Sierra Nevada Mixed-Conifer Species Response to Gap Openings Jedediah Parr Abstract Canopy openings have been used for years as a means of improving tree growth and health. However, little information exists about particular species’ responses to the size of the gap opening, and in comparison to each other. Given that the Sierra Nevada has been experiencing a radical change in ratio between shade-tolerant and shade-intolerant species due to fire suppression, such information could help forest managers correct the imbalance through selective encouragement of shade-intolerant stands. White fir, a shade-tolerant tree species, and Ponderosa pine, a shade-intolerant species, were sampled from a research station in the Sierra Nevada. Core samples were taken from three replicates of four different gap sizes (0.1, 0.3, 0.6, 1.0 ha) opened in 1996. Core samples were used to measure growth rings and calculate relative growth rates over the five post-gap years and ten pre-gap years. Relative growth rate was calculated as the percentage of the tree’s total basal area added that year. Ponderosa pine’s response to gap creation (measured by the change in relative growth rate) was about 9% higher than white fir’s. That difference was not a significant factor in the whole-model tests. Likewise, changes in gap size did not significantly affect either species’ growth rate.Introduction The use of canopy openings has been explored for years as a means of improving tree growth and health. Canopy openings are created either through natural means such as fire and windfall, or artificially by removing trees to create a “gap” in the forest. The trees surrounding the gap then have greater access to sunlight, and suffer less from competition. Gap models have been developed for many different kinds of forest habitat, including some of those present in the Sierra Nevada, as well as the Appalachians of the eastern United States, northwestern Canada, Scandanavia, and Great Britain (Miller and Urban 1999, Mailly and Kimmons 2000). From a forest management standpoint, however, these studies have been little used. Actual relationships between species, gap size, and competitors are much less common than general studies of gap size and stand growth as a whole, and the potential for matching species to appropriate gap size and orientation has yet to be realized (Coates 2000). While canopy openings are generally recognized to benefit all edge trees to some degree, many past experiments have focused on seedling performance after a gap is opened (Gray and Spies 1997, Oliver and Dolph 1992). Furthermore, many of these studies have taken place in other parts of the world, on species other than those found in the Sierra Nevada (Kneeshaw et al. 1998, Sundkvist 1994). The question of whether shade-tolerant or shade-intolerant species benefit more, and in what size opening, still remains. Answering this question could lead to changes in forest management practices. The forests of the Sierra Nevada are currently experiencing a shift from shade-intolerants to shade-tolerants, due mainly to decades of fire suppression (Minnich et al. 1995). White fir in particular is overtaking stands formerly dominated by species such as Ponderosa pine and giant sequoia, resulting in dense understories and a corresponding increase in risk of severe wildfires (Roy and Vankat 1999). More knowledge about species performance in relation to gap size could give forest managers a useful tool for correcting this growing imbalance, as an alternative or complement to prescribed burning and understory thinning. This project focused on a shade-tolerant species, Abies concolor (white fir), and a shade-intolerant species, Pinus ponderosa (Ponderosa pine) (Burns and Honkala 1990). Two hypotheses were tested. The first was that Ponderosa pine’s response to gap opening would be greater, given that Ponderosa pine prefers less shade than white fir and would therefore respond more when released from shade. The second hypothesis was the smaller gaps would favor theshade-tolerant species, and that as gap size increased, the more shade-intolerant species would be favored, due to the increased availability of light in the larger gaps. Methods This study was carried out at Blodgett Forest Research Station (BFRS). BFRS is at about 4000’ elevation, near the towns of Cool and Georgetown in the Sierra Nevada. The forest is primarily composed of mixed conifer species, as well as some tan and black oak. Core samples were taken with a hollow core drill approximately 1/4” in diameter. There were four discrete gap sizes: 0.1 ha, 0.3 ha, 0.6 ha, and 1.0 ha. Each gap size was replicated in three different places, for a total of 12 gap openings. All gaps were within a few hundred yards of each other. The cores were taken at regular intervals around the edge of each gap, using a compass to determine even spacing between samples. Samples were also drawn from an approximately equal number of trees in the “matrix” (the forest proper, or areas where no gaps were opened), to compare growth rates with the trees sampled from the gaps. A total of 84 trees were sampled. White fir was far more prevalent than Ponderosa pine—there were a total of 188 white firs available for sampling as edge trees, but only 49 Ponderosa pines. Of those trees, 18 Ponderosa pines and 35 white firs were sampled as edge trees. Within the 0.1 ha gaps, 2 Ponderosa pines and 14 white firs were sampled; within the 0.3 ha gaps, 5 and 5, respectively; within the 0.6 ha gaps, 7 and 7, respectively; within the 1.0 ha gaps, 4 and 9, respectively. Within the matrix, 13 Ponderosa pines and 18 white firs were sampled. Relevant growth factors were controlled for. The trees were all at about the same elevation, and used the same watershed and soil type. The side of the mountain where the gaps were cut had a northward orientation, with little variation. The slope was likewise relatively constant. In one part of the forest, trees were thinned (approximately 20% by basal area) some years before any gaps were cut. One gap of each size (4 total) was located within this area, while the other two of each size (8 total) were opened in unthinned areas. This was taken into account (see Results). Trees that showed signs of disease (flagging, bleeding, etc.) were avoided. Only trees with crown classes of codominant or dominant were sampled. Each tree’s
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