ALBERT

Description: Small differences in the sensitivity of stomatal conductance to light intensity on leaf surfaces may lead to large differences in total canopy transpiration ( E C ) with increasing canopy leaf area ( L ). Typically, the increase of L would more than compensate for the decrease of transpiration per unit of leaf area ( E L ), resulting in concurrent increase of E C . However, highly shade-intolerant species, such as Larix principis-rupprechtii Mayr., may be so sensitive to increased shading that such compensation is not complete. We hypothesized that in such a stand, windfall-induced spatial variation at a decameter scale would result in greatly reduced E L in patches of high L leading to lower E C than low competition patches of sparse canopy. We further hypothesized that quicker extraction of soil moisture in patches of lower competition will result in earlier onset of drought symptoms in these patches. Thus, patches of low L will transition from light to soil moisture as the factor dominating E L . This process should progressively homogenize E C in the stand even as the variation of soil moisture is increasing. We tested the hypotheses utilizing sap flux of nine trees, and associated environmental and stand variables. The results were consistent with only some of the expectations. Under non-limiting soil moisture, E L was very sensitive to the spatial variation of L , decreasing sharply with increasing L and associated decrease of mean light intensity on leaf surfaces. Thus, under the conditions of ample soil moisture maximum E C decreased with increasing patch-scale L . Annual E C and biomass production also decreased with L , albeit more weakly. Furthermore, variation of E C among patches decreased as average stand soil moisture declined between rain events. However, contrary to expectation, high L plots which transpired less showed a greater E L sensitivity to decreasing stand-scale soil moisture, suggesting a different mechanism than simple control by decreasing soil moisture. We offer potential explanations to the observed phenomenon. Our results demonstrate that spatial variation of L at decameter scale, even within relatively homogeneous, single-species, even-aged stands, can produce large variation of transpiration, soil moisture and biomass production and should be considered in 1-D soil–plant–atmosphere models.

Description: Manipulating tree belowground carbon (C) transport enables investigation of the ecological and physiological roles of tree roots and their associated mycorrhizal fungi, as well as a range of other soil organisms and processes. Girdling remains the most reliable method for manipulating this flux and it has been used in numerous studies. However, girdling is destructive and irreversible. Belowground C transport is mediated by phloem tissue, pressurized through the high osmotic potential resulting from its high content of soluble sugars. We speculated that phloem transport may be reversibly blocked through the application of an external pressure on tree stems. Thus, we here introduce a technique based on compression of the phloem, which interrupts belowground flow of assimilates, but allows trees to recover when the external pressure is removed. Metal clamps were wrapped around the stems and tightened to achieve a pressure theoretically sufficient to collapse the phloem tissue, thereby aiming to block transport. The compression's performance was tested in two field experiments: a 13 C canopy labelling study conducted on small Scots pine ( Pinus sylvestris L.) trees [2–3 m tall, 3–7 cm diameter at breast height (DBH)] and a larger study involving mature pines (~15 m tall, 15–25 cm DBH) where stem respiration, phloem and root carbohydrate contents, and soil CO 2 efflux were measured. The compression's effectiveness was demonstrated by the successful blockage of 13 C transport. Stem compression doubled stem respiration above treatment, reduced soil CO 2 efflux by 34% and reduced phloem sucrose content by 50% compared with control trees. Stem respiration and soil CO 2 efflux returned to normal within 3 weeks after pressure release, and 13 C labelling revealed recovery of phloem function the following year. Thus, we show that belowground phloem C transport can be reduced by compression, and we also demonstrate that trees recover after treatment, resuming C transport in the phloem.

Description: Uncertainties in ecophysiological responses to environment, such as the impact of atmospheric and soil moisture conditions on plant water regulation, limit our ability to estimate key inputs for ecosystem models. Advanced statistical frameworks provide coherent methodologies for relating observed data, such as stem sap flux density, to unobserved processes, such as canopy conductance and transpiration. To address this need, we developed a hierarchical Bayesian State-Space Canopy Conductance (StaCC) model linking canopy conductance and transpiration to tree sap flux density from a 4-year experiment in the North Carolina Piedmont, USA. Our model builds on existing ecophysiological knowledge, but explicitly incorporates uncertainty in canopy conductance, internal tree hydraulics and observation error to improve estimation of canopy conductance responses to atmospheric drought (i.e., vapor pressure deficit), soil drought (i.e., soil moisture) and above canopy light. Our statistical framework not only predicted sap flux observations well, but it also allowed us to simultaneously gap-fill missing data as we made inference on canopy processes, marking a substantial advance over traditional methods. The predicted and observed sap flux data were highly correlated (mean sensor-level Pearson correlation coefficient = 0.88). Variations in canopy conductance and transpiration associated with environmental variation across days to years were many times greater than the variation associated with model uncertainties. Because some variables, such as vapor pressure deficit and soil moisture, were correlated at the scale of days to weeks, canopy conductance responses to individual environmental variables were difficult to interpret in isolation. Still, our results highlight the importance of accounting for uncertainty in models of ecophysiological and ecosystem function where the process of interest, canopy conductance in this case, is not observed directly. The StaCC modeling framework provides a statistically coherent approach to estimating canopy conductance and transpiration and propagating estimation uncertainty into ecosystem models, paving the way for improved prediction of water and carbon uptake responses to environmental change.

Description: To depict the wet (April with a soil water content, SWC, of 37 %) and dry (October with a SWC of 24.8 %) seasonal changes in the water use and physiological response of a Eucalyptus urophylla plantation in subtropical South China characterized by monsoon climate, the whole-year (June 2012 to May 2013) transpiration of E. urophylla was monitored using the TDP method. Daily transpiration (ET) in October averaged 5.7 ± 2.9 kg d−1 and was 58.0 % higher than that in April (3.6 ± 2.3 kg d−1). The difference is consistent with that of the radiation and evaporative demand of the two months, while the nocturnal transpiration (ET-NOC) in the wet season (0.18 ± 0.021 kg d−1) was almost twice that in the dry season (0.11 ± 0.01 kg d−1). Trees displayed a higher stomatal conductance (GS) (53.4–144.5 mmol m−2 s−1) in the wet season and a lower GS (45.7–89.5 mmol m−2 s−1) in the dry season. The leaf-soil water potentials (ΨL) of the two months (April and October) were −0.62 ± 0.66 and −1.22 ± 0.10 MPa, respectively. A boundary line analysis demonstrated that the slight improvement in the GS by SWC in wet season was offset by a significant decrease in D, and the slope of GS sensitivity to D (dGS/dlnD) in response to GSref (references GS at D = 1 kPa) was affected by the variance of radiation instead of SWC. Specific hydraulic conductivity (ks) of trees of different sizes decreased by 45.3–65.6 % from the wet to the dry season. Combining the decreased maximum reference GS at D = 1 kPa (GSref-max) by 22.4 % with the constant max GS (GSmax) when ΨL 〈 −1.2 MPa, we shed some light on the mechanism underlying the high water-use efficiency (WUE) of this Eucalyptus specie. With a slight change in GSref-max and high sensitivity of ks to decreasing ΨL, large trees used water more efficiently than small ones did. In addition, the −m in the dry season (0.53 ± 0.007) was lower than that in the wet season (0.58 ± 0.01) due to the difference in the ratio of GS to the boundary layer conductance (gb) in the two months. The negative relationship between −m (except when light is limited) and Q proved to be a plastic response to environmental changes for E. urophylla but did not change with decreased ks as expected.