and phloem anatomies, undermining the hypothesis that successful acclimation to drought would primarily involve increased resistance against air embolism

Dataset of "No xylem phenotypic plasticity in mature Picea abies and Fagus sylvatica trees after five years of throughfall precipitation exclusion"

This collection contains the data used in the article entitled "No xylem phenotypic plasticity in mature Picea abies and Fagus sylvatica trees after five years of throughfall precipitation exclusion" published in Global Change Biolog

Switching the light off: A break in photosynthesis and sap flow of forest trees under total solar eclipse

In mature trees of Picea abies, Fagus sylvatica and Quercus robur, photosynthesis and transpiration were assessed in response to the total solar eclipse that occurred in Central Europe during the late morning hours of August 11, 1999, a day with changing cloudiness. Measurements were conducted at three forest sites located in the totality zone and the 99% area of the eclipse within a radius of about 100 km around the city of Munich (southern Germany). The eclipse lasting 164 minutes lowered the photosynthetic photon flux density (PPFD) to about 1 μmol m-2 s-1 during the 2-minute totality period, when the sky was clear. During totality, photosynthesis was reduced to an extent that allowed CO2 release to dominate the gas exchange of leaves. Effects on transpiration were less pronounced as the totality was apparently too short to induce distinct stomatal closure in response to low PPFD. Transpiration was strongly reduced, however, by increased air humidity and wet leaf surfaces during sporadic rain showers which preceded or succeeded the eclipse during the same day, whereas low PPFD through intermittent cloudiness during rain only moderately reduced photosynthesis. Although transpiration was lowered to a minor extent only by the eclipse, the latter affected the water transport through the whole tree, as reflected in a decline in the sap flow rate through the basal stem part with a time delay depending on the species. Nevertheless, trees responded in a synchronous way, regardless of the site, species or the percent degree of the eclipse

What Are We Missing? Occlusion in Laser Scanning Point Clouds and Its Impact on the Detection of Single-Tree Morphologies and Stand Structural Variables

Laser scanning has revolutionized the ability to quantify single-tree morphologies and stand structural variables. In this study, we address the issue of occlusion when scanning a spruce (Picea abies (L.) H.Karst.) and beech (Fagus sylvatica L.) forest with a mobile laser scanner by making use of a unique study site setup. We scanned forest stands (1) from the ground only and (2) from the ground and from above by using a crane. We also examined the occlusion effect by scanning in the summer (leaf-on) and in the winter (leaf-off). Especially at the canopy level of the forest stands, occlusion was very pronounced, and we were able to quantify its impact in more detail. Occlusion was not as noticeable as expected for crown-related variables but, on average, resulted in smaller values for tree height in particular. Between the species, the total tree height underestimation for spruce was more pronounced than that for beech. At the stand level, significant information was lost in the canopy area when scanning from the ground alone. This information shortage is reflected in the relative point counts, the Clark–Evans index and the box dimension. Increasing the voxel size can compensate for this loss of information but comes with the trade-off of losing details in the point clouds. From our analysis, we conclude that the voxelization of point clouds prior to the extraction of stand or tree measurements with a voxel size of at least 20 cm is appropriate to reduce occlusion effects while still providing a high level of detail

Post-drought hydraulic recovery is accompanied by non-structural carbohydrate depletion in the stem wood of Norway spruce saplings

Abstract Hydraulic failure and carbon starvation are recognized as main causes of drought-induced forest decline. As water transport and carbon dynamics are strictly interdependent, it is necessary to clarify how dehydration-rehydration cycles are affecting the relations between stem embolism and non-structural carbohydrates (NSC). This is particularly needed for conifers whose embolism repair capability is still controversial. Potted Norway spruce saplings underwent two drought-re-irrigation cycles of same intensity, but performed in two consecutive summers. During the second cycle, stem percent loss of hydraulic conductivity (PLC) and NSC content showed no carry-over effects from the previous drought, indicating complete long-term recovery. The second drought treatment induced moderate PLC (20%) and did not affect total NSCs content, while starch was converted to soluble sugars in the bark. After one week of re-irrigation, PLC recovered to pre-stress values (0%) and NSCs were depleted, only in the wood, by about 30%. Our data suggest that spruce can repair xylem embolism and that, when water is newly available, NSCs stored in xylem parenchyma can be mobilized over short term to sustain respiration and/or for processes involved in xylem transport restoration. This, however, might imply dependency on sapwood NSC reserves for survival, especially if frequent drought spells occur

The influence of microclimate and tree age on the defense capacity of European beech (Fagus sylvatica L.) against oxidative stress

Microclimate and tree age have been suggested to be factors influencing the defense capacity against oxidative stress. Therefore, 5-year-old Fagus sylvatica seedlings were grown on a scaffolding in the sun and shade crown of 55-year-old trees throughout one growing season. Independent of tree age sun leaves had a lower specific leaf area, lower pigment contents and a more capacitive antioxidative system than shade leaves. In addition, in the sun crown leaves of seedlings displayed a higher specific leaf area than leaves of adult trees. Age dependent changes in leaf morphology were related to changes in the defense capacity against oxidative stress, with area based concentrations of antioxidants and pigments increasing with tree age. Thus our results suggest that differences in the response to oxidative stress may be attributed to age and crown position related differences in the specific leaf area, the latter influencing the biochemical and physiological performance of Fagus sylvatica leaves.L'influence du microclimat et de l'âge de l'arbre sur la capacité défensive du hêtre européen (Fagus sylvatica L.) contre le stress oxidatif. Le microclimat et l'âge de l'arbre sont considerés comme des facteurs qui influencent la capacité défensive contre le stress oxidatif. Par conséquent, des plants de Fagus sylvatica de cinq ans ont grandi sur un échafaudage à la couronne des arbres âgés de 55 ans, une fois au soleil, une autre fois à l'ombre pendant une période de croissance. Indépendamment de l'âge de l'arbre, le feuillage exposé au soleil avait une surface spécifique de la feuille inférieure, un contenu de pigment inférieur et un système antioxidatif plus puissant que le feuillage à l'ombre. De plus, au niveau de la couronne la plus haute, le feuillage des plants a développé une surface spécifique plus élevée que celui des arbres adultes. Des transformations de morphologie du feuillage, qui dépendent de l'âge des feuilles, ont été comparées avec des transformations de la capacité contre le stress oxidatif. À cela la teneur en substances antioxidatives relative à la surface de la feuille a augmenté avec l'âge de l'arbre. Donc nos résultats suggèrent que les différences de réaction au stress oxidatif peuvent être attribuées aux différences de la surface spécifique de la feuille - dépendant de l'âge et de la position à la couronne - ce qui influence la performance biochimique et physiologique des feuilles du Fagus sylvatica

Effects of chronic elevated ozone exposure on gas exchange responses of adult beech trees (Fagus sylvatica) as related to the within-canopy light gradient

The effects of elevated O3 on photosynthetic properties in adult beech trees (Fagus sylvatica) were investigated in relation to leaf mass per area as a measure of the gradually changing, within-canopy light availability. Leaves under elevated O3 showed decreased stomatal conductance at unchanged carboxylation capacity of Rubisco, which was consistent with enhanced δ13C of leaf organic matter, regardless of the light environment during growth. In parallel, increased energy demand for O3 detoxification and repair was suggested under elevated O3 owing to enhanced dark respiration. Only in shade-grown leaves, light-limited photosynthesis was reduced under elevated O3, this effect being accompanied by lowered Fv/Fm. These results suggest that chronic O3 exposure primarily caused stomatal closure to adult beech trees in the field regardless of the within-canopy light gradient. However, light limitation apparently raised the O3 sensitivity of photosynthesis and accelerated senescence in shade leaves. Across leaf differentiation in adult beech crowns, elevated ozone acted through stomatal closure on gas exchange although enhancing photosynthetic sensitivity of shaded leaves

The lesson learnt from two long-term precipitation exclusion experiments: how intensity and duration of drought may influence xylem and phloem plasticity

The last decades have been characterized by an increased number of extreme events, such as droughts and heatwaves which had a clear impact on plants, with effects on tree status, vigor decline and mortality. Mechanism of acclimation and adaptation to drought are still to be fully understood, but they play a critical role in the evaluation of climate change effects on vegetation’s survival. Throughfall exclusion experiments may give great insights about all the possible reactions to drought in standing vegetation. We investigated xylem and phloem anatomical traits and leaf/branch biomass from mature Piñon Pine (Pinus edulis Engelm.), Norway spruce (Picea abies Karst.) and beech (Fagus sylvatica L.) in two throughfall exclusion experiments(TEEs). One TEE is the “Kranzberg Roof Project” (KROOF) in Bavaria and the second is the Sevilleta Long Term Ecological Research (LTER) project (New Mexico, USA). Sampling in Sevilleta was carried out on 1,5m long branches in 4 different experimental plots of Piñon Pines: control (CO), long term 45% rain-off (Legacy, 10 years), short term 45% rain-off (New45, 1 year) and short term 90% rain-off (New90, 1 year). Sampling in Kranzberg Forest was carried out on 1,5m long branches in 2 experimental plots made by mixed stands of spruce and beech: control (CO) and troughfall-excluded (TE, 5 years -70% rain). Experimental results show that the biomass allocation and the ratio between needles and branches did not differ between treatments with a clear isometric relationship for Sevilleta’s Piñon Pines and KROOF’s Norway Spruces. On the contrary F. sylvatica trees allocated proportionally more biomass to leaves than biomass compared to control trees. For Sevilleta the xylem tracheid diameter of the outermost ring is larger only in New90 treatment, but the overall trend in the last decade shows no difference between legacy and control. The same trend is seen in the apical branches of KROOF experiment, where for both species the lumen diameter (or area) of xylem conduits did not differ between TE and CO trees. Phloem cell diameter display larger sieve cell perimeter only in Legacy treatment (Sevilleta) but show no changes in other treatments and in KROOF experiment. These results would suggest only few phenotypic plasticity changes for xylem, phloem, and biomass allocation. Furthermore, these plastic changes appear to be different for each species and treatment intensity, but they all push towards an increased efficiency of the overall hydraulic transport system

The lesson learnt from two long-term precipitation exclusion experiments: xylem/phloem plasticity is not a real option in trees

The last decades have been characterized by an increased number of extreme events, such as droughts and heatwaves which had a clear impact on plants, with effects on tree status, vigor decline and mortality. Mechanism of acclimation and adaptation to drought are still to be fully understood, but they play a critical role in the evaluation of climate change effects on vegetation’s survival. Throughfall exclusion experiments may give great insights about all the possible reactions to drought in standing vegetation. We investigated xylem and phloem anatomical traits and leaf/branch biomass from mature Piñon Pine (Pinus edulis Engelm.) in a throughfall exclusion experiment at the Sevilleta Long Term Ecological Research (LTER) project (New Mexico, USA). Sampling was carried out on 1,5m long branches in 4 different experimental plots : control (CO), long term 45% rain-off (Legacy), short term 45% rain-off (New45) and short term 90% rain-off (New90). Preliminary results show that the biomass allocation and the ratio between needles and branches did not differ between treatments with a clear isometric relationship. The xylem tracheid diameter of the outermost ring is larger in Legacy and New90 treatments. Phloem cell diameter follows the same pattern, with larger sieve cell perimeter in Legacy treatment, but appears lower in the New45 treatment. These results would suggest an adaptation to drought by a modification of the vascular structure, without any significant change in biomass allocation for needles and branches