The Ecology of Celtis reticulata Torr. (Netleaf hackberry) in Idaho Public Deposited

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  • The ecology of netleaf hackberry (Celtis reticulata Torr.) is poorly known throughout its broad western range. To gain some understanding of the species' basic ecology, three components of its life history were studied. First, seed germination requirements were analyzed under laboratory conditions. Second, hackberry population dynamics were evaluated from tree size and age structures. Lastly, an examination of hackberry's response to fire was made, using tree-level post-fire data. A higher percentage of non-stratified hackberry seed germinates if the seed is mechanically scarified. While germination percent is low, the treatment may eliminate the 120-day stratification period recommended by some authorities. Even with cold stratification, germination is usually quite low for Celtis species. Fermenting fruits for 72 hours and subsequent depulping prior to scarification did not significantly increase germination levels, as hypothesized. However, seed that was not scarified but was fermented with its pulp intact germinated at a higher rate than the non-fermented, non-scarified seed. This result may be significant to its biology. Seed is typically cleaned prior to planting by seed distributors. However, hackberry seed with its pulp retained may in fact have a superior germination rate, as it is allowed to undergo natural processes of fermentation resulting from moist field conditions. In Idaho, hackberry's uniquely fragmented distribution appears to be controlled by a variety of factors. Hackberry is generally restricted to semi-arid portions of the state where temperatures are least severe during the winter months, such as in moderated river canyons, or at elevations below 1600 m. It occurs in a variety of habitats, from riparian to upland, however it is most abundant on sites with a southeast-to southwesterly aspect and a rocky surface cover. The presence of rock below the surface may be equally important, but this factor was not quantified. Hackberry is a slow-growing species in Idaho, averaging 3.9 m tall at 50 years. Differences in hackberry growth rate under a variety of site conditions were evaluated by a site index constructed from the log-log regression of height on age. Trees reach maximum heights where topographic shelter is greatest, such as in draws, and where soils are loamy. However, well-drained soils may be important, as nearly all soils had some sand or skeletal component. Hackberry can be long-lived, with the maximum age recorded at 374 years; the average age was 66 years (N = 959 trees). Hackberry appears to be reproducing, in spite of habitat degradation caused by livestock overgrazing, alien plant invasion, and increasing fire frequencies. However, newly established even-aged stands are rare and are generally found along waterways on stream terraces or at high water lines. In these areas, surface rock cover was typically high, with the rock providing a moderated below-ground microclimate, as well as protection from herbivores. Recruitment is favored by rockiness at the surface, but growth rates of established individuals are not higher with more surface rock. Under severe livestock grazing pressure, stands had a single dominant cohort and lowered recruitment. Fire-caused mortality of hackberry growing in low density stands appears to be uncommon in Idaho. In nine burns, only 8% of the trees (N = 161) were dead after the fires. This mortality, however, cannot be attributed solely to fire, as pre-burn vigor was not evaluated. Individuals typically persist or resprout following fire. Mortality rates were highest for individuals with a high percent cover of live vegetation below their canopy. Temperatures were probably more intense within the live vegetation, where the fire could feasibly linger long enough to kill the roots of the associated hackberry. While burns in high density stands were not observed, these burns could result in greater mortality, due to their greater continuity of plant cover.
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