The hydraulic limitation hypothesis (Ryan and Yoder 1997) proposes that leaf-specific hydraulic conductance (kl) and stomatal conductance (gs) decline as trees grow taller, resulting in decreased carbon assimilation. We tested the hydraulic limitation hypothesis by comparison of canopy-dominant Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) trees in stands that were approximately 15 m (20 years old), 32 m (40 years old) and 60 m (> 450 years old) tall in Wind River, Washington, USA. Carbon isotope discrimination (Delta) declined with tree height (18.6, 17.6 and 15.9 per thousand for stands 15, 32 and 60 m tall, respectively) indicating that gs may have declined proportionally with tree height in the spring months, when carbon used in the construction of new foliage is assimilated. Hydraulic conductance decreased by 44% as tree height increased from 15 to > 32 m, and showed a further decline of 6% with increasing height. The general nonlinear pattern of kl versus height was predicted by a model based on Darcy's Law. Stemwood growth efficiency also declined nonlinearly with height (60, 35 and 28 g C m-2 leaf area for the 15-, 32- and 60-m stands, respectively). Unlike kl and growth efficiency, gs and photosynthesis (A) during summer drought did not decrease with height. The lack of decline in cuvette-based A indicates that reduced A, at least during summer months, is not responsible for the decline in growth efficiency. The difference between the trend in gs and A and that in kl and D may indicate temporal changes (spring versus summer) in the response of gas exchange to height-related changes in kl or it may be a result of measurement inadequacies. The formal hydraulic limitation hypothesis was not supported by our mid-summer gs and A data. Future tests of the hydraulic limitation hypothesis in this forest should be conducted in the spring months, when carbon uptake is greatest. We used a model based on Darcy's Law to quantify the extent to which compensating mechanisms buffer hydraulic limitations to gas exchange. Sensitivity analyses indicated that without the observed increases in the soil-to-leaf water potential differential (DeltaPsi) and decreases in the leaf area/sapwood area ratio, kl would have been reduced by more than 70% in the 60-m trees compared with the 15-m trees, instead of the observed decrease of 44%. However, compensation may have a cost; for example, the greater DeltaPsi of the largest trees was associated with smaller tracheid diameters and increased sapwood cavitation, which may have a negative feedback on kl and gs.

An investigation of hydraulic limitation and compensation in large, old Douglas-fir trees.

To examine the predictability of leaf physiology and biochemistry from light gradients within canopies, we measured photosynthetic light-response curves, leaf mass per area (LMA) and concentrations of nitrogen, phosphorus and chlorophyll at 15-20 positions within canopies of three conifer species with increasing shade tolerance, ponderosa pine [Pinus ponderosa (Laws.)], Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], and western hemlock [Tsuga heterophylla (Raf.) Sarg.]. Adjacent to each sampling position, we continuously monitored photosynthetically active photon flux density (PPFD) over a 5-week period using quantum sensors. From these measurements we calculated FPAR: integrated PPFD at each sampling point as a fraction of full sun. From the shadiest to the brightest canopy positions, LMA increased by about 50% in ponderosa pine and 100% in western hemlock; Douglas fir was intermediate. Canopy-average LMA increased with decreasing shade tolerance. Most foliage properties showed more variability within and between canopies when expressed on a leaf area basis than on a leaf mass basis, although the reverse was true for chlorophyll. Where foliage biochemistry or physiology was correlated with FPAR, the relationships were non-linear, tending to reach a plateau at about 50% of full sunlight. Slopes of response functions relating physiology and biochemistry to ln(FPAR) were not significantly different among species except for the light compensation point, which did not vary in response to light in ponderosa pine, but did in the other two species. We used the physiological measurements for Douglas fir in a model to simulate canopy photosynthetic potential (daily net carbon gain limited only by PPFD) and tested the hypothesis that allocation of carbon and nitrogen is optimized relative to PPFD gradients. Simulated photosynthetic potential for the whole canopy was slightly higher (<10%) using the measured allocation of C and N within the canopy compared with no stratification (i.e., all foliage identical). However, there was no evidence that the actual allocation pattern was optimized on the basis of PPFD gradients alone; simulated net carbon assimilation increased still further when even more N and C were allocated to high-light environments at the canopy top.

Foliage physiology and biochemistry in response to light gradients in conifers with varying shade tolerance.

https://nature.berkeley.edu/biometlab/pdf/Law%20et%20al%20Ag%20met%202001%20pipo%20respiration.pdf

Spatial and temporal variation in respiration in a young ponderosa pine forest during a summer drought

Stand density reductions have been proposed as a method by which old-growth ponderosa pine ( Pinus ponderosa ) forests of North America can be converted back to pre-1900 conditions, thereby reducing the danger of catastrophic forest fires and insect attacks while increasing the productivity of the remaining old-growth individuals. However, the duration of productivity response of individual trees and the physiological mechanisms underlying such a response remain speculative issues, particularly in old trees. Treering measurements of carbon isotope ratios ( d 13 C) and basal area increment (BAI) were used to assess the response of intrinsic water-use efficiency (the ratio of photosynthesis, A to stomatal conductance, g ) and growth of individual > 250-year-old-ponderosa pine trees to stand density reductions. It was hypothesized that reductions in stand density would increase soil moisture availability, thus decreasing canopy A / g and increasing carbon isotope discrimination ( D ). Cellulose- d 13 C of annual tree rings, soil water availability (estimated from pre-dawn leaf water potential), photosynthetic capacity, stem basal growth and xylem anatomy were measured in individual trees within three pairs of thinned and un-thinned stands. The thinned stands were treated 7 to 15 years prior to measurement. The values of d 13 C and BAI were assessed for 20 consecutive years overlapping the date of thinning in a single intensively studied stand, and was measured for 3 years on either side of the date of thinning for the two other stands to assess the generality of the response. After thinning, D D D increased by 0.89‰ ( ± 0.15‰). The trees in the un-thinned stands showed no change in D D D (0.00‰ ± 0.04‰). In the intensively studied trees, significant differences were expressed in the first growing season after the thinning took place but it took 6 years before the full 0.89‰ difference was observed. BAI doubled or tripled after disturbance, depending on the stand, and the increased BAI lasted up to 15 years after thinning. In the intensively studied trees, the BAI response did not begin until 3 years after the D response, peaked 1 year after the D D D peak, and then BAI and D oscillated in unison. The lag between BAI and D was not due to slow changes in anatomical properties of the sapwood, because tracheid dimensions and sapwood-specific conductivity remained unchanged after disturbance. The D response of thinned trees indicated that A / g decreased after thinning. Photosynthetic capacity, as indexed by foliar nitrogen ([N]) and by the relationship between photosynthesis and internal CO 2 ( A ‐ C i curves), was unchanged by thinning, confirming our suspicion that the decline in A / g was due to a relatively greater increase in g in comparison with A . Model estimates agreed with this conclusion, predicting that g increased by nearly 25% after thinning relative to a 15% increase in A . Pre-dawn leaf water potential averaged 0.11 MPa ( ± ± ± 0.03 MPa) less negative for the thinned compared with the un-thinned trees in all stands, and was strongly correlated with D post-thinning ( R 2 = 0.91). There

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2003.00999.x

Carbon isotope discrimination and growth response of old Pinus ponderosa trees to stand density reductions

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin synthesis, i.e. targeted to senescing leaves and negatively auto-regulated (PSAG12‐IPT), thus preventing developmental abnormalities. Plants were grown with growthlimiting nitrogen supply. Compared to the wild-type, endogenous levels of free zeatin (Z)- and Z riboside (ZR)type cytokinins were increased up to 15-fold (total ZR up to 100-fold) in senescing leaves, and twofold in younger leaves of PSAG12‐IPT. In these plants, the senescenceassociated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more ( 15 N-labelled) N than younger leaves, associated with reduced shoot N accumulation (‐60%) and a partially inverted canopy N profile in PSAG12‐IPT plants. While root N accumulation was not affected, N translocation to non-senescing leaves was progressively reduced. We discuss potential consequences of these modified sink‐source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2000.00544.x

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

The three dimensional distribution of intercepted radiation, intercellular CO2 concentration (Ci) and late summer needle nitrogen (N) concentration were determined at the tips of all 54 branches in a 6·2-m-tall Pinus radiata D. Don tree growing in a New Zealand plantation. Measurements included above- and below-canopy irradiance, leaf stable carbon isotopic composition (δ13C) and tree canopy architecture. The radiation absorption component of the model, MAESTRO, was tested on site and then used to determine the branch tip distribution of intercepted radiation. We hypothesized that in branch tip needles: (i) the allocation of nitrogen and other nutrients would be closely associated with the distribution of intercepted radiation, reflecting carbon gain optimization theory, and (ii) Ci would predominantly reflect changes in photosynthetic rate (A) rather than stomatal conductance (gs), indicating that the increase in A for a given increase in N concentration was larger than the corresponding increase in gs. Needle nitrogen concentration was poorly related to intercepted radiation, regardless of the period over which the latter was calculated. At a given height, there was a large azimuthal variation in intercepted radiation but N concentration was remarkably uniform around the tree canopy. There was, however, a linear and positive correspondence between N concentration and δ13C and needle height above ground (r2 = 0·73 and 0·68, respectively). The very strong linear correspondence between N concentration and Ci (r2 = 0·71) was interpreted, using gas exchange measurements, as supporting our second hypothesis. Recognizing the strong apical control in P. radiata and possible effects of leaf nitrogen storage in an evergreen species, we propose that the tree leader must have constituted a very strong carbon sink throughout the growing season, and that the proximity of branch tip needles to the leader affected their photosynthetic capacity and nutrient concentration, independent of intercepted radiation. This implies an integrated internal determination of resource allocation within the tree and challenges the current convention that resources are optimally distributed according to the profile of intercepted radiation.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.1998.00314.x

J. C. Grace

Papers

Nitrogen allocation and carbon isotope fractionation in relation to intercepted radiation and position in a young Pinus radiata D. don tree