Forest Investment Account (FIA) - Forest Science Program
FIA Project Y092066

    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
Project lead: Krzic, Maja (University of British Columbia)
Contributing Authors: Zhao, Yihai; Krzic, Maja; Bulmer, Charles E.; Schmidt, Margaret G.; Simard, Suzanne W.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Please note that there are no deviations from the originally approved project.

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 soilís 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 MBD for each bulk density sample collected in the field and this could be done by relating certain properties (e.g., soil organic matter, plastic and liquid limit, particle size distribution, Al- and Fe- oxides, and particle density) of the sample to a model of how MBD varies with those properties. Such an approach would increase the analytical cost associated with each bulk density sample, but would drastically reduce the point to point variation in RBD because the choice of reference bulk density would always be determined by the properties of each individual bulk density sample.

This project aims to advance recent studies of soil productivity on natural and disturbed forest ecosystems in the Interior of BC by evaluating thresholds for RBD in relation to tree growth. Work carried out under FSP funded project Y073250 (Dr. Bulmer) has shown that MBD can be predicted with reasonable success using easily measured soil properties. Liquid limit, particle size distribution, Al- and Fe- oxides, and particle density were strongly correlated to MBD (R2=0.89) (Zhao et al. 2006). Studies carried out in other countries (Howard et al. 1981, Smith et al.1997, Aragon et al. 2000, Ball et al. 2000) have also attempted to predict MBD from easily tested soil properties. In the proposed study, our approach involves performing these laboratory tests on bulk density samples previously collected from 8 sites (see attached Table 1) in the Interior of BC, and supplementing the resulting information with further field evaluations and updated tree growth to enable the prediction of MBD, determination of RBD, and evaluation of relationships between RBD and tree growth. The 8 sites were chosen since they cover a range of ecological and soil physical conditions expected in natural soils in BC, as well as typical conditions resulting from forest harvesting operations. For example, in moist areas of BC, landing construction on medium- and fine-textured soils often exposes subsoil horizons with more clay and less organic matter than the overlying surface horizons. On such sites, the RBD not only could offer improved productivity interpretations, but may also provide information to distinguish effects of induced compaction from those effects that reflect the original character of a particular soil material.

This study addresses the need for more refined indicators and thresholds of sustainability on disturbed and undisturbed forest soils. In many cases, limited compaction is not expected to impact tree growth or ecosystem stability in the short term. Our work will help managers who want to know (1) which ecosystems may be at risk from soil disturbance, (2) whether operations on a particular site are creating soil conditions that approach or exceed thresholds that could be detrimental to tree growth over the long term, and (3) at what sites changing climate or other factors would expose ecosystems to additional risk.
Related projects:  FSP_Y081066


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Updated August 16, 2010 

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