Projected drought effects on the demography of Ashe juniper populations inferred from remote measurements of tree canopies

Authors: Polley, Herbert; JOHNSON, DANIEL - University Of Georgia; JACKSON, ROBERT - Stanford University

Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2018
Publication Date: 8/24/2018
Citation: Polley, H.W., Johnson, D.M., Jackson, R.B. 2018. Projected drought effects on the demography of Ashe juniper populations inferred from remote measurements of tree canopies. Plant Ecology. 219(10):1259-1267. https://doi.org/10.1007/s11258-018-0876-5.
DOI: https://doi.org/10.1007/s11258-018-0876-5

Interpretive Summary: Droughts are killing an increasing number of trees worldwide and are anticipated to increase in frequency or severity. The impact of tree mortality on ecosystem processes, including hydrology and carbon storage, depends on both the number and sizes of trees that die. Yet, there is limited capacity to predict how many or which sizes of trees might die because of limited capacity to predict mortality risk for individual trees. We used an airborne sensor to remotely measure leaf area of living individuals of Ashe juniper at three sites in central Texas, USA in which mature juniper trees (> 4 m height) are dispersed within grassland vegetation (juniper savanna). Ashe juniper is the dominant woody species in central Texas having increased in density during the last century following overgrazing and fire suppression. Remote measurements were used to evaluate tree number (frequency) and mortality risk as a function of tree size at each site. We used the difference between the leaf area per measured tree and that of a similarly-sized individual with near maximal leaf area to assess mortality risk. Risk was determined by using correlations between leaf area and mortality measured during a prior drought. We found that most mature juniper trees in savannas were of intermediate size, with fewer trees that were either smaller or larger. Mortality risk from drought exceeded 25% of all juniper trees, but was highest for trees of intermediate and larger size. Results imply that severe drought could kill a substantial fraction of large Ashe juniper trees in Texas savannas with likely impacts on ecosystem hydrology and carbon storage. More generally, our analysis demonstrates a novel use of remote measurements of tree canopies to link mortality risk from drought to the size structure of populations of a key woody species.

Technical Abstract: Tree mortality from drought is anticipated to increase as climate change promotes more frequent or severe water limitation. Ecosystem impacts of woody mortality depend on both the number and sizes of trees that die, but a limited capacity to predict mortality risk for individual trees hinders the capacity to forecast drought effects on tree population demography and ecosystem processes. We remotely measured leaf area of living individuals of Ashe juniper trees at three savanna sites in central Texas, USA to characterize the frequency-size distribution (FSD) of juniper populations and evaluate mortality risk from drought as a function of tree size. Mortality risk was assessed from the deviation in leaf area per tree from that of a similarly-sized individual with near maximal leaf area using correlations among leaf area, growth rate, and mortality measured during a prior drought. We found that the FSD of juniper trees is bell-shaped at each site. Mortality risk from drought exceeded 25% of emergent (> 4 m height) trees in savanna juniper populations, but was highest for trees of intermediate and large size. Mortality risk was greatest at a grazed savanna, exceeding 50% of trees with projected canopy area > 20 m2. Results imply that severe drought could kill a large fraction (18-85%) of intermediate- to large-sized Ashe juniper trees in central Texas savannas. Our analysis demonstrates a novel use of remote measurements of canopy foliation to link mortality risk from drought to the demography of Ashe juniper populations through properties of individual trees.