Krassilovia: An Amazing Cretaceous Conifer

Reconstructing extinct organisms based on fossils is no simple task. Rarely do paleontologists find complete specimens. More often, reconstructions are based on fragments of individuals found either near one another or at least in similar rock formations. This is especially true for plants as their growth habits frequently result in fragmentary fossilization. As such, fossilized plant remains of a single species are often described as distinct species until subsequent detective work pieces together a more complete picture.

Such was the case for the fossil remains of what were described as Krassilovia mongolica and Podozamites harrisii. Hailing from the Early Cretaceous (some 100-120 million years ago), Krassilovia was only known from oddly spiny cone scales and Podozamites was only known from strap-shaped leaves found in a remote region of Mongolia. Little evidence existed to suggest they belonged to the same plant. That is, until these structures were analyzed using scanning electron micrographs.

(A–C) Articulated seed cones, (D) Isolated cone axis, (E) Incomplete leafy shoot showing a cluster of three attached leaves, (F) Three detached strap-shaped leaves, G) Detail of A showing tightly imbricate interlocking bract-scale complexes, (H) Detail of leaf apex showing converging veins, (I) Three isolated bract-scale complexes showing abaxial (top) and adaxial (bottom) surfaces, (J) Two isolated seeds showing narrow wings. [SOURCE]

These fossilized plant remains were preserved in such detail that microscopic anatomical features such as stomata were visible under magnification. By studying the remains of these plants as well as others, scientists discovered some amazing similarities in the stomata of Krassilovia and Podozamites. Unlike other plant remains associated with those formations, the Krassilovia cone scales and Podozamites leaves shared the exact same stomate morphology. Though not without some uncertainty, the odds that these two associated structures would share this unique morphological trait by chance is slim and suggests that these are indeed parts of the same plant.

The amazing discoveries do not end with stomata either. After countless hours of searching, fully articulated Krassilovia cones were eventually discovered, which finally put the strange spiky cone scales into context. It turns out those spiked scales interlocked with one another, with the two bottom spikes of one scale interlocking with the three top spikes of the scale below it. In life, such interlocking may have helped protect the developing seeds within until they had matured enough to be released. Also, the sheer volume of cone scales coupled with other minute anatomical details I won’t go into here indicate that, similar to Abies and Cedrus cones, Krassilovia cones completely fell apart when fully ripe.

Though not related, the cone scales of the extinct Krassilovia (left) show similarities with the cone scales of modern day Cryptomeria species (right).

Interestingly, the ability to resolve microscopic structures in these fossils has also provided insights into some modern day taxonomic confusion. It turns out that Krassilovia shares many minute anatomical similarities with present day Gnetales. Gnetales really challenge our perception of gymnosperms and their superficial resemblance to angiosperms have led many to suggest that they represent a clade that is sister to flowering plants. However, more recent molecular work has placed the extant members of Gnetales as sister to the pines. Evidence of shared morphological features between extinct conifers like Krassilovia and modern day Gnetales add some interesting support to this hypothesis. Until more concrete evidence is described and analyzed, the true evolutionary relationships among these groups will remain the object of heated debate for the foreseeable fture.

What we can say is that Krassilovia mongolica was one remarkable conifer. Its unique morphology clearly demonstrates that conifers were once far more diverse in form and function than they are currently. Even the habitat in which Krassilovia once lived is not the kind of place you can find thriving conifer communities today. Krassilovia once grew in a swampy habitat. However, whereas only a few extant conifers enjoy swamps, Krassilovia once shared its habitat with a wide variety of conifer species, the likes of which we are only just beginning to appreciate. I for one am extremely excited to see what new fossil discoveries will uncover in the future.

LISTEN TO EPISODE 300 OF THE IN DEFENSE OF PLANTS PODCAST TO LEARN MORE ABOUT THIS FOSSIL AND THE ECOSYSTEM IN WHICH IT ONCE EXISTED.

Photo Credits: [1] [2] [3]

Further Reading: [1]

Maxipiñon: One of the Rarest Pines in the World

Photo by Ruff tuff cream puff licensed under public domain

The maxipiñon (Pinus maximartinezii) is one of the rarest pines on Earth. A native of southern Sierra Madre Occidental, Mexico, nearly all individuals of this species can be found scattered over an area that collectively spans only about 3 to 6 square miles (5 – 10 km²) in size. Needless to say, the maxipiñon teeters on the brink of extinction. As a result, a lot of effort has been put forward to better understand this species and to develop plans aimed at ensuring it is not lost forever.

The maxipiñon has only been known to science for a few decades. It was described back in 1964 after botanist Jerzy Rzedowski noted some exceptionally large pine seeds for sale at a local market. He named the species in honor of Maximino Martínez, who contributed greatly to our understanding of Mexican conifers. However, it was very obvious that the maxipiñon was well known among the residents of Zacatecas.

The reason for this are its seeds. The maxipiñon is said to produce the largest and most nutritious seeds of all the pines. As such, it is a staple of the regional diet. Conversations with local farmers suggest that it was much more common as recent as 60 years ago. Since then, its numbers have been greatly reduced. It soon became apparent that in order to save this species, we had to learn a lot more about what threatens its survival.

The most obvious place to start was recruitment. If any species is to survive, reproduction must outpace death. A survey of local markets revealed that a lot of maxipiñon seeds were being harvest from the wild. This would be fine if maxipiñon were widespread but this is not the case. Over-harvesting of seeds could spell disaster for a species with such small population sizes.

Indeed, surveys of wild maxipiñon revealed there to be only 2,000 to 2,500 mature individuals and almost no seedlings. However, mature trees do produce a considerable amount of cones. Therefore, the conclusion was made that seed harvesting may be the single largest threat to this tree. Subsequent research has suggested that seed harvests actually may not be the cause of its rarity. It turns out, maxipiñon population growth appears to be rather insensitive to the number of seeds produced each year. Instead, juvenile tree survival seems to form the biggest bottleneck to population growth.

Photo by Krzysztof Ziarnek, Kenraiz licensed under CC BY-SA 4.0

You see, this tree appears to be more limited by suitable germination sites than it does seed numbers. It doesn’t matter if thousands of seeds are produced if very few of them ever find a good spot to grow. Because of this, scientists feel that there are other more serious threats to the maxipiñon than seed harvesting. However, humans are still not off the hook. Other human activities proved to be far more damaging.

About 50 years ago, big changes were made to local farming practices. More and more land was being cleared for cattle grazing. Much of that clearing was done by purposefully setting fires. The bark of the maxipiñon is very thin, which makes it highly susceptible to fire. As fires burn through its habitat, many trees are killed. Those that survive must then contend with relentless overgrazing by cattle. If that wasn’t enough, the cleared land also becomes highly eroded, thus further reducing its suitability for maxipiñon regeneration. Taken together, these are the biggest threats to the ongoing survival of this pine. Its highly fragmented habitat no longer offers suitable sites for seedling growth and survival.

As with any species this rare, issues of genetic diversity also come into play. Though molecular analyses have shown that maxipiñon does not currently suffer from inbreeding, it has revealed some interesting data that give us hints into the deeper history of this species. Written in maxipiñon DNA is evidence of an extreme population bottleneck that occurred somewhere between 400 and 1000 years ago. It appears that this is not the first time this tree has undergone population decline.

There are a few ways in which these data can be interpreted. One is that the maxipiñon evolved relatively recently from a small number of unique and isolated individuals. Perhaps a hybridization event occurred between two closely related piñon species - the weeping piñon (Pinus pinceana) and Nelson piñon (Pinus nelsonii). Another possibility, which does not rule out hybridization, is that the maxipiñon may actually be the result of artificial selection by agriculturists of the region. Considering the value of its seeds today, it is not hard to imagine farmers selecting and breeding piñon for larger seeds. It goes without saying that these claims are largely unsubstantiated and would require much more evidence to say with any certainty, however, there is plenty of evidence that civilizations like the Mayans were conserving and propagation useful tree species much earlier than this.

Despite all we have learned about the maxipiñon over the last few decades, the fate of this tree is far from secure. Ex situ conservation efforts are well underway and you can now see maxipiñon specimens growing in arboreta and botanical gardens around the world. Seeds from these populations are being used for storage and to propagate more trees. Sadly, until something is done to protect the habitat on which it relies, there is no telling how long this species will last in the wild. This is why habitat conservation efforts are so important. Please support local land conservation efforts in your area because the maxipiñon is but one species facing the loss of its habitat.

Photo Credits: [1] [2] [3] [4]

Further Reading [1] [2] [3]

The Celery-Topped Conifers

Photo by RTBG licensed under CC BY-NC-SA 2.0

I am only just starting to fully appreciate the diversity in form and habit exhibited by the gymnosperm lineages alive today. What I once thought of as a unidimensional group of plants is proving to be wonderfully diverse, despite being overshadowed by the angiosperms. For instance, imagine my surprise when I first laid eyes on a member of the genus Phyllocladus.

At first glance, these weird conifers look more like a broad-leaf angiosperm. This similarity is superficial, of course. Before we get to why they look the way they do, it is worth considering this group from a as a whole. The genus Phyllocladus comprises roughly 5 species spread out among New Zealand, Tasmania, and Malesia. They are somewhat variable in form but usually settle out somewhere between a good sized shrub and a medium sized tree. Where exactly this genus of oddball gymnosperms fits on the tree of life is subject to some debate.

Phyllocladus trichomanoides licensed under public domain

For many years after its initial description, Phyllocladus was placed in a family of its own - Phyllocladaceae. Subsequent molecular work has only managed to add to the confusion. Despite its unique morphological characteristics, some authors feel this genus fits nicely into the family Podocarpaceae. At least one other study suggests that it doesn’t belong in Podocarpaceae but rather is situated as sister to the family. By the looks of it, this will not be cleared up any time soon. So, for now, let’s focus in on why these plants are so strange.

For starters we have the “leaves.” I place the word ‘leaves’ in quotes because they are not true leaves. The correct term for these structures are phylloclades (hence the generic name). A phylloclade is a flattened projection of a branch that takes on the form and function of a leaf. What we know of as leaves have been greatly reduced in the genus Phyllocladus. If you want to see them, you must look closely at the tips of the phylloclades. Early on in their development, the leaves exist as tiny brown scales. These scales are gradually lost over time as they serve no function for the plant.

Phyllocladus alpinus. Photo by MurielBendel licensed under CC BY-SA 4.0

Though no one has tested this directly (that I am aware of), the evolution of phylloclades over leaves likely has to do with energy conservation in one form or another. Why produce stems and leaves when you can co-opt stem-like structures to do the work for you? Oddly enough, some suggest that to consider them stems in the truest sense of the word is erroneous. Morphologically speaking, they share traits that are intermediate between branches and stems. However, I am going to need to do more homework before I feel comfortable elaborating on this point.

Only when it comes time for reproduction does their place among the gymnosperms become readily apparent, that is before the ovules are fertilized. All members of the genus Phyllocladus produce cones. Male cones are tiny, cylindrical structures located at the ends of their side branches whereas female cones are clustered into groups along the axils or margins of the phylloclades. Once fertilized, however, these plants offer another point of confusion for the casual observer.

Phyllocladus is yet another genus of conifers that has converged on a fruit-like seed dispersal strategy. As the seed cones mature, the scales gradually swell and become berry-like. Poking out of the bright red and/or white aril is a single seed. These fleshy arils function in a similar way to fruit in that they attract birds, which then consume them, dispersing the seeds later on in their feces.

Another intriguing aspect of their morphology occurs below ground. The roots of this genus form nodules, which provide a home for bacteria that specializing in fixing atmospheric nitrogen. In return for a home and some carbohydrates from photosynthesis, these bacteria pay these trees with nitrogen that would otherwise be unavailable. Pretty remarkable stuff for a such an esoteric group of conifers!

Photo Credits: [1] [2]

Further Reading: [1] [2] [3] [4] [5]

The Only True Cedars

Cedrus deodara. Photo by PabloEvans licensed under CC BY 2.0

The only true cedars on this planet can be found growing throughout mountainous regions of the western Himalayas and Mediterranean. All others are cedars by name only. The so-called “cedars” we encounter here in North America are not even in the same family as the true cedars. Instead, they belong to the Cypress family (Cupressaceae). The true cedars all belong to the genus Cedrus and are members of the family Pinaceae. They are remarkable trees with tons of ecological and cultural value.

J. White,1803-1824.

The true cedars are stunning trees to say the least. They regularly reach heights of 100 ft. (30 m.) or more and can live for thousands of years. Cedars exhibit a dimorphic branching structure, with long shoots forming branches and smaller shoots carrying most of the needle load. The needles themselves are borne in dense, spiral clusters, giving the branches a rather aesthetic appearance. Each needle produces layers of wax, which vary in thickness depending on how exposed the tree is growing. This waxy layer helps protect the tree from sunburn and desiccation.

Cedrus libani. Photo by Zeynel Cebeci licensed under CC BY-SA 4.0

Cedrus libani. Photo by Leonid Mamchenkov licensed under CC BY 2.0

One of the easiest ways to identify a cedar is by checking out its cones. All members of the genus Cedrus produce upright, barrel-shaped cones. Male cones are smaller and don’t stay on the tree for very long. Female cones, on the other hand, are quite large and stay on the tree until the seeds are ripe. Upon ripening, the entire female cone disintegrates, releasing the seeds held within. Each seed comes complete with blisters full of distasteful resin, which is thought to deter seed predators.

Male cones of Cedrus atlantica. Photo by Meneerke bloem licensed under CC BY-SA 3.0

Female Cedrus cones. Photo by Zeynel Cebeci licensed under CC BY-SA 4.0

In total, there are only four recognized species of cedar - the Atlas cedar (Cedrus atlantica), the Cyprus cedar (C. brevifolia), the deodar cedar (C. deodara), and the Lebanon cedar (C. libani). I have heard arguments that C. brevifolia is no more than a variant of C. libani but I have yet to come across any source that can say this for certain. Much more work is needed to assess the genetic structure of these species. Even their place within Pinaceae is up for debate. Historically it seems that Cedrus has been allied with the firs (genus Abies), however, work done in the early 2000’s suggests that Cedrus may actually be sister to all other Pinaceae. We need more data before anything can be said with certainty.

Regardless, two of these cedars - C. atlantica & C. libani - are threatened with extinction. Centuries of over-harvesting, over-grazing, and unsustainable fire regimes have taken their toll on wild populations. Much of what remains is not considered old growth. Gone is the heyday of giant cedar forests. Luckily, many populations are now located in protected areas and reforestation efforts are being put into place throughout their range. Still, the ever present threat of climate change is causing massive pest outbreaks in these forests. The future for these trees hangs in the balance.

Photo Credit: Wikimedia Commons

Further Reading: [1] [2] [3]

Why Trees Have Rings (and why they are so useful)

Dendrochronology is a field of study that focuses on tree rings. Though it may not be obvious, the amount of information we gain from looking at these rings is astounding. This research goes far deeper than simply finding out how old a tree was when it died. Dendrochronological data can be used to investigate paleoclimates, paleoecologies, and the archaeological dating of buildings and artwork. It is amazing how a practiced eye can look back in time. To date, we have an unbroken dendrochronological record for the northern hemisphere dating back some 12,000+ years!

All of this would not be possible if it were not for tree rings. But what exactly are they and how do they form? The answer is physiological. Essentially tree rings result from patterns in vascular tissues. Early in the spring, before the leaves start to grow, a layer of tissue just under the bark called the cambium begins to divide. In this cool, water-laden time of the growing season the vessels that are produced are large and less dense. This is the beginning of the spring or early wood. Although they are not as strong as vessels that are produced later in the season, they sure can move a lot of water. Things are a bit different for conifers. Because they do not produce vessel elements in their wood, this large cell growth is initiated instead by large amounts of a growth hormone called auxin that is produced by the new buds. This causes the cells of the early wood in conifers to grow large in a similar way to that of the hardwoods. 

As summer heats up, things start to change. The cambium starts producing smaller, thicker cells. The vessels that result from this are much stronger than those of the early wood. This late wood as it is called gives trees much of their rigidity and strength. Late wood is also resistant to what is called cavitation, a process in which water within the tree can literally vaporize, causing a damaging embolism during the hottest months of summer. In conifers, bud growth stops by mid to late summer and with it much of the production of auxin. This results in smaller vessels as well. 

In temperate regions, this cycle of growth occurs over the course of a growing season. As such, each ring demarcates a year in that trees life. Because so much of a trees growth is determined by environmental conditions, the size and shape of the rings can tell a lot about the conditions in which that tree was growing. That is why dendrochronology is such a useful tool. By looking at tree rings from all over the world, researchers can tell what was going on at that point in time. And, though it was long thought that this was a phenomenon restricted to seasonal forests, we are finding that even some tropical trees produce annual growth rings. This is especially true in regions that have a measurable dry season. It just goes to show you that data comes in many shapes, sizes, and forms.

LEARN MORE ABOUT DENDROCHRONOLOGY IN EPISODE 247 OF THE IN DEFENSE OF PLANTS PODCAST

Further Reading: [1] [2] [3]