Frank G. Hawksworth

Arceuthobium (Viscaceae) in Mexico and Guatemala: Additions and range extensions

Three new dwarf mistletoes are described:Arceuthobium globosum subsp.grandicaule (Mexico and Guatemala),A. aureum subsp.aureum (Guatemala) andA. aureum subsp.petersonii (Chiapas, Mexico).Arceuthobium guatemalense is recorded for the first time in Mexico. Significant range extensions are recorded forA. abietisreligiosae, A. divaricatum,A. gillii subsp.nigrum, andA. rubrum. New hosts are reported for several taxa. Nineteen members of the genus are presently known from Mexico, and three (possibly four) from Guatemala. Chromosome counts are reported for the first time for 3 taxa.

Embryonic and host-associated skewed adult sex ratios in dwarf mistletoe

Embryonic sex ratios were determined for the first time in dioecious flowering plants by utilizing malate dehydrogenase (Mdh-3) as a genetic marker. In three populations of Arceuthobium tsugense ssp. tsugense (hemlock dwarf mistletoe) the embryonic sex ratio was 1:1, as it also was at first reproduction. Mixed-age, adult sex ratios of these and other populations from Oregon to south-eastern Alaska, however, were significantly female-biased (59 per cent) on western hemlock, its primary host. We suggest that these female-skewed, adult sex ratios are the result of increased longevity among females. On a secondary host, noble fir, the embryonic and the adult sex ratios were both 1:1. On still another secondary host, shore pine, the embryonic sex ratio is unknown, but the adult sex ratio was significantly male-biased (55.1 per cent). Host environment apparently influences adult sex ratios in hemlock dwarf mistletoe. However, in the European mistletoe Viscum album, a gametic system apparently controls the production of the adult female-predominant sex ratios (69.9 per cent) at fertilization (or perhaps by abortion during embryogenesis). In V. album, the sex ratio of progeny raised from seed is already similarly biased at first reproduction (67.3 per cent), and the sex ratios are not influenced by host shifts.

Phloem in Arceuthobium globosum (Viscaceae)

Our studies reveal that Arceuthobium globosum, a primitive member of the genus, has highly specialized phloem tissue. The sieve elements of this large Mexican species have transverse to oblique, usually simple sieve plates and small but numerous diffuse lateral pores. The sieve elements show the normal pattern of callose deposition and subsequent removal reported for other dicotyledons. Sieve elements were not found in A. douglasii or A. tsugense, two specialized, temperate-region species.

A new species of Arceuthobium (Viscaceae) from central Mexico

Arceuthobium pendens, a parasite of the pinyonsPinus discolor andP. cembroides, is described from San Luis Potosí and Veracruz, Mexico. This andA. divaricatum Engelm. are the only dwarf mistletoes known to parasitize pinyons.Arceuthobium pendens is distinct fromA. divaricatum Engelm. in its long, slender, greenish shoots, formation of systemic witches brooms, and frequency on various species of pinyons. With the discovery of this new species, 19 dwarf mistletoes are known from Mexico.

SWYLD2, yield tables for even-aged and two-storied stands of southwestern ponderosa pine, including effects of dwarf mistletoe /

Myers, Clifford A., Carleton B. Edminster, and Frank G.Hawksworth. 1976. SWYLD2: Yield tables for even-aged and two-storied stands of southwestern ponderosa pine, including effects of dwarf mistletoe. USDA For. Serv. Res. Pap. RM-163, 25 p. Rocky Mt. For. and Range Exp. Stn., Fort Collins, Colo. 80521. Presents a procedure for computation of yield tables for ponderosa pines in Arizona and New Mexico. Possible alternatives include: even-aged or two-storied stands, healthy or diseased stands, and managed or unmanaged stand densities. Stand conditions and severity of dwarf mistletoe infestation change with time and in response to intermediate cuttings. Supersedes SWYLD, published in 1972 as USDA For. Serv. Res. Pap. RM-87. A concise users guide for program SWYLD2 is available as USDA For. Serv. Gen. Tech. Rep. RM-23, 1976.