Water relations, survival and growth of Douglas-fir seedlings at a pinegrass dominated site in the interior Douglas-fir zone of south-central British Columbia

Abstract

A field study was conducted to determine the impact of microclimate and vegetation on survival and growth of planted Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) at a pinegrass (Calamagrostis rubescens Bucki.) dominated clearcut in the Interior Douglas-fir Zone of south-central British Columbia. The study focussed on (1) the water balance of the site, (2) the response of Douglas-fir and pinegrass to moisture deficits, and (3) the influence of environmental factors and the pinegrass-dominated vegetation on Douglas-fir survival and growth. Survival and growth, physiological and microclimate measurements were made within the framework of a replacement series and neighbourhood experiment. Evapotranspiration and surface resistance were calculated based on data including air temperature, solar radiation, rainfall and soil water content measures using gravimetric sampling and the neutron moderation method. After three growing seasons, survival of the Douglas-fir in the absence of pinegrass was 43%. Mortality resulted from a variety of factors including drought, frost, and rodent damage. Poor seedling quality at the time of planting was also suspected to have contributed to mortality. In the presence of the native vegetation Douglas-fir seedling growth was reduced and risk of death increased. Results suggested that: (i) reducing the leaf area index around the seedlings to below 0.5 effectively improved seedling performance and (ii) removing all vegetation immediately adjacent to a seedling was preferable to partial removal. A comparison of the relative growth rates of the Douglas-fir and pinegrass indicated that rapid growth of pinegrass early in the growing season when moisture is least limiting gave it a competitive advantage. Soil temperatures beneath pinegrass dominated vegetation were cooler and warmed more slowly in the spring than beneath bare soil suggesting that native vegetation may cause restricted root growth of Douglas-fir by influencing soil temperature. Longterm climate records showed that amount and distribution of precipitation near the study site varies greatly from year-to-year. Unfavourable early spring growing conditions may be expected 80% of the time. Furthermore, soil water content profiles suggested water deficits develop rapidly at the beginning of the growing season. Predawn twig xylem water potentials suggested that, even in the absence of pinegrass, Douglas-fir were moderately stressed by mid-June in all three growing seasons. At this time, the roots of the Douglas-fir remained in the top 15 cm of soil where moisture deficits were the greatest. In contrast, predawn leaf xylem water potentials of the pine9rass were found to be more responsive to varying weather conditions because pinegrass' roots extend throughout the soil profile. Although the pattern and timing of soil moisture depletion differed in each of the three growing seasons, measurements of total growing season evapotranspiration were similar. Because minor precipitation evnts resulted in substantially elevated soil evaporation at times, it was difficult to separate surface evaporation from transpiration using a technique which treated the entire soil profile as one layer. Regardless, it appears that pinegrass transpiration is responsible for removing most of the water from below 10cm. Pinegrass in fact played a major role in the water balance of the site. Several implications of study results to silvicultural practices are discussed. These include: (i) the importance of considering longterm climatic data when formulating rehabilitation/regeneration strategies; (ii) the increased importance of planting stock handling on harsh sites; (iii) the appropriate selection of species and stocktype; (iv) planting strategies; and (v) site treatment to reduce competition and ameliorate environmental restraints. Finally, the author discusses the limitations of the study and devotes a chapter to suggestions for studying vegetation competition with respect to reforestation problems