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Climate warming may stimulate growth and reproduction in cold-adapted plants, but also reduce their performance due to warming-induced drought limitation. We tested this theory using a unique experiment with the alpine forb Rumex alpinus. We examined how climate warming over the past four decades affected its annual rhizome growth, leaf production and flowering, and whether responses varied between alpine, subalpine and montane populations. Before the period of accelerated warming in the 1970s and 1980s, the primary limitation on growth had been cold temperatures and short growing seasons. Increased summer temperatures in the 1990s and 2000s enhanced rhizome growth and leaf production, but not flowering. Alpine and subalpine plants profit more than montane plants, currently producing three times longer annual rhizome increments and twice as many leaves as 40 yr ago, and achieving nearly the same values as montane plants. During the warmest 2005-2015 period, growth became contingent on summer precipitation and began to decrease across all populations, likely due to an increasing water shortage in dense monospecific stands. Warming releases plants from cold limitations but induces water shortage. Rumex alpinus exceeds its thermal optimum and becomes water-limited as the climate warms. Our results suggest that warming-induced responses in alpine plants will not be onesided shifts to higher growth and reproduction, but rather multidimensional and spatiotemporally variable.

Alpine plants growing along altitudinal gradients have been traditionally considered to have reduced flowering and growth and enhanced clonality toward higher altitudes. This pattern, however, has seldom been studied for multiple characteristics in one plant species and over several years, and thus its generality is uncertain. In the present study, we used annual growth markers on perennial rhizomes (herbchronology) to analyze the long-term growth of Rumex alpinus. By determining the width and length of rhizome segments (annual increments) and the numbers of leaf scars, inflorescence scars, dormant buds, and branches preserved on rhizomes, we analyzed past growth for more than 10 years at seven sites along an altitudinal gradient (950-1,900 m a.s.l.) in the Low Tatra Mts., West Carpathians, Slovakia. We determined where growth was optimum on the gradient and whether the data supported the common opinion that clonal growth is enhanced and flowering is reduced with increasing altitude. Although R. alpinus is a light-demanding species occurring preferentially along small streams above the treeline, the most vigorous growth (highest number of leaves and inflorescences) occurred at low altitudes in forest clearings. According to expectation, R. alpinus exhibited less flowering and growth but increased clonality with increasing altitude. Low temporal inequality in growth indicates that the perennial rhizome system plays a role in buffering year to year climatic variability in the harsh environment along altitudinal gradients.

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