Sap velocity as an indicator of diurnal and long-term hydraulic resistance changes in mature ‘Walker’ hybrid poplar treesThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada.
Bibliographic record
Abstract
Hydraulic resistance directly limits sap flow in trees so that understanding water-use patterns in poplar trees requires knowledge about diurnal and long-term changes in resistances in the soil–plant–atmosphere continuum. Granier thermal dissipation probes were used for continuous measurements of sap velocity (V) changes over time and whole-tree water-use patterns in the main stems of ‘Walker’ hybrid poplar trees. When atmospheric demand was low, V depended only on solar radiation (Rg) and vapour pressure difference (VPD). However, on warm, sunny days, midday V decreased, despite a constant leaf water potential (Ψl) of –1.5 MPa. Because bulk soil water potential (Ψs) did not change significantly, the constant Ψl implied that the water potential difference (ΔΨ) remained diurnally constant. Although stomatal control would explain changes in V when VPD was low, an increase in hydraulic resistance was required to explain the decrease in V when VPD was high. Xylem cavitation was ruled out as a cause because the apparent increase in resistance reversed in late afternoon, while VPD was still high. Mid-August irrigation immediately increased daily V, which was attributed to a change in ΔΨ because of less negative Ψs. A gradual increase in sap flow over two weeks following irrigation implied a decrease in plant hydraulic resistance, most likely attributable to new root growth. It was concluded that hydraulic resistance changes, both diurnal and in the longer term, occurred primarily at the rhizosphere or root level.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".