title: The plasticity of masting: understanding how and why temporal reproductive patterns vary in trees
Creators: name: Foest, Jessie
Publisher: name: University of Liverpool Repository
Year: 2024

description: Long-lived plants like trees often exhibit highly variable, yet synchronised reproduction, commonly referred to as ‘masting’. The boom-and-bust patterns of seed production have complex and far-reaching effects on regeneration and food webs. Understanding how and why masting varies across space and time is crucial, as masting is linked to a spectrum of ecological and environmental challenges, including mitigating climate change-induced tree mortality. Masting plasticity (here used as a synonym of masting variability) signals that regeneration potential might differ between populations, and is related to the stability of food supply for wildlife across species ranges. 

Studying the plasticity of masting has historically been complicated by the difficulty of acquiring long-term records across species ranges. This thesis, which examines the plasticity of masting and its drivers, would have been impossible without collating the many observations collected by observers across the world. These data are part of MASTREE+ (Hacket-Pain et al., 2022), the English beech mast survey (Packham et al., 2008), ongoing beech seed sampling efforts across Europe, and my own fieldwork. This thesis also builds on open-access climate (Cornes et al., 2018; Fick &amp; Hijmans, 2017), and species range data (e.g. EUFORGEN, 2022).

I found that intraspecific variation in masting can exceed interspecific variation, and that the commonly held environmental stress hypothesis does not explain how masting varies across space. When examining the temporal variability of masting, I found that climate warming is driving the decline in masting across the species range of European beech (Fagus sylvatica L.). My work reveals that seed collection methods contribute to observed masting plasticity. Not only was seed collection method an important predictor in the spatio-temporal analyses, but the loglinear relationship I found between seed crop estimates of two seed collection methods can lead to different masting metrics.

The insights from this thesis are relevant for a broad spectrum of professionals including biologists, ecologists, foresters, land managers, and public health experts, and inform our understanding of ecosystem dynamics. The findings illustrate the need to acknowledge, incorporate and examine masting plasticity. Masting effects on regeneration and the food web may be more variable than previously assumed. The breakdown of variability in European beech seed production across the species range warns that a covert mechanism is underway which may hinder the regeneration potential of this species under climate change, with great potential to alter forest functioning and community dynamics. Lastly, the effects of sampling methodologies on masting metrics underpin the importance of caution when combining datasets, and the need for calibration. This work also helps to translate between two sampling approaches, and the wider adoption of the highly efficient ground-based timed count may help to expand data sampling efforts and improve our understanding of masting plasticity.