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How stand age affects transpiration in trees of the Pacific Northwest

December 7, 2014
a large Douglas fir tree, also known as an Oregon pine or Douglas spruce

The Douglas fir, scientifically named Pseudotsuga menziesii, is an evergreen conifer species native to western North America. Moore et al. (2003) studied these trees in relation to transpiration rates and stand age. Photo credit: morguefile.com

Introduction

Plants have an uncanny ability to pull on their environment and draw in the needed resources for their survival.  One of the most important mechanisms they utilize to achieve this is transpiration, which is the loss of water through the stomata (pores) of a plant into the surrounding atmosphere.  Transpiration, which could be considered the process of a plant sweating, is the phenomenon that functions as the driving force for the pull of water and dissolved minerals upward and into plant tissues.  Due to cohesion (molecules of the same type “sticking” to each other), tension builds on a leaf’s inner water molecules as the outmost water molecules continue to evaporate into the air.  This results in an overall pulling, or negative pressure gradient, and can be deemed a cascade effect, as water molecules in the plant’s tissues are drawn together in an upward tow.  This incredible occurrence is what allows a plant to distribute water and minerals throughout its entire body without expending energy; it is truly a solar-powered process.

Transpiration rates vary considerably, depending on the particular plant, time of day, atmospheric temperature, weather conditions, humidity, sunlight and/or wind intensity, soil type, water availability, etc.  A large oak tree is capable of transpiring over 100 gallons of water a day, while an acre of corn can transpire anywhere from 3,000 to 4,000 gallons a day (United States Geological Survey 2014).  Besides these factors, however, researchers are discovering that there are other more subtle factors that can significantly affect transpiration rates.  These are considered “drivers” of transpiration.  Understanding these drivers is crucial as it can greatly influence how we as humans implement forest management policies and handle our water budget, which, in this rapidly growing world, is of ever-increasing significance.  One driver in particular, stand age (stand, meaning a group of plants growing together) is striking in its implications.  In the ensuing discussion, I will illuminate recent findings in regards to the relationship between stand age and transpiration rates of trees in the Pacific Northwest region, especially Douglas firs, and the resulting affects upon stream flow.

Young vs. Old

Recently, due to various forest management endeavors, large parts of forests in the Pacific Northwest, and even around the world have becoming increasingly younger (Moore et al. 2003).  Obviously, harvesting trees is crucial for us in replenishing our supply of timber for building, paper products, and cleaning products, among the many other thousands of products that tree components are useful for.  However, it may be that this practice is more hurting than helping our environment in the long run.  In relation to transpiration, it has been suggested in previous research that old trees utilize less water per unit leaf area than young trees of the same species do (Moore et al. 2003).  As forests are being cut down and new trees planted, it could be that these new, young, and fast-growing trees are, in essence, sucking up more water than loggers and forest management implementers previously bargained for.  The water yield after logging an area may at first be greater, according to Oregon State University researcher Georgianne Moore, but the area may then experience periods of low yield as the new forest grows (2003).  This could have great meaning for establishing new and potentially more appropriate forest management approaches. 

forest stand in autumn

Young and old forest stands have very different dynamics in regards to transpiration. Photo credit: morguefile.comworld

Moore et al. (2003) further investigated this supposition regarding stand age and transpiration by studying various populations and harvest plots in the H.J. Andrews Forest of the western Cascade Range of Oregon.  In one of their focuses, they compared a 40-year-old Douglas fir population with a 450-year-old Douglas fir population.  Leaf area index (defined as the one-sided green leaf area per unit of horizontal ground area and often used to predict the amount of organic compounds the particular species is synthesizing), was quite similar in both of these forests, but the 40-year-old forest was measured to use about 3.3 times more water between late June and October of 2000 than the 450-year-old forest.  Moore et al. (2003) explained that this occurs because, as trees get older, they cannot transport water as efficiently as they used to.  As they age, the “sapwood” component of a tree (referring to its outermost “living” wood through which sap flows, as opposed to the inner, “dead” heartwood) becomes less and less as the “heartwood” becomes more proportionally dominant.  This can be seen in their findings that while there was twice as much total basal area (a function basically measuring forest stand density) in the old stand as in the young stand, the sapwood basal area in the young stand showed to be 21% greater (Moore et al. 2003).  More active sapwood indicates increased water flow.  They also suggest that the decrease in water use in the old Douglas firs could be partially due to size and height factors; the pressure gradient due to gravity could become greater as the trees age and assume their expansive heights, causing water flow to experience more resistance. 

In another study conducted by Rachhpal Jassal et al. (2009), involving Pacific Northwest Douglas firs as well, an intermediate-aged stand (58 years old) exhibited less variability in its evapotranspiration (evaporation and transpiration taken together) rates and low sensitivity to summertime soil water deficits in comparison with the younger (7- and 19-year-old) stands.  From these findings, it appears that older trees may show more stability transpiration-wise (and, consequently, water flow-wise) than their young and hastily growing counterparts.

cascading stream in a forest

Transpiration rates of surrounding vegetation, particularly in summertime, can have a significant affect on stream flow. Photo credit: morguefile.com

Affect on stream flow

Water flow in these areas has been indicated to be affected by these differing rates.  Moore examined stream flow patterns at differing intervals at the old Douglas fir stand (2003).  She found that there was a relationship between transpiration and watershed stream flow during the summer season.  On the hourly scale, the stream flow was negatively correlated with transpiration, suggesting that the more trees transpire, the less ample the stream flow.  However, it was positively correlated on the daily scale, because higher stream flow, seasonally, allowed the trees to utilize more water at that time (2003). Seasons other than summer were not so linked to transpiration rates, but were more dominated by factors of soil moisture and precipitation.  She completed this research as supplement to previous work done by Bond et al. (2002) on a younger Douglas fir stand, which had, in fact, indicated strong, daily correlation between transpiration and stream flow, with 4-8 hour lags during the summer period.  Moore indicates that her measurements may suggest a reduced transpiration-stream flow relationship in the old stand vs. the relationship in Bond et al.’s young stand (2003).  This would demonstrate that transpiration plays a greater role in stream flow when a young tree stand is involved than if an old stand is involved.  Further research must be done to more adequately understand the relationship between vegetation and stream flow. 

Implications and Conclusion

black & white photo of area recently deforested

Harvesting too early and too often could have detrimental affects on our water flow. Photo credit: morguefile.com

If young forest stands more harshly affect stream flow in the summertime than do old stands (Bond et al. 2002, Moore 2003), and if they exhibit less stability in regards to evapotranspiration and higher sensitivity to soil water deficits (Jassal et al. 2009), and if they use potentially over three times as much water as old stands use (Moore et al. 2003), then the level to which our forests are being harvested and then replanted must be reevaluated.  Since water has become such a valuable commodity, even in our wet Pacific Northwest climate, we must do whatever is in our power to conserve and properly budget that resource.  Perhaps we must let our forests grow longer before harvesting.  Perhaps we should bring a greater limitation on tree-harvesting in general.  More research must be done to further clarify these relationships, but, as it stands, the current forest management approach is not working.  Transpiration is an incredible mechanism, one which we should employ to our advantage in hopes of better comprehending how hydrology and ecology fit together, and then in putting that knowledge to action.         

References

1. Bond, BJ, Jones JA, More G, Philips N, Post DA, McDonel J (2002). The zone of vegetation influence on base flow revealed by diel patterns of streamfiow and vegetation water use in a headwater basin. Hydrological Processes.16:1671-167.

2. Jassal RS, Black A, Spittlehouse DL, Brummer C, Nesic Z (2009). Evapotranspiration and water use efficiency in different-aged Pacific Northwest Douglas-fir stands. Agricultural and Forest Meteorology 149:1168-1178. doi:10.1016/j.agrformet.2009.02.004

3. Moore GW, Bond BJ, Jones JA, Phillips N, Meinzer FC (2003). Structural and compositional controls on transpiration in the 40- and 450-year-old riparian forests in western Oregon, USA. Tree Phys 24:481-491.

4. Moore GW (2003) Drivers of variability in transpiration and implications for stream flow in forests of western Oregon. Dissertation, Oregon State University

5. United States Geological Survey (2014) Transpiration – the water cycle. http:// water.usgs.gov/edu/watercycletranspiration.html. Accessed 28 Oct 2014

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