Hiking through Colorado's San Juan Mountains almost 10 years ago, botanist Herbert Kronzucker was presented with a picturesque puzzle: to his left arose a mountainside of blue spruce; to the right, a slope of trembling aspen.
A ranger mentioned that the aspen side had been logged 15 years earlier and replanted with young spruce seedlings. But if the spruce thrived before, thought Kronzucker, why were they being so totally out-competed by aspen?
After nearly a decade of research, the UBC-based scientist thinks he's found an answer, one which may have dramatic implications for B.C.'s efforts at reforestation.
A paper of Kronzucker's, based on his UBC PhD thesis, appeared in the Jan. 2 issue of the science journal, Nature. The paper states that a different form of nitrogen found in clearcut areas from that found in mature forests, combined with spruce's inability to make use of that form of nitrogen, might be a very important factor in determining the success of replanting conifers. The paper also says that in B.C. alone, replanting has failed on more than one-and-a-half-million hectares.
Soil samples taken from under the Colorado spruce and aspens were analysed and found to contain different types of nitrogen, the most important of plant nutrients. Ammonium was the chemical variation of nitrogen found under the spruce while nitrate predominated under the aspens.
Kronzucker, a visiting scientist in UBC's Dept. of Botany, explains that when a forest undergoes a clearcut, forest fire or landslide, the population of micro-organisms in the soil changes. Gone are the conifers which used to shed their needles and leave behind a class of acidic compounds known as phenolics. This forest litter leaches into the soil over time and has been shown to be highly toxic to certain micro-organisms.
As the phenolics are washed away by rain, new bacteria gain a foothold in the clearcuts and convert nitrogen from its previous form of ammonium to nitrate -- a form foreign to conifers.
"In the course of evolution somehow these species (conifers) adapted to ammonium as their one source of food and lost the ability to use nitrate," says Kronzucker. "White spruce seedlings replanted in B.C. are being forced to try to grow on the wrong food."
Kronzucker says his nitrogen hypothesis holds true in soil samples taken from forests in Sweden, Norway and his home country, Germany.
Kronzucker proved his theory in a UBC botany lab where he grew white spruce seedlings hydroponically. He then fed the seedling roots cocktails of ammonium and nitrate with a radioactive tracer produced by the TRIUMF cyclotron. The tracer allowed Kronzucker and colleagues to measure exactly how the roots transported and stored ammonium and nitrate. They found white spruce seedlings consumed up to 20 times more ammonium than nitrate.
Botany Prof. Tony Glass, in whose lab Kronzucker undertook his PhD studies, says Forest Renewal B.C. is funding further comparative studies looking at aspen, white spruce, Douglas fir and lodgepole pine. Results, so far, indicate that aspen seedlings prefer nitrate and the others, ammonium. Future plans in the UBC lab entail changing the genetic code of spruce seedlings using genes from species whose roots take up nitrates more readily.
Kronzucker suggests, short of a profound change to current forest practices away from large-scale clearcutting, a wheat or barley mulch may be an answer to the problem. Tissues in these mulches contain the phenolics which would prohibit nitrate production and help regenerate ammonium in severely disturbed areas.
The appearance of Kronzucker's paper in Nature touched off a fire-storm of media interest from no less than 17 major daily newspapers in Europe as well as the Christian Science Monitor.