Maybe it's kind of like - loose sphagnum probably allows for pools of water to be formed among the media. That is, when enough water gets on the bits of the loose sphagnum segments, the water and loose sphagnum combination can form localised layers or pools/clumps of water through out the pot region. And when you have some (or a lot) of these water clumps, then it becomes a bunch of pools.
Pathways, directions and speed of aerated water movement in the media and pot will be affected by the pool regions. Perhaps the more pools there are, or the bigger these pools are, the more it affects the movement (or no movement) of oxygenated (aerated) water, which the roots (or parts of roots) need to survive.
The non-uniformity of this loosely-packed water soaking media could mean significant variability (variations) in aerated water moving activity - some regions might have slow water movement - or hardly any aerated water movement.
But - when the sphagnum is firmly packed and the medium approaches a tighter network or has more uniform water spreading and propagation (movement) properties throughout - working collectively as a wick, then aerated moisture/water can keep propagating (at some desirable rate) without getting hung up too much in any particular region - provided the amount of water we add each time (when it is time for watering) doesn't overload the system - in which case - drowning of roots could still occur.
When packed firmly, the water that is being evaporated (away from the surface of the pot and through holes etc) probably pulls/draws the water from the inner regions (of the packed sphagnum) much more uniformly than loose sphagnum - maintaining some 'suitable' rate of water movement throughout the pot. And - as we know - moving water (even slowly) that contains oxygen, will sustain the roots. But water moving at too slow a pace, or hardly any movement at all (if any medium gets water-logged or saturated) can result in roots somewhere in the pot running out of oxygen, and dying.
The key factor is - adequate oxygenated water movement. The roots generally occupy a fraction of the region of the pot/medium. So we can assume that there will be lots of oxygen carrying water in the medium. The main thing is that the oxygen carrying water in the various regions of the pot keeps moving adequately around any roots regions, and also keeps moving adequately in general - to avoid stagnation. Firmly packed sphagnum can provide the necessary conditions for this needed water movement.
If there is pooling of water (eg. water-logged loose packed sphagnum), then the movement of oxygenated water in the medium might have more variability in speed and directions of movement. Sections of roots in certain regions might use up all the oxygen where oxygenated water movement is very slow (or none at all) - and the roots could take a bad turn after a while.
It could also turn out that loosely packed sphagnum is fine too. It likely depends on how much water we add (and how we apply it - such as controlled spray doses or pouring it into the pot etc), and how we control the system - so that water logging in any region(s) of the pot doesn't occur.
It may be harder to get things right (or do what we need) with loosely packed sphagnum - such as working towards keeping the bulk of the sphagnum moist (without water pooling, without saturation of any region) when watering the media.
Also, good air-movement in the growing environment is likely to help the movement of oxygenated (aerated) water along (better than still-air environment), as well as providing other healthy benefits for the orchids.
We might also need to consider what happens if the medium has some pathways of aerated water movement slow down considerably, or no movement at all --- maybe the possibility of some kind of unwanted bacterial activity or some kind of unwanted fungal activity in certain regions or pockets of media.
One other consideration - pot contents isn't going to be just sphagnum. After a while, there's going to be lots of roots. So how the water will move among roots plus sphagnum is something to consider as well.
In short, when inside the pot, we don't want sphagnum to have water looking watery - like this:
Sphagnum - Wikipedia
Linear Mode
