Controls on the distribution and resilience ofQuercus garryana: ecophysiological evidence of oak's water‐limitation tolerance

Abstract

Citation: Hahm, W. J., W. E. Dietrich, and T. E. Dawson. 2018. Controls on the distribution and resilience of Quercus garryana: ecophysiological evidence of oak's water-limitation tolerance. Ecosphere 9(5):e02218. 10.1002/ecs2.2218Abstract. The composition of forests in Western North America is changing. The decline in the shadeintolerant Oregon white oak (Quercus garryana var. garryana) is attributed to increased competition with the tall-growing Douglas fir (Pseudotsuga menziesii var. menziesii) as a result of widespread fire exclusion. In a warmer, drier future, both species will experience increased water stress, and their distribution will depend on ecophysiological adaptations to water limitation, of which little is known for Q. garryana. Here, we report a suite of new ecophysiological observations to better understand the oak's water-limitation tolerance, in order to predict its fate in a changing climate. Our study site in the Central Belt of the Franciscan Formation in the Northern California Coast Ranges receives almost no rain in the leaf-on growing season and has limited subsurface water storage capacity. In spite of low pre-dawn water potentials (Ψ; to below À3.0 MPa), mature trees maintained high rates of sapflow. Sapflow continued due to a high Ψ gradient (%1.6 MPa on average) at mid-day between shoots and the subsurface (inferred from pre-dawn measurements) throughout the dry season. Depletion and recharge of stored water in stem tissue and leaves helped to sustain transpiration. Leaves experienced low Ψ (below À4 MPa), and declining hydraulic conductance yet remained functional. Pressure-volume curve analyses revealed that the maintenance of positive turgor pressures in leaves at low Ψ may be attributable to dynamic adjustment due to changes in cell wall elasticity. The turgor loss point may be of limited use in delineating ecophysiological limits to growth and reproduction, as transpiration and apparently normal physiological behavior continued after pre-dawn water potentials declined below turgor loss limits inferred from rehydrated leaves. These findings indicate that Q. garryana is a water-limitation-tolerant tree species that maintains hydraulic function as subsurface water supply and atmospheric demand conditions exceed the ranges at which P. menziesii can operate. These observations can be used to explain Q. garryana's extant species range and anticipate its likely resilience in a warmer climate.

Lithologically Controlled Subsurface Critical Zone Thickness and Water Storage Capacity Determine Regional Plant Community Composition

“…This could suggest that the higher tree canopy cover on north‐facing slopes at both sites indicates that vegetation is more water limited than energy limited. However, fires were common at both sites (Hahm et al, ; Johnson, ), and drier vegetation on south‐facing slopes may have burned more readily, also potentially explaining the aspect‐related canopy cover differences.…”

“…At both sites, augering and drilling observations of roots, as well as repeat neutron probe measurements (Hahm, Rempe, et al, ; Rempe & Dietrich, ), indicate that plants use soil and rock moisture. Sap flow rates in Oregon white oak in the Central Belt remain high throughout the summer dry season, declining to only 70–90% of their maximum with shorter day lengths in the autumn (Hahm et al, ). In the Coastal Belt, madrone—and to a lesser degree live oak—similarly transpire at high rates in September, whereas Douglas fir exhibits greater decline during the summer (Link et al, ).…”

“…garryana , is also well suited to these conditions. It is winter deciduous, with a leaf‐off period that closely matches the sustained wet‐season subsurface saturation (Hahm et al, ) and is known to inhabit riparian areas elsewhere prone to saturation (Stein, ). Q .…”

“…In contrast, Sagehorn, in the Central Belt, is predominantly inhabited by winter deciduous oaks and annual grasses. The water limitation‐tolerant Oregon white oak (also known as Garry oak; Quercus garryana ) is the dominant species (Hahm et al, ) and is concentrated with higher density on north‐facing slopes, occasionally forming woodlands with contiguous canopy. Dense evergreen forest areas without an herbaceous understory are found on large sandstone blocks, dominated by Pacific madrone and Douglas fir.…”

Quantification of the seasonal hillslope water storage that does not drive streamflow

“…The shallow depth to fresh bedrock and the consequent winter saturation and rapid water delivery to channels leave little storage to sustain summer baseflows, causing Dry Creek to stop flowing typically by the end of May or beginning of June. Despite receiving annual rainfall totals similar to Elder Creek, Dry Creek's small stores of subsurface water are insufficient to support dense forest, and the dominant land cover type is winter‐deciduous oak annual grassland savanna (Hahm, Dietrich, et al, ; Hahm, Dralle, et al, ; Hahm et al, ). The sandstone blocks suspended in the mélange support patches of mixed broadleaf–needleleaf evergreen forest similar to those found in the Elder Creek on the Coastal Belt (Hahm, Dralle, et al, ; Lovill et al, ).…”