Seed Set, Phenology, and the Redbud Tree
This extrapolation may be inappropriate;
Regarding seed set in redbud, and using where-seeds-should-be-but-aren't in a pod to measure seed set, one additional caution.
In another legume tree, honey locust, there is a seed predator (the inappropriately named bruchid beetle Amblycerus robiniae) that will destroy seeds so that some successful seed-set would be undetectable just by looking at apparent "vacancies" in the pod. You would think that in such a common tree that the biololgy/bionomics of seed predation (OK, herbivory) would be well understood, but that is not the case.
So be on the lookout for seed predators that might confound seed set calculations. It matters in honey locust, but apparently there are no black locust seed predators. I am not familiar enough with redbud - it ain't where I roam too much - to know if this applies.
Redbud is also a legume. So there should be less maintenance (nitrogen). It seems particularly attractive to H. laboriosa, which is an important pollinator of wild and cultivated vaccinum in the southeast. So, it would be very useful data set for this very important bee. I have been planting them and crucifers to enhance H. laboriosa habitat for better pollination on my farm. According to the NC State Redbud Breeding Site it is may be self incompatible. It has to be cross pollinated for seed development. Additionally, Don Orton (Coincide), Lists it as “a useful indicator plant with concise blooming Stages observable over a period of 3-4 weeks”. There is a lot of genetic variation. I have found in working with Crape myrtle that certain bees prefer certain cultivars of this plant. From and ecological standpoint, it is also a chance to find cultivars or combinations that would have the most benefit to native bees as well as just monitoring the bees themselves. Seed development may occur as pollination occurs. This provides another opportunity to access pollinator preference and efficacy over time as well as from north to south. I do want to see cultivars developed that help pollinators.
I have found the tree hard to collect from because they can get tall enough that netting fast flying bees can be a problem.
There are probably seed predators. I have worked with weevils and stinkbugs and usually you can tell when a seed has been eaten. A seed predator would not always eat the same seed at the same time. For example a high number of missing or small seed that should be developing at a certain time would be more probably a pollinator issue.
Redbud is Cercis canadensis, at least in eastern N. America; varieties/other species are found elsewhere. In addition to possible pollinator issues, redbud does suffer from a number of fungal diseases, especially as it ages and/or is stressed. As we put these trees under more stress (by planting them as street trees when they're actually forest understory plants as well as what we are doing to alter climate patterns), they are more susceptible to disease and death. There is a graduate student at Clemson (Isaac Park) who is working on a phenology project using redbud as one of his subjects; I'll certainly pass the ideas along to him.
From: firstname.lastname@example.org [email@example.com] On Behalf Of Sam Droege [sdroege@...]
Sent: Friday, August 20, 2010 5:38 PM
To: firstname.lastname@example.org; lebuhn@...; Jake Weltzin
Subject: [beemonitoring] Seed Set, Phenology, and the Redbud Tree
I was looking at some of the copious seed pods produced on Redbud Trees (Celtis americana? ... Internet not handy) this summer in Kentucky and noticed that it was very easy to both count the number of seeds produced directly through the pod (the pods were green) and to note the number of places in the pod that should have had a seed, but, did not. Since the Redbud is a widely occurring understory/edge tree in at least Eastern North America and widely planted as an ornamental (not sure about the West) and short in stature (making the seed pods easy to inspect) and providing abundant nectar and pollen in the spring (see email from Mark Kraemer below); I would propose that it has the characteristics that would make it a good candidate for monitoring both phenology and to investigate patterns and changes in seed set which should be related to pollinator abundance. That said, I am not a pollination ecologists and am thus likely am over-simplifying the situation in regards to interpretation of rates of pollination. However, it seems likely to me that the primary factor decreasing the seed set of Redbuds (over years) would be loss of its pollinators.
The following characters seem to make Redbud an appealing (at least to me) candidate for looking at geographic and habitat patterns of phenology and seed set
- It is widely planted in urban/suburban/rural human landscapes
- It can be readily found in the wild in some parts of North America
- The tree is easy to identify, short in stature and the pods are low hanging
- It is easy to identify and count the seeds (and easy to photograph them)
- The seeds hang around for a long time
- It blooms are highly attractive to spring bees
- It can be used to gauge spring phenology
I would be interested in what people think might be the interpretational issues of variations (both long and short and geographic) of seed set in Redbud. Another issue might have to do with whether there is useful information in the absolute number of possible seeds in a pod...is this fixed or, if not, what are the possible controlling variables?
Email from Mark Kraemer
Here are my thoughts on using Osmia as a monitor for climate change. I’ve worked with O. lignaria in Virginia since about 2002.
It is likely that each geographical population of bees has adapted to local climate conditions and has its own chilling and perhaps heating degree day requirements for spring emergence. Thus, sending cohorts from one geographical region to other regions may be a problem. For example, if bees from a colder winter climate area are sent to a warmer one and they do not get enough chilling they will have poor emergence extended over a longer period of time, similar to fruit tree bloom when not enough chill units have been accumulated. Nest building would be extended late into the season but have nothing to do with climate change. If bee tubes are sent out in late winter after chilling requirements are met, you still have the problem of variable and unknown temperatures during transit. It’s best to use local bee populations if possible.
I’d use eastern red bud bloom for phenological comparisons, at least in the Eastern U.S. I found female emergence in Virginia and North Carolina closely coincides with initial bloom of Eastern redbud. Eastern redbud provides not just pollen but nectar early in the season when few if any other trees/shrubs are flowering. It’s a very common forest understory tree and an ornamental found throughout most of the eastern U.S. The vast majority of pollen (>90%) that I sampled from nest cells created in the first week was redbud (4 sites), even when the trees were not nearby the shelter boxes. Nesting was very poor and late in one site where redbuds were not found. I would expect first nest completions to be 7-10 days after initial redbud bloom, depending on air temps. Some of these phenology results should appear soon in Environmental Entomology.
I’d like to know whether our observed phenological link of female emergence with Eastern redbud bloom occurs elsewhere, and the pollen composition of the early nest cells. If this relationship is true, and because climate has changed so radically during the past few million years, I’d speculate that the bees and trees have co-evolved a very similar mechanism for coordinating their emergence/bloom through a broad range of climate change.
Sam Droege sdroege@...
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USGS Patuxent Wildlife Research Center
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Wanting Sumptuous Heavens
No one grumbles among the oyster clans,
And lobsters play their bone guitars all summer.
Only we, with our opposable thumbs, want
Heaven to be, and God to come, again.
There is no end to our grumbling; we want
Comfortable earth and sumptuous Heaven.
But the heron standing on one leg in the bog
Drinks his dark rum all day, and is content.
- Robert Bly