TB: "The hemlock/hardwood hypothesis is that stream chemstries should be less acidic than in the past because of a shift in vegetation.
But for that to be true the hypothesized hemlock/hardwood effect would have to be stronger than that of the effects of acid precipitation."
Steam chemistries, like water temps, just have to be appropriate for trout. Trout only care about the outcome, not the process or some "natural" or historical level, or the "balance" with acid rain. if trout insisted on living in pristine environments unchanged by man, we might not have many wild trout
big picture:
1) Many PA streams lost first growth canopies to logging and now have mixed canopies. Read about logging: where hemlocks were taken, mixed forests of maple, cherry, etc now live. The insect pest now damaging hemlocks could also cause hemlock loss, with hardwood replacement. Much written lately because of the adelgid.
2) A stream that shifts from hemlock- to hardwood- drained forest would see changes in multiple variables. Yes, a small reduction in water acidity based on things I have read, in one case of paired study of hemlock and hardwood draining streams, the one under hardwood had a pH .25 higher. The pH difference was attributed to fewer acidic hemlock needles. Hemlocks were harvested because of tannic acid, right they produce acid. However, the pH level only has to be appropriate for trout, they don't calculate some balance w/ acid rain or anything else.
3) Along with a change in acidity, a switch from denser hemlock to thinner hardwood would allow light more light in and tend to raise water temps. The higher water temps in many cases were too much for brookies. That's the biggest issue in brookie decline with forest change and removal right there: higher water temps. However, we have all seen that some PA streams, usually small ones, have enough ground water to still provide cool enough water for brookies under thinner hardwood tree canopies. People see this in studies of streams under hemlocks that are being reduced by adelgids: With enough ground water input, streams can still be cold enough for brookies under suffering hemlocks.
4) My real point is that the well documented switch from hemlock to hardwood with logging altered a food chain involving trout. Hardwood leaf litter is more productive for in-stream invertebrates than hemlock leaf litter. Invertebrates eat leaf litter, and trout eat invertebrates.
why do I think think hardwood draining streams will have more inverbrates than eastern hemlock draining streams?
"The conversion of forests from hemlock to deciduous species is predicted to impact the hydrology, chemistry, and biology of associated headwater streams. In this study, we examined the macroinvertebrate communities of two adjacent headwater streams with differing hemlock influence in central Massachusetts. Abundance, taxa richness, diversity, and unique taxa were generally greater in the deciduous stream."
"Stream Macroinvertebrate Communities in Paired Hemlock and Deciduous Watersheds"
http://www.bioone.org/doi/pdf/10.1656/045.016.0108
Similar effects were seen - 2.7 times higher aquatic invertebrate densities under hardwood than hemlock - right here in the DWGNRA in the thorough Snyder study I have cited here. (No free online version of that one, but a university library would have it.)
A shift from hemlock to hardwood happened over many PA streams. It logically lead to pH, water temp, and invertebrate density influences. The bottom line is that trout don't care about the historical or natural water temp, pH, or invertebrate level of an environment. Hemlock logging no doubt killed many more brook trout than it helped through higher invertebrate levels under replacement hardwood canopies.
5) So yes stream pH and water temps would both probably rise a bit when hemlock canopies are lost through logging or adelgid infestation. The water temp issue can be a great threat for brookies, but as in limestoners, where there is enough ground water, trout still live. Small headwater streams are often spring fed.
But no, trout don't worry about balancing an acid rain effect, or the historical or natural temp or acidity regime of a stream. They just live - if they can - in the water and food envt they find themselves in. We wouldn't have many wild brook trout in PA if they only lived in unchanged "natural" environments!
And there is good reason to believe that where the water temp is still char-friendly after logging (or adelgids) cause hemlock canopies to change to hardwood, there will be higher in-stream invertebrate levels.
food chain!=> trees drop leaf litter of varying productivity for aqauatic invertebrates, which eat leaf litter; hemlock needles are slow decaying and a poorer food source (sorry eeull gibbons
) than hardwood leaf litter; trout eat aquatic invertebrates.
so imho a small minority of pa brook trout streams may have higher biomass now than in 1492. I am not defending logging, or the adelgid, just trying to be thorough about potential effects.