Let's Talk About Recruitment

July 15, 2019

Photo by --Tico-- licensed by CC BY-NC-ND 2.0

For any species to be considered successful, it must replace itself generation after generation. We call this process recruitment and it is very important. After all, reproduction is arguably the most fundamental aspect of life in a Darwinian sense. For plants, this can be done either vegetatively or sexually via seeds and spores. Though vegetative reproduction is a fundamental process for many plants around the globe, seed or spore germination is arguably the most important. To truly understand what a plant needs, we have to understand its germination requirements.

Recruitment is a considerable limiting factor for plant populations. In fact, it is the first major bottleneck plants must pass through. It is estimated that a majority of plant mortality occurs during the germination and seedling stages. However, not all plants are equal in this way. Some plants are considered seed or propagule limited whereas others are habitat limited.

If a plant is seed limited, it means that its ability to expand its population or colonize new habitats its limited by the ability of seeds (or spores) to make it to a new location. Once there, nature takes its course and germination occurs with little impediment. If a plant is habitat limited, however, things get a bit more tricky. For habitat limited plants, simply getting seeds to a new location is not enough. Some other aspect of the environment (soil moisture, texture, temperature, disturbance, etc.) limit successful germination. Only when the right conditions are present can habitat limited plants germinate and begin to grow.

Habitat limitation is probably the most common limit to plant establishment. Simply put, not all plants will be successful everywhere. Even the successful growth and persistence of adult plants can be poor predictors of seedling success. Many plants can live for decades or even centuries and the conditions that were present when they germinated may have long since changed. Even the presence of the adults themselves can make a site unsuitable for germination. Think of all of those fire adapted species out there that require the entire community to burn before their seeds will ever germinate.

In reality, it is likely that most plants are habitat limited to some degree. These are not binary categories after all, rather they are aligned along a spectrum of possibilities. The fact that most plants don’t completely take over an area once seeds or spores arrive is proof of the myriad limits to plant establishment. As such, recruitment limitation is extremely important to study. It can make a huge difference in the context of conservation and restoration. Even the successful establishment of adult plants is no guarantee that seedlings stand a chance. Without successful recruitment, all you have left is a nice garden that is doomed to run its course. By understanding the limits to plant recruitment, we can do much more than just improve on our ability to protect and bolster plant populations, we can also gain insights into why so many plants remain rare on the landscape and so few ever rise to dominance.

Photo Credits: [1]

Seed Anchor

March 28, 2017

Epiphytic plants live out their entire lives on the trunks or branches of trees. Using their roots, they attach themselves tightly to the bark. Spend any amount of time in the tropics and it will become quite clear that such a lifestyle has been very successful for a plethora of different plant families. Still, living on a tree isn't easy. Epiphytic plants must overcome harsh conditions among or near the canopy.

Photo by faatura licensed under CC BY-NC-ND 2.0

One of the biggest challenges these plants face starts before they even germinate. This is especially true for orchids. Orchid seeds are more like spores than they are seeds. They are so small that thousands could fit inside of a thimble. Upon ripening, the dust-like seeds waft away on the slightest breeze. In order for epiphytic species to germinate and grow, their seeds must somehow anchor themselves in place on a trunk or branch. Inevitably most seeds are doomed to fail. They simply will not land in a suitable location. It stands to reason then that any adaptation that increases their chances of finding the right kind of habitat will be favored. That's where the strange coils on the tip of Chiloschista seeds, a genus of leafless orchids native to southeast Asia, New Guinea, and Australia, come in. For these orchids, this process is aided by some truly unique seed morphology.

Unlike most orchid seeds that are nothing more than a thin sheath surrounding a tiny embryo, the seeds of Chiloschista have additional parts. These "appendages," which are specialized seed coat cells, are tightly wound into coils. Upon contact with water, these coils shoot out like tiny grappling hooks that grab on to moss and bark alike. In doing so, they anchor the seed in place. By securing their hold on the trunk or branch of a tree, the seeds are much more likely to germinate and grow. This is one of the most extreme examples of seed specialization in the orchid family.

Photo Credit: [1] [2]

Germinating a Seed After 32,000 Years

November 4, 2015

What you are looking at are plants that were grown from seeds buried in permafrost for nearly 32,000 years. The seeds were discovered on the banks of the Kolyma River in Siberia. The river is constantly eroding into the permafrost and uncovering frozen Pleistocene relics. Upon their discovery, researchers took the seeds and did the unthinkable - they grew them into adult plants. To date, this is the oldest resurrected plant material.

The key to their extreme longevity lies in the permafrost. They were found inside the frozen burrow of an Arctic ground squirrel. The state of the burrow suggests that everything froze quite rapidly. As such, the seeds remained in a state of suspended animation for 32,000 years. This is not the first time viable plant materials have been recovered from Pleistocene permafrost. Spores, mosses, as well as seeds of other flowering plants have been rejuvenated to some degree in the past but none of these were grown to maturity.

Using micropropagation techniques coupled with tissue cultures, researchers were able to grow and flower the 32,000 year old seeds. What they discovered was that these seeds belonged to a plant that can still be found in the Arctic today. It is a small species in the family Caryophyllaceae called Silene stenophylla. However, there were some interesting differences.

As it turns out, the seeds taken from the burrow proved to be a phenotype quite distinct from extant S. stenophylla populations. For instance, their flowers were thinner and less dissected than extant populations. Also, whereas the flowers of extant populations are all bisexual, individuals grown from the ancient seeds first produced only female flowers followed by fewer bisexual flowers towards the end of their blooming period.Though there are many possible reasons for this, it certainly hints at the different environmental parameters faced by this species through time. What's more, such findings allow us a unique window into the world of seed dormancy. Researchers are now looking at such cases to better inform how we can preserve seeds for longer periods of time.

Photo Credit: Svetlana Yashinaa, Stanislav Gubin, Stanislav Maksimovich, Alexandra Yashina, Edith Gakhova, and David Gilichinsky

Further Reading: [1]

On Orchids and Fungi

October 12, 2015

It is no secret that orchids absolutely need fungi. Fungi not only initiate germination of their nearly microscopic seeds, the mycorrhizal relationships they form supplies the fuel needed for seedling development. These mycorrhizal fungi also continue to keep adult orchids alive throughout their lifetime. In other words, without mycorrhizal fungi there are no orchids. Preserving orchids goes far beyond preserving the plant. Despite the importance of these below-ground partners, the requirements of many mycorrhizal fungi are poorly understood.

Researchers from the Smithsonian Environmental Research Center have recently shone some light on the needs of these fungi. Their findings highlight an important concept in ecology - conservation of the system, not just the organism. Their results clearly indicate that orchid conservation requires old, intact forests.

Their experiment was beautifully designed. They added seeds and host fungi to dozens of plots in both young (50 - 70 years old) and old (120-150 years old) forests. They continued to monitor the progress of the seeds over a period of 4 years. Orchid seeds only germinated in plots where their host fungi were added. This, of course, was not very surprising.

The most interesting data they collected was data on fungal performance. As it turns out, the host fungi displayed a marked preference for older forests. In fact, the fungi were 12 times more abundant in these plots. They were even growing in areas where the researchers had not added them. What's more, fungal species were more diverse in older forests.

The researchers also noted that host fungi grew better and were more diverse in plots where rotting wood was added. This is because many mycorrhizal fungi are primarily wood decomposers. Nutrients from the decomposition of this wood are then channeled to growing orchids (as well as countless other plant species) in return for carbohydrates from photosynthesis. It is a wonderful system that functions at its best in mature forests.

This research highlights the need to protect and preserve old growth forests more than ever. Replanting forests is wonderful but it may be centuries before these forests can ever support such a diversity of life. Also, this stands as a stark reminder of the importance of soil conservation. Less obvious to most is the importance of decomposition. Without dead plant material, such fungal communities would have nothing to eat. Clearing a forest of dead wood can be just as detrimental in the long run as clearing it of living trees.

Research like this is made possible by the support of organizations such as the Native North American Orchid Conservation Center. Head on over to www.indefenseofplants.com/shop and pick up an In Defense of Plants sticker. Part of the proceeds are donated to this wonderful organization, which helps support research such as this! As this research highlights: What is good for orchids is good for the ecosystem.

Trillium

May 11, 2015

Trillium. The very name is synonymous with spring wherever they grow. Even the non-botanically minded amongst us could probably pick one out of a lineup. This wonderful genus holds such a special place in my heart and I anxiously await their return every year. The journey from seed to flowering plant is an arduous one for a trillium and some may take for granted just how much time has elapsed from the moment the first root pushed through the seed coat to the glorious flowers we admire each spring. The story of a Trillium, like any other plant, starts with a seed.

As with many other spring ephemerals, Trilliums belong to that group of plants that utilize ants as seed dispersers. Once underground in an ant midden, a Trillium seed plays the waiting game. Known as double dormancy, their seeds germinate in two phases. After a year underground, a root will appear followed by an immature rhizome and cotyledon. Here the plant remains, living off of the massive store of sunlight saved up in the endosperm for yet another year. Following this second year underground, the plant will throw up its first leaf.

In its fourth year of growth, the Trillium seedling will finally produce the characteristic whorl of 3 leaves we are familiar with. Now the real waiting game begins. Growing for such a short period of time each year and often in shady conditions, Trilliums must bide their time before enough energy is saved up to produce a flower. In an optimal setting, it can take a single Trillium 7 to 8 years to produce a flower. If conditions aren't the best, then it may take upwards of 10 years! Slow and steady wins the race in the genus Trillium. A large population of flowering Trillium could easily be 40 or 50 years old!

Sadly, when you couple this slow lifestyle with their undeniable beauty, you begin to spell disaster for wild trillium populations. A plant that takes that long to germinate and flower isn't the most marketable species for most nurseries and, as a result, Trillium are some of the most frequently poached plants in the wild. Because of their slow growth rate, poached populations rarely recover and small plots of land can quickly be cleared of Trilliums by a few greedy people. Leave wild Trilliums in the wild!

Microclimates

February 24, 2015

How can we know exactly when and where a seed will germinate? The simple answer is we can't. Much to the chagrin of anyone who has ever tried to grow plants from seed, there seems to be an endless amount of obstacles between deposition of seed and whether or not it will germinate. At the end of the day, it seems that the land will decide where a plant is going to grow. Because of this, the seed to seedling stage of a plants life is the first and greatest bottleneck in the patterns we see in plant communities around the world.

For some plants its easy. If seed makes it to a location, the plants will grow. For many (and I mean many) plant species, habitat means everything. Anyone interested in growing plants must never overlook microclimates. We all know what climate means. At least we should. Just like entire regions can have their own climates, so too can different parts of the landscape. It could be that snow melt happens slower in one area, making the ground slightly more saturated. Perhaps there is a layer of clay underneath helping to hold water longer. If you are a high desert or alpine species, perhaps a small rock or boulder provides just enough protection from violent winds. It could even be a clump of moss or a shrubs that shelters a seedling long enough for it to establish itself. Heck, it could just as easily be a rusted out old can in the middle of the Mojave Desert.

The are limitless variations on the theme but they are all too often overlooked. There are few ways of predicting what will work and what won't. On more than one occasion I have forgotten about seeds planted years ago only to have the plants suddenly appear out of nowhere. As with everything in nature, these things are dynamic. What ecology is in need of are more studies that investigate the recruitment limitations of individual or groups of species. We need ecologists speaking with restoration practitioners and vice versa. We need to keep in mind that organisms can inform theory and then some.

Photo Credit: Zachary Cava (https://www.flickr.com/photos/101789078@N06/)

Seeds That Plant Themselves

February 18, 2015

Photo by Matt Lavin licensed under CC BY-SA 2.0

With March (and hopefully spring) just around the corner here in the northern hemisphere, I have been thinking a lot about my garden plans. Winter is a time to get your hands on seeds. What a wonderful thing seeds are. They carry within them the genetic blueprints for building a plant. They are also the means by which most plants get around. Each seed has the potential to start a new generation somewhere else. We are well aware of the myriad ways in which plants equip their seeds for dispersal but the investment doesn't stop there. Many plant species produce seeds that maximize the likelihood of successful germination once dispersed. The ways in which this is done are as diverse as they are interesting.

One of the most remarkable examples of this involves hair-like structures called awns. Awns can be bristly and thus can become tangled in the fur or feathers of an animal. Once on the ground, some awns serve a different purpose. They can be rather sensitive to humidity. This is referred to as "hygroscopic." Hygroscopic awns will begin to twist when humidity levels rise. This movement will actually drill the seed down into the soil where it can safely germinate. Many grasses as well as some geranium seeds behave in this way.

Other seeds have awns or pappuses (hairs) that point backward at an angle which, once driven into the soil, prevent the seed from being pushed back out. This is especially useful when the young roots begin pushing their way down into the soil. Others have pappuses that expand and contract with humidity, placing the seed at a favorable angle for germination when moisture levels are just right. These adaptations are commonly found in species of Leontodon, Taraxacum, Sonchus, Senecio, and Erigeron. Some plants even produce seeds with hairs that become mucilaginous when wet, literally gluing them to the surrounding soil. This adaptation can be seen in Polemonium viscosum.

A seed is an investment for the future. Being static organisms, plants rely on subsequent generations to maintain their presence in and migrate into new habitats. Countless seeds are produced and only a handful will ever survive to flower and repeat the process. Despite these odds, plants are nonetheless incredibly successful.

Photo Credit: Matt Lavin (http://bit.ly/1Bnh4oq)

Further Reading:
http://www.jstor.org/stable/2258879…

http://www.jstor.org/stable/1940966…