Plant temperatures and heat flux in a Sonoran Desert ecosystem

Abstract

In the extreme desert environment the potential energy load is high, consequently high temperatures might be a limiting factor for plant survival. Field measurements of plant temperatures in a Sonoran Desert ecosystem were made using fine thermocouples. Temperatures of six desert species were measured: Opuntia engelmannii, Opuntia bigelovii, Opuntia acanthocarpa, Echinocereus engelmannii, Larrea tridentata and Franseria deltoidea. Daily temperature profiles were used to compare the different responses of cacti and shrubs to the desert heat load and also to compare spring and summer responses. Leaf temperature of shrubs was at or near air temperature during both the mild, spring season and the hotter dry season. The cacti, on the other hand, absorbed and stored heat, thus temperatures were often above air temperature. The energy absorbed is determined largely by plant orientation and surface area exposed to the sun. Actual energy absorbed by the plants was estimated from energy diagrams.The flat stem pads of Opuntia engelmannii plants are oriented to receive maximum sunlight without long periods of continuous heating. Opuntia bigelovii spines reflect and absorb much of the environmental energy load, thereby protecting the thick, succulent stems from overheating. The smaller stems of Opuntia acanthocarpa dissipate heat more effectively by their large surface area exposed to convective air currents. Leaves on desert shrubs remain nearer to air temperature than do succulent stems of cacti, because their very large surface to volume ratio allows them to dissipate much heat by convection.

Thermal Energy Exchange Model and Water Loss of a Barrel Cactus, Ferocactus acanthodes

“…The convection coefficient (h,) was determined using a plant 40 cm tall and 26 cm in diameter in which an 800-w immersion (3) (4) heater was inserted. Thermocouples were placed in the tissue to determine the radial temperature gradient from which the heat flux can be calculated (7); also, thermocouples at various locations in the ribs were used to estimate the local h.. A 45-cm-diameter fan was used to create various wind speeds (measured with Thornthwaite 106 lightweight cup anemometers) under turbulence intensities approximating those found in the field.…”

“…THEORY Although information on transpiration and photosynthesis by cacti has become available (1,14,16), relatively little progress has been made in understanding their thermal relations. Gibbs and Patten (4) indicate that the north-south orientation of the vertical cladophylls of Opuntia engelmannii reduces their surface temperature compared with an orientation of pad surface more perpendicular to the solar irradiation. Hadley (5) speculated that the surface projections of cacti reflect solar irradiation and thereby reduce the stem temperature.…”

“…Hadley (5) speculated that the surface projections of cacti reflect solar irradiation and thereby reduce the stem temperature. Indeed, Gibbs and Patten (4) (11,13) are needed to account for the energy balance of a barrel cactus: (1): shortwave irradiation from the sun; (2): net longwave radiation exchange; (3): convection; (4): conduction; (5): latent heat loss; and (6): heat storage. The total shortwave irradiation absorbed by the surface can be represented by:…”

“…By the Stefan-Boltzmann law the longwave radiation emitted by a surface equals earT. 4, where e is the emittance; a-, the Stefan-Boltzmann constant; and T, the surface temperature (in degrees Kelvin). The longwave irradiation absorbed is aT,4n, where Tenv is the effective temperature of the environment facing the particular part of the plant being considered.…”

“…Due to the nonlinear nature of the latent heat and longwave radiation terms for external nodes, usual means for solving simultaneous equations proved to be inappropriate. Consequently, an initial estimate was made based on the previous hour's temperature distribution and this was followed by a series of consecutive corrections on each of the nodal temperatures until the net flow of energy for each node was zero (the algebraic sum of terms represented by equations [1][2][3][4][5][6]. This is essentially a modified relaxation technique used in numerical analysis of complex systems by convergent iteration.…”