Modelling phenology to probe for
trade-offs between frost and heat risk in lentil and faba
Extreme temperatures at critical
developmental phases reduce grain yield. Combinations of sowing
date and cultivar that favour faster development reduce the likelihood
of heat stress but increase the risk of frost at critical phases. Current
models are unable to predict pulse yield in response to frost and heat, hence
our focus on phenology. Our aim was to model phenological variation with sowing
date and cultivar for lentil and faba
bean against the climatic patterns of frost and heat in 45 Australian
locations that spanned 29 °S-41 °S, 11−340 m.a.s.l.,
and 1−423 km to the coast.
For both crops, modelled mean and
standard deviation of time to flowering were
close to actuals and mean prediction error was below 5%. Comparison of actual
and modelled time to flowering returned: r = 0.89 (n = 121, P < 0.0001) and
modelling efficiency = 0.73 for lentil, and r = 0.96 (n = 123, P < 0.0001)
and modelling efficiency = 0.84 for faba bean.
The critical period for yield
determination was assumed to span from flowering to 200 °Cd after flowering.
Curves fitted to the time-course of frost (< 0 °C) and heat (> 34 °C)
probabilities between 1957 and 2018 were used to estimate the date of 10 %
frost probability and the date of 30 % heat probability as the boundaries of a
frost-heat risk window for the critical period. Out of the 45 locations, 12
were frost-free but with risk of heat, 7 were heat-free but with risk of frost,
3 were frost- and heat-free, and 23 featured a window defined by both frost and
heat boundaries. Frost variables discriminated locations more strongly than
heat variables. Geographical patterns in thermal regimes emerged that were
associated with latitude, altitude and continentality.
Realised warming between 1957 and 2018
advanced the time to 200 °Cd after flowering and shortened the critical period
in most locations, particularly in early-sown crops.
Comparisons of the probability curves of frost and heat between 1957–1985 and
1986–2018 showed, with few exceptions, an asymmetry between delayed late frost
(up to 44 d) and earlier heat onset (up to 11 d), with a narrowing of the
frost-heat risk window from 46 to 90 d for the period 1957–1985 to 34–64 d for
We identified a dominant role of frost
as (i) the main discriminating factor among geographically distinct locations,
(ii) the main source of variation of the frost-heat window, and (iii) a
putatively increased risk factor with climate change. Adaptation to frost in
the critical period for yield is important for pulses despite warming trends.
Increased frost tolerance can directly improve yield and indirectly contribute
to reduce risk of heat and drought later in the season.