Tree Regeneration in Response to Prescribed Fire, Thinning, and Microsite Conditions

1. BC3 BS3 BC2 US3 UN3 US2 UC3 US1 UC2 UC1 UN2 Cabin Garage BN3 BC1 BN2 BS2 UN1 BS1 BN1 a c d b Tree Regeneration in Response to Prescribed Fire, Thinning, and Microsite Conditions in a Sierran Mixed Conifer Forest Harold Zald (541-750-7299, hzald@fs.fed.us, PNW Research Station, 3200 SW Jefferson Way, Corvallis, OR 97333), Andrew Gray (PNW Research Station, Corvallis, OR), and Malcolm North (Sierra Nevada Research Center, Davis, CA) Introduction Results: Seedling Abundance and Treatment Combinations in Relation to Microsite Environment Conditions Methods: Seedling and Sapling Data Collection The Healthy Forests Initiative and the Sierra Nevada Ecosystem Project have proposed using prescribed fire and thinning to restore pre-settlement composition and structure and reduce the risk of catastrophic wildfire in mixed conifer forests of the Sierra Nevada Range, California. Rrelatively little is known about how alternative management approaches will impact mixed conifer forest ecosystems. The Teakettle Ecosystem Experiment was initiated to examine the multiple ecological effects of prescribed fire and thinning treatments in a mixed conifer forest of the Southern Sierra Nevada Range. Sapling mortality and seedling regeneration are important processes that can drive future stand structure, overstory composition, and susceptibility to wildfire (via fuel loading and ladder fuels). Highly clustered patterns of trees that are not based on tree age cohorts or shade tolerance gradients suggest that within stand microsite conditions will influence forest regeneration, in addition to stand level treatments. The primary objectives of this study were to: • Describe the effects of prescribed fire and thinning treatments on seeding and sapling mortality and regeneration. • Examine the role of microsite environmental gradients in determining species composition in the regeneration pool. • Observe how environmental variable important to seedling composition vary in response to treatments at the stand scale. • Determine the role of treatments and patchy vegetation types in tree germinant survivorship. At each grid point in the intensive and replicate plots, seedlings and saplings (trees with diameter at breast height (DBH) < 5cm, and height > 5cm) were tallied within 3.5 m radius (38 m2) plots centered on each grid point (N=402). Each plot was divided into 4 quadrants. The total number of individuals by size class was recorded for each tree species in each quadrant. The size classes were: Seedlings (5-50 cm in height). Saplings (greater than 50 cm in height, and less than 5 cm DBH). • Nonmetric Multidimensional Scaling (NMS) ordination • suggests species occupy different microsites that are • based on a handful of environmental gradients. • The joint plot overlay indicates the light levels • (TSF and DSF), soil moisture (H2O_03 and H2O_04), • and shrub cover (SHRUB and CECO) are the most • important environmental factors in determining • regeneration composition and abundance. • Many of the species are arranged in the ordination on a • gradient from high soil moisture, low light levels, and low • shrub cover to low moisture, high light levels, and higher • shrub cover. • White fir and incense-cedar (ABCO and CADE) occupied • moist, relatively shaded sites. Sugar pine (PILA) occupied • slightly drier and more open sites, while Jeffrey pine and • bitter cherry (PIJE and PREM) occupied progressively • drier and more open sites. • Sites with no tree seedling (NOSPP) were drier, more • open, and had higher shrub cover than sites with • seedlings. Methods: Seed Sowing Manipulative Experiment A seed-sowing experiment was used to determine the influence of treatments combinations and dominant vegetation types on tree germination and survival. Within each of the six treatment combinations, 9 grid points (54 total grid points) were chosen representing the three dominant vegetation types (closed tree canopy, Ceanothus cordulatus, and bare ground). At each grid point two 0.49 m2 exclosures were constructed from 19 gauge hardware cloth with a 1.25 cm mesh size. In the fall of 2002 seeds from the five dominant overstory species were sown in numbers to equalize the number of viable seed per species. Total germinants and germinant mortality were recorded multiple times during the growing season (June-September) in 2003 and 2004. Methods: Microsite Environmental Variables The solar radiation above each grid point was estimated using digital hemispherical images taken during September 2003. Estimated radiation variables were indirect site factor (ISF), direct site factor (DSF), and total site factor (TSF), or the indirect, direct, and total radiation compared to an open site at the same latitude. Volumetric soil water content in the top 15 cm of soil was measured multiple times during the growing seasons of 2002, 2003, and 2004 using calibrated time domain reflectometry (TDR, model 1502C, Tektronix Inc., Beaverton, OR). Percent cover of shrubs, coarse woody debris by size class, mineral soil, litter cover, litter depth, and rock were estimated within a 10 m2 micro plot centered on each grid point. • Thinning heavily effected • solar radiation levels • Thinning also influenced • overall and species specific • levels of shrub cover • Moisture levels appear to be • lower in burned versus • unburned treatments, • burned/shelterwood • treatment combinations tended • to have low soil moisture relative to other treatment combinations • Results suggest that solar radiation can be successful altered by silvicultural treatments, but soil moisture • is highly variable within treatment combinations, with no strong patterns present Methods: Study Area Location and Description The study conducted at the Teakettle Experimental Forest. Data collected during the summers of 2000, 2002, 2003, and 2004. Teakettle is located 80 km east of Fresno, California in the north drainage of the Kings River (Sierra National Forest, Kings River Ranger District). Teakettle consisted of 1300 ha of old-growth mixed-conifer and red fir (Abies magnifica) forest. • Elevation range: 1980-2590 m • Annual precipitation: 110 cm, primarily snow, from November to April • Mean, maximum and minimum July temperatures are 17C, 30C, and 3C. Teakettle grades from a mix of white fir (Abies concolor), sugar pine (Pinus lambertiana), incense-cedar (Calocedrus decurrens), and Jeffrey pine (Pinus jeffreyi) at the lower elevations to red fir, lodgepole pine (Pinus contorta) and western white pine (Pinus monticola) at higher elevations. Soils are generally Xerumbrepts and Xeropsamments typical of the southwestern slopes of the Sierra Nevada. Results: Pretreatment Seedling and Sapling Composition and Abundance • Firs and incense-cedar dominate both the seedling and sapling components of the regeneration pool. • Extremely low Jeffrey pine component in the regeneration pool, with scattered sugar pine. Results: Effects of Treatment Combinations and Vegetation Types on Germinant Survivorship Methods: Study Design • • 2 by 3 factorial design of treatments, thinning occurred during 2000 and 2001, burning • occurred during the fall of 2001 • • For each treatment: one intensive plot containing 49 grid points, and 2 replicate plots • containing 9 grid points each. • 4 ha plots for each treatment combination, 18 total plots, 72 total ha. Results: Changes in Sapling and Seedling Abundance in Relation to Treatments • Frequency of white fir seedlings • increased across all treatment • combinations. • White fir sapling densities declined • dramatically in shelterwood thinnings, • moderately in understory thinnings, • and increased slightly due to growth • of seedlings in the unthinned • treatments. UN UC US BN BC BS BARE CLOSED_CNPY CECO Vegetation Type Treatment Combination • Germinant survivorship varies by species, treatment combination, and vegetation type • Jeffrey pine and sugar pine overall higher survivorship than the firs or incense-cedar • Both pine species had higher germinant survivorship in burned and shelterwood treatments • All species had low survivorship on bare sites Sacramento UN UC US BN BC BS UN UC US BN BC BS Fresno • Frequency of incense-cedar • seedlings increased across most • treatment combinations. • The shelterwood treatments had • essentially unchanged seedling • frequency after three years compared • to pretreatment levels (2001). • Incense-cedar sapling densities • declined dramatically in • burned/shelterwood treatments, • and returned to approximately • pretreatment levels in all other • treatment combinations. Conclusions • Immediate short term reductions in ladder fuels (fir and incense-cedar saplings) best accomplished by the • burned/shelterwood treatment combination. • The combination of burning and shelterwood silvicultural practices may promote the abundance of • pine seedlings, but the two species appear to respond very differently to a range of burning and thinning • activites. • Shelterwood treatments resulted in the lowest abundance of incense-cedar, but white fir appears to • regenerate well in all treatment combinations. Multiple entries over time maybe required in these forests if • reductions in shade tolerant species is a management objective. • Analysis of environmental conditions suggests light levels, soil moisture, and shrub cover are most • important in determining regeneration composition and abundance. • Most species fit along a gradient from high soil moisture and low light, to dry and open sites. • Thinning treatments resulted in well defined changes in solar radiation, but soil moisture did not display well • defined patterns based on treatment combinations. • Considerable variability exists in soil moisture and shrub cover conditions between treatments, suggesting • that two important environmental factors driving species composition and abundance cannot be • manipulated by treatments with any degree of certainty. • Manipulative seed sowing experiment indicates that burning and thinning should result in higher germinant • survivorship for both pine species. • The wide discrepancy between germinant survivorship and observed seedling responses in treatments • leads us to speculate that recruitment limitation (i.e. seed predation, low seed crops, etc.) may be partly • responsible for the low overall regeneration of pines in response to treatment combinations. Teakettle Experimental Forest Plot Map • Each plot identified by two letter and one digit code indicating the treatment for each block • Burn Treatments Thinning Treatments • U-unburned B-burned N-no thinning C-understory thin S-overstory thin • For each treatment combination, plots are numbered 1-3 starting from the south. • Brown plots have 49 internal grid points (25 m ground distance spacing with a 25 m buffer) for intensive spatial sampling. Blue plots have 9 grid points (50 m spacing with a 50 m buffer). UN UC US BN BC BS UN UC US BN BC BS • Frequency of Jeffrey pine and sugar • pine seedlings increased across most • treatment combinations. • The highest increases for • Jeffrey pine were in shelterwood • thinning and burned/understory • thinned treatment combinations. • The highest increases for sugar • pine were in shelterwood thinned • and burned/unthinned treatment • combinations. • Low densities prohibited analysis • of sapling data for both pine • species UN UC US BN BC BS UN UC US BN BC BS Treatment Combination Treatment Combination Primary funding provided by the Joint Fire Sciences Program Additional support provided by The USDA Forest Service PNW Research Station, and the USDA Forest Service PSW Research Station Selected treatment images: a.burned/shelterwood, b. unburned/shelterwood, c. unburned/understory thin, and d. burned/understory thin.