Visitor revenue from Scotland's state-owned historic properties has "bucked the trend" by increasing 15% last year to £31 million in 2011-12, according to Ruth Parsons, chief executive of the heritage agency Historic Scotland.

In her first-ever press interview, in which she hit back at critics of her tenure of the £100,000-a-year post, the controversial quango boss pointed to the strong performance by Historic Scotland (HS) as "evidence that it is going in the right direction and performing well in difficult economic conditions".

Earnings from its 78 staffed properties around Scotland defied the slowdown to rise £4.2m or 15% from last year's £27.8m, and are up £5.8m or 22% on the £26.2m figure for 2007-08, the year preceding Parsons's appointment.

But there was also evidence of HS's bruising reorganisation of senior management in figures showing heightened costs from departure of staff. In 2011-12 the cost of staff accepting severance or early retirement terms was £477,985. While this is half of the previous year's £934,000, it compares with only £64,157 in 2007-08, the last year of the former regime. Parsons said the departure of senior staff would "generate ongoing savings" enabling her to secure frontline jobs: "That's how we can double our apprentices by creating 30 new places," she said.

The new revenue growth partly offsets a real-terms cut of 24% in Scottish Government funding to the agency, which is currently £47m, phased over the next three years.

Parsons, 50, would not take personal credit for the visitor growth, citing the "many years of investment" for the rise, and praised the strength, depth of the expertise, and passion of agency staff. She hinted that her own contribution was to increase focus on the benefit of heritage to Scotland's economy, particularly in the tourism and construction sectors.

"When I started working with Historic Scotland I saw just what a contribution it was making to Scotland in terms of its economic, social, cultural and environmental impact and yet [HS] hadn't been able to present that effectively," she said.

The former HM Customs & Excise civil servant, who has worked to instil a more business-aware culture in HS, said the rising figures reflect the investment in refurbishing HS attractions as well as in ensuring rare "high-end" traditional skills are maintained.

Parsons strongly refuted press allegations of discontent within the organisation, including accusations of "bullying" levelled at her personally. While no formal complaints are under consideration, a spate of departures by experienced senior managers since her appointment in October 2009 include the chief inspector, the director of properties in care, the director of outreach and education, head of the press office, director of human resources, director finance and director of policy.

Parsons declined to comment on the departures, but claimed her "unsettling" changes were designed to "bring out the best in people" and improve corporate decision-making. She admitted she found public criticism of her management style difficult, but claimed strong internal support for her changes, which she said had given HS's conservation department its seat at the senior management table along with visitor services, commercial and tourism, and heritage management.

"It was difficult to read those things in the newspapers but you have to understand that when you are taking an organisation through change, not everyone is going to agree with those changes or accept those changes."

In return, Parsons claimed to have been offended on behalf of the remaining staff, whose contribution, she said, is slighted by the suggestion that the exit of key conservation personnel was putting Scotland's heritage at risk.

"I would absolutely refute that we have lost significant amounts of expertise and knowledge. I think our staff feels that was grossly unfair as well because we have such a strength and depth in the organisation."

Parsons pointed to the widely-acclaimed £12m refurbishment of the royal apartments at Stirling Castle, opened by the Queen last summer, as an example of HS's commitment to the highest quality standards, and the forthcoming National Conservation Centre, also at Stirling, due to open in 2015. The centre, a collaboration with Stirling Council and Forth Valley College, will showcase and preserve those high-end skills, some of which will be passed on to the 30 extra apprentices.

The other major project under way, in partnership with the National Trust for Scotland, is the new £10m Bannockburn visitor attraction, to be completed in time for the 700th anniversary of the battle in June 2014.

Cabinet Secretary for Culture and External Affairs Fiona Hyslop said: "I have been very impressed by the knowledge, expertise, skills and commitment of Historic Scotland staff, and the high quality and range of the services they provide.

"I have full confidence in Ruth Parsons and the demonstrated ability of the agency to meet the challenges of promoting and safeguarding our historic environment."