We all know an old paddock tree when we see one: broad, deep, canopy; sagging, tangled branches; broken boughs full of hollows. The classic woodland tree. But how do you recognize an old tree that grew – not in the open – but in a closed, dense stand? It won’t have a big, wide canopy nor a thick, wide trunk if its growth was suppressed by neighbors.
One of the world’s top dendro-ecologists, Neil Pederson, has a great web site with lots of fantastic posts about trees, tree rings and climate history. My favorite is a tale of how his research group used tree rings to show that unusually warm and wet weather helped Genghis Khan to conquer the world. It’s a great example of how climate change can alter human history, all silently witnessed by long-lived trees.
Neil’s site also has a great post on how to recognize a really old tree. Neil calls old trees, charismatic megaflora, which is pretty cool. A characteristic feature of many old hardwood trees in the northern hemisphere is a distinctively bent and sinuous trunk, as shown in his photo of a 300-year old chinkapin oak.
What drives a trunk to bend? It’s probably the same process that causes a seedling to bend towards the sun, phototropism. The wonderful process that makes seedlings ‘dance to the light’ in time lapse videos. In Neil’s words,
One somewhat common characteristic of old broadleaf trees… is a sinuous trunk…. this trait, the sinuous stem, is an important clue of age in dense forests, no matter if they have been cut or are uncut…. I like to think (hypothesize) that each twist is a record of decadal-scale phototropism. Individual plants track or move with the sun as it moves through the sky….
So, what do you do if you are an understory tree and a small gap has created a fleck of light in your general vicinity? You reach for it, almost literally. Your solar panels “detect” the higher light levels, intercept more light, and grow in that general direction. If you are successful in that pursuit, you then gain more energy and stretch more in that direction. This becomes a positive feedback loop: as you gain more energy, you can grow more. As you grow more in that direction, your mass will “move” in that direction. And, in opposition, “trailing” or more heavily-shaded branches and leaves might lose out as you “move” toward the more resource-rich area of the forest.
Old Eucalyptus and Callitris trees that grow in dense stands often have bent, sinuous trunks, especially the small, suppressed trees that struggle to get enough light to grow tall and fast. This patch of Grey Box (Eucalyptus microcarpa) is over 70, and possibly over 90, years old according to old air photos. The area was probably cleared in the late 1800s and the trees regenerated densely afterwards. It is visible as a dense patch of regrowth on the earliest air photo, taken in the 1940s, so I assume the trees regenerated at least a decade or two before then. Given their age, these bent old things haven’t grown tall or wide, and they certainly aren’t very straight, are they?
I was amazed when I found this patch on the ground. I’ve visited many patches of dense, young regrowth forests (see the dirt-ring photo blog), but this was the first unequivocally ‘old’ patch of dense, post-agricultural regrowth that I’d knowingly visited. Unlike younger patches, the trees are bent and twisted, and the forest floor is covered with dense leaf litter and fallen trunks and boughs. Once you’ve found a stand like this, you can use your newly-learned ‘spotting characters’ to interpret past changes in other stands. Another neat example of learning to read the bush in an informed way.
Suppressed old Callitris trees that grow in dense stands often have bent, twisted trunks too. In the late 1800s, White Cypress-pine (Callitris glaucophylla) regenerated densely over large areas in eastern Australia. Many stands were thinned and managed for timber production, forming the evenly spaced ‘plantation-style’ forests in Terrick Terrick National Park in northern Victoria and many forests across central NSW.
Occasionally you find an old stand that never got thinned. In the late 1800s, some improvement leases apparently required landholders to retain an uncleared strip of dense Callitris regrowth around the border of their property. The strip was called a ‘green break’ and the goal was to prevent fires from spreading.
How could this work? Dense stands of Callitris can suppress grass beneath them, and by reducing ground fuels, can inhibit the passage of low intensity fires. You can read about this in an earlier blog on alternative stable states in Callitris-Eucalyptus forests.
Most green breaks were apparently cleared, thinned or logged long ago, but a great example survives along the edge of a small forest north of Corowa in southern NSW, called Lonesome Pine State Forest. The dense old trees have grown and self-thinned very slowly over 130-odd years, and many of the suppressed trees have the same willowy, sinuous trunks that Neil Pederson describes from the hardwood forests in North America. All the trees in the green break below (and in the manipulated photo at the top of the post) are over 130 years old; the same age as those in the thinned stand above.
So next time you think about old trees, what will you picture? A large and spreading woodland tree, or some bitter and twisted, charismatic megaflora like the wonderful tree below? That’s 130 years of slow, slow growth – and a superb example of survival under harsh conditions – all wrapped in the palm of your hand.
If my daughter’s, grandpa’s, grandmother was still alive, she’d be as old as this small tree.