Resiliency or restoration: management of sudden oak death before and after outbreak

Abstract

Forests at risk to diseases caused by invasive Phytophthora pathogens can be grouped into two broad classes: those already invaded by the focal pathogen where disease has emerged or those at significant risk of invasion and subsequent emergence of disease. This dichotomy represents distinct management scenarios -treating after or before disease emerges -with a set of epidemiological, ecological, and practical management characteristics that determine optimal actions and associated costs. Here we present the initial outcomes from two management experiments aimed at restoring forest structure or protecting against changes to forest structure following invasion of Phytophthora ramorum, the cause of sudden oak death (SOD). We conducted a stand-level restoration experiment on Mount Tamalpais (Marin County, CA) at three sites where the disease has killed most overstory tanoak and dramatically increased understory vegetation density and fuels. A separate experiment in an uninvaded, at risk forest was conducted near Lacks Creek, in Humboldt County. This treatment is an attempt to increase forest resiliency to catastrophic loss of tanoak, increased fuels associated with tree mortality, and densification of the understory that are expected to accompany disease in these stands within the coming decade. Restoration experiments employed two types of mastication of understory vegetation and hand-crew thinning with pile burning; resiliency experiments employed hand crews. Treatments were compared to a set of reference conditions representing overly dense stands where intervention is needed and extensively managed stands that serve as management targets. Both restoration and resiliency treatments greatly reduced density of key sporulation supporting hosts with modest-to-minimal effects on stand basal area. Prior land use, especially past harvesting, was a primary factor determining treatment costs and potential disease impacts in treated vs untreated stands. Although both restoration and resiliency management strategies are likely to reduce disease impacts, treatment costs vary substantially and will greatly influence when and where each approach is optimal.

Ongoing surveillance protects tanoak whilst conserving biodiversity: applying optimal control theory to a spatial simulation model of sudden oak death

“…dependence on the state of the local epidemic. We have found that thinning 193 of bay laurel is very important to the success of disease management, echoing results of previous modelling studies 194 [18,44]. Management advice from the US Forest Service [24] also suggests removal of bay trees, and even complete 195 area-wide removal of bay in some cases.…”

“…P. ramorum can sporulate prolifically on bay trees, but that host is not killed by the disease [17]. Field studies 61 have shown that removal of bay laurel, and thinning of susceptible trees to reduce host density, can control disease 62 spread [18]. Environmental niche models have been developed to classify SOD invasion risk [19,20], and dynamic 63 mathematical models have been used to predict SOD spread at large scales [21,22].…”

“…Only one model captures 64 dynamics at a small enough scale to show the decline of overstorey tanoak at the forest stand level ( 20 ha) [16]. 65 Studies have used this model to demonstrate the effectiveness of one-off interventions [18,23], but do not assess 66 time-dependent controls, and strategies have not been optimised in order to protect tanoak. Whilst a clear objective for control is to protect tanoak, this must not be at the cost of the overall health of the 68 forest.…”

“…234 We have shown that by repeated surveillance and re-optimisation of control, the damaging effects of the 235 worst-case scenarios of pathogen spread can be limited. As found by Cobb et al [18], disease control is only 236 effective when there is long-term commitment to management projects. Effort put into this long-term surveillance 237 has to be cost-effective though.…”

“…These decisions must be made in the context of the local area, as well as other forest management goals such as fire 221 reduction and timber production [18]. Decisions about management goals however, either locally or through larger 222 scale regulation, can lead to conflicts that may impact the effectiveness of control [45].…”