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4.5 MOISTURE STRESS AS AN INDICATOR OF THE EFFECTS OF SNOW ON BIOMASS RODUCTION OF FOREST TREES (Dr. C.P.P. ~1d and A.K. Evans) I 4.5.1. Specific Ob1ecttves The overall objective of this investigation, in con- junction with the Tree Biomass and Tree Phenology projects, is to determine and evaluate the effects of snow quantity on the growth of Engelmann spruce (Pices en~elmannii Parry) and quaking aspen (Populus tremulo1des Michx.). Increases in snowpack should affect the moisture status of the tree which should, in turn, affect tree growth. The Moisture Stress Project is designed specifically to provide informa- tion concerning the effects of microclimate and sno~ pack changes on the water status of the tree. The three basic hypotheses being investigated, as stated in the Interim Progress Reports of 1971 and 1972 (Reid and Evans, 1971; Reid and Evans, 1973), are: - Increases in snowpack will cause delayed snowmelt which will provide increased soil moisture later in the growing season and therefore increased biomass production through reduced moisture stress. - Increases in snowpack will cause delayed snoWmelt resulting in lower air and soil temperatures later into the spring causing decreased water uptake. incressed water stress. and reduced growth. - Changes in water stress will reflect changes in growth patterns and these changes can be efficiently monitored by the pressure chamber technique. This project is attempting to answer specifically the following questions: (1) How does moisture stress of spruce ~d aspen relate to varying snowpack? (2) How is moisture stress related to all environ- mental variables? (3) How can moisture stress related to spring snow- pack be separated from that related to summer weathE:;r patterns? In conjunction with Tree Biomass and Tree Phenology projects the following questions are being pursued: (4) How is moisture stress in spruce and aspen related to periodic growth through the season and subsequent annual increment? (5) Will a significant increase or reduction in growth of spruce and aspen occur with a one week delay in snowmelt? 4.5.2 Methode Moisture stress was monitored by the,pressure chamber technique on each of six study sites (4 spruce. 2 aspen) through the growing seasons of 1971 and 1972. Moisture stress was determined weekly in the early sesson and biweekly in the late season by sampling both the north and south sides of the crown of five trees on each site. In 1973. intensive study of one site during the snowmelt period was carried out rather than the continued monitoring of all sites. Site 2 (M3200-Spruce) was monitored every third day from May 15 through the end of snowmelt. and weekly through the remainder of the growing season. A laboratory experiment was conducted in early 1974 to determine the effects of cold soil temperatures on root resistance to water f1 seedlings. The pressure cbs large water bath at three di (0. 7.5. and 150C). Intact tops removed were placed in chamber. Three bars pressur rate of water movement throu stem was measured by collect procedure gave an excellent resistance to water flow at 4.5.3. Results and Discussi n General Model Considerations The growing season for trees under study can be divided i to moisture stress, the earl the later or midsummer perio (3) above. it can be stated environmental factors most i water stress are different i tne early period water stres by factors relating to snow, period. stress is primarily weather. Therefore. a water subalpine forest must consis for each of the two periods. that the emphasis in this st Engelmann spruce. Early sea difficult to obtain because use of leaf samples. A 1im! season twig samples have bee the emerging foliage and bra depleted. Because it will b conclusive relationships for of the aspen model. our emph on spruce. The two submodel considered separately begion season. Midsummer Water Stress I in Engelmann spruce er was immersed in a ferent temperatures oot systems with the ater in the pressure was applied and the h the roots and up the ng the exudate. This eans of assessing root arying temperatures. I I I in the subalpine forests to two parts in relation or snowmelt period and In answer to question imply that the sets of portant in controlling the two periods. Also. is primarily controlled while in the midsummer ontrolled by daily stress model for the of two submodelsJ one It is important to note dy has been placed on on data for aspen are ur technique is based on ed number of early taken from aspen. but ches are rapidly difficult to establish the early season part sis will continue to be for spruce will now be ng with the midsummer I I I I , I~ " o " ~ Iii 1 I~.j n II .] I , Ij I I I IJ I I I Ij I I~ Iii Extensive statistical analys s of the 1972 season data on Site 1 (M3500-Spruce has resulted in a simple model and significant insigh into the environmental parameters controlling water stress in the midsummer season (Evans. 1973). Other sites, including aspen. show similar relationships a though the Missionary Ridge spruce and aspen sites appear to be more similar to each other than ether is to the Wolf Creek spruce sites. The significance of a low t perature effect. as introduced in the 1972 Inter m Progress Report (Reid and Evans. 1973). became app rent. Air or soil temperatures which approache freezing caused a reversal in the normal patte n of decreasing water stress with decreasing tempe ature (evaporative demand). Because this reve~rl was rather abrupt. it was possible to remove these data from further mid- summer analysis. The low te erature effect data are a combination of two additio al data populations. Some of the high stress dat may be attributed to reduced water uptake from co d soils and should there- fore be included in the dat;poPulation of early season water stress. The re ainder is due to an error in the pressure chambe- technique when working at temperatures near freezin . which has been reported by Evans and Reid (1974). dsummer water stress and 25