Suppressing Scotch broom establishment by manipulating microclimate with logging debris and herbicides

Timothy B. Harrington and David H. Peter, USFSRobert A. Slesak, University of Minnesota

3 years after debris treatments

2 weeks after debris treatments

Heavy debris Light debrisMatlock Long-Term Soil Productivity Study

Harrington & Schoenholtz CJFR 2010

Matlock Long-Term Soil Productivity Study

Dry Bed Creek StudyStudy objective: compare operational logging debris and herbicide treatments for their ability to control competing vegetation and improve performance of planted Douglas-fir.

Scotch broom in pre-harvest stand

Soil seed bank:

• 3800 viable broom seeds acre-1

Forest harvesting - November 2011

Logging debris treatments - December 2011

Light debris: 9 Mg ha-1 (4 t acre-1)

• A tracked excavator + clamshell bucket created two levels of logging debris on 0.19-ha main plots.

• Machine was restricted to designated trails between plots to isolate soil disturbance.

Heavy debris: 20 Mg ha-1 (9 t acre-1)

1. Aminopyralid: Milestone® @ 0.5 L ha-1 (7 oz acre-1) + 0.25% Syl-Tac® surfactant

2. Triclopyr ester: Forestry Garlon® XRT @ 2.9 L ha-1 (40 oz acre-1) + 2.5% SuperSpread® MSO

3. Aminopyralid + triclopyr

4. No herbicides

Herbicide treatments - August 2012Three herbicide treatments and a non-treated check were assigned to 0.04-ha split plots within each main plot:

Dry Bed Creek Study, Google Earth, August 2016

Microclimate responses

Soil temperature• During 2012-2015, soil

temperatures were 1.2°-1.5°C cooler under heavy debris (summer only).

• Debris effects NS in 2016 due to vegetation recovery.

• Temperature reductions sufficient to limit Scotch broom germination (Harrington, Weed Sci., 2009).

Soil water• During 2012-2014, more

soil water under heavy debris.

• Debris effects NS during 2015-2016 due to intense droughts and vegetation recovery.

• More mesic conditions under heavy debris favor native species?

Light environment

Measuring PAR under different debris levels

Etiolated seedling development under heavy debris

Lightdebris

Heavydebris

Lightdebris

Heavydebris

Vegetation responses

July

201

2Ju

ly 2

014

Heavy debris, no herbicides Light debris, triclopyr

Scotch broom seedling density

• Seedling density varied 8-fold between debris levels.

• Largest increase: 2nd

year after forest harvesting.

0.1-m2 frame for estimating broom seedling density

Plant cover responses to logging debris

Scotch broom

Hairy catsear

Trailing blackberry Trailing snowberry

Heavy debris facilitated:

• 71% decrease in Scotch broom cover.

• 63% increase in cover of native vines.

Plant cover responses to herbicides

Serviceberry

Salal

Trailing snowberry Trailing blackberry

• Herbicide effects < debris effects, in general.

• Triclopyr reduced woody cover, especially in combination with aminopyralid.

• Herbicides reduced Scotch broom cover but differences NS.

Douglas-fir survival responses to logging debris

• Overall, Douglas-fir survival averaged 8 percentage points greater in heavy debris than in light debris.

• Considerable mortality in 2015 – a year of low growing-season precipitation.

Longer-term responses

Sustained vegetation responses to debris

Changes in logging debris mass

• 5 years after forest harvesting, mass in heavy debris similar to initial level in light debris.

• Heavy debris treatment: ≈5 years of increased wildfire risk.

Thanks to our collaborators and sponsors!

USFS S&PF