“Near term response of surface soil nitrogen cycling and pools to forest clearing and burning” Heather E. Erickson Recent soils research from the Teakettle Experimental Forest, Sierra Nevada Mountains, California

Background: Fire suppression over the last century has altered western forest ecosystems (increased tree densities, changed species compositions, and increased fuel loading) To return forest stands to “pre-suppression” conditions and to reduce the likelihood of catastrophic fires, fire and cutting are being re- introduced

Background: Nitrogen is a critical element in many forest ecosystems: likely influences post-disturbance successional dynamics and recovery of forest primary productivity and other key processes Given that forest restoration will occur over large areas, important to know how soil N (and C) pools and N transformations are affected by burning and cutting treatments

Experiment at Teakettle: Designed to assess ecosystem effects of burning and thinning in mixed-conifer old growth forest (2200 m, west slope Sierra Nevada mountains) Large (4 ha) plots afforded opportunity to examine effects of restoration treatments on soil processes

What others have found: Total N (and C) in forest soils either increase, decrease or remain the same after fire Inorganic N increases after fire but remains elevated for usually less than a year Wan et al. 2001, Ecol Appl

Teakettle is a typical mixed-conifer forest Conifers in discreet patches (CC) Shrub thickets (Ceanothus cordulatus) (CECO) Open areas mostly lacking vegetation (OC) CC OC CECO

Pre-treatment soils data (0-15 cm) show patches matter Under CECO, total N pools ~ 30% greater and net N mineralization more than double compared to other patch types Under OC, soils slightly more acidic, have a higher bulk density, and greater rock fraction percentage compared to other patch types Erickson et al., in revision, Forest Science

Main research questions What are the effects of restoration treatments on N (and C) pools and N dynamics? Do differences among 3 patch types (CC, CECO, OC) carry over after restoration treatments?

4 Treatments (codes): burning (B) burning plus light cutting (BC) burning plus heavy cutting (BS) and an un-restored control (U) 2 (4 ha) plots for each treatment

200 m 3 patch types 3 reps per type 6 sub-samples per patch Single composite sample per patch 9 patches x 8 plots = 72 patches = CC= CECO= OC * * * * * *

ANOVA Table (Split plot) Source of variationdf Treatment 3 Plot (Treatment) 4 Patch 2 Patch x Treatment6 Patch x Plot (Treatment) 8 Residual48 Total71

Restoration treatments completed in 2001 Mineral horizon soils (0-15 cm) 1 and 2 years post-treatment Total N and C (dry combustion), inorganic N, pH (water), bulk density

Treatment effects (soil pools): Burning with cutting increased total N a a b b a a bb N

Treatment effects (soil pools): Burning with cutting increased total N and C a aa a b b a a a b b b a a bb NC

Patch effects (soil pools): Total N varied by patch pre- and post-treatment a a b Total N a a a ab b b Patch NS

Treatment effects (soil pH): Burning with cutting increased pH a a aa b b b b

Patch effects (soil pH): OC patches were the most acidic pre- and post-treatment a a a b b a

Treatment effects (inorganic N): Burning with cutting increased NH 4 -N NH 4 -N T x P *

Treatment effects (inorganic N): Burning with cutting increased NH 4 -N and NO 3 -N NH 4 -NNO 3 -N T x P *

Interaction in 2003 suggests effect of treatment depends on patch a b a b b c c c c c c

a b a b b c c c c c c High nitrate in soils under CECO could be due to new N from fixation

Conclusions Burning and cutting treatments (BC, BS) significantly increase soil N pools, C pools, pH, and inorganic N relative to controls Burning alone (B) does little to affect soil properties Near-term effects are noted two years post-treatment Previous patch type also explains post- treatment variation in soil properties

Acknowledgements Malcolm North USFS, PSW Range and Experiment Station International Institute of Tropical Forestry Model Institutions for Excellence (NSF) grant to Universidad Metropolitana Undergraduates from Universidad Metropolitana