Shelters Affect Tree Seedling Establishment Under Grass Competition

Methods

This experiment was conducted at the Utah State University research farm in Logan, Utah (USDA hardiness zone 5a) with bigtooth maple (Acergrandidentatum) and gambel oak (Quercus gambellii), native to the Intermountain region. In early April, 1993, we planted one-yearold, 0.1 m (4 in) high, seedling plants, nursery grown in 160 ml tubes, in a well-drained Millville silt loam (coarse-silty, carbonatic, mesic, Typic Haploxeroll) that had 0.17 m/m (2 inches/foot) available water content to a depth of 2 m (6.5 ft). Treatments included species, +/- grass competition and +/- shelters. The experimental design was a threeway randomized complete-block, with each species x competition x shelter block replicated seven times, and individual trees were randomly assigned to treatments. Immediately after planting 1.25 (4.1 ft) high brown-transluscent plastic shelters (TreeEssentials, Inc, St. Paul, MN) were placed over treatment trees, and ‘Newhy’ wheatgrass (Agropyron cristatum x A. repens) was seeded in a 0.5 m (1.6 ft) radius around the trees with competition. Trees were watered the first growing season with one drip emitter applying 2 liters of water per tree once a week. The experiment was not irrigated the second year. Weeds were removed by cultivation or tilling from the entire plot as they appeared.

Surviving trees were counted at the end of the first season, at the start the second season prior to budbreak, and finally at the end of the second season prior to leaf fall. We measured total tree height in fall 1994 on the single leader for trees in shelters, and for non-sheltered trees under competition. Because non-competition trees not in shelters were multi-stemmed, all dominant shoots originating from the root crown were measured, and the average height calculated. All leaves for each tree were collected in September, 1994, with the exception of non-sheltered maples under competition because all trees had died and leaves had senesced and fallen off, and measured for total leaf area with a leaf area meter (Model CI-203, CID Inc., Vancouver WA).

Eight trees without grass competition, four oak and maple replicates +/- shelters, were randomly selected for root studies. Roots of trees under competition were not investigated since few plants survived and fine roots of the trees could not be distinguished from the grass. In September 1994 a pit was excavated immediately to the south and under each of the selected trees to a depth and width of 1 m (3.2 ft). A 1-m²(11 ft²) grid divided into 100 cm² (16 in²) cells was placed over the soil profile face nearest the tree, and the number of fine (non-woody, < 1 mm [0.04 in]) and woody roots (nonflexible > 1mm diameter) observable in each cell was counted.

On July 7 and August 25, 1994, midday (between 12 noon and 2 PM) leaf water potential (Ψ) was measured with a pressure chamber (series 3000, Soil Moisture Inc., Santa Barbara, CA) to assess plant water status. Predawn Ψ was not measured because prior work had shown that Ψ at predawn was higher than those not in shelters even when well watered (5). A single leaf was excised from each tree before dawn, immediately sealed in an aluminum bag (4), and returned to the laboratory for measurement with a pressure chamber, usually within an hour.

Water potential, leaf area, and shoot elongation data were subjected to analysis of variance appropriate for a three-way complete block design. Survival was calculated as the mean of the replicates for each competition x species x shelter treatment combination. Consequently, statistical comparison with analysis of variance could only be performed on the main treatment effects. The effect of treatment interactions could only be compared observationally. Differences in fine and coarse root number were first analyzed by comparing number of woody and fine roots among competition and species combined for the whole profile. Change in average root number for the same treatments by depth was then compared with woody and fine roots combined, and the change in total root number was plotted against depth for species x shelter treatments.

Results and Discussion

Shelters generally improved tree survival and growth under competition, but the species responded differently (Table 1). After the first growing season with irrigation, only two oaks overall had died. Competition evidently reduced winter hardiness, as a number of trees of both species had died between the fall and spring counts. Survival, then, of trees with competition was significantly lower than for the trees without competition. Observationally, three oak and two maple without shelters under competition died, however, while in shelters, only one tree of each species under competition died. The summer of 1994 was exceptionally hot and dry, as average high temperature was 2 C above normal for June, July, and August; and rainfall was only 29 mm (1.13 in or 40% of normal) for this period. At the end of the second season, overall survival of trees under competition was significantly lower than those without grass competition, while no maple or oaks without competition had died after the first season. While the interactive effect of shelters and competition on survival could not be statistically tested, observation showed that shelters clearly benefited maple survival under competition. By fall of the second year, all maples without shelters were dead while only one in shelters had died. Shelters did not appear to benefit oak under competition, however, two survived without shelters but only three with shelters.

Shelters reduced water stress through the second season, particularly during early summer for trees under competition (Table 2). On July 7 leaf water potential (Ψ) was significantly less negative for all trees in shelters, and under competition sheltered trees of both species were less water stressed than those without shelters. By late summer, most non-sheltered trees under competition had died, making water stress comparisons impossible. Again, however, shelters reduced overall water stress, as trees in shelters had less negative Ψ than those without, consistent with prior reports (6). Less water stress in shelters was likely due to reduced transpiration (5) that reduced depletion of soil moisture.

Competing vegetation suppressed stem elongation and total leaf area, most likely due to competition for light (Table 2). Non-sheltered oak and maple under competition were unable to elongate above the competing grass stems and thus received little light. If leaves cannot absorb enough sunlight for photosynthesis, there is no carbon for elongation and additional leaves are not produced (1), and in the case of maples, all non-sheltered trees in grass died. In contrast, sheltered maples under competition utilized the additional space provided by the shelters to elongate and develop new leaves beyond competing grass foliage.

Maple and oak above-ground growth responded very differently to shelters (Table 2). Oaks were much slower growing than maple, as they had much less leaf area and elongation across all treatments, which could explain why fewer sheltered oak than maple survived competition. For maple without competition, shel-tered trees elongated more than non-sheltered, but nonsheltered trees had nearly twice the total leaf area. A trade-off between enhanced elongation and leaf area in shelters was consistent with reports elsewhere (5). This trade-off benefited trees under competition, but means less productive leaf area for those trees without competition. However, oak apparently was unable to utilize conditions in shelters as much as maple. Both leaf area and elongation of oaks with no competition was greater with shelters than without.

Differences in top growth were reflected in root growth (Figure 1, Table 3). Consistent with more leaf area and shoot elongation, maple had more fine and woody roots than the oaks (Table 3). Shelters had no effect on fine root growth, but the number of woody maple roots compared to nonsheltered was reduced, suggesting that less total leaf area reduced transport of carbon to structural roots. Shelters had no significant effect on either woody or fine root number in oak. We saw no treatment differences in root numbers down to 0.5 m (1.6 ft) depth (Figure 1). Below 0.5 m (1.6 ft) depth, species differences in above-ground growth were reflected in root growth, as maple had significantly more roots than oaks. Noncompetition maples had significantly more total roots than non-competition oaks by 0.8 (2.6 ft) m depth, and significantly more than all other treatments by 0.9 m (3 ft) depth. These results would suggest that maple without shelters, at least initially, would provide more resistance to soil shear given its greater number of woody roots (9, 10).

Figure 1.

Total root number by depth observable in a 1 m wide soil profile immediately under bigtooth maple and gambel oak with and without treeshelters grown without competition. Error bars are included only for those depths where the species x shelter interaction term was significant.

Conclusions

Shelters have some promise in establishing seedling trees in arid environments with competing herbaceous vegetation, depending on the species. When a species can utilize the space and light provided by shelters to increase stem elongation, the increased leaf area and reduction in transpiration rate (5) afforded by shelters can allow a tree more access to soil water and sunlight, and it will be better able to compete with surrounding vegetation. A slow-growing species like gambel oak that was unable to elongate enough to utilize shelters would not benefit as much as a fastergrowing species such as bigtooth maple. Indeed, oaks without competition did not grow as much as bigtooth maple. Even if shelters do not enhance growth for all species, the protection from external damage, such as animal browse, can enhance establishment (7). The presence of shelters could also facilitate weed control by eliminating potential damage from drift of herbicides.