  Distributed Ledger Technologies (DLTs) are playing a major role in building
security and trust in Internet of Things (IoT) systems. However, IoT
deployments with a large number of devices, such as in environment monitoring
applications, generate and send massive amounts of data. This would generate
vast number of transactions that must be processed within the distributed
ledger. In this work, we first demonstrate that the Proof of Work (PoW)
blockchain fails to scale in a sizable IoT connectivity infrastructure. To
solve this problem, we present a lightweight distributed ledger scheme to
integrate PoW blockchain into IoT. In our scheme, we classify transactions into
two types: 1) global transactions, which must be processed by global blockchain
nodes and 2) local transactions, which can be processed locally by entities
called witnesses. Performance evaluation demonstrates that our proposed scheme
improves the scalability of integrated blockchain and IoT monitoring systems by
processing a fraction of the transactions, inversely proportional to the number
of witnesses, locally. Hence, reducing the number of global transactions.
