A 100,000-square-foot office tenant relocating in the downtown market is like a Tyrannosaurus Rex crashing through the brush: Every other creature in the vicinity – predator or prey – pays attention, even if nobody is exactly sure what the Tyrant Lizard is up to.

Between 2013 and 2015, some of Boston’s largest corporate dinosaurs will see their leases in the Financial District’s most high-profile towers expire. But large tracts of contiguous space are scarce, potentially creating upward pressure on rents in the coming year.

Corporate leases from firms as diverse as Arnold Communications, Brown Brother Harriman, Iron Mountain and Blue Cross Blue Shield will all expire in the next few years. There are roughly a dozen or so large firms “potentially” on the hunt for new space, and all currently occupy spaces in the 75,000- to 200,000-square-foot range. But there are only an estimated half dozen or so contiguous spaces expected to be available that can handle that kind of capacity.

Moving The Market

Obviously, the best spaces at the best prices will be absorbed first. As that space is taken off the market, it reduces the number of options for a group of tenants that “can really move a market,” said Benjamin Heller, managing director for Jones Lang LaSalle in Boston.

That, in turn, puts leverage in the hands of landlords – a rare occurrence after years of tenant-driven markets. In the most in-demand, high-rise portions of the city’s office towers, rents are likely to climb by as much as 5 percent in the next year or two. Mid-rise spaces may see a 2 percent to 3 percent bump in 2012 or 2013, according to Joe Sciolla, managing director for CresaPartners.

And if rents are pushed too high, or space becomes so tight there’s none left at the top, the market could finally find a cure for the glut of low-rise tower space that formerly housed back office-type jobs at large financial services companies, law firms and insurers. Those jobs have since been shipped to cheaper digs, if not eliminated entirely.

This outsourcing has created a bifurcation in the tower market in recent years. Space on the more prestigious upper floors – the spaces with million-dollar views and client-wowing cache – have experienced strong demand and are fetching healthy rents. But that demand hasn’t trickled down, so to speak, to lower floors, where vacancies persist and rents remain anemic.

But that’s nothing a good ol’ game of big-tenant musical chairs can’t fix.

“This may be the trend that gets those tower owners through that tough period,” Heller said. “Some low-rise could be absorbed as a result of this. There’s certainly some value [large tenants will] find in being in low-rise space in a very good building, because of the efficiencies of floor plate size.”

Not So Fast

But talk of movement in 2013 and beyond is hard to quantify in 2011. In order to move at all, companies need to either grow – or contract – to be forced to relocate. Otherwise, they may opt to simply stay put, albeit at revised terms.

Complicating that aspect, however, is the fact that many of the largest blocks of space likely don’t have renewal options, because landlords want the freedom of renting those valuable tracts to new tenants when it looks like they can raise rents, according to real estate executives.

Another wrench in the works is the possibility that many of these tenants could end up looking at the suburbs, as well as downtown, and could split operations between the two.

And the scarcity of new construction has placed additional pressure on large users as they enter the marketplace.

“The result, as has been seen in the Back Bay and Cambridge, are some reasonably dramatic increases in rental rate over the last 12 months,” said one industry expert that wished to remain anonymous.

Charles O’Connor, a partner in FHO Partners’ corporate advisory services group, said some of the big boys are coming into the market too early, looking to secure favorable deals today, often at the expense of potential landlords – especially those with leases expiring in 2014 and 2015.

“There are tenants with leases up in ’14 and ’15 that are going to properties now and the landlords are saying, ‘Well, what are we going to do about your [current] lease?’” O’Connor opined. “Unless the [new] landlord is prepared to eat the [remainder of the] potential tenant’s lease, especially if there are current vacant spaces out there, it’s hard to solve for X in that equation. Some tenants think the market is improving and want to get a jump on it. The people advising these large tenants need to ask, ‘How does the tenant best leverage its occupancy?’ We’re constantly telling our clients to sit tight, you’re paying the right rent where you are now.”

In the end, relocations and absorption – and resulting growth in rents – come from job growth, and without job growth, the Financial District will just be playing musical chairs, filling some vacancies but creating others, said another broker that wished to stay anonymous.

“A lot of the major tenants have 200,000 square feet and are out looking for 200,000 square feet. And a lot of law firms are 200,000 square feet looking for 150,000 square feet. Until it’s not musical chairs and we have some job growth, I don’t know how rents will grow.”