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ARS Home » Plains Area » Temple, Texas » Grassland Soil and Water Research Laboratory » Research » Publications at this Location » Publication #328694
Title: Canopy foliation and area as predictors of mortality risk from episodic drought for individual trees of Ashe juniper

Author
item Polley, Herbert
item JOHNSON, DANIEL - University Of Idaho
item JACKSON, ROBERT - University Of Idaho

Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2016
Publication Date: 7/26/2016
Publication URL: http://handle.nal.usda.gov/10113/5421328
Citation: Polley, H.W., Johnson, D.M., Jackson, R.B. 2016. Canopy foliation and area as predictors of mortality risk from episodic drought for individual trees of Ashe juniper. Plant Ecology. 217(9):1105-1114.

Interpretive Summary: Drought is killing an increasing number of trees globally with significant impacts on ecosystem carbon balance and hydrology, but it remains difficult to predict which or how many trees of a given species are especially vulnerable to drought mortality. We sought to identify readily-measureable traits of living individuals that could be used to estimate mortality risk of Ashe juniper (Juniperus ashei) trees under drought. Ashe juniper is the most abundant woody species in the Edwards Plateau region in west-central Texas, USA. We analyzed tree rings from both living and dead trees to determine growth rate prior to an historic drought in 2011, and measured morphological, physiological, and stand-level variables hypothesized to link growth rate and mortality risk. Slowly-growing trees were especially vulnerable to drought. Drought killed >85% of trees for which pre-drought growth dipped below a threshold value. Slowly-growing trees, in turn, had reduced leaf area and larger than average canopies. By contrast, growth of juniper individuals did not depend on the amount of shading by neighboring trees, soil depth, or plant water status. Mortality risk of Ashe juniper in the Edwards Plateau region can be estimated from non-destructive measurements of leaf and canopy area of individual trees by using the relationships among risk, growth, and leaf and canopy area that we report.

Technical Abstract: Drought is killing an increasing number of trees globally, yet mortality risk remains difficult to predict at fine spatial scales. We sought to identify readily-measureable metrics of living individuals that could be used to estimate mortality risk of Ashe juniper (Juniperus ashei) trees under episodic drought. Ashe juniper is a keystone woody species in the Edwards Plateau region in west-central Texas, USA. We analyzed tree rings from both living and dead trees to determine growth rate prior to an historic drought in 2011, and measured morphological, physiological, and stand-level variables hypothesized to link growth rate and mortality risk. Slowly-growing trees were disproportionately vulnerable to mortality. Fractional mortality of sampled trees was significantly related to a size-standardized index of growth, deviation in the mean pre-drought basal area increment (BAI) per tree from the pre-drought BAI of minimally-stressed trees growing on deep soil (=BAI90 – BAI). Slowly-growing trees, in turn, had sparsely-foliated canopies. BAI90 – BAI was positively correlated to two variables: the difference between leaf area per unit of projected canopy area per tree (LA) and the LA of minimally-stressed trees and the difference between projected canopy area (CA) and the mean CA of comparably-sized trees. By contrast, there was no correlation between growth of living trees and light interception by neighboring trees, soil depth, or two functional metrics, the stem-leaf ' gradient and leaf light use efficiency. Mortality risk in Ashe juniper populations in the Edwards Plateau region can be estimated from non-destructive measurements of leaf and canopy area of individual trees by using relationships among risk, growth, and leaf and canopy area.