| Shrubs have replaced arid grasslands over large parts of the American southwest. Multiple mechanisms are proposed to explain this shift. One hypothesis invokes allelopathy as a mechanism for maintenance of shrub dominance in arid grasslands. This paper focuses on secondary compounds found within creosotebush and tarbush, their distribution, and possible losses from plant tissues through precipitation and leaf decay.; Total phenolics, condensed tannins and nordihydroguaiaretic acid (NDGA) were measured in nine categories of tissue. Total phenolic and condensed tannin concentrations were determined using colorimetric methods, NDGA content was determined with high performance liquid chromatography. Total phenolics were present throughout the plant with highest concentrations in leaves (36.2 mg/g), green stems (40.8 mg/g) and roots (mean for all root categories = 28.6 mg/g). Condensed tannins were in all tissues except larger stems (5 to 12 mm in diameter), with highest concentrations in flowers (1.7 mg/g), seeds (1.1 mg/g), and roots less than 5 mm in diameter (1.1 mg/g). Flowers, leaves, green stems, and small stems (<5 mm in diameter) all contained NDGA, with highest concentrations in leaves (38.3 mg/g).; Stemflow, throughfall, and interspace precipitation collections were analyzed for total phenolics and nordihydroguaiaretic acid. Phenolics were found in throughfall and stemflow; 0.088 and 0.086 mg/ml for stemflow and 0.022 and 0.014 mg/ml for throughfall in two creosotebush morphotypes; 0.044 and 0.005 mg/ml for stemflow and throughfall, respectively, in tarbush. Nordihydroguaiaretic acid was not found in any precipitation samples.; Bags containing leaf litter were placed under shrubs. One set was placed on the surface, one set buried at 5 cm. They were then collected at intervals from 0 to 90 days. Sets 1 and 2 (creosotebush and tarbush, respectively) were in place from February to April. Set 3, creosotebush, was in place from April to July. Percent organic matter loss was 1.99% and 3.23% for Set 1 surface and buried samples, respectively, 2.31% and 2.49% for Set 2 surface and buried and 34.10% and 75.27% for Set 3 surface and buried, respectively. Set 1 lost total phenolics (surface 6%), NDGA (19% surface, 11% buried), and condensed tannins (53% surface and 44% buried). Set 2 had losses in total phenolics (27% surface, 38% buried) but concentration of condensed tannins increased slightly (1.41% surface, 1.47% buried). Set 3, spring creosote, lost total phenolics (13% surface, 48% buried), nordihydroguaiaretic acid (52% surface, 82% buried), and condensed tannins (55% surface, 68% buried).; This paper demonstrates the presence and distribution of secondary phenolic compounds in creosotebush and the movement of these compounds from production sites in creosotebush and tarbush to sites of possible allelopathic activity in soil. Phenolic compounds, other than NDGA, are moved from plant surfaces to soil through precipitation. Phenolic compounds are lost from leaf litter as the leaf decays. Within the duration of this study, this process is most effective from April through July. The fate of these compounds once in the soil, and their interactions with other plants, needs to be examined before a complete picture of possible allelopathic activity in creosotebush dominated systems in the northern Chihuahuan Desert can be presented. |
