Sensitivity of the Colorado plateau to change: climate, ecosystems, and society

Journal ArticleThe Colorado Plateau is located in the interior, dry end of two moisture trajectories coming from opposite directions, which have made this region a target for unusual climate fluctuations. A multidecadal drought event some 850 years ago may have eliminated maize cultivation by the first human settlers of the Colorado Plateau, the Fremont and Anasazi people, and contributed to the abandonment of their settlements. Even today, ranching and farming are vulnerable to drought and struggle to persist. The recent use of the Colorado Plateau primarily as rangeland has made this region less tolerant to drought due to unprecedented levels of surface disturbances that destroy biological crusts, reduce soil carbon and nitrogen stocks, and increase rates of soil erosion

Seedling responses to water pulses in shrubs with contrasting histories of grassland encroachment.

Woody plant encroachment into grasslands has occurred worldwide, but it is unclear why some tree and shrub species have been markedly more successful than others. For example, Prosopis velutina has proliferated in many grasslands of the Sonoran Desert in North America over the past century, while other shrub species with similar growth form and life history, such as Acacia greggii, have not. We conducted a glasshouse experiment to assess whether differences in early seedling development could help explain why one species and not the other came to dominate many Sonoran Desert grasslands. We established eight watering treatments mimicking a range of natural precipitation patterns and harvested seedlings 16 or 17 days after germination. A. greggii had nearly 7 times more seed mass than P. velutina, but P. velutina emerged earlier (by 3.0±0.3 d) and grew faster (by 8.7±0.5 mg d⁻¹). Shoot mass at harvest was higher in A. greggii (99±6 mg seedling⁻¹) than in P. velutina (74±2 mg seedling⁻¹), but there was no significant difference in root mass (54±3 and 49±2 mg seedling⁻¹, respectively). Taproot elongation was differentially sensitive to water supply: under the highest initial watering pulse, taproots were 52±19 mm longer in P. velutina than in A. greggii. Enhanced taproot elongation under favorable rainfall conditions could give nascent P. velutina seedlings growth and survivorship advantages by helping reduce competition with grasses and maintain contact with soil water during drought. Conversely, A. greggii's greater investment in mass per seed appeared to provide little return in early seedling growth. We suggest that such differences in recruitment traits and their sensitivities to environmental conditions may help explain ecological differences between species that are highly similar as adults and help identify pivotal drivers of shrub encroachment into grasslands

Sensitivity of the Colorado Plateau to change: climate, ecosystems and society

The Colorado Plateau is located in the interior, dry end of two moisture trajectories coming from opposite directions, which have made this region a target for unusual climate fluctuations. A multi-decadal drought event some 850 years ago may have eliminated maize cultivation by the first human settlers of the Colorado Plateau, the Fremont and Anasazi people, and contributed to the abandonment of their settlements. Even today, ranching and farming are vulnerable to drought and struggle to persist. The recent use of the Colorado Plateau primarily as rangeland has made this region less tolerant to drought due to unprecedented levels of surface disturbances that destroy biological crusts, reduce soil carbon and nitrogen stocks, and increase rates of soil erosion. The most recent drought of 2002 demonstrated the vulnerability of the Colorado Plateau in its currently depleted state and the associated costs to the local economies. New climate predictions for the southwestern United States include the possibility of a long-term shift to warmer, more arid conditions, punctuated by megadroughts not seen since medieval times. It remains to be seen whether the present-day extractive industries, aided by external subsidies, can persist in a climate regime that apparently exceeded the adaptive capacities of the Colorado Plateau’s prehistoric agriculturalists

Total amount of water applied under watering treatments.

<p>Watering treatments were initial pulse duration (four levels, from 2 to 5 days with 10 mm water per day) and subsequent maintenance regime (two levels, applying with 5 mm water either every day or on alternate days) over a combined duration of 16 or 17 days. Treatments were based on long-term precipitation records at the Santa Rita Experimental Range, Tucson, AZ, USA.</p

Growth rate and biomass responses of young shrub seedlings to watering treatments.

<p>Mean (± S.E.M.) AGR (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0087278#pone.0087278.e002" target="_blank">equation 2</a>; a, b, c) and oven-dry root mass (d, e, f) and shoot mass (g, h, i) of <i>Prosopis velutina</i> (n = 96) and <i>Acacia greggii</i> (n = 91) seedlings 16 and 17 days after imbibition, in response to pulse duration (number of days at start of experiment with 10 mm water per day; panels a, d and g) and follow-up watering (post-pulse frequency of 5 mm watering events; panels b, e and h). Panels c, f, and i depict regressions against day of emergence (time between imbibition and cotyledon emergence) of harvested seedlings for <i>P. velutina</i> (dashed lines) and <i>A. greggii</i> (solid lines). Means with different letters were significantly different (Tukey-Kramer test, <b>α</b> = 0.05).</p

Mean time to emergence, absolute growth rate (AGR) and dry biomass in young shrub seedlings.

<p>Species were <i>Prosopis velutina</i> (n = 96) and <i>Acacia greggii</i> (n = 91). Time to emergence was evaluated by within-pot averages from 4 seeds pot⁻¹. Biomass was measured 16 and 17 days after imbibition. AGR was calculated according to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0087278#pone.0087278.e002" target="_blank">equation 2</a>, excluding seed coat mass. <i>P</i> values are from <i>t</i> tests.</p

Summary of ANCOVA analyses for absolute growth rate (AGR, eq. 2) and dry biomass of <i>P. velutina</i> (n = 96) and <i>A. greggii</i> (n = 91).

<p>Seedlings were harvested 16 and 17 days after imbibition was initiated.</p

ANOVA analyses for days to emergence, absolute growth rate (AGR; eq. 2), total seedling dry mass and taproot length.

<p>Days to emergence was analysed for within-pot averages (4 seeds pot⁻¹). AGR, total mass and taproot length were evaluated on the first seedling to emerge per pot. Watering treatments were pulse duration (number of days at start of experiment with 10 mm water per day) and follow-up watering frequency (post-pulse frequency of 5 mm watering events). Seedlings of <i>P. velutina</i> (n = 96) and <i>A. greggii</i> (n = 91) were thinned as needed to one per pot and harvested 16 and 17 days after imbibition was initiated.</p

Taproot length responses of young shrub seedlings to watering treatments.

<p>Mean (±S.E.M.) taproot length of <i>Prosopis velutina</i> (n = 96) and <i>Acacia greggii</i> (n = 91) seedlings at harvest (16 and 17 days post-imbibition) in response to pulse duration (number of days at start of experiment with 10 mm water per day) and follow-up watering (post-pulse frequency of 5 mm watering events). Values with different letters differed significantly (Tukey-Kramer test, <b>α</b> = 0.05).</p

Canopy cover of <i>Acacia greggii</i> and <i>Prosopis velutina</i> shrubs in a grassland upland, Arizona, USA.

<p>Data were taken from the Santa Rita Experimental Range pasture 8 (from the Santa Rita Experimental Range Digital Database, <a href="http://ag.arizona.edu/SRER/longterm/ltcover.xls" target="_blank">http://ag.arizona.edu/SRER/longterm/ltcover.xls</a>.). This pasture was grazed year-round by cattle at a stocking rate of 250–300 Animal Unit Months.</p