Trees do not waste their energy growing branches and twigs whose leaves will not reach the sunlight. In this little grove of trees, the two on the left and right edges have put all their efforts into growing branches on the exterior of the clump, and almost none in the middle. The trees in the middle have simply grown straight up, adding twigs and leaves at the top. As a result, the entire copse has the same shape as a single tree growing all alone in the middle of a field, and each individual tree gets a decent share of the solar power.
It somehow reminds me of Indian dancers with many arms:
[Photo credit: By Will Folsom [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)%5D, via Wikimedia Commons]
In writing this, it occurred to me that English has many names for a small group of trees: coppice, copse, spinney, stand, grove. I chose “grove”, “clump”, and “copse”, just for variety. Unsurprisingly, we also have lots of names for rain: shower, drizzle, downpour, deluge, .. and indeed for coffee.