Mr Buchan, who chairs the well-regarded £556 million Personal Assets Trust, said trusts should hold back from the expensive business of appointing advisers to support relocation because the industry will have a window of at least 18 months if there is a "yes" vote in September's referendum.
His comments come after broker Winterflood said some trusts have examined changing jurisdiction amid concerns about taxation and regulation in the event of independence.
Mr Buchan said: "It is an issue that has been raised at shareholder meetings I was at recently.
"If it (the vote) says 'yes' we will have at least 18 months to evaluate the various pronouncements to see whether it has any kind of impact on taxation, regulation and costs and those sorts of things. All one is doing is keeping a careful watching brief."
He added: "People like lawyers and accountants, auditors, advisers and brokers are all keeping an eye on this. It will continue to be on the agenda and the foot will go on the accelerator at some stage. We are on amber rather than red."
While other funds such as unit trusts and open-ended investment companies face similar issues, the investment trust industry is of particular importance because its roots in Scotland date back to the 19th century and Scottish companies account for around 18% of the UK industry.
Mr Buchan, former chairman of industry trade body the Association of Investment Companies, said a key issue for the industry is taxation. Investment trusts escape tax on capital gains.
But he cautioned against trusts acting too quickly by hiring advisers.
"You could be spending tens of thousands of pounds and the whole thing is a lemon because it is a 'no' vote," he said.
Mr Buchan, who serves on the boards of the venerable £807m Scottish Investment Trust and £1.8 billion Templeton Emerging Markets Trust, said: "There is going to be a heck of a lot of horse trading (after any 'yes' vote).
"I think it is virtually impossible to get it done in 18 months. Even if it was done in 18 months and it was going to disadvantage companies, we would reorganise then."
He said that there had been little sign of investors eschewing Scottish-based trusts as the referendum campaign hots up.
"I do not think people are shunning Scottish trusts.
"We haven't seen any sign of a drop off in activity or turnover," he added.
Mr Buchan, a former analyst with NatWest Securities, said trust directors would act to defend investors' interests.
"We would do our damndest to protect our shareholders, particularly those down South, and make sure the status quo remains for them."
Some Scottish trusts are run by investment managers south of the Border, such as Personal Assets Trust's arrangement with London-based Troy Asset Management. Others, including the £1.1bn Monks investment trust, are English-registered but have Scottish managers, in its case Baillie Gifford of Edinburgh.