Chad S. Horman and Dr. Val Jo Anderson, Botany and Range Science
Juniper (Juniperus spp.) forests cover a large percentage of the West and are continuing to invade grass/shrub ecotypes. This eventually leads to a reduced understory and lower productivity for grazing animals. This reduction in understory biomass is due to the competitiveness of this species. The negative affects of Juniper on the understory have been attributed to one or more of the following: 1) shading, 2) rainfall interception, 3) litter accumulation, 4) allelopathy, and 5) competition for water and nutrients.
Juniper litter may negatively affect the understory either by its depth, a physical barrier, or by the production of allelochemic compounds. Allelopathy occurs when one plant produces secondary compounds that negatively affect the surrounding vegetation. This study examined the effect of litter depth and allelopathy on a grass, bluebunch wheatgrass (Agropyron spicatum), and a shrub, antelope bitterbrush (Purshia tridentata), normally found in the grass/shrub ecotype. Two null hypotheses were tested. First, litter depth does not reduce seedling emergence or vigor. The second, that juniper litter is not allelopathic.
A greenhouse trial was done using eight inch diameter pots filled with a soil mixture of one part sand, one part Pearlite, and two parts soil from a pinyon pine (Pinus edulis)- Utah juniper (Juniperus osteosperma) woodland located five miles east of Sheep Creek canyon in Spanish Fork canyon. 100 wheatgrass seeds and 20 bitterbrush seeds were planted in each pot. The bluebunch seeds were broadcast seeded on one half of the pot, while the bitterbrush seeds were planted in four caches of five seeds at a depth of 2.5 cm in the other half. The experiment was a two factor complete factorial design with three litter depths (Control i.e. no litter, 3cm, and 5cm) and three leachate concentrations ( Control i.e. distilled water, 1%, and 10%) replicated five times in a completely randomized block design. The juniper litter was collected from the same area as the soil. The 1% and 10% leachate concentrations were prepared by soaking 150 9 and 1500 g, respectively, of juniper litter in 15 liters of distilled water a room temperature (20* C) for 24 hours.’ The pots were watered to field capacity every five days. Seedling emergence was recorded during the eight week study. At the conclusion of the study, vigor and biomass were recorded. Vigor was based on the average number of leaves per plant. ANOVA was used to determine if the various treatments had any significant affect on the emergence, vigor and biomass.
The leachate treatments had no significant effect on total emergence for either species. However, total emergence was reduced by litter depth. Bluebunch wheatgrass emergence was only .6% and .006% for 3cm and 5cm depths, respectively. This was significantly lower than the 57.3% emergence in the control pots. Bitterbrush emergence was similarly affected by litter depth, emergence of control, 3cm and 5cm depths were 78%, 22.7% and 1%, respectively.
Litter depth also negatively affected emergence rate. Bluebunch wheatgrass never reached 25 or 50% emergence in the 3cm and 5cm pots. In the control pots, bluebunch reached 25% emergence in 2.5 days and 50% in 20.5 days. Litter depth also significantly affected the emergence rate of bitterbrush. Days to 25% emergence was 1.5 and 29.4 days for the control and 3cm litter treatments, respectively. The 5cm litter depth treatments never reached 25% emergence. Bitterbrush seeds in the control pots reached 50% emergence after 5.7 days, whereas 46.2 days were required by the 3cm pots to reach 50%.
The leachate treatments had no significant effect on plant biomass. As with emergence, litter depth negatively affected biomass. Bluebunch wheatgrass in the control pots produced an average of .12 g, whereas, the 3cm and 5cm pots only produced .003 g and 0 g, respectively. Bitterbrush in the control pots produced and average of .54 g and only .18 g and .01 g in the 3cm and 5cm pots, respectively.
It was interesting to note that neither leachate concentrations or litter depth had any significant effect on survival of the plants once they emerged. Likewise there was no significant difference in the vigor of the plants among the various treatments.
The results of this study indicate that bluebunch wheatgrass and antelope bitterbrush are not affected by juniper litter leachate. This means that other factors are responsible for the reduced understory beneath juniper trees. One factor appears to be litter depth. Both of the litter depths tested, effectively prevented bluebunch emergence. Increasing litter depth also acts as a deterrent to bitterbrush emergence as seen by the fact that the 3cm depth had 22.7% emergence and the 5cm depth had 1%.
There are two possible reasons for lower emergence with increasing litter depth. First, litter may drain and dry out too quickly for seed germination to occur. This environment does not provide enough moisture for water imbibition to occur into the seed and thus the seed coat will not rupture. If the seed coat does not rupture then the radicle can not emerge. Secondly, the emerging radicle may never be able to reach mineral soil in order to obtain water and nutrients. In trying to reach mineral soil the seed uses up all of its energy stores and dies.
The fact that vigor and seedling survival following emergence were not statistically different seems to indicate that once the plant has emerged that litter has no effect. This would mean that if the seed radicle can reach mineral soil, then the litter ceases to by a barrier. It was also interesting to note that not only did the leachate not affect plant emergence, but it did not reduce plant vigor or survivability either.
In conclusion, juniper litter was found to effect emergence of bluebunch wheatgrass and bitterbrush. Allelochemical activity did not influence these two species. Increasing juniper litter depth showed a strong negative relationship with emergence, but had no impact on survival.5
- R. Jobidon, Forest Science, 32(1986)112-118.
- T.N. Johnsen Jr., Ecological Monograph, 32(1962)187-207.
- E.L. Rice, Allelopathy, Academic Press, Inc, Orlando (1984)
- M.R. Vaitkus and L.E. Eddleman, Proc. Pinyon-Juniper Conf. General Technical Report INT-215, (1987)..
- The help and support of Dr. Val Jo Anderson was gratefully appreciated.