Using a tree seedling mortality budget as an indicator of landscape-scale forest regeneration security

Abstract

Securing desirable forest regeneration outcomes is an essential component of sustainable forest management. When natural reproduction is preferred over planting, achieving desirable outcomes may be the principal challenge for forest managers, as reports of struggles and even failures are common across many regions and forest ecosystems. Informing managers and policymakers of the prospects of regeneration success before practices and policies are implemented promotes long-term sustainability because rehabilitating undesirable regeneration outcomes is often lengthy, expensive, and uncertain. In 2012, the USDA Forest Service, Northern Research Station, Forest Inventory and Analysis program implemented Regeneration Indicator (RI) protocols that added detailed seedling measurements and browse impact assessments to a subsample of inventory plots across 24 states in the northern United States. The goal of this expanded sampling effort is to improve the ability to monitor broad scale regeneration trends and better inform forest management planning and policy. Modeling probable regeneration outcomes is difficult and the rarity of vetted models that can fully utilize RI inventory data highlights an immediate need for flexible methods to evaluate regeneration of different taxa at large scales. This manuscript is premised on estimation of a tree seedling mortality budget for inventoried reproduction. The method offers a transparent structure for leveraging existing literature and expert knowledge to gain provisional insight into plausible regeneration outcomes. The resulting tool provides flexibility for users to examine regeneration for multiple species, site conditions, and user-defined quantitative regeneration objectives. The approach is demonstrated by applying a suite of multispecies regeneration objectives to RI data for two case studies with different forest composition and geographic scales, the Ozark Highland Ecological Section (OHES) and the Monongahela National Forest (MNF). Within the Quercus/Carya dominated OHES, analyses indicate that desirable regeneration outcomes are more likely than not based on current plot conditions. Regeneration events were projected to produce new fully stocked forests on 76% of OHES plots, produce a sizable component of characteristic overstory species on 57%, and produce a sizable component of commercially important species on 59%. Within the Acer/Fagus/Betula and Quercus/Carya forests of the MNF, analyses indicated that difficulties in achieving desirable regeneration outcomes were likely. Only 36% of MNF plots were projected to produce new fully stocked forests following a regeneration event and only 29% were projected to regenerate a sizable component of either characteristic overstory species or commercially important species.