Total aboveground net primary production (ANPP, black line) and the component from annual plants (red line) and perennials (blue line) averaged across all treatments, from 1998 through 2014. Treatments comprised full-factorial warming, added precipitation, elevated CO2, and nitrogen deposition. Graphic: Zhu, et al., 2016 / PNAS

STANFORD, California, 5 September 2016 (Carnegie Science) – One of the world’s longest-running, most comprehensive climate change experiments produced some surprising results. The extensive experiment subjected grassland ecosystems to sixteen possible future climates and measured many aspects of ecosystem performance and sustainability. This study, appearing in the September 5, 2016, Early Online Edition of the Proceedings of the National Academy of Sciences, reports on 17 years of plant growth, an important bellwether of ecosystem health. Plant growth varied tremendously from year to year, reaching a peak under conditions near the average over the last several decades. As conditions move away from the averages, as happens with climate change, plant growth fell.

The findings are from the Jasper Ridge Global Change Experiment, which is directed by Chris Field, the founding director of Carnegie’s Department of Global Ecology. Unlike most climate-change experiments that altered one or maybe two aspects of the environment, the Jasper Ridge Global Change Experiment altered four aspects of climate change—temperature, precipitation, atmospheric composition (carbon dioxide concentration), and atmospheric deposition (nitrogen pollution). With all possible combinations of ambient and elevated levels of the four factors, the study explored ecosystem responses to sixteen different possible futures.

The study ecosystem, a typical California grassland, is ideal for this kind of experiment because it has many species, even small plots express a wide range of ecosystem processes, and the short lifetime of most species means that an experiment can encompass many generations of the most important organisms.  

“Plant growth varied by more than three times over the years and the range of treatments,” remarked lead author Kai Zhu who was at Carnegie and Stanford when the work was conducted and is now at Rice University. “Good conditions tend to look like the recent past, and bad conditions look more common in a world of climate change. But we did not see progressive effects, meaning that we did not see that one bad year makes the next year bad as well. We think year-to-year variability acts as a reset button.”

Field said, “For understanding impacts of climate change and options for dealing with it, one important result was the absence of a strong response to elevated atmospheric carbon dioxide. Data from some ecosystems indicate that elevated atmospheric carbon dioxide might compensate for negative effects of warming or drought, sustaining ecosystem health and potentially removing carbon dioxide from the atmosphere. The absence of such compensation in this long-term, comprehensive experiment emphasizes that solving the carbon dioxide problem will require cutting emissions and planting forests. We can’t count on a free helping hand from nature.”

Analysis of the impacts of individual factors showed that warming had negative effects on plant growth. Plant growth peaked when precipitation was close to historic averages. There was no consistent response to elevated atmospheric carbon dioxide. Nitrogen pollution led to a 23% increase in plant growth, a typical response in nitrogen-limited ecosystems.

Scenarios with combined factors mostly resulted in this same pattern, but with a few surprises. Specifically, the response to combined warming and increased precipitation was larger than the sum of the individual responses when they happened in isolation. But the response of plant growth to warming and nitrogen pollution was smaller than the sum of each individual effect occurring alone. The maximum plant growth occurred when both temperature and precipitation were at levels typical of average conditions over the last several decades. Plant growth declined with rising temperature and with precipitation either lower or higher than long-term averages.

Field commented, “In Jasper Ridge grasslands, we see an ecosystem finely adapted to historic conditions. Providing a chance for places like this will require ambitiously tackling climate change so that we stabilize warming at the low end of the possible range. That is our challenge for the future.”

Grassland tuned to present suffers in a warmer future

ABSTRACT:Global changes in climate, atmospheric composition, and pollutants are altering ecosystems and the goods and services they provide. Among approaches for predicting ecosystem responses, long-term observations and manipulative experiments can be powerful approaches for resolving single-factor and interactive effects of global changes on key metrics such as net primary production (NPP). Here we combine both approaches, developing multidimensional response surfaces for NPP based on the longest-running, best-replicated, most-multifactor global-change experiment at the ecosystem scale—a 17-y study of California grassland exposed to full-factorial warming, added precipitation, elevated CO2, and nitrogen deposition. Single-factor and interactive effects were not time-dependent, enabling us to analyze each year as a separate realization of the experiment and extract NPP as a continuous function of global-change factors. We found a ridge-shaped response surface in which NPP is humped (unimodal) in response to temperature and precipitation when CO2 and nitrogen are ambient, with peak NPP rising under elevated CO2 or nitrogen but also shifting to lower temperatures. Our results suggest that future climate change will push this ecosystem away from conditions that maximize NPP, but with large year-to-year variability.

SIGNIFICANCE: Global environmental change involves many factors that occur simultaneously, yet they are usually studied in isolation. Here we report a long-term global change experiment that subjected California grassland to multiple individual and simultaneous changes in temperature, precipitation, carbon dioxide, and nitrogen. Our analysis revealed nonlinear and interactive effects of temperature and precipitation on grassland net primary production (NPP), which defined a ridge-shaped NPP response surface to these two variables. Added nitrogen raised the peak of the NPP response surface, and added CO2 shifted the peak to lower temperatures. Our approach was validated by tests showing an absence of progressive effects over the years. In other ecosystems, our approach may be similarly powerful for probing the effects of multifactor global change.

Nonlinear, interacting responses to climate limit grassland production under global change



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