Use of live and dead pinyon pine Pinus edulis and juniper Juniperus utahensis trees by four phrynosomatid lizard species, and implications for fire fuel management, Colorado National Monument, Colorado, USA


In the USA, management to decrease fire risk in forest includes the removal of dead vegetation and debris, removed through thinning, prescribed burning and other removal methods. However, dead vegetation provides a key microhabitat for a range of animals including many invertebrates, reptiles, cavity-nesting birds and small mammals.

A study of the use of living, dead standing and dead prone trees by four species of phrynosomatid lizard was undertaken in Colarado, USA. The objectives were to assess tree usage and differences in use among species, and look at the conservation implications of the removal of dead wood.

Study area: The study was undertaken at the Colorado National Monument on the Colorado Plateau in three years (1990, 1992, 2000). The habitat is mostly pinyon pine Pinus edulis and juniper Juniperus utahensis woodland, with a sandy substrate interspersed with flat rocky areas.

Lizard species: Within this open woodland, microhabitat use of four sympatric phrynosomatid lizards was assessed: eastern fence lizard Sceloporus undulatus and tree lizard Urosaurus ornatus are both rock-dwelling and arboreal; sagebrush lizard Sceloporus graciosus and side-blotched lizard Uta stansburiana are primarily terrestrial. All four species however, use dead trees as 'perch' sites.

Lizard surveys: During late May 1990, late May and early June 1992, and August 2000, 10, 21, and 15 person-days, respectively, were spent searching for lizards. Features likely to be included in lizard territories were concentrated upon to maximize lizard observations. Lizards were censused using mark and recapture. Each captured individual was sexed, given a distinct temporary paint mark and released at the initial point of observation. Each time a lizard was observed or captured, a description of the habitat was recorded within a 5 m radius of the initial observation point i.e. capture point tree type, and the number of living, dead standing, and dead prone trees (pinyon and juniper).

For each lizard species, the proportion of each tree category (living, dead prone and dead standing) used was calculated from the number of captures on each type of tree and the total number of available trees at the capture site and within its radius.

Use of trees: Average use and total percent availability of living, dead standing, and dead prone trees during 1990, 1992, and 2000 are summarised in Table 1 (attached). Percentage use was highly significantly different among the three tree categories but not among lizard species.

Despite greater availability of live trees, all species used dead (prone and standing) trees far more than live ones. The arboreal U.ornatus, was the only species that differed in the use of dead standing as opposed to dead prone trees, using dead standing juniper and pine more often than prone. A similar but not significant preference was apparent for S.undulatus. U.ornatus and S.undulatus were observed using arboreal microhabitat, often in conjunction with rocks, for perching and basking, territory surveillance, feeding, shelter and as refugia. As dead pinyon and juniper are preferred to living trees, these are considered to be an important component of these species' territories. As dead trees become available, lizards shift their territory and home range boundaries to exploit this microhabitat.

The two mostly terrestrial species S.graciosus and U.stansburiana, were expected to use dead prone trees more frequently than dead standing trees (as dead prone trees are more accessible and act as refugia) but infact no significant preference was found. Despite the small sample sizes of U.stansburiana (N = 22), those that were captured in trees showed the same preferential pattern of use as the two arboreal species U.ornatus (N = 102) and S.undulatus (N = 71), and also S.graciosus (N=80) of dead compared to live trees.

During the study, concentrations of all four species were found in areas with numerous dead juniper and pine, with comparatively few individuals in the surrounding habitat with relatively few dead trees. This suggests that concentrations of dead prone and standing trees represent an important habitat component for these lizards, concurring with several other studies in southern USA.

Conclusions: All four study species used dead trees (both prone and standing) more frequently than live ones, despite an opposite pattern of availability. U.ornatus used dead standing more often than dead prone trees, reflecting its arboreal nature. In addition to trees, the study lizards also exploited other substrates (rock ledges and rock faces) which almost certainly also influenced patterns of lizard distribution and numbers. During the study, 54.4% of all lizards encountered were perched on live or dead trees, while the remaining 45.6% were observed on rock or sandy substrate.

Note: If using or referring to this published study, please read and quote the original paper.

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