Thermal time - concepts and utility

Abstract

This paper (i) reviews temperature/development rate relationships in plants and poikilothermic invertebrates, (ii) argues that the relationship is often linear over much of the range up to the thermal optimum (T o ) and provides a possible mechanism, (iii) provides evidence of a trade-off between the base temperature (T b ) and the thermal constant (DD) that enables each species to adapt to its thermal environment, and (iv) indicates some of the practical and ecological implications. Where a linear relationship has been characterised it is possible to estimate the base temperature for development (T b , expressed in °C) and the thermal constant for development (DD, the reciprocal of the temperature coefficient (a), expressed in degree [°C] days accumulated above T b ). A possible basis for the linear relationship between rate and temperature is proposed based on the Arrhenius and Sharpe-Schoolfield equations involving activation enthalpy and progressive inactivation of the reactant molecules at both low and high temperatures. Knowledge of T b and DD enables rates of development of organisms/ processes to be calculated and compared at any given temperature between T b and T o . An analysis of published results for differentiation processes (differentiation = a change of state) in species of insects, Collembola, spiders, nematodes and plants showed that T b tended to vary with the temperature of the niche to which the organism is adapted, and that there was a trade-off between T b and DD. Tropical species had higher values of T b than temperate and DD decreased as T b increased (and vice versa). This conferred a competitive advantage on each species in the thermal environment to which it was adapted. The decrease in DD tended to be relatively greater than the increase in T b, further favouring a high T b in tropical species. A mechanism for the trade-off is suggested whereby DD and T b were shown to be correlated (P < 0.01) with the activation enthalpy (H A ) of an assumed, rate-limiting enzyme. Thermal time can also be applied to processes involving growth (= an increase in dry weight) when the DD requirement for development to maturity is the sum of the requirements for differentiation and growth. Rates of both differentiation and growth can vary greatly between species, depending upon the niche they inhabit, and the implications of such differences for resource requirements are considered. In insects and nematodes, but not in annual plants, development is usually coupled to growth. Consequently, when resources are inadequate, mature size in these animals varies less than in plants. Thermal time is shown to provide insight into the life strategies of species within their communities and to have practical implications.

Growth of champa fruit under agroecological conditions of Miraflores, Boyacá, Colombia

“…In contrast, as the temperature rises, the development rates rise to an optimal temperature, after which they decline and eventually cease at their maximum development temperature (Trudgill et al, 2005). This observation indicates that the temperatures that prevailed during the present study were more favorable for fruit development so that the development cycle was shortened compared with that reported by Balaguera et al (2009).…”

Genetic control of flowering time in Eucalyptus globulus ssp. globulus

“…Climatic data from the Hobart airport weather station (42˚49'59.16"S 147˚30'11.88"E, 24 elevation 3m), which is only 6.5 km from the seed orchard and at a similar elevation, were 25 provided by the Bureau of Meteorology, Hobart, for the period from October 2002 to 26 accumulation of temperature above a base temperature (Trudgill et al 2005). In this study, 1 five degrees was used as a base temperature as this was identified as the base temperature in a 2 study of E. globulus growth (Reed et al 2003).…”

Genetic control of flowering in spotted gum, Corymbia citriodora subsp. variegata and C. maculata

“…Studies of introduced annuals in the northern hemisphere revealed a common pattern in flowering times that emerged in response to climatic gradients during historical times across a wide range of plant species (Kollmann and Banuelos 2004;Bastlová et al 2006;Montague et al 2008). Unlike annuals, however, where the need to reproduce within one growing season, and photoperiod, appears to be the main driver of adaption (Riihimaki and Savolainen 2004;Hancock et al 2011), there is increasing evidence that photoperiod is not a main driver for flowering time in perennials (Keller et al 2012) where temperature, and more particularly heat sum, better predicts floral development and anthesis (Reader 1983;Yeh et al 1999;Trudgill et al 2005).…”

“…For example, photosynthates necessary for bud development can be allocated in the shoots only at a temperature higher than 18 C among subtropical species (Specht and Brouwer 1975). Exposure to warm temperatures can be expressed as heat sum, which is the linear accumulation of temperature above a base temperature measured in degree-days (Trudgill et al 2005). Heat sum has been found to determine the timing of bud break and subsequent reproductive development in temperate eucalypts (Moncur et al 1994;Jones et al 2011), and heat-sum models have been used to predict anthesis time in other trees, e.g.…”