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. 1. Introduction Quaking aspen (Populus tremuloides Michx.) communities are being replaced by conifers throughout many forests and rangelands of North America [1–9]. Succession to coniferous species is expected in the absence of disturbances such as wildfires that kill young conifers establishing within aspen stands [10]. Pioneering features such as lightweight seed, shade intolerance, and rapid growth of vegetative root-sucker regeneration indicate that aspen is adapted to disturbance [10, 11]. Having the widest distribution of any native tree in North America suggests that aspen has benefitted from a long history of natural disturbances. Fire suppression throughout much of the 20th Century has lengthened fire return intervals, allowing conifers time to establish and develop thicker fire-resistant bark with advancing size and age. Conifers eventually overtop aspen and suppress aspen regeneration and herbaceous vegetation. Loss of aspen forest area merits concern because aspen is considered a keystone species [12, 13], and aspen is the foundation species in stands that are “hotspots” of biodiversity [14]. At the
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