B4WarmED forest warming experiment: phenological responses of dominant tree species at the temperate-boreal ecotone
Climate is a major driver determining the location of biome boundaries worldwide. Such ecotones often encompass the range boundaries of more broadly distributed species. In northeastern MN, the southern boreal forest meets the eastern deciduous forest with numerous species at either their northern or southern range limits. Based solely on climate envelope models, many of the species found in this region are considered likely to decline or increase in abundance, and contract or extend their range during the 21st century. Yet little information exists to evaluate the validity of such predictions or the mechanisms that might underlie such changes. The Boreal Forest Warming at an Ecotone in Danger (B4WarmED) experiment explores the potential for projected climate warming to alter tree function and species composition at the boreal-temperate forest ecotone through effects on juvenile physiology, growth, and survival. Here, we present results that suggest the potential for major alterations in tree leafing phenology associated with +1.7°C and +3.4°C warming. We warmed small plots using infrared heat lamps and resistance cables buried in the soil. We measured phenology twice weekly throughout the growing season on seedlings of five boreal and five temperate tree species (~150 individuals per species were monitored).
In general, warming caused earlier leaf out in spring and later leaf senescence in fall, extending the photosynthetic growing season for all species by ≈10 and ≈20 days on average for +1.7°C and +3.4°C warming, respectively. Changes of 10-20 days in the “leaf on” period could have substantial positive impacts on annual gross primary production and possibly net ecosystem exchange. In spring, all species responded to spring warming by advancing the absolute date of budburst, but several lines of evidence support the role of other factors, such as photoperiod or winter chilling, in co-determining observed responses. First, a number of species showed non-linear responses in absolute day of year of budburst across levels of warming: specifically, some species did not advance the date of budburst in +3.4°C compared to +1.7°C treatments. Second, using thermal time to budburst rather than absolute date of budburst, we found that all species required more warming to break bud in warmed treatments. Better data on phenological responses at the population and community level as well as elucidation of species-specific environmental cues for phenology will further our understanding of the consequences of phenological shifts for ecological processes now and in the future.