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

Light meas­ure­ments inside the shade-houses dur­ing a sunny day (9 September 2009) used for char­ac­ter­iz­a­tion of both light regime treat­ments in the present study. Light was aver­aged every minute (from 5-s meas­ure­ments) from sun­rise to sun­set. The high-light gap treat­ment pro­duced two high-light events per day, each last­ing ∼51 min (for a total of ∼102 min per day), for an integ­rated global light intens­ity of 21% (with respect to incid­ent light). The low-light under­story treat­ment pro­duced four high-light events per day, each only last­ing ∼8 min (∼33 min total), for a global light intens­ity of 4.9%.

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.


Eric Ward.

Post Doctoral Researcher, North Carolina State University, PINEMAP

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