Abstract
An enhanced understanding of plant water uptake is critical for making better-informed management decisions involving vegetative manipulation practices aimed to improve site productivity. This is particularly true in arid and semiarid locations where water is a scarce, yet precious commodity. In this project, we evaluated the interannual and seasonal variability of soil moisture and transpiration in sapling, juvenile, and mature western juniper (Juniperus occidentalis) trees in a semiarid rangeland ecosystem of central Oregon, USA. Transpiration levels were greatest in mature juniper trees in an untreated juniper watershed (Jensen WS), while the lowest transpiration levels were observed in juniper saplings in a treated watershed (Mays WS) where most mature juniper trees were removed in 2005. Significant differences (p ≤ 0.05) in leaf water potential levels observed between predawn and midday readings for all juniper growth stages indicated water is lost over the course of the day. Results showed seasonal precipitation was highly variable over the course of the study (2017 through 2019) and this was reflected in soil water available for tree uptake. This resulted in considerable intra- and inter-annual variation in transpiration. In years with greater winter precipitation amounts (2017 and 2019), juniper transpiration rates were highest during the summer, followed by spring, autumn, and winter. On average, transpiration rates during the summer in the wettest (329 mm) year 2017 were 115 and 2.76 L day−1 for mature and sapling trees, respectively. No data were collected for juvenile trees in 2017. In the drier (245 mm) year 2018, higher transpiration rates were observed in the spring. On average, spring transpiration rates were 72.7, 1.61, and 1.00 L day−1 for mature, juvenile, and sapling trees, respectively. Study results highlight the sensitivity of western juniper woodlands to variations in seasonal precipitation and soil moisture availability.
Highlights
The replacement of grassland vegetation by encroaching woody species in rangelands is a global phenomenon occurring during the past century [1,2,3]
This study shows that western juniper transpiration for sapling, juvenile, and mature growth stages follow a similar pattern, that is consistent with soil moisture availability in response to seasonal precipitation in cool climate rangeland ecosystems of semiarid central Oregon, USA
This study evaluated western juniper transpiration and soil moisture relationships at different tree growth stages
Summary
The replacement of grassland vegetation by encroaching woody species in rangelands is a global phenomenon occurring during the past century [1,2,3]. Juniper (Juniperus spp.) encroachment is one of the most large-scale changes occurring in North American rangelands [7,8]. The spatial distribution of juniper has increased by one order of magnitude since the mid-19th century throughout the Great Basin [9]. The expansion of juniper is commonly attributed to a mix of biophysical and anthropogenic factors including overgrazing, increases in atmospheric carbon dioxide, and fire suppression [11,12]. Since the late 1890s, western juniper (Juniperus occidentalis spp. occidentalis Hook.)
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