Plastic Responses Contribute to Explaining Altitudinal and Temporal Variation in Potential Flower Longevity in High Andean <i>Rhodolirion montanum</i>

The tendency for flower longevity to increase with altitude is believed by many alpine ecologists to play an important role in compensating for low pollination rates at high altitudes due to cold and variable weather conditions. However, current studies documenting an altitudinal increase in flower longevity in the alpine habitat derive principally from studies on open-pollinated flowers where lower pollinator visitation rates at higher altitudes will tend to lead to flower senescence later in the life-span of a flower in comparison with lower altitudes, and thus could confound the real altitudinal pattern in a species´ potential flower longevity. In a two-year study we tested the hypothesis that a plastic effect of temperature on flower longevity could contribute to an altitudinal increase in potential flower longevity measured in pollinator-excluded flowers in high Andean Rhodolirium montanum Phil. (Amaryllidaceae). Using supplemental warming we investigated whether temperature around flowers plastically affects potential flower longevity. We determined tightly temperature-controlled potential flower longevity and flower height for natural populations on three alpine sites spread over an altitudinal transect from 2350 and 3075 m a.s.l. An experimental increase of 3.1°C around flowers significantly decreased flower longevity indicating a plastic response of flowers to temperature. Flower height in natural populations decreased significantly with altitude. Although temperature negatively affects flower longevity under experimental conditions, we found no evidence that temperature around flowers explains site variation in flower longevity over the altitudinal gradient. In a wetter year, despite a 3.5°C temperature difference around flowers at the extremes of the altitudinal range, flower longevity showed no increase with altitude. However, in a drier year, flower longevity increased significantly with altitude. The emerging picture suggests an increase in flower longevity along the altitudinal gradient is less common for potential flower longevity than for open-pollination flower longevity. Independently of any selection that may occur on potential longevity, plastic responses of flowers to environmental conditions are likely to contribute to altitudinal variation in flower longevity, especially in dry alpine areas. Such plastic responses could push flowers of alpine species towards shorter life-lengths under climate change, with uncertain consequences for successful pollination and plant fitness in a warming world.许多高山生态学家认为，花朵寿命随海拔升高而延长的趋势，对弥补高海拔地区因寒冷多变天气导致的低授粉率具有重要作用。然而，目前记录高山生境中花朵寿命随海拔升高而增加的研究，主要基于开放授粉花朵的观测：高海拔地区访花者到访率更低，会导致花朵相较于低海拔个体更晚进入衰老阶段，因此可能混淆物种潜在花朵寿命的真实海拔分布模式。本研究为期两年，以安第斯山区高海拔的山地管花石蒜（*Rhodolirium montanum* Phil.，石蒜科Amaryllidaceae）的排除访花者花朵为研究对象，检验了“温度对花朵寿命的表型可塑性（plastic）效应，可能促成潜在花朵寿命随海拔升高而增加”这一假说。通过人工增温处理，我们探究了花朵周围的温度是否会通过表型可塑性影响潜在花朵寿命。我们对分布在海拔2350至3075米样带上的3个高山自然种群的潜在花朵寿命及花朵高度进行了严格控温测定。实验中花朵周围温度升高3.1℃后，花朵寿命显著缩短，表明花朵对温度存在表型可塑性响应。自然种群的花朵高度随海拔升高显著降低。尽管实验条件下温度对花朵寿命存在负向影响，但我们未发现证据表明，花朵周围的温度可以解释海拔梯度上不同样地间花朵寿命的差异。在较为湿润的年份，尽管海拔两端的花朵周围温度差可达3.5℃，花朵寿命并未随海拔升高而增加；但在较为干旱的年份，花朵寿命则随海拔升高显著增加。现有研究结果表明，相较于开放授粉花朵的寿命，潜在花朵寿命随海拔梯度升高而增加的情况更为少见。无论潜在寿命是否经历自然选择作用，花朵对环境条件的表型可塑性响应都可能促成花朵寿命的海拔差异，在干旱高山地区尤为如此。在气候变化背景下，这类表型可塑性响应可能导致高山物种的花朵寿命缩短，进而在变暖的全球环境中，对成功授粉与植物适合度产生尚不明确的影响。