Productivity Archives | JuneauNature

Nature’s GNP

It’s impossible to understand watershed-to-watershed variability, or island-to-island distinctions, without considering the drivers and pathways of productivity. In ecology, productivity is rate of production (eg grams per year) of some component of the community such as biomass of vegetation. As a mostly unschooled seat-of-the-pants naturalist I should repeat the cautionary note in Ecology 101 that a quantitative biologist might find my hand-waving pronouncements on productive versus unproductive landscapes rather sloppy.

Contrasting wetland productivity on Xutsnoowú, (Admiralty Island). Bogs and fens are both ancient peatlands, but annual growth of plants such as sedges is much higher on fens.

On the Tongass National Forest, and the corporate ‘working forests,’ we mostly hear about productivity in the context of tree growth. In aquatic systems, salmon dominate the conversation. Combined—in terms of annual tonnage—timber and salmon far outweigh all other Southeast commodities. In most of my work, I define productive watersheds as those that—relative to their biogeographic province—-rapidly grow very large trees and associated fauna, and/or host exceptional runs of salmon. This kind of productivity often appears to spring from the nature of underlying bedrock, with carbonate rocks at the rich end, and granitics toward the lean end of the spectrum.

But, as Juneau ecologist Mary Willson points out, it’s important to be clear that timber or fish productivity

“doesn’t translate directly into productivity of anything else, because habitat structure and exposure and plant defensive compounds and a zillion other things may regulate other community components.”

For example, shrub production is often inversely proportional to forest canopy production. So it could be there’s higher production of some shrub-nesting birds and blueberry parasites in the types of scrubby-forest watersheds I call “less productive” in contexts such as the Ground-truthing reports (links below). Diversity and even biomass within many taxa (eg bryophytes) is also probably higher in our ‘less productive’ watersheds with less timber and more peatland.

Even more confounding—some conditions productive of certain fish species need not be productive of forests. Anan Creek watershed has extraordinary pink salmon productivity but almost zero large-tree forest. The entire watershed is mapped as granitic. Evidently, this relatively sterile parent rock is a deal-breaker for tree production, but not for a fish that barely feeds in the freshwater portion of its life cycle. Anan’s pink productivity may have less to do with in-stream fish food than with flow-stabilization provided by the many lakes in the system.

Still, for better or worse, timber and salmon productivity tend to go together in most watersheds. Since our greatest large-tree forests have mostly been cut, the best single indicator of watershed productivity is probably the province ranking for pink and chum salmon. Sadly, an even more reliable productivity index is percent loss of productive old growth (POG) and large-tree forest.

Reports linked below address the implications of variable productivity in conservation design.