Christa M. Dagley

Geographic Patterns and Stand Variables Influencing Growth and Vigor of Populus tremuloides in the Sierra Nevada (USA)

Awareness of geographic patterns and stand variables that influence tree growth will help forest managers plan appropriate management and monitoring strategies. We quantified influences of stand location, species composition, stand density, and tree size on aspen tree growth and vigor around the Lake Tahoe Basin in the Sierra Nevada Mountains of California and Nevada, USA. Radial growth data were taken from increment cores. Aspen trees on the south and west sides of the lake grew 20–25% faster than aspen in north and east side stands. Diameter growth at 2,400 m elevation was 58% of growth at 1,900 m near lake level. Aspen grew faster with less competition from neighbor trees. At any level of competition, aspen growth was slower beside conifer neighbors and correlated with crown ratio (CR: length of live crown relative to total tree height, a proxy for tree vigor). Analysis of independent CR data for 707 aspen trees in nine additional stands indicated that aspen had smaller crowns in the presence of greater competition, and that composition of neighbor trees also affected CR: aspen trees had shorter crowns in the presence of conifer at higher stand densities. Taken collectively, our analyses point towards a cascading decline in aspen growth and vigor incited by succession of aspen stands to conifers. Our findings suggest that conifer removal and stand density control in aspen-conifer stands at Lake Tahoe will enhance aspen growth and vigor.

Assessing longleaf pine (Pinus palustris) restoration after southern pine beetle kill using a compact experimental design

Abstract A compact experimental design and analysis is presented of longleaf pine (Pinus palustris) survival and growth in a restoration project in the Piedmont region of Georgia, USA. Longleaf pine seedlings were planted after salvage logging and broadcast burning in areas of catastrophic southern pine beetle (Dendroctonus frontalis) attacks on even-aged mixed pine–hardwood forests. The split-plot design with randomized complete blocks was replicated on four sites. Main treatments were: chemical control of competing vegetation with herbicides; mechanical mowing to remove above-ground competition; and an untreated control. Nested within fixed-area treatment plots were single-tree plots centered on longleaf pine seedlings where adjacent vegetation was assessed immediately before treatment in years 1 and 2. Treatments applied to single-tree plots comprised three sizes of sprayed area around seedlings in chemical treatment plots. Logistic mixed-effects models indicated that initial survival was impacted by herbaceous vegetation but remained unaffected by the various vegetation control treatments applied later in year 1. Control of above- and below-ground competition around planted seedlings by mowing in conjunction with herbicide treatments resulted in significantly greater frequency of emergence from the grass stage, accelerating development of longleaf pine planted to restore beetle-killed areas in the Piedmont region of Georgia.

Regeneration and Recruitment Correlate with Stand Density and Composition in Long-Unburned Aspen Stands Undergoing Succession to Conifer in the Sierra Nevada, USA

Succession of aspen stands to conifer in the Sierra Nevada Mountains of California and Nevada, USA, is being interrupted by forest managers thinning conifers to sustain aspen stands in situ. However, patterns of stand density, species composition, and regeneration prior to management intervention have scarcely been described. We established a grid of sample plots throughout nine aspen stands encircling Lake Tahoe in the central Sierra Nevada. The degree of succession to conifer throughout each aspen stand was spatially heterogeneous. Patches of pure aspen were rare. Stand density index (SDI) reached an upper limit of 1700 in pure aspen. As composition shifted in favor of conifer, SDI approached or attained a maximum of 2500 in some plots. Stand density and species composition data were tested as predictors of conifer and aspen regeneration densities in each plot. Conifer seedlings had an average density of 3261 ha-1; they were most abundant in aspen-dominated areas of any density, and in conifer-dominated areas of higher density. Aspen regeneration had an average density of 3211 ha-1 and was one order of magnitude less frequent in areas of pure conifer versus areas of pure aspen, but remained relatively abundant in most areas. Aspen saplings were rare (average density of 42 ha-1 for saplings 10-15 cm DBH), especially at high stand densities or in areas where conifers dominated. Our findings suggest that forest managers interested in sustaining aspen stands in situ will need to control stand density to promote recruitment of younger aspen to the overstory.

Predicting Treatment Longevity after Successive Conifer Removals in Sierra Nevada Aspen Restoration

Populus tremuloides (quaking aspen) stands throughout the Sierra Nevada Mountains are undergoing succession to conifers. Removal of conifers is being tested, however, little is known about treatment longevity—the time taken for stand density to return to pretreatment levels. To determine longevity of treatments removing conifers below different size limits, we developed tree growth equations from data collected in 1 ha plots around Lake Tahoe in P. tremuloides stands with varying amounts of conifer, and simulated stand development after treatment in two stands. At Ward Creek, cutting all conifer < 35 cm diameter at breast height (DBH) generated the most wood that could practically be piled and burned inside the stand, but only reduced stand density by 16%. After 13 years of predicted treatment longevity, a second treatment was simulated with options of light, medium, or heavy cutting (50, 60, or 75 cm DBH limits). This gave treatment longevity of 23, 29, and 40 years respectively but did not restore P. tremuloides dominance. At Cookhouse Meadow, cutting conifer < 35 cm DBH had 16-year treatment longevity, after which time two treatments were compared. Cutting conifers < 50 cm DBH enhanced P. tremuloides’ representation from 27% to 37% of stand basal area and had 23-year treatment longevity. Raising the DBH limit to 60 cm left P. tremuloides representing 45% of stand basal area, and extended treatment longevity to 36 years. Our findings indicate that a series of treatments will be needed to restore and maintain P. tremuloides communities, and will eventually require removal of large conifers (> 75 cm DBH).

Vegetative Response to Weed Control in Forest Restoration

Longleaf pine (Pinus palustris Mill.) stands once occupied an estimated 24 million ha in the southeastern USA (Stout & Marion, 1993). Fire suppression, timber harvest, and land conversion reduced its extent to around one million ha (Outcalt & Sheffield, 1996). In recent times, widespread interest in restoring longleaf pine ecosystems or planting the species for timber production has motivated private landowners, industrial forest owners, and public agencies to establish more longleaf pine forest. Over 33 million longleaf pine seedlings were produced for the 2005-2006 planting season in the southeastern United States (McNabb & Enebak, 2008), and 54 million produced in 2008-2009 (Pohl & Kelly, 2011).