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 highresolution thermal camera, we performed a midsummer 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.

Leaf and floral heating in cold climates: do sub-Antarctic megaherbs resemble tropical alpine giants?

“…The densely packed, dark purple disc florets were observed to be much warmer than the loosely arrayed, lighter purple ray florets. This result supports Dietrich & Kö rner's (2014) observation that dense Asteraceae structures are particularly efficient at heating under solar radiation. In addition, this species has the deepest coloured inflorescences of all megaherb species, with very dark purple pigmented inflorescences that show the so-called Ageratum effect: anomalous reflectance due to some pigments reflecting infra-red light that is picked up as red by a camera (Eckert 2004).…”

“…The heating of the inflorescence is therefore generated by the combination of the dark purple disc florets that absorb radiation and the densely packed structure of the capitulum that retains heat. Other studies in cold climates have also proven strong links between solar radiation and floral heating (Kevan 1975;Young 1984;Dietrich & Kö rner 2014).…”

“…Dietrich & Kö rner (2014) found that flowering units were around 38C warmer than foliage (68C above air temperature), with the exception of flowers from cushion plants, which were cooler than the background vegetation. The floral display of S. polaris was situated within the large, elliptical, sparsely hairy leaves, where the inflorescences are likely to benefit from the shelter provided by the large, corrugated leaves, resulting in similar heating effects between inflorescence and leaves.…”

“…Leaf and floral hairiness of giant herbs in the Himalayas, Hawaii and equatorial Andes has been shown to increase temperatures at the leaf surface and inside the flower by increasing the boundary layer and reflecting long wave radiation into the centre of the giant rosette (Miller 1986;Meinzer et al 1994;Yang et al 2008). Further, it has recently been determined that temperatures inside continental alpine flowers are dependent on solar radiation, with size and density of the floral unit being significant co-factors (Dietrich & Kö rner 2014). Giant herbaceous species in the tropical alpine regions have similar plant traits across continental regions, and are an example of functional convergent evolution (Smith & Young 1987;Rundel et al 1994;Givnish 2010).…”

Temperature Microclimates of Plants in a Tropical Alpine Environment: How Much does Growth Form Matter?

“…Because these climatic data mostly come from weather stations, however, they usually depart significantly from particular site microclimates and are of limited value for ecological inference (Scherrer and Körner, 2011;Graae et al, 2012;Graham et al, 2012). The temperature microclimate of tropical alpine plants, in contrast to those of temperate ones (Körner, 2003;Dietrich and Körner, 2014), has been examined only occasionally, by short-term observations (Hedberg and Hedberg, 1979;Young, 1984;Beck, 1994;Diemer, 1996;Ramsay, 2001;Sklenář, 1999Sklenář, , 2007. These studies document the capability of the tropical alpine plants to decouple from the ambient air temperature, but no comparison has been made of microclimates of various tropical alpine growth forms in different climatic conditions.…”