Limits to understory plant restoration following fuel-reduction treatments in a piñon-juniper woodland

Adaptive management of shrub-encroached woodlands: assessing the short-term impact of alternative mastication techniques on fuel and biodiversity

Woody shrub encroachment is a growing problem in many ecosystems across the globe, causing declines in biodiversity and elevated fire risk. Mastication can be used to remove the dense shrub layer, however decisions regarding the optimal mastication technique are hampered by a lack of information about how different mastication techniques influence fuel, vegetation structure and floristic composition. We applied an adaptive management approach to trial various mastication techniques. The trial was co-designed with land managers and conducted in eucalypt woodlands of southeastern Australia which have a dense understorey of Coast Wattle (Acacia longifolia var. sophorae). We tested two techniques, a high intensity (greater mechanical processing) and low intensity (less mechanical processing) treatment. We measured surface fuel, vegetation structure and floristic diversity pre-mastication, then 1-, 6- and 12-months post-mastication. The high intensity technique produced higher surface fuel loads compared to the low intensity technique. By 12-months post-mastication, cover of short (<0.3 m tall) vegetation had recovered to pre-treatment levels, with no difference between techniques. Both techniques yielded higher native species richness and diversity than the control, driven mainly by increases in native forb species. The low intensity technique was faster and less expensive and had similar impacts on floristic diversity but had lower surface fuel load. Therefore, we recommend the application of the low intensity technique to manage shrub encroachment. Our study used adaptive management to understand the short-term (1-year) impacts of different mastication techniques. Future trials (with additional treatments) and ongoing monitoring are planned to further improve our understanding of how to best to manage shrub encroachment.

Understory Vegetation Change Following Woodland Reduction Varies by Plant Community Type and Seeding Status: A Region-Wide Assessment of Ecological Benefits and Risks

Woodland encroachment is a global issue linked to diminished ecosystem services, prompting the need for restoration efforts. However, restoration outcomes can be highly variable, making it difficult to interpret the ecological benefits and risks associated with woodland-reduction treatments within semiarid ecosystems. We addressed this uncertainty by assessing the magnitude and direction of vegetation change over a 15-year period at 129 sagebrush (Artemisia spp.) sites following pinyon (Pinus spp.) and juniper (Juniperus spp.) (P-J) reduction. Pretreatment vegetation indicated strong negative relationships between P-J cover and the abundance of understory plants (i.e., perennial grass and sagebrush cover) in most situations and all three components differed significantly among planned treatment types. Thus, to avoid confounding pretreatment vegetation and treatment type, we quantified overall treatment effects and tested whether distinct response patterns would be present among three dominant plant community types that vary in edaphic properties and occur within distinct temperature/precipitation regimes using meta-analysis (effect size = lnRR = ln[posttreatment cover/pretreatment cover]). We also quantified how restoration seedings contributed to overall changes in key understory vegetation components. Meta-analyses indicated that while P-J reduction caused significant positive overall effects on all shrub and herbaceous components (including invasive cheatgrass [Bromus tectorum] and exotic annual forbs), responses were contingent on treatment- and plant community-type combinations. Restoration seedings also had strong positive effects on understory vegetation by augmenting changes in perennial grass and perennial forb components, which similarly varied by plant community type. Collectively, our results identified specific situations where broad-scale efforts to reverse woodland encroachment substantially met short-term management goals of restoring valuable ecosystem services and where P-J reduction disposed certain plant community types to ecological risks, such as increasing the probability of native species displacement and stimulating an annual grass-fire cycle. Resource managers should carefully weigh these benefits and risks and incorporate additional, appropriate treatments and/or conservation measures for the unique preconditions of a given plant community in order to minimize exotic species responses and/or enhance desirable outcomes.

Long-term vegetation responses to pinyon-juniper woodland reduction treatments in Nevada, USA

Expansion of native pinyon-juniper (Pinus monophylla-Juniperus osteosperma) woodlands can decrease shrub and herbaceous cover in the Intermountain West, U.S., affecting habitat quality and biodiversity. Removing pinyon-juniper woodlands in former sagebrush ecosystems to increase understory cover has a long management history, and short- and long-term monitoring reveal different understory plant community responses. We revisited a 500 mm average precipitation site in the sagebrush steppe of western Nevada, 32 years after three types of tree thinning treatments and seeding had occurred in a mature, closed-canopy woodland. We measured vegetation foliar cover and density within plots arranged in a 3-block randomized design. We found significantly lower cover of P. monophylla in treated plots (average of 2-8%), relative to controls (32%). However, P. monophylla seedlings (<0.5 m tall) were detected throughout all plots (average of 86-160 trees/ha in treated plots, 111 in controls). Cover of perennial graminoids and shrubs was higher in all treatments (600-870% higher grass cover and 470-570% higher shrub cover) than controls. Cover of invasive annual species, primarily Bromus tectorum, was highly variable and not significantly different among plots, but B. tectorum had the highest cover of all species in two of the three woodland removal treatment types. Control plots contained significantly larger perennial canopy gaps compared to all treatments (average of 318 cm vs. 104-133 cm), and had significantly more woody litter cover than clear cut plots (average of 14% vs. 3%). These results suggest tree thinning and removal in tree dominated woodlands can increase shrub and perennial grass cover and reduce litter and canopy gaps, especially in conjunction with seeding, but that tree recolonization over the long-term is inevitable. Perennial forbs did not respond well to treatments (<1% average foliar cover in all plots), and seeding or other treatments may be needed to improve their response. Further, if tree seedlings survive, these plots will likely return to tree dominance without additional treatments.