I'd like to start my node js application on boot. Therefore I start a service from Systemd:

[Unit]
Description=Node.js server
After=network.target

[Service]
ExecStart=/usr/bin/node /var/www/Raspberry-Pi-Status/js/server.js
Restart = always
RestartSec=10
StandardOutput=syslog
StandardError=syslog
SyslogIdentifier=nodejs-server
Environment=NODE_ENV=production PORT=8000
Environment=PYTHONPATH=/usr/bin/python

[INSTALL]
WantedBy=multi-user.target

The server.js looks like this:

var util = require('util'),
spawn = require('child_process').spawn,
py = spawn('python',['temperature.py'],{detached: true});
var mysql = require('mysql');
var connection = mysql.createConnection({
host : 'localhost',
user : 'monitor',
password : 'password',
database : 'temps'});


var app = require('http').createServer(handler),
  io = require('socket.io').listen(app),
  fs = require('fs'),
  sys = require('util'),
  exec = require('child_process').exec,
  child;

// Listen on port 8000
app.listen(8000);
// If all goes well when you open the browser, load the index.html file
function handler(req, res) {

  fs.readFile(__dirname+'/../index.html', function(err, data) {
    if (err) {
    // If no error, send an error message 500
        console.log(err);
        res.writeHead(500);
        return res.end('Error loading index.html');
    }
    res.writeHead(200);
    res.end(data);
  });

}


py.stdout.on('data', function(data){
  console.log('testing');
  date = new Date().getTime();
  temp = parseFloat(data);
  io.sockets.emit('temperatureUpdate',date,temp);
});


// When we open the browser establish a connection to socket.io.
// Every 5 seconds to send the graph a new value.

io.sockets.on('connection', function(socket) {
   console.log('user connected');
});

The node.js application should start a python script which reads out a temperature sensor. When I start node.js via the console everything works fine. However, when I start from Systemd, the python script is not spawned. What's the problem there? Am I missing something?

Thanks in advance Alexander