HARVEST PAST MULCH LAYERS BEFORE APPLYING NEW!! Any gardener continually mulching over last 5 or 10 years, will know correct answer to this posted question. As long as they are interested citizen scientists, who actually observe how things develop with Mulch over time. Fact is, decomposing Mulch actually turns into rich soil. And goal of mulching, is to improve condition of original soil levels. top "O" horizon. Otherwise rather than Mulching, gardeners could just put down new layers of rich soil instead every spring. Gardeners need to be aware of most important horticultural rule, which states "Never raise existing grade" over trees original root systems underneath. Otherwise suffocated feeder roots growing just under soil line, produce new adventitious roots extending upward to get above obstruction. Like drowning swimmers, adventitious roots become disorientated, and grow in all directions. Including back toward tree, becoming SGR's or stem girdling roots. Along with causing soil to become compacted over time from confused networks of knitted crossing roots. Which can result in tripping roots growing above smothered ground.

I've never heard of such a thing, but I can tell you pine needle mulch doesn't fall down the cliff. I've used that on the side of my road, which is only about 4' high, but steep enough regular mulch ends up in what would be gutters if my country road had them.

agree with above but, i would also stress how important organic matter is... i use tons of "chop+drop... all the extra leaves, vines, twigs, grass, fruit peels, coffee grounds and anything else i can use. i use cardboard or newspaper as a ring around my plants to keep in moisture and keep weeds down... you can use a woody mulch on top of that which has a nice appearance... i generally do this in my front yard, while, in the back, i use leaves, twigs, compost etc... its about a foot deep in many areas, but i make sure to have 3 inches or so clear around the trunk. in my backyard i had used newspaper and cardboard with a little grass clippings and coffee grounds UNDERNEATH. then, leaves on top. the next year i had to add more newspaper , grounds, grass and leaves. since then, i have enough chop+drop leaves and twigs to keep 6in to a foot of mulch. i get about 100lb of grounds from starbucks every week. this attracted worms like crazy. and i have built several inches of soil. the top layer of soil is almost black, soft, moist, and full of worms and other bugs. all those bugs eat and poop.... that waste stream (including dead insects) is excellent fertilizer, and keeps the soil loose and aerated. its basically composting-in-place. i started with sand... plain sand... the house was built on it. if you start with poor soil, you may need to use a small amount of fertilizer to start, but i kept mine as organic as i could... kelp, fish emulsion etc... until the soil is right. no one fertilizes the forest, and it does quite well.

Mulch, though some folks recommend not within a few feet of the house, and weed as needed. You don’t want erosion. You may need to do some modification of flow if it is concentrated in an area, such as creating a rain garden or dry well or French drain. If there is a slope, use highly texture mulch such as “gorilla hair” or arborist’s trimmings to help keep it in place. Where you want established moss, don’t mulch. However, it will be best for small patches in flat areas since it won’t stop erosion, and eventually will be shaded out by groundcovers IME.