Nicholas G. Smith

Plant respiration and photosynthesis in global-scale models: incorporating acclimation to temperature and CO2.

To realistically simulate climate feedbacks from the land surface to the atmosphere, models must replicate the responses of plants to environmental changes. Several processes, operating at various scales, cause the responses of photosynthesis and plant respiration to temperature and CO2 to change over time of exposure to new or changing environmental conditions. Here, we review the latest empirical evidence that short-term responses of plant carbon exchange rates to temperature and CO2 are modified by plant photosynthetic and respiratory acclimation as well as biogeochemical feedbacks. We assess the frequency with which these responses have been incorporated into vegetation models, and highlight recently designed algorithms that can facilitate their incorporation. Few models currently include representations of the long-term plant responses that have been recorded by empirical studies, likely because these responses are still poorly understood at scales relevant for models. Studies show that, at a regional scale, simulated carbon flux between the atmosphere and vegetation can dramatically differ between versions of models that do and do not include acclimation. However, the realism of these results is difficult to evaluate, as algorithm development is still in an early stage, and a limited number of data are available. We provide a series of recommendations that suggest how a combination of empirical and modeling studies can produce mechanistic algorithms that will realistically simulate longer term responses within global-scale models.

Temperature acclimation of photosynthesis and respiration: A key uncertainty in the carbon cycle-climate feedback

Earth System Models typically use static responses to temperature to calculate photosynthesis and respiration, but experimental evidence suggests that many plants acclimate to prevailing temperatures. We incorporated representations of photosynthetic and leaf respiratory temperature acclimation into the Community Land Model, the terrestrial component of the Community Earth System Model. These processes increased terrestrial carbon pools by 20 Pg C (22%) at the end of the 21st century under a business-as-usual (Representative Concentration Pathway 8.5) climate scenario. Including the less certain estimates of stem and root respiration acclimation increased terrestrial carbon pools by an additional 17 Pg C (~40% overall increase). High latitudes gained the most carbon with acclimation, and tropical carbon pools increased least. However, results from both of these regions remain uncertain; few relevant data exist for tropical and boreal plants or for extreme temperatures. Constraining these uncertainties will produce more realistic estimates of land carbon feedbacks throughout the 21st century.

Toward a better integration of biological data from precipitation manipulation experiments into Earth system models

The biological responses to precipitation within the terrestrial components of Earth system models, or land surface models (LSMs), are mechanistically simple and poorly constrained, leaving projections of terrestrial ecosystem functioning and feedbacks to climate change uncertain. A number of field experiments have been conducted or are underway to test how changing precipitation will affect terrestrial ecosystems. Results from these experiments have the potential to vastly improve modeled processes. However, the transformation of experimental results into model improvements still represents a grand challenge. Here we review the current state of precipitation manipulation experiments and the precipitation responses of biological processes in LSMs to explore how these experiments can help improve model realism. First, we discuss contemporary precipitation projections and then review the structure and function of current-generation LSMs. We then examine different experimental designs and discuss basic variables that, if measured, would increase a field experiments usefulness in a modeling context. Next, we compare biological processes commonly measured in the field with their model analogs and find that, in many cases, the way these processes are measured in the field is not compatible with the way they are represented in LSMs, an effect that hinders model development. We then discuss the challenge of scaling from the plot to the globe. Finally, we provide a series of recommendations aimed to improve the connectivity between experiments and LSMs and conclude that studies designed from the perspective of researchers in both communities will provide the greatest benefit to the broader global change community.

Rainfall variability and nitrogen addition synergistically reduce plant diversity in a restored tallgrass prairie

Summary 1. Climate change is expected to bring fewer, larger rainfall events and prolonged droughts (i.e. increased rainfall variability). Concurrently, the burning of fossil fuels and reliance on nitrogen (N) fertilizers are expected to continue to increase N availability in many ecosystems. These changes in water and N availability have the potential to alter plant community composition and structure. 2. We manipulated rainfall variability and N inputs in a restored tallgrass prairie over the course of two growing seasons. 3. Greater rainfall variability led to wetter soils throughout the majority of both growing seasons and provided punctuated relief from a severe drought that occurred during the first three months of the experiment. 4. Both rainfall variability- and fertilization-induced increases in resource availability favoured fast-growing, deeply rooted C3 forbs, particularly the dominant Solidago canadensis, at the expense of species adapted to low resource conditions, particularly C4 grasses and N-fixing forbs. 5. This change in community composition decreased plant community diversity and evenness in plots receiving both supplemental N and more variable rainfall. 6. Synthesis and applications. These results suggest that future increases in rainfall variability and nitrogen (N) deposition could synergistically alter the structure of prairie restorations and jeopardize restoration targets related to increasing floral diversity. Mitigating N availability in restoration sites may help to maintain prairie diversity as rainfall patterns become more variable.

Responses of aboveground C and N pools to rainfall variability and nitrogen deposition are mediated by seasonal precipitation and plant community dynamics

Plant productivity and tissue chemistry in temperate ecosystems are largely driven by water and nitrogen (N) availability. Although changes in rainfall patterns may influence nutrient limitation, few studies have considered how these two global change factors could interact to influence terrestrial ecosystem productivity and stoichiometry. Here, we examined the influence of experimentally-increased intra-annual rainfall variability and low-level nitrogen addition on aboveground productivity, C and N pools, and C:N ratios in a restored tallgrass prairie across two growing seasons. In the drier first year of the experiment, increased rainfall variability boosted productivity and C pools. In the wetter second year, aboveground productivity and C pools increased with N addition, suggesting a switch in primary resource limitation from water to N. Increased rainfall variability also reduced aboveground N pools in the second year. Community-level C:N increased under increased rainfall variability in the wetter second year and N addition slightly reduced community C:N in both years. These changes in element pools and stoichiometry were mostly a result of increased forb dominance in response to both treatments. Overall, our findings from a restored prairie indicate that increased rainfall variability and N addition can enhance aboveground productivity and C pools, but that N pools may not have a consistent response to either global change factor. Our study also suggests that these effects are dependent on growing season precipitation patterns and are mediated by shifts in plant community composition.

Biophysical consequences of photosynthetic temperature acclimation for climate

Photosynthetic temperature acclimation is a commonly observed process that is increasingly being incorporated into Earth System Models (ESMs). While short-term acclimation has been shown to increase carbon storage in the future, it is uncertain whether acclimation will directly influence simulated future climate through biophysical mechanisms. Here, we used coupled atmosphere-biosphere simulations using the Community Earth System Model (CESM) to assess how acclimation-induced changes in photosynthesis influence global climate under present-day and future (RCP 8.5) conditions. We ran four 30 year simulations that differed only in sea surface temperatures and atmospheric CO2 (present or future) and whether a mechanism for photosynthetic temperature acclimation was included (yes or no). Acclimation increased future photosynthesis and, consequently, the proportion of energy returned to the atmosphere as latent heat, resulting in reduced surface air temperatures in areas and seasons where acclimation caused the biggest increase in photosynthesis. However, this was partially offset by temperature increases elsewhere, resulting in a small, but significant, global cooling of 0.05°C in the future, similar to that expected from acclimation-induced increases in future land carbon storage found in previous studies. In the present-day simulations, the photosynthetic response was not as strong and cooling in highly vegetated regions was less than warming elsewhere, leading to a net global increase in temperatures of 0.04°C. Precipitation responses were variable and rates did not change globally in either time period. These results, combined with carbon-cycle effects, suggest that models without acclimation may be overestimating positive feedbacks between climate and the land surface in the future.

Characterizing the drivers of seedling leaf gas exchange responses to warming and altered precipitation: indirect and direct effects

Climate change is expected to bring warmer temperatures and more variable precipitation patterns worldwide, patterns that will depend on the ability of the worlds flora to take up carbon under these new conditions. We subjected deciduous tree seedlings growing in an old-field ecosystem in Massachusetts, USA to warming and altered precipitation. We found that leaf carbon uptake was greatest under the coolest, wettest conditions, an effect driven by increased soil water availability in these plots. Our findings suggest that warming may reduce leaf carbon uptake by decreasing soil moisture, an effect that will be exacerbated during drought periods.

Warming increases the sensitivity of seedling growth capacity to rainfall in six temperate deciduous tree species

Using a fully factorial precipitation by warming experiment in an old-field ecosystem in the northeastern USA we studied the climatic sensitivity of seedlings of six native tree species. Warm and dry conditions suppressed seedling growth, but affected species differently by increasing mortality, enhancing rates of herbivory or decreasing foliar carbon uptake. Our results indicate that, in the northeastern USA, dry years in a future warmer environment could have damaging effects on the growth capacity of early secondary successional forests, through species-specific effects on leaf production, herbivory and mortality.

Drivers of leaf carbon exchange capacity across biomes at the continental scale

Realistic representations of plant carbon exchange processes are necessary to reliably simulate biosphere-atmosphere feedbacks. These processes are known to vary over time and space, though the drivers of the underlying rates are still widely debated in the literature. Here, we measured leaf carbon exchange in >500 individuals of 98 species from the Neotropics to high boreal biomes to determine the drivers of photosynthetic and dark respiration capacity. Covariate abiotic (long- and short-term climate) and biotic (plant type, plant size, ontogeny, water status) data were used to explore significant drivers of temperature-standardized leaf carbon exchange rates. Using model selection, we found the previous weeks temperature and soil moisture at the time of measurement to be a better predictor of photosynthetic capacity than long-term climate, with the combination of high recent temperatures and low soil moisture tending to decrease photosynthetic capacity. Non-trees (annual and perennials) tended to have greater photosynthetic capacity than trees, and, within trees, adults tended to have greater photosynthetic capacity than juveniles, possibly as a result of differences in light availability. Dark respiration capacity was less responsive to the assessed drivers than photosynthetic capacity, with rates best predicted by multi-year average site temperature alone. Our results suggest that, across large spatial scales, photosynthetic capacity quickly adjusts to changing environmental conditions, namely light, temperature, and soil moisture. Respiratory capacity is more conservative and most responsive to longer-term conditions. Our results provide a framework for incorporating these processes into large-scale models and a data set to benchmark such models.

Effect of microtopography on soil respiration in an alpine meadow of the Qinghai-Tibetan plateau

Background and aimsSoil respiration is an important component of terrestrial carbon cycling and is sensitive to environmental change. Most previous studies focus on the effect of soil temperature and moisture on soil respiration, whereas the impact of spatial heterogeneity (e.g., microtopography) is seldom studied.MethodsTo test the impact of microtopography on soil respiration, we performed a field investigation to examine soil respiration, soil temperature, soil water content, soil total porosity, soil organic content, and plant biomass at a hummock site (composed of grass hummocks and inter-hummock areas) and an adjacent flat meadow of the Qinghai-Tibetan plateau.ResultsSimilar seasonal dynamics of soil respiration in the grass hummocks, inter-hummock areas, and flat meadow were found in the alpine meadow of the Qinghai-Tibetan plateau. However, soil respiration of the grass hummocks was 79.3% and 413.9% higher than that of the flat meadow during the growing (April, June, August) and non-growing seasons (October, December, February), respectively. Although there was no difference in soil respiration between the inter-hummock areas and the flat meadow during the non-growing season, soil respiration was 42.5% higher at the inter-hummock areas than the flat meadow during growing season. Larger soil porosity, greater surface area, and more substrate supply, but not more root growth, likely contributed to the higher soil respiration of grass hummocks.ConclusionsOur findings suggest that the impact of spatial heterogeneity on soil respiration should be taken into consideration to facilitate the accurate estimation of soil carbon fluxes at ecosystem and regional scales.