Spring phenophases of larch are more sensitive to spring warming than to year-round warming: Results of a seasonally asymmetric warming experiment

Abstract

Plant phenology is a highly sensitive indicator of climate change. The advancement in time of spring phenology due to climate warming is well documented. However, although the progressive warming process is asymmetrical across seasons in temperate and boreal regions, the consistency of spring phenological response to non-uniform seasonal warming has not been well studied. Furthermore, the different responses of budburst and leaf unfolding to seasonally uneven warming have not been fully explored. We investigated the responses of budburst and leaf unfolding in two-year-old larch (Larix principis-rupprechtii) seedlings to seasonally asymmetric warming treatments (with passive warming applied in open-top chambers) in northern China. We found that spring warming significantly advanced budburst and leaf unfolding by 7 and 2 days, respectively; year-round warming advanced budburst by 4 days but had no impact on the timing of leaf unfolding. Summer and winter warming had no effects on the timing and duration of spring phenophases. Notably, spring leaf unfolding was significantly delayed, by 2 days, following an autumnal warming, i.e., there was a carry-over effect (a 1-day delay in leaf senescence delayed leaf unfolding by 0.3 day in the next spring). The temperature sensitivities of budburst and leaf unfolding in the spring warming treatment (9.56 days/°C and 2.66 days/°C, respectively) were significantly greater than those in the year-round warming treatment (6.43 days/°C and 0.66 days/°C, respectively). We also found a trade-off relationship between the durations of budburst and leaf unfolding in the spring and year-round warming treatments. Our demonstration of (i) differences in the magnitude of responses between budburst and leaf unfolding, and (ii) the carry-over effect under non-uniform seasonal warming, should be incorporated into modeling simulations of spring phenological responses to future warming scenarios. Overall, our findings would have some critical implications for resistance and performance (e.g., frost damage risk and carbon uptake) in the seedling stage of plantation establishment under ongoing climate warming scenarios.