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

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

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

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

CCA Ordination (canonical correspondence analysis) results ordered in multivariate space along the first two canonical axes, separately depicting relationships among (A) tree-stature species (Table 1) and soil variables; and (B) communities in each 6 m × 6 m sample plot (symbols indicate community type) along a hill-slope transect.

<p>Species centroids (+) indicate center of distribution among sample plots for each tree species coded as follows: acf = <i>Acacia farnesiana</i>; acr = <i>Acacia rigidula</i>; cel = <i>Celtis pallida</i>; con = <i>Condalia hookeri</i>; dio <i>= Diospyros texana</i>; kar = <i>Karwinskia humboltiana</i>; pro = <i>Prosopis glandulosa</i>; zan = Zanthoxylum <i>fagara</i>. Lines are vectors indicating direction of increasing value (from center outward) for soil variables as follows: BD = bulk density; Sand and clay = soil particle percentages; SOC = soil organic carbon; TN = total nitrogen. </p

CCA Ordination (canonical correspondence analysis) results ordered in multivariate space along the first two canonical axes, separately depicting relationships among (A) shrub-stature species and soil variables (Table 1); and (B) 2 m × 2 m sample plots (symbols indicate community type) along a hill-slope transect.

<p>Species centroids (+) indicate center of distribution among sample plots for each shrub species coded as follows: acf = <i>Acacia farnesiana; acg = A. greggeii</i>; acr = <i>A. rigidula</i>; alo = <i>Aloysia gratissima</i>; bem = <i>Bernardia myricaefolia</i>; ber <i>= Bernardia myricaefolia</i>; cel <i>= Celtis pallida</i>; col = <i>Colubrina texensis</i>; con <i>= Condalia hookeri</i>; dio <i>= Diospyros texana</i>; eph = Ephedra <i>antisyphilitica</i>; eys = <i>Eysenhardtia texana</i>; for = <i>Forestiera angustifolia</i>; gym = <i>Gymnosperma</i> spp.; kar <i>= Karwinskia humboltiana</i>; par <i>= Parkinsonia aculeata</i>; pro <i>= Prosopis glandulosa</i>; sch = <i>Schaefferia cuneifolia</i>; zan = Zanthoxylum <i>fagara</i>; ziz = <i>Ziziphus obtusifolia</i>. Lines are vectors indicating direction of increasing value (from center outward) for soil variables coded as follows: BD = bulk density; Sand and clay = soil particle percentages; SOC = soil organic carbon; TN = total nitrogen.</p

Variation decomposition shown as a percentage of total variation explained (TVE) for separate analyses of tree (Figure 3) and shrub (Figure 4) plots, that were uniquely related to variable groupings that included (a) soil organic carbon and total nitrogen, (b) % sand and % clay; or (c) an equal sharing by a and b.

<p>Variation decomposition shown as a percentage of total variation explained (TVE) for separate analyses of tree (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0084364#pone-0084364-g003" target="_blank">Figure 3</a>) and shrub (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0084364#pone-0084364-g004" target="_blank">Figure 4</a>) plots, that were uniquely related to variable groupings that included (a) soil organic carbon and total nitrogen, (b) % sand and % clay; or (c) an equal sharing by a and b.</p