DOI: 10.5061/dryad.8cz8w9gkz
title: Interactive effects of global change factors on terrestrial net primary productivity are treatment length- and intensity-dependent

description: 1. Individual effects of co‐occurring global change factors on net primary
 productivity (NPP) have been widely studied; however, their interactive
 effects remain highly debated. 2. Here, we conducted a global
 meta‐analysis based on 919 multifactor observations from 120 published
 studies to examine the interactive effects on NPP of global change factors
 including elevated [CO2], warming, nitrogen addition, irrigation, drought,
 and changes in species diversity. 3. On average, of the factors studied,
 six pairs of factors had additive and two pairs had synergistic
 interactions. Importantly, some of those interaction types changed over
 time and with treatment intensity. The synergistic interaction between
 elevated [CO2] and nitrogen addition became additive at high nitrogen
 addition rates, whereas the synergistic interaction between irrigation and
 warming diminished at higher temperatures. Over time, the additive effect
 between elevated [CO2] and increased species richness switched to
 synergistic. Other global change factor pairs—including elevated [CO2] and
 warming, nitrogen addition and increased richness, irrigation and N
 addition, as well as drought and increased richness—remained additive
 regardless of their treatment intensity or experimental duration.
 Interaction types of those global change factor pairs did not vary with
 ecosystem types assessed in our study. 4. Synthesis. Our results
 suggest that assumptions of static effects through time or ignoring
 treatment intensity effects will provide inaccurate predictions of the
 interactive effects of global change factors on terrestrial NPP.
 Understanding the context‐dependent nature of interactive effects is
 crucial for validating Earth system models and predicting future NPP
 responses to co‐occurring global change drivers.
description: We searched peer-reviewed journal articles that evaluated the
 response of plant growth or ecosystem NPP under experimental manipulations
 of multiple global change factors using the Web of Science and Google
 Scholar up to 1<sup>st</sup> August 2018. Different keyword
 combinations were used for the search, such as “(climate change OR
 elevated [CO<sub>2</sub>] OR warming OR temperature OR
 nitrogen addition OR fertilization OR irrigation OR precipitation
 manipulation OR drought OR diversity OR richness OR mixture OR interaction
 effect) AND (plant growth OR production OR productivity OR biomass
 production)”. Only primary studies that satisfied the following criteria
 were included in this meta-analysis: 1) at least a 2 × 2 full-factorial
 design was used to examine the effects of global change factors including
 elevated [CO<sub>2</sub>], warming, N deposition, irrigation,
 and/or drought, and change in species diversity; 2) at least one component
 of production (e.g., aboveground, belowground, or total) was examined
 under all treatments and the control at the same temporal and spatial
 scales; 3) the plots used for all treatments were of the same ecosystem
 type and environmental condition as the control; and 4) the mean and
 sample size of the selected variables were available or could be
 calculated from data provided in the publications. All original data were
 extracted from the text, tables, figures, and appendices of the
 publications. When data were graphically presented, SigmaScanPro version 5
 (Systat Software Inc., Point Richmond, CA, USA) was used to obtain numeric
 data. Furthermore, data regarding experimental duration (years), treatment
 intensity (i.e., increased or decreased quantities applied in the
 treatment as compared with the control), ecosystem type (forest,
 grassland, cropland, tundra, wetland), production component (aboveground,
 belowground, or total), and growth form (woody or herbaceous), were also
 recorded from detailed descriptions in the Materials and Methods sections
 of papers included in the analysis.