Anson Call and Dr. Samuel St. Clair, Department of Plant and Wildlife Science
Introduction
Clonal plants are important contributors to natural environments and can have disproportionate effects on ecosystem function. In the mountain west, the clonal species Populus tremuloides (quaking aspen) is critical in helping to regulate ecosystem function; healthy aspen stands support a diverse group of plants and animals. Aspen are in decline across the west, due to climate change (increased drought), changing fire cycles, and increasing pressure from herbivores like deer and elk. Understanding the natural processes that enable aspen to survive and grow will help us protect aspen in the future. Clonality, or the ability to reproduce asexually through root suckering, is a hallmark trait of aspen that is not fully understood. Aspen can share nutrients between different trees (called ramets) in a clone through an interconnected root system. However, the degree of nutrient sharing, or clonal integration, may vary throughout the developmental stages of ramets. The relative importance of nutrient sharing at each developmental stage is unknown. In this study, we addressed the question of how nursing affects ramet development across different stages of development. We used physical separation of ramets to test the importance of the nursing effect in growth, mortality rate, and photosynthesis rate. Results show that clonal integration affects some – but not all – aspects of ramet health and the effects are dependent on ramet size.
Methodology
Study location
The study included four aspen clones in the Uinta National Forest. The clones were located along the SR-92 Alpine Loop Road. Elevations ranged from 2290m to 2400m. Annual precipitation averages about 570 cm and the parent soil is a roundy loam. The clones were identified through genetic analysis. The average ages of the clones range from 39 to 49 years, with no significant differences in mean ramet age across clones. Point-quarter method was used to determine the stand density, composition, and basal area at each site. All clones were located within stands that were predominately (>80%) aspen.
Study design
We assessed the functional influence of root integration by severing root connections of ramets in four size classes in a randomized block design with three treatments. In the first, or “cut” treatment, we unearthed the ramet’s lateral root system and severed a 3 cm section of the root to eliminate vascular connections to the parent root system. The cut was made 40 cm from the stem, on both sides of the sucker. Finally, we replaced the soil around the sucker. In the second, or “dig” treatment, we removed the soil and replaced it but left root connections intact. This treatment allowed us to account for any confounding caused by soil disturbance in the cutting treatment. In the third treatment, or control, no digging or cutting was applied to the ramet. All treatments were applied on May 19. The ramet size classes were: small (20-50 cm tall), medium (70-100 cm), large (100-130 cm), and extra-large (200-230 cm). There were 112 total ramets, usually with two suckers for each treatment and size class in every clone. Physical measurements were collected 5 times during the growing season, in the second week of each week in May through September. Measurements included height, stem diameter at base, and stem diameter at 1 meter height (for ramets over 1m tall). Photosynthesis measurements were conducted in late July, mid-August, and early September using a Li-Cor 6400 Infrared Gas Analyzer (Li-Cor Biosciences, Lincoln, NE). We also recorded mortality in each visit to the field sites.
Results
Results show that both the digging and cutting treatments had considerable effects on all the variables we measured. Over the course of the growing season, ramets in the control treatment grew an average of 18.78 cm, while “dug” ramets grew 12.89 cm on average (68.6% of control) and “cut” ramets grew only 7.58 cm on average (40.4%). When analyzed by size class, the biggest differences between treatments were found in the medium and extra-large size classes. Diameter analysis also showed large treatment effects in the extra-large size class. However, there was great variation in diameter measurements across the board, suggesting that height growth may be a more reliable indicator of treatment effects.
Overall, average photosynthesis rates for cut trees was 32% lower than the control, while photosynthesis of the dig trees was 20% lower. Small, medium, and large size trees had similar readings for cut and dig treatments, and higher readings in the control treatment. In the extra-large trees, the dig and control treatments had similar values, while the average value of the cut treatment was somewhat lower.
Mortality rates for all treatments were <5% through mid-July. Towards the end of July, a significant number of cut ramets began to die. By early September, the mortality rate of cut ramets reached 21%, while the rate remained <10% for the dig and control treatments.
Discussion
Our results suggest that parental nourishment is important for developing ramets in all stages of growth. Surprisingly, the strongest treatment differences were in the larger size classes, with extra-large ramets showing the greatest difference between the cut treatment and other treatments. This finding, along with high variability in the smaller size classes, suggest that parental nourishment supports ramet growth throughout all stages of growth, while other factors confound the effects of parental nourishment at the earliest stages of growth. Some of those confounding factors may be trampling, herbivory, or natural thinning, which are quite common in small ramets and much less common in larger ones. In either case, clonal integration is key in enhancing the prospects of ramet survival and recruitment in to the forest overstory.
Conclusion
Our study shows that parental nourishment is a key to aspen’s survival strategy. Land managers must enable aspen stands to grow in a way that allows for fully-developed ramets to support the growth of young and adolescent ramets. This is especially true for areas of constant fire suppression and high herbivory rates, where aspen are strained to compete with other plant species for resources.