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Effect of shading, mulching and vegetation control on Douglas-Fir seedling growth and soil water supply
Forest Ecology and Management, 18 (1987) 189-203
189
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effect of Shading, Mulching and Vegetation Control on D o u g l a s - F i r S e e d l i n g G r o w t h and Soil Water Supply L.E. FLINT and S.W. CHILDS
Department of Soil Science, Oregon State University, Corvallis, OR 97331 (U.S.A.) (Accepted 1 August 1986)
ABSTRACT Flint, L.E. and Childs, S.W., 1987. Effect of shading, mulching, and vegetation control on Douglasfir seedling growth and soil water supply. For. Ecol. Manage., 18: 189-203. Harsh environmental conditions on many harvested sites in southwest Oregon necessitate site modifications for successful regeneration of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco]. We conducted a 2-year study with 350 seedlings to assess the effects of twelve soil-surface shading, mulching, and vegetation control techniques on seedling growth and soil temperature and moisture environments. Treatments modified a variety of environmental conditions. Major effects of treatments were to lower soil surface temperature, reduce soil surface evaporation, and reduce vegetative competition for soil water. These modified conditions affected seedlings by adjusting the timing of seedling growth and reducing soil water loss to increase available water for seedling use. Final seedling shoot volume and stem diameter both differed among treatments. Seedlings in treatments where competing vegetation was controlled showed significantly greater growth than seedlings in other treatments. Soil water loss in treatments where either soil surface evaporation was controlled by mulching, or where competing vegetation was controlled, was significantly less than water loss from the shaded and control treatments. Soil water loss in treatments with vegetation controlled by herbicide was significantly less than in treatments with vegetation controlled by scalping. Seedlings showed greatest growth with treatments that elicited the most efficient use of available microsite water either by reducing soil surface evaporation or vegetation competition.
INTRODUCTION
Of publicly owned commercial forest land in southwest Oregon, there are 283 000 ha, about 12%, identified by land managers as having regeneration problems (Hobbs et al., 1983). Reforestation problems due to limited water Oregon Agricultural Experiment Station Technical Paper No. 7378. Contribution from the Dep. of Soil Science and the Dep. of Forest Engineering, Oregon State University. This work was supported as part of the Forestry Intensified Research Program (PNW-80-85), a cooperative project of Oregon State Univ., U.S.D.A. Forest Sevice, and U.S.D.I. Bureau of Land Management.
0378-1127/87/$03.50
© 1987 Elsevier Science Publishers B.V.
190
supply are usually encountered in shallow, skeletal soils on steep slopes with southerly aspects. These soils have low water-holding capacities and experience long, hot, dry periods during the growing season. While regeneration failures have been reduced in southwest Oregon with an increased awareness of improved stock quality and proper handling of seedlings ( Duryea and Landis, 1984 ), particularly harsh sites require special management techniques for artificial r~generation. Although the general effects of shading and mulching are known, there is little quantitative information available to aid in selection among the range of reforestation treatments. This report provides information regarding the effects of 12 site-modification techniques on seedling growth, soil moisture depletion, and soil temperature. High soil temperatures can limit seedling growth and survival. Conifer seedlings can tolerate soil temperatures up to 54 ° C, but higher soil temperatures will damage seedling tissue and impede vascular transport ( Silen, 1960; Seidel, 1986). Temperatures up to 76 °C are common on south-facing slopes in southwest Oregon and maximum soil surface temperatures of 86°C have been recorded (Hallin, 1968). These conditions can be alleviated by limiting the solar radiation load to the soil surface around seedlings by either shading or mulching. This reduces soil surface temperatures and helps to maintain water in the soil by reducing evaporation rates. Soil water is important not only for plant use but also for maintenance of the soil temperature regime. Water increases soil thermal conductivity which, along with the high specific heat of water, makes the soil a more effective heat sink. The seedling root zone is, therefore, less susceptible to excessive heating when the soil is wet (De Vries, 1975). In mediterranean climates, available water in the seeding root zone is used by the end of the growing season in most years (Youngberg, 1959; Flint and Childs, 1984). The soil water supply for seedling use can be effectively increased by reducing vegetative competition for water in the root zone, limiting radiation input, and mulching to reduce soil surface evaporation. Many methods for modifying the seedling microclimate have been tried with various results. Artificial shade created by cardboard shadecards is used to reduce radiation input to the soil surface. Shadecards do not significantly reduce average soil temperatures but they do reduce m a x i m u m soil temperatures and soil heat flux ( Childs et al., 1985). Mulching is a treatment which has increased seedling survival by reducing water loss due to surface evaporation and competing vegetation (Hermann, 1964a, 1965 ). Black plastic mulches have been shown to minimize temperature extremes in agricultural situations (Takatori et al., 1964; Smith et al., 1968). Waggoner et al. (1960) suggested that black plastic mulch absorbs most of the incoming radiant energy but transmits little of it to the soil due to the insulating effect of the still air layer between the mulch and the soil. The net effects of insulation plus decrease in surface evaporation can be seen in reductions in
191
soil temperature extremes with little change in average temperatures. High mulch surface temperatures would, however, be of concern if the mulch was in direct contact with seedlings. Techniques designed only for vegetation control have resulted in dramatic increases in seedling growth and survival (Newton, 1964; Passof, 1978), in addition to reduced seedling water stress (Sands and Nambiar, 1983). One such method is to 'scalp' the soil surface free of vegetation; an alternative is herbicide application, which kills vegetation in place and maintains the soil surface layer. The objective of this study was to assess the effects of various soil surface shading and mulching treatments and two vegetation control techniques on soil temperature, availability of soil water, and growth of Douglas-fir seedlings; in order to accomplish this detailed measurements of growth, moisture, and temperature were replicated for all treatments on a single reforestation site. METHODS
This study was conducted near Wolf Creek, Oregon, (latitude 42 °43' N, longitude 123 ° 17'W, 715 m elevation). The site is a steep, south-facing slope (190 degrees azimuth, 30% slope) of approximately 16 ha that had been clearcut and burned during summer, 1981. The soil is a moderately deep, loamyskeletal, mesic mixed Typic Xerochrept. Average soil depth is 660 mm ranging between 480 and 685 mm. Available water in the seedling root zone (0-250 mm) averages 40.6 mm water, or 0.16 m3/m 3. Vegetation that grew during the 2 years following harvest included the following: Arbutus menziesii, 66% of total vegetative cover after 2 years; Ceanothus sanguineus, 12%; Holodiscus discolor, 4%; Penstemon sp., 3%; Rubus ursinus, 3%; and Rhus diversiloba, 1%. Three hundred and fifty 2-0, bareroot, Douglas-fir seedlings were selected for uniformity and planted in a 1.6 × 1.6-m spacing, in March 1982. These seedlings averaged 267 mm height, 5 mm diameter at root collar, 42 250 mm ~ shoot volume, 10 000 mm 3 root volume, for a shoot:root ratio of 4:1. After two growing seasons (September 1983), all seedlings were harvested for final growth measurements.
Treatments Three hundred and fifty seedlings were experimental units in a completely randomized experimental design. Twelve treatments were assessed as protection from high soil temperatures or rapid soil water loss. A group of untreated seedlings was used as experimental control, and soil temperature and moisture measurements were made on other sites with no seedlings. T r e a t m e n t s were selected from current operational techniques based on hypothesized effects of the treatments on the soil water and temperature environment (Table 1).
192 TABLE 1 Treatments and expected effects on soil temperature and soil and plant water use Treatment
Seasonal seedling transpir2
Soil surface evap.
Soil temp.
Shadecards Mulches Black plastic White plastic Paper mulch
Comp. veg.b
Water available for seedling use
++
+ + +
----
+ ---
+ + +
+ ÷ +
+
--
__
++ ++
+
Scalp
+
+
+
None
+
Herbicide
+
--
+
None
+
Stem shade
Styrofoam cup Pyramid
Control
++
aAll responses relative to control treatment, ( + ) increased response, (--) decreased response. Blanks show no expected effect due to treatment. b+ + Indicates much competing vegetation, + indicates some competing vegetation. T r e a t m e n t s were r a n d o m l y applied to all seedlings over t h e entire site with 2 4 - 2 8 replicates per t r e a t m e n t . D e t a i l e d d e s c r i p t i o n s are as follows:
Shadecards. A c o n v e n t i o n a l 2 0 0 X 3 0 0 - m m s h a d e c a r d was s t a k e d into t h e g r o u n d n e x t to the seedling. Five d i r e c t i o n a l o r i e n t a t i o n s were used: W, SW, S, SE, a n d E. S h a d e c a r d s s h a d e the soil surface n e a r t h e seedling to limit radiation load a n d reduce t e m p e r a t u r e s at d i f f e r e n t t i m e s of t h e day.
Mulches. 760 X 7 6 0 - m m s h e e t s of b l a c k plastic, w h i t e plastic, a n d c a r d b o a r d p a p e r were p l a c e d a r o u n d seedlings. T h e s e t r e a t m e n t s were e x p e c t e d to reduce soil surface e v a p o r a t i o n , reduce r a d i a t i o n p e n e t r a t i o n , a n d help to c o n t r o l competing vegetation.
S t e m shade. A 240-ml s t y r o f o a m cup with t h e b o t t o m c u t out was i n v e r t e d a n d p l a c e d a r o u n d t h e s t e m of t h e seedling at the soil surface. T h i s t r e a t m e n t is designed to p r o t e c t a g a i n s t lethal tissue d a m a g e f r o m possible high soil surface t e m p e r a t u r e s . A n o t h e r f o r m of t h e s t e m s h a d e was a t h r e e - s i d e d c a r d b o a r d p y r a m i d with a 350 X 3 5 0 - m m base a n d 70 X 7 0 - m m hole in t h e top. P y r a m i d s
193 were placed around the seedling stem (with the open side to the north ) in order to decrease soil surface evaporation and temperature.
Scalp. The soil surface layer and any accompanying vegetation were scraped away in a 1.2 X 1.2-m square around the seedling. The area was maintained vegetation-free throughout the growing season in order to decrease vegetation competition in the soil volume surrounding the seedling. Removal of the soil surface covering is likely to increase surface evaporation. Herbicide. A mixture of 20 g Atrazine and 30 g 2,4-D per kg water was used to spray a 3.3 X 3.3-m area around the seedling. One application in the spring of the second season kept the soil free of vegetation throughout the season. This t r e a t m e n t eliminated vegetation competition while leaving the soil surface and any dead vegetation intact. Standard techniques in herbicide application usually involve broadcast spraying an entire site. A larger surface area was treated by the herbicide spray t h a n by the scalp to simulate this practice. Seedling growth measurements In-situ height and diameter measurements were taken on all seedlings. Other measurements taken to examine t r e a t m e n t responses in more detail were: (1) Initial, first year, second year, and final height; (2) Initial and first year total (outside) diameter, measured with calipers at the root collar. At the end of the experiment, we cut all seedlings at the root collar and measured total and inside bark diameters and first and second-year radial growth; (3) Initial and final root and shoot volumes (final root volumes done on a 45-tree subset) were determined using water displacement; (4) First and second year needle lengths and final bud size. Yearly occurrence of browse was noted. In addition, periodic observations of survival, budburst and budset of seedlings were made. These observations include identification of a second above-ground growth period for some seedlings. This phenomenon, called double flushing, is a response to good environmental conditions for growth. Double flushing was identified as a second occurrence of budburst after the first budburst of the season culminated in budset.
Soil water loss Soil water and temperature measurements were made approximately every 2 weeks from early May through September. Soil water loss was determined using a two-probe gamma ray attenuation device ( Model 2376, Troxler Instrum e n t Co., Research Triangle Park, NC ), which was calibrated daily before use.
194
Water loss in the seedling root zone was measured across a 300-mm path length in 25-mm depth increments to either 760 mm or bedrock. This soil volume was considered to be the microsite from which seedlings extracted water. Measurements were made during each site visit beneath 60 seedlings distributed across five treatments (shadecard, mulch, scalp, pyramid, and herbicide), plus control and no-tree locations. N e t water extracted from a given microsite over the season was calculated from measured data. Rainfall values were added to net water use over the season to determine total water use; only rainfall that did not wet the soil above field capacity was used in these calculations. Available soil water storage (average of 26 mm3/mm 2 over the entire soil profile for all seedlings in 1983) is the difference between the soil water content as field capacity (41 mm) and the water left in the soil at the driest part of the year (15 mm for 1983). Precipitation exceeding field capacity occurred three times during the season, in early May, early June, and early September, totaling 28 mm.
Soil temperature Soil surface temperatures were measured at hourly intervals for 2-3 days every visit with an infra-red thermometer (Raynger II, Raytek, Inc., Santa Cruz, CA ). Soil temperature profiles were measured with thermistor probes at 20, 40, 80, 160, and 320 mm below soil surface beneath the seedlings. Temperatures were monitored every 15 min on data loggers (CR5 Digital Recorder, Campbell Scientific, Inc., Logan, U T ) for 2-3 days every site visit. Measurements were made beneath 18 seedlings on control, herbicide, mulch, shadecard, and pyramid treatments.
Statistical analyses Chi-square analyses were done on data from periodic observations of budburst, budset, and double flushing. All growth data were subjected to analysis of variance. Least significant differences were determined using total analysis of variance to compare all treatments. The same methods were used for analyses of soil water loss. Evaluations of growth data were done at a significance level of P - - 0.20, as we feel this is acceptable for management decisions based on measurements taken in a highly variable environment. Soil water measurements were made using a precision instrument and analyzed at a significance level of P--0.05. A correlation matrix was calculated for all growth measurements on the total population. Standard deviations are presented with temperature data because sample sizes were too small for statistical analysis.
195 o
~Total
]
diameter
CO
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,
<
+
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9
bc
Herbicide
Scalp
Mulch Control Shade card
c
70
x
Pyramid
Fig. 1. Final seedling total diameters and shoot volumes for six treatment groups. Mean values between bars with same letters do not differ significantly at the level of P = 0.20. RESULTS AND DISCUSSION
Growth Weather was mild during the 1982 and 1983 growing seasons with no extreme temperature events. Daytime temperatures in 1983 ranged from 6.4°C to 35.7°C, and 59 mm of precipitation fell between 1 M a y and 15 September. There was 98% seedling survival during the 2-year study and growth trends were similar in both years. Although the first-year growth data did not show statistical differences among treatments, significant differences were measured in the second year. This is probably due to a combination of effects of nursery conditions and transplanting stresses on first-year outplanted seedlings. The lack of much vegetative competition until the second season is likely to have reduced the impact of different treatments. The small differences in height growth among treatments may have been due to deer browse which occurred on 49% of all seedlings in 1982. The severity of browse damage and its even distribution among all treatments probably contributed to the smaller differences in seedling response among treatments that first growing season. Browse was successfully controlled with a repellent spray in 1983. Therefore, for clarity of presentation, we discuss only second-year results. The six t r e a t m e n t groups shown in Fig. 1 are logical groups based on our hypotheses (Table 1 ). Variation within groups was small while differences between groups were large as determined by least significant difference analyses. The styrofoam cup treatment, designed solely for protection from high soil surface temperatures, resulted in growth trends which were not statistically different from those of the controls and was included with the control
196
TABLE 2 Correlations { r) of growth measurement data Total diameter vs. inside diameter Initial S:R vs. final S:R Initial root volume vs. final root volume Final shoot volume vs. inside diameter Final shoot volume vs. total diameter Second year height growth vs. total height
0.958 0.910 0.898 0.843 0.820 0.809
All possible correlations were performed on the entire data set. Only correlations above r = 0.8 are listed
treatment for analysis. The pyramid treatment, originally conceived to provide combined protection against high temperatures and soil surface evaporation, did produce different growth results from the controls, and was thus maintained as a separate group. Total diameters and shoot volumes were the most sensitive indicators of differences among treatments. The seedlings with surrounding vegetation controlled (herbicide and scalp) grew the most and the shaded seedlings ( shadecard and pyramid) grew the least. Calculations of percentage increases in total diameter and top volume showed the same significant differences among treatments as did the direct measurements.
Additional growth data Additional growth measurements were taken to assess the possibility of relating various nonstandard growth measurements to the treatments and to each other. The ability to predict growth and determine the adequacy of standard field measurements were major considerations in the choice of additional measurements. Correlations between all pairs of growth measurements were performed and those with r values above 0.80 are shown in Table 2. Correlation of inside bark diameters and total diameters was high indicating that diameter measurements made after destructive sampling provide no more information than standard field caliper measurements. Also, final diameters were well correlated with shoot volume. Shoot:root ratios before planting were highly correlated to final shoot:root ratios, and treatments had little effect on the final ratios. This suggests that shoot:root ratios during the first few years after outplanting are influenced strongly by the dimensions of the nursery stock used. Since smaller shoot:root ratios have been shown to be beneficial on droughty sites (Hermann, 1964b), the data presented here corroborate the importance of selecting appropriately sized seedlings for outplanting. Relative root growth was similar for all seedlings, regardless of treatment. As larger root biomass increases both the absorptive capacity and the volume
197
~%budburst I00
I----I%budset
by 5 / 4 / 8 3 by 7 / 1 3 / 8 3
p=O.05 p=O. 10
8oi I-Z
60 r~ W O-
40
20
% actively
growing seedlings
Herbicide
Scalp
69.0
61.2
Mulch Control
Shade cord
Pyramid
58.8
45.0
43.6
51,9
Fig. 2. Percentage of seedlings undergoing budburst by 4 May 1983, and budset or double flushing by 13 July 1983. Chi-square analyses were done and indicate differences among treatments at corresponding p values of 0.05 for budburst, 0.10 for budset, and 0.20 for double flushing. Also shown is that percentage of seedlings in each treatment actively growing between the representative dates.
of soil utilized for water and nutrient collection, the importance of planting seedlings with initially large root systems is apparent.
Timing of seedling growth The growth patterns for seedlings in treatments that resulted in more or less growth than the controls may be partially explained by the length of time seedlings in different treatments were actively growing. This was determined by the percentage of seedlings in each treatment that had undergone budburst (started active growth for the season) by 4 May, and the percentage that had set their buds (stopped active growth for the season) by 14 July (Fig. 2). These seedlings were in a growth stage rather than in dormancy induction or dormancy. These dates were chosen to show a representative percentage of seedlings in different treatments to have undergone budburst, as about 80% of all seedlings had achieved budburst by 4 May, and about 50% had set buds by 14 July. The treatments are displayed in the same order as in Fig. 1, from those resulting in the most growth to those with the least. Chi-square analysis showed a significant, although perhaps inconsequential, trend in percentage budbust. Soil temperature, which has been found to influence the date of early season growth initiation ( Sorensen and Campbell, 1978), differed little among treatments, so treatments had little effect on burdburst. Soil temperatures (dis-
198
cussed in the next section ) for bare soil plots, which simulate a scalp treatment, were higher throughout the season. This observation, although inconclusive, helps explain the trend of earlier budburst for the scalp treatment. The same phenomenon was also noted in a shadecard, clearcut, shelterwood comparison (Childs and Flint, 1987), in which treatment did not alter budburst but, as in our study, did have a significant effect on the timing of budset. Our data indicate that treatments under which seedlings which showed most growth had lower percentages of trees with buds set (more seedlings actively growing) by mid-July. The shadecard treatment contained more seedlings that had set buds and stopped active growth by this time. The data for double flushing (Fig. 2 ) also support this conclusion. There is a trend in these data showing that seedlings which set buds later in the season also produced a second flush of growth. This trend is shown most clearly by seedlings with the herbicide, shadecard, and pyramid treatments. Doubleflushing is generally related to availability of water, whereas budset is primarily in response to moderate moisture stress ( Greaves et al., 1978). In our study, seedlings with more available water left in the soil had the opportunity to undergo additional growth. The result of double flushing is an increase in height and shoot biomass which supplies more leaf area for photosynthesis, therefore additional diameter growth. There may be disadvantages to double flushing in harsh years when high soil and air temperatures occur in late summer. Seedlings undergoing a second flush in the nursery show less survival on droughty sites (Lavender, 1984). Second-flush tissue often does not develop frost hardiness early enough to escape damage during fall frosts. In addition, winter resting buds need time to harden so that growth will not resume with fall rains and they will be subjected to chilling, which increases vigor (Lavender and Cleary, 1974). In our study, the shaded treatments produced seedlings with less growth, but had more seedlings undergoing dormancy induction in midJuly. Data on timing of budburst and budset were used to calculate the percentage of seedlings in each treatment that had achieved budburst but had not undergone budset during the main part of the growing season. Ranking the percentages of seedlings in each treatment that were actively growing between early May and mid-July shows the treatments to be again in the same order of most to least growth. This supports the analysis of Fig. 2, and the assertion that seedling growth was influenced by growing-season length - - the time between budburst and budset that was influenced by treatment.
Soil temperature Treatments giving the best seedling growth also had the highest surface soil temperatures throughout the season (Fig. 3 ). It is noteworthy that in the herbicide and bare-soil treatments the soil surface reached nearly lethal temper-
199
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I
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(2) ( I) (4) (6) (3} (3)
AUGUST
Fig. 3. Soil surface high temperatures for five treatments and bare soil for each sampling date for the 1983 season.
atures. In a hotter growing season, surface temperatures would be greater and shading treatments would provide protection for seedlings from the heat stress. It is also of note that pyramid-treated seedlings had lowest soil surface temperatures for most of the season. This is in accord with the measured smaller growth of those seedlings. Soil temperature profiles to 320 mm were not significantly different among treatments. Microsite w a t e r use a n d allocation
Seasonal soil water loss data (Fig. 4) show that treatments with no vegetation or evaporation control resulted in the most microsite water loss over the season, the remaining treatments all using less water. The treatments that controlled similar amounts of vegetation resulted in different water use patterns due to either surface shading or evaporation control. In the case of the no-tree treatment, however, the difference is due to no seedling water use. A notable difference in water use due to evaporation is seen between the total vegetation control treatments, scalp and herbicide. The low water use in the herbicide t r e a t m e n t illustrates the effectiveness of an undisturbed soil surface layer in controlling evaporative loss of soil water. Rate of water loss was greatest from early J u n e to mid-July. During this period, the majority of aboveground growth occurred and t r e a t m e n t differences in soil water loss became pronounced. Total soil profile water loss trends do not, however, match the trends in seedling growth (Fig. 5). Differences are due to the partitioning of available microsite water among seedlings, competing vegetation and surface evaporation. As an example, the scalp t r e a t m e n t resulted in large diameter growth but also resulted in more water use than the other treatments. This is likely due to increased surface evaporation caused by higher surface tempera-
200
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T "r .-'~C°ntr°l ']" .s ~ " ....eShadecard [ ,,r" ..'" Mulch _ - ~ - e... .-'e .~-I Scalp ="-" ." al~:"~'j~ No tree /" ...." . ~ e Pyrarnid /," .e"" ..~" e ~ . . . ~ ' " Herbicide
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Fig. 4. Seasonal cumulative water loss from the soil profile for seven treatments. Vertical bars are
least significant differences determined on cumulative water used by each date at a significance level of P = 0.05.
tures (indicated by bare soil surface temperatures in Fig. 3) and a disturbed soil surface. The herbicide treatment, by losing less water to evaporation and competing vegetation, resulted in more soil water available for increased plant growth. The effect of treatment on the partitioning of water to seedlings can be demonstrated as water use efficiency ( Table 3 ), which is shown in this case as total shoot volume ( m m 3), divided by total microsite water used ( m m ) . The herbicide treatment elicited the most efficient use of site-available water for seedling growth. The scalp, mulch and pyramid treatments were less efficient and
ITota "~
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Fig. 5. Final seedling total diameter vs. percentage of the total seasonal water lost by seven treatments between 6 June 1983 and 13 July 1983.
201 TABLE 3 Water use efficiencies (WUE) for six treatments, calculated as shoot volume (mm 3) divided by total seasonal microsite water lost (mm ) Treatment
WUE
Herbicide Scalp Mulch Pyramid Control Shadecard
140 126 115 103 99 94
the control and shadecard treatments had the poorest microsite allocation of available soil water. The percentage of vegetative cover occurring in each treatment was estimated periodically throughout the growing season. A ranking of cover corresponds to the water use efficiency ranking. The herbicide spray and scalp treatments had no vegetative cover, mulch and pyramid treatments partially controlled vegetation, and the control and shadecard treatments had no effect on competing vegetation. SUMMARY
This study was designed to make a comparison of common site-modification techniques used in reforestation of harsh sites. Testing of 12 treatments on one site for 2 years allowed unambiguous t r e a t m e n t comparisons. In addition, the nature of the effects ( growth timing, soil temperature, water supply, etc. ) can be assessed because there are no growing-season or site differences to confound the experiment. This study shows that during a mild growing season, Douglas-fir seedlings under different treatments had different water loss patterns and seasonal growth. These differences were due to various interactive effects of temperature, timing of growth, and increased supply of water available to seedlings. Increased water supply was due to t r e a t m e n t effects on either soil surface evaporation or water use by competing vegetation. Competing vegetation seemed to be most instrumental in influencing water availability for seedlings as the degree of t r e a t m e n t control of vegetation correlated well with seedling growth, seasonal water loss, and water use efficiency. The importance of surface evaporation as a mechanism of water loss was clearly shown by herbicide-treated sites which lost significantly less water, yet had a higher water-use efficiency than scalp t r e a t m e n t sites. T r e a t m e n t s that increased the efficient use of microsite water, whether by vegetation or evaporation control, allowed more water to be allocated to increasing seedling growth. T r e a t m e n t s that affected
202
the timing of budburst and budset had a large influence on seedling growth. Temperature seemed to influence budburst and earlier seedling growth. Increased water availability and decreased moisture stress later in the season delayed budset, increased double flushing, and therefore increased growth. Standard field techniques for measurement of seedling growth were found to be as reliable as more intensive measurements requiring destructive sampling. While the necessary first step in understanding the environmental dynamics of a site and reforestation treatments is on a microsite level, these approaches should also be considered from a management perspective. An important consideration in selecting treatments for regenerating harsh sites is the risk of infrequent severe drought and heat stress seasons which may greatly reduce the survival of the tree crop. A study in southwest Oregon during the very harsh 1981 season showed that shadecards increased seedling survival by 50% (Flint and Childs, 1984 ). This is supported by our study of a mild year in which earlier budset and lower soil temperatures occurred for shaded treatments. These were also accompanied by less seedling growth and high soil water loss resulting in lower water-use efficiencies. Shading alone, as a safeguard for survival in the event of a harsh year, is at the expense of enhanced growth and efficient water use in good years. Survival is, of course, of ultimate concern, but treatments that increase microsite water use efficiency may also help to increase seedling survival beyond that of standard shading techniques by allowing enhanced seedling growth.
REFERENCES Childs, S.W. and Flint, L.E., 1987. Effect of shadecards, shelterwoods and clearcuts on temperature and moisture environments. For. Ecol. Manage., 18: 205-217. Childs, S.W., Holbo, H.R. and Miller, E.L., 1985. Shadecard and shelterwood modification of the soil temperature environment. Soil Sci. Soc. Am. J., 49: 1018-1023. De Vries, D.A., 1975. Heat transfer in soils. In: D.A. De Vries and N.H. Afgan (Editors), Heat and Mass Transfer in the Biosphere. Scripta Book Co., Washington, D.C., pp. 5-28. Duryea, M.L. and Landis, T.D., 1984. Development of the forest nursery manual: a synthesis of current practices and research. In: M.L. Duryea and T.D. Landis (Editors), Forest Nursery Manual: Production of Bareroot Seedlings. Nijhoff/Junk, The Hague, pp. 3-8. Flint, L.E. and Childs, S.W., 1984. Seedling responses to heat and moisture environments in clearcuts and shelterwoods. For. Intensif. Res. Rep., 6 (2) : 3-4. Greaves, R.G., Hermann, R.K. and Cleary, B.D., 1978. Ecological principles. In: B.D Cleary, R. D. Greaves and R. K. Hermann ( Editors ), Regenerating Oregon's Forests. Oregon State Univ. Extension Service, Corvallis, OR, pp. 1-26. Hallin, W.E., 1968. Soil surface temperatures on cutovers in southwest Oregon. USDA For.Serv. Res. Note PNW-78. PNW Forest and Range Experiment Station, Portland, OR, 17 pp. Hermann, R.K., 1964a. Importance of top-root ratios for survival of Douglas-fir seedlings. Tree Plant. Notes, 64: 7-11. Hermann, R.K., 1964b. Paper mulch for reforestation in southwest Oregon. J. For., 62: 98-101. Hermann, R.K., 1965. Survival of planted Ponderosa pine in southern Oregon. Forest Research Laboratory, School of Forestry, Oregon State Univ., Corvallis, OR, Res. Pap. 2, 31 pp.
203 Hobbs, S.D., Gordon, J.G. and Brown, G.W., 1983. Research and technology in southwest Oregon. J. For., 81: 8. Lavender, D.P., 1984. Plant physiology and nursery environment. In: M.L. Duryea and T.D. Landis (Editors), Forest Nursery Manual: Production of Bareroot Seedlings. Nijhoff/Junk, The Hague, pp. 133-142. Lavender, D.P. and Cleary, B.D., 1974. Coniferous seedling production techniques to improve seedling establishment. In: R.W. Tinus, W.I. Stein and W.E. Balmer (Editors), Proc. North Am. Containerized For. Tree Seed. Symp. Great Plains Agricultural Council Pub. No. 68, pp. 177-180. Newton, M., 1964. The influence of herbaceous vegetation on coniferous seedling habitat in old field plantations. Ph.D. thesis. Oregon State Univ., Corvallis, OR, 114 pp. Passof, P., 1978. Report in Forestation Notes. USDA Forest Service, Pacific Northwest Region State and Private Forestry, No. 54-1-2. Sands, R. and Nambiar, E.K.S., 1983. Water relations ofPinus radiata in competition with weeds. Can. J. For. Res., 14: 233-237. Seidel, K.W., 1986. Tolerance of seedlings of Ponderosa pine, Douglas-fir, Grand fir and Engleman spruce for high temperatures. Northwest Sci., 60: 1-7. Silen, R.R., 1960. Lethal surface temperatures and their interpretation for Douglas-fir. Ph.D. thesis, Oregon State Univ., Corvallis, OR, 170 pp. Smith, E.M., Skaggs, R.W. and Casada, J.H., 1968. Potential of heat transfer in soil covered with plastic mulches. Trans. Am. Soc. Agric. Eng., 67: 546-548. Sorensen, F.C. and Campbell, R.K., 1978. Comparative roles of soil and air temperatures in the timing of spring bud flush in seedling Douglas-fir. Can. J. Bot., 56: 2307-2308. Takatori, F.H., Lippert, L.F. and Whiting, F.L., 1964. The effect of petroleum mulch and polyethylene films on soil temperature and plant growth. Proc. Am. Soc. Hortic. Sci., 85: 532-548. Waggoner, P.E., Miller, P.M. and DeRoo, H.C., 1960. Plastic mulching-- principles and benefits. Connecticut Agric. Exp. St., New Haven, Bull. No. 634, 44 pp. Youngberg, C.T., 1959. The influence of soil moisture on the survival of planted Douglas-fir seedlings on clay soils. J. For., 57: 842-844.
189
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effect of Shading, Mulching and Vegetation Control on D o u g l a s - F i r S e e d l i n g G r o w t h and Soil Water Supply L.E. FLINT and S.W. CHILDS
Department of Soil Science, Oregon State University, Corvallis, OR 97331 (U.S.A.) (Accepted 1 August 1986)
ABSTRACT Flint, L.E. and Childs, S.W., 1987. Effect of shading, mulching, and vegetation control on Douglasfir seedling growth and soil water supply. For. Ecol. Manage., 18: 189-203. Harsh environmental conditions on many harvested sites in southwest Oregon necessitate site modifications for successful regeneration of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco]. We conducted a 2-year study with 350 seedlings to assess the effects of twelve soil-surface shading, mulching, and vegetation control techniques on seedling growth and soil temperature and moisture environments. Treatments modified a variety of environmental conditions. Major effects of treatments were to lower soil surface temperature, reduce soil surface evaporation, and reduce vegetative competition for soil water. These modified conditions affected seedlings by adjusting the timing of seedling growth and reducing soil water loss to increase available water for seedling use. Final seedling shoot volume and stem diameter both differed among treatments. Seedlings in treatments where competing vegetation was controlled showed significantly greater growth than seedlings in other treatments. Soil water loss in treatments where either soil surface evaporation was controlled by mulching, or where competing vegetation was controlled, was significantly less than water loss from the shaded and control treatments. Soil water loss in treatments with vegetation controlled by herbicide was significantly less than in treatments with vegetation controlled by scalping. Seedlings showed greatest growth with treatments that elicited the most efficient use of available microsite water either by reducing soil surface evaporation or vegetation competition.
INTRODUCTION
Of publicly owned commercial forest land in southwest Oregon, there are 283 000 ha, about 12%, identified by land managers as having regeneration problems (Hobbs et al., 1983). Reforestation problems due to limited water Oregon Agricultural Experiment Station Technical Paper No. 7378. Contribution from the Dep. of Soil Science and the Dep. of Forest Engineering, Oregon State University. This work was supported as part of the Forestry Intensified Research Program (PNW-80-85), a cooperative project of Oregon State Univ., U.S.D.A. Forest Sevice, and U.S.D.I. Bureau of Land Management.
0378-1127/87/$03.50
© 1987 Elsevier Science Publishers B.V.
190
supply are usually encountered in shallow, skeletal soils on steep slopes with southerly aspects. These soils have low water-holding capacities and experience long, hot, dry periods during the growing season. While regeneration failures have been reduced in southwest Oregon with an increased awareness of improved stock quality and proper handling of seedlings ( Duryea and Landis, 1984 ), particularly harsh sites require special management techniques for artificial r~generation. Although the general effects of shading and mulching are known, there is little quantitative information available to aid in selection among the range of reforestation treatments. This report provides information regarding the effects of 12 site-modification techniques on seedling growth, soil moisture depletion, and soil temperature. High soil temperatures can limit seedling growth and survival. Conifer seedlings can tolerate soil temperatures up to 54 ° C, but higher soil temperatures will damage seedling tissue and impede vascular transport ( Silen, 1960; Seidel, 1986). Temperatures up to 76 °C are common on south-facing slopes in southwest Oregon and maximum soil surface temperatures of 86°C have been recorded (Hallin, 1968). These conditions can be alleviated by limiting the solar radiation load to the soil surface around seedlings by either shading or mulching. This reduces soil surface temperatures and helps to maintain water in the soil by reducing evaporation rates. Soil water is important not only for plant use but also for maintenance of the soil temperature regime. Water increases soil thermal conductivity which, along with the high specific heat of water, makes the soil a more effective heat sink. The seedling root zone is, therefore, less susceptible to excessive heating when the soil is wet (De Vries, 1975). In mediterranean climates, available water in the seeding root zone is used by the end of the growing season in most years (Youngberg, 1959; Flint and Childs, 1984). The soil water supply for seedling use can be effectively increased by reducing vegetative competition for water in the root zone, limiting radiation input, and mulching to reduce soil surface evaporation. Many methods for modifying the seedling microclimate have been tried with various results. Artificial shade created by cardboard shadecards is used to reduce radiation input to the soil surface. Shadecards do not significantly reduce average soil temperatures but they do reduce m a x i m u m soil temperatures and soil heat flux ( Childs et al., 1985). Mulching is a treatment which has increased seedling survival by reducing water loss due to surface evaporation and competing vegetation (Hermann, 1964a, 1965 ). Black plastic mulches have been shown to minimize temperature extremes in agricultural situations (Takatori et al., 1964; Smith et al., 1968). Waggoner et al. (1960) suggested that black plastic mulch absorbs most of the incoming radiant energy but transmits little of it to the soil due to the insulating effect of the still air layer between the mulch and the soil. The net effects of insulation plus decrease in surface evaporation can be seen in reductions in
191
soil temperature extremes with little change in average temperatures. High mulch surface temperatures would, however, be of concern if the mulch was in direct contact with seedlings. Techniques designed only for vegetation control have resulted in dramatic increases in seedling growth and survival (Newton, 1964; Passof, 1978), in addition to reduced seedling water stress (Sands and Nambiar, 1983). One such method is to 'scalp' the soil surface free of vegetation; an alternative is herbicide application, which kills vegetation in place and maintains the soil surface layer. The objective of this study was to assess the effects of various soil surface shading and mulching treatments and two vegetation control techniques on soil temperature, availability of soil water, and growth of Douglas-fir seedlings; in order to accomplish this detailed measurements of growth, moisture, and temperature were replicated for all treatments on a single reforestation site. METHODS
This study was conducted near Wolf Creek, Oregon, (latitude 42 °43' N, longitude 123 ° 17'W, 715 m elevation). The site is a steep, south-facing slope (190 degrees azimuth, 30% slope) of approximately 16 ha that had been clearcut and burned during summer, 1981. The soil is a moderately deep, loamyskeletal, mesic mixed Typic Xerochrept. Average soil depth is 660 mm ranging between 480 and 685 mm. Available water in the seedling root zone (0-250 mm) averages 40.6 mm water, or 0.16 m3/m 3. Vegetation that grew during the 2 years following harvest included the following: Arbutus menziesii, 66% of total vegetative cover after 2 years; Ceanothus sanguineus, 12%; Holodiscus discolor, 4%; Penstemon sp., 3%; Rubus ursinus, 3%; and Rhus diversiloba, 1%. Three hundred and fifty 2-0, bareroot, Douglas-fir seedlings were selected for uniformity and planted in a 1.6 × 1.6-m spacing, in March 1982. These seedlings averaged 267 mm height, 5 mm diameter at root collar, 42 250 mm ~ shoot volume, 10 000 mm 3 root volume, for a shoot:root ratio of 4:1. After two growing seasons (September 1983), all seedlings were harvested for final growth measurements.
Treatments Three hundred and fifty seedlings were experimental units in a completely randomized experimental design. Twelve treatments were assessed as protection from high soil temperatures or rapid soil water loss. A group of untreated seedlings was used as experimental control, and soil temperature and moisture measurements were made on other sites with no seedlings. T r e a t m e n t s were selected from current operational techniques based on hypothesized effects of the treatments on the soil water and temperature environment (Table 1).
192 TABLE 1 Treatments and expected effects on soil temperature and soil and plant water use Treatment
Seasonal seedling transpir2
Soil surface evap.
Soil temp.
Shadecards Mulches Black plastic White plastic Paper mulch
Comp. veg.b
Water available for seedling use
++
+ + +
----
+ ---
+ + +
+ ÷ +
+
--
__
++ ++
+
Scalp
+
+
+
None
+
Herbicide
+
--
+
None
+
Stem shade
Styrofoam cup Pyramid
Control
++
aAll responses relative to control treatment, ( + ) increased response, (--) decreased response. Blanks show no expected effect due to treatment. b+ + Indicates much competing vegetation, + indicates some competing vegetation. T r e a t m e n t s were r a n d o m l y applied to all seedlings over t h e entire site with 2 4 - 2 8 replicates per t r e a t m e n t . D e t a i l e d d e s c r i p t i o n s are as follows:
Shadecards. A c o n v e n t i o n a l 2 0 0 X 3 0 0 - m m s h a d e c a r d was s t a k e d into t h e g r o u n d n e x t to the seedling. Five d i r e c t i o n a l o r i e n t a t i o n s were used: W, SW, S, SE, a n d E. S h a d e c a r d s s h a d e the soil surface n e a r t h e seedling to limit radiation load a n d reduce t e m p e r a t u r e s at d i f f e r e n t t i m e s of t h e day.
Mulches. 760 X 7 6 0 - m m s h e e t s of b l a c k plastic, w h i t e plastic, a n d c a r d b o a r d p a p e r were p l a c e d a r o u n d seedlings. T h e s e t r e a t m e n t s were e x p e c t e d to reduce soil surface e v a p o r a t i o n , reduce r a d i a t i o n p e n e t r a t i o n , a n d help to c o n t r o l competing vegetation.
S t e m shade. A 240-ml s t y r o f o a m cup with t h e b o t t o m c u t out was i n v e r t e d a n d p l a c e d a r o u n d t h e s t e m of t h e seedling at the soil surface. T h i s t r e a t m e n t is designed to p r o t e c t a g a i n s t lethal tissue d a m a g e f r o m possible high soil surface t e m p e r a t u r e s . A n o t h e r f o r m of t h e s t e m s h a d e was a t h r e e - s i d e d c a r d b o a r d p y r a m i d with a 350 X 3 5 0 - m m base a n d 70 X 7 0 - m m hole in t h e top. P y r a m i d s
193 were placed around the seedling stem (with the open side to the north ) in order to decrease soil surface evaporation and temperature.
Scalp. The soil surface layer and any accompanying vegetation were scraped away in a 1.2 X 1.2-m square around the seedling. The area was maintained vegetation-free throughout the growing season in order to decrease vegetation competition in the soil volume surrounding the seedling. Removal of the soil surface covering is likely to increase surface evaporation. Herbicide. A mixture of 20 g Atrazine and 30 g 2,4-D per kg water was used to spray a 3.3 X 3.3-m area around the seedling. One application in the spring of the second season kept the soil free of vegetation throughout the season. This t r e a t m e n t eliminated vegetation competition while leaving the soil surface and any dead vegetation intact. Standard techniques in herbicide application usually involve broadcast spraying an entire site. A larger surface area was treated by the herbicide spray t h a n by the scalp to simulate this practice. Seedling growth measurements In-situ height and diameter measurements were taken on all seedlings. Other measurements taken to examine t r e a t m e n t responses in more detail were: (1) Initial, first year, second year, and final height; (2) Initial and first year total (outside) diameter, measured with calipers at the root collar. At the end of the experiment, we cut all seedlings at the root collar and measured total and inside bark diameters and first and second-year radial growth; (3) Initial and final root and shoot volumes (final root volumes done on a 45-tree subset) were determined using water displacement; (4) First and second year needle lengths and final bud size. Yearly occurrence of browse was noted. In addition, periodic observations of survival, budburst and budset of seedlings were made. These observations include identification of a second above-ground growth period for some seedlings. This phenomenon, called double flushing, is a response to good environmental conditions for growth. Double flushing was identified as a second occurrence of budburst after the first budburst of the season culminated in budset.
Soil water loss Soil water and temperature measurements were made approximately every 2 weeks from early May through September. Soil water loss was determined using a two-probe gamma ray attenuation device ( Model 2376, Troxler Instrum e n t Co., Research Triangle Park, NC ), which was calibrated daily before use.
194
Water loss in the seedling root zone was measured across a 300-mm path length in 25-mm depth increments to either 760 mm or bedrock. This soil volume was considered to be the microsite from which seedlings extracted water. Measurements were made during each site visit beneath 60 seedlings distributed across five treatments (shadecard, mulch, scalp, pyramid, and herbicide), plus control and no-tree locations. N e t water extracted from a given microsite over the season was calculated from measured data. Rainfall values were added to net water use over the season to determine total water use; only rainfall that did not wet the soil above field capacity was used in these calculations. Available soil water storage (average of 26 mm3/mm 2 over the entire soil profile for all seedlings in 1983) is the difference between the soil water content as field capacity (41 mm) and the water left in the soil at the driest part of the year (15 mm for 1983). Precipitation exceeding field capacity occurred three times during the season, in early May, early June, and early September, totaling 28 mm.
Soil temperature Soil surface temperatures were measured at hourly intervals for 2-3 days every visit with an infra-red thermometer (Raynger II, Raytek, Inc., Santa Cruz, CA ). Soil temperature profiles were measured with thermistor probes at 20, 40, 80, 160, and 320 mm below soil surface beneath the seedlings. Temperatures were monitored every 15 min on data loggers (CR5 Digital Recorder, Campbell Scientific, Inc., Logan, U T ) for 2-3 days every site visit. Measurements were made beneath 18 seedlings on control, herbicide, mulch, shadecard, and pyramid treatments.
Statistical analyses Chi-square analyses were done on data from periodic observations of budburst, budset, and double flushing. All growth data were subjected to analysis of variance. Least significant differences were determined using total analysis of variance to compare all treatments. The same methods were used for analyses of soil water loss. Evaluations of growth data were done at a significance level of P - - 0.20, as we feel this is acceptable for management decisions based on measurements taken in a highly variable environment. Soil water measurements were made using a precision instrument and analyzed at a significance level of P--0.05. A correlation matrix was calculated for all growth measurements on the total population. Standard deviations are presented with temperature data because sample sizes were too small for statistical analysis.
195 o
~Total
]
diameter
CO
bc
I
+ ,ollq
,
<
+
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9
bc
Herbicide
Scalp
Mulch Control Shade card
c
70
x
Pyramid
Fig. 1. Final seedling total diameters and shoot volumes for six treatment groups. Mean values between bars with same letters do not differ significantly at the level of P = 0.20. RESULTS AND DISCUSSION
Growth Weather was mild during the 1982 and 1983 growing seasons with no extreme temperature events. Daytime temperatures in 1983 ranged from 6.4°C to 35.7°C, and 59 mm of precipitation fell between 1 M a y and 15 September. There was 98% seedling survival during the 2-year study and growth trends were similar in both years. Although the first-year growth data did not show statistical differences among treatments, significant differences were measured in the second year. This is probably due to a combination of effects of nursery conditions and transplanting stresses on first-year outplanted seedlings. The lack of much vegetative competition until the second season is likely to have reduced the impact of different treatments. The small differences in height growth among treatments may have been due to deer browse which occurred on 49% of all seedlings in 1982. The severity of browse damage and its even distribution among all treatments probably contributed to the smaller differences in seedling response among treatments that first growing season. Browse was successfully controlled with a repellent spray in 1983. Therefore, for clarity of presentation, we discuss only second-year results. The six t r e a t m e n t groups shown in Fig. 1 are logical groups based on our hypotheses (Table 1 ). Variation within groups was small while differences between groups were large as determined by least significant difference analyses. The styrofoam cup treatment, designed solely for protection from high soil surface temperatures, resulted in growth trends which were not statistically different from those of the controls and was included with the control
196
TABLE 2 Correlations { r) of growth measurement data Total diameter vs. inside diameter Initial S:R vs. final S:R Initial root volume vs. final root volume Final shoot volume vs. inside diameter Final shoot volume vs. total diameter Second year height growth vs. total height
0.958 0.910 0.898 0.843 0.820 0.809
All possible correlations were performed on the entire data set. Only correlations above r = 0.8 are listed
treatment for analysis. The pyramid treatment, originally conceived to provide combined protection against high temperatures and soil surface evaporation, did produce different growth results from the controls, and was thus maintained as a separate group. Total diameters and shoot volumes were the most sensitive indicators of differences among treatments. The seedlings with surrounding vegetation controlled (herbicide and scalp) grew the most and the shaded seedlings ( shadecard and pyramid) grew the least. Calculations of percentage increases in total diameter and top volume showed the same significant differences among treatments as did the direct measurements.
Additional growth data Additional growth measurements were taken to assess the possibility of relating various nonstandard growth measurements to the treatments and to each other. The ability to predict growth and determine the adequacy of standard field measurements were major considerations in the choice of additional measurements. Correlations between all pairs of growth measurements were performed and those with r values above 0.80 are shown in Table 2. Correlation of inside bark diameters and total diameters was high indicating that diameter measurements made after destructive sampling provide no more information than standard field caliper measurements. Also, final diameters were well correlated with shoot volume. Shoot:root ratios before planting were highly correlated to final shoot:root ratios, and treatments had little effect on the final ratios. This suggests that shoot:root ratios during the first few years after outplanting are influenced strongly by the dimensions of the nursery stock used. Since smaller shoot:root ratios have been shown to be beneficial on droughty sites (Hermann, 1964b), the data presented here corroborate the importance of selecting appropriately sized seedlings for outplanting. Relative root growth was similar for all seedlings, regardless of treatment. As larger root biomass increases both the absorptive capacity and the volume
197
~%budburst I00
I----I%budset
by 5 / 4 / 8 3 by 7 / 1 3 / 8 3
p=O.05 p=O. 10
8oi I-Z
60 r~ W O-
40
20
% actively
growing seedlings
Herbicide
Scalp
69.0
61.2
Mulch Control
Shade cord
Pyramid
58.8
45.0
43.6
51,9
Fig. 2. Percentage of seedlings undergoing budburst by 4 May 1983, and budset or double flushing by 13 July 1983. Chi-square analyses were done and indicate differences among treatments at corresponding p values of 0.05 for budburst, 0.10 for budset, and 0.20 for double flushing. Also shown is that percentage of seedlings in each treatment actively growing between the representative dates.
of soil utilized for water and nutrient collection, the importance of planting seedlings with initially large root systems is apparent.
Timing of seedling growth The growth patterns for seedlings in treatments that resulted in more or less growth than the controls may be partially explained by the length of time seedlings in different treatments were actively growing. This was determined by the percentage of seedlings in each treatment that had undergone budburst (started active growth for the season) by 4 May, and the percentage that had set their buds (stopped active growth for the season) by 14 July (Fig. 2). These seedlings were in a growth stage rather than in dormancy induction or dormancy. These dates were chosen to show a representative percentage of seedlings in different treatments to have undergone budburst, as about 80% of all seedlings had achieved budburst by 4 May, and about 50% had set buds by 14 July. The treatments are displayed in the same order as in Fig. 1, from those resulting in the most growth to those with the least. Chi-square analysis showed a significant, although perhaps inconsequential, trend in percentage budbust. Soil temperature, which has been found to influence the date of early season growth initiation ( Sorensen and Campbell, 1978), differed little among treatments, so treatments had little effect on burdburst. Soil temperatures (dis-
198
cussed in the next section ) for bare soil plots, which simulate a scalp treatment, were higher throughout the season. This observation, although inconclusive, helps explain the trend of earlier budburst for the scalp treatment. The same phenomenon was also noted in a shadecard, clearcut, shelterwood comparison (Childs and Flint, 1987), in which treatment did not alter budburst but, as in our study, did have a significant effect on the timing of budset. Our data indicate that treatments under which seedlings which showed most growth had lower percentages of trees with buds set (more seedlings actively growing) by mid-July. The shadecard treatment contained more seedlings that had set buds and stopped active growth by this time. The data for double flushing (Fig. 2 ) also support this conclusion. There is a trend in these data showing that seedlings which set buds later in the season also produced a second flush of growth. This trend is shown most clearly by seedlings with the herbicide, shadecard, and pyramid treatments. Doubleflushing is generally related to availability of water, whereas budset is primarily in response to moderate moisture stress ( Greaves et al., 1978). In our study, seedlings with more available water left in the soil had the opportunity to undergo additional growth. The result of double flushing is an increase in height and shoot biomass which supplies more leaf area for photosynthesis, therefore additional diameter growth. There may be disadvantages to double flushing in harsh years when high soil and air temperatures occur in late summer. Seedlings undergoing a second flush in the nursery show less survival on droughty sites (Lavender, 1984). Second-flush tissue often does not develop frost hardiness early enough to escape damage during fall frosts. In addition, winter resting buds need time to harden so that growth will not resume with fall rains and they will be subjected to chilling, which increases vigor (Lavender and Cleary, 1974). In our study, the shaded treatments produced seedlings with less growth, but had more seedlings undergoing dormancy induction in midJuly. Data on timing of budburst and budset were used to calculate the percentage of seedlings in each treatment that had achieved budburst but had not undergone budset during the main part of the growing season. Ranking the percentages of seedlings in each treatment that were actively growing between early May and mid-July shows the treatments to be again in the same order of most to least growth. This supports the analysis of Fig. 2, and the assertion that seedling growth was influenced by growing-season length - - the time between budburst and budset that was influenced by treatment.
Soil temperature Treatments giving the best seedling growth also had the highest surface soil temperatures throughout the season (Fig. 3 ). It is noteworthy that in the herbicide and bare-soil treatments the soil surface reached nearly lethal temper-
199
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Fig. 3. Soil surface high temperatures for five treatments and bare soil for each sampling date for the 1983 season.
atures. In a hotter growing season, surface temperatures would be greater and shading treatments would provide protection for seedlings from the heat stress. It is also of note that pyramid-treated seedlings had lowest soil surface temperatures for most of the season. This is in accord with the measured smaller growth of those seedlings. Soil temperature profiles to 320 mm were not significantly different among treatments. Microsite w a t e r use a n d allocation
Seasonal soil water loss data (Fig. 4) show that treatments with no vegetation or evaporation control resulted in the most microsite water loss over the season, the remaining treatments all using less water. The treatments that controlled similar amounts of vegetation resulted in different water use patterns due to either surface shading or evaporation control. In the case of the no-tree treatment, however, the difference is due to no seedling water use. A notable difference in water use due to evaporation is seen between the total vegetation control treatments, scalp and herbicide. The low water use in the herbicide t r e a t m e n t illustrates the effectiveness of an undisturbed soil surface layer in controlling evaporative loss of soil water. Rate of water loss was greatest from early J u n e to mid-July. During this period, the majority of aboveground growth occurred and t r e a t m e n t differences in soil water loss became pronounced. Total soil profile water loss trends do not, however, match the trends in seedling growth (Fig. 5). Differences are due to the partitioning of available microsite water among seedlings, competing vegetation and surface evaporation. As an example, the scalp t r e a t m e n t resulted in large diameter growth but also resulted in more water use than the other treatments. This is likely due to increased surface evaporation caused by higher surface tempera-
200
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Fig. 4. Seasonal cumulative water loss from the soil profile for seven treatments. Vertical bars are
least significant differences determined on cumulative water used by each date at a significance level of P = 0.05.
tures (indicated by bare soil surface temperatures in Fig. 3) and a disturbed soil surface. The herbicide treatment, by losing less water to evaporation and competing vegetation, resulted in more soil water available for increased plant growth. The effect of treatment on the partitioning of water to seedlings can be demonstrated as water use efficiency ( Table 3 ), which is shown in this case as total shoot volume ( m m 3), divided by total microsite water used ( m m ) . The herbicide treatment elicited the most efficient use of site-available water for seedling growth. The scalp, mulch and pyramid treatments were less efficient and
ITota "~
12
I diameter
I'~'l%Total water loss 6 / 6 - 7/13
60
o3 O3 O
re
LU
50
~-
.J
40
0 F-LI0
t" Herbicide Scalp
Mulch Control Shade Pyramid No card tree
Fig. 5. Final seedling total diameter vs. percentage of the total seasonal water lost by seven treatments between 6 June 1983 and 13 July 1983.
201 TABLE 3 Water use efficiencies (WUE) for six treatments, calculated as shoot volume (mm 3) divided by total seasonal microsite water lost (mm ) Treatment
WUE
Herbicide Scalp Mulch Pyramid Control Shadecard
140 126 115 103 99 94
the control and shadecard treatments had the poorest microsite allocation of available soil water. The percentage of vegetative cover occurring in each treatment was estimated periodically throughout the growing season. A ranking of cover corresponds to the water use efficiency ranking. The herbicide spray and scalp treatments had no vegetative cover, mulch and pyramid treatments partially controlled vegetation, and the control and shadecard treatments had no effect on competing vegetation. SUMMARY
This study was designed to make a comparison of common site-modification techniques used in reforestation of harsh sites. Testing of 12 treatments on one site for 2 years allowed unambiguous t r e a t m e n t comparisons. In addition, the nature of the effects ( growth timing, soil temperature, water supply, etc. ) can be assessed because there are no growing-season or site differences to confound the experiment. This study shows that during a mild growing season, Douglas-fir seedlings under different treatments had different water loss patterns and seasonal growth. These differences were due to various interactive effects of temperature, timing of growth, and increased supply of water available to seedlings. Increased water supply was due to t r e a t m e n t effects on either soil surface evaporation or water use by competing vegetation. Competing vegetation seemed to be most instrumental in influencing water availability for seedlings as the degree of t r e a t m e n t control of vegetation correlated well with seedling growth, seasonal water loss, and water use efficiency. The importance of surface evaporation as a mechanism of water loss was clearly shown by herbicide-treated sites which lost significantly less water, yet had a higher water-use efficiency than scalp t r e a t m e n t sites. T r e a t m e n t s that increased the efficient use of microsite water, whether by vegetation or evaporation control, allowed more water to be allocated to increasing seedling growth. T r e a t m e n t s that affected
202
the timing of budburst and budset had a large influence on seedling growth. Temperature seemed to influence budburst and earlier seedling growth. Increased water availability and decreased moisture stress later in the season delayed budset, increased double flushing, and therefore increased growth. Standard field techniques for measurement of seedling growth were found to be as reliable as more intensive measurements requiring destructive sampling. While the necessary first step in understanding the environmental dynamics of a site and reforestation treatments is on a microsite level, these approaches should also be considered from a management perspective. An important consideration in selecting treatments for regenerating harsh sites is the risk of infrequent severe drought and heat stress seasons which may greatly reduce the survival of the tree crop. A study in southwest Oregon during the very harsh 1981 season showed that shadecards increased seedling survival by 50% (Flint and Childs, 1984 ). This is supported by our study of a mild year in which earlier budset and lower soil temperatures occurred for shaded treatments. These were also accompanied by less seedling growth and high soil water loss resulting in lower water-use efficiencies. Shading alone, as a safeguard for survival in the event of a harsh year, is at the expense of enhanced growth and efficient water use in good years. Survival is, of course, of ultimate concern, but treatments that increase microsite water use efficiency may also help to increase seedling survival beyond that of standard shading techniques by allowing enhanced seedling growth.
REFERENCES Childs, S.W. and Flint, L.E., 1987. Effect of shadecards, shelterwoods and clearcuts on temperature and moisture environments. For. Ecol. Manage., 18: 205-217. Childs, S.W., Holbo, H.R. and Miller, E.L., 1985. Shadecard and shelterwood modification of the soil temperature environment. Soil Sci. Soc. Am. J., 49: 1018-1023. De Vries, D.A., 1975. Heat transfer in soils. In: D.A. De Vries and N.H. Afgan (Editors), Heat and Mass Transfer in the Biosphere. Scripta Book Co., Washington, D.C., pp. 5-28. Duryea, M.L. and Landis, T.D., 1984. Development of the forest nursery manual: a synthesis of current practices and research. In: M.L. Duryea and T.D. Landis (Editors), Forest Nursery Manual: Production of Bareroot Seedlings. Nijhoff/Junk, The Hague, pp. 3-8. Flint, L.E. and Childs, S.W., 1984. Seedling responses to heat and moisture environments in clearcuts and shelterwoods. For. Intensif. Res. Rep., 6 (2) : 3-4. Greaves, R.G., Hermann, R.K. and Cleary, B.D., 1978. Ecological principles. In: B.D Cleary, R. D. Greaves and R. K. Hermann ( Editors ), Regenerating Oregon's Forests. Oregon State Univ. Extension Service, Corvallis, OR, pp. 1-26. Hallin, W.E., 1968. Soil surface temperatures on cutovers in southwest Oregon. USDA For.Serv. Res. Note PNW-78. PNW Forest and Range Experiment Station, Portland, OR, 17 pp. Hermann, R.K., 1964a. Importance of top-root ratios for survival of Douglas-fir seedlings. Tree Plant. Notes, 64: 7-11. Hermann, R.K., 1964b. Paper mulch for reforestation in southwest Oregon. J. For., 62: 98-101. Hermann, R.K., 1965. Survival of planted Ponderosa pine in southern Oregon. Forest Research Laboratory, School of Forestry, Oregon State Univ., Corvallis, OR, Res. Pap. 2, 31 pp.
203 Hobbs, S.D., Gordon, J.G. and Brown, G.W., 1983. Research and technology in southwest Oregon. J. For., 81: 8. Lavender, D.P., 1984. Plant physiology and nursery environment. In: M.L. Duryea and T.D. Landis (Editors), Forest Nursery Manual: Production of Bareroot Seedlings. Nijhoff/Junk, The Hague, pp. 133-142. Lavender, D.P. and Cleary, B.D., 1974. Coniferous seedling production techniques to improve seedling establishment. In: R.W. Tinus, W.I. Stein and W.E. Balmer (Editors), Proc. North Am. Containerized For. Tree Seed. Symp. Great Plains Agricultural Council Pub. No. 68, pp. 177-180. Newton, M., 1964. The influence of herbaceous vegetation on coniferous seedling habitat in old field plantations. Ph.D. thesis. Oregon State Univ., Corvallis, OR, 114 pp. Passof, P., 1978. Report in Forestation Notes. USDA Forest Service, Pacific Northwest Region State and Private Forestry, No. 54-1-2. Sands, R. and Nambiar, E.K.S., 1983. Water relations ofPinus radiata in competition with weeds. Can. J. For. Res., 14: 233-237. Seidel, K.W., 1986. Tolerance of seedlings of Ponderosa pine, Douglas-fir, Grand fir and Engleman spruce for high temperatures. Northwest Sci., 60: 1-7. Silen, R.R., 1960. Lethal surface temperatures and their interpretation for Douglas-fir. Ph.D. thesis, Oregon State Univ., Corvallis, OR, 170 pp. Smith, E.M., Skaggs, R.W. and Casada, J.H., 1968. Potential of heat transfer in soil covered with plastic mulches. Trans. Am. Soc. Agric. Eng., 67: 546-548. Sorensen, F.C. and Campbell, R.K., 1978. Comparative roles of soil and air temperatures in the timing of spring bud flush in seedling Douglas-fir. Can. J. Bot., 56: 2307-2308. Takatori, F.H., Lippert, L.F. and Whiting, F.L., 1964. The effect of petroleum mulch and polyethylene films on soil temperature and plant growth. Proc. Am. Soc. Hortic. Sci., 85: 532-548. Waggoner, P.E., Miller, P.M. and DeRoo, H.C., 1960. Plastic mulching-- principles and benefits. Connecticut Agric. Exp. St., New Haven, Bull. No. 634, 44 pp. Youngberg, C.T., 1959. The influence of soil moisture on the survival of planted Douglas-fir seedlings on clay soils. J. For., 57: 842-844.