By Karen Hills
In non-irrigated areas that are too dry to support annual cropping, fallow (the practice of leaving land unplanted) preserves soil moisture for future crops. However, annual fallow combined with conventional tillage has resulted in a net decrease in soil carbon over time in our region, with negative impacts to soil health across large areas. And even when tillage is eliminated, it is very difficult to maintain soil carbon over time in a wheat-fallow system. For this reason, the impact of climate change on the frequency of fallow in crop rotations has important future implications both for soil health and for opportunities for carbon sequestration.
Two papers published last year by Kaur et al. and Karimi et al. use modeling to project the impacts of climate change on dryland cropping systems. Of specific interest in both studies was the anticipated effect of climate change on the frequency of fallow in crop rotations in the inland Pacific Northwest. Reading these papers revealed interesting, yet divergent, findings that seemed worth sharing here.
In the first paper, Kaur and colleagues use modeling to predict shifts in agroecological classes (AECs) in dryland areas in response to climate change scenarios. Agroecological classes are defined based on actual land use/cover from 2007 and 2014 and consist of: (1) annual crop (<10% fallow); (2) annual crop-fallow-transition (10—40% fallow); and (3) grain fallow (>40% fallow) (Huggins et al. 2014). Dynamic AECs are defined as those areas that changed classes. The AEC of a particular area, thus, indicates the frequency of fallow for crop rotations in that area.
Kaur and colleagues determined the most important bioclimatic variables driving the current distribution of dryland AECs. Then, they projected how current AECs would shift if they imposed future climate change projections (and associated changes in these bioclimatic variables) on top of today’s technological, environmental, and cropping system situations. Projections were made for the years 2030, 2050, and 2070 (Figure 1). Their results suggest an increase in the amount of cropland in dynamic AECs and fallow-based AECs. An increase in fallow would have negative implications for soil health, increasing the potential for erosion and decreasing soil organic matter levels and biological activity.

Figure 1. Projected geo-spatial distribution of three agroecological classes (AECs) using three individual Random Forest models of bioclimatic variables under future climate scenario (2070, representative concentration pathway [RCP] 8.5). (source: Kaur et al. 2017)

Figure 2. Percent of grid cells in the Inland Pacific Northwest assigned to a given agroecological class (crop/fallow [CF], annual crop/fallow transition [CCF], and continuous cropping [CC]) for historical baseline (1979-2010) and future periods (2030s, 2050s and 2070s) under representative concentration pathway (RCP) 8.5. Results are the average of 12 global climate models. (modified from Karimi et al. 2017; results for RCP 4.5 shown in Karimi et al. 2017)
These two papers, published in the same journal issue, remind us of the importance of considering multiple studies on any given topic. It is also important not to underestimate the complexity of the effort involved here, in going beyond forecasting growth for one crop, to attempting to forecast cropping patterns across the landscape. Much uncertainty remains regarding the anticipated effects of climate change on agricultural systems in our region and there is a need for ongoing rigorous, peer-reviewed studies. As shown by the papers by Kaur, Karimi and their teams, two such studies can result in vastly different predictions. While I hope that the Karimi prediction is more accurate with its rosier outlook for farmers, soil health, and carbon sequestration potential, I am reminded of some sage advice for approaching uncertain situations: “Hope for the best but prepare for the worst.” And, importantly, continue to follow the research results.
Stay tuned for a future post exploring a related paper by Jason Morrow on the topic of the impacts of climate change on soil organic matter in cropping systems in the inland Pacific Northwest.
References
Huggins, D. R., R. Rupp, H. Kaur, and S. Eigenbrode. 2014. Defining agroecological classes for assessing land use dynamics, in Regional Approaches to Climate Change for Pacific Northwest Agriculture, eds. K. Borrelli, D. Daley-Laursen, S. Eigenbrode, B. Mahler, and B. Stokes (Moscow: University of Idaho), 4–7.
Karimi, T., C.O. Stockle, S.S. Higgins, R.L. Nelson, and D. Huggins. 2017. Projected Dryland Cropping System Shifts in the Pacific Northwest in Response to Climate Change. Frontiers in Ecology and Evolution, 5: 20. http://journal.frontiersin.org/article/10.3389/fevo.2017.00020/abstract
Kaur, H., D.R. Huggins, R.A. Rupp, J.T. Abatzoglou, C.O. Stockle, and J.P. Reganold. 2017. Agro-ecological Class Stability Decreases in Response to Climate Change Projections for the Pacific Northwest, USA. Frontiers in Ecology and Evolution 5: 74. https://doi.org/10.3389/fevo.2017.00074
This article is also posted on the CSANR Perspectives on Sustainability blog.