Heat resistance and energy budget in different Scandinavian plants
Authors: Gauslaa Yngvar 
Published: July 2006

Alpine vascular plants seem to use other strategies in surviving a cold environment than reducing the reflectance in level leaves. Pubescence in alpine plants has small effect upon total reflectance, but may increase the amount of photosynthetic active radiation within the sheltered canopy. Alpine cushion plants like Silene acaulis, Diapensia lapponica and Loiseleuria procumbens maximize the absorptance of radiant energy with minimum heat losses, probably as an effect of the dense canopy structure. The young inflorescences of Eriophorum vaginatum were found to be extremely efficient absorbators, while the reflectance in Salix catkins was close to that of green leaves.

In lichens, a great variation both in visible (400–700 nm) and infrared (700–1400 nm) reflectance was found: (A) The Pseudephebe pubescence group consists of species with very low reflectance at all measured wavelengths. The species are chionophobous, probably because of the high absorptance which makes growth possible during the cold season. (B) The Sticta sylvatica group, characterized by very low visible reflectance and very high infrared reflectance, is well adapted to shade. (C) The Cetraria nivalis group consists of fruticose species with high reflectance both in the visible and the near infrared. The intense visible reflectance probably makes net photosynthesis possible in well protected layers of the canopy. (D) The Nephroma arcticum group with spectral properties resembling green leaves in vascular plants. (E) The Haematomma ventosum group and the Parmelia perlata group with spectral properties intermediate between group C and D.

A modified method determining lethal temperatures and energies of activation in the process leading to death during a heat shock, is described. The two parameters are rather species specific in many of the 118 Scandinavian plants investigated. The lethal temperatures completely overlap the values in hotter parts of the world. However, habitat specific lethal temperatures were found; low values in wet‐ or shade‐growing species and high values in dry‐growing species. In Picea abies lethal temperatures and energies of activation showed pronounced, but diverging, year cycles in 12 ecotypes from different parts of Europe. Only negligible differences between the ecotypes exist, and cycles are probably photoperiodically determined.

Heat hardening can be achieved quickly, both in an active and dormant stage, by increasing the temperature. A linear correlation between hardening temperature, both in the optimal and supraoptimal temperature range, and hardening capability was found. In most species, but especially in cold adapted species, the hardening capability at supraoptimal temperatures decreases with increasing cultivation temperature.

Diffusion resistances with open (rs) and closed stomata (rc) are measured on excised leaf samples in 72 species. A positive correlation between rs and rc was found. rc ranges from 2.2‐62.5 s cm−1 in mesophytes and from 19‐425 s cm−1 in xerophytes, the highest values were found in succulents. Some of the alpine species had extremely low rc, falling within the rs‐range. Some habitat specific differences in rc were found, but the relatively few significant differences in rs between different habitats indicate that a lot of different drought avoidance mechanisms exist. The greatest variation in rc between different species was found in periodically dry habitats, though a few species like Epilobium alsinifolium (rc= 62.5 s cm−1) growing in constantly wet habitats had remarkably high rc.

In Saussurea alpina leaf size increases with improved moisture conditions. Calculations of leaf temperatures with closed stomata and somewhat extreme meteorological conditions showed that the mean leaf size in the wettest part of the transect was below, but very close to the size giving lethal leaf temperatures. In Rubus chamaemorus leaf size increases with increasing artificial shading. The leaves growing in sunexposed sites will be only 0.5°C below the lethal limit when the stomata are closed. All the shade‐leaves would exceed the lethal limit if the screen was removed and closing of stomata occurred. The northern distribution of this species is probably due to its low ability to avoid heat stress.

In Silene acaulis heat damage was observed under natural conditions at an air temperature of only 21°C. Leaf temperatures about 20°C above air temperature was often found in prostrate alpine vascular plants during sunny periods. The highest overtemperature (25.5°C) was observed in the broad leaves of Rubus chamaemorus. A comparison with maximum leaf temperatures measured in different parts of the world revealed rather uniform maximum leaf temperatures in spite of very contrasting air temperatures. Thus, vascular plants seem to control the leaf temperatures to a great extent by means of morphological modifications.

Leaf temperatures in a hot and dry period were calculated and compared with the heat resistance in 69 Scandinavian, mainly alpine, plants. In 14 wet growing species the lethal limits were exceeded if closing of the stomata occurred. In the remaining species calculated temperatures in single leaves never exceeded the lethal limit. Most of these species have leaves densely crowded in cushions or prostrate rosettes. Hence they get warmer than indicated by the calculated temperatures in single leaves, and will probably be heated close to the lethal limit. A highly significant correlation between lethal temperatures and cuticular diffusion resistances was found, probably illustrating the importance of transpirational cooling during a hot period. A combination of cuticular diffusion resistances and lethal temperatures segregates the species better in natural groups than only one parameter alone.

Factors involved in limiting the downward distribution of alpine plants are discussed. Some species avoid lowlands since they are drought sensitive (low cuticular diffusion resistance), others, mainly cushion plants with low heat exchange capacity, are probably overheated in lowlands.