When this tree fell in the middle of the forest, everybody heard it. It was a giant redwood. It was a mighty oak. It was an evergreen.

There is no way to see the recent and abrupt resignation of longtime reporter and commentator David Brinkley as anything better than a disaster for NBC News, and some NBC News insiders are also viewing it as a disgrace. They say the man chiefly responsible for the loss of Brinkley -- who was nearing his fourth decade with the network -- is NBC News president William J. Small, considered by some so insensitive and corrosive a leader that he might have learned personnel management from the Ayatollah Khomeini.

Since Small took over the news division in September 1979, it hasn't exactly rollicked through a heyday. "NBC Nightly News" has not been successful in rebuffing competition from ABC's "World News Tonight," NBC's "Today" show continues to lose ratings weeks to ABC's "Good Morning, America," and there have been such additional embarrassments as NBC's slip-and-slide coverage of the shooting of Pope John Paul II.

But NBC spokesmen insist Small is not the reason for Brinkley's startling departure. And, for the record, Brinkley says that, too. From his New York office -- which he'll vacate Sept. 20 -- Brinkley says that he feels NBC News no longer offers an opportunity for him to do what he does best, especially now that Tom Brokaw and Roger Mudd have been signed up to be the Huntley and Brinkley of the '80s.

Brinkley wants to be the Brinkley of the '80s, just as he was the Brinkley of the '50s, '60s and '70s.

"I'm leaving NBC because there is nothing here that I really want to do," Brinkley says. "I'm not all that crazy about the Eastern Shuttle. I want to work in Washington, where I've spent my life. I'm not quitting work. I'm just changing employers . . .

"The company has changed, moved along, as it should have, I suppose. The news is wrapped up with Mudd and Brokaw, which is fine. It just hasn't left me much to do." The dubious honor of anchoring NBC's doomed weekly "Magazine" show is obviously not enough for someone with Brinkley's experience and talents.

Brinkley's contract with NBC still had four years to go. Under normal circumstances, he could have resigned but still been held to the terms of the contract, at least those terms that forbade him from working for another network. But -- reputedly in return for foregoing massive doses of severance pay -- Brinkley will be a free agent as of Sept. 21. He can work for ABC or CBS. He says he might even do that. He is also thinking about returning to the prehistoric medium of print.

What could have happened to Brinkley at NBC that would cause him to forsake the alma mater of 38 years that even he refers to as his "womb"? The halls of the network are abuzz with speculation, some of it indignant. It is said Brinkley got fed up after a long series of "slights and hurts" from Small. Among them: the one segment of "Magazine" that Brinkley enjoyed doing, and that made some use of his abilities, was a round-table chat with reporters on the show about their stories. Small killed the round table after one month.

According to insiders, the final insult was when Small proposed that the fee Brinkley is paid for the program be cut (Brinkley's ample base salary contractually could not be touched). The fee included a bonus payment for commuting expenses that Small, under intense budget pressures from NBC and parent corporation RCA, suggested be dropped. NBC president Robert E. Mulholland reportedly intervened and had the fee restored, but the wound to Brinkley's pride did not heal. Official network spokesmen deny this occurred.

Brinkley says, "Small is not really my reason for leaving," and "We haven't even discussed money. Money had nothing to do with it." But he does not deny the story about Small's attempt to trim his fee. He says only, "The fee is so small, it doesn't make any difference."

The pettiness of the attempted fee-cutting is what angers some NBC insiders, especially when they contemplate such recent extravagances as the multimillion-dollar deal Brokaw cut with the network -- causing at least one NBC veteran to mutter, "Millions for Brokaw, not one cent for Brinkley."

A source close to Brinkley says that at a meeting with new NBC chairman Grant Tinker, Brinkley indicated he would stay with NBC if he knew that Small would be leaving soon, but that in the interests of maintaining the illusion of "stability" at the network, Tinker refused to fire Small. Official network spokesmen deny this occurred.

Even Small's defenders at the network concede that he is a difficult man -- "abrasive" and blunt, and not adept at handling people -- and that Brinkley did not like him (Small's longtime feud with Mulholland is almost legendary). But they also say that "strong leaders" often develop passionate factions, one way or the other. Ancil Payne, former chairman of the NBC affiliates, said yesterday that he was extremely saddened by Brinkley's decision, but said of Small, "He's a very direct and forceful guy, a very frontal person. Someone new in a company comes in and shakes things up like he has done, and there are bound to be some dissatisfied people."

Small's critics at the network say he has divided the news division into two camps: dissatisfied people and his own coterie of yes-persons.

On another and somewhat melodramatic level, the almost casual way the network has treated Brinkley's decision to leave suggests another case of the old guard being dislodged by Young Turks -- in this case, the likes of Chris Wallace, one of Small's fair-haired tots. Insiders say Wallace was in fact offered the job of replacing Brinkley on "Magazine," but since "Magazine" will air opposite "The Dukes of Hazzard" on Friday nights this fall -- and has virtually no chance of earning a civil rating -- Wallace turned the offer down. He is considered by some, including himself, a top prospect to replace Brokaw in the cushier anchor chair of the "Today" show.

A network spokesman says it is "not true" that Wallace was offered the job and turned it down.

Even Young Turks see in Brinkley a man of incomparable skills and influence. His trademark wry, clippety delivery is no affectation, but a style ideally suited to broadcasting; he is also almost unanimously considered the best news writer ever in network TV. Brinkley is probably a more important figure in television journalism than Walter Cronkite, partly because Brinkley wrote all his own words; partly because, though he may occasionally have been dull, he never really got pompous.

Roger Mudd, who will not discuss Brinkley's departure, is willing to say of the man, "He brought a level of political sophistication and literary craftsmanship and a lively sense of humor that television had never known before and that hasn't been equaled since."

Tom Brokaw, who also will not discuss Brinkley's resignation, says, "I literally grew up with him. When we moved to a town in South Dakota that got television, the first thing I was conscious of was this program called 'The Huntley-Brinkley Report.' I watched him all my adolescent and adult life.

"What people do not fully appreciate about him is the amount of wisdom he has dispensed over the years; he's not one who demands a drum roll for everything. There was a lot of insight and a healthy skepticism from him. I enjoy his company."

Richard C. Wald, a former NBC News president who is now a top executive at ABC News, says of Brinkley's skills as a political reporter, "I don't think anybody does it better, and I think he's just at the height of his game now. More than any present broadcaster, he is the person that has defined political and Washington coverage for us."

And Reuven Frank, the former NBC News president who teamed Chet Huntley and Brinkley in the first place for their long domination of network news, recalls first admiring Brinkley for his coverage along the parade route when President Eisenhower returned from a hospital stay in the '50s. "You didn't have to be told that was Eisenhower waving to the crowd, but everybody else along the parade route did tell you," says Frank. "Not Brinkley.

"So far as I know, he's the best writer for television in the field of news that there's ever been. He's really a creature of the medium -- he hadn't been sullied by print -- but what people may forget is that he is a terrific special-events man. He knows how to shut up, and anybody who watches any sports event on television knows how rare that is.

"Because he was a star, and a stylist, he gets too little recognition for being a professional. He knows television. It's in his bones."

All this sounds faintly, maybe loudly, like eulogy. Brinkley does not want to be eulogized because, well, he isn't dead, and he doesn't intend to stop working soon. One reason he is leaving NBC, he says, is that when another political campaign rolls around, he would go mad on the sidelines. "To sit home next time, and look at the elections and conventions on the air -- I just couldn't stand it."

It seems mandatory to call this the end of an era. To that notion, Brinkley, surely among the least pretentious towering figures ever in journalism, says, "Oh, there's the end of an era of some kind about every three days."