Let's think a bit more about this dichotomy. On one hand, trees are at the heart of this "natural civilization" with its own purposes, wishes, and history; they have personalities, are fallible, and are not omniscient. Treebeard remembers much of the ancient past, but cannot say why the Ents have lost the Ent-wives. The trees of the Old Forests have hearts and "thoughts, which were often dark and strange, and filled with a hatred of things that go free upon the earth," their memory runs far into the past to the times of the "fathers of trees, remembering times when they were lords." (The Fellowship of the Ring I, "In the House of Tom Bombadil") Indeed they have thoughts and memories, but cannot be called intelligent - their knowledge seems to be organically accumulated through time and experience, rather through deduction and creation. They are wholly another sort of being, which seems to become sentient simply by witnessing the changes of their time and inheriting the organic "memory" of their forefathers.
In comparison, the "worship-trees" which derive from the Two Trees of Valinor do not seem sentient. They exist only in relation to the civilizations of men, elves; a sort of link between Middle-Earth's inhabitants and the Ainur and Valinor. They are personality-less, but beings take wisdom and strength from them. Consider the imagery evoked in Fellowship II: "Lothlórien" :
They were standing in an open space. To the left stood a great mound, covered with a sward of grass as green as Springtime in the Elder Days. Upon it, as a double crow, grew two circles of trees: the outer had bark of snowy white, and were leafless but beautiful in their shapely nakedness; the inner were mallorn-treees of great height, still arrayed in pale gold. High amid the branches of a towering tree that stood in the centre of all there gleamed a white flet. At the feet of the trees, and all about the green hillsides the grass was studded with small golden flowers...Lothlórien is portrayed as a place of worship by elves and plants alike. Frodo's serene awe is akin to the feeling of entering some great cathedral, mosque, or temple, and being struck by a central symbol or symbol. He lays is hand upon a tree, and "felt a delight in wood and the touch of it (...) it was the delight of the living tree itself." These trees are planted, they are worked: they are a part of another civilization, rather than ruling over their own.
So, here's an interesting dichotomy. On one hand, we have trees like the cross that narrates the Dream of the Rood - timeless, jewel-like, an object of worship and source of renewal, wisdom, and strength. On the other hand, we have trees like the Green Knight - unpredictable, dangerous, yet civilized and patient. Their "intelligence" and "morality" are defined in their own terms, they cannot be converted to human terms.
Let me digress slightly to draw parallels between the Ents and the Green Knight. Treebeard is described as "a large Man-like, almost Troll-like figure" fourteen feet tall. The hobbits cannot tell if he was "clad in stuff like green and grey bark, or whether that was its hide, was difficult to say." Yet the most striking feature is his "deep eyes (...) slow and solemn, but very penetrating. They were brown, shot with a green light." Pippin feel as as if "there was an enormous well behind them (....) but their surface was sparkling with the present; like sun shimmering on the leaves of a vast tree..." (The Two Towers I: "Treebeard") Let's summarize this description: anthropomorphic but definitively inhuman, full of green light. Now, how is the Green Knight introduced "his loins and limbs so long and so huge / that half a troll upon earth I trow that he was..." (sound familiar?) "All of green were they made, both garments and man: a coat tight and close that clung to his sides" (almost as if it is a hide, a skin, a bark). And his gaze! "his glance was lightning bright / so did all that saw him swear / no man would have the might / they thought, his blows to bear." His glance alone communicates such power. The supernatural man glows grass-green, he is a "lord gazing". Like the sentient Middle-Earth trees, he is a witness, an occasional participant in human history, but his wisdom and thought is alien to the humans.
Here's what I think. At the center of creation are the sacred Telperion and Laurelin. They are species-less (but gendered!) and life-giving. Even after their destruction (is their poisoning akin to a crucifixation?), their power and imagery is reiterated and echoed in jewels (the Silmarils) and other sanctified trees which are sources of strength for humans, elves and Ainur (e.g. the mallorn-trees of Lothlórien, as we have mentioned, as well as the White tree of Gondor, which itself is descended by a tree created by a Vala.) We can call them "jewel-trees," "cross-trees" - they facilitate sub-creation by passing down the gifts of creation, most fundamentally, through light and strength.
The "Green Knight" trees are entirely separate. They are diverse and specied, they have personalities and faults. They have civilizations and realms of their own, as well as their own ways of forming "intelligence," memory, and power. The manner of their thought is alien to us, which is why we see them alternately as too patient and unpredictably impulsive - we cannot reason with them or solve their problems. However, by interacting with them, we come to better understand the world that we live in, and perhaps our own selves. This is certainly the experience of Gawain, it is also the experience of the hobbits and all others who witness the Ents' march upon Isengard. By acknowledging this alternate civilization - which overlaps but does not merge with ours - we come to recognize that we, children of Ilúvatar, are surrounded by beings whose forms recall those most sacred objects of our world, the Two Trees of Valinor.
- Elaine Yao