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Cost-effective indicators of soil physical condition: natural variation in the relative bulk density and associated tree growth as measures of forest productivity and ecosystem resilience
Zhao, Yihai
Healthy soils are an essential part of productive forest ecosystems, and many of the processes underpinning ecosystem stability and resilience are located below ground. Reliable methods are needed to determine the natural range of soil conditions in healthy ecosystems and the likely range that would be conducive to tree growth. By applying such methods, managers would be in a better position to mitigate any negative effects of forest practices on current and future productivity. A common consequence of modern forest management is disturbance by machine traffic on harvested areas, and the effects on soil physical conditions are often obvious. Soil physical characteristics strongly affect ecosystem stability in the extensively managed forests of British Columbia (BC): very few plants can thrive in soils with severely degraded pore structures. Despite this, there are also numerous examples throughout BC of soils that have experienced considerable disturbance but still maintain a healthy pore structure and plants of all types are able to grow. Recent advances in characterizing and interpreting soil physical conditions have provided land managers with an opportunity to evaluate ecosystem sustainability in a cost-effective way. In particular, the concept of relative bulk density (RBD) could significantly improve interpretations of soil physical condition compared to traditional methods (Carter 1990, Lipiec et al. 1991, Topp et al.1997). Bulk density is the most common measure of a soil?s compaction state, but is strongly influenced by soil texture, organic matter content, and other soil properties. It has been difficult to determine thresholds for ecosystems that are at risk based on bulk density measurements alone due to wide variations of soil texture and organic matter that occur naturally. The RBD has the potential to be a more reliable indicator, requiring fewer samples to achieve similar levels of precision, and having the advantage that threshold values may be more easily determined. Evaluating the RBD may eliminate the need for determination of a unique growth-limiting threshold for every soil with different organic matter contents and texture. Relative bulk density is determined as the ratio of field bulk density and a site-specific reference bulk density, usually referred to as maximum bulk density-MBD (Stengel et al. 1984). The reference bulk density can be determined experimentally using the Proctor compaction test (ASTM 2000), or it could be predicted based on the set of soil properties. In either case, the RBD is a measure of compaction state with values in nature ranging from about 0.65 to 0.95 as a proportion of the (theoretical) maximum density. For any soil/site, values of RBD less than approximately 0.80 are expected to be consistent with productive forest growth and ecosystem stability. Values higher than 0.80 have been associated with limiting values for water availability, soil mechanical resistance, and air-filled porosity (da Silva et al. 1994, Bulmer and Simpson 2005) and signal degraded soil physical conditions where plants may grow poorly, or the ecosystem may be more vulnerable to external factors like climate change or the introduction of stressors such as disease or insects (Hakansson and Lipiec 2000, Blouin et al. 2005). Although RBD has the potential to improve evaluations of soil productivity for disturbed sites, reliably determining RBD is challenging. One of the greatest challenges is determining the appropriate MBD to use as the reference density, even though the ASTM Proctor method is reliable and provides repeatable results for a given sample. The challenge arises because of high natural soil variability in properties like organic mater content and texture, which strongly influence MBD. Because of this variability, one could envision requiring a very large number of Proctor samples to capture site variation, and this would be impractical. A better approach would be to reliably calculate MB ...
Report Number
Executive Summary
Sustainable Soil Management Course Description
Soil Properties Poster
Development of Assessment Method for Forest Soil Compaction
Soil Compaction and Tree Growth Abstract

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