Forest trees play a major role in influencing the flow of water resources. Trees, like most plants, undergo a process called evapotranspiration. This is where water taken up from the roots moves through the plant to be utilized for photosynthesis in the leaves. The water is then released from the leaves into the air as water vapor. In forests with a large amount of trees, the effects of this process are readily seen in the changes in streamflow and soil water. For example, harvesting or cutting down forests substantially increases streamflow because fewer trees are able to draw up and cycle the water back into the atmosphere.
Forests with different types of trees vary in their capacity for water interception and transpiration. Deciduous trees are species that shed their leaves when conditions are unfavorable (such as too cold, not enough water, etc.). Since the evapotranspiration process occurs through the trees’ leaves, deciduous trees transpire much less during their dormant seasons. This means that deciduous trees such as oak, maple, and hickory allow more rain to seep into the soil (rather than into roots and through the leaves into the air) and eventually flow downstream. In contrast, evergreen trees (like pines) retain their leaves year-round, and have been shown to have higher annual evapotranspiration and therefore reduce streamflows.
Deciduous trees play an important role in ecosystem maintenance by seasonally generating higher water yields. This increase in soil water and streamflow provides a valuable resource to the community. The higher flow can help fill storage reservoirs and mitigate water shortages during drought seasons. Thus, in municipal watersheds and other areas where water resources are of concern, it is important to consider the effects of forest conversion. If forests are converted from deciduous trees to evergreen trees, then local communities will surely see some form of water yield reduction.
This summary was contributed by Leon Wang.
“Will any forest tree work the same on a given site? Are trees completely interchangeable? The questions hinged on whether trees vary in their capacity for water interception and transpiration. To get answers, loggers clear-cut mature hardwood forests of native oak and hickory in two watersheds in the Southern Appalachian Mountains of North Carolina, then replanted them with white pine seedlings. By 1973. when the pines had filled out their canopies, those two watersheds were yielding 20 percent less streamflow than equivalent catchments still forested by hardwoods…transpiration and evaporative losses from pines are greater because this species retains most of its needles year-round. In contrast, the dormant hardwoods stand leafless through the fall and winter, and their bare trunks and branches allow more rain to reach the soil and seep to the streams…the conversion of just sixteen hectares of forest from oak-hickory to pines cost 23 million liters in lost water in a single year.” (Baskin 1997:85)
“These experiments provide the first conclusive evidence of how streamflow changes when hardwood stands are converted to white pine. The implications are far-reaching. Streamflow levels begin dropping six years after conversion, and, after only ten years of growth, annual evapotranspiration losses were greater from white pine than from the hardwoods it replaced. A greater, perhaps much greater, reduction in streamflow is expected as the plantations mature. We suggest that the reduced flow results primarily from increased interception loss by the pine. Summaries of forest interception studies indicate that higher interception losses could be expected from white pine during dormant season, since the intercepting surface in a hardwood stand is drastically reduced following leaf fall.” (Swank, Miner 1968: 950-951)
“The results on these watersheds have important implications for the management of water resources. It is clear that the quantity of streamflow can be substantially altered by changing the type of forest vegetation. On municipal watersheds in the east, white pine has long been a favorite species for planting but this practice will reduce water supplies. On watershed 1, water available for downstream use in 1972 was reduced by 23.7 x 106 liters by converting just 16 hectares from deciduous hardwood to white pine. Identical water yield reductions would not be expected everywhere because of differences in climate and vegetation. But a summary of interception by conifers in North America (9) indicates greater interception loss for pine species and other conifers than for deciduous forests. Thus, since evaporative processes involved are universal, a trend toward streamflow reductions when deciduous hardwood stands are converted to pine might be expected in other regions.” (Swank, Douglass 1974:858-859)