Seed masting and fire disturbance synchronisation in white spruce

David Ascoli, Andrew Hacket-Pain, … & Giorgio Vacchiano just had their paper entitled “Climate teleconnections synchronize Picea glauca masting and fire disturbance: Evidence for a fire‐related form of environmental prediction” published in Journal of Ecology (DOI:10.1111/1365-2745.13308). In this blog post, David explains how seed masting in white spruce (Picea glauca) matches fire disturbance and can be an important mechanism of climate change adaptation.

Many plants do not produce regular annual seed crops, but switch between years of abundant seed crops (known as “mast years”) and years with low seed production. Intriguingly, these mast years occur simultaneously in plants living alongside each other, but synchronisation can also extend across hundreds of kilometres. 

This highly variable production of seeds is an important process in ecosystems. Bumper seed crops generate pulses in the populations of seed-eating in small mammals and birds, which in turn cascade across trophic levels down to humans, for example via ticks as Lyme disease vectors. Understanding what triggers the occurrence of mast years, and how the relevant factors respond to climate change, is therefore crucial for the management of natural and semi-natural ecosystems that rely on masting species. 

In ecosystems influenced by disturbances, such as wildfires or windstorms, the timing of seed years is crucial to the ability of plants to regenerate. Disturbances favour seed germination, seedling emergence and establishment by exposing mineral soil, increasing light at the forest floor, and reducing competition by understory vegetation. Since this “window of opportunity” may close quickly, plants have evolved strategies to reclaim disturbed sites and gain a competitive advantage. If a tree is not equipped with specific adaptations for disturbance – such as resprouting or serotiny in fire-prone ecosystems – its only hope for success is to produce producing abundant seeds in the immediate aftermath of the disturbance.

But how do masting species “feel” (“know”) that a disturbance is going to occur? And how can this ability be selected and persist though their evolutionary history? Both masting and natural disturbances can be triggered by specific weather conditions: for example, droughts increase vegetation flammability, while dry and warm summers initiate the development of floral buds. Weather is synchronized on large scales by synoptic climate patterns, such as ENSO or the Atlantic Multidecadal Oscillation (AMO). In our paper, we test the hypothesis that climate patterns conducive to severe large-scale natural disturbances also trigger masting and synchronize seed crops over wide areas.

We analyzed annual time series of the Palmer Drought Severity Index (PDSI), area burned by fire and seed production by white spruce (Picea glauca) in boreal forests of western North America. Burned area was significantly higher during drought events (July PDSI <20th percentile). Also, masting occurred more frequently after years with lower-than-average PDSI and higher-than-average burned area. During AMO+/ENSO+ events, drought and burned area in the whole neoarctic region experienced significant departures from the mean, as did masting synchrony and intensity in the year after any event.

The ability to synchronize seed crops with large-scale disturbances is a key asset for the persistence of Picea glauca in fire-prone landscapes, providing it with higher than expected plasticity and climate change adaptation capacity. Whether other plant species in disturbance-prone forest ecosystems of the world globally have the same ability is an intriguing and new question open for further research.

David Ascoli, University of Torino, Italy

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