Interactive effects of elevated CO2 , warming, reduced rainfall, and nitrogen on leaf gas exchange in five perennial grassland species.

Global changes can interact to affect photosynthesis and thus ecosystem carbon capture, yet few multi-factor field studies exist to examine such interactions. Here, we evaluate leaf gas exchange responses of five perennial grassland species from four functional groups to individual and interactive global changes in an open-air experiment in Minnesota, USA, including elevated CO2 (eCO2 ), warming, reduced rainfall and increased soil nitrogen supply. All four factors influenced leaf net photosynthesis and/or stomatal conductance, but almost all effects were context-dependent, i.e. they differed among species, varied with levels of other treatments and/or depended on environmental conditions. Firstly, the response of photosynthesis to eCO2 depended on species and nitrogen, became more positive as vapour pressure deficit increased and, for a C4 grass and a legume, was more positive under reduced rainfall. Secondly, reduced rainfall increased photosynthesis in three functionally distinct species, potentially via acclimation to low soil moisture. Thirdly, warming had positive, neutral or negative effects on photosynthesis depending on species and rainfall. Overall, our results show that interactions among global changes and environmental conditions may complicate predictions based on simple theoretical expectations of main effects, and that the factors and interactions influencing photosynthesis vary among herbaceous species.

Strong photosynthetic acclimation and enhanced water-use efficiency in grassland functional groups persist over 21 years of CO2 enrichment, independent of nitrogen supply.

Uncertainty about long-term leaf-level responses to atmospheric CO2 rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO2 (eCO2 ) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long-lived perennials. Here, we report the effects of eCO2 on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free-Air CO2 Enrichment experiment, BioCON. Monocultures of species belonging to C3 grasses, C4 grasses, forbs, and legumes were exposed to two levels of CO2 and nitrogen supply in factorial combinations over 21 years. eCO2 increased photosynthesis by 12.9% on average in C3 species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO2 was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO2 was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C4 grasses the least as expected, but did not further diverge over time. Leaf-level water-use efficiency increased by 50% under eCO2 primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO2 in nitrogen-limited systems, and that significant and consistent declines in stomatal conductance and increases in water-use efficiency under eCO2 may allow plants to better withstand drought.

Lifetime return on investment increases with leaf lifespan among 10 Australian woodland species.

• Co-occurring species often differ in their leaf lifespan (LL) and it remains unclear how such variation is maintained in a competitive context. Here we test the hypothesis that leaves of long-LL species yield a greater return in carbon (C) fixed per unit C or nutrient invested by the plant than those of short-LL species. • For 10 sympatric woodland species, we assessed three-dimensional shoot architecture, canopy openness, leaf photosynthetic light response, leaf dark respiration and leaf construction costs across leaf age sequences. We then used the YPLANT model to estimate light interception and C revenue along the measured leaf age sequences. This was done under a series of simulations that incorporated the potential covariates of LL in an additive fashion. • Lifetime return in C fixed per unit C, N or P invested increased with LL in all simulations. • In contrast to other recent studies, our results show that extended LL confers a fundamental economic advantage by increasing a plant's return on investment in leaves. This suggests that time-discounting effects, that is, the compounding of income that arises from quick reinvestment of C revenue, are key in allowing short-LL species to succeed in the face of this economic handicap.

Controls on declining carbon balance with leaf age among 10 woody species in Australian woodland: do leaves have zero daily net carbon balances when they die?

* Here, we evaluated how increased shading and declining net photosynthetic capacity regulate the decline in net carbon balance with increasing leaf age for 10 Australian woodland species. We also asked whether leaves at the age of their mean life-span have carbon balances that are positive, zero or negative. * The net carbon balances of 2307 leaves on 53 branches of the 10 species were estimated. We assessed three-dimensional architecture, canopy openness, photosynthetic light response functions and dark respiration rate across leaf age sequences on all branches. We used YPLANT to estimate light interception and to model carbon balance along the leaf age sequences. * As leaf age increased to the mean life-span, increasing shading and declining photosynthetic capacity each separately reduced daytime carbon gain by approximately 39% on average across species. Together, they reduced daytime carbon gain by 64% on average across species. * At the age of their mean life-span, almost all leaves had positive daytime carbon balances. These per leaf carbon surpluses were of a similar magnitude to the estimated whole-plant respiratory costs per leaf. Thus, the results suggest that a whole-plant economic framework, including respiratory costs, may be useful in assessing controls on leaf longevity.

Response to Comment on "Unexpected reversal of C3 versus C4 grass response to elevated CO2 during a 20-year field experiment".

Nie and colleagues suggest a key role for interannual climate variation as an explanation for the temporal dynamics of an unexpected 20-year reversal of biomass responses of C3-C4 grasses to elevated CO2 However, we had already identified some climate-dependent differences in C3 and C4 responses to eCO2 and shown that these could not fully explain the temporal dynamics we observed.

Response to Comment on "Unexpected reversal of C3 versus C4 grass response to elevated CO2 during a 20-year field experiment".

Wolf and Ziska suggest that soil and species attributes can explain an unexpected 20-year reversal of C3-C4 grass responses to elevated CO2 This is consistent with our original interpretation; however, we disagree with the assertion that such explanations somehow render our results irrelevant for questioning a long-standing paradigm of plant response to CO2 based on C3-C4 differences in photosynthetic pathway.

Unexpected reversal of C3 versus C4 grass response to elevated CO2 during a 20-year field experiment.

Theory predicts and evidence shows that plant species that use the C4 photosynthetic pathway (C4 species) are less responsive to elevated carbon dioxide (eCO2) than species that use only the C3 pathway (C3 species). We document a reversal from this expected C3-C4 contrast. Over the first 12 years of a 20-year free-air CO2 enrichment experiment with 88 C3 or C4 grassland plots, we found that biomass was markedly enhanced at eCO2 relative to ambient CO2 in C3 but not C4 plots, as expected. During the subsequent 8 years, the pattern reversed: Biomass was markedly enhanced at eCO2 relative to ambient CO2 in C4 but not C3 plots. Soil net nitrogen mineralization rates, an index of soil nitrogen supply, exhibited a similar shift: eCO2 first enhanced but later depressed rates in C3 plots, with the opposite true in C4 plots, partially explaining the reversal of the eCO2 biomass response. These findings challenge the current C3-C4eCO2 paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.

Legume species identity and soil nitrogen supply determine symbiotic nitrogen-fixation responses to elevated atmospheric [CO2].

In nitrogen (N)-limited systems, the response of symbiotic N fixation to elevated atmospheric [CO2] may be an important determinant of ecosystem responses to this global change. Experimental tests of the effects of elevated [CO2] have not been consistent. Although rarely tested, differences among legume species and N supply may be important. In a field free-air CO2 enrichment (FACE) experiment, we determined, for four legume species, whether the effects of elevated atmospheric [CO2] on symbiotic N fixation depended on soil N availability or species identity. Natural abundance and pool-dilution 15N methods were used to estimate N fixation. Although N addition did, in general, decrease N fixation, contrary to theoretical predictions, elevated [CO2] did not universally increase N fixation. Rather, the effect of elevated [CO2] on N fixation was positive, neutral or negative, depending on the species and N addition. Our results suggest that legume species identity and N supply are critical factors in determining symbiotic N-fixation responses to increased atmospheric [CO2].

Legume presence increases photosynthesis and N concentrations of co-occurring non-fixers but does not modulate their responsiveness to carbon dioxide enrichment.

Legumes, with the ability to fix atmospheric nitrogen (N), may help alleviate the N limitations thought to constrain plant community response to elevated concentrations of atmospheric carbon dioxide (CO(2)). To address this issue we assessed: (1) the effects of the presence of the perennial grassland N(2 )fixer, Lupinus perennis, on biomass accumulation and plant N concentrations of nine-species plots of differing plant composition; (2) leaf-level physiology of co-occurring non-fixing species (Achillea millefolium, Agropyron repens, Koeleria cristata) in these assemblages with and without Lupinus; (3) the effects of elevated CO(2) on Lupinus growth and symbiotic N(2) fixation in both monoculture and the nine-species assemblages; and (4) whether assemblages containing Lupinus exhibit larger physiological and growth responses to elevated CO(2 )than those without. This study was part of a long-term grassland field experiment (BioCON) that controls atmospheric CO(2) at current ambient and elevated (560 micromol mol(-1)) concentrations using free-air CO(2) enrichment. Nine-species plots with Lupinus had 32% higher whole plot plant N concentrations and 26% higher total plant N pools than those without Lupinus, based on both above and below ground measurements. Co-occurring non-fixer leaf N concentrations increased 22% and mass-based net photosynthetic rates increased 41% in plots containing Lupinus compared to those without. With CO(2) enrichment, Lupinus monocultures accumulated 32% more biomass and increased the proportion of N derived from fixation from 44% to 57%. In nine-species assemblages, Lupinus N derived from fixation increased similarly from 43% to 54%. Although Lupinus presence enhanced photosynthetic rates and leaf N concentrations of co-occurring non-fixers, and increased overall plant N pools, Lupinus presence did not facilitate stronger photosynthetic responses of non-fixing species or larger growth responses of overall plant communities to elevated CO(2). Non-fixer leaf N concentrations declined similarly in response to elevated CO(2) with and without Lupinus present and the relationship between net photosynthesis and leaf N was not affected by Lupinus presence. Regardless of the presence or absence of Lupinus, CO(2) enrichment resulted in reduced leaf N concentrations and rates of net photosynthesis.