Seasonal Variations in Soil and Plant Water Status in a Quercus suber L. Stand: Roots as Determinants of Tree Productivity and Survival in the Mediterranean-type Ecosystem

Abstract

Studies were conducted to examine changes in soil (Ys) and plant water status during summer in a 16-year old Quercus suber plantation in southern Portugal. . Intensive measurements were conducted on five trees with mean height and DBH of 5.3 m and 11.6 cm, respectively, growing at close proximity to each other. Weather conditions and soil water potential (Ys) at the rhizosphere of each of the trees measured at 0.3 and 1 m soil depth were continuously monitored. Predawn (Ypd) and midday (Ymd) leaf water potentials were determined every month. Soil and plant samples were also collected in June and September from different locations within the study site for d 18 O isotope composition analysis. Pressure-volume (p-v) curves were constructed from plant shoots at different times during the vegetative period to determine osmotic potential at full saturation (P 100 ), water potential at turgor loss point (Y tlp ), relative water content at turgor loss point (R* tlp ) and bulk modulus of elasticity (e). Significant P < 0.05 decline in Ys occurred between May and September, the lowest value recorded being -2.0 MPa. Decline in soil moisture affected tree water status, but decline in leaf water potential varied significantly (P < 0.05) among the trees. At the end of summer drought, lowest Ypd measured was -1.7 MPa while the highest measured during this time was -0.8 MPa. Differences among trees were attributed to differences in rooting depth, as shown by regression analysis of 18 O isotopes. Radial stem growth ceased when Ys within the upper 0.3 m depth approached -1.5 MPa. The upper soil layers contributed approximately 33% of the total tree water requirement, between spring and mid summer when drought was experienced by trees. Deep soil layers however, supplied most of the water required during drought and no growth was recorded during this time. Stressed trees increased solute concentration of their tissues by a Magnitude of 0.7 MPa while bulk tissue elastic modulus increased by about 17 MPa. The study emphasizes the significance of roots as determinants of tree productivity and survival in the Mediterranean ecosystems.

Herbaceous layer development during spring does not deplete soil nitrogen in the Portuguese montado

“…The aboveground biomass development of the herbaceous vegetation in the montado is confined to the period between February and May due to low winter temperatures and low soil moisture in summer that hinder plant growth (Otieno et al, 2006). The mean peak aboveground biomass observed in this study was 200 g m conditions, favourable temperatures and availability of ample soil nutrients.…”

“…Due to its extent, the herbaceous layer provides a larger photosynthetic surface compared to the trees and could play a significant role in carbon fixation and ecosystem productivity (Pereira et al, 2007;Aires et al, 2008;Li et al, 2008) as well as influence soil nutrient and water balance (Joffre, 1990;Moreno et al, 2007). Due to the semiarid nature of the montado, the life cycle of the herbaceous plant community is confined to spring when soil moisture and temperatures favor their growth and development (Otieno et al, 2006;Pereira et al, 2007).…”

“…Woodmansee and Duncan (1980), Joffre (1990) and Gallardo et al (2000) have shown a rapid increase in nitrogen (N) in the herbaceous layer biomass between February and April in the Mediterranean regions, while soil extractable nitrogen was at its lowest. This however, is also the period when maximum growth of trees occurs (Otieno et al, 2006). The proliferation of the herbaceous layer in spring results in the accumulation of nitrogen in the aboveground biomass and may lead to a significant reduction in soil N and its limitation.…”

“…Monthly biomass determinations were conducted during spring of the three years. Active growth of the herbaceous layer was confined to the spring period between March and May, due to low temperatures between November and February and drought that occurs from June to September (Otieno et al, 2006;Pereira et al, 2007). The choice of sampling period was, therefore, based on the active vegetative period (spring) of the herbaceous layer.…”

Xylem water potentials of native shrubs from northeastern Mexico

“…This response may be viewed as a mechanism to conserve water and thus avoid tissue dehydration (Veneklaas & Poot, 2003;Otieno et al, 2006).…”