Since the start of the Industrial Revolution, human activities have changed the composition of the atmosphere at an accelerating rate, with increasingly recognized consequences for Earth's climate and biogeochemical cycles [1
]. Ecosystem responses to these changes may further affect climate and biogeochemical cycling [3
], and alter the character of ecosystem services provided to society [5
]. During the past two decades, researchers have studied ecosystem responses to changes in climate, nitrogen (N) deposition, and atmospheric carbon dioxide (CO2
]. In some natural systems, responses of plant growth and resource use to one of these global changes have been extensively quantified. However, few studies have examined responses of ecosystems to the simultaneous and interacting global changes likely to be seen later this century. Even fewer studies have observed these responses over many years.
Production responses to single environmental changes vary widely among systems, and by year.
First, doubled atmospheric CO2
increased aboveground biomass production by an average of 14% across nine herbaceous systems [6
]. However, CO2
enrichment suppressed production in some systems, while increasing it in others by as much as 85%. Some grasslands responded more positively in dry years than wet years [7
], possibly because plants narrow their stomatal openings under elevated CO2
, which leads to water savings.
Second, observed patterns of plant growth across natural gradients of precipitation and across years within locations suggest that increases in precipitation have the most positive effect on plant growth in systems with the lowest annual inputs [10
]. Where precipitation exceeds about 3,000 mm per year, additional precipitation may suppress growth [11
Third, warming increases aboveground biomass production in many systems, with the strongest effects in colder climates. Across 20 experimental warming sites in tundra, grassland, and forest, increases in aboveground productivity averaged 19% [12
]. Across natural systems, production tends to increase with increasing mean annual temperature [11
]. Within some productive systems, aboveground growth is correlated with maximum growing season temperature [10
Fourth, responses to N additions are generally positive across temperate, boreal, and arctic systems [13
While all terrestrial systems are experiencing a fairly uniform increase in CO2
, the character of other global changes varies from one region to the next. Thus, the mix of global changes impacting a given region will depend on both space and time. Understanding the responses of ecosystems to potentially interacting global changes is critical to predicting ecosystem feedbacks to climate and biogeochemical cycles. In particular, the response of carbon (C) storage in ecosystems is dependent on (and proportionally related to) two ecosystem processes: C inputs from primary production, and the residence time of C in the system [15
Several previous studies have examined interactions between N availability and other global change factors [16
], and some have examined interactions between CO2
and changes in water availability [8
], climate [19
], or loss of biodiversity [22
]. However, we are still developing a conceptual framework to describe the conditions under which a given interaction is most important. For instance, mineral element availability may progressively limit positive CO2
responses in some systems, but other systems are unlikely to develop such an interaction [24
]. Similarly, where elevated CO2
leads to important soil moisture savings [25
], increases in precipitation might negate any CO2
effect. Temperature and CO2
responses are frequently assumed to be additive, although few ecosystem-scale experiments exist [26
]. No previous studies, to our knowledge, have simultaneously tested responses to enhanced CO2
, warming, increased precipitation, and increased N deposition.
Since 1998, the Jasper Ridge Global Change Experiment (JRGCE) has exposed a moderately fertile grassland to atmospheric and climate conditions expected later this century, and to enhanced nitrate deposition. Because small-statured, annual species dominate California grasslands, this ecosystem is well suited for the study of responses to global changes. Thousands of individual plants can be examined within a small area, and changes in the chemistry of plants and plant litter quickly reach the soil as the plants die. Additionally, the plants complete one generation each year, so competition and selection can “tune” the performance of the grassland to new environmental conditions more quickly than would occur in systems with longer-lived species. While systems dominated by larger, longer-lived organisms might adjust to a step change in CO2 or N deposition over a span of decades, annual grassland can be expected to reach a steady, “representative” response more quickly.
With a wide range of treatments and treatment combinations, the JRGCE provides a foundation for characterizing how ecosystems may perform in the future in a range of possible scenarios. Of particular interest is determining whether ecosystem responses to individual factors are additive. How reliably can we predict ecosystem responses to many concurrent environmental changes based on responses to individual changes? Previously, Shaw et al. [27
] focused on CO2
responses in this grassland and found an unexpected result: elevated CO2
suppressed positive production responses to other global changes during the third year of the JRGCE. Here, we present a comprehensive description of the responses of grassland production to all four global changes over the first 5 y of experimental treatments, and discuss these responses in the context of natural, as well as experimental, climate variation. With this expanded dataset, we are able to put the results from Shaw et al. [27
] in a larger context, and determine whether there have been consistent changes in grassland net primary production (NPP) that could directly affect the amount of C stored in this ecosystem.