Oaks 'n' Folks Newsletter Acorn Production by California Oaks: A Role for Pollen Limitation? by Walt Koenig, Research Zoologist, Hastings Reservation, UC Berkeley Like most other flowering plants, the flowers of oaks must be fertilized in order to produce seeds. Oaks are "monoecious," which means that both the relatively showy male flowers (catkins) producing the pollen required for fertilization and the relatively obscure female flowers that eventually turn into acorns are grown by each individual. However, unlike many plants, oaks are generally "self-incompatible," and the flowers produced by one individual must be fertilized by pollen produced by another. How does pollen get from the catkins to the female flowers? It will come as no surprise to any of you who suffer from hay fever that, for both oaks and a few other major groups such as grasses, this task is performed by the very air we breath. In other words, these species are "wind pollinated" and pollen simply blows from one tree to the next. This being the case, it was generally thought that there was always enough pollen out there to fertilize any females flowers that were produced, and that whatever the cause of the strikingly high variability in the number of acorns produced from one tree to the next or from one year to another, it wasn't due to differences in pollen availability. Recent research has changed all this in the last several years. In my own study of acorn production by California oaks centered at Hastings Reservation in Monterey County, I ignored the possibility of pollen limitation until a colleague of mine convinced me that there was an easy way to test whether it might be important or not by comparing the phenology of trees with the number of acorns they produce. Why phenology? As anyone who's looked carefully at oaks in the spring knows, there's a lot of variation in when individual trees leaf out and flower (Fig. 1). If pollen is not always present in superabundance, then trees flowering in the middle of the season, when the majority of other trees are flowering as well, would suffer the least from pollen limitation while trees flowering early and late in the season, when few other trees are producing pollen, would suffer the most. Although we currently have no direct way of measuring the pollen of a particular oak species present in the air, or even how many oak flowers are fertilized, the prediction of this "pollen limitation" hypothesis is that early- and late-flowering trees should end up having relatively few female flowers fertilized and thus produce few acorns, while trees flowering in the middle of the season should enjoy relatively high fertilization success and produce lots of acorns. 
Fig. 1. Variation in phenology in valley oaks at Hastings Reservation: two trees on 7 March 2005, one of which (in the foreground on the right) is almost completely leafed out while the other (in the background on the left) is still completely leafless. Starting in spring 2003 I initiated a test this prediction using the 86 valley oaks (Quercus lobata) at Hastings that we've been surveying for acorn production since 1980. Starting at the beginning of March, when the first trees are beginning to leaf out, I checked trees once a week, keeping track of when trees leafed out and whether the catkins were sufficiently mature to be shedding pollen. Then in the fall we conducted a visual survey of the acorn crop on each of the trees, counting as many acorns as we can in a 30 second period. These data allow us to relate phenology of individual trees to their acorn productivity in order to see what kind of role pollen limitation might be playing. After three years the results are still incomplete,
but indications suggest that phenology, and by inference pollen limitation,
is considerably more important to acorn production than we would have
previously imagined. So far the best indication of this comes from the
relationship between when trees leaf out and their long-term productivity
(Fig. 2). In both years shown in the graph there was a strong relationship
between phenology and productivity, but the relationships were very
different in the two years. In 2003, first leaf date was spread out
over nearly two months, and trees that leafed out later were generally
more productive, with the exception of the single tree that leafed out
during the week of 19 April. In contrast, trees leafed out synchronously
over only a 5-week period in 2004 (mostly during the weeks of 15 and
22 March), and those that did so during the middle of the season were
on average the most productive, while those leafing out earlier and
later were less productive, increasingly so the farther from the middle
of the season they did so. 
Fig. 2. The relationship between when individual valley oaks leafed out at Hastings Reservation during 2003 and 2004 and their long-term productivity in terms of the mean number of acorns counted on the trees during fall visual surveys, 1980 – 2004. This latter pattern is exactly that predicted
by the pollen limitation hypothesis. What I find particularly exciting, however, is the possibility of a completely unexpected relationship between phenology and annual acorn productivity. As is evident from the figure, variance of when trees leaf out and flower was very high in 2003 and quite low in 2004. Assuming that pollen limitation occurs and that other things are equal, this difference would be expected to result in there being considerably less pollen available at any one point in time in 2003 compared to 2004, and intriguingly, it turns out that 2003 was a very poor year for acorn production of valley oaks while 2004 was a very good year. Variance in first leaf date in spring 2005 was intermediate compared to the prior two years; will the acorn crop this fall be intermediate as well? By the time you read this we’ll have conducted the 2005 California Acorn Survey and know the answer. In any case, the possibility that synchrony of phenology might be determining not only variation in productivity among individual trees but also variability among years is one that I would have dismissed out of hand only a few years ago. Even after 25 years of counting acorns, we remain a long way from understanding the factors affecting acorn production by California oaks. Return to current issue of Oaks 'n' Folks Read Previous issues of Oaks 'n' Folks Editor: Adina Merenlender
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