Question: The desmodromic valve engine, widely used by Ducati, uses cams and levers to close its valves in place of what conventional mechanism?

Context paragraphs:
Active valve lift system: The I-Active Valve Lift System (i stands for intelligence) or i-AVLS is a valvetrain technology implemented by Subaru in the 2.5L naturally aspirated engines SOHC to improve emissions, efficiency and performance.  Note that AVLS is different from AVCS used on other Subaru engines.  AVLS improves performance and efficiency by changing which camshaft is operating which of the two intake valves.  The camshafts on all AVLS Subaru engines have specially designed lobes for intake valves.  They feature two different cam profiles: a low/mid lift profile or a high lift profile.  The two intake valves in each cylinder are operated by a rocker arm with its own cam lobe.  The cam utilized is selected by the Engine Control Unit (ECU).  To select different valve lift modes, oil pressure generated by the engine moves a pin which locks the two lobes together.  At low engine speeds the low/mid lift camshafts increases the speed of air rushing into the engine thereby increasing torque and efficiency.  At higher engine speeds the high lift camshafts fully open the intake valves, reducing resistance to incoming air and improving power.  AVLS only operates one of the intake valves in each cylinder as the other is always open to promote swirl.

Ducati singles: The Ducati singles were single cylinder motorcycles, made by Ducati from 1950 to 1974.  Chief Engineer Fabio Taglioni developed a desmodromic valve system in these years, a system that opens "and" closes the valves using the camshaft, without the need for valve springs.  This valve system has become a trademark feature of Ducati motorcycles.

Overhead valve engine: An overhead valve engine (OHV engine) is an engine in which the valves are placed in the cylinder head.  This was an improvement over the older flathead engine, where the valves were placed in the block next to the piston.  Overhead camshaft (OHC) engines, while still overhead valve by definition, are usually categorized apart from other OHV engines.

Ducati 749: The Ducati 749 is a 90° V-twin Desmodromic valve actuated engine sport bike built by Ducati Motor Holding between 2003 and 2006.  Designed by Pierre Terblanche, the 749 was available as the 749, 749 Dark, 749S, and 749R.  It shared many of its parts with the 999, with the exception of a slightly smaller 180/55 profile rear tire, smaller cylinders, and different cylinder heads, giving it a smaller displacement of 748 cc.  This smaller engine is higher revving and produces a lower peak horsepower than the larger engine used in the 999.  The 749S model, like the 999, incorporates features rarely found on production motorcycles, including adjustable rake (23.5° or 24.5°) and five-position adjustable rearset mounts.

Desmodromic valve: A desmodromic valve is a reciprocating engine valve that is positively closed by a cam and leverage system, rather than by a more conventional spring.

Ducati 998: The Ducati 998, a successor to the Ducati 996 and the final variation on the Ducati 916, was produced in various models from 2002 to 2004.  The new Testastretta engine shared many similarities with the previous Desmoquattro engine in the 996, although it was completely new from the crankshaft up.  Testastretta means narrow head and refers to a complete redesign of the cylinder heads.  The desmodromic valve actuation method was retained.

Ducati 888: The Ducati 888 was a motorcycle manufactured by Ducati as an upgrade to the Ducati 851.  The earlier 851 had introduced liquid cooling, computerized fuel injection and four-valve heads to the company's two cylinder motors.  In 1991 Ducati increased the capacity of the 851 to 888 cc to create the 888.  Both engines featured the Desmoquattro valvetrain concept in which a four valve per cylinder motor was given desmodromic valve actuation, with cams both opening and closing the valves.  Ducati's desmodromic system reduces the frictional penalty from conventional valve springs.

Ducati Superquadro engine: The Ducati Superquadro engine is a 90° V-twin four-stroke motorcycle engine made by Ducati since 2011.  It has Ducati's signature desmodromic valve system, with four valves per cylinder and gear/chain driven double overhead camshafts.  It has been made in four displacements ranging from , with power as high as in the largest version.

Valve actuator: A valve actuator is the mechanism for opening and closing a valve.  Manually operated valves require someone in attendance to adjust them using a direct or geared mechanism attached to the valve stem.  Power-operated actuators, using gas pressure, hydraulic pressure or electricity, allow a valve to be adjusted remotely, or allow rapid operation of large valves.  Power-operated valve actuators may be the final elements of an automatic control loop which automatically regulates some flow, level or other process.  Actuators may be only to open and close the valve, or may allow intermediate positioning; some valve actuators include switches or other ways to remotely indicate the position of the valve.

Gate valve: A gate valve, also known as a sluice valve, is a valve which opens by lifting a round or rectangular gate/wedge out of the path of the fluid.  The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow of fluid and minimum restriction is desired.  The gate faces can be parallel, but are most commonly wedge-shaped.  Gate valves are primarily used to permit or prevent the flow of liquids, but typical gate valves shouldn't be used for regulating flow, unless they are specifically designed for that purpose.  Because of their ability to cut through liquids, gate valves are often used in the petroleum industry.  For extremely thick fluids, a specialty valve often known as a knife gate valve is used to cut through the liquid.  On opening the gate valve, the flow path is enlarged in a highly nonlinear manner with respect to percent of opening.  This means that flow rate does not change evenly with stem travel.  Also, a partially open gate tends to vibrate from the fluid flow.  Most of the flow change occurs near shutoff with a relatively high fluid velocity causing gate and seat wear and eventual leakage if used to regulate flow.  Typical gate valves are designed to be fully opened or closed.  When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low friction loss.

Supporting paragraphs: Ducati 749 Desmodromic valve
Answer: spring