Thermal imaging reveals massive heat accumulation in flowers across a broad spectrum of alpine taxa

Abstract

By their small size, alpine plants escape free atmospheric convection and operate in a warm canopy climate depending on exposure and solar radiation. Inflorescences commonly protrude from that aerodynamically sheltered microenvironment and, thus, can be expected to experience a comparatively cooler climate. Using a high-resolution thermal camera, we performed a mid-summer field survey in the Swiss Alps at 300 m above the climatic treeline to examine floral temperatures as related to solar radiation and flower diameter, height, brightness of flowering units and their area density across a sample of 43 alpine species. We show that flowers of alpine species are actually warmer (mostly 3 K) rather than cooler than canopy foliage on bright days. The degree of heating largely depends on solar radiation, with flowering unit diameter and their area density as significant co-factors. Only in cushion plants, we found flowers to be slightly cooler than the compact leaf canopy. Abrupt shading of flowers revealed the dynamics of heat loss, with two-thirds of the extra heat lost in about 2–3 min. The study evidences that the size and shape of alpine flowers overcompensate the expected negative effects of greater wind exposure on experienced temperature as long as the sun is out. The thermal conditions for floral processes are, thus, not much different from lower elevation conditions during good weather. The remaining reproductive constraints are related to the duration of such favourable periods and season length in general.