Controls on gross production by a semiarid forest growing near its warm and dry ecotonal limit

Aaron W Fellows,Michael L Goulden

doi:10.1016/j.agrformet.2012.10.001

Aaron W Fellows, Michael L Goulden

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https://doi.org/10.1016/j.agrformet.2012.10.001

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Journal: Agricultural Meteorology	Publication Date: Nov 27, 2012
Citations: 16	License type: cc-by

Affiliation: University of California, Irvine

Abstract

Climate change is expected to move the spatial patterns of temperature and water availability poleward and upslope, with concomitant shifts in vegetation distribution. Vegetation growing near its southern or low-elevation range limit may prove especially susceptible to mortality and displacement. We investigated the controls on Gross Primary Production (GPP) by an oak and pine stand located just above the lower forest limit in Southern California's San Jacinto Mountains. The local climate was montane Mediterranean, and the stand experienced extensive mortality in the early 1990s and from 2002 to 2004 coincident with extended droughts. The maintenance of high rates of CO2 uptake in winter, and access to water deep in the soil column in summer, allowed for a year-round growing season. The evergreens at the site remained photosynthetically active year-round despite frequent freezing nights. High rates of CO2 uptake were observed at air temperatures below 8°C, which is colder than has been reported for other ecosystems. Winter cold exerted a minor limitation on GPP, and winter warming would have a small effect on GPP. Vegetation withdrew water from the soil, saprolite, and fractured granitic bedrock to support transpiration and CO2 uptake during the dry summer, which further expanded the growing season. Access to a reliable supply of moisture deep in the soil and regolith appears critical for the survival of large trees at the site. These trees may prove vulnerable to climate change if increasing evaporation rates or interannual precipitation variability causes a more frequent or severe depletion of deep regolith moisture.

Highlights

Air temperatures in the Southwestern United States are projected to rise 1.5–4.5 ◦C by 2070–2099 with global climate change (Cayan et al, 2008)
Vegetation growing near its southern or low elevation range limit may prove especially vulnerable to mortality and displacement with climate change, underscoring the need to better understand the mechanisms that contribute to the resilience, resistance and persistence of these ecosystems
We focused on forest CO2 uptake (referred to as Gross Ecosystem CO2 Exchange at half-hourly intervals (GEE), and Gross Primary Production at annual intervals (GPP)), which is presumably linked to tree survival, persistence, and stress evasion through the availability of carbohydrates for growth and the avoidance of stomatal closure (Adams et al, 2009)

Summary

Introduction

Air temperatures in the Southwestern United States are projected to rise 1.5–4.5 ◦C by 2070–2099 with global climate change (Cayan et al, 2008). Widespread tree mortality was observed throughout western North America in the last decade coincident with severe drought and warmer temperatures (Allen et al, 2010; Breshears et al, 2005; Raffa et al, 2008; Walker et al, 2006).

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