Arboriculture & Urban Forestry 34(4): July 2008 Table 2. Mean integrated net radiation at 1 m (3.3 ft) over landscape tree trimmings (LTT), ponderosa pine residue (PPR), decomposing granite (DG), or bare soil without a mulch cover (control) by day (0600 to 1900 HR) and night (1900 to 0600 HR) during August 2004 and July 2005. Net radiation (MJ/m2) 2004 Treatment LTT PPR DG Control Day 10.77z ay 9.35 a 10.43 a 10.35 a Mean values within the same column followed by the same letter were not significantly different using Tukey’s honestly significant difference test ( 0.05). zValues are treatment means, n 4. y Although radiation and mulch surface temperature data were collected seasonally or continuously for 2 years, we have chosen to present only the daily mean patterns for the summer months of August 2004 and July 2005 because they best demonstrate the ability of different surface mulch treatments to buffer soil from intense radiation energy typical in a hot desert climate. Gener- ally, integrated mean values of net radiation over the surface mulch treatments were positive during the day and negative dur- ing the night (Table 2). During both August 2004 and July 2005, daytime net radiation values over all mulch treatments were similar, and positive daytime values were positively correlated with mean insolation (r 0.9188). During both August 2004 and July 2005, nighttime (1900 HR to 0600 HR, no direct inso- lation) net radiation values over LTT and PPR mulches were less negative than over DG mulch of soil without mulch. Concomi- tantly, nighttime long wave radiation values emitted by the LLT and PPR mulches were significantly less than over DG mulch of soil without mulch (Table 3). Patterns of mulch surface temperatures for both August 2004 and July 2005 were similar (Figure 1A–B). The LTT and PPR mulches had the highest surface temperatures at 1000 HR and 1400 HR and the lowest surface tempera- tures at 2200 HR. At 1400 HR, the mean surface temperatures of the two organic mulches were approximately 62°C (143°F) and 68°C (154°F) during August 2004 and July 2005, respec- tively. By 2200 HR, mean surface temperatures of the two Table 3. Mean nighttime (1900 to 0600 HR) long wave radiation over landscape tree trimmings (LTT), ponderosa pine residue (PPR), decomposing granite (DG), or bare soil without a mulch cover (control) during August 2004 and July 2005. Long wave radiation (W/m2) Treatment LTT PPR DG Control 2004 38.8z by 37.9 b 61.6 a 61.2 a zValues are treatment means, n 4. yMean values within the same column followed by the same letter were not significantly different using Tukey’s honestly significant difference test ( 0.05). 2005 49.2 b 45.5 b 83.0 a 70.6 a Figure 1. Effect of landscape tree trimmings (LTT), ponderosa pine residue (PPR), decomposing granite (DG), and bare soil without a mulch cover (control) on mean landscape surface temperatures (0600 to 2200 HR) during: (A) August 2004 and (B) July 2005. Values are treatment means (n = 36 for LTT and PPR mulch, n = 18 for control soil without mulch cover). Vertical lines represent ± SE of the means; where not visible, ± SE is smaller than symbol size. ©2008 International Society of Arboriculture Night −1.61 a −1.60 a −2.57 b −2.75 b Day 12.97 a 12.78 a 11.23 a 12.97 a 2005 Night −1.59 a −1.66 a −2.73 b −2.67 b 233 organic mulches were approximately 4°C(7°F) and 8°C (14°F) cooler during August 2004 and July 2005, respectively, than the surface temperature of the DG mulch or soil without mulch. For all treatments, soils were warmest during the seasonally hot summer months and were coolest during the winter months. Moreover, the amplitudes of the daily patterns of soil tempera- tures were greatest during the summer months and at the 5 cm depth. Mean daily temperature fluctuations tended to be lower during the spring and fall months and at the 30 cm depth and were lowest in soils covered by the LTT and PPR mulches. Although soil temperatures at the two depths were recorded con- tinuously for 2 years, we present only the mean daily soil tem- perature data for the months of August 2004 and July 2005 because these data are sufficient to show the capacity of different surface mulch treatments to moderate the microclimate of land- scape soil (Figure 2A–D). At 5 cm (2 in) depth, temperatures of soil without a mulch cover exceeded 40°C (104°F) for approximately 12 hrs and 9 hrs per day during August 2004 and July 2005, respectively (Figures
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