Norway vs sugar maples: a silent struggle in the understory.

I grew up in Michigan, where both Norway and sugar maples are commonly found in parks and backyards.  In the sixth grade, we had a unit on tree identification and one of the hardest questions was: Norway or sugar maple?  It’s a question that still stumps me if I’m caught without a guidebook or smartphone.  The differences seem small, but what a difference a little difference can make.

Norway maple (Acer platanoides L.), originally introduced to North America as a street tree, is now invading many natural areas in the eastern parts of the continent.  As a shade-tolerant species, its seedlings compete with those of native trees, especially the sugar maple (Acer saccharum Marsh).  A similarly shade-tolerant species, it is unclear why sugar maple is often displaced by its European relative, even in relatively intact forests.  My mental archetype of an invasive plant is probably best typified by kudzu, a large-leaved vine that blankets roadside trees near my new home in North Carolina. Norway maple is a much less conspicuous invader, quietly establishing itself in the understory of mature forests.

A recent study by Paquette et al. tackles one dimension of the competition between seedlings of these species–the responses of each to different light regimes.  Instead of the temporally homogenous light reductions provided by most experimental manipulations (such as shade netting), Paquette et al. used shade-houses with adjustable roof openings to provide realistic diurnal variations in light availability.  This mimics the light received by seedlings in the understory (a few intense periods per day) or in a tree-fall gap (two somewhat longer periods of full sun).  Such short, intense events are often referred to as “sunflecks”, and have different implications for photosynthesis than temporally homogenous shading.  All other conditions of growth were near-optimal, so that the researchers’ results reflect maximal photosynthesis and growth rates under these light regimes.

Seedlings were monitored for light saturated photosynthetic rates, as well as above- and below-ground growth.  The authors analyze these data not just for mean responses by species and treatment, but also compare the variation of responses within each species and the plasticity exhibited by each species between the two light treatments.  The authors find that Norway maple exhibited 13% higher photosynthetic rates than sugar maple, but no difference in biomass in the understory light regime, confirming that the two species may indeed be very close competitors in this environment.

In contrast, Norway maple had 47% higher photosynthetic rates and nearly four times the biomass of sugar maple in the gap light regime.  Furthermore, Norway maple continued to grow in height into late autumn, while sugar maple exhibited only minor stem extension after midsummer.  Thus, Norway maple’s competitive advantage over  sugar maple is largely a product of phenology and plasticity in its response to light availability.

This experiment serves as an excellent reminder that growth is a cumulative process and thus phenology is important.  Although it is difficult to directly relate photosynthesis to current year growth in larger trees, presumably storage is not as great during seedling establishment.  Differences in phenology are particularly important in the context of climate change, leading to the authors’ suggestion that Norway maple’s competitive advantage over sugar maple could increase with warmer fall temperatures.  While the mysteries of forest community composition and species invasion cannot be settled simply by controlled experiments comparing pairs of species under two levels of an environmental variable, such experiments do provide information important to modeling more complex scenarios and interpretation of field data.

Seedling establishment is a messy thing in nature.  Seedlings may persist for years in the understory, so that competition in a tree-fall gap is not simply between new germinants, but also older seedlings, saplings and stump sprouts.  Disturbances vary in spatial extent, intensity and return interval.  We mustn’t forget that–out there in the woods–competition is between individuals and individual responses vary within a species.  Indeed, some believe that this individual variability is one reason we have so many species competing for relatively few resources.  This is why it is encouraging to see experimentalists such as Paquette et al. address not only the mean population responses in their research, but also variability and plasticity within each population.