Arboreally Speaking, the Good Old Growth Curve Is a Delusion

Discussions of the nature and definition of old growth and primary forests.

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Neil
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Re: Arboreally Speaking, the Good Old Growth Curve Is a Delu

Post by Neil » Mon Jan 09, 2012 4:31 pm

thanks Lee - that helps.

neil

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KoutaR
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Re: Arboreally Speaking, the Good Old Growth Curve Is a Delu

Post by KoutaR » Mon Jan 09, 2012 5:55 pm

Lee,

The difference between Finnish and north Minnesota forests you suggested makes sense. Indeed, there is no such a short insect related disturbance cycle in Finland, and the fire return interval in Finland's "semi oceanic" climate before intensive human influence (slash-and-burn agriculture etc.) has been 200–500 years. Also, not so violent storms.

How about tropical rainforests on old highly weathered bedrock, which occur extensively in the Amazon Basin, for example? Those forests are very old, don't burn naturally but are still luxuriant forests. According to Peltzer et al., they are probably "retrogressed systems"; at least, they write so about a tropical forest in north Queensland (page 523). Now, will they retrogress further? Figure 5 of Peltzer et al. seems to indicate that (I didn't read the whole paper, perhaps I missed something important). What these forests will become eventually? It is hard to believe so old forests would not have reached a stabile state. Or did I miss some disturbance agent?

How about the temperate rainforests of the Pacific Northwest? In higher elevations there are stands which have not seen fire since thousands of years but still have high biomass. As the forests are only thousands of years old (after the last glaciation), are they still on their way towards their collapse? What will they become eventually?

Kouta

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Lee Frelich
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Re: Arboreally Speaking, the Good Old Growth Curve Is a Delu

Post by Lee Frelich » Tue Jan 10, 2012 2:49 pm

Kouta:

Old tropical rainforests can have mechanisms and plant species adaptations that allow them to retain and very tightly recycle nutrients, and losses to the ecosystem may be so small that atmospheric inputs can replace them, even for P (see page 512 of the article). This would allow retrogressed tropical rainforests such as in the Amazon and Australia to maintain high biomass, possibly for millions of years. However, as the paper states, clearing might push them past the maximal biomass state.

In the Pacific Northwest soils are still quite young, and ecosystem acidification, (a type of retrogression that occurs in colder climates where Sphagnum moss takes over, and over thousands of years acidifies the soil, water and entire ecosystem, forming peatlands, moors, and bogs) has not affected the vast majority of the landscape yet. It probably won't retrogress for many thousands more years, if ever, because there is substantial input of nutrients from windborn cations from the ocean, and from volcanic ash (the latter affects most areas every several hundred to a few thousand years).

Lee

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KoutaR
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Re: Arboreally Speaking, the Good Old Growth Curve Is a Delu

Post by KoutaR » Wed Jan 11, 2012 8:01 am

Lee,

Thanks for the explanation!

Kouta

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KoutaR
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Re: Arboreally Speaking, the Good Old Growth Curve Is a Delu

Post by KoutaR » Sun Jan 29, 2012 6:46 am

Lee,

I have read the whole paper of Peltzer et al. and two additional paper on the topic. Now I understand the concept of retrogression better. Of course, I had read that soils of the Amazon Basin and Australia, for example, are old and poor, but I hadn't understood the inevitability of soil retrogression. The concept is actually very logical and even simple: during millennia nutrients are "washed" away or below the rooting zone. I would have two questions to you.

First, I find the boreal retrogression, you described for Minnesota and Petzer et al. for Sweden, is fundamentally different from most of the others Petzer et al. describe. In these boreal forests, the reason for retrogression is the development of a thick humus layer which prevents roots from reaching mineral soil, but the nutrients are still there. Add million years without glaciations/volcanic events, and the mineral soil would retrogess even if there were fires with a sufficient return period which remove the humus layer. Is this correct? Perhaps this "boreal retrogression" could be called "secondary retrogression" and that in Australia, for example, "primary retrogression".

Second, how about the prairies of the US? Large areas were unglaciated but they are still very fertile. Petzer et al. say dry areas do not necessarily retrogess, but I have read there were boreal forests in those areas in the glacial time, and thus they were not so dry during most of the last 2 million years. So, why are the prairies so fertile? Or is some of the facts above incorrect?

Kouta

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Lee Frelich
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Re: Arboreally Speaking, the Good Old Growth Curve Is a Delu

Post by Lee Frelich » Thu Feb 02, 2012 4:01 pm

Kouta:

I agree with your assessment (in many cases) of boreal retrogression in Sweden and North America. Perhaps secondary retrogression is a valid concept. However, some of those sites also have almost no soil on top of granite, so in some cases its not just that the nutrients are unavailable in the soil, there may be almost no nutrients to begin with, and low pH and decomposition rates allow what small amounts of nutrients that are available to be tied up in the moss mat.

Regarding some of the prairie areas that were nor glaciated (as well as southeastern Minnesota and southwestern Wisconsin, which also were not glaciated or minimally glaciated), there has been a lot of erosion and deposition of nutrients by wind blown particles at very long distances. This has especially been the case during departure of each glacier, accompanied by major vegetation change across the entire continent. Southeastern Minnesota and southwestern Wisconsin, for example have from 1-2 feet up to 50 or 60 feet of wind blown loess on top of the old soils, which is quite nutrient rich and sets the clock back to zero with regard to retrogression.

Lee

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