Unable to use SVN SCM wagon to upload a site

Even though the SCM project is apparently able to correctly pick up the SVN provider in the attached POM, the site plugin is unable to deploy to a site with an 'scm:svn...' URL and warns that

[WARNING] No SCM providers configured.

Here the abbreviated output of two commands - see the attachment for the full output.

> mvn scm:status
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building SCM deploy:site test 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] — maven-scm-plugin:1.4:status (default-cli) @ scm-deploy-test —
[INFO] Executing: cmd.exe /X /C "svn --non-interactive status"
[INFO] Working directory: XXX
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------

> mvn site:deploy -X

[DEBUG] The site will be deployed to url 'scm:svn:https://acme.com/svn/site' with id 'scm-deploy-test-site'
[DEBUG] repository protocol scm
[DEBUG] getProxy 'protocol' : scm
[DEBUG] getProxy 'protocol' : scm no ProxyInfo found
[DEBUG] found proxyInfo null
[DEBUG] authenticationInfo with id 'scm-deploy-test-site' : null
[WARNING] No SCM providers configured.
[DEBUG] configureWagon
[DEBUG] configureWagon server XXX
...
[DEBUG] configureWagon server XXX
[DEBUG] connect with authenticationInfo and without proxyInfo
scm:svn:https://acme.com/svn/site - Session: Opened
Transfer error: org.apache.maven.scm.manager.NoSuchScmProviderException: No such provider: 'svn'.
scm:svn:https://acme.com/svn/site - Session: Disconnecting
scm:svn:https://acme.com/svn/site - Session: Disconnected
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------