COS 154-6 - The timing of spring green-up affects tree reproduction in the temperate and boreal forests

Abstract: Understanding the drivers of tree reproduction that measures the regeneration potential of forests is a goal of global change research. Phenology is the intra-annual rhythm of the start, progression, and ending of vegetation activity. The timing of spring green-up could affect seed production through its effects on pollen production (e.g., the phenological synchrony hypothesis). However, evidence supporting this hypothesis is still limited to a few locations and few species, due to the large investment needed for field measurements on both variables. Are wind-pollinated species more sensitive to spring green-up than animal-pollinated species? Are masting species (i.e., volatile, quasi-synchronous seed production at lagged intervals) responding more to spring green-up than non-masting species? Which year of spring green-up is the most important for genera that have multiple years of seed development (e.g., 2-year red oaks, 3-year pines)? To answer these questions, we synthesized seed production data from 144,770 trees, 2,329,608 tree-years, and 103 species within the Masting Inference and Forecasting (MASTIF) network with the remotely sensed timing of spring green-up from the Moderate Resolution Imaging Spectroradiometer (MODIS) in the temperate and boreal forests.

Species concentrates more of its reproductive output in years with earlier spring green-up in a large majority of genera, with notable exceptions in Fagus, Liriodendron, and Juglans. Consistent with this finding, seed production also increases at sites that have advanced timings of green-up. We did not find differences in within-species response to spring green-up between wind- and animal-pollinated species. Similarly, the most volatile species are not those having the highest sensitivity. Spring green-up in the current year has a larger influence on seed production in the white oaks compared to that in red oaks, perhaps related to the 2-year seed development in red oaks. For conifers with multiple years of seed development, spring green-up in the current year (t), the prior year (t-1), and the 2 years prior (t-2) have similar effects on the 3-year Pinus. Spring green-up in t-1 and t is equally important for the 2-year genera Tsuga and Picea but not for Abies. The timing of spring green-up is expected to be further advanced due to contemporary warming, these findings have broad impacts on the understanding of tree production and how it might influence future forest regeneration in a changing climate.