|Abstract or Summary
- 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
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.