Crown carbon gain and elevated CO2 responses of understory saplings with differing allometry and architecture

1. Attempts at determining the physiological basis of species' differences, such as the ability to grow in deep shade, have been of limited success. However, this basis is fundamental to predicting species' responses to rising atmospheric CO2 in the forest understorey. We linked a leaf photosynthesis and a tree architecture model to predict the effects of dynamic and steady state photosynthetic characteristics, crown architecture and elevated atmospheric CO2 concentration ([CO2]) on crown-level carbon gain (Acrown). Twenty-four-h Acrown was modelled for shade-tolerant Acer rubrum and shade-intolerant Liriodendron tulipifera saplings growing for three years in a forest understorey under ambient and elevated [CO2] in free-air CO2 enrichment. 2. Two factors best explained Acrown in ambient [CO2]: tree light environment and sapling allometry. Microsite light environment influenced carbon gain via daily photosynthetic photon flux (PFD), average diffuse PFD and sunfleck characteristics. Species differences in specific leaf area (SLA) and size-related biomass allocation to leaves affected the effective leaf area and hence Acrown. 3. At a common above-ground biomass, small saplings (100 g above-ground dry mass) of L. tulipifera had higher Acrown than A. rubrum samples due to larger SLA and greater biomass allocation to leaves. Larger saplings of the two species had similar Acrown due to greater carbon allocation to leaves with increasing plant size in A. rubrum vs L. tulipifera. For saplings > 800 g, Acrown was greater in A. rubrum than in L. tulipifera. Enhancement of Acrown by elevated [CO2] on sunny days was similar for both species. 4. Overall, though the shade-tolerant species had lower Acrown than the shade-intolerant species at a common small size, our results indicate that the relative performance of these species can reverse at larger sizes due to allocational differences. These results suggest that elevated [CO2] may accelerate competition for light between A. rubrum and L. tulipifera as these species grow larger in the understorey

Leaf to landscape

One fundamental problem for maximizing carbon gain at the leaf and higher organizational levels entails the link between light aperture and leaf energy budgets. The balance between the two processes, however, depends on the environment. For example, shade environments limit carbon gain due to low light levels, and so we would expect plants to display traits that maximize light interception and traits that facilitate survival in a low-resource environment (relative to photosynthesis). These topics have received wide attention and have been reviewed numerous times (for the most relevant contributions in the context of this book, see Ackerly 1996, Poorter 1999, Poorter and Werger 1999, Valladares 1999, Walters and Reich 1999; see also Chapter 2). Conversely, in high-light environments, leaf energy balance becomes the driving force for structural traits. This is particularly true for environments that are at the same time limited by water availability. Similarly, although in the opposite direction, leaf energy balance is a major factor in carbon gain in cold environments. A third significant environmental axis is nutrient availability which, when limiting, can result in evergreen, often sclerophyllous, leaves which have potentially low photosynthetic capacity but high efficiency of resource utilization

Modeling dynamic understory photosynthesis of contrasting species in ambient and elevated carbon dioxide

Dynamic responses of understory plants to sunflecks have been extensively studied, but how much differences in dynamic light responses affect daily photosynthesis (Aday) is still the subject of active research. Recent models of dynamic photosynthesis have provided a quantitative tool that allows the critical assessment of the importance of these sunfleck responses on Aday. Here we used a dynamic photosynthesis model to assess differences in four species that were growing in ambient and elevated CO2. We hypothesized that Liriodendron tulipifera, a species with rapid photosynthetic induction gain and slow induction loss, would have the least limitations to sunfleck photosynthesis relative to the other three species (Acer rubrum, Cornus florida, Liquidambar styraciflua). As a consequence, L. tulipifera should have the highest Aday in an understory environment, despite being the least shade tolerant of the species tested. We further hypothesized that daily photosynthetic enhancement by elevated CO2 would differ from enhancement levels observed during light-saturated, steady-state measurements. Both hypotheses were supported by the model results under conditions of low daily photosynthetic photon flux density (PFD; <3% of the above-canopy PFD). However, under moderate PFD (10-20% of the above-canopy PFD), differences in dynamic sunfleck responses had no direct impact on Aday for any of the species, since stomatal and photosynthetic induction limitations to sunfleck photosynthesis were small. Thus, the relative species ranking in Aday under moderate PFD closely matched their rankings in steady-state measurements of light-saturated photosynthesis. Similarly, under elevated CO2, enhancement of modeled Aday over Aday at ambient CO2 matched the enhancement measured under light saturation. Thus, the effects of species-specific differences in dynamic sunfleck responses, and differences in elevated CO2 responses of daily photosynthesis, are most important in marginal light environments

Photosynthetic responses of Mojave Desert shrubs to free air CO2 enrichment are greatest during wet years

It has been suggested that desert vegetation will show the strongest response to rising atmospheric carbon dioxide due to strong water limitations in these systems that may be ameliorated by both photosynthetic enhancements and reductions in stomatal conductance. Here, we report the long-term effect of 55 Pa atmospheric CO2 on photosynthesis and stomatal conductance for three Mojave Desert shrubs of differing leaf phenology (Ambrosia dumosa—drought-deciduous, Krameria erecta—winter-deciduous, Larrea tridentata—evergreen). The shrubs were growing in an undisturbed ecosystem fumigated using FACE technology and were measured over a four-year period that included both above and below-average precipitation. Daily integrated photosynthesis (A(day)) was significantly enhanced by elevated CO2 for all three species, although Krameria erecta showed the greatest enhancements (63% vs. 32% for the other species) enhancements were constant throughout the entire measurement period. Only one species, Larrea tridentata, decreased stomatal conductance by 25–50% in response to elevated CO2, and then only at the onset of the summer dry season and following late summer convective precipitation. Similarly, reductions in the maximum carboxylation rate of Rubisco were limited to Larrea during spring. These results suggest that the elevated CO2 response of desert vegetation is a function of complex interactions between species functional types and prevailing environmental conditions. Elevated CO2 did not extend the active growing season into the summer dry season because of overall negligible stomatal conductance responses that did not result in significant water conservation. Overall, we expect the greatest response of desert vegetation during years with above-average precipitation when the active growing season is not limited to ∼2 months and, consequently, the effects of increased photosynthesis can accumulate over a biologically significant time period

Photosynthetic responses of Larrea tridentata to seasonal temperature extremes under elevated CO2

Elevated CO2 potentially decreases the effects of temperature stress on photosynthesis. Under both freezing and high temperatures previous studies have shown that elevated CO2 can particularly enhance photosynthetic rates, although results from freezing studies are more variable. Here we show gas exchange responses of Larrea tridentata to elevated CO2 over a 6-yr period when temperature stress events may have had a significant effect on photosynthesis in the field. Nighttime freezing air temperatures decreased subsequent daytime photosynthetic rates, stomatal conductance, and the maximum yield of PSII similarly under ambient and elevated CO2. Further, we found no statistically significant relationship between leaf temperature and photosynthetic enhancement. Overall, the degree of photosynthetic enhancement under elevated CO2 was directly proportional to the response of stomatal conductance to CO2. Thus, elevated CO2 does not significantly affect apparent physiological responses of Larrea to temperature extremes. However, because of the tight relationship between stomatal conductance and photosynthetic enhancement, potential climate change effects on stomatal conductance will significantly influence Larrea performance in the future

Increases in desert shrub productivity under elevated carbon dioxide vary with water availability

Productivity of aridland plants is predicted to increase substantially with rising atmospheric carbon dioxide (CO2) concentrations due to enhancement in plant water-use efficiency (WUE). However, to date, there are few detailed analyses of how intact desert vegetation responds to elevated CO2. From 1998 to 2001, we examined aboveground production, photosynthesis, and water relations within three species exposed to ambient (around 38 Pa) or elevated (55 Pa) CO2 concentrations at the Nevada Desert Free-Air CO2 Enrichment (FACE) Facility in southern Nevada, USA. The functional types sampled—evergreen (Larrea tridentata), drought-deciduous (Ambrosia dumosa), and winter-deciduous shrubs (Krameria erecta)—represent potentially different responses to elevated CO2 in this ecosystem. We found elevated CO2 significantly increased aboveground production in all three species during an anomalously wet year (1998), with relative production ratios (elevated:ambient CO2) ranging from 1.59 (Krameria) to 2.31 (Larrea). In three below-average rainfall years (1999–2001), growth was much reduced in all species, with only Ambrosia in 2001 having significantly higher production under elevated CO2. Integrated photosynthesis (mol CO2 m−2 y−1) in the three species was 1.26–2.03-fold higher under elevated CO2 in the wet year (1998) and 1.32–1.43-fold higher after the third year of reduced rainfall (2001). Instantaneous WUE was also higher in shrubs grown under elevated CO2. The timing of peak canopy development did not change under elevated CO2; for example, there was no observed extension of leaf longevity into the dry season in the deciduous species. Similarly, seasonal patterns in CO2 assimilation did not change, except for Larrea. Therefore, phenological and physiological patterns that characterize Mojave Desert perennials—early-season lags in canopy development behind peak photosynthetic capacity, coupled with reductions in late-season photosynthetic capacity prior to reductions in leaf area—were not significantly affected by elevated CO2. Together, these findings suggest that elevated CO2 can enhance the productivity of Mojave Desert shrubs, but this effect is most pronounced during years with abundant rainfall when soil resources are most available