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Fraxinus ornus trees differ in biomass allocation and phloem anatomy along an environmental gradient
No full textTree growth responses to different environmental conditions
No full textIncreasing frequency and intensity of droughts affect forest ecosystems. There are already higher rates of tree mortality and forest drought-induced dieback. Since climate change affects the soil conditions, the precipitation rates and the growth seasons, it is expected that trees will develop acclimation and adaptation strategies in order to maintain vigor. Since the two most important reasons of mortality are carbon starvation and hydraulic failure, adjustments in physiological traits are expected to be related to the maintenance of the tree carbon and water balances.
This thesis focused on understanding how trees respond under different environmental conditions (e.g. wet vs dry) both at the intraspecific and interspecific level. The conducted studies focused on biomass allocation, xylem and phloem anatomies of stem and branches, xylem anatomy of leaves, plant hydraulics, and wood density. Mostly, data from personal measurements were used, but also published data to compare the results. The aim was to understand which structural traits are linked to the acclimation and adaptation strategies of trees and how.
The structure of the thesis is: (i) general introduction, (ii) five main chapters based on different studies, (iii) one main chapter describing in short three more studies, (iv) overall conclusion, (v) additional information for each of the main chapters (if applicable), (vi) acknowledgments, (vii) anatomical images for some chapters, (viii) literature cited listed per chapter
The total path length hydraulic resistance according to known anatomical patterns: what is the shape of the root-to-leaf tension gradient along the plant longitudinal axis?
Full text linkXylem conduit diameter widens from leaf tip to stem base and how this widening affects the total
hydraulic resistance (RTOT) and the gradient of water potential (Wxyl) has never been thoroughly investigated.
Data of conduit diameter of Acer pseudoplatanus,Fagus sylvatica and Picea abies were used to model the axial variation of RTOT and Wxyl.
The majority of RTOT (from 79 to 98%) was predicted to be confined within the leaf/needle. This means that the xylem conduits of stem and roots, accounting for nearly the total length of the hydraulic path, theoretically provide a nearly negligible contribution to RTOT. Consequently, a steep gradient of water potentials was predicted to develop within the leaf/needle base, whereas lower in the stem water potentials approximate those of rootlets.
Our results would suggest that the strong partitioning of RTOT between leaves/needles coupled with
basal conduit widening is of key importance for both hydraulic safety against drought-induced embolism formation and efficiency, as it minimizes the exposure of stem xylem to high tensions and makes the total plant’s conductance substantially independent of body size
Vulnerability to xylem embolism correlates to wood parenchyma fraction in angiosperms but not in gymnosperms
No full textUnderstanding which structural and functional traits are linked to species’ vulnerability to embolism formation (P50) may provide fundamental knowledge on plant strategies to maintain an efficient water transport. We measured P50, wood density (WD), mean conduit area, conduit density, percentage areas occupied by vessels, parenchyma cells (PATOT) and fibers (FA) on branches of angiosperm and gymnosperm species. Moreover, we compiled a dataset of published hydraulic and anatomical data to be compared with our results. Species more vulnerable to embolism had lower WD. In angiosperms, the variability in WD was better explained by PATOT and FA, which were highly correlated. Angiosperms with a higher P50 (less negative) had a higher amount of PATOT and total amount of nonstructural carbohydrates. Instead, in gymnosperms, P50 vs PATOT was not significant. The correlation between PATOT and P50 might have a biological meaning and also suggests that the causality of the commonly observed relationship of WD vs P50 is indirect and dependent on the parenchyma fraction. Our study suggests that angiosperms have a potential active embolism reversal capacity in which parenchyma has an important role, while in gymnosperms this might not be the case
Similarities and differences in the balances between leaf, xylem, and phloem structures in Fraxinus ornus along an environmental gradient
No full textThe plant carbon balance depends on the coordination between photosynthesis and the long-distance transport of water and
sugars. How plants modify the allocation to the different structures affecting this coordination under different environmental conditions has been poorly investigated. In this study, we evaluated the effect of soil water availability on the allocation to leaf, xylem
and phloem structures in Fraxinus ornus L. We selected small individuals of F. ornus (height ~2 m) from sites contrasting in soil
water availability (wet vs dry). We measured how the leaf (LM) and stem + branch biomass (SBM) are cumulated along the stem.
Moreover, we assessed the axial variation in xylem (XA) and phloem tissue area (PA), and in lumen area of xylem vessels (CAxy)
and phloem sieve elements (CAph). We found a higher ratio of LM:SBM in the trees growing under drier conditions. The longdistance transport tissues of xylem and phloem followed axial patterns with scaling exponents (b) independent of site conditions.
PA scaled isometrically with XA (b ~ 1). While CAxy was only marginally higher at the wet sites, CAph was significantly higher at
the drier sites. Our results showed that under reduced soil water availability, F. ornus trees allocate relatively more to the leaf biomass and produce more conductive phloem, which is likely to compensate for the drought-related hydraulic limitations to the leaf
gas exchanges and the phloem sap viscosity
Scots pine trees react to drought by increasing xylem and phloem conductivities
Full text linkDrought limits the long-distance transport of water in the xylem due to the reduced leaf-to-soil water potential difference and possible embolism-related losses of conductance, and of sugars in the phloem due to the higher viscosity of the dehydrated sugary solution. This condition can have cascading effects in water and carbon fluxes that may ultimately cause tree death. We hypothesize that the maintenance of xylem and phloem conductances is fundamental for survival also under reduced resource availability, when trees may produce effective and low C cost anatomical adjustments in the xylem and phloem close to the treetop where most of the hydraulic resistance is concentrated.
We analyzed the treetop xylem and phloem anatomical characteristics in coexisting Scots pine trees symptomatic and non-symptomatic of drought-induced dieback. We selected the topmost 55 cm of the main stem and selected several sampling positions at different distances from the stem apex to test for differences in the axial patterns between the two groups of trees. We measured the annual ring area (RA), the tracheid hydraulic diameter (Dh) and cell wall thickness (CWT), the conductive phloem area (PAcond) and the average lumen diameter of the 20 largest phloem sieve cells (Dph).
Declining trees grew less than the non-declining ones, and despite the similar axial scaling of anatomical traits, had larger Dh and lower CWT. Moreover, declining trees had wider Dph.
Our results demonstrate that even under drought stress, maintenance of xylem and phloem efficiencies is of primary importance for survival, even if producing fewer larger tracheids may lead to a xylem more vulnerable to embolism formation
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