Beyond the Bioclimatic Law: Geographic adaptation patterns of temperate plant phenology

Almost a century ago, observed geographic patterns of plant phenology (such as leaf-out and flowering) were summarized in Hopkins' Bioclimatic Law. This law describes phenology as varying along climatic gradients by latitude, longitude, and altitude. Yet phenological patterns are not only affected by contemporary climatic differences across space, but also by underlying geographic variations in plant genetics that arise from long-term climatic adaptation. The latter influence on geographic patterns in phenology has been undervalued to this day, mainly due to the difficulty of quantifying it. This study outlines a methodology for bridging this knowledge gap through delineating geographic adaption patterns using common garden and cloned plant phenology. Through synthesizing existing literature, typical geographic adaptation patterns in both spring and autumn phenology of many temperate tree species are identified. Under uniform environment, spring leaf-out of colder climate-adapted populations of a certain species is either earlier than warmer climate-adapted ones due to lower thermal requirements, or later because of higher chilling (for dormancy release) demands. The former leads to a countergradient pattern as it is opposite to an in situ observation, while the latter leads to a cogradient pattern. Autumn leaf senescence, on the other hand, expresses a consistent cogradient pattern that is related to latitude and constrained by the populations' varied photoperiod requirements. These geographic adaptation patterns allow a clearer understanding of geographical variations in phenological responses to climate change, and provide a theoretical basis for spatially explicit phenological models. In addition, given that these adaptive patterns reveal genotype-based variabilities, they are potentially useful for more accurately tracking phenology-dependent ecosystem processes (e.g. species distribution) and non-weather-related vegetation changes. As a unique subfield of physical geography with broad environmental implications, this line of research needs to be further developed by furnishing a stronger and more explicit spatial structure into current phenological studies.