Stand structure and aridity alter tree mortality risk in Nevada s PJ woodlands Sam Flake and Peter Weisberg Dept. of Natural Resource and Env. Science, Univ. Nevada, Reno 10/12/2016 Pinyon-Juniper Symposium Albuquerque, NM
Drought and woodland mortality
Predicting drought resistance High mortality Low mortality
Questions 1. Where (what environmental factors) is mortality greatest? 2. What tree characteristics are associated with mortality? 3. Are stand structure effects consistent with competition?
Abiotic environment
Biotic factors Age Size
Stand structure Changes through time Manipulated through management Feedbacks (density-dependent mortality)
Reduced Mortality Feedbacks and stability Negative feedback with negative interactions (competition) Positive feedback with positive interactions (facilitation) High density Increased Mortality Mortality Reduced density Reduced competition Reduced facilitation Reduced density
Past density studies Greenwood and Weisberg (2008). Forest Ecology and Management
Recent Nevada drought 50 years since last severe drought
Study design 2005 2015
102 plots in 26 clusters 45 original clusters Study area
Nevada PJ woodlands
Sparse, patchy trees
Closed canopy, more even distribution
Stand structure measurements Total trees measured (resampled): Pinyon: 3672 (2545) Juniper: 1926 (1419) Mahogany: 162 (120) (not analyzed) 4m local-neighborhood basal area Nearest-neighbor distance Ʃ Local-neighborhood basal area Nearest-neighbor distance
Widespread canopy loss and tree mortality between 2005 and 2015 Pinyon: 10.9% mortality and 23% canopy loss Juniper: 0.6% mortality and 10% canopy loss Defoliation versus beetle mortality
Dramatic declines in tree health on dry sites Pinyon dieback Pinyon mortality Juniper dieback
Stand structure effects consistent with competition Pinyon dieback Pinyon mortality Juniper dieback
Stand structure Differences between uncommon severe drought and common droughts in spatial pattern of mortality? When should we expect positive densitydependence in mortality? Intensity: moderate drought Cause: driven by water limitation
Drought resistance summary More resistant Junipers Mesic climate Small diameter trees Shallow soil Low basal area Widely spaced trees Less resistant Pinyon Arid climate Large diameter trees Deep soil High basal area Closely spaced trees
Future of Nevada s woodlands More junipers, smaller pinyons Arid sites suceptible to future drought Greater resilience at higher elevations, shallower soils (where old growth is found) Negative feedback with continued drought? Understory reinitiation?
Silviculture to enhance resilience? Thinning changes basal area and tree spacing Small effect compared to environmental changes Potential decrease in mortality from ~10% to ~5%
Managing past canopy closure Phase 1 High resistance Phase 2 Moderate resistance Phase 3 Low resistance Severe drought Phase 4? Transition to old-growth structure Understory reinitiation
Summary Widespread defoliation and mortality Arid sites, dense patches Long-term consequences of drought Effects of repeated severe drought Feedbacks: drought legacies (tree-scale) and drought hardening (stand-scale)
Acknowledgements Great Basin Landscape Ecology Lab: Ali Urza, Miranda Redmond, Tom Dilts Field and lab assistants: Elana Ketchian, Jonathan Vivet, Annie Baker Beth Leger, Steve Vander Wall Ben Sullivan USDA Hatch Project accession #1003021
Questions?
Implications for wildlife Transition to more juniper and smaller pinyon pine pinyon specialists more impacted Likely decrease in pine nuts in individual trees (reduced leaf area) as well as stands (reduced live pinyon) Species that use lower-elevation ecotones more impacted Changes in understory Changes to stand structure and habitat heterogeneity Changes to tree architecture