T.A. Black, N.J. Grant, S. Graham, A. Balakshin, R. Ketler. Biometeorology and Soil Physics Group, University of British Columbia. N.J.

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1 Variable retention, microclimate, experimental and modelling projects in TFL 39: Microclimate in Variable Retention: preliminary results from a 24 transect study. T.A. Black,.J. Grant, S. Graham, A. Balakshin, R. Ketler Biometeorology and Soil Physics Group, University of British Columbia.J. Smith Weyerhaeuser BC Coastal Group, 65 Front St, anaimo, BC March 25, 5

2 Introduction Variable retention (VR) systems cover substantial parts of the forest land base in BC and elsewhere. Many forest companies and organizations are examining the advantages of leaving more trees as unharvested individuals (dispersed retention) or in groups (group retention). VR systems maintain structure across the harvested area: the latter having greater than 5% of its area within one tree length of a residual tree or group of trees for at least one rotation. It is hypothesized that this edge influence substantially alters the microclimate compared to a clearcut. This has implications for the growth of the next crop- which is considered in elsewhere. The intent of this project is to examine the impact of VR on stand microclimate collected from an experimental site in Coastal BC. Following clearcut harvesting, radiant energy exchange shifts to near the ground surface so that planted seedlings are subjected to microclimate (temperature, atmospheric humidity and soil water content) extremes (Spittlehouse and Stathers, 199), which affect their survival and growth rates (Fleming et al., 1998). On the other hand, variable retention harvesting results in higher spatial variability in microclimate. Low light levels near the unharvested trees may limit the growth of planted seedlings. VR techniques also affect air and soil temperatures, soil water content, and solar irradiance; these, in turn, affect soil biological and chemical processes (e.g. mineralization of organic matter; (Prescott et al. 1997)), which are critical to the establishment and growth of planted seedlings. Research on the effects of forestry harvesting practices on microclimates has most often focused on the differences between open, forest edge and forest interior microclimates created by forest gaps and clear-cut strips. Little is known, however, about the effects of the type and extent of VR on microclimate and hence, on the growth and survival of planted seedlings in west coast forests. Only with this information can foresters select the VR system that optimizes seedling survival and growth while preserving biodiversity and wildlife habitat. The results presented herein represent a preliminary investigation into these effects at one site. Funding was not approved until July limiting us to one site. Data The field site selected was at Powell River, BC. (See Fig 1 for a description). These comprised a full Campbell Scientific weather station in the clearcut coupled with a Decagon BF3 (diffuse light) meter. For each transect at each sample point we used Hoskins RH and temperature metres and Apogee PAR sensors. We established a north-south transect in early July and an east-west transect (the latter was referred to as Xtransect) on Aug 12 (PAR sensors only) in a 3% (by leave basal area, see Figure 2a) dispersed VR plot at Powell River. The transects were later moved to a 5% dispersed area (Figure 2b) in September 24. Sensors were installed at selected distances from a retained tree or patch of trees to measure PAR, temperature (3-cm depth) and soil moisture content (3-cm depth) to examine seedling growth responses to the observed microclimate. The fixed sensors were spaced 5 m apart near the tree or patch of trees extending to 1 m apart beyond 15 m. Precise placement of the sensor nodes is depicted in Figures 2a and 2b. A climate station, consisting of a data logger and associated sensors to measure half-hourly mean values of precipitation, solar irradiance, total and diffuse PAR, wind speed, air temperature and relative humidity, was installed in an adjacent clearcut. 2

3 Fig 1. The Powell River Study Site. Comprises a clearcut, 1% retention, 5%, 1% and 3% retention (by basal area) in a 7 year old site index 34m Douglas-fir stand. This is part of Weyerhaeuser s BC Coastal Group series of VR Experimental Sites that examine group and dispersed retention. The area was logged 21 and planted 22 with 12 85% 1+ Douglas-fir and 15% cedar 1+ seedlings. On average for the 5% retention a 38m spacing between trees was maintained. For 1% the same 38m spacing was maintained but 2 trees were left. For 3% the same 38m spacing was maintained but between 6-8 trees were left. This created a clumping that is visible on the map. Analyses Photosynthetically active radiation and soil moisture and temperature are presented along the -S and E-W transect for the 3% and 5% retention. Our hypothesis are that there is no difference in microclimate along these transects. Half-hourly measurements were made from July to December. 3

4 3% Figure 2a. 3% transect at Powell River showing the sensor locations and surrounding trees. 5% Figure 2b. 5% transect at Powell River showing the sensor locations and surrounding trees. 4

5 Results I. DIEL PHOTOSYTHETICALLY ACTIVE RADIATIO (PAR) Six diel (i.e., full 24 hour diurnal) periods were chosen for this report; three for the period in which the transect was installed in the 3% retention stand (July 23, August 16, September 2), and three for the period in which the transect was installed at the 5% retention stand (September 19, October 14, December 11). Each diel period was chosen for the presence of relatively cloudfree conditions, as indicated by low diffuse to total PAR ratios. Diel pattern, 3% transect: The contoured sections of half-hour mean PAR make clear that both north and south sections of the transect experienced significant periods of full sunshine and shade during daylight hours. On July 23, sensors in the middle of the north-end of the transect ( and ) were exposed to a short period of direct early morning sunlight from about 8 to 9 am, before being cast in shadow. The middle of the south end of the transect () received PAR of about 12 µmol m -2 s -1 for a half-hour to 4 minutes between 9 and 1 am. The highest PAR readings observed during the morning hours (approx. 16 µmol m -2 s -1 ) were observed at to in the hours from 1:3 am till about noon, with (5 m north of the clump) and (inside the clump) experienced mostly shaded conditions during this time. At midday, the north end of the transect was cast in shadow, from the centre of the clump () to the northernmost node (); the southern part of the transect ( and ) saw full sun conditions for a couple of hours following noon. In the northern part of the transect,,,, and spent an hour in shade at midday but experienced more direct sunlight again in the later afternoon at, this late afternoon sun was observed after 3 pm. The highest sustained PAR readings were observed at, the most southerly node on the transect as expected. PAR readings of between 12 and 16 µmol m -2 s -1 were measured between noon and 3 pm. The Xtransect (i.e., east-west) data that was collected beginning with the installation of the EW transect on August 12, shows that the western end of the transect experiences the longest sustained periods of high PAR in the late morning into mid-afternoon. The entire EW transect experienced high PAR values for the half-hour period beginning at about 11 am. The clump of trees at the centre of the transect places the eastern end of the transect in shade before midday, and again in the late afternoon hours. Diel pattern, 5% transect: The influence of the single tree at the centre of the transect in the 5% retention stand is immediately apparent. PAR levels are uniform along the S transect except at midday, when the tree located at casts its shadow northward. The shadow from the central tree lower PAR values recorded at for approximately an hour at midday. Moving southward from, the shadow causes a more significant decrease on PAR values recorded at stations closest to the tree. The Xtransect data recorded at the 5% retention stand did not show the influence of the single central tree on PAR values recorded on the EW transect, as we would expect. For the most part, all four Xtransect sensors showed the same diel PAR pattern. However, the sensor at experienced lower PAR values at midday than the surrounding sensors; this was likely due to the influence of a single tree directly south of this sensor (i.e. tree 177, see Figure 2b). 5

6 Powell River 3% Retention Transect: Diel PAR, July 23, Sensor : PAR July 23,24 18 Sensor : PAR July 23,24 18 Sensor : PAR July 23, Hour of Day PAR (µmol m -2 s -1 ) S 6

7 Powell River 3% Retention Transect: Diel PAR, August 16, Sensor : PAR August 16,24 18 Sensor : PAR August 16,24 18 Sensor : PAR August 16, Hour of Day PAR (µmol m -2 s -1 ) S 7

8 Powell River 3% Retention Xtransect: Diel PAR, August 16, Sensor : PAR August 16,24 18 Sensor : PAR August 16,24 18 Sensor : PAR August 16, Hour of Day E PAR (µmol m -2 s -1 ) W 8

9 Powell River 3% Retention Transect: Diel PAR, September 2, Sensor : PAR September 2,24 18 Sensor : PAR September 2,24 18 Sensor : PAR September 2, Hour of Day S 9

10 Powell River 3% Retention Xtransect: Diel PAR, September 2, Sensor : PAR September 2, Sensor : PAR September 2, Sensor : PAR September 2, Hour of Day E PAR (µmol m -2 s -1 ) W 1

11 Powell River 5% Retention Transect: Diel PAR, September 19, Sensor : PAR September 19,24 18 Sensor : PAR September 19,24 18 Sensor : PAR September 19, Hour of Day PAR (µmol m -2 s -1 ) S 11

12 Powell River 5% Retention Xtransect: Diel PAR, September 19, Sensor : PAR September 19, Sensor : PAR September 19, Sensor : PAR September 19, E Hour of Day PAR (µmol m -2 s -1 ) W 12

13 Powell River 5% Retention Transect: Diel PAR, October 14, Sensor : PAR October 14,24 18 Sensor : PAR October 14,24 18 Sensor : PAR October 14, Hour of Day S 13

14 Powell River 5% Retention Xtransect: Diel PAR, October 14, Sensor : PAR October 14, Sensor : PAR October 14, Sensor : PAR October 14, Hour of Day E PAR (µmol m -2 s -1 ) W 14

15 Powell River 5% Retention Transect: Diel PAR, December 11, Sensor : PAR December 11,24 18 Sensor : PAR December 11,24 18 Sensor : PAR December 11, Hour of Day PAR (µmol m -2 s -1 ) S 15

16 Powell River 5% Retention Xtransect: Diel PAR, December 11, Sensor : PAR December 11, Sensor : PAR December 11, Sensor : PAR December 11, E Hour of Day PAR (µmol m -2 s -1 ) W 16

17 II. SOIL TEMPERATURE 3% transect: Mean daily air temperatures and the corresponding soil temperatures are shown in the two panels on page 18. Mean daily air and soil temperatures climbed from about 15 C early in the measurement period, to between 2 to 25 C for several weeks. A 4 to 5 day cold spell occurred beginning on DOY 216, before the temperature recovered to its previous level. Finally, a cooling trend began on about DOY 233 and mean air temperatures of 12 C were observed by the end of the experiment. Perhaps surprisingly, the highest consistent soil temperatures were measured at sensor, whereas the southernmost sensor () was consistently among the coldest stations on the S transect. Between DOY 195 and 235, the transect experienced the greatest variability in soil temperatures between sensors, a likely result of the dominance of direct over diffuse PAR conditions. Through all of the diel periods examined, the warmest soil temperatures were observed in two islands surrounding sensors and, located in the middle of the northern and the southern parts of the transect, respectively (see contoured sections, page 19). Elevated soil temperatures in these areas persisted well into the night and early into the morning. At the beginning of September, 24, soil temperatures at, 1 m south of the centre of the clump, were the warmest along the transect. 5% transect: Soil temperatures in the 5% transect showed much less variability between the sensor stations - a result consistent with the uniform light conditions reported for the transect above. However, the influence of the single tree at the centre of the transect can still be seen in the contoured sections presented on page 2. Between the hours of 11 am and 5 pm, soil temperatures measured 5 m north of the tree were 2-3 C above those measured at the other stations. 17

18 Powell River 3% Retention Transect: Soil Temperature 18

19 Powell River 3% Retention Transect: Diel Soil Temperature Soil Temperature ( C ) 19

20 Powell River 5% Retention Transect: Soil Temperature 2

21 Powell River 5% Retention Transect: Diel Soil Temperature Soil Temperature ( C ) 21

22 III. SOIL MOISTURE 3% transect: Daily mean soil moisture from the 8 transect stations is presented on page 23. Soil moisture values varied considerably across the transect, with the southernmost station,, consistently showing the highest soil volumetric water content exceeding even that measured at the clearcut climate station. Station, 15 m to the north, consistently showed among the lowest soil volumetric water contents. Over 5 mm of rain fell between DOY 235 and 24, and this caused soil moisture to rise abruptly over the transect during this period; volumetric water contents range from approx..5 () to.2 () by the end of the measurement period in the 3% retention stand. 5% transect: The soil moisture pattern was markedly different for the 5% retention transect. In part, this is due to the more frequent precipitation events. Two stations showed significantly higher soil moisture than all of the other stations:, roughly equidistant from the trees at the centre and the northern extreme of the transect, and the clearcut climate station. Soil volumetric water contents recorded by probes at these locations ranged from approx..15 to.17 () and.15 to.22 (clearcut-climate station). The amplitude of variation in the clearcut is consistently higher than at the transect stations. The other transect stations recorded soil volumetric water contents between.2 () and.75 (). 22

23 Powell River 3% Retention Transect: Soil Moisture 23

24 Powell River 5% Retention Transect: Soil Moisture 24

25 Powell River 5% Retention Transect: Soil Moisture Volumetric Water Content (m 3 /m 3 ) 25

26 IV. Transect Relationships The following analyses are based on mean monthly values for PAR, soil moisture and soil temperature for each transect station (see Fig 2a and 2b). Both the 3% and 5% showed a decrease in PAR to the north of the trees or tree. Soil temperatures closely followed PAR. Soil moisture did not recover as fast as either PAR or soil temperature from south to north. See Figs IV General Conclusions The preliminary results presented here show considerable variability in microclimate in the 3% dispersed area: less variability was detected in the 5% area due to the fewer trees and lateness in the season when measurements were made. A clear pattern is detected in all cases however: light, temperature and (to some extent) moisture are highly related which suggest a good basis for model development when trying to predict the relationships here to other sites. The trends are also very similar. These data have been used to improve the microclimate model, FORGE (see attached review). 3% (6 tree clump): south->north transect 5% (single tree): south->north transect PAR micromoles m 2 sec MOTH July August September distance from clump center (-=south; +=north) PAR micromoles m 2 sec distance from tree (-=south; +=north) MOTH September October ovember December Fig IV. 1. Relationship between PAR and distance along south-north transect for a 6 tree clump (3% dispersed) and single tree (5%) at TM 188 Powell River. The dotted vertical lines show the extent of the tree crowns. 3% (6 tree clump): south->north transect 5% (single tree): south->north transect soil temperature, o CC MOTH 15 July August 14 September distance from clump center (-=south; +=north) soil temperature, o C distance from tree (-=south; +=north) MOTH September October ovember December Fig IV. 2. Relationship between soil temperature and distance along south-north transect for a 6 tree clump (3% dispersed) and single tree (5%) at TM 188 Powell River. The dotted vertical lines show the extent of the tree crowns 26

27 3% (6 tree clump): south->north transect.4 5% (single tree): south->north transect.4 soil moisture, m 3 m MOTH. July August September distance from clump center (-=south; +=north) soil moisture, m 3 m distance from tree (-=south; +=north) MOTH September October ovember December Fig IV. 3. Relationship between soil moisture and distance along south-north transect for a 6 tree clump (3% dispersed) and single tree (5%) at TM 188 Powell River. The dotted vertical lines show the extent of the tree crowns. (There is an apparent wet microsite causing high readings at 15m in the 5%.) REFERECES Fleming, R.L., Black, T.A., Adams, R.S. and Stathers, R.J Silvicultural treatments, microclimatic conditions and seedling response in Southern Interior clearcuts. Can. J. Soil Sci. 78: Prescott, C.E Effects of clearcutting and alternative silvicultural systems on rates of decomposition and nitrogen mineralization in a coastal montane coniferous forest. Forest Ecol. Mngmnt. 95: Spittlehouse, D.L. and Stathers, R.J Seedling Microclimate. BC Ministry of Forests, Land Management Report 65, p