During my recent research stay at CREAF in Barcelona, I was analysing long-time data series of seed production of Quercus ilex, an evergreen oak species widespread in Spain. We were testing several recent hypotheses linking weather variation and seed production and found a lot of interesting trends. Our story was just accepted in the oldest biological journal: Proceedings of the Royal Society B!
What we found? Among others:
- phenology synchrony, driven by spring temperature, affects acorn production. In accordance with the phenology synchrony hypothesis recently proposed by Walt Koenig, and already tested by us in other European oak. It seems like this mechanism may be at work in oaks in general, resolving quite a long dispute about the ‘odd’ masting behaviour of oaks.
- phenology synchrony interacts with drought in driving seed production. When long pollen seasons (low temperature = low synchrony = low pollination efficiency) and spring drought are concurrent the crop is the smallest (and vice versa). It is my favourite result because a bunch of past studies looking at seeding and weather correlates across different oak species found that sometimes acorn production correlates with rain in spring, sometimes with temperature. Our results suggest that it may vary depending on which veto (pollination failure or drought-related acorn abortion) is most important in particular system. I believe that both vetoes work in most oaks. However, in water-limited systems, the drought will be most important therefore rain will have the strongest effect in correlative studies. In mesic forests, water is less of a problem so temperature-driven pollination efficiency will appear in correlative studies.
- Veto from year T interacts with veto from year T-1. In short, if the reproduction in the last year was vetoed by drought (or pollination failure) then more resources are available for current season reproduction. It supports recent hypothesis that in oaks resource storage might be passive.
The paper will be online soon, but here is the abstract 🙂
Masting is the highly variable production of synchronized seed crops, and is a common reproductive strategy in plants. Weather has been long recognized as centrally involved in driving seed production in masting plants. However, the theory behind mechanisms connecting weather and seeding variation has only recently been developed, and still lacks empirical evaluation. We used 12-year long seed production data for 255 holm oaks (Quercus ilex), as well as airborne pollen and meteorological data, and tested whether masting is driven by environmental constraints: phenological synchrony and associated pollination efficiency, and drought-related acorn abscission. We found that warm springs resulted in short pollen seasons, and length of the pollen seasons was negatively related to acorn production, supporting the phenological synchrony hypothesis. Furthermore, the relationship between phenological synchrony and acorn production was modulated by spring drought, and effects of environmental vetoes on seed production were dependent on the last year environmental constraint, implying passive resource storage. Both vetoes affected among-trees synchrony in seed production. Finally, precipitation preceding acorn maturation was positively related to seed production, mitigating apparent resource depletion following high crop production in the previous year. These results provide new insights into mechanisms beyond widely reported weather and seed production correlations.