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1. The important but weakening maize yield benefit of grain filling prolongation in the US Midwest

   https://doi.org/10.1111/gcb.14356  

   Zhu, Peng ; Jin, Zhenong ; Zhuang, Qianlai ; Ciais, Philippe ; Bernacchi, Carl ; Wang, Xuhui ; Makowski, David ; Lobell, David ( July 2018 , Global Change Biology)

   A better understanding of recent crop yield trends is necessary for improving the yield and maintaining food security. Several possible mechanisms have been investigated recently in order to explain the steady growth in maize yield over theUSCorn‐Belt, but a substantial fraction of the increasing trend remains elusive. In this study, trends in grain filling period (GFP) were identified and their relations with maize yield increase were further analyzed. Using satellite data from 2000 to 2015, an average lengthening ofGFPof 0.37 days per year was found over the region, which probably results from variety renewal. Statistical analysis suggests that longerGFPaccounted for roughly one‐quarter (23%) of the yield increase trend by promoting kernel dry matter accumulation, yet had less yield benefit in hotter counties. Both official survey data and crop model simulations estimated a similar contribution ofGFPtrend to yield. If growing degree days that determines theGFPcontinues to prolong at the current rate for the next 50 years, yield reduction will be lessened with 25% and 18% longerGFPunder Representative Concentration Pathway 2.6 (RCP2.6) andRCP6.0, respectively. However, this level of progress is insufficient to offset yield losses in future climates, because drought and heat stress during theGFPwill become more prevalent and severe. This study highlights the need to devise multiple effective adaptation strategies to withstand the upcoming challenges in food security.

    

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2. Decreasing, not increasing, leaf area will raise crop yields under global atmospheric change

   https://doi.org/10.1111/gcb.13526  

   Srinivasan, Venkatraman ; Kumar, Praveen ; Long, Stephen P. ( November 2016 , Global Change Biology)

   Without new innovations, present rates of increase in yields of food crops globally are inadequate to meet the projected rising food demand for 2050 and beyond. A prevailing response of crops to rising [CO₂] is an increase in leaf area. This is especially marked in soybean, the world's fourth largest food crop in terms of seed production, and the most important vegetable protein source. Is this increase in leaf area beneficial, with respect to increasing yield, or is it detrimental? It is shown from theory and experiment using open‐air whole‐season elevation of atmospheric [CO₂] that it is detrimental not only under future conditions of elevated [CO₂] but also under today's [CO₂]. A mechanistic biophysical and biochemical model of canopy carbon exchange and microclimate (MLCan) was parameterized for a modernUSMidwest soybean cultivar. Model simulations showed that soybean crops grown under current and elevated (550 [ppm]) [CO₂] overinvest in leaves, and this is predicted to decrease productivity and seed yield 8% and 10%, respectively. This prediction was tested in replicated field trials in which a proportion of emerging leaves was removed prior to expansion, so lowering investment in leaves. The experiment was conducted under open‐air conditions for current and future elevated [CO₂] within the Soybean Free Air Concentration Enrichment facility (SoyFACE) in central Illinois. This treatment resulted in a statistically significant 8% yield increase. This is the first direct proof that a modern crop cultivar produces more leaf than is optimal for yield under today's and future [CO₂] and that reducing leaf area would give higher yields. Breeding or bioengineering for lower leaf area could, therefore, contribute very significantly to meeting future demand for staple food crops given that an 8% yield increase across theUSAalone would amount to 6.5 million metric tons annually.

    

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3. Climate change impacts on selected global rangeland ecosystem services

   https://doi.org/10.1111/gcb.13995  

   Boone, Randall B. ; Conant, Richard T. ; Sircely, Jason ; Thornton, Philip K. ; Herrero, Mario ( December 2017 , Global Change Biology)

   Rangelands are Earth's dominant land cover and are important providers of ecosystem services. Reliance on rangelands is projected to grow, thus understanding the sensitivity of rangelands to future climates is essential. We used a new ecosystem model of moderate complexity that allows, for the first time, to quantify global changes expected in rangelands under future climates. The mean global annual net primary production (NPP) may decline by 10 g C m⁻² year⁻¹in 2050 under Representative Concentration Pathway (RCP) 8.5, but herbaceousNPPis projected to increase slightly (i.e., average of 3 g C m⁻² year⁻¹). Responses vary substantially from place‐to‐place, with large increases in annual productivity projected in northern regions (e.g., a 21% increase in productivity in theUSand Canada) and large declines in western Africa (−46% in sub‐Saharan western Africa) and Australia (−17%). Soil organic carbon is projected to increase in Australia (9%), the Middle East (14%), and central Asia (16%) and decline in many African savannas (e.g., −18% in sub‐Saharan western Africa). Livestock are projected to decline 7.5 to 9.6%, an economic loss of from $9.7 to $12.6 billion. Our results suggest that forage production in Africa is sensitive to changes in climate, which will have substantial impacts on the livelihoods of the more than 180 million people who raise livestock on those rangelands. Our approach and the simulation tool presented here offer considerable potential for forecasting future conditions, highlight regions of concern, and support analyses where costs and benefits of adaptations and policies may be quantified. Otherwise, the technical options and policy and enabling environment that are needed to facilitate widespread adaptation may be very difficult to elucidate.

    

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4. Declines in northern forest tree growth following snowpack decline and soil freezing

   https://doi.org/10.1111/gcb.14420  

   Reinmann, Andrew B. ; Susser, Jessica R. ; Demaria, Eleonora M. C. ; Templer, Pamela H. ( November 2018 , Global Change Biology)

   Changes in growing season climate are often the foci of research exploring forest response to climate change. By contrast, little is known about tree growth response to projected declines in winter snowpack and increases in soil freezing in seasonally snow‐covered forest ecosystems, despite extensive documentation of the importance of winter climate in mediating ecological processes. We conducted a 5‐year snow‐removal experiment whereby snow was removed for the first 4–5 weeks of winter in a northern hardwood forest at the Hubbard Brook Experimental Forest in New Hampshire,USA. Our results indicate that adverse impacts of reduced snowpack and increased soil freezing on the physiology ofAcer saccharum(sugar maple), a dominant species across northern temperate forests, are accompanied by a 40 ± 3% reduction in aboveground woody biomass increment, averaged across the 6 years following the start of the experiment. Further, we find no indication of growth recovery 1 year after cessation of the experiment. Based on these findings, we integrate spatial modeling of snowpack depth with forest inventory data to develop a spatially explicit, regional‐scale assessment of the vulnerability of forest aboveground growth to projected declines in snowpack depth and increased soil frost. These analyses indicate that nearly 65% of sugar maple basal area in the northeastern United States resides in areas that typically experience insulating snowpack. However, under theRCP4.5 and 8.5 emissions scenarios, we project a 49%–95% reduction in forest area experiencing insulating snowpack by the year 2099 in the northeastern United States, leaving large areas of northern forest vulnerable to these changes in winter climate, particularly along the northern edge of the region. Our study demonstrates that research focusing on growing season climate alone overestimates the stimulatory effect of warming temperatures on tree and forest growth in seasonally snow‐covered forests.

    

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5. Evidence for divergence of response in Indica , Japonica , and wild rice to high CO 2 × temperature interaction

   https://doi.org/10.1111/gcb.13279  

   Wang, Diane R. ; Bunce, James A. ; Tomecek, Martha B. ; Gealy, David ; McClung, Anna ; McCouch, Susan R. ; Ziska, Lewis H. ( April 2016 , Global Change Biology)

   HighCO₂and high temperature have an antagonistic interaction effect on rice yield potential and present a unique challenge to adapting rice to projected future climates. Understanding how the differences in response to these two abiotic variables are partitioned across rice germplasm accessions may be key to identifying potentially useful sources of resilient alleles for adapting rice to climate change. In this study, we evaluated eleven globally diverse rice accessions under controlled conditions at two carbon dioxide concentrations (400 and 600 ppm) and four temperature environments (29 °C day/21 °C night; 29 °C day/21 °C night with additional heat stress at anthesis; 34 °C day/26 °C night; and 34 °C day/26 °C night with additional heat stress at anthesis) for a suite of traits including five yield components, five growth characteristics, one phenological trait, and four photosynthesis‐related measurements. Multivariate analyses of mean trait data from these eight treatments divide our rice panel into two primary groups consistent with the genetic classification ofINDICA/INDICA‐like andJAPONICApopulations. Overall, we find that the productivity of plants grown under elevated [CO₂] was more sensitive (negative response) to high temperature stress compared with that of plants grown under ambient [CO₂] across this diversity panel. We report differential response toCO₂× temperature interaction forINDICA/INDICA‐like andJAPONICArice accessions and find preliminary evidence for the beneficial introduction of exotic alleles into cultivated rice genomic background. Overall, these results support the idea of using wild or currently unadapted gene pools in rice to enhance breeding efforts to secure future climate change adaptation.

    

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