A plant’s reproductive cycle doesn’t end with pollination, but continues with seed set and fruiting, then seed drop and dispersal. Finally, conditions must be right for the germination of a new plant. This cycle is conditioned by both environmental factors and associations with other living creatures that run a continuum from antagonistic to mutualistic. Arctostaphylos or manzanita is a genera of plants in the family Ericaceae in which most species are adapted to a dry climate of sporatic rainfall and a regime of fire. The plant sets seed in a drupe, a fruit where the seed is encased in a hard resinous endocarp. The fruit is eventually stripped away to the endocarp which can stay as a single stone or can break apart into various pieces down to a single seed. Seed images from Rancho Santa Ana Botanical Garden (Rancho Santa Ana Botanical Gardens, 2004) suggest that whether the endocarp stays as a single stone or breaks apart is species dependent. A. pungens and A. pringlei are two species found throughout Arizona. The seed images suggest that A. pringlei has a single stone while A. pungens breaks apart. I investigate this claim by gathering berries from various plants of both species and examining the endocarp. I have found that although each species shows a tendency that matches the seed image claims, there is great variety from location to location. I speculate on this variety and look into possible adaptive reasons for it. A statistical analysis will follow in a future post.
Materials and Methods
Since February I have been going to various locations in Arizona looking for A. pungens and A. pringlei. I have taken images, a voucher sample, leaf samples and berry samples, coordinates and elevation whenever possible. I currently have 12 A. pungens, 5 A. pringlei, and 1 Arbutus arizonica berry samples. For each berry counted I removed the exocarp and the mesocarp to get to the endocarp. The endocarp either fell apart immediately or from a slight pressure. Stones would not break even when I put the flat end of a knife on top of it and applied pressure. The endocarp either did not break or broke apart into from 2 to 7 pieces. Only once did I count 7 pieces. On one or two occasions a seed was missing or rotted away and I discarded the berry. I glued all the endocarp pieces of every berry to a sheet, one sheet for each sample location. The counts were put in a database [here current]. I used the R statistical language (R Core Team, 2012) to create the graphs.
The number of berries for each sample varied as sometimes there weren’t that many berries on the plant. The counts were normalized so the graphs show a normalized distribution, the total adds up to 1.
Map of berry specimens collected.
In California and Baja, A. pringlei ssp pringlei is characterized as having a single stone (Jepson, 2013), (Rancho Santa Ana Botanical Gardens, 2004) while in Baja and Arizona A. pringlei ssp drupacea is characterized as having unfused stones (Vasey, 1999). Looking at a graph of all A. pringlei ftuit and a set of graphs for each individual plant sampled it can be seen that there is a great deal of variety although two plants out of the five clearly have a majority of single stones.
A. pungens sampled look less “stoney” than A. pringlei with one plant out of twelve having single stones in the majority and a combined graph showing a majority of “broken” endocarps as opposed to single endocarps.
As can be seen in the series of three images above, seed and endocarp development takes place simultaneously. By the third image the fruit was already hard to cut with a dissecting knife. There has been some study of the genetics of what is called fruit patterning in Arabidropsis (Liljegren, Roeder, Kempin, Gremski, Østergaard, Guimil, Reyes, Yanofsky, 2004) but the pathways still remain a mystery and Arabidropsis fruit are very different from Arctostaphylos. What seems to be happening is that tissue that looks like the same mealy tissue that fills the mesocarp somehow makes it’s way between the seeds, in some berries the endocarp is visibly separated, in others the stone comes apart with slight finger pressure, in still others the stone is like a small rock. Even if there is a tendency towards stoniness in the A. pringlei species, there is a great deal of variation both between it and A. pungens and within each species. Is there a mechanism to this variation or is it just random fluctuation?
A. pringlei occurs in three other isolated regions besides Arizona: across extreme southern Nevada and Utah north of the Grand Canyon, in the Transverse Ranges of southern California and in parts of Baja California Norte (SEINet, 2013). In the Transverse Ranges A. pringlei has been found to hybridize with other species of Arctostaphylos (Chester, 2008). A. pungens shares the same range and in addition, extends deep into Mexico and across New Mexico and southern Transpecos Texas (SEINet, 2013). A. pungens generally grows at a lower elevation than A. pringlei but it can extend into higher elevations and there are large areas in sympatry. I have seen A. pungens growing among A. pringlei at it’s highest elevations in the Santa Catalina Mts. So far, I know of no evidence that A. pungens and A. pringlei hybrids exist although the opportunity is there. Twice I have found the two species growing so close together their root systems had to be intertwined and every time I have found A. pringlei I have found A. pungens growing in part or all of it’s range.
Knowledge of character variation is important as it helps differentiate species. Arctostaphylos is believed to have originated in the California Floral Provence and most Arctostaphylos species are endemic to that region (Hileman, Vasey, Parker, 2010). One species, A. uva-ursi, has a large nearctic range throughout North America, Europe and Asia. Besides many know cases of hybrids there is evidence of hybrid species (Wahlert, 2005). This range of character variation, known hybrids and hybrid species is much like another complex genera, Quercus, the oaks. The biological species concept (Mayr, 1942) states that reproductive viability is the true test of species. Two populations are considered two species if they cannot reproduce. This is an important concept but as with many biological concepts it represents two ends of a continuum. Another important species concept is the ecological species concept (Simpson, 1961), (Valen, 1976) which states that two species can be reproductively viable but never reproduce because of life history differences brought about by different environments. One example of this are species that grow on specialized soils like the serpentine soils in California. Seeds from a plant growing in these soils would not grow in other soils and vice versa. In addition to species there are the categories of variety and subspecies. What exactly these two terms mean is vague and contentious (Tribe.net, 2006). In an important planting experiment with East Coast A. uva-ursi (ROSATTI, 1983), it was shown that various local varieties and/or subspecies of the plant could be turned into other subspecies and/or varieties by simply growing them with different soil and water conditions. Thus there was a failure to reject the null hypothesis that they were the same species. I would think that experiments such as this one along with more information on character variation and molecular data will eventually sort out a robust phylogeny.
Sources of seed fusion variation
Plants are affected by various environmental factors and by associations with different lifeforms. These factors and associations change with the seasons and and fluctuations of the seasons and with the reproductive state of the plant. A study of greenleaf manzanita (A. patula) found associations with over 500 arthropod species (Valenti, Ferrell, Berryman, 1997). One says that a plant is “adapted” to it’s environment but what this means biologically is an important question. A plant starts out with a single genetic structure but the germination of a single seed is the result of a myriad of factors that come into play during it’s life history. Arctostaphylos are long-lived plants that are adapted to a regime of fire. Seeds only sprout after a fire and some species have a burl, a large mass just below the surface of the ground to regrow the burned plant. All species use a seed bank, seeds are stored in the ground below the plant where they wait for the combination of fire heat and charate, burned plant matter, to sprout (Meyer, 2008). The two Arizona species in question don’t have a burl so they use the seed bank exclusively, they are obligate seeders.
Below is a crude map of possible environmental factors and associations during the different reproductive states of a plant. The fusion state of endocarp and the viability of the seed is related to three dynamic processes: fire, seed bank and dispersal. A fused endocarp would mean survival from hotter fires, less chance of being digested by large animals or eaten after storage by smaller ones. On the other hand, a fused endocarp or a piece of a fused endocarp is considered one seed as only one plant survives the competition between seedlings (Keeley, Zedler, 1978) though a larger amount of seeds in the chunk expands the chances of one surviving. Notice from the data that a completely unfused endocarp containing five to seven individual seeds is very rare in both of the two species sampled.
Seed banks are very dynamic, being replenished by fallen berries and reduced mainly by predation and some decay (Kelly, Parker, 1990). The number of viable seeds found in different seed banks studies has been low (Keeley, 1977), (O'Neil, 2009), (Tyler, Odion, Meade, 1998), in the order of a few percent. The endocarp, even if not fused doesn’t fall apart until the exocarp and mesocarp rots away or is eaten. Predation suggests dispersal but I can’t find any direct research on this for manzanita although rodents have been shown to eat the seeds on the spot (Keeley, Hays, 1976).
Sources of Error and possible additions
Mainly I need more power in that I need more plants sampled, especially A. pringlei. Also, I need to find what the optimal amount of fruit that need to be collected. Some plants had hardly any fruit. I have to also test for prejudice in selecting seeds. Am I taking just the bigger seeds cause they are easy to grab and easier to peel? Testing for seed size variation would be easy if I had a scale but I can get an idea with digital images of the vouchers and measuring the area of fully fused endocarps and individual seeds. Does the height on the plant I am collecting at mean anything? Do the endocarp of fruit on lower branches differ from higher branches? At the moment I have elevation and lat/long as other variables but I’m wondering what else would be needed. Some, but only some of the collection locations have weather data. Possible patterns in how the stone splits would be interesting to look at as a stone that divides into two sections can contain a five seed piece and a one seed piece or a two seed piece and a four seed piece or two three seed pieces. This can be found by visually inspecting the vouchered endocarps.
There is some question as to the structure of the model to be used. The natural tendency would be to nest the species within the genera but as there are only two species there is really no need. Also, there is no guarantee that in a study of several species there would be any pattern that followed a phylogeny. So the highest level would be species with each plant nested within. With each plant having a set of berries each berry could be considered a repeated count, or they could be taken as a single count. I think there is reason to look at both these models.
The endocarp is the final strategy the plant has to protect the seed in the seed bank. It seems that with Arctostaphylos, this strategy has two extremes, protecting all the seeds in a single stone at one end and encasing each seed separately on the other. These strategies relate to viability, the ability of a seed to germinate. Recall that each chunk will only produce a single plant . One strategy relies on many chunks of endocarp containing less seed material while the other strategy relies on less chunks containing more seed material. Preliminary data on seed fusion in Arizona suggests that both A. pungens and A. pringlei have a strategy that falls somewhere in between with A. pringlei tending towards a single stone and A. pungens falling somewhere in the middle.