The Author is Lead Scientist, Ecology of Intensive Plantations, with the Ontario Forest Research Institute in Sault Ste. Marie, Ontario, Canada. OFRI develops new scientific knowledge to support the sustainable management of forests, wildlife habitats and biodiversity. → See also:
Density control is an important silvicultural intervention in the life of a stand of trees. Its reduction results in multiple ecological effects, especially in terms of water extraction by tree roots and space for crown development. This paper analyzes soil moisture availability, as determined using the Time Domain Reflectometry, TDR, technique in a mature red pine thinning experiment.
Hourly soil water content was determined in each of four treatments HT (heavy thinning), MT (moderate thinning), LT (light thinning), and UC (unthinned control), in tree profiles in each treatment, up to 150 cm depth. The stand density at end of 1992 was 41.04, 44.35, 47.31 and 62.98 m² ha-1, respectively, in HT, MT, LT and UC plots. The automated soil moisture measurements were performed during the 1993 to 1995 growing seasons. Weather information was collected concomitantly in an adjacent open area for calculation of hourly Penman standard potential evapotranspiration, ETp.
Analysis of soil moisture revealed important differences between the treatments, with the highest water consumption occurring in UC and the lowest in HT. In comparison with a water consumptive use of about 670 mm in the UC, the stand density reduction interventions decreased water uptake up to 73%, 69% and 63% in LT, MT and HT, respectively.
The reduction of stand density, a periodic silvicultural intervention in managed stands, with the ensuing decrease of stand water consumption, is important in the context of the impending climate change, which is characterized by increased evaporative demand and climatic variability, potentially leading to accentuated hydric stress. This research provides an assessment of the extent to which extreme water stress can be avoided in existing plantations, thus increasing their ecological stability in the scenario of climate change.
Keywords: Evapotranspiration; Time domain reflectometry; Soil water balance; Drought; Pinus resinosa; Thinning.
Forests play an important role in hydrology at landscape and regional scales. At the same time, water availability strongly affects forest productivity, as is demonstrated by the narrow growth rings formed during dry years (Fritts 1976).
Few attempts have been made in forestry to study the soil moisture regime in relation to stand density. Notable exceptions are those of Hoover et al. (1953), McClurkin (1958) and Zahner and Whitmore (1960), that mentioned the effect of stand density control on the soil water balance of pine stands. Comparable results were obtained by Sanesi and Sulli (1973) with a young natural stand of fir and Aussenac et al. (1982) with a douglas fir plantation. Later, increases in soil moisture following thinning in various intensities were explained through the positive influence of increased throughfall precipitation and reduced evapotranspiration on the soil water regime (Cregg et al. 1990; Stogsdill et al. 1992).
The study of soil water regime of forest stands has become more important in recent years because there is growing evidence of an increased annual variability of precipitation and a higher frequency of climatic extremes as a result of global climate change (Karl et al. 1995; Francis and Hengeveld 1998). Even in areas not affected by climatic extremes, the distribution of rainfall during a growing season changes from one year to another, affecting both stand growth and the ecological stability of forest ecosystems (Overpeck et al. 1991). Apart of the direct physical effect of increased temperature, climate warming interferes with ecosystems through changes of soil moisture regime (Manabe and Wetherald 1986). Quantifying the effects of drought on forest transpiration and increment may help to assess the potential associated production losses. In this new context, stand-density control should be looked upon as one effective management tool for reducing the risks due to hydric stress, which, certainly, are going to be intensified by accrued average temperature and climatic variability.
Studies of soil water in stands of trees are necessary to determine growing conditions at the stand level, especially during the growing season and for the purposes of intensive silviculture, or when exotics are grown in a new environment. In particular, the soil water balance budget technique can provide useful information on actual evapotranspiration, ETa, soil water availability or hydric stress (Drissen 1986; Campbell and Diaz 1988; Brisson et al. 1992). The accuracy of soil water budgets derived from soil moisture measurements performed with the new Time Domain Reflectometry technique, TDR, depends on the frequency of actual soil moisture measurements (Mastrorilli et al. 1998). At any rate, it represents a considerable improvement over the classic, gravimetric method. Bertuzzi et al. (1994) and Leenhardt et al. (1994) stressed the importance of using a soil sampling procedure that takes into account spatial variability in estimating the terms of water balance and showed that the error in estimating soil moisture is inversely proportional to the number of soil samples. However, such methodological requirements could not be materialized as long as the soil moisture was determined, in the best case, through limited, weekly sampling, as in the case of classic, gravimetric method.
In recent years, the use of the TDR technique, pioneered by Topp et al. (1982), made possible the automation of soil moisture measurements (Wraith and Baker 1991). Today, TDR is considered a proven method to measure soil water content for a large number of research topics (Baker and Lascano 1989; Ledieu et al. 1986), and various hardware embodiments are commercially available. Perhaps the greatest advantage of this technique is its capability to provide instant soil moisture measurements, that allow for a continuous assessment of changes in soil water content. Other advantages are that probes do not disturb soil structure and/or root distribution (Wraith and Baker, 1991). As well, there is no risk involved to human health.
Red pine was selected for this research because it is widely planted in mid Canada, from southern Ontario to the Great Lakes-St. Lawrence-Boreal transition zone. It is also one of the species considered in Ontario for the establishment of intensive plantations. Based on red pine tolerance of episodic water stresses, stands of red pine are frequently established on sandy sites (Rudolf 1950; Horton and Bedell. 1960), which have limited water storage capacity and, temporarily, might experience severe water deficits.
Reported in this paper is the influence of thinning intensity on soil water regime of a mature, uniform red pine plantation, using soil water content variations measured by TDR. Finally, hourly ETp values, determined micro-meteorologically with Penman standard potential evapotranspiration model (Jensen 1983), are compared with soil moisture recorded using TDR, for two episodes - one dry and one wet.
The trial was situated in Kirkwood forest, on the northern shore of Lake Huron, close to Thessalon (46o 14' N. Lat., 83o 26' W. Long.), in Ontario, Canada. The soil at the site is a deep, uniform sand deposit of glacial origin, with groundwater not accessible to the forest vegetation. The soil moisture measurements were performed in a red pine plantation established in 1927. Starting with 1949, the plantation was thinned four times at intervals of 10 to 15 years. At every intervention, an attempt was made to maintain the initial thinning intensity, designated as: heavy thinning, HT, medium thinning, MT, light thinning, LT, and unthinned control, UC, which were replicated four times.
At end of 1992 an inventory of trees was performed in every treatment. Stand density, expressed as basal area per hectare was 41.04, 44.35, 47.31 and 62.98 m² ha-1, respectively, in HT, MT, LT and UC plots.
A very large clearing in the nearby of trial was instrumented with an automatic weather station, recording air temperature, at 200 and 20 cm, air relative humidity, at 200 and 20 cm, global solar radiation, wind speed at 360 cm and eight rain gauges. The sensors were connected to a 21X Campbell Scientific micrologger, programmed to take a scan every 10 seconds and log a record every 10 minutes. The diurnal pattern of air saturation deficit and Penman standard evapotranspiration were determined as reference values.
The TDR sensors were deployed in three profiles of seven probes, in one replication of each of the thinning treatments. The profiles were positioned in pits dug at 1 m from the center of an average tree. Sensor were positioned horizontally at 8, 16, 24, 35, 60, 90 and 120 cm in the wall of the pit, with almost no disturbance of the soil volume sampled. Sensors of the three profiles were connected through coaxial cables and a multiplexer to one micrologger. The distance between pits was about 3 m. The sensors, composed of three parallel rods of stainless steel of 0.3 m in a solid plastic mount, multiplexers and microloggers form the Trase-1 soil moisture system, manufactured by Soil Moisture Co., California, U.S.A.
The TDR soil moisture measurements are based on the shape of high frequency electromagnetic waves, as modified by the sensors. Deformation depends on soil properties, especially electrical conductivity, and is used to calculate the momentary dielectric constant of the soil, Ka. Using measurements of the propagation velocity of electromagnetic waves, the TDR technique determines Ka directly. Since water has a relatively high dielectric constant, about 80, compared with the dry soil with less than 5, the bulk soil Ka varies with the volume of water present in the soil (Topp and Davis 1985; Ledieu et al. 1986; Zegelin et al. 1989).
In operation, the rods act as wave-guides, for the signals which are reflected from the end of the transmission lines in the soil back to the TDR micrologger. The electromagnetic characteristic used for determining Ka, is the propagation velocity of the reflected signal, measured by a timing device in the micrologger, as time between sending and receiving the reflected signal. This time yields the volumetric water content, Θv, according to the formula (Topp et al. 1982):
Θv = -25.83 + 12.7 Ka - 1.077 Ka^2 + 0.034 Ka^3 (1)
On the screen of Trase-1 micrologger, it is possible to visualize the beginning and the end of each measurement or record its time. However, all this signal analysis is done automatically by specialized software provided with Trase-1. A soil moisture value is logged in about 13 seconds, after which the multiplexer commutes the micrologger to the next sensor. Such a short time per measurement indicates that this equipment can be used for studies in which the soil moisture is highly dynamic. For this monitoring, the sensors were scanned every 60 minutes, which resulted in daily files with 24 scans. No additional validation against gravimetrically determined soil moisture of this commercially released equipment was performed, relaying on validations performed by many authors, including Topp et al. (1996) and Mastrorilli et al. (1998).
Soil moisture was measured continuously, concomitant with the operation of the weather station. In the absence of information about the spatial variability of hydrological characteristics, the choice of three profiles per treatment was made arbitrarily. However, in view of recommendations by Topp et al. (1996), who found that replicate profiles of measurement points in soil have to be distanced at more than 1 m and less than 25 m, for a whole group of profiles measured by a single instrument, it appears to be adequate. The soil moisture measurements resulting from the three pits for each treatment were averaged by depth.
Soil moisture monitoring was conducted between days-of-year, DOY, 113 and 289, in 1993, DOY 128 and 298, in 1994, and DOY 101 and 285, in 1995. To illustrate the differences between treatments and for ease of comparison between years, in each year, the interval between DOY 152 and DOY 243, corresponding with the months of June, July and August, was selected for a more detailed analysis. Among years, the sum of precipitation for the interval between DOY 152 and DOY 243 amounted to 256.05 mm in 1993, 225.70 mm in 1994 and 135.65 mm in 1995.
For one dry and one wet episode, after smoothing with the procedure «kernel» (SPSS 1998), hourly examinations of soil moisture data was performed and are presented in Figure 8a / Figure 8b and Figure 9a / Figure 9b.
2.4.1. Atmospheric demand for evaporation
An independent estimate of ETp demand was obtained from data collected by the weather station through the Penman model (Jensen 1983):
| Δ | Y | ||||
| ETp = | --------- | (Rn + G) + | --------- | 15.36 Wf (ea - ed) | |
| Δ + Y | Δ + Y | (2) |
where ETp is the potential evapotranspiration of «standard grass» in mm day-1; Δ is the slope of the vapor pressure-temperature curve, in mbar oC-1; Y is the psychrometer constant, in mbar oC-1; Rn is the net radiation, in cal sq.cm-1 day-1; G is the soil heat flux to the surface, in cal sq.cm-1 day-1; Wf is the wind function, dimensionless; (ea - ed) is the mean daily vapour deficit, in mbar; and 15.36 is a constant of proportionality, in cal sq.cm-1 day-1 mbar-1. This general model is used extensively in agriculture. Here, it is used only as a reference value, with no assumption that it expresses the ETa of red pine stand. A Bowen ratio system, which would had provided direct information about stand ETa (Baldocchi et al. 1988), was not available.
This formula has been modified to accept 10 minute weather measurements and to result in 10 minute values of ETp.
2.4.2. Soil available water and soil saturation with available water
Ten samples per treatment were collected, with special steel cylinders with precise geometry, to assess soil dry bulk density and derive volumetric soil moisture contents. The soil dry bulk density averaged 1.264 g cm3-1. Also, four samples of sand were taken in each pit, from each depth of the TDR sensors to assess the wilting point, WP, which was determined with sunflower (Jensen 1983). Soil moisture values corresponding to WP served for calculation of soil water content at WP, in mm of water, for each strata represented by a sensor, between 0 and 150 cm soil. The sum of water content corresponding to WP was 80.29 mm, for 150 cm soil depth. Current soil moisture measurements were treated in a similar manner to yield the momentary soil water content. Subsequently, the water content was summed for: layer 1 (0 - 30 cm), layer 2 (31 - 150 cm) and total soil profile (0 - 150 cm).
The volume of water stored in soil up to 1.50 m was determined from the current soil moisture measurements, using the same methodology as Ratliff et al. (1983). Finally, available water was determined, by strata, as the difference between the amount of water in the profile at a certain point in time and the amount of water in the profile at WP.
2.4.3. Soil water balance
The soil water reserve (R) fluctuates over time owing to the effects of precipitation (P), evapotranspiration (ETp), and drainage (DR), as per equation 2.
DR/dt =P - ETp + DR (3)
For each day, measurements made in the stand provided P and the variation of soil water between two successive moisture determinations, DR/dt. In this experiment runoff and capillarity rise should be negligible due to the soil matrix (sand) and topographic characteristics (flat terrain).
Nevertheless, the drainage, DR, of the sandy soil is very active, especially during and immediately after a rain event. Since DR was not measured, the water balance could not be calculated according to equation 3. Instead, reported here are only the elements of the balance that were measured, i.e. soil water content at the end of each day and the rainfall received in the same interval.
Soil water content measurements in the four treatments differed considerably, with the HT having always the greatest moisture content and the UC the lowest; soil moisture in MT was always closer but inferior to HT, while it was always superior to LT. Summing up the influence of stand density on the actual evapotranspiration, it can be unequivocally asserted that the densest stand resulted in the highest soil water depletion. Except during rainfall events, the moisture readings evolved in a smooth manner.
The soil water regimes of the thinning treatments are illustrated in Figure 1, Figure 2 and Figure 3, given here only for the 24 cm depth. In general, the patterns of soil water content are similar as form, with HT consistently having the greatest amount of soil water.
Each rainfall event increased soil moisture content. The relative maxima and minima of soil water content are coincidental among treatments. As can be seen, the TDR-determined soil water budget reacts instantly to variations in soil moisture, either caused by rainfall or resulting from drainage, as opposed to classic, gravimetric determinations which are repeated, in the best case, at fixed weekly intervals, and not always in the same spot. Consequently, the TDR approach grasps the real soil moisture variation, which otherwise is obscured in gravimetrically-based research. Another advantage of this method is the possibility to measure numerous sensors with a sole instrument, thus allowing for several replicated profiles per treatment. Consistently, the highest values of soil moisture were attained during or immediately after rainfall events.
On the basis of this recorded patterns, it appears that stand density played a major role in the soil water balance of a treatments; the lower the stand density, the greater the soil moisture content. However, the daily values of ETa could not be determined in this manner on this very permeable soil, because the drainage term in equation 3 remains unknown. In order to get an idea of the evolution of atmospheric demand for evapotranspiration we present in Table 1 the values of ETp for the same interval, calculated with the Penman model. Due to the active drainage of the site, it can be inferred that stand ETa was considerably lower than Penman ETp. Drainage will likely be easier to be determined a posteriori, using a physical model, locally calibrated through measured saturated hydraulic conductivity (Belmans et al, 1983; De Jong and Hayhoe 1984).
| Year | Rainfall (mm) | ETp (mm) | ||||||
| 1993 | number of events | 34 | avg. | .373 ± .017 | ab. std. | .166 | ||
| total | 253.40 | max. event | 30.25 | total | 342.87 | daily max. | .708 | |
| 1994 | number of events | 50 | avg. | .379 ± .019 | ab. std. | .181 | ||
| total | 237.50 | max. event | 29.05 | total | 348.85 | daily max. | .865 | |
| 1995 | number of events | 37 | avg. | .353 ± .015 | ab. std. | .147 | ||
| total | 183.50 | max. event | 35.70 | total | 324.98 | daily max. | .710 | |
Soil moisture measurements allowed the calculation of available soil water at various points in time. Daily amounts of available water reflect the consumption of canopy, which was greatest in UC during all three months, in each of the three years (Figure 4, Figure 5 and Figure 6). It is to be noted that available water at end of dry episodes reached very low values, under 50 mm water per 150 cm soil depth.
During a dry episode, between DOY 208 and DOY 234 in 1993, the soil water content and the available water fell markedly in all treatments (Table 2). Compared with the amount of available soil water at the beginning of the dry episode, the reduction was more pronounced in the first layer, from 56.10 to 87.04% for HT and UC, respectively, likely, because of the superficial rooting of red pine. The reduction was milder in the second layer, 35.65 to 43.39% for HT and UC, respectively. Throughout the same dry episode, in the first layer, soil saturation with accessible water fell to an acceptable 33.45% in HT, and to a severe 6.83% in UC (Table 3).
| Stratum | Heavy Thinning (mm) | Moderate Thinning (mm) | Light Thinning (mm) | Unthinned Control (mm) | |
| Layer 0 - 30cm | begin | 31.14 | 25.44 | 21.36 | 14.04 |
| end | 13.64 | 9.78 | 6.80 | 1.82 | |
| Layer 31 - 150cm | begin | 135.50 | 116.00 | 102.10 | 77.10 |
| end | 87.20 | 72.50 | 62.20 | 43.65 | |
| Profile 0 - 150cm | begin | 164.64 | 141.44 | 123.46 | 91.14 |
| end | 100.84 | 82.28 | 69.00 | 45.47 | |
| Stratum | Heavy Thinning (%) | Moderate Thinning (%) | Light Thinning (%) | Unthinned Control (%) | |
| Layer 0 - 30cm | begin | 55.63 | 50.60 | 46.23 | 36.11 |
| end | 35.45 | 28.25 | 21.49 | 6.83 | |
| Layer 31 - 150cm | begin | 70.96 | 67.66 | 64.80 | 58.17 |
| end | 61.13 | 56.66 | 52.87 | 44.05 | |
| Profile 0 - 150cm | begin | 67.48 | 63.79 | 60.59 | 53.16 |
| end | 55.67 | 50.61 | 46.22 | 36.16 | |
When the soil water depletion during a dry episode is related to the stand density it is apparent that this relation is exponential, for both layers of the soil (Figure 7). It is obvious that, at the beginning of the episode, in both layers, the amount of water is highest in HT, with 41.04 m² ha-1 stand basal area, and diminishes gradually in MT, with 44.35 m² ha-1, and LT, with 47.31 m² ha-1. At the opposite end of researched densities, in UC, with a stand basal area of 62.98 m² ha-1, the situation of available water is precarious from the beginning of the episode.
At the end of episode, in the soil of each treatment, we found between 20 (HT) and 60 (UC) mm less water in the soil than at the beginning, in the first and second layer, respectively.
We interpret these findings as clear proofs that reduction of stand density through thinning can result in a significant relaxation of the demand for consumptive water use. At the same time, the relation between soil available water and stand density opens some predictive abilities. However, these will only be realised after a considerable amount of practical experience is gained.
Two episodes were selected in 1993 for a more detailed, hourly examination:
(i) DOY 196 to 199, after abundant rains, a succession of four days with clear sky, in which the daily ETp was extensive (Figure 8a), and (ii) DOY 205 to 208, after a long dry interval, two days with sizable precipitation resulted in reduced ETp (Figure 9a).
The change of soil moisture during the first episode, illustrated in Figure 8b for the depths of 8 cm and 60 cm, is typical. Soil moisture declines slowly, more visibly at 8 cm, where the daily peaks of ETp result in barely visible depressions. At 60 cm there is just constant content decline. The soil moisture content is always greater near the surface, probably explainable by the fact that in sandy soils under forest the presence of organic matter improves more the water holding capacity of the first layer (0 - 30 cm).
During the second episode, soil moisture starts with about 17% in both layers and, before rain, has a slow, normal decline. At the time of the first rainfall, only the soil moisture at 8 cm increases, while the moisture at 60 cm continues to decline due to infiltration. After about 20 hours, due to the second rain event, at the surface, soil moisture increases even more, reaching up to 28% of volume, for a short interval. Soil moisture at 60 cm follows a similar path, only flattened and slightly delayed.
The hourly analysis shows also that the TDR equipment can reflect adequately the moisture variations due to rain intensity changes, provided that scans of sensors are taken more often, e.g. every 10 minute. The peaks visible after rains in Figure 9b are only short time influences. Subsequently, water has time to reach a more even distribution along the profile, while continuing to percolate. This type of data, especially when multiple depths are involved, can be illustrated even more clearly if a smoothing procedure is applied, e. g. the smoothing procedure «kernel» (SPSS 1998). However, an undesirable effect of smoothing might also appear. A false impression, apparent in Figure 9b, could be conveyed that the soil moisture increase began even before the start of the rain event. Yet, smoothing remains a very useful procedure for a clear illustration of the change of soil moisture content in days with multiple rain events. Finally, it is evident that, in the last day of the episode, the drainage removes a part of water reserve through percolation to deeper soil layers.
This research has shown the influence of stand density on water consumption in red pine stands of varying densities. It is obvious that soil moisture is depleted much more actively where the stand is denser. Due to combined effects of drainage and ETa, at the end of a dry episode, the amount of available water in the first soil layer of UC, the densest treatment, fell the most, less than 2 mm (Table 2). Estimated water uptake was highest in the upper soil layer of UC.
Comparison of available soil water among treatments showed that water uptake by red pine was more accentuated from the top soil layer, compatible with its shallow rooting pattern. In other words, when during an intense rain event, the values of soil moisture increase beyond the root zone, that water is lost gravitationally from the soil water reservoir explored by roots and percolates to groundwater. That is why a water balance in which the drainage term is neglected, or not accurately assessed, cannot lead to correct information about the actual soil water regime. This situation is aggravated for shallow rooted species and soils with poor water-related properties. Conversely, in the case of deeply rooted species, the fraction of water that percolates to the groundwater is reduced, while the water can be retrieved for consumptive use from deeper strata.
Apart of the impact such situations have on stand mortality, other much more important ecological implications result from these findings. The fact that water supply is affected by stand density is critical for the existence and growth of trees that compose the stand. If we accept the fact that in the scenario of climate change the frequency and severity of hydric deficits will increase (Bolin et al. 1986; Houghton 1997), with the inherent summer soil moisture reduction (Manabe and Wetherald 1986), it is likely that the stand of trees will have to survive even more severe dry episodes. In time, this will exert pressure on existing ecosystems for a northward migration (Overpeck et al 1991). However, the immediate reaction of stands that are too dense might be massive dieback. It results that the only realistic possibility to improve the soil water balance of such stands is an urgent density reduction, thus preventing situations in which during long dry episodes the hydric stress depresses growth and, possibly, increases mortality. Given the increasing climatic variability, to avoid growth reduction and/or dieback, periodic thinnings remain the only tool on hand for reducing the stress level and adapting existing forest to cope with future stresses, while continuing to secure almost regular increments.
In the interval when the soil moisture was monitored, 1993 - 1995, the driest interval was between DOY 208 and DOY 234 of 1993, in which almost all available water was exhausted in UC, the densest stand. However, in the circumstances of newly increased climatic variability, we may expect to face longer dry intervals, clearly with increased ETp. Conceivably, in such a case and on such soils, the hydric stress could be much more severe. It is therefore reasonably to think that reducing the stand density will be, first and foremost, an insurance policy, preventing massive diebacks when a stressful episode appears. Even when stress may not be that dangerous, density reduction will at least secure that increment is less affected by a prolonged drought.
Because the influence of one intervention gradually diminishes in the subsequent few years, it is to be conceived that a policy of increasing the frequency of interventions will be beneficial in the future. As a consequence, reduction of stand density is likely to result in a certain relaxation of the demand for water, especially important in mono-cultures, where the competition for this primary resource occurs at the same level in soil.
In this context, it is imperative that more information is obtained about the soil water uptake rates of various species and stand structures. Especially important, for the success of future carbon sequestering intensive silviculture, is the understanding of relation between stand structure and physiological parameters. This can be achieved only through careful experimentation in characteristic stands, especially in uniform plantations. From this standpoint, an ecologically significant study area is offered by sandy soils, where high permeability accentuates the scarcity of water.
In the climate change context, already occurring, the study of soil water will most likely lead to a diversification of silvicultural solutions adopted for the vast expanses with permeable soils existent in Ontario. An immediate implication would be the extension of some deeply rooted species, indigenous or exotic (oaks and European larch being suggested), in new areas. It is to be noted that due to climate warming the forest formations are likely to suffer a pressure to move northward. Although this successional trend is already occurring, the rate at which an ecosystem can move naturally is much less than the recorded pace of warming, 0.76 oC per century in Ontario (Papadopol 2000). This will likely result in recommendations to extend towards north the deeply rooted species, while withdrawing from the southern edges of their areas. As climatic change is followed by a gradual expansion of the growing season, we may expect the regional differences to gain in importance. Thus, while the water balance of sites in northwestern Ontario will worsen, a contrasting situation might occur in northeastern Ontario. In the clay belt, the existing and future sufficient water supply, combined with a warmer and longer growing season, will probably result in an amelioration of growing conditions for forest vegetation.
Finally, studies of soil water balance will be greatly helped by the validation of a comprehensive, physically based soil water transport model for forested sites, using as input on-site energy balances (De Jong and Hayhoe 1984). Such a model would allow «explorations» of the water balance for species with various physiological traits, in areas where now only the standard climatic parameters are known.
This research has revealed the important differences induced by the thinning of a mature red pine stand on the soil water regime and water availability. In general, during the growing season, soil water content varied from field capacity down to values close to wilting point. For similar water supply conditions, the soil water content increased as the stand density decreased, being greatest in HT and lowest in UC. Also, during stress episodes, the lower limits are attained more frequently in UC.
The TDR technique provides accurate soil moisture data, within a wide range of variations of soil water content. Compared with the classic, gravimetric method, its operational use is greatly enhanced by automation. Also, because the soil profile is not disturbed, results are more reliable than those obtained gravimetrically. Results reported here show that TDR offers the possibility of studying the time distribution of water in the soil and is suitable for investigating the effects of stand density on available soil moisture.
The topic of forest stand density influence on soil water regime is new and not widely studied. In the context of already occurring climate change, it deserves thorough evaluation. Periodic stand density reduction remains the only realistic means through which the forest manager can reduce the influence of hydric stresses, that are sure to be experienced in the future with increased frequency. Through regular stand density control, risks due to climatic variability to existing stands can be decreased while higher quality timber can be brought in the economic circuit.
Acknowledgements are due to Al. Beckwith (retired from Ontario Ministry of Natural Resources) who originated the thinning research at this location as early as 1949.
Funding for this research was been provided by Ontario Ministry of Natural Resources under the Sustainable Forestry Initiative. The author is indebted to M. Gaetz and J. Smith, for field data collection and to Lisa Buse for manuscript review.
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