2006-2011 Mission Kearney Foundation of Soil Science: Understanding and Managing Soil-Ecosystem

Functions Across Spatial and Temporal Scales Progress Report: 2007031, 1/1/2008-12/31/2008

1University of California, Irvine *Principal Investigator

Does Thinning Improve the Soil Water Balance and Vegetation Health of Southern Californian Forest? Michael L. Goulden*1, Aaron W. Fellows1

Project Objectives High levels of tree mortality occurred in Southern California forests in 2002-03 (Minnich 2007). This study uses two objectives (obj) to understand this mortality. Obj. 1 is to characterize the spatial patterning of dead trees and determine the role that plant and soil water availability played in structuring mortality patterns at multiple scales. Obj. 2 is to understand the physiological mechanisms driving this mortality and the importance of stand thinning on improving soil-water availability at the tree level. We have made significant progress on obj. 1 and have begun work on obj. 2 (see table 1).

Approach and Procedures Fourteen 300m long belt transects spaced 400 feet in elevation spanned an elevation gradient along the north-west aspect of the San Jacinto mountains (fig 1a; transects in red). Cover was recorded along each transect by species for live and dead vegetation. Nearly all dead-trees from the 2002-03 mortality event remained standing and retained branches down to ~.25 inches at the time of the survey. We use these standing dead trees to quantify mortality (the cover that is dead). Diameter at Breast Height (DBH) was recorded in 0.1 ha plots located at each transect and seedlings were counted in the plots. Cloud-free, summer Landsat images from 1984 to 2006 were geometrically and radiometrically corrected for the region.

Ten separate vegetation indices constructed from Landsat imagery were compared with measured forest mortality. The differences in Normalized Burn Ratio (dNBR) between 2000 and 2004 images (ones that bracketed the 2002-03 mortality event) performed best by explaining ~63% of the measured mortality. We used dNBR to construct a regional mortality map for 2002-03 (fig. 1b; red=high mortality). Twenty-four additional 60m belt transects located without regard to elevation or aspect in high and low mortality forests assured us that our calibrations effectively documented the spatial pattern and relative magnitude of mortality over the San Jacinto Mountains. We then used these mortality maps to determine how tree death varied as a function of aspect, elevation, curvature of the ground surface, cover before mortality, and species present. Measurements of plant-water status, including pre-dawn water potential (soil-water potential), mid-day water potential, and leaf gas exchange were made during October and November 2008, and January 2009.

Results Obj. 1: Mortality differed between species. Mortality was greatest in mid-elevation conifer species (Abies concolor (abco), Pinus jeffreyi (pije), Pinus ponderosa (pipo), Pinus coulteri (pico), Pinus lambertiana (pila), Calocedrus decurrens (cade)). White fir showed the highest

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mortality. Oak species (Q. chrysolepis, Q. wislizeni, Q. kelloggii) showed low mortality. Mortality was greatest for mid-elevation conifers species at the lower elevations of their range (fig. 2). Forest that underwent high mortality entered 2002 with a higher fraction cover than nearby forest that showed low mortality (fig. 3). Mortality derived from remote sensing was greatest between 7,000 feet and 8,000 feet in elevation (data not shown). This corresponds with peak conifer cover measured by belt transect. The highest mortality occurred on north and north–west aspects. High mortality occurred most frequently on upwardly concave surfaces. Regional analysis of soil type and structure using the soil survey geographic database SSURGO (http://www.ncgc.nrcs.usda.gov/products/datasets/ssurgo/) are being investigated. Obj. 2: There is a strong correlation between predawn water potential (Ψpd; a measure of soil water potential) and carbon assimilation rates (A) for mid-elevation conifers during late summer (fig. 4). Trees that showed lower predawn water potentials showed lower carbon assimilation. Mid-elevation conifers show similar Ψpd to oaks but very low A (fig. 5).

Discussion Differential mortality across both space and species type has resulted in a shift in species distributions and forest structure. Mortality at low elevations may have retracted species ranges upslope. High tree mortality may reduce live basal area and carbon storage in live biomass. High leaf cover may predispose these forests to mortality (fig. 3). Taken together, initial analysis of observed trends suggests that plant-water availability is important in structuring patterns of forest mortality in these forests. Furthermore, the strong control of Ψpd on A implicates drought as a driver of mortality (fig. 4).

Completed and future work address’s the Kearney mission by investigating the role of soil water balance on forest health and the impacts of management on California’s forests. Also addressing the Kearney mission is the use of multiple techniques to focus on these questions at multiple scales.

Future Direction Obj. 1: Regional mortality patterns will be analyzed more extensively by including patterns of springtime snow presence and estimates of soil water availability. Obj. 2: Species-level strategies for coping with seasonal drought will be investigated with ongoing physiological measurements. Future xylem cavitation susceptibility curves and sap exudation pressure measurements will be measured to inform possible mechanisms of drought-driven tree mortality.

Future δ13C isotope measurements, a proxy for intrinsic Water Use Efficiency (A/g; McCarroll and Loader 2004), will be taken from trees in thinned and unthinned stands. Carbon will be taken from tree cores or leaves at the end of the 2009 growing season. These δ 13C measurements will be used to test the importance of thinning on plant water availability and how this impacts forest health. Potential field sites have been established.

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Table 1: Components of Research. Remote sensing (Remote sens.), field surveys, and physiological measurements (phys.) are being used to address objective 1 using three different spatial domains (large to small). Physiological measurements and Thinning will be used to address objective 2. Year 2007 2008 2009 2010

Field surveys (Obj 1) X X Remote sens. (Obj 1) X X Phys. (Obj 1 and 2) X X Thinning/ C13 (Obj 2) X X Analysis X X X

Figure 1: San Jacinto Mountains a)belt transects (red) b) forest mortality (red=high).

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Figure 2: Live and dead cover for six mid-elevation conifer species (see text) centered at the species’ mean elevation.

Figure 3: Cover from 1984 to 2006 for high and low mortality regions (~40 landsat pixels in each grouping).

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Figure 4: Carbon assimilation (A; micromol CO2/m^2/s^1) vs. predawn water (Mpa).

Figure 5: Predawn water potential (Mpa) and Carbon assimilation (A; micromol CO2/m^2/s^1) by species (see text for codes); error bars are 95% CI.

References McCarroll D. and N.J. Loader. 2004. Stable isotopes in tree rings. Quarternary Science Reviews.

23: 771-801. Minnich R.A. 2007. Chapter 18: Southern California conifer forests. In: Terrestrial vegetation

of California 3rd edition. (eds) Barbour M.G., Keeler-Wolf T., and A. S. Schoenherr pp:502-538. University of California Press.

This research was funded by the Kearney Foundation of Soil Science: Understanding and Managing Soil-Ecosystem Functions Across Spatial and Temporal Scales, 2006-2011 Mission (http://kearney.ucdavis.edu). The Kearney Foundation is an endowed research program created to encourage and support research in the fields of soil, plant nutrition, and water science within the Division of Agriculture and Natural Resources of the University of California.