Reduction in phosphorus loading to streams is one goal of utilizing hybrid poplars in riparian buffers in agricultural regions such as the Palouse. This study documented the growth of 360 trees until they were six years old so that others may make reasonable expectations about the performance of hybrid poplars under similar dryland conditions. The phosphorus concentration of tree tissues was used along with biomass to quantify the amount of phosphorus sequestered within trees. The study presented a mathematical approach to designing riparian plantings in terms of the quantities of biomass produced and/or phosphorus removed when trees are harvested.
- The objectives of this proposal are to develop sustainable farming systems in the Palouse region by:
Promoting use of hybrid poplar riparian buffers to reduce water pollution.
Enhancing ecological functions by increasing total landscape diversity in agricultural ecosystems.
Developing fundamental information on poplar nutrient requirements, nutrient recycling and nutrient removal essential to proper design of riparian systems for pollution control.
Establishing a core cadre of Palouse farmers skilled in poplar cultivation and with linkages to corporate producers and markets.
The rich, fertile loess soils of eastern Washington and the northern Idaho Palouse Region have prompted the removal of much of the original vegetation to make way for farms and homes. The combination of farming and development on steep slopes that lack vegetation and have vulnerable soils has led to an erosion rate among the highest in the country and streams ranked by EPA as among the most polluted in Idaho and Washington.
To address these concerns, many see establishing riparian vegetation as important to restoring terrestrial and aquatic ecosystems in the Palouse.
Riparian zones function as “filters” or “buffers” between upland agricultural regions and waterways (Gregory et al. 1991, Welsch 1991). Runoff from agricultural fields may include sediment, nutrients such as phosphorus and nitrogen, pesticides or other contaminants.
Phosphorus is a necessary element required for crop growth, so it is routinely added to fields as fertilizer. However, in streams and lakes, even small amounts of phosphorus can cause the stimulation of unwanted aquatic weeds and algae (Wetzel 2001). Therefore, phosphorus is a component of agricultural runoff that is of particular concern for aquatic ecosystems.
It has been proposed that hybrid poplars be utilized for buffer applications in the Palouse region. The addition of streamside plantings of trees could help reduce sediments, nutrients, and runoff reaching streams and lakes, while also providing additional environmental factors, such as shading, supply of organic material to streams, and wildlife shelter in a nearly monocultural landscape. Hybrid poplars could give economic benefits to landowners by providing pulpwood harvesting. The potential for economic return could be an important factor in overcoming producers’ resistance to removing land from crop production to create riparian buffers.
More information about the growth of hybrid poplars under arid dryland conditions was needed. Much of the existing literature concerns trees growing under optimum conditions (mild climate and/or under irrigation) where tree growth often is extremely rapid. In order for landowners to make decisions about implementing hybrid poplar plantings under arid dryland conditions, more reasonable expectations of growth were needed. Performance data from irrigated sites would encourage unrealistic expectations for trees under arid, dryland cultivation.
The relative performance of various clones is critical to selection of appropriate hybrids. This study documented the performance of nine hybrid poplar clones over six years in a typical Palouse riparian setting. Clones selected were among the earliest ones thought to be suitable for dryland environments.
Phosphorus concentrations within various tree tissues were analyzed in order to calculate the amount of phosphorus sequestered within the trees. Removal of trees during harvest should benefit aquatic ecosystems by the physical removal of that amount of phosphorus, reducing loading to streams and lakes.
The study site was located 9 kilometers (5.6 miles) to the northeast of Pullman, Washington, U.S.A [46°45’57”N, 117°04’42”W], along the southern bank of Missouri Flat Creek, a 3rd order stream. The surrounding terrain is gently rolling hills on either side of the stream; nearby fields are cultivated with dryland cereal crops up to the banks of the stream. Soil at the site is Palouse Silt Loam, characterized by high fertility but potentially high erosion rates on steep slopes. Climate is that of the sub-Pacific type, typified by the predominance of precipitation in winter months, moist springs, with dry summers and autumns. Annual precipitation in the Palouse ranges from 15 to 21 inches (USDA-SCS 1980). At the site, the annual precipitation averages closer to the higher end of this range.
Plant Materials and Cultivation
In 1995, the Palouse Soil Conservation District, in conjunction with private landowners, established the test plot. Nine hybrid poplar clones were planted in rows of four-tree blocks, with ten replicates for each block to minimize edge effects. A total of 360 trees were planted but subsequent mortality reduced this number.
Listed below are the nine clones planted at the site. Names of clones are from the WSU-UW Clone Register (i.e. 49-177 or PC-6). Parentage is indicated by abbreviations: P. trichocarpa = T; P. deltoides = D; and P. nigra = N. Clones #1 through 9 are numbers assigned to the clones throughout the study for reporting convenience.
Clone 1 49-177 T X D
Clone 2 OP-367 D X N
Clone 3 15-029 T X D
Clone 4 50-194 T X D
Clone 5 58-280 T X D
Clone 6 50-197 T X D
Clone 7 52-225 T X D
Clone 8 Lombardy
Clone 9 PC-6 D X N
A few of the clones were inadequately labeled or planted incorrectly; their growth characteristics were included in results as “Miscellaneous clones”.
Growth and Biomass Accumulation
Tree heights, diameters, and survival were recorded at the end of each growing season until the trees were 6 years old. A telescoping rod was used to measure the heights. Diameters were determined using a D-tape (Forestry Supplies Model 347D). Heights and diameters were used to calculate biomass with an equation derived from selected harvested trees.
A subset of surviving trees was sampled for biomass determinations at the end of the fifth growing season (1999). Two trees of average size were randomly selected for harvest but were not tracked by clone identity. Each tree was partitioned into three-foot (0.914m) increments along the main stem. Leaves originating from each section were placed into large paper bags; branches were labeled and b bundled together. Green weights were recorded and all materials, including boles, were dried to constant mass in a large oven set at 60°C.
Estimates of total biomass were prepared using the following equation, based on additional hybrid poplar data (unpublished data, Jon Johnson, W.S.U. Puyallup Research Station):
B = 0.9D * 0.9D * H * 127.1 
where, B is aboveground biomass dry weight (kg), H is height (m), and D is basal diameter (m). Biomasses of all surviving trees at the end of the sixth year were calculated.
A number of analyses were conducted on the different plant tissues in order to quantify phosphorus found within the aboveground portions of the tree. All plant tissues were solubilized using a perchloric acid method (McCurdy 1990) in the laboratory and subsequently analyzed with a Win-Flow Analyzer (O-I-Analytical 1996) in order to determine the phosphorus concentration.
Boles, or main stems, were subdivided into bark, wood and the central xylem (“heart sap”). Tissues were digested and analyzed separately for phosphorus, with replications for each tissue type. In a similar fashion, branches and first- and second-year twigs were separated into bark, woody tissue and xylem. Composite samples were also made, where entire sections of branches or twigs were digested in their entirety. Phosphorus concentration for each tissue type was recorded from the base to the top of the tree.
A large number of leaves were analyzed from the harvested trees, as well as from the living trees at the site. Type of leaf and position on the tree were considered. A telescoping pruner was used to collect leaves. Comparisons between clones were made from similar leaves from a number of trees for each clone. Young- and old-aged leaves were assessed by analysis of green living leaves versus older leaves dropped during the fall months, as described below.
Autumn Leaf Data
Data were collected for one year (Fall 2000) to assess leaf biomass in litter remaining on the site over winter. This information is important for calculating nutrient budgets and to quantify phosphorus retention on site versus that removed when the trees are harvested.
Twelve screened, 0.3 m2 litter boxes were placed randomly in the plot to collect autumn leaves. From October to December, leaf material was collected daily to weekly depending upon weather conditions. Collections were made more frequently when windy to prevent loss of material from boxes. After collection, leaf material was oven dried at 60°C and weighed. Leaves were then analyzed for phosphorus, using procedures described above.
Overland Flow Collectors and Piezometers
Twelve overland flow collectors were placed at the site. Half were 0.25 square meter boxes; the remaining half consisted of 0.75 square meter triangles. These were designed to capture overland flow moving across the ground surface. Both designs consisted of wooden boxes carefully emplaced so as not to disturb the soil surface. Bottles for collection were located at the lowest end of the collector and the collection area was covered with a light mesh.
Three piezometers were located across the center of the site, ranging from the upland side of the strip, the middle, and the downhill side, closest to the stream. Samples of groundwater from piezometers were intended to indicate depths to the water table, changes over time, and the amount of phosphorus present in the groundwater.
In addition to the overland flow collectors and the piezometers, a number of small bottles were placed randomly at the site to collect samples of rain. All water samples were analyzed for phosphorus concentration as described above.
Growth and Biomass
Height and basal diameters were compiled for each of the nine clones from the 10 replications at the site. There were no statistically significant differences between the clones. The mean tree height was 10.5 m (34 ft.) with a basal diameter of 14.6 cm (5.7 inches) at the end of six years.
In order to compare basal diameters to DBH (diameter at breast height or 1.5 m), a small sub-sample of trees (30 trees of various clones) were measured. It was found that the trees had DBH values between 73 to 80% of the basal diameters.
From the harvested subsample of trees, it was found that boles or main stems comprised 62% of total biomass, branches formed 21%, and leaves formed 17% of total biomass. Dry weights were 44% of the total wet weight for the entire tree (46% for boles alone). Bark formed 7% of the bole volume, and wood 92.4%. Xylem averaged only 0.6% of the volume of each bole section. The relative percent of xylem increased proportionally up the tree.
Biomass was estimated using Equation 1, as described above. All biomasses were reported as aboveground dry weights for individual trees. Considering all poplars at the site, the average six-year old tree was 26.7 kg. The highest average biomass was found from Clone #2 with 33.2 kg. Clone #8 had the lowest biomass; it was only 22.2 kg. Tree to tree variation was high enough that the differences were not statistically significant.
The amount of biomass gained yearly was summarized and may be used to predict biomass accumulations beyond six years with the understanding that uncertainty increases the further out predictions are made. Biomass values were reported in terms of biomass per tree as well as on a spatial basis (biomass per meter squared, by hectare, or per acre).
Survivability and mortality were documented throughout the study. Winter damage in the early years was significant. A number of trees died back to the ground but some of these subsequently sprouted secondary branches (coppicing), producing multiple-boled trees.
Later in the study, insect and/or vole damage killed additional numbers of trees. Of the original 360 trees planted, 313 survived to the conclusion of the study, yielding an overall mortality of 13% (87% survivability). Clone #2 had the best survival (97.5%), followed by Clones 4 and 5 with 93% and 95% surviving, respectively.
The numbers of multiple-boled trees at the site were recorded since multiple-boled trees can be more difficult to harvest. The overall average of all trees was 20% with multiple boles, leaving 80% of the trees with the single, straight trunks that are ideal for harvesting purposes. The Lombardy poplar was the clone with the highest percentage of multiple leaders, at 45%. (It is a diagnostic characteristic of this tree.)
Spacing of hybrid poplars is an active area of research, with a number of factors involved in decisions for a given site (Heilman et al. 1995). Spacing in our plots was approximately 2.33 m by 2.33 meters; each tree occupied 5.42 m2 (1 tree per 58 ft.2). Total plot area was 1950 m2, 0.195 hectares (ha), or 0.48 acres. With 360 trees on the plot, original planting density was equivalent to 1,846 trees per ha (747 trees per acre).
Overland Flow Collectors and Piezometers
Results from water samples (overland flow and precipitation) were recorded throughout the fall and winter of 2000 and into spring of 2001. Collection times were centered during and immediately after storm events, thus attempting to collect overland flow while it was occurring. It was observed repeatedly that overland flow was sporadic at the site. It only occurred after lengthy precipitation when soils were quite saturated and only occurred in portions of the site.
Results from the overland flow collectors (and piezometers) were disappointing in this study. Despite the care with which the overland flow collectors were placed into the soil, runoff in a number of the boxes washed out along the sides and bottom, rather than flowing into the collection bottle. In other cases, it appeared that the collection box itself provided a perch for birds (the square boxes) or a travelway through the grass and snow (the larger triangles) for mice and/or voles. (Additional runways in the snow and grass were quite noticeable at times.) Animal activity appeared to have contributed to the erosion along sides and bottoms of collectors. There were visible amounts of bird and mouse/vole droppings deposited in some collectors. In addition, various insects and numerous spiders took up residence and/or became trapped in collection bottles.
Throughout most of the year, piezometers were dry, confirming the relative aridity of the region. The lowest piezometer (closest to the stream) was the most likely to contain water, but even it was most often damp rather than containing liquid. Piezometers appeared to contain sediment; even after bailing them out repeatedly and waiting, sediment continued to contaminate the water samples.
Phosphorus concentrations were consistently correlated with the amount of sediment or other contaminants present in the sample. This was true of all the water samples (the overland flow samples, rain samples and piezometer samples). When sediment, bird droppings, spiders or other contaminants were present in the sample, the resulting phosphorus concentration in the water sample would be high even after centrifuging to remove sediment. Phosphorus tends to move with sediment because of the way it chemically binds (adsorbs) to sediment.
Amounts of rainfall collected in bottles were converted into resulting inches of precipitation. For larger storm events, the values correlated well with amounts reported at the Pullman-Moscow airport located only a few kilometers from the site. However, quantities did not correlate as well for smaller storms; in addition, results were more varied across the site when rain was light.
Phosphorus concentrations in precipitation ranged from less than detectable up to 0.4 ppm. The average concentration for all precipitation samples was 0.07 ppm. Contamination with dust or sediment in collection bottles could not be ruled out. However, it has been noted that precipitation may carry more phosphorus than once thought, particularly when storms are “washing out” particulate matter carried in the atmosphere (Holmes 2001, Wetzel 2001). In the past, it has been thought that phosphorus loading from the atmosphere was “negligible.” (Stevenson et al. 1999).
Results from overland flow collectors and piezometers were thought to be too inconsistent to make quantifications over the entire study site. Without useful quantities and concentrations from overland flow collectors, it was not possible to calculate the complete mass balance of phosphorus entering and moving through the site.
Phosphorus Concentrations in Plant Material
All results of phosphorus analyses of plant material were reported in parts per million (ppm) or % by dry weight. Combining results from all tissues in proportion to their contribution to biomass, the overall percentage (by dry weight) was 0.11% (1,100 ppm) phosphorus.
Leaves had the highest phosphorus concentration, at 0.21% (2,100 ppm). Xylem contains relatively high concentrations of phosphorus (0.12% or 1,200 ppm) but it comprises only a small amount of the tree’s total aboveground biomass (0.6%). Bark phosphorus concentration averaged 0.104% (1,040 ppm) and wood was 0.067% (670 ppm). Bark is approximately 6% of a tree’s total aboveground biomass; woody tissue comprises about 77%.
Phosphorus concentrations in leaves and xylem were highly variable. Wood and bark data were less variable. Phosphorus concentration of leaves was compared according to their relative position on the tree. Leaves near the top of the tree had consistently higher phosphorus concentrations compared to lower leaves. Tree to tree variation was also quite high.
Comparison of leaf phosphorus concentrations was made between the nine clones. For this comparison, mid-level leaves from each tree were utilized. Clones #4 and #6 had the highest leaf phosphorus concentrations. Leaves of Clone #4 (#50-194) contained 3,760 (+/- 510) ppm and Clone #6 (#50-197) had 3,090 (+/- 160) ppm. The mean leaf phosphorus concentration for all clones was 2,420 (+/- 600) ppm.
Green leaves versus autumn leaves also were assessed. Autumn leaves contained less phosphorus than green leaves. The average phosphorus content of all green leaves analyzed was 2100 (+/- 700) ppm; this was from over 100 samples. Autumn leaves on average contained 1760 (+/- 490) ppm. These results indicate somewhat less phosphorus content in older leaves, but the variability of phosphorus content is too high to make it statistically significant.
Biomass Data Combined with Phosphorus Concentrations
Phosphorus concentration data and tree biomass estimates allowed calculation of total phosphorus (in kg) present in a typical tree and in a typical “best” clone (Clone #2 OP-367). Biomass values were partitioned into categories of leaves, boles, branches, and were multiplied times the appropriate phosphorus concentration to yield estimates of phosphorus associated with each plant tissue component.
Results combining all clones and all plant tissues indicated that for a typical hybrid poplar tree, the overall phosphorus content was 0.11% by dry weight (1,100 ppm). A range of possible biomasses may be matched to phosphorus concentrations to provide a range of phosphorus content on a spatial basis (amount of phosphorus in kg per meter squared, per hectare, and per acre).
It was hypothesized that certain clones would perform better than others at this site. It was found that the variation in growth was large enough that there were no statistically significant differences in biomass of clones. Six years of biomass estimates should provide reasonable expectations for future plantings of hybrid poplars. The trees performed adequately, but further hybrid poplar breeding strategies indicate potentially greater promise for clones in the future.
In addition to measuring the growth of the hybrid poplars at the study site, it was observed that weed control is a factor to consider under dryland conditions. Hybrid poplar plantings grown under irrigated conditions tend to have rapid growth, which soon suppresses weed growth among the trees because of the shading effects of the leaves (Stanturf et al. 2001). Under irrigation, canopies tend to close by the second year of growth under irrigation. Canopy closure versus weeds versus planting density is not a clear-cut equation under dryland conditions.
Weed growth was not tremendous in the later years of this study, but it was significant enough that it may have affected the growth of the trees. The site was weeded manually in the first several years after planting. At the conclusion of the study, the canopy was closed, but not densely closed. Light levels at the ground surface appeared adequate to grow significant weeds every year of the study. Closer spacing (i.e., increased planting density) would not solve the problem of weed growth in non-irrigated situations as the individual trees were probably in direct competition for whatever water does exist. In dryland regions, some weed control is probably desirable even in later years of the project (Personal observation).
Deer browsing on young hybrid poplars may cause considerable damage in the first several years after planting. In milder winters, the deer are not so apt to browse the trees, but in harsher winters, browsing damage may be significant. Fencing may be necessary in the first few years after planting the trees.
Several factors must be considered in addition to growth rate when considering a given clone’s suitability for a specific site. It is not enough to want the best growth rate. Disease and insect resistance are among other factors to be considered. In western Washington, disease resistance is particularly important. In eastern Washington, disease might not be as much a problem, but drought-resistance and/or winter hardiness appear to be major factors.
As discussed earlier, a number of trees died over the six years of the study. The overall survival rate was 87%. Some of the dead trees appeared to have suffered insect and/or vole damage. Voles consume bark along the base of the trees, particularly under the cover of winter snow or dense grass; once the main stem is girdled, the tree may die. Even partial injury to bark may allow insect (or disease) to cause subsequent death of trees. Significant differences between clones were seen in mortality at the site.
Clone #2 had the best survival rate; 97.5% of the original 40 trees were still alive at the conclusion of the study. Clones 4 and 5 also had good survival with 93% and 95%, respectively. The rest of the clones ranged in survival from 88 to 82%.
The Miscellaneous clone category had by far the lowest survival; this category included the trees that were “incorrectly planted” or “improperly labeled” at the time of planting. Being “incorrectly planted” was obviously terminal for a number of trees; the “Miscellaneous” clone category had only 74% survival. This emphasizes the need for care when establishing new plantings of trees.
Disregarding the Miscellaneous category, the overall survival was 88% for the nine clones combined (299 trees out of 341 correctly planted). Clone #2 had 97.5% survival (39 trees out of the original 40 planted). Clones #4 and 5 were almost as good.
High survival combined with strong growth performance indicates that Clone #2 (OP-367) was the best of the nine clones selected for the site. Considering height, basal diameter, and biomass, Clone #2 was consistently the leader, although tree-to-tree variation was too large to allow for statistically significant differences between the nine clones.
Selection of future clones even more appropriate to arid, dryland conditions is expected to further enhance performance. The clones at the site were among the earliest to be tried in the region and further selections are ongoing.
Overland Flow Collectors and Piezometers
Performance of the overland flow collectors and piezometers was discussed in the Results section above. It is hoped that a different, more sophisticated design for collecting runoff would yield more useful results.
Phosphorus Content of Individual Trees
Average overall phosphorus content was 0.11% (1,100 ppm). This value is within the range reported for most deciduous tree species (Raven et al. 1999, Marschner 1995). Green leaves and new growth had higher phosphorus concentrations than the woody tissue. This is to be expected, as actively growing tissues usually have relatively high phosphorus because of the vital role that ATP (the “energy currency” of the cell) plays in replicating new cells. Phosphorus also is a component in nucleic acids and phospholipid membranes within and between cells.
Xylem also tends to carry relatively high phosphorus levels since these tissues are involved with the transport of materials from roots to the leaves and active growth areas. Wood, principally composed of lignin and cellulose, tends to have lower concentrations and less variability in phosphorus than leaves or new growth. The mean phosphorus concentration in woody tissue from the hybrid poplars was 0.067% (670 ppm). Bark was somewhat higher, at 0.104% (1,040 ppm) phosphorus.
Differences between Clones in Phosphorus Concentration
The original hypothesis of the study was that the nine hybrid poplar clones would vary in the amount of phosphorus sequestered. Results from leaf analyses taken from similar positions in the trees do suggest significant variation between clones. Concentrations in green leaves ranged from 0.18 to 0.38% (1,800 to 3,800 ppm) by dry weight. Clone #4 (#50-194) had the highest concentration and this value was significantly different from the other clones; the next lowest concentration was 0.31% (3,090 ppm) for Clone #6.
Limitations due to the length of the acid digestion process and subsequent phosphorus analyses prevented full-scale comparisons of woody tissues from all nine clones. Preliminary results, however, indicated that differences in phosphorus content of wood between the various clones was not nearly as significant as the variation in the leaf concentrations. In addition, it appeared that high leaf concentrations did not necessarily correspond to higher phosphorus content in wood. Additional analyses are needed to fully address the phosphorus content of woody tissue.
A Conceptual Procedure of Riparian Buffer Design
On average, a typical six-year-old poplar had 0.03 kg of phosphorus, and this amount would be removed when the tree is harvested. Phosphorus removed may be figured in terms of individual trees or more usefully, on a spatial basis. The amount of phosphorus removed per hectare or per acre could then be related to an estimate of the amount of phosphorus to be removed. It would then be possible to design riparian strips adequate to reduce phosphorus loading to receiving streams by a certain amount.
For example, phosphorus is typically applied to Palouse agricultural fields at an annually rate of 2.2 kg of phosphorus per hectare (equivalent to 12 lbs. per acre) (Pan et al. 2001). Calculations may be made of how many trees would be necessary to remove enough phosphorus such that harvest of the poplar trees at ten years, say, would remove similar amounts of phosphorus from the site. Predictions of biomass at ten years of age ranged from 47 kg to 80 kg, with an average of 60 kg of dry weight biomass. With a typical phosphorus concentration of 0.11% by dry weight, each tree would contain approximately 0.066 kg of phosphorus (60 kg of biomass times 0.11% phosphorus).
With a tree spacing of 2.33 meters (the same as the study site) a riparian strip 100 meters long (the length of a hectare on one side) would contain 43 trees in each row (100 meters divided by 2.33 meters). Each row would theoretically remove 2.84 kg of phosphorus at harvest time (43 trees times 0.066 kg of phosphorus in each tree). Ten rows of trees would remove ten times as much phosphorus or 28.4 kg of phosphorus. This amount is greater than the cumulative amount of phosphorus (22.0 kg) phosphorus added over ten years to the surrounding fields. Thus, ten rows of trees are probably more than necessary to reduce phosphorus inputs to streams. A more appropriate selection would be eight or even six rows.
Strips may be designed with the ability to remove phosphorus factored in. In addition to allowing calculations for individual riparian strips in relation to neighboring fields, calculations may be made as more and more of a watershed is planted with riparian buffers. Reduction of phosphorus input to streams will benefit aquatic systems in proportion to the amount of riparian buffer strips added to the agricultural landscape.
In terms of overall phosphorus concentrations, hybrid poplars appear to be typical of many, if not most, tree species (Raven et al. 1999, Marschner 1995). Their value in terms of phosphorus removal for aquatic ecosystems does not lie in particularly high phosphorus content but in the fact that these trees grow rapidly and can be removed from the site at the time of harvest. By physically removing phosphorus along with the trees, aquatic ecosystems will benefit.
By providing an economic benefit to agricultural growers in the Palouse region, some of the reluctance to use riparian buffers may be overcome. Hybrid poplars should serve well for this purpose. Benefits to aquatic systems and to wildlife would be in proportion to the amount of hybrid poplars added to the landscape.
Educational & Outreach Activities
Hybrid Poplars in Dryland Riparian Agricultural Buffers in Eastern Washington: Performance and Phosphorus Removal, Ph.D. Dissertation by Laurie Flaherty, Environmental and Natural Resource Sciences, Washington State University, December 2002.
The performance of hybrid poplars and their ability to retain nutrients from edge-of-field runoff has been the primary focus of this research, rather than direct economic benefits to producers.
The potential for some economic return to landowners in the Palouse will be an important factor in determining whether or not the utilization of hybrid poplar buffers will become widespread in the future. Currently riparian zones are often some of the most productive lands in terms of dryland cereal crop yields. By offering an alternative economic benefit to landowners, producers are more likely to consider riparian plantings. The development of markets for pulpwood harvesting would increase as more and more trees were planted in the region.
Benefits to aquatic ecosystems and the surrounding landscape also will accrue in proportion to the number of streamside plantings.
Waterways would receive less sediment, nutrients, and contaminants, and would receive beneficial organic material (leaves and branches that have fallen in streams provide needed structure and complexity for aquatic insects and fish). Streamside trees would provide much needed shading; aquatic communities will benefit from cooler water temperatures.
This project began with five producers. Although all have cooperated and remain interested, only two sites proved suitable for the research portion of the project. Management decisions have remained with the landowners and all project activities were cleared with them.
Cooperative agreements with the Palouse Soil Conservation District have aided communication to and from farmers interested in establishing hybrid poplar buffers. In addition, the Spokane office of the Natural Resources Conservation Service visited several sites in the Palouse, including the research sites, where presentations and discussion ensued. Judging from comments during the tour, producer interest in hybrid poplars is very high, particularly if markets could be increased or if access to markets were more convenient.
Areas needing additional study
More study is needed concerning the most appropriate hybrid poplar clones to use in the Palouse region under dryland conditions. This study involved some of the earliest selections chosen for the region. More recent clones appear to have improved performances. Ongoing information disseminated widely will help encourage further planting of the most appropriate clones.
A comment made by a number of producers, indicates that they would like to be sure that hybrid poplars will, in fact, return some economic benefit before planting them on their properties. Thus, some specific demonstrations of the economic benefits would be helpful in encouraging others to plant these trees in riparian settings.