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I recently flew into New Jersey over the Delaware river and took this photo of the ridgeline at the Delaware Water Gap. The trees are mostly green (early October) but on the south-east side of the ridge there is a bright streak of reddish foliage along the entire length of the ridge. (I noticed this on other hills in the area as well.)

What could be causing this color change? My first guess would be that it is related to sun exposure.

enter image description here

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Your photo doesn't show the strip of autumn-coloured trees very well; without your description I would have taken it for a strip of bare earth. All sorts of things could account for the autumn shades, there are too many unknowns to give a definite answer. We don't know whether the red-leaved trees are the same species as the others, for example.

I doubt if sun exposure alone could account for it, though it might be a combination of sun exposure and soil conditions, or soil conditions alone. The red trees seem to follow a line where strata different from the rest reach the surface. This would affect the soil conditions, nutrients and perhaps available moisture. Without an investigation on the ground it is impossible to be sure what the cause is, but I think a difference in soil conditions is the most likely explanation.

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  • $\begingroup$ Thanks for the insight, and sorry about the photo quality. Taken with a phone camera out an airplane window through some haze. I might try to get out there for a hike and take a closer look. $\endgroup$ – Nat Oct 8 at 19:07
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    $\begingroup$ All of these conditions could also lead to a different species dominating on the south side of the ridge. $\endgroup$ – Brian Borchers Oct 9 at 1:50
  • $\begingroup$ I'd support the "different soil" theory. Reading up a bit on the Delaware Water Gap and having a look at satellite images (Google Earth) indicate different geological layers near the top of the southeast facing ridge. $\endgroup$ – Erik Oct 9 at 7:22

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