Influence of climate-driven shifts in biomass allocation on water transport and storage in ponderosa pine

Abstract

Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (A L /A S ) in desert versus montane trees could increase leaf-specific hydraulic conductance (K L ). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (J S ) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and J S . In the summer, mean maximum J S was 31.80±5.74 and 24.34±3.05 g m -2 s -1 for desert and montane trees (a 30.6% difference), respectively. In the fall, J S was 25.33±8.52 and 16.36±4.64 g m -2 s -1 in desert and montane trees (a 54.8% difference), respectively. J S was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher K L than montane trees in the summer (2.41×10 -5 versus 1.05×10 -5 kg m -2 s -1 MPa -1 , P<0.001) and a 162% higher K L in the fall (1.97×10 -5 versus 0.75×10 -5 kg m -2 s -1 MPa -1 , P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum g C was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high K L in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase K L and not to increase the contribution of stored water to transpiration.

Leaf Size and Leaf Area Index in Fagus sylvatica Forests: Competing Effects of Precipitation, Temperature, and Nitrogen Availability

“…The large majority of studies in mature stands reported a positive relationship between precipitation or soil water availability and LAI, when moisture transects included a turnover of tree species or forest communities (for example, Grier and Running 1977;Kozlowski and Pallardy 1997;Bussotti and others 2000;Eamus 2003;Prior and others 2005). In studies with only one tree species, a reduction in LAI with decreasing rainfall was found in certain temperate coniferous stands (Gower and others 1992;Mencuccini and Grace 1994;Hebert and Jack 1998;Maherali and DeLucia 2001) and in species-poor tropical forests in Hawaii (Harrington and others 1995;Austin and Vitousek 1998;Ares and Fownes 1999). However, we are only aware of one study (Bussotti and others 2000) along a rainfall gradient focusing on the LAI of a temperate broad-leaved tree species.…”

Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences

“…Stored water may contribute to transpiration in trees during diurnal cycles [60] and to the ability of plants to thrive in dry habitats [137]. However, field comparisons of ponderosa pine trees growing in contrasting habitats (desert vs. mountain) did not evidence any difference in water storage between habitats [93]. In pine dominated forests, the strategy in response to increasing atmospheric evaporative demand is to shift the relative allocation to leaves vs. sapwood and therefore to rise K L .…”

Transpiration and stomatal conductance across a steep climate gradient in the southern Rocky Mountains

“…Sensitivity analyses of varying k s with elevation (e.g. Maherali and DeLucia, 2001) did not result in significant differences in the model output, and so, for simplicity we held k s constant across sites. Comparison to the observations was facilitated by normalizing predictions relative to the highest observed values, constraining the upper limit of the predictions (Oren et al, 1999).…”