The evidence for decreasing author submissions is only indirect, as we do not have access to those data. We really looked at journal output in pages/year.* An alternative explanation could be that Elsevier drastically increased rejection rates by beiing more selective, but there is no indication that they did so. Looking at ISI impact factors and article counts, we find:

Nuclear Physics B:
2008: 4.158 (350), 2007: 4.645 (313), 2006: 5.199 (405), 2005: 5,522 (524), 2004: 5.819 (476), 2003: 5.297 (534)

A SCOAP3 core journal. Impact factor has decreased by 30% since 2004, size by 1/3.

Physics Letters B:
2008: 4.034 (929), 2007: 4.189 (840), 2006: 5.043 (999), 2005: 5.301 (955), 2004: 4.619 (1038), 2003: 4.066 (968)

A SCOAP3 core journal. Impact factor has decreased by 25% to 30% since 2004/05 for both journals. This means that also the better papers have gone to other journals.

We have to revise our first estimate of a 50% reduction in output for PLB, as the picture is complicated by changes in frequency and discrepancies between scheduled volumes for a subscription year and the printing of indices in former volume. We find now that the reduction in output in pages was similar to NPB, about 30%-40%. The reduction in the number of papers published for PLB was less, around 10%-20%

Nuclear Instruments and Methods in Physics Research A (Elsevier):
2008: 1.019 (884), 2007: 1.114 (1888), 2006: 1.185 (1650), 2005: 1.244 (1359), 2004: 1.349 (1692), 2003: 1.166 (1551)

A SCOAP3 "broadband journal" with 25% HEP content. The sharp decrease in article numbers for this title comes about because Elsevier published much less conference proceedings in NIMA from 2008 on. No of pages published decreased from 10792 to 6340 (40%), whereas the price was lowered only by 15% (from 2007 to 2008). Impact factor has decreased by 25% over the last 4 years.

For comparison the other SCOAP3 core journals:

Journal of High Energy Physics (SISSA):
2008: 5.375 (1284), 2007: 5.659 (1247), 2006: 5.393 (1027), 2005: 5.944 (859), 2004: 6.503 (885), 2003: 6.057 (809)

JHEP has grown by 45% in the last 4 years, but impact factor has decreased by 20% probably because it captured a larger part of the mss. lost by Elsevier.

Physical Review D (APS):
2008: 5.050 (2863), 2007: 4.696 (2268), 2006: 4.896 (2375), 2005: 4.852 (2247), 2004: 5.156 (2277), 2003: 4.599 (1964)

PRD is also going strong (25% more published than 4 years ago) but has maintained its high impact factor.

European Physical Journal C (Springer):
2008: 2.805 (323), 2007: 3.255 (369), 2006: 3.251 (214), 2005: 3.209 (306), 2004: 3.486 (302), 2003: 3.580 (287)

Springer's EPJ C has also lost 30% in Impact factor, while its size has not changed substantially.

To set this in perspective: The aggregate impact factors for the ISI category: Physics, Particles & Fields were (Year: aggregate IF (median IF, #articles):
2008: 3.165 (2.015, 9178), 2007: 3.034 (1.894, 9720), 2006: 3.058 (1.564, 8209), 2005: 2.988 (1.659, 9471), 2004: 3.137 (1.589, 8751)

This is only a preliminary analysis. Caution is also necessary because it is unclear how much of the real citations are captured by ISI if many early citations on HEP articles are on their preprint version at arxive instead of the final published article. This could considerably distort the picture. (I still have to check what has been reported in the literature on this, and I'm glad if people direct me to such studies.)

Elsevier's analysis of the same data (Research Trends, August 2008) is highly misleading and selfserving, as they compare total citations to a journal in a given year over all articles (the citations can be to any previous year, but largely since 1996, as Scopus has only incomplete data before this) to the journal's article output in that year and call this the "average journal citation per article" [=(cumulated) cites to all articles divided by number of current articles]. It is therefore ridiculous if Publishing Director David Clark maintains “Nuclear Physics B has consistently maintained its high standards despite the reduction in the number of papers being published in particle physics”, emphasizing the upward trend and the fact that Nuclear Physics B has the highest "average journal citation per article", 77.15 in 2007. The sure recipe to drive this factor even higher is to rapidly reduce the output in papers [the denominator, number of current articles] even further because it is well known that the citation half life of these journals is around 10 years and it will take some time for the cumulated citations to come down. In fact, the trendline for average journal citation per article would go through the roof if Elsevier stopped publishing altogether in HEP. The impact factor, on the other hand, measures the average citation rate with much less latency, namely through a citation window of only the previous two years and takes into account the citations and number of articles published in both years.

Bernd-Christoph Kämper (@bckaemper), Stuttgart University Library