Tree encroachment on meadows of the North Rim, Grand Canyon National Park, Arizona, U.S.A.

We analyzed attributes of tree encroachment on montane meadows and subalpine grasslands on the North Rim of Grand Canyon National Park. Species composition, age, density, height, diameter, condition class, and patterns of tree establishment were examined on belt transects at 12 meadow sites. Of the 3481 live and dead trees sampled, 52% of all trees were Populus tremuloides, 20% were Picea spp., 11% were Abies lasiocarpa, 10% were Abies concolor, and 7% were Pinus ponderosa. Tree densities averaged 4703 trees ha^-1. Sixty-two percent of the trees sampled were less than 20 yr of age, although some species established in the early and mid-1800s. Ninety-one percent of all trees, however, established after the mid-1930s, with the greatest establishment occurring after the early 1970s. Of this 91%, Populus tremuloides composed the majority (90%) of recent tree recruitment, with a mean establishment date of 1983-1984. A general pattern of progressively younger trees from the closed forest toward the meadow interiors was detected for some species, and together with little evidence of historical tree occurrence (e.g., large standing dead, downed logs, etc.) on the transects, indicated that trees had been encroaching on these meadows since the 1800s. These trends suggest a loss of important meadow habitat and landscape biodiversity within Grand Canyon National Park during the past century.