Indirect prediction of bud flushing from ex situ observation in hybrid Larch (Larix decidua x L. kaempferi) and their parents

Phenology of bud burst was studied for larches (Larix) using ex situ and in situ observations. The main objective of the study was to test the possibility to predict flushing in the field (in situ) from ex situ parameters obtained in growth chambers. Several progenies of European (EL), Japanese (JL) and hybrid (HL) larches were assessed in progeny trials for dates of terminal bud flushing and of overall “greening” of the tree (proportion of green crown). Twigs collected on the same trees during the previous winter were observed under controlled environments in two growth chambers, one at 11 °C and the other at 16 °C. Dates of flushing were recorded and from these, threshold temperatures and heat-sum requirements to flush were estimated. Correlations between in situ and ex situ parameters were studied and several linear models were tested to predict in situ flushing parameters using flushing days and heat-sums from ex situ experimentation.On average, HL and EL flushed after 61–63 days from January 1st against 56–61 days for JL in the growth chamber at 16 °C, while at 11 °C, HL needed 70–80 days, EL 70–78 days and JL 63–73 days. Estimated mean threshold temperatures were 1.7–3.0 °C for JL, 2.2–3.6 °C for HL and 2.3–3.3 °C for EL. Heat-sum requirements reached 250–320 degree-days for JL, 257–315 for HL and 307–326 for EL. In the field, terminal bud flushing took 98–109 days and crown greening of trees took around 101–102 days. In terms of heat-sums, 237–270 degree-days were needed for terminal bud flushing and between 186 vs 294 degree-days for crown greening.Relationships between heat-sum requirements ex situ and in situ were stronger across progeny trials than those found for dates of flushing and also more stable for greening (0.71–0.83) than for terminal bud flushing (0.54–0.86). As well, for both ex situ and in situ   flushing assessment, the heat-sum variables presented a higher genetic control (hfam2=0.70−0.95) than the dates of flushing (hfam2=0.57−0.59).The two best predictive models involved only heat-sums parameters. Although they are simple, some of them looked successful enough in predicting field flushing parameters with an acceptable accuracy. Heat-sum estimated in growth chambers from twigs could be used as a reliable indirect predictor of bud burst or flushing in the forest.

Research highlights▶ Phenology of bud burst was studied for larches (Larix) using ex situ and in situ observations. Several progenies of European (EL), Japanese (JL) and hybrid (HL) larches were assessed in progeny trials for dates of terminal bud flushing and of overall “greening” of the tree. Twigs collected on the same trees during the previous winter and they were observed under controlled environments in two growth chambers. As result, threshold temperatures and heat-sum requirements to flush were estimated. ▶ Correlations between in situ and ex situ parameters were studied and several linear models were tested to predict in situ flushing parameters using flushing days and heat-sums from ex situ experimentation.