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Range limits of temperate high-altitude plants may be controlled by tradeoffs between physically severe but uncrowded conditions, versus mild but crowded conditions. Because up-slope migration of lowland species may accompany global warming and earlier snowmelt, I tested whether crowding by neighbors vs. timing of snowmelt limit Potentilla diversifolia to later-melting sites in the Rocky Mountains (U.S.A.). I transplanted individuals from two altitudes to experimental plots within and below the species' range limit, and experimentally altered snowmelt timing and the density of neighboring vegetation. In contrast to theoretical predictions for biotic control of lower range limits, higher temperatures and drier soils contributed to markedly reduced survivorship and reproduction below the native-range regardless of treatment. Neighbor removal only marginally increased performance both within and below the native-range, suggesting that interactions with neighboring vegetation are much less important for distribution than abiotic factors. Populations from the subalpine zone had longer growth intervals and grew larger than those from the alpine tundra in their native sites, but did not outperform alpine tundra populations when grown below the species' range. Although earlier snowmelt enhances growth, phenotypic plasticity may enable P. diversifolia to persist in later- melting tundra sites while higher temperatures and drought restrict it from much of the subalpine zone. [ABSTRACT FROM AUTHOR]