Soil drying and nitrogen availability modulate carbon and water exchange over a range of annual precipitation totals and grassland vegetation types

Increased intensity in precipitation events and longer periods of water deficit are predicted as a general trend under future climate scenarios with potentially large effects on terrestrial ecosystem function. The primary objective of this study was to understand how variation in the intensity of precipitation inputs followed by intermittent soil drying events influence leaf and ecosystem carbon dioxide (CO₂) and water exchange in a California annual grassland mesocosm experiment. We further examined how nitrogen (N) availability, and differences in plant community composition (grass-forb combinations) affected gas exchange responses to the precipitation treatments. Net ecosystem CO₂ exchange (NEE) and evapotranspiration (ET) increased significantly with greater precipitation and were positively correlated with soil moisture. A repeated 10-day soil drying period, following 11 days of watering, strongly depressed NEE over a range of annual precipitation totals (297, 657 and 987 mm), and plant community types. Ecosystem dark respiration ( R _e) and leaf level photosynthesis ( A _max) showed greater sensitivity to periods of soil drying in the low precipitation plots (297 mm). N additions significantly increased NEE and R _e, particularly as water availability was increased. Across the range of precipitation totals and plant community types, intermittent periods of soil moisture deficit and native soil N availability constrained leaf and ecosystem level CO₂ exchange, while the influence on water vapor exchange was less pronounced.     

Canopy development of a model herbaceous community exposed to elevated atmospheric CO2 and soil nutrients

To test the prediction that elevated CO₂ increases the maximum leaf area index (LAI) through a stimulation of photosynthesis, we exposed model herbaceous communities to two levels of CO₂ crossed with two levels of soil fertility. Elevated CO₂ stimulated the initial rate of canopy development and increased cumulative LAI integrated over the growth period, but it had no effect on the maximum LAI. In contrast to CO₂, increased soil nutrient availability caused a substantial increase in maximum LAI. Elevated CO₂ caused a slight increase in leaf area and nitrogen allocated to upper canopy layers and may have stimulated leaf turnover deep in the canopy. Gas exchange measurements of intact communities made near the time of maximum LAI indicated that soil nutrient availability, but not CO₂ enrichment, caused a substantial stimulation of net ecosystem carbon exchange. These data do not support our prediction of a higher maximum LAI by elevated CO₂ because the initial stimulation of LAI diminished by the end of the growth period. However, early in development, leaf area and carbon assimilation of communities may have been greatly enhanced. These results suggest that the rate of canopy development in annual communities may be accelerated with future increases in atmospheric CO₂ but that maximum LAI is set by soil fertility.     

Respiratory responses of higher plants to atmospheric CO2 enrichment

Although the respiratory response of native and agricultural plants to atmospheric CO₂ enrichment has been reported over the past 75 years, only recently have these effects emerged as prominent measures of plant and ecosystem response to the earth's changing climate. In this review we discuss this rapidly expanding field of study and propose that both increasing and decreasing rates of leaf and whole-plant respiration are likely to occur in response to rising CO₂ concentrations. While the stimulatory effects of CO₂ on respiration are consistent with our knowledge of leaf carbohydrate status and plant metabolism, we wish to emphasize the rather surprising short-term inhibition of leaf respiration by elevated CO₂ and the reported effects of long-term CO₂ exposure on growth and maintenance respiration. As is being found in many studies, it is easier to document the respiratory response of higher plants to elevated CO₂ than it is to assign a mechanistic basis for the observed effects. Despite this gap in our understanding of how respiration is affected by CO₂ enrichment, data are sufficient to suggest that changes in leaf and whole-plant respiration may be important considerations in the carbon dynamics of terrestrial ecosystems as global CO₂ continues to rise. Suggestions for future research that would enable these and other effects of CO₂ on respiration to be unravelled are presented.     

Increasing growth temperature reduces the stimulatory effect of elevated CO2 on photosynthesis or biomass in two perennial species

We examined how anticipated changes in CO₂ concentration and temperature interacted to alter plant growth, harvest characteristics and photosynthesis in two cold-adapted herbaceous perennials, alfalfa ( Medicago sativa L. cv. Arc) and orchard grass ( Dactylis glomerata L. cv. Potomac). Plants were grown at two CO₂ concentrations (362 [ambient] and 717 [elevated] μmol mol⁻¹ CO₂) and four constant day/night temperatures of 15, 20, 25 and 30°C in controlled environmental chambers. Elevated CO₂ significantly increased total plant biomass and protein over a wide range of temperatures in both species. Stimulation of photosynthetic rate, however, was eliminated at the highest growth temperature in M. sativa and relative stimulation of plant biomass and protein at high CO₂ declined as temperature increased in both species. Lack of a synergistic effect between temperature and CO₂ was unexpected since elevated CO₂ reduces the amount of carbon lost via photorespiration and photorespiration increases with temperature. Differences between anticipated stimulatory effects of CO₂ and temperature and whole plant single and leaf measurements are discussed. Data from this study suggest that stimulatory effects of atmospheric CO₂ on growth and photosynthesis may decline with anticipated increases in global temperature, limiting the degree of carbon storage in these two perennial species.     

Acquisition and allocation of carbon and nitrogen by Danthonia richardsonii in response to restricted nitrogen supply and CO2 enrichment

Dry weight (DW) and nitrogen (N) accumulation and allocation were measured in isolated plants of Danthonia richardsonii (Wallaby Grass) for 37 d following seed imbibition. Plants were grown at ≈ 365 or 735 μ L L^–1 CO₂ with N supply of 0·05, 0·2 or 0·5 mg N plant^–1 d^–1. Elevated CO₂ increased DW accumulation by 28% (low-N) to 103% (high-N), following an initial stimulation of relative growth rate. Net assimilation rate and leaf nitrogen productivity were increased by elevated CO₂, while N concentration was reduced. N uptake per unit root surface area was unaffected by CO₂ enrichment. The ratio of leaf area to root surface area was decreased by CO₂ enrichment. Allometric analysis revealed a decrease in the shoot-N to root-N ratio at elevated CO₂, while the shoot-DW to root-DW ratio was unchanged. Allometric analysis showed leaf area was reduced, while root surface area was unchanged by elevated CO₂, indicating a down-regulation of total plant capacity for carbon gain rather than a stimulation of mineral nutrient acquisition capacity. Overall, growth in elevated CO₂ resulted in changes in plant morphology and nitrogen use, other than those associated simply with changing plant size and non-structural carbohydrate content.     

Canopy photosynthesis of crops and native plant communities exposed to long-term elevated CO2

Abstract. There have been seven studies of canopy photosynthesis of plants grown in elevated atmospheric CO₂: three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO₂ increased canopy photosynthesis in all cases. The relative effect of CO₂ was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO₂ were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO₂ treatment to date, monospecific stands of a C₃ sedge, Scirpus olneyi (Grey), and a C₄ grass, Spartina patens (Ait.) Muhl., have been exposed to twice normal ambient CO₂ concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO₂ exchange per unit green biomass (NCE_b) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO₂ increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community.     

Seasonal Changes in Diel CO2 Exchange of Three Central European Moss Species: a One-Year Field Study

Abstract: Diel courses of net CO₂ gas exchange, water relation parameters and microclimatic conditions of three bryophyte species (Grimmia pulvinata, Schistidium apocarpum, Tortula ruralis) were studied in the Botanical Garden Würzburg (Franconia, Germany) over the course of one year. At the exposed study site, i.e., the upper surface of an anthropogenic limestone wall, photosynthetic active radiation (PFD) was hardly limiting for carbon gain, while plant water relations were of much greater importance: the time of metabolic activity correlated rather closely with the diurnal carbon budget. Carbon gain was highly seasonal in all three species, with the highest diel carbon budgets (NP_24h) being observed in autumn, while near zero NP_24h were typical for summer. Annual carbon gain differed considerably between the three species, and Tortula ruralis was by far the most productive. The estimates of annual productivity derived from gas exchange measurements in Grimmia pulvinata were compared with direct measurements of growth: the results of these different approaches yielded quite similar values.     

Changes in land plant function over the Phanerozoic: reconstructions based on the fossil record

Major fluctuations in the concentrations of atmospheric CO₂ and O₂, are predicted by historical long-term carbon and oxygen cycle models of atmospheric evolution and will have impacted directly on past climates, plant function and evolutionary processes. Here, palaeobotanical evidence is presented from the stomatal density record of fossil leaves spanning the past 400 Myr supporting the predicted changes in atsmopheric CO₂. Evidence from experiments on plants exposed to long-term high CO₂, environments and the newly-assembled fossil data indicate the potential for genetic modification of stomatal characters. The influence of the changes in fossil stomatal characteristics and atmospheric composition on the rates of leaf gas exchange over the course of land plant evolution has been investigated through modelling. Three contrasting eras of plain water economics emerge in the Devonian (high), Carboniferous (low) and from the Upper Jurassic to the present-day (high but declining). These patterns of change result from structural changes of the leaves and the impact of atmospheric CO₂, and O₂, concentrations on RuBisCo function and are consistent with the fossil evidence of sequential appearances of novel plant anatomical changes. The modelling approach is tested by comparing predicted leaf stable carbon isotope ratios with those measured on fossil plant and organic material. Viewed in a geological context, current and future increases in the concentration of atmospheric CO₂, might be considered as restoring plant function to that more typically experienced by plants over the majority of their evolutionary history.     

Low carbon dioxide concentrations can reverse stomatal closure during water stress

Leaf water potentials below threshold values result in reduced stomatal conductance (g_s). Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of g_s by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until g_s was reduced and then lowering the [CO₂] to determine whether g_s and transpiration rate could be increased and leaf water potential decreased and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low [CO₂] could reverse the stomatal closure because of drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. The relative sensitivity of g_s to internal [CO₂] in the leaves of dry plants of each species averaged three to four times higher than in leaves of wet plants. Two species in which g_s was reputed to be insensitive to [CO₂] were examined to determine whether high leaf to air water vapor pressure differences (D) resulted in increased stomatal sensitivity to [CO₂]. In both species, stomatal sensitivity to [CO₂] was indeed negligible at low D, but increased with D, and low [CO₂] partly or fully reversed closure caused by high D. In no case did low leaf water potential or low hydraulic conductance during drying of the air or the rooting medium prevent low [CO₂] from increasing g_s and transpiration rate.     

Leaf structure and chemical composition as affected by elevated CO2: genotypic responses of two perennial grasses

Genotypic variability was studied in two Mediterranean grass species, Bromus erectus and Dactylis glomerata , with regard to the response to CO₂ of leaf total non-structural carbohydrate concentration ([TNC]_lf), specific leaf area (SLA), and leaf carbon and nitrogen concentrations ([C]_lf and [N]_lf, respectively). Fourteen genotypes of each species were grown together on intact soil monoliths at ambient and elevated CO₂ concentrations (350 and 700 μmol mol⁻¹, respectively). In both species, the most consistent effect of elevated CO₂ was an increase in [TNC]_lf and a decrease in leaf nitrogen concentration when expressed either as total dry mass [N_m]_lf, structural dry mass [N_mst]_lf or leaf area [N_a]_lf. The SLA decreased only in D. glomerata , due to an accumulation of total non-structural carbohydrates and to an increase in leaf density. No genotypic variability was found for any variable in B. erectus , suggesting that genotypes responded in a similar way to elevated CO₂. In D. glomerata , a genotypic variability was found only for [Cst], [N_m]_lf, [N_mst]_lf and [N_a]_lf. Since [N_m]_lf is related to plant growth and is a strong determinant of plant–herbivore interactions, our results suggest evolutionary consequences of elevated CO₂ through competitive interactions or herbivory.     

Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate
ABA, abscisic acid
Ci , intercellular space CO₂ concentration
g , leaf conductance
WUE, water use efficiency

Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well-watered air-grown or double CO₂-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO₂ concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre-treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO₂ for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO₂. Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO₂. Identical experiments were performed with plants fully grown at double CO₂. Levels of leaf conductance and carbon assimilation rate measured at double CO₂ were similar for air-grown and elevated CO₂-grown plants. An enhanced response to ABA was still observed at high CO₂ in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO₂ slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO₂ and ABA which is not modified by growth at elevated CO₂.     

The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source-sink relations and respiration

Abstract. Herbaceous C₃ plants grown in elevated CO₂ show increases in carbon assimilation and carbohydrate accumulation (particularly starch) within source leaves. Although changes in the partitioning of biomass between root and shoot occur, the proportion of this extra assimilate made available for sink growth is not known. Root:shoot ratios tend to increase for CO₂-enriched herbaceous plants and decrease for CO₂-enriched trees. Root:shoot ratios for cereals tend to remain constant. In contrast, elevated temperatures decrease carbohydrate accumulation within source and sink regions of a plant and decrease root:shoot ratios. Allometric analysis of at least two species showing changes in root: shoot ratios due to elevated CO₂ show no alteration in the whole-plant partitioning of biomass. Little information is available for interactions between temperature and CO₂. Cold-adapted plants show little response to elevated levels of CO₂, with some species showing a decline in biomass accumulation. In general though, increasing temperature will increase sucrose synthesis, transport and utilization for CO₂-enriched plants and decrease carbohydrate accumulation within the leaf. Literature reports are discussed in relation to the hypothesis that sucrose is a major factor in the control of plant carbon partitioning. A model is presented in support.     

Adaptation to high CO2 concentration in an optimal environment: radiation capture, canopy quantum yield and carbon use efficiency

The effect of elevated [CO₂] on wheat (Triticum aestivum L. Veery 10) productivity was examined by analysing radiation capture, canopy quantum yield, canopy carbon use efficiency, harvest index and daily C gain. Canopies were grown at either 330 or 1200 μ mol mol^–1[CO₂] in controlled environments, where root and shoot C fluxes were monitored continuously from emergence to harvest. A rapidly circulating hydroponic solution supplied nutrients, water and root zone oxygen. At harvest, dry mass predicted from gas exchange data was 102·8 ± 4·7% of the observed dry mass in six trials. Neither radiation capture efficiency nor carbon use efficiency were affected by elevated [CO₂], but yield increased by 13% due to a sustained increase in canopy quantum yield. CO₂ enrichment increased root mass, tiller number and seed mass. Harvest index and chlorophyll concentration were unchanged, but CO₂ enrichment increased average life cycle net photosynthesis (13%, P < 0·05) and root respiration (24%, P < 0·05). These data indicate that plant communities adapt to CO₂ enrichment through changes in C allocation. Elevated [CO₂] increases sink strength in optimal environments, resulting in sustained increases in photosynthetic capacity, canopy quantum yield and daily C gain throughout the life cycle.     

Is there a trade‐off between the plant's growth response to elevated CO2 and subsequent litter decomposability?

Under elevated levels of atmospheric CO₂, leaf N concentration usually decreases due to dilution of N by excess carbon. Thus, the larger the growth response to elevated CO₂, the larger the decrease in leaf N concentration should be. This should, in turn, lead to a proportional decline in litter N concentration and litter decomposition rate. Thus, we hypothesize a trade-off between a plant's growth response to elevated CO₂ and subsequent litter decomposability. We tested this hypothesis by measuring the growth response, green leaf and leaf litter chemistry and litter respiration of six plant species grown under ambient and elevated atmospheric CO₂ concentrations in the greenhouse.
Growth response increased in the order Calluna vulgaris2 and litter decomposability. This implies that the productivity response of plant species to elevated CO₂ is, in general, uncoupled from the decomposition response.     

Photosynthesis, growth and nutrient changes in non-nodulated Phaseolus vulgaris grown under atmospheric and elevated carbon dioxide conditions

The response of Phaseolus vulgaris L. cv. Contender grown under controlled environment at either ambient or elevated (360 and 700 μmol mol^-1, respectively) CO₂ concentrations ([CO₂]), was monitored from 10 days after germination (DAG) until the onset of senescence. Elevated CO₂ had a pronounced effect on total plant height (TPH), leaf area (LA), leaf dry weight (LD), total plant biomass (TB) accumulation and specific leaf area (SLA). All of these were significantly increased under elevated carbon dioxide with the exception of SLA which was significantly reduced. Other than high initial growth rates in CO₂-enriched plants, relative growth rates remained relatively unchanged throughout the growth period. While the trends in growth parameters were clearly different between [CO₂], some physiological processes were largely transient, in particular, net assimilation rate (NAR) and foliar nutrient concentrations of N, Mg and Cu. CO₂ enrichment significantly increased NAR, but from 20 DAG, a steady decline to almost similar levels to those measured in plants grown under ambient CO₂ occurred. A similar trend was observed for leaf N content where the loss of leaf nitrogen in CO₂-enriched plants after 20 DAG, was significantly greater than that observed for ambient-CO₂ plants. Under enhanced CO₂, the foliar concentrations of K and Mn were increased significantly whilst P, Ca, Fe and Zn were reduced significantly. Changes in Mg and Cu concentrations were insignificant. In addition. high CO₂ grown plants exhibited a pronounced leaf discoloration or chlorosis, coupled with a significant reduction in leaf longevity.     

Interaction of elevated CO2 and O3 on growth, photosynthesis and respiration of three perennial species grown in low and high nitrogen

Seedlings of three species native to central North America, a C₃ tree, Populus tremuloides Michx., a C₃ grass, Agropyron smithii Rybd., and a C₄ grass, Bouteloua curtipendula Michx., were grown in all eight combinations of two levels each of CO₂, O₃ and nitrogen (N) for 58 days in a controlled environment. Treatment levels consisted of 360 or 674 μmol mol^-1 CO₂, 3 or 92 nmol mol^-1 O₃, and 0.5 or 6.0 m M N. In situ photosynthesis and relative growth rate (RGR) and its determinants were obtained at each of three sequential harvests, and leaf dark respiration was measured at the second and third harvests. In all three species, plants grown in high N had significantly greater whole-plant mass, RGR and photosynthesis than plants grown in low N. Within a N treatment, elevated CO₂ did not significantly enhance any of these parameters nor did it affect leaf respiration. However, plants of all three species grown in elevated CO₂ had lower stomatal conductance compared to ambient CO₂-exposed plants. Seedlings of P. tremuloides (in both N treatments) and B. curtipendula (in high N) had significant ozone-induced reductions in whole-plant mass and RGR in ambient but not under elevated CO_2. This negative O₃ impact on RGR in ambient CO₂ was related to increased leaf dark respiration, decreased photosynthesis and/or decreased leaf area ratio, none of which were noted in high O₃ treatments in the elevated CO₂ environment. In contrast, A. smithii was marginally negatively affected by high O_3.     

Partitioning of dry mass and leaf area within plants of three species grown at elevated CO2

1. We tested the hypothesis that the net partitioning of dry mass and dry mass:area relationships is unaltered when plants are grown at elevated atmospheric CO₂ concentrations.
2. The total dry mass of Dactylis glomerata, Bellis perennis and Trifolium repens was higher for plants in 700 compared to 350 μmol CO₂ mol^–1 when grown hydroponically in controlled-environment cabinets.
3. Shoot:root ratios were higher and leaf area ratios and specific leaf areas lower in all species grown at elevated CO₂. Leaf mass ratio was higher in plants of B. perennis and D. glomerata grown at elevated CO₂.
4. Whilst these data suggest that CO₂ alters the net partitioning of dry mass and dry mass:leaf area relationships, allometric comparisons of the components of dry mass and leaf area suggest at most a small effect of CO₂. CO₂ changed only two of a total of 12 allometric coefficients we calculated for the three species: ν relating shoot to root dry mass was higher in D. glomerata , whilst ν relating leaf area to total dry mass was lower in T. repens .
5. CO₂ alone has very little effect on partitioning when the size of the plant is taken into account.     

Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol⁻¹ CO₂) and double ambient (720 μmol mol⁻¹ CO₂) carbon dioxide (CO₂) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO₂ concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO₂ concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO₂ concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation ( V _cmax) and the light saturated rate of electron transport ( J _max) between the control and elevated CO₂ treatment.     

Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature

Ananas comosus L. (Merr.) (pineapple) was grown at three day/night temperatures and 350 (ambient) and 700 (elevated) μ mol mol^–1 CO₂ to examine the interactive effects of these factors on leaf gas exchange and stable carbon isotope discrimination ( Δ ,‰). All data were collected on the youngest mature leaf for 24 h every 6 weeks. CO₂ uptake (mmol m^–2 d^–1) at ambient and elevated CO₂, respectively, were 306 and 352 at 30/20 °C, 175 and 346 at 30/25 °C and 187 and 343 at 35/25 °C. CO₂ enrichment enhanced CO₂ uptake substantially in the day in all environments. Uptake at night at elevated CO₂, relative to that at ambient CO₂, was unchanged at 30/20 °C, but was 80% higher at 30/25 °C and 44% higher at 35/25 °C suggesting that phosphoenolpyruvate carboxylase was not CO₂-saturated at ambient CO₂ levels and a 25 °C night temperature. Photosynthetic water use efficiency (WUE) was higher at elevated than at ambient CO₂. Leaf Δ -values were higher at elevated than at ambient CO₂ due to relatively higher assimilation in the light. Leaf Δ was significantly and linearly related to the fraction of total CO₂ assimilated at night. The data suggest that a simultaneous increase in CO₂ level and temperature associated with global warming would enhance carbon assimilation, increase WUE, and reduce the temperature dependence of CO₂ uptake by A. comosus .     

Effects of tree height on branch hydraulics, leaf structure and gas exchange in California redwoods

We examined changes in branch hydraulic, leaf structure and gas exchange properties in coast redwood ( Sequoia sempervirens ) and giant sequoia ( Sequoiadendron giganteum ) trees of different sizes. Leaf-specific hydraulic conductivity ( k _L) increased with height in S. sempervirens but not in S. giganteum , while xylem cavitation resistance increased with height in both species. Despite hydraulic adjustments, leaf mass per unit area (LMA) and leaf carbon isotope ratios ( δ ¹³C) increased, and maximum mass-based stomatal conductance ( g _mass) and photosynthesis ( A _mass) decreased with height in both species. As a result, both A _mass and g _mass were negatively correlated with branch hydraulic properties in S. sempervirens and uncorrelated in S. giganteum . In addition, A _mass and g _mass were negatively correlated with LMA in both species, which we attributed to the effects of decreasing leaf internal CO₂ conductance ( g _i). Species-level differences in wood density, LMA and area-based gas exchange capacity constrained other structural and physiological properties, with S. sempervirens exhibiting increased branch water transport efficiency and S. giganteum exhibiting increased leaf-level water-use efficiency with increasing height. Our results reveal different adaptive strategies for the two redwoods that help them compensate for constraints associated with growing taller, and reflect contrasting environmental conditions each species faces in its native habitat.