Metoro did not say niu at Aa1-13, but kua tuu, but otherwise he fairly consistently said niu at GD18-glyphs. What does he mean by kua tuu? I think he means that the text has arrived at an important pillar standing erect:

Consulting Churchill 2 I found that in Mangareva niu means the coconut palm when young, ripening into nikau. I draw the conclusion that when the palm has grown tall and the crown is high above, then it is natural to make a wordplay using nika (above). Although another explanation is offered:

"... the ni of New Caledonia leads us to infer that niu was anciently a composite in which ni carried at least some sort of generic sense, it being understood that this refers to those characteristics which might strike the islanders as indicating a genus.

In composition with kau tree we should then see nikau, the ni-tree, serving in Mangareva for the coconut palm, in New Zealand for the characteristic palm (Areca sapida) of that land, in Tahiti as niau for coconut leaf, and as niau in Hawaii for the leaf stalk of the coconut. The ni-form is found in Micronesia, and in the Marshall Islands ni is the coconut."

The word kau - from the Melanesian Efaté language(s) - at first sight evidently is a borrowing from the latter part of Polynesian rakau:

How extraordinary wide a field of meaning rakau has! The meaning goes to the extreme in Marquesan akau 'anything in general'.

In the Mamari moon calendar Metoro identifies (as I think) the night immediately before full moon as rakau:

Ca7-23	Ca7-24
Rakau	Omotohi

For me it appears that rakau primarily has to do with 'wealth', a meaning which could easily then be transferred upon the important kinds of trees, not only on Easter Island but on every island with limited resources (as on the global island nowadays). Without efficient medical resources rakau would also quickly be transferred upon 'medicine'. I remember Thor Heyerdahl's comment that modern medicine was the only thing he really missed when living as a native on Nuku Hiva.

'... They came on again, they came to Te Manavai, to Canoe Bay, to Anavaero and all those places along the south coast. They came to Onetea where the white sand is, and said, 'This is the land for the ariki to live on.' But one said, 'No, this is poor land, our breadfruit and our coconuts will not grow here ...'

In Churchill 2 we can read:

"The stem kau does not appear independently in any language of Polynesian proper. For tree and for timber we have the composite lakau in various stages of transformation. But kau will also be found as an initial component of various tree names. It is in Viti that we first find it in free existence. In Melanesia this form is rare. It occurs as kau in Efaté, Sesake, Epi, Nguna, and perhaps may be preserved in Aneityum; as gau in Marina; as au in Motu and somewhere in the Solomon islands.

The triplicity of the Efaté forms [kasu, kas, kau] suggests a possible transition. Kasu and kas are easy to be correlated, kasu and kau less easy. They might be linked by the assumption of a parent form kahu, from which each might derive. This would appear in modern Samoan as kau; but I have found it the rule that even the mildest aspirate in Proto-Samoan becoming extinct in modern Samoan is yet retained as aspiration in Nuclear Polynesia and as th in Viti, none of which mutations is found on this record."

Words can be studied in two ways: either the way Churchill does - by following the 'roots' back through the centuries - or by trying to see what wordplays would be irresistible at a given locality in time and space (how the 'canopy of the tree' appears). The coconut palm, how does it 'look'? We have no possible chance of seeing the whole picture, but sometimes we find clues. Three such are reiterated here:

1. [Ku-kolu - Tu-toru] '... is the first night of the rising of the Moon. It is valueless to the farmer for planting potatoes, bananas, gourds; they would just shoot up like coconuts ...' (from the Hawaiian Moon calendar)

2. '... Maui came out again; he cut off Tuna's head to take it to his ancestor. But Huahega his mother took it from him and she said: 'You must bury this head of Tuna beside the post in the corner of our house.' Maui did so, and that head grew up, it sprouted, it became a coconut tree. On the nut which is its fruit we see the face of Tuna, eyes and mouth. All coconuts have this ...' (Legends of the South Seas)

3. '... The Ackawois of British Guyana say that in the beginning of the world the great spirit Makonamia (or Makunaima; he is a twin-hero; the other is called Pia) created birds and beasts and set his son Sigu to rule over them. Moreover, he caused to spring from the earth a great and very wonderful tree, which bore a different kind of fruit on each of its branches, while round its trunk bananas, plantains, cassava, maize, and corn of all kinds grew in profusion; yams, too, clustered round its roots; and in short all the plants now cultivated on earth flourished in the greatest abundance on or about or under that marvellous tree. 

In order to diffuse the benefits of the tree all over the world, Sigu resolved to cut it down and plant slips and seeds of it everywhere, and this he did with the help of all the beasts and birds, all except the brown monkey, who, being both lazy and mischievous, refused to assist in the great work of transplantation. So to keep him out of mischief Sigu set the animal to fetch water from the stream in a basket of open-work, calculating that the task would occupy his misdirected energies for some time to come.

In the meantime, proceeding with the labour of felling the miraculous tree, he discovered that the stump was hollow and full of water in which the fry of every sort of fresh-water fish was swimming about. The benevolent Sigu determined to stock all the rivers and lakes on earth with the fry on so liberal a scale that every sort of fish should swarm in every water. But this generous intention was unexpectedly frustrated. For the water in the cavity, being connected with the great reservoir somewhere in the bowels of the earth, began to overflow; and to arrest the rising flood Sigu covered the stump with a closely woven basket. This had the desired effect. But unfortunately the brown monkey, tired of his fruitless task, stealthily returned, and his curiosity being aroused by the sight of the basket turned upside down, he imagined that it must conceal something good to eat. So he cautiously lifted it and peeped beneath, and out poured the flood, sweeping the monkey himself away and inundating the whole land. 

Gathering the rest of the animals together Sigu led them to the highest points of the country, where grew some tall coconut-palms. Up the tallest trees he caused the birds and climbing animals to ascend, and as for the animals that could not climb and were not amphibious, he shut them in a cave with a very narrow entrance, and having sealed up the mouth of it with wax he gave the animals inside a long thorn with which to pierce the wax and so ascertain when the water had subsided. After taking these measures for the preservation of the more helpless species, he and the rest of the creatures climbed up the palm-tree and ensconced themselves among the branches ...' (Hamlet's Mill)

There is a cave (ana) in which helpless creatures unable to climb the tall tree (ana) - and not able to swim - can hide waiting for the waters to subside. They are hidden like nuts in the womb of mother earth during winter.