Tuesday, November 10, 2015

Manzanita Reproduction

I recently came across Fall 2007 edition of Fremontia, the Journal of the California Native Plant Society. This edition was a special issue devoted to Chaparral. I love Fremontia and I especially love this special edition. There were two articles that particularly impressed me. One dealt with the Manzanitas and Ceanothus (primarily Arctostaphylos spp.) and the other dealt with geophytes, ie. plants that arise from bulbs or corms. In this post I'm going to summarize the article on Manzanitas and talk about how it applies to gardening with natives and to my garden in particular.

New Growth on Del Mar Manzanita (Arctostaphylos glandulosa ssp. crassifolia) in my front yard
The Manzanita article was titled Diversity and Evolution of Arctostaphylos and Ceanothus by V. Thomas Parker. The author explained that there are two main lineages of Arctostaphylos, and these are differentiated by how they respond to fire. The two types are burl-formers and obligate seeders. This in itself is a fascinating fact. It suggests that there was a parent taxon somewhere in the past 1.5 million years that was subjected to episodic fire, with some of its offspring responding one way and others responding another way. 

Burl-formers are also called stump-sprouters and crown-sprouters. The burl is an enlarged woody rootstock or junction between the trunk and the roots. Here's how the sprouters work: these species are capable of producing new growth from the burl after a fire as long as the burl is not irrevocably damaged. The top of the burl usually gets charred, but the rest of it is very tough and mostly underground, so it's usually okay unless the fire is unusually hot. New growth comes out from the undamaged part of the burl. All branches can be destroyed, but if the burl is not totally burned up new branches will sprout out almost immediately after the fire, often within days. Burl-formers do produce seeds, and some of them do germinate after a fire, but the majority of seedlings die within a year or two (unless the fire is exceedingly hot and a large number of burls are destroyed). In general, seedlings can't complete with re-sprouting burls that have huge, intact root systems.

I should mention here that Arctostaphylos isn't the only genus that uses stump-sprouting after fires. Heteromeles arbutifolia (Toyon) and Xylococcus bicolor (Mission Manzanita) also stump-sprout, as do many others. It's a fairly common chaparral adaptation to fire.

Arctostaphylos rudis (Sand Mesa Manzanita) in San Luis Obispo County.
Photo by Akos Kokai [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)]
Manzanitas that are obligate seeders do not form a burl and are typically killed outright by fire. However, obligate seeding species of Arctostaphylos outnumber the sprouters, and their response to fire is equally effective because they produce lots of seeds that are primed to germinate by the chemicals in smoke. Germination is not immediate, but when the first rains come after the fire, germination is rapid and widespread. There are no old plants to compete with the seedlings, and an entirely new generation is created. Thus, both lineages of Arctostaphylos are able to come back after a fire. There are limitations to each approach, but in a system that is not too screwed up by people both lineages do well. The key difference between the two lineages is what it means for their genetics. This is where it gets really amazing.

Los Padres National Forest, Aug. 2009
Photo by Airman 1st Class Andrew Lee
The burl-formers don't experience a lot of genetic change over time because the original plants are simply regenerating themselves. The fire has changed nothing in the genes of these plants. On the other hand, obligate seeders experience a lot of genetic change with each fire. It isn't that fire directly causes genetic change. Rather, fire kills all the adult plants in the burned area, wiping out that generation in that place. The thousands of seeds which have lain dormant in the soil for many years or decades will have slightly different genetics from their parents, the result of unique combinations of pollen and ovules. It makes for essentially 100% genetic turnover in the local population. The new genetic diversity created by this event creates some plants that are very well suited to their environment and others that are not so well suited. It should also be noted that Manzanitas of both lineages tend to hybridize readily, and over time all this cross-fertilization and hybridization by obligate seeders will produce enough genetic variation to form new taxa (species, subspecies and varieties). Hybridization doesn't mean as much for the burl-formers because production of new plants from seed is less important to their short-term survival. 

The behavior of obligate seeders is most dramatically seen in the Arctostaphylos-dominated chaparral communities of the central coast. I'm going to throw around some numbers from Parker's article. California is home to about 95 taxa of Arctostaphylos; 42 of these occur in the narrow coastal band between the San Francisco Bay and Monterey Bay; 32 of these are referred to as "narrow endemics" that are restricted to very small geographic areas. Plus, there are several more narrow endemics between Monterey and Santa Barbara. Many of these are are on the CNPS Inventory of Rare and Endangered Plants. Why is this a big deal? Virtually all of these narrow endemic Manzanitas are obligate seeders, and this is the key to their rarity. 

In Monterey County alone the following 14 rare taxa are found (each species name below is clickable and will take you to the appropriate page on Calflora):

What the above points out is the fairly incredible amount of genetic diversity generated by the obligate seeders of the central coast. In every little micro-climate and micro-niche there is a Manzanita that is specifically adapted to the conditions found there. It is also noteworthy that in this central coastal strip fire is currently not as common as it is in warmer and more arid parts of the state. Burl-formers would have no particular advantage in the absence of fire, but also the obligate seeders would have fewer opportunities to get rid of the old generation and start a new one. Parker states that these narrow endemics have evolved quite recently, in the last 10,000 to 20,000 years, suggesting that this amount of speciation occurred very rapidly as a result of just a few major fire events.

One implication of the foregoing is that the obligate seeder taxa are potentially not very stable in the long run. A big fire in Monterey County could wipe out one or more rare species, and the seedlings that would emerge after the fire would be different from the parents in ways we cannot predict. All of these rare species will have hybridized with each other and with more common species. Each seedling would have slightly different genetics, and it would take years to see which ones would survive and dominate. The chances of the original species reestablishing themselves after such an event is slim.

Manzanita in My Garden. I live in San Diego County, not the central coast, so the Arctostaphylos species here are naturally going to be different from those in Monterey or Santa Cruz. There are some rare species of Manzanita in my region, but only a handful compared with the 14 found in Monterey County. One of San Diego's rare taxa is A. glandulosa ssp. crassifolia (Del Mar Manzanita), and I am fortunate to have a nice specimen of it (shown in the photo below). The basic species A. glandulosa is quite widespread, occurring in the coastal strip from Oregon to Baja. However, ssp. crassifolia is limited to a very small ribbon of southern maritime chaparral between Torrey Pines and Carlsbad. You can see it on the Calflora map. Zoom in on San Diego County to see exactly where it is found. A. glandulosa has a number of other subspecies as well. Some are common and some are rare. An academic paper on the species states: "Particularly in the southern half of its range it exhibits complex patterns of morphological variation that have long presented taxonomic challenges." (Subspecific Variation in the Widespread Burl-Forming Arctostaphylos glandulosa, Jon E. Keeley, Michael C. Vasey and V. Thomas Parker. Madrono, Vol. 54, No. 1, pp. 42–62, 2007).

A. glandulosa ssp. crassifolia with flowers and fruit in my front yard
Is ssp. crassifolia a sprouter or a seeder? It does both. It has a burl and after fire it sprouts from the burl. But the seeds are also activated in response to fire. This dual approach is called Facultative Seeder. Perhaps this helps explain the rarity of ssp. crassifolia, as well as the taxonomic challenges of the many subspecies. Under ordinary conditions it would be expected that stump sprouting would dominate after fires, producing genetic stability. Seedlings would come up but these would typically not survive. But suppose there were some fires in the past which killed all the mature A. glandulosa plants in a given area, destroying all the burls. This can happen in an extremely hot fire. Then recruitment from seeds would have taken over. The new genetic diversity contained in these seeds would have created numerous new versions of A. glandulosa, some of which would have been especially well suited to a particular soil or micro-climate. The one that became ssp. crassifolia would have been a seedling that was best adapted to the sandstone soil and mild, maritime climate where it occurs, while other versions of the same species would be better adapted elsewhere. In response to subsequent low-intensity fires, individuals of ssp. crassifolia would stump sprout, maintaining stable genetics in the subspecies. I would expect ssp. crassifolia to remain stable until the next catastrophic fire in which most or all of the ssp. crassifolia burls are destroyed. Then we might get entirely new subspecies or varieties arising from all the seeds in the soil. In this scenario, unburned individuals of ssp. crassifolia, such as those in gardens, might be the only source for the original genes of the taxon. This is just speculation on my part and I may have it completely wrong, but I think it makes sense.
Mojave Yucca (in bloom), Dudleya, Summer Holly, Del Mar Manzanita, and Toyon in my front yard
Taxonomists must go crazy trying to sort out all these variations in Arctostaphylos. Or maybe they love it. In addition to all of these species, subspecies and varieties, we have other genera that exhibit similar traits, including Comarostaphylos, Ornithostaphylos, and Xylococcus, all in the same family. In a more recent Fremontia (May 2015), Lee Gordon and several co-authors examined the reproductive behavior of Mission Manzanita (X. bicolor). In view of its apparently low seedling survival rate, they asked whether Mission Manzanita is in decline. It, too, is concentrated in San Diego County. Over the eons it seems to have been more stable genetically than Arctostaphylos. I say that because there is only one species in the genus, and no subspecies or varieties. My guess is that it is a Facultative Seeder that can reproduce from seed but seldom needs to, much like Del Mar Manzanita. But unlike Del Mar Manzanita it never experienced a fire that wiped out a whole generation and triggered massive seedling production, with its attendant explosion of genetic diversity. Is this possible? I don't know. Once again I am just speculating. But it is indisputable that Mission Manzanita relies heavily on stump-sprouting, rarely reproducing from seed, and appears to remain extremely stable genetically. Or at least it looks that way to me.

The Bottom Line is that plants must use sexual reproduction (seeds) in order to achieve genetic variability. They can't get any variation from stump-sprouting. The same could be said of plants that reproduce vegetatively, such some of the cholla cacti which produce clones. But stump-sprouting isn't even reproduction, it's just continued growth of the same individual. Stump-sprouting gives individual plants the ability to live a long time and to withstand most (but not all) fires. Seed reproduction, on the other hand, gives the species as a whole the opportunity to develop new genetic strains which may have enhanced survival value in a given place and time. So the Arctostaphylos species in California give us an interesting picture of the adaptive trade-offs between these two different strategies for responding to fire.

In Part II I will talk about the geophytes and their unique response to fire.

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