Heat transfer of rhyniophytic plant axes

Abstract

Heat transfer is important for plants being sedentary organisms and exposed fully or partly to direct sunlight. It comprises three different mechanisms: (1) emission of IR (infra-red) radiation, (2) heat conduction and convection (sensible heat) and (3) evaporative cooling by transpiration (latent heat). Transpiration has been shown to act as an efficient cooling device in the case of extant land plants. The earliest known land plants consist of a simple branching axis system without leaves or roots. This paper addresses the question of how rhyniophytic plants dissipated heat as well as the significance of evaporative cooling for these organisms. This is particularly interesting in light of the fact that rhyniophytic land plants show a low stomatal density compared to extant plants. Using formulae (representing approximative approaches) for forced convection (heat is ‘carried away’ by wind movements), the results suggest that if wind velocity is high enough for this heat transfer mechanism, then transpiration does not play a role in heat dissipation. This is due to the fact that the slender habit of rhyniophytic plant axes lead to high boundary layer conductance and that the transpiration rate is too low to significantly contribute to heat transfer. During low wind velocities, the regime of mixed convection develops which leads to heat transfer both by forced convection and free convection (heat transfer by buoyancy plumes). Computer simulations were applied in order to study mixed convection for rhyniophytic plants due to the complexity of this heat transfer regime. Slight air movements significantly decrease the plant temperature due to the high boundary layer conductance. Although the transpiration may be significant for heat transfer during low wind velocities if the plant surface temperature is very high, convective heat transfer is expected to dominate heat dissipation. Further detailed investigations of the interactions between a rhyniophytic plant stand and its micrometeorological environment would be of great interest, because these plants differ from extant land plants in various properties which also affect microclimatic factors. Gaining new information about the ecophysiological behaviour of rhyniophytic plants and their interactions with the microclimate created by these plants also concern other organisms associated with rhyniophytes, such as fungi or arthropods.