Mossing over the issue

Wood Focus magazine
2 Sep 2011
spruce tree

One of the great failings of rolling stones is their failure to gather moss, which makes them useless in forests. However, 800-year-old Sitka spruces in coastal boreal forests are excellent at gathering moss. As a result, they are effective at mitigating nutrient depletion and carbon sequestration.

According to a study conducted by Zoe Lindo at McGill University in Montreal, Canada, the canopy mosses in old-growth trees have nitrogen-fixing cyanobacteria associated with them. The study also says the nitrogen fixing occurring is twice as high per unit area than on the forest floor, and that the moss-cyanobacteria relationship is found across many species of moss.

She hopes this research could be used at industry level to change how harvesters view retention patches, and to lobby for the protection of large, old-growth trees, which are effective at harbouring high moss biomass.

Lindo explains, ‘Most temperate coniferous and boreal forests are nitrogen limited. The growth of trees and the productivity of the forest will increase with more available nitrogen in the system. Most nitrogen within the forest is unavailable for plant growth either because it is retained within an organism or because it occurs in a form that plants cannot take-up. Only a few organisms can take nitrogen from the atmosphere and convert it to a bio-available nitrogen. Cyanobacteria can perform this conversion.’

While scientists previously made cyanobacterial associations in canopy systems with cyanolichens, Lindo asserts that no one had explored whether there were nitrogen-fixing cyanobacteria associated with moss in coastal temperate rainforest systems before.

With this in mind, Lindo collected moss samples on the forest floor, and at two heights within the canopy of 18 Sitka spruce trees (15m and 30m) across three watersheds in Clayoquot Sound UNESCO Biosphere Reserve on Vancouver Island, Canada.

The moss samples were incubated and measured for nitrogen fixing and cyanobacterial density. Nitrogen fixation and cyanobacterial density values were standardised to the amount (mass) of moss to derive mass value per unit.

The values were then used to estimate the amount of nitrogen fixing across a per hectare area based on 80% moss cover on the forest floor. Canopy moss biomass estimates per tree were multiplied by the number of larger, old trees per hectare for the canopy estimate.

The findings were surprising. Lindo notes, ‘Cyanobacterial densities were approximately three times greater at 15m and nearly four times greater at 30m than on the forest floor. Nitrogen fixation rates were nearly 10 times greater at 30m than on the forest floor, but only marginally greater at 15m.’

On future endeavours, Lindo says, ‘The next phase of this research is to better understand the “fate” of the nitrogen that is fixed, and to incorporate this into soil nutrient and foodweb models. What factors control the regulation of nitrogen fixing is another area of active research and how these factors might differ among different species of nitrogen-fixing cyanobacteria.’