Temperature

Temperature governs almost every metabolic reaction a plant carries out, and every physiological process has an optimum temperature at which it runs fastest. Three temperature thresholds define an organism's response curve: the minimum temperature (below which physiological activity is at its lowest), the optimum temperature (at which activity peaks), and the maximum temperature (above which activity stops altogether). Based on how wide a temperature range an organism can tolerate, organisms are classed as eurythermal (tolerating a wide range of temperature fluctuation — e.g. the marine angiosperm Zostera and Artemisia tridentata) or stenothermal (tolerating only a narrow range — e.g. mango and palm, both terrestrial angiosperms that simply cannot survive in temperate countries like Canada or Germany). Water bodies show a distinctive vertical temperature pattern called thermal stratification, dividing into three layers: the warm upper epilimnion, a middle metalimnion where temperature drops steadily with depth, and the cold bottom hypolimnion. Variation in latitude and altitude also drives temperature-based zonation of vegetation across the globe — moving from the equator to the poles, or from the base of a mountain to its summit, reproduces essentially the same sequence of vegetation belts, because both changes ultimately mean falling temperature. Raunkiaer classified world vegetation into four temperature-based types — megatherms, mesotherms, microtherms and hekistotherms — spanning everything from the hottest tropical rain forest to the coldest alpine zones (with the odd exception of thermal springs and deep-sea hydrothermal vents, where average temperature can exceed 100°C despite the surrounding cold). Temperature's downstream effects are wide-ranging: it drives enzyme activity in every biochemical reaction ins …