The planned cuts involve 270 Government staff, many of whom hold the key to tackling climate change, flood management, urban drainage and coastal management, according to the institute.
The Northern Ireland Planning Service aims to "redeploy" a third of staff because of an £8m funding cut caused by a reduction in planning application receipts.
The institute spoke out after environment minister Edwin Poots announced the need to make savings late last month. Northern Ireland branch chair Simon Bell said: "Redeployment won't save money. Reading between the lines, he means job losses."
Landscape Institute chief executive Alastair McCapra warned that cuts would "leave us ill-prepared to adapt to climate change".
The Climate Change Act required the Northern Ireland Assembly to draw up programmes to fight climate change through good public spaces and grey water recycling. "Without the expertise of landscape architects in planning, how can this be done?" asked McCapra.
Landscape Institute policy and communications director Paul Lincoln said the plans flew in the face of a recent Government consultation that placed huge emphasis on green infrastructure.
"Severe cutbacks will affect landscape architects and make it extremely difficult for the directorate to meet its climate change and European obligations," he added.
A spokesman for the executive's Department of Finance & Personnel said no targets had been set on staff reductions. But the need to make savings "will inevitably place downward pressures on workforce numbers", he added.