Empirical geographic modeling of switchgrass yields in the United States

Switchgrass (Panicum virgatum L.) is a perennial grass native to the United States that has been studied as a sustainable source of biomass fuel. Although many field‐scale studies have examined the potential of this grass as a bioenergy crop, these studies have not been integrated. In this study, we present an empirical model for switchgrass yield and use this model to predict yield for the conterminous United States. We added environmental covariates to assembled yield data from field trials based on geographic location. We developed empirical models based on these data. The resulting empirical models, which account for spatial autocorrelation in the field data, provide the ability to estimate yield from factors associated with climate, soils, and management for both lowland and upland varieties of switchgrass. Yields of both ecotypes showed quadratic responses to temperature, increased with precipitation and minimum winter temperature, and decreased with stand age. Only the upland ecotype showed a positive response to our index of soil wetness and only the lowland ecotype showed a positive response to fertilizer. We view this empirical modeling effort, not as an alternative to mechanistic plant‐growth modeling, but rather as a first step in the process of functional validation that will compare patterns produced by the models with those found in data. For the upland variety, the correlation between measured yields and yields predicted by empirical models was 0.62 for the training subset and 0.58 for the test subset. For the lowland variety, the correlation was 0.46 for the training subset and 0.19 for the test subset. Because considerable variation in yield remains unexplained, it will be important in the future to characterize spatial and local sources of uncertainty associated with empirical yield estimates.     

半干旱黄土丘陵沟壑区柳枝稷（Panicum virgatum）的生物质形成

为深入认识半干旱黄土丘陵沟壑区引种能源植物柳枝稷生物质生产的开发潜力及其约束机制,调查了农田、植丛尺度上早熟和晚熟柳枝稷年度生命周期内生物量累积、分株建成动态,以及土壤水分供求平衡过程。研究发现,植丛尺度早熟柳枝稷抽穗比例近100％,分株生殖发生大小阈值依赖基本丧失,高度大小分布近似正态,种群内光资源竞争强度明显弱化,与晚熟类型形成明显差异。农田尺度晚熟柳枝稷生物质产量可以达到15t／hm^2,高出早熟类型近1倍,但是其立地80～400cm土层的含水量稳定在7％以下,土壤干旱已经发生,早期干旱胁迫导致的生长停滞,以及生长中后期的成片倒伏现象在两年的观测周期内连续出现。早熟柳枝稷立地则形成相对稳定的白草、柳枝稷复合优势植被结构,深层土壤水分含量稳定在10％以上,实现了跨年度的土壤水分供求平衡。植丛尺度的生物质形成在一定程度上取决于分株生殖发生的大小依赖程度和分株间竞争关系格局,基于植丛尺度普遍的生殖发生和明显弱化的光资源竞争,早熟柳枝稷表现出更为高效的生物质形成机制。农田尺度晚熟柳枝稷尽管在雨热同步期的降水资源利用效率上存在明显比较优势,但是在降水资源利用分配策略和效应上,早熟柳枝稷表现出了综合的生态适宜性优势。保证雨热同步期降水资源利用和保蓄的平衡,是半干旱黄土丘陵沟壑区生物质生产应该遵循的基本原则之一。     

Does greater leaf‐level photosynthesis explain the larger solar energy conversion efficiency of Miscanthus relative to switchgrass?

C₄ perennial grasses are being considered for bioenergy because of their high productivity and low inputs. In side-by-side replicated trials, Miscanthus ( Miscanthus x giganteus ) has previously been found more than twice as productive as switchgrass ( Panicum virgatum ). The hypothesis that this difference is attributable to higher leaf photosynthetic rates was tested on established plots of switchgrass and Miscanthus in central Illinois with >3300 individual measurements on 20 dates across the 2005 and 2006 growing seasons. Seasonally integrated leaf-level photosynthesis was 33% higher in Miscanthus than switchgrass ( P  < 0.0001). This increase in carbon assimilation comes at the expense of additional transpiration since stomatal conductance was on average 25% higher in Miscanthus ( P  < 0.0001). Whole-chain electron transport rate, measured simultaneously by modulated chlorophyll fluorescence, was similarly 23% higher in Miscanthus ( P  < 0.0001). Efficiencies of light energy transduction into whole chain photosynthetic electron transport, leaf nitrogen use and leaf water use were all significantly higher in Miscanthus. These may all contribute to its higher photosynthetic rates, and in turn, productivity. Systematic measurement of photosynthesis over two complete growing seasons in the field provides a unique dataset explaining why the productivity of these two species differs and for validating mechanistic production models for these emerging bioenergy crops.     

黄土丘陵半干旱区引种禾草柳枝稷的生物量与水分利用效率

加强优良引种禾草植物的生态适应性研究对促进我国黄土高原半干旱区草地建设和草业科学发展具有十分重要的意义。比较研究了引种禾草柳枝稷(Panicumvirgatum)在黄土丘陵半干旱区不同立地条件下地上生物量的大小和季节累积差异及其水分利用特征。2001~2002年川地柳枝稷草地地上生物量达13000~16000kg/hm2,山地梯田和坡地为2300~2650kg/hm2。不同立地条件柳枝稷返青后的生物量累积过程呈二次或三次多项式。柳枝稷的绝对生长速率(AGR)在整个生长季内呈双峰曲线变化,川地柳枝稷草地的现存量和总量AGR最大值分别为158.93和169.83kg/(hm2·d),梯田分别为27.31和38.25kg/(hm2·d),坡地为37.0和36.69kg/(hm2·d)。坡地柳枝稷生物量较大值和AGR最大值出现时间最早。不同立地柳枝稷生物量相对生长速率(RGR)在整个生长季内呈双峰曲线变化,均以返青后的20d内最大,平均AGR以川地最大,坡地和梯田相近,但坡地两峰值高于梯田。不同立地柳枝稷草地土壤水分主要利用层次为0~2m,月平均含水量顺序为梯田>坡地>川地。川地柳枝稷叶片和整体生物量水分利用效率均最高,梯田整体生物量水分利用效率大于坡地,但二者叶片水分利用效率相近。川地和山地地表下5cm生育期平均地温均为17.60℃,但4~5月份川地地温高出山地1.2~2.8℃,川地4~10月份平均气温较山地高1.5℃,这些差异影响不同立地条件柳枝稷草地水分利用和生长进程。     

An integrated biogeochemical and economic analysis of bioenergy crops in the Midwestern United States

This study integrates a biophysical model with a county‐specific economic analysis of breakeven prices of bioenergy crop production to assess the biophysical and economic potential of biofuel production in the Midwestern United States. The bioenergy crops considered in this study include a genotype of Miscanthus, Miscanthus×giganteus, and the Cave‐in‐Rock breed of switchgrass (Panicum virgatum). The estimated average peak biomass yield for miscanthus in the Midwestern states ranges between 7 and 48 metric tons dry matter per hectare per year ( t DM ha^?1 yr^?1), while that for switchgrass is between 10 and 16 t DM ha^?1 yr^?1. With the exception of Minnesota and Wisconsin, where miscanthus yields are likely to be low due to cold soil temperatures, the yield of miscanthus is on average more than two times higher than yield of switchgrass. We find that the breakeven price, which includes the cost of producing the crop and the opportunity cost of land, of producing miscanthus ranges from $53 t^?1 DM in Missouri to $153 t^?1 DM in Minnesota in the low‐cost scenario. Corresponding costs for switchgrass are $88 t^?1 DM in Missouri to $144 t^?1 DM in Minnesota. In the high‐cost scenario, the lowest cost for miscanthus is $85 t^?1 DM and for switchgrass is $118 t^?1 DM, both in Missouri. These two scenarios differ in their assumptions about ease of establishing the perennial crops, nutrient requirements and harvesting costs and losses. The differences in the breakeven prices across states and across crops are mainly driven by bioenergy and row crop yields per hectare. Our results suggest that while high yields per unit of land of bioenergy crops are critical for the competitiveness of bioenergy feedstocks, the yields of the row crops they seek to displace are also an important consideration. Even high yielding crops, such as miscanthus, are likely to be economically attractive only in some locations in the Midwest given the high yields of corn and soybean in the region.