Aspen Recovery Since Wolf Reintroduction on the Northern Yellowstone Winter Range (original) (raw)

Abstract

Quaking aspen (Populus tremuloides Michx.) recruitment and overstory stem densities were sampled in 315 clones in 1991 and 2006 on 560 km 2 of the Northern Yellowstone Winter Range (NYWR). A primary objective was to observe if aspen status had improved from 1991 to 2006: evidence of a wolf (Canis lupus) caused trophic cascade. Recruitment stems (height. 2 m and diameter at breast height , 5 cm) represent recent growth of aspen sprouts above elk (Cervus elaphus) browsing height, whereas overstory stems (all stems. 2 m) represent the cohort of stems, which will insure the sustainability of the clone. Overstory stem densities declined by 12% (P 5 0.04) on the landscape scale when compared with paired t-tests. Overstory stems declined in 58% of individual clones and in 63% of the 24 drainages of the study area. The second objective was to determine which factors influenced changes in aspen density. Winter ungulate browsing (P 5 0.0001), conifer establishment (P 5 0.0001), and cattle (Bos spp.) grazing (P 5 0.016) contributed to the decline in overstory stem densities when analyzed using a mixed effects model of log transformed medians. Eighty percent of the clones were classified as having medium to high browsing levels in 1991, whereas 65% of the clones received a similar rating in 2006, possibly due to the reduced NYWR elk population. Aspen recruitment has increased in some 2-10 km 2 areas, but not consistently. Our study found that a trophic cascade of wolves, elk, and aspen, resulting in a landscape-level recovery of aspen, is not occurring at this time. Resumen Se muestreó el reclutamiento y la densidad de tallos aéreos de Populus tremuloides Michx. de 315 clones en 1991 y 2006 en un área de 560 km 2 en la Invernada Norte de Yellowstone (NYWR). Un objetivo primario de este estudio fue observar si el estatus de P. tremuloides mejoró entre 1991 y 2006 evidenciando una cascada trófica causada por el lobo (Canis lupus). El reclutamiento de tallos nuevos (altura. 2 m y diámetro a la altura del pecho , 5 cm) representa crecimiento de brotes recientes por encima de la línea de ramoneo de los ciervos (Cervus elaphus), mientras que los tallos aéreos (todos los tallos. 2 m) representan la cohorte de tallos que asegurarán la sustentabilidad del clon. Las densidades de tallos aéreos decreció en un 12% (P 5 0.04) a la escala de paisaje según una comparación realizada con un prueba de ''t'' apareada. Los tallos aéreos decrecieron en un 58% de los clones individuales y en un 63% de las 24 micro-cuencas del área de estudio. El segundo objetivo de este estudio fue determinar cuáles factores influenciaron los cambios en la densidad de P. tremuloides. El ramoneo invernal de los ungulados (P 5 0.0001), el establecimiento de coníferas (P 5 0.0001), y el pastoreo bovino (Bos spp.; P 5 0.016) contribuyeron a la disminución en la densidad de tallos aéreos según un análisis de efectos mixtos utilizando una transformación logarítmica de las medianas. El 80% de los clones fueron clasificados en la categoría de nivel de uso medio a elevado en 1991, mientras que el 65% de los clones recibieron una valoración similar en el 2006, posiblemente debido a la reducción en la población de ciervos en el NYWR. El reclutamiento de P. tremuloides ha aumentado en dos áreas de 10 km 2 , pero no de modo consistente. Nuestro estudio demostró que una cascada trófica de lobos, ciervos, y P. tremuloides, que resulte en una recuperación de esta especie leñ osa a escala de paisaje, no está ocurriendo en este momento.

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