How temperate and boreal trees' leaves respond to climate change remains uncertain. Now, a new study of northern forests reports that while early-season climate warming – that occurring before the summer solstice – tends to be associated with earlier autumn leaf senescence, late-season warming (after the summer solstice) has the opposite impact, delaying onset of leaf senescence in fall. "Improved models of plant development and growth under climate change will need to incorporate the reversal of warming effects after the summer solstice," write Constantin Zohner and colleagues, authors of the study. Climate change has resulted in changes to the growing seasons of plants. For example, research shows that the start of the growing season for boreal and temperate trees – when leaves emerge during the spring and trees begin to photosynthesize – occurs, on average, two weeks earlier than it did during the 19th and 20th centuries. Similarly, the end of season (EOS) – when autumn leaves die and fall – is being delayed. Not only do these shifts affect tree performance, but they can also lead to changes in ecosystem structure and functioning, impacting global biogeochemical cycles. A longer growing season could mean greater carbon sequestration in forests, for example. The timing of EOS largely depends on dynamic environmental and biological interactions, which aren't well understood. Using a combination of satellite, ground, carbon flux, and experimental data, Zohner and colleagues evaluated how leaf senescence relates to various environmental cues, including day length, temperature, and early-season photosynthesis, across northern forests. Across 84% of the study area, Zohner et al. found that warming had opposing effects on leaf senescence depending on when the warming occurred. According to the findings, increased temperatures and leaf growth before the summer solstice was correlated with earlier onset of leaf senescence by a rate of roughly 2 days per degree Celsius (°C) of warming. Warmer temperatures following the solstice delayed autumn leaf senescence by ~2.5 days per °C. "The solstice switch in trees' physiological responsiveness to temperature calibrates their seasonal rhythms and mediates how they react to warm or cool temperatures, now and in the future," say the authors.
For reporters interested in trends, a 2020 Science study reporting results of a large-scale analysis of European trees reported that climate warming was causing tree leaves to fall earlier in autumn. The results were built on growing evidence that plant growth is limited by the ability of tree tissues to use and store carbon.
