Forest Productivity as a Function of Root Growth in Compacted Forest Soils

1. 1 3 FS LTSP – VT Research Plots 5 7 6 5 7 4 FS LTSP 3 2 1 Industry & Academia OM0 OM1 OM2 • Forest Productivity as a Function of Root Growth in Compacted Forest Soils • C.M. Siegel-Issem1, J.A. Burger1, D.A. Scott2, F. Ponder3,R.F. Powers4, and S.C. Patterson5 • 1Forestry Dept., Virginia Tech; 2USFS Southern Res. Stn., LA, 3USFS North Central Res. Stn., MO, 4USFS Pacific Southwest Res. Stn.,CA, 5MeadWestvaco Hypothesis Methods Results Phase II-Objective 1. Model Validation (On-going) • Field-validate root growth models derived under controlled greenhouse conditions in Phase I of this research and determine the extent to which these models predict tree growth by comparing model predictions with measured seasonal net tree productivity. • Phase I. Model Development • (Completed 2002) • Optimum water content for compaction, maximum bulk density, soil texture and OM determined. Each soil had unique compaction characteristics although they had similar textures. There was no single soil property that best predicted soil compactibility. • Soil and species specific models developed from a greenhouse experiment showed that root growth response to VW and BD was significant and soil- and species specific. Productivity • General Procedure • Soil bulk density and strength were measured on each plot. Continuous soil water content was measured during one growing season. Tree net productivity for one growing season was measured by sequential root corings, diameter growth and litterfall. • Root length density (RLD) models will be developed from collected field data (Fig. 5) and will be used to determine soil sufficiency for root growth (SSRG). • VW and BD associated with the greatest RLD will be defined as a sufficiency index (SI) of 1.00. • Seasonal SSRG will be determined by summing the intervals where daily sufficiency is determined as a function of in situ daily soil VW and soil BD (Fig. 6). • Predicted SSRG will be regressed with measured seasonal root production, tree growth and NPP (Fig. 7). Bulk Density level Moisture level Figure 1. Hypothesized model showing productivity as a function of soil moisture level and bulk density where growth is best at lower bulk densities and moderate water contents. Growth becomes limited by soil strength in high density dry soils, and limited by inadequate aeration in very wet soils. Figure 7. Hypothetical presentation of root production as a function of predicted SSRG, illustrating that this predicted relationship is significant and soil and species specific C0 C1 C2 Root Length Density Models 0.2 ha veg. control 160 trees Objectives Phase II- Objective 2. Soil Compactibility Characterization (On-going) United States Forest Service Long-Term Soil Productivity Study Nationwide Field Research Plots • Examine the key soil properties determining soil compactibility such as OM, clay mineralogy, texture, plasticity and chemistry. Figure 3. LTSP Site Network and study design. Each site has 9, 0.4ha plots with a factorial arrangement of three levels of organic matter removal (bole only to whole tree +forest floor) and 3 levels of compaction (none to severe) with a competing vegetation control subplot. We worked on the OM2, C0, C1 and C2 plots at the MS, MO, CA and SC LTSP sites. Previous Research Greenhouse Study with Representative LTSP soils: 7x7 factorial; seedlings grown in columns with 7 levels of bulk density (BD) and volumetric water (VW) Model Development: Root Length Density = bo +b1BD + b2VW2 Study Outcomes • Further our understanding of fundamental soil processes affecting productivity across several soil and forest types • Further our understanding of those soil properties most related to soil compactibility and how this information can be used for predictive purposes. • Incorporate the validated models and soil compactibility assessment as management guideline tools Model validation: Measure in situ: Bulk density, soil strength, volumetric water content, net primary productivity measured over a growing season Soil Physical Characterization for Compactibility Assessment: Texture, clay mineralogy, particle shape, shear strength, plasticity index, proctor limits, OM measured Objective 1 Objective 2 Acknowledgements Assessment of validated models, soil compaction properties and growth and climate data Objective 3 This research supported by: The United States Forest Service Research Stations (Southern, Northcentral and Pacific Southwest), MeadWestvaco and Agenda 2020 Contact: Cristina Siegel-Issem csiegel@vt.edu Figure 5. Root length density of shortleaf pine seedlings on Clarksville soil, loblolly pine seedlings on Argent soil and ponderosa pine seedlings on Dome and Cohasset soils, as a function of soil water content and bulk density. Figure 6. Hypothetical presentation of soil sufficiency for tree growth based on soil moisture and density conditions determined across a growing season for a soil at three different compaction levels. Input to USFS for determining appropriate management guidelines Figure 4. Soil strength being measured on the OM2 C0 plot at the Mississippi LTSP site. Figure 2. Diagram of research objectives and activities.