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The leaf area to sapwood area ratio ($A_{1}\\colon A_{\\text{s}}$) of trees has been hypothesized to decrease as trees become older and taller. Theory suggests that$A_{1}\\colon A_{\\text{s}}$must decrease to maintain leaf-specific hydraulic sufficiency as path length, gravity, and tortuosity constrain whole-plant hydraulic conductance. We tested the hypothesis that$A_{1}\\colon A_{\\text{s}}$declines with tree height. Whole-tree$A_{1}\\colon A_{\\text{s}}$was measured on 15 individuals of Douglas-fir (Pseudotsuga menziesii var. menziesii) ranging in height from 13 to 62 m (aged 20-450 years).$A_{1}\\colon A_{\\text{s}}$declined substantially as height increased (P=0.02). Our test of the hypothesis that$A_{1}\\colon A_{\\text{s}}$declines with tree height was extended using a combination of original and published data on nine species across a range of maximum heights and climates. Meta-analysis of 13 whole-tree studies revealed a consistent and significant reduction in$A_{1}\\colon A_{\\text{s}}$with increasing height (P<0.05). However, two species (Picea abies and Abies balsamea) exhibited an increase in$A_{1}\\colon A_{\\text{s}}$with height, although the reason for this is not clear. The slope of the relationship between$A_{1}\\colon A_{\\text{s}}$and tree height ($\\Delta A_{1}\\colon A_{\\text{s}}/\\Delta h$) was unrelated to mean annual precipitation. Maximum potential height was positively correlated with$\\Delta A_{1}\\colon A_{\\text{s}}/\\Delta h$. The decrease in$A_{1}\\colon A_{\\text{s}}$with increasing tree size that we observed in the majority of species may be a homeostatic mechanism that partially compensates for decreased hydraulic conductance as trees grow in height.

International audience

Plant phenological development is orchestrated through subtle changes in photoperiod, temperature, soil moisture and nutrient availability. Presently, the exact timing of plant development stages and their response to climate and management practices are crudely represented in land surface models. As visual observations of phenology are laborious, there is a need to supplement long-term observations with automated techniques such as those provided by digital repeat photography at high temporal and spatial resolution. We present the first synthesis from a growing observational network of digital cameras installed on towers across Europe above deciduous and evergreen forests, grasslands and croplands, where vegetation and atmosphere CO2 fluxes are measured continuously. Using colour indices from digital images and using piecewise regression analysis of time series, we explored whether key changes in canopy phenology could be detected automatically across different land use types in the network. The piecewise regression approach could capture the start and end of the growing season, in addition to identifying striking changes in colour signals caused by flowering and management practices such as mowing. Exploring the dates of green-up and senescence of deciduous forests extracted by the piecewise regression approach against dates estimated from visual observations, we found that these phenological events could be detected adequately (RMSE < 8 and 11 days for leaf out and leaf fall, respectively). We also investigated whether the seasonal patterns of red, green and blue colour fractions derived from digital images could be modelled mechanistically using the PROSAIL model parameterised with information of seasonal changes in canopy leaf area and leaf chlorophyll and carotenoid concentrations. From a model sensitivity analysis we found that variations in colour fractions, and in particular the late spring 'green hump' observed repeatedly in deciduous broadleaf canopies across the network, are essentially dominated by changes in the respective pigment concentrations. Using the model we were able to explain why this spring maximum in green signal is often observed out of phase with the maximum period of canopy photosynthesis in ecosystems across Europe. Coupling such quasi-continuous digital records of canopy colours with co-located CO2 flux measurements will improve our understanding of how changes in growing season length are likely to shape the capacity of European ecosystems to sequester CO2 in the future.

Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg$\\text{day}^{-1}\\ \\text{tree}^{-1}$). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m2of sapwood per kg of water transpired over one hour. Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s-1that is equivalent to 112 mmol m-2s-1at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m-2s-1at the top and 150 mmol m-2s-1at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was -27.76±0.27‰ (average ± s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.

Widespread tree species must show physiological and structural plasticity to deal with contrasting water balance conditions. To investigate these plasticity mechanisms, a meta-analysis of Pinus sylvestris L. sap flow and its response to environmental variables was conducted using datasets from across its whole geographical range. For each site, a Jarvis-type, multiplicative model was used to fit the relationship between sap flow and photosynthetically active radiation, vapour pressure deficit (D) and soil moisture deficit (SMD); and a logarithmic function was used to characterize the response of stomatal conductance (Gs) to D. The fitted parameters of those models were regressed against climatic variables to study the acclimation of Scots pine to dry/warm conditions. The absolute value of sap flow and its sensitivity to D and SMD increased with the average summer evaporative demand. However, relative sensitivity of Gs to D (m/Gs,ref, where m is the slope and Gs,ref is reference Gs at D = 1 kPa) did not increase with evaporative demand across populations, and transpiration per unit leaf area at a given D increased accordingly in drier/warmer climates. This physiological plasticity was linked to the previously reported climate- and size-related structural acclimation of leaf to sapwood area ratios. Gs,ref, and its absolute sensitivity to D (m), tended to decrease with age/height of the trees as previously reported for other pine species. It is unclear why Scots pines have higher transpiration rates at drier/warmer sites, at the expense of lower water-use efficiency. In any case, our results suggest that these structural adjustments may not be enough to prevent lower xylem tensions at the driest sites

UNDERSTANDING mass and energy exchange between vegetation and the atmosphere is essential for determining the future state of the climate system 1,2 and responses of plant communities. Plant water use is at present described by steady-state transport models 3,4 , even though transport in the boundary layer is turbulent 5,6 . This is especially true for forests, where the canopy air space is a chaotic environment where large turbulent events alternate with smaller-scale mixing 3,4,7 . Here we demonstrate that the turbulent nature of the atmosphere affects plant processes. We propose that the dynamic nature of plant-atmosphere coupling represents a previously unrecognized feedback that influences plant water use and transport.

Introducing climate quotients for the growing season (Qgs) provides a way to quantify effects of climate trends with respect to Potential Natural Vegetation (PNV), especially beech forests (Fagus sylvatica L.) in Central Germany. What is crucial in this regard is the great influence of the dominant decrease in the amount of precipitation (up to 40% in the last 50 years) during the growing season versus the dormant season. However, precipitation during the dormant season (which is predominantly increasing: up to 40% in the last 50 years) is also important for replenishing the soil water supply. The Qgs values of the Climatic Normal period of 1971-2000 are generally higher (up to 12% in lowland areas) compared with the Climatic Normal period of 1961-1990, the extent of the difference being in general inversely proportional to elevation above sea level. What this means for the area under investigation is that humidity conditions, which generally improve as the elevation above sea level increases, have a positive effect on the site potential. However, a comparison of the climatologically important period of 1991-2003 with the period of 1961-1990 (area-wide increase between 12% and 16%) could not identify this positive effect of elevation on precipitation for the area under investigation. With regard to the recent climate-based trends of PNV, we have shown that all natural spatial units in Central Germany are affected by progressing continentality (i.e., dryness) during the growing season and the resulting deterioration of the site potential. The area of potential beech forest at lower elevation has decreased in favour of oak forest as PNV, while less change is observed in the montane area.