Were the Punan really nomadic or had nomadic origins

The headwaters of Punan River. This is the river that become the ethnonym "Punan'.

The single most common misconception about Punan is that they were nomad or had a nomadic origins. This misconception has been found in many literature. Charles Hose lump the Punan, he described as 'Punan Bah' in a list of tribes of Borneo (see Henry Ling Roth 1896).

Then in 1976, this misconception was repeated by Ida Nicolaisen in an article "Form and Function of Punan Bah Ethno-historical Tradition". Among others, according to Nicolaisen Punan had a history of being subsistence on sago not rice and were without canoes (Nicolaisen 1976:76).

And in 1990 Jerome Rousseau (1990:25) went a step further by openly suggest Punan has nomadic origins. This theory is based on the writings of Cense and Uhlenbeck (1958) and Hilderbrand (1982) who contended Punan [Ba] was related to the Punan Sajau, a nomadic group found in Kalimantan Timur. Subsequently, this nomadic origins theory also furthered by Bernard Sellato who linked the Punan [Ba] to another nomad variously known as "Semukung" "Uheng" or "Hangai".

Liju went back to the Mahakam with an enormous amount of loot and a great number of captive slaves. Only then could the Kayan Uma' Aging settle the Mendalam in peace (Enthoven 1903:79). It was during that same period that the Semukung (or Uheng, from the Punan name of the Kapuas), a nomadic group coming from Sarawak, began to establish themselves in the upper Kapuas, on the left bank. They are known to have had clashes with the Aoheng, but nothing is known of their relations with the Bukat. Hovongan and Kereho traditions, as well as that of the Semukung, report that Liju's military logistics entailed a demand for the services of these nomadic groups (Sellato 1994:25). 

 And this:

The Semukung (aka Hangai) also called themselves Uheng, from local name of the upper Kapuas River. They came from central Sarawak via Balleh, and there are reasons to believe that they are related to the Kajang (Punan Bah) of the Balui. They possibly followed the Kayan Uma' Suling into their migration to Kapuas. They first settled on the Hangai River, at the sources of the Kapuas, then split into three groups in about 1840-1850. One group moved across to the sources of the Mahakam and merged with the local Aoheng; another moved downstream the Kapuas, along with a group of Aoheng Huvung, and settled at Nanga Ira, near these Aoheng; and the remainder settled on the Bungan River, later to mix with Kereho nomads. There are very few Semukung left today, as those in Nanga Ira are heavily mixed with Bukat, Kereho, Mandai, and others; and while a limited Semukung wordlist could be collected in 1980, there is no Semukung speaker now left among the Aoheng in the Mahakam area (Sellato and Soriente 2015:345).

MAP 1: Showing known Punan migrations. No migration has been known to the south.

Firstly, Sellato is actually relying on a shaky reconstruction. Ida Nicolaisen admitted that "although a number of facts demonstrate that there are similarities between the Punan Bah in Sarawak and the Upper Mandai groups... no ultimate proof of their relationship has been established yet" (Nicolaisen 1976:80).

Secondly, the fact is that, other than Nicolaisen, no else actually ever known of Punan migration to Kapuas! "Madai River" mentioned by Nicolaisen is likely referring to Dabai River - a small tributary of Kakus River. There is no Mandai River in the oral tradition.

Thirdly, all known Punan migrations were actually northbound into Mukah (Sitieng) area and Baram - see Map 1. The group at Sitieng known as "Punan Lelak". This group is also Punan Ibiek to many Melanau and today an extinct people. Into the 19th century, their offshoot has become known as 'Melanau Sitieng'.

According to a scholar, the Melanau Sitieng speak a language closely related to Punan [Ba] (Ian Clayre, 1969). Intermittent contact with the Punan Lelak (Melanau Sitieng) was sustained up until early 21st century.  Another northbound migration was into Baram. Today offshoot of this group had intermingled with the Berawan of Long Jegan, Long Uko and Long Pata. They are known as "Punan Berom".

The language link 

Actually, linguistically there is wide disagreement on the classification of “Punan” (Alexander David Smith 2017 - unpublished PhD thesis). According to Smith, Blust (2010) does not address the subgroup for lack of data and leaves them out of his discussion. Hudson (1978)created a misleading subgroup “Punan-Nibong” which consists of several so-called Punan languages.

The data that he provides for these languages, however, suggests that they are Penan,not Punan. For example, Hudson lists tujak ‘seven’ for Punan Gang (Penan Gang), which is most likely a mishearing of Western Penan tujək ‘seven’.

Smith said the "actual Punan languages in Hudson’s classification are split between two groups, some appear in his Rejang-Bintulu subgroup and others in his RejangSajau group. His reasons for doing so are a small set of shared lexical innovations in Rejang Sajau, and the presence of glide fortition in Rejang-Bintulu.

Sellato (1980, 1986, 1994) has argued for some time that Punan and Müller-Schwaner languages form a subgroup, while the Ethnologue classification splits Punan dialects into three separate subgroups: Müller-Schwaner Punan (part of Kayanic), Rejang-Sajau, and Punan Tubu, which is grouped as an isolate within North Sarawak.

Blust (2015:204) provides the most recent statement on the classification of Punan, although he does so as a side note. There, Blust suggests that Bukitan, Ukit, Punan Bah, Punan Batu, Punan Busang, and Punan Sajau form a subgroup which excludes the Penan (which Hudson mislabeled Punan). This classification, based on limited data, is the most consistent with data collected by Smith for this dissertation.

The similarity between Punan [Ba] with the languages spoken by Punan Sajau and related group Punan Tuvu (see Césard, Nicolas, Antonio Guerreiro, and Antonia Soriente, 2015) is the Kayan-Kenyah languages. Punan language as spoken today are heavily corrupted by Kayan-Kenyah languages.

Punan first contact with the Kenyah group (Seping and Sebob) as early as in the 18th century and followed by the Kayan into the middle of 18th century. Intermarriages between Kayan and Kenyah are common thereafter and persisted until today.

The language spoken by Punan prior to the 18th century was closely related to a Melanau dialect called "Sitieng" (see Ian Clayre 1969). The Sitieng Melanau is known to Punan as "Punan Lelak or Punan Ibiek". It is distinct from the Kayan-Kenyah dialect of today.

Hence, it is misleading to suggest this language relatedness as evidence of Punan [Ba] nomadic origins. And equally misleading the contention that the Semukung was related to Punan [Ba] as suggested by Bernard Sellato.

Furthermore, all known Punan migrations were northbound to Mukah and Baram Rivers. Contact with these groups continues to be upheld until today. Ida Nicolaisen ultimately said there is no proof of Punan relationship with the group on upper Mandai (Nicolaisen 1976:80).

Finally, here is a list of article you probably want to read and review.

• Nicolaisen, Ida. Form and Function on Punan Bah Ethno-historical Tradition. Sarawak (The) Museum Journal Sarawak 24.45 (1976): 63-95.
• Rousseau, Jerome. Central Borneo: ethnic identity and social life in a stratified society. Oxford University Press, 1990.
• Sellato, Bernard. Nomads of the Borneo rainforest: the economics, politics, and ideology of settling down. University of Hawaii press, 1994.
• Sellato, Bernard, and Antonia Soriente. The Languages and Peoples of the Müller Mountains: A Contribution to the Study of the Origins of Borneo’s Nomads and their Languages. Wacana 16.2 (2015): 339-354.