The Interaction Between Climate Change, Soil Moisture Stress and Stand Density in a Red Pine Plantation

By Dr. Chris S. Papadopol
June 2001

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:

Abstract

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.

Contents

1. Introduction

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.

2. Materials and methods

3. Results

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.

4. Discussion

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.

5. Conclusions

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.

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Acknowledgments

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.

References

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