Walter G. Thies

Season and severity of prescribed burn in ponderosa pine forests: Implications for understory native and exotic plants

Abstract We investigated herbaceous richness and cover in relation to fire season and severity, and other variables, five growing seasons following prescribed fires. Data were collected from six stands consisting of three randomly applied treatments: no burn, spring burn, and fall burn. Fall burns had significantly more exotic/native annual/biennial (an/bi) species and greater cover of these species (6.5% exotic; 1.7% native) compared to spring and unburned areas. These patterns are likely related to indirect fire effects associated with fire severity and resource availability, rather than direct fire effects due to burn timing. CART models indicated that high native and exotic an/bi richness and cover were associated with overstory gaps and higher fire severity areas, conditions common to fall burns. Exotics may be more successful at exploiting these environments. No treatment differences were found for native perennials. Location was important for explaining native perennial patterns, but richness and cover were also positively associated with lower fire severity, greater tree cover, and coarse woody debris. Expectations for increased native perennial plant diversity and abundance following prescribed fires may not necessarily be met and exotic species spread may compromise other ecosystem attributes. Restoration in these forests presents a challenge as prescribed fires interact with present environmental conditions that are very different from historical ones.

Prediction of delayed mortality of fire-damaged ponderosa pine following prescribed fires in eastern Oregon, USA

Prescribed burning is a management tool used to reduce fuel loads in western interior forests. Following a burn, managers need the ability to predict the mortality of individual trees based on easily observed characteristics. A study was established in six stands of mixed-age ponderosa pine (Pinus ponderosa Dougl. ex Laws.) with scattered western junipers at the south end of the Blue Mountains near Burns, Oregon, USA. Stands were thinned in either 1994 or 1995. Three treatments, a fall burn, a spring burn, and an unburned control, were randomly assigned to 12-ha experimental units within each stand. Prescribed burns occurred during mid-October of 1997 or mid-June of 1998 and were representative of operational burns, given weather and fuel conditions. Within each experimental unit, six 0.2-ha plots were established to evaluate responses to the burns. Ponderosa pine plot trees (n = 3415) alive 1 month after the burns were evaluated and observed for four growing seasons. Nine fire damage and tree morphological variables were evaluated by logistic regression. A five-factor full model and a two-factor reduced model are presented for projecting probability of mortality. Significant variables in the full model included measures of crown, bole, and basal damage.

Season of prescribed burn in ponderosa pine forests in eastern Oregon: impact on pine mortality

A study of the effects of season of prescribed burn on tree mortality was established in mixed-age ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at the south end of the Blue Mountains near Burns, Oregon. Each of six previously thinned stands was subdivided into three experimental units and one of three treatments was randomly assigned to each: fall 1997 burn, spring 1998 burn, and no burning (control). Burns were conducted as operational prescribed burns. Trees within six 0.2-ha circular plots on each experimental unit were observed for four post-burn growing seasons to determine fire damage and to detect immediate and delayed mortality and occurrence of black stain root disease (BSRD). There were 5321 tagged ponderosa pines alive at the time of the burns. The percentage of ponderosa pine dying was higher after fall burns than after spring burns. Differences in percentages of fire-caused mortality may be because fall burns are inherently more severe than spring burns. Although present in many trees, BSRD appeared to have little impact on mortality. The lion’s-tail appearance, thought to be a symptom of BSRD, was found to be an unreliable indicator of BSRD in the six test stands.

Sapwood and crown symptoms in ponderosa pine infected with black-stain and annosum root disease.

Abstract Crown growth parameters, extent of disease in roots, and volatile compounds from sapwood were measured for diseased and healthy ponderosa pine (Pinus ponderosa Dougl. ex Laws.). The pathogens were black-stain root disease, caused by Leptographium wageneri var. ponderosum (Harrington & Cobb), annosum root disease, caused by Heterobasidion annosum (Fr.) Bref., or both. Samples of sapwood collected near the root collar were heated in sealed vials and volatile compounds analyzed by gas chromatography. Acetaldehyde, acetone, ethanol, methanol, and 2-propanol were quantified. Acetaldehyde was selected by logistic regression to be the best predictor for distinguishing healthy from diseased trees. Acetaldehyde concentrations were significantly lower in healthy trees. Concentrations of methanol and acetaldehyde from sapwood were positively correlated to the percentage of non-functional root area caused by disease, while length of the terminal leader and water content of sapwood from near the root collar were negatively related. Crown growth parameters did not change significantly until more than one-third of the root system was diseased. The mean ethanol concentration from diseased trees was significantly higher than that from healthy trees. About half of the diseased pine contained high ethanol concentrations, with substantial variation in concentrations among cardinal positions sampled around the root collar. While high ethanol concentrations were associated with diseased trees, low ethanol concentrations were not a good indicator of health. All volatiles, except ethanol, were partially or entirely generated by thermal decomposition of sapwood constituents when heated in sealed vials during analysis. Nevertheless, some may be useful chemical markers for identifying trees with root disease before the disease reaches an advanced stage. Also, they might be useful in estimating what proportion of a root system is diseased and non-functional.

Responses of soil foodweb organisms in the first year following clearcutting and application of chloropicrin to control laminated root rot

Abstract Alteration in soil foodweb organism populations (numbers, biomass and activity) were monitored following clearcutting of a mature 70-year old Douglas-fir forest and compared with an uncut mature Douglas-fir stand. Changes in soil foodweb organisms were assessed following application of chloropicrin to stumps in the clearcut to reduce infection by Phellinus weirii, which causes laminated root rot. The effect of chloropicrin was measured using two different spatial scales: (1) around treated stumps, and (2) within 0.2 ha areas where all stumps within the area were treated with chloropicrin. A 10-fold decrease in total fungal biomass, between 2- and 3-fold reductions in total bacterial biomass, a 2-fold reduction in nematode numbers and a nearly 10-fold reduction in the ratio of total fungal to total bacterial biomass were observed between 9 months to a year following clearcutting. Active fungal biomass in the 20% chloropicrin treatment was reduced 1 year (October) after application. Reduction in amoebae numbers were observed between April and June in the 100% chloropicrin treatments, while no effects of chloropicrin on total fungal biomass, total bacterial biomass, flagellate, ciliate or nematode numbers were observed. The ratio of total fungal to total bacterial biomass was reduced as compared to the control in the 100% chloropicrin-treated infected areas in April following chloropicrin treatment. Chloropicrin decreased soil foodweb organisms in only one out of 45 random soil samples from 0.2 ha areas. In samples collected from around stumps, three points out of 180 had significant decreases in all organism numbers as compared to non-chloropicrin treated stumps. Chloropicrin-sensitive tomato and alfalfa seedlings, planted at 2 m from stumps to monitor the release of chloropicrin from stumps, indicated there was no significant release of chloropicrin in the first year following clearcutting and chloropicrin application.

Ectomycorrhizae establishment on Douglas-fir seedlings following chloropicrin treatment to control laminated-root rot disease: Assessment 4 and 5 years after outplanting

Abstract Laminated-root rot, caused by Phellinus weirii (Murr.) Gilb., is a serious disease affecting Douglas-fir and other commercially important species of conifers in northwestern North America. Recent work has shown that this fungus is successfully reduced or eliminated by the fumigant chloropicrin. However, the effect of this biocide on nontarget organisms, including ectomycorrhizae, is uncertain. Following an initial assessment of organisms 2 years after application of the fumigant, a reassessment of the establishment of ectomycorrhizae on Douglas-fir seedlings was undertaken 4.5 and 5.5 years following chloropicrin application. Our findings show that in areas around stumps treated with 20% and 100% of the labeled dosage and in areas around non-treated stumps, chloropicrin did not adversely affect the formation of ectomycorrhizae on young Douglas-fir seedlings by naturally occurring fungi. No significant effect on the abundance or type of mycorrhizae were detected. On this site, chloropicrin did not affect these mycorrhizal associations, at least for 5 years following application.

Validating the Malheur model for predicting ponderosa pine post-fire mortality using 24 fires in the Pacific Northwest, USA

Fires, whether intentionally or accidentally set, commonly occur in western interior forests of the US. Following fire, managers need the ability to predict mortality of individual trees based on easily observed characteristics. Previously, a two-factor model using crown scorch and bole scorch proportions was developed with data from 3415 trees for predicting the probability of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) mortality following prescribed fire. Here, we report validation of that model for broader application using data from 10 109 ponderosa pines in 17 prescribed fires and 7 wildfires, observed for 3 years post-fire, from east of the Cascade Range crest in Washington, Oregon and northern California. The overall rate of correct classification was 87.1% and the rate of correctly predicting mortality was 80.1%. Similar accuracy is reported when testing the model for small trees (<53.3-cm diameter at breast height), wildfire, prescribed fire, and when using a field guide that simplifies application of the model. For large trees (≥53.3-cm diameter at breast height), the overall rate of correct prediction was 93.6% and the rate of correctly predicting mortality was 65.2%. These results suggest the Malheur model is useful for predicting ponderosa pine mortality following fires in this region.