Xeric Limit Research Articles

Heritable variation in the survival of seedlings from Patagonian cypress marginal xeric populations coping with drought and extreme cold

The rear edges of tree species have begun to be perceived as highly valuable for genetic resources conservation and management. In view of expected climatic changes, the responses of trees at their xeric limits may largely be determined by their capacity to cope with augmented environmental variance. We assess the heritability of early survival of Patagonian cypress in two common-garden field tests with contrasting summer water deficits, comprising 140 and 163 open-pollinated families from 10 marginal xeric populations. The first experiment underwent less rigorous conditions than the average mesic, Mediterranean climatic conditions, which were sufficient to reveal additive genetic effects of summer drought on seedling survival. The second trial suffered strong summer water-deficit stress and a winter extreme cold event. In this harsher environment, the heritabilities of survival under summer water-deficit stress were high in all the populations (h 2 = 0.84 on average), while the heritabilities of seasonal, extreme cold survival were moderate or even nil (h 2 = 0.28 on average). We did not find evidence of genetic differentiation among populations in their capabilities to survive droughts and cold extremes. Our results indicate that even when climatic changes were strong enough to cause the extinction of the most threatened populations, heritable variation for traits underlying drought and cold tolerances may allow the marginal xeric edge of cypress to persist under augmented environmental variance, without losing overall genetic diversity.

Forests at the limit: evolutionary — genetic consequences of environmental changes at the receding (xeric) edge of distribution. Report from a research workshop

Although they most probably contain original genetic combinations, rear-edge, xeric limit populations are poorly known and have received limited interest from scientists and forest managers alike. Much effort is needed to better understand and manage these populations which will prove critical for the benefits of economically and ecologically important forest species under climate change.

Influences of gap microheterogeneity on the regeneration of Nothofagus pumilio in a xeric old-growth forest of northwestern Patagonia, Argentina

We experimentally examined the influences of within-gap environmental heterogeneity on regeneration patterns of Nothofagus pumilio (Poepp. & Endl.) Krasser near the xeric limit of its distribution in northern Patagonia, Argentina. Results from this xeric old-growth forest are compared with patterns previously described for the same species in mesic forests. Survival of N. pumilio seedlings beneath tree-fall gaps in this relatively xeric forest appears to be strongly influenced by moisture availability. Seedlings and saplings that have survived this demographic bottleneck are found at microsites where soil water potentials are higher, such as in the shady northern edges of tree-fall gaps (Ψ = -0.46 MPa compared with less than -0.6 MPa in other gap positions) and on coarse woody debris (Ψ = -0.29 MPa, compared with -0.51 MPa on the forest floor). Although gap creation in this dry N. pumilio forest is favorable to tree regeneration by releasing light resources, decreased water resources may switch the system from a light- to a water-limited system in some positions of the gap. This may explain the lack of regeneration of N. pumilio often observed after creation of large gaps towards the xeric end of its range and needs to be considered in the management of this important timber species.

Influences of gap microheterogeneity on the regeneration of <i>Nothofagus pumilio</i> in a xeric old-growth forest of northwestern Patagonia, Argentina

We experimentally examined the influences of within-gap environmental heterogeneity on regeneration patterns of Nothofagus pumilio (Poepp. & Endl.) Krasser near the xeric limit of its distribution in northern Patagonia, Argentina. Results from this xeric old-growth forest are compared with patterns previously described for the same species in mesic forests. Survival of N. pumilio seedlings beneath tree-fall gaps in this relatively xeric forest appears to be strongly influenced by moisture availability. Seedlings and saplings that have survived this demographic bottleneck are found at microsites where soil water potentials are higher, such as in the shady northern edges of tree-fall gaps (Ψ = -0.46 MPa compared with less than -0.6 MPa in other gap positions) and on coarse woody debris (Ψ = -0.29 MPa, compared with -0.51 MPa on the forest floor). Although gap creation in this dry N. pumilio forest is favorable to tree regeneration by releasing light resources, decreased water resources may switch the system...

A 22,000-year record of vegetation change in the north-central Sonoran Desert

Plant macrofossils from eleven packrat ( Neotoma sp.) middens provide a history of vegetation change over the last 22,140 years at Eagle Eye Mountain, near the northern border of the Sonoran Desert in Arizona. This site is located in the Arizona Upland subdivision. Departures and arrivals of various perennial plant species during the Holocene were similar to other records from more southerly sites and support earlier generalizations of Van Devender [Van Devender, T.R., 1990. Late Quaternary vegetation and climate of the Sonoran Desert, United States and Mexico. In: Betancourt, J.L., Van Devender, T.R., Martin, P.S. (Eds.), Packrat Middens: The Last 40,000 Years of Biotic Change. University of Arizona Press, Tucson, pp. 134–165.] regarding the tempo and mode of late Quaternary changes in the Arizona Upland. Evergreen woodland containing Pinus monophylla (singleleaf pinyon), Juniperus osteosperma (Utah juniper), and Quercus turbinella (shrub live oak) occupied the site during full glacial times. The latter two species, but not P. monophylla, persisted into the early Holocene and were joined by a limited number of desertscrub species, including Acacia greggii (catclaw), Carnegiea gigantea (saguaro), and several other succulents. With the arrival of many other species during the middle Holocene, a desertscrub community that differed considerably from the modern vegetation developed. Acacia greggii and Cercidium floridum (blue paloverde) were the principal large woody species on exposed hillslopes. Assemblages similar in composition to the modern desertscrub did not appear until approximately 4000 years ago, when the more xerophytic Cercidium microphyllum (foothills paloverde) arrived and C. floridum disappeared from hillslopes. The delayed arrival of C. microphyllum in comparison to other Sonoran Desertscrub species with similar climatic tolerances is attributed to the limited dispersal distance of its seeds by seed-eating heteromyid rodents. At 4540 and 6425 yr B.P., the cold-intolerant shrub Encelia farinosa was present on northeasterly aspect exposures where it is absent today, suggesting slightly warmer winter conditions during the middle Holocene. The presence of the tree Cercidium floridum and the perennial, C-4 grass Setaria leucopila on exposed slopes during the middle Holocene indicates substantially moister conditions during that time than in the late Holocene or today. At sites on the xeric limits of the modern distributions of both species, average annual precipitation is from 50% to 100% greater than that presently received at Eagle Eye Mountain. In particular, the responses of S. leucopila and cacti during the middle Holocene indicate substantial increases in warm-season precipitation.