Forest Stewardship: Reproductive Strategies

It’s been a spectacular fall for foliage, so I should be writing about that, probably. Instead, I’m writing about something less obvious, but pretty important - trees making more trees – and there’s a lot going on in the fall.

How’d You Get Started?

A theme that I’ll emphasize in this series of columns is “species strategy,” which in an evolutionary sense, is the sum total of what has worked over millennia or millions of years, with continued survival as the proof.  

Using some combination of leaves, bark, and form, you are probably able to distinguish one species from another very quickly. The differences go beyond appearance - species vary in how they arrive, compete for growing space, reproduce and in their longevity.  Broadly speaking, the advanced vascular plants – think trees and shrubs – in the woods of the North Quabbin began their lives in one of three ways – from seed (sexually), as sprouts (asexually), or from “layering” (far less common).

I Started as a Winged Samara

The seeds of all species are the same in that they are all embryonic plants, pollinated, fertilized and developed within a fruit. The seeds ending up in a particular patch of forest might come from:

• trees already in that patch;
• trees adjacent to or some distance away, especially if wind-dispersed;
• seed deposited in previous years and resident in the so-called “seed bank”. The duration of their viability varies by species. The seed of a few species, especially in forests prone to frequent or severe disturbance, may remain viable for decades or more than a century. Pin cherry is one such New England species. Although uncommon, pitch pine can be found in the North Quabbin area. Some of its seeds are held in what are called “serotinous cones”, opening only when the surrounding temperature reaches a certain point and melts the resin gluing the cone shut. This is an adaptation to fire. In the North Quabbin, pitch pine is most likely to occur near railroad tracks.

Let’s look briefly at several of the key species common in North Quabbin forest stands and how they might get their start.

Red maple is kind of a superstar when it comes to reproducing. Hefty seed crops are common most years; the fruit is winged and the seeds light, numbering more than 20,000 per pound, dispersed primarily by wind (Fig. 1). Seeds are dropped in late spring and early summer, usually germinating very soon after release, minimizing exposure to predation. The so-called “germinative capacity”, or percentage of viable seed, tends to be quite high; the seed is able to germinate and survive the perilous first few weeks on most types of seedbed. All in all, a strong strategy. Red maple is also a strong sprouter – more on that later.

Black birch also produces frequent heavy seed crops, seed averaging upwards of 600,000 per pound. The winged fruit is released in the fall months, but tends to travel a shorter distance than red maple (Fig. 2). Look for black birch seed atop packed snow in the winter; dispersal is increased significantly when the seed is blown along the surface of a hard snow pack. Germination occurs in the spring following dispersal. Black birch sometimes germinates on moist down logs which subsequently rot and leave the new tree with “stilt roots”, a phenomenon you’ve probably observed (Fig. 3).

Red oak and beech have a very different seed strategy than birch and maple. Red oak acorns number about a hundred per pound and are moved around primarily by animals, and sometimes by running water (Fig. 4). Acorn crops are quite irregular and it takes two full growing seasons for red oak to produce seed. If you’ve been hit on the head by a falling acorn, you probably know that they come down in autumn. Beech nuts are much lighter, averaging about 1,500 per pound, usually falling to the ground in late autumn (Fig. 5). Seed crops of both red oak and beech are irregular. In our area, beech seed crops are especially unpredictable because most of the beech here is afflicted with beech bark disease. Animals and insects move, bury, and consume oak and beech mast; acorn germination takes place below the soil surface. Beech and oak don’t begin to produce seed until a much later age than red maple and black birch, again a contrast in “strategy.”

White pine and eastern hemlock are the most abundant conifers in the North Quabbin. It takes two growing seasons for white pine to produce a mature cone. Wind-dispersed seeds of white pine number about 25,000 per pound (Fig. 6) and typically don’t travel more than a couple of hundred feet from the tree, but squirrels help out and triple that distance with their gathering and caching activities. White pine seed year frequency is variable, even within a small geographic area.

The production of seed by eastern hemlock provides an example of how things may not always be as they seem. From the appearance of cones on trees, good seed crops seem to be very frequent. Small, but fully-developed cones open in fall and disperse seed into the winter. Wind-dispersed seed averages about 175,000 per pound (Fig. 7). But a few factors may limit the effectiveness of hemlock reproduction by seed. Dispersal distance is limited, often to about one tree height, a limitation overcome when seed can be blown further across the surface of snow. Both seed and very young seedlings are sensitive to dry conditions which may cause very high mortality. Finally, and least visibly, hemlock seed seems to have low viability relative to other species (germinative capacity).

I Started as a Clone

The second important method of reproduction in the North Quabbin region is sprouting, either from a stump or the roots, most notably by red maple, the oaks, beech and American chestnut. A sprout is a clone of the stump or root system from which it sprouts. Typically, sprouting is triggered by a major disturbance to the tree, such as being cut, top breakage from a storm, blowdown, defoliation, or serious disease.

Red maple’s status as a reproductive superstar is due in part to its ability to produce stump sprouts.  Anywhere from a few to a dozen or more sprouts may burst from around a stump, each arising from what had been a dormant bud – stimulated by the disturbance to the tree and a change in the flow of the hormone auxin (Fig. 8). The oaks generally are also strong stump sprouters (Fig. 9). The ability to stump sprout declines as trees get older, varying by species. Typically, the competition between stump sprouts will eventually produce from two to five or so winners.

In the North Quabbin, American chestnut trees are still remarkably common in the understory. (A lot of towns have a “Chestnut Hill”). These sprouts, arising from the still living root system of overstory trees felled by the Chestnut blight, reach a certain size, get hit by the blight, die back, and are replaced by a new sprout (Fig. 10). Most North Quabbin American beech suffers from beech bark disease thanks to an unholy alliance between a tiny insect and a virulent fungus. In response to cutting or advanced disease, sprouts, sometimes in the thousands, will sprout from the root system. Because these sprouts are genetically identical to the sick parent tree, they too will eventually succumb to the disease (Fig. 11).