Sustaining plantation forest productivity in Sumatra over three decades: From acacias to eucalypts

The pulp and paper industry in Sumatra, Indonesia, completely changed its key plantation forest species during 2012-2017 from their previous mainstay, Acacia mangium (which had become severely damaged by diseases) to Eucalyptus pellita and related hybrids. This rapid wholesale change posed major challenges to management and ongoing wood production. We present here long-term experimental results and operational pre-harvest inventory of the two species covering five successive rotations, spanning three decades. This is the first such report of longterm, multi-rotation productivity trends for plantations from a tropical equatorial environment. Highlights include: (i) the trends in productivity in the experiments and inventory are parallel and were characterised by an initial increase, followed by a decrease and then recovery of productivity in response to ecosystem stress, species change and management interventions, (ii) the growth rates of E. pellita are significantly lower than those of A. mangium across soils and sites (iii) the conservation of site organic matter during the inter-rotation phase is as critical for eucalypt as it was for acacia, (iv) the response to applied P is more widespread and stronger in E. pellita than in A. mangium, and (v) significant production losses are occurring due to high tree mortality. We also highlight some of the emerging issues warranting attention. Productivity varies widely across sites, so does the relative responses to management inputs in this environment. The reasons for this need to be better understood so that site-specific management practices can be applied. For example, the depth at which an impeding horizon occurs in some soil profiles is a strong factor influencing growth, regardless of species. The legacy effects of A. mangium, especially of fixed N and nutrient cycling are benefitting eucalypts. The adoption of research outcomes by managers have enabled gains of 8-19% in the productivity of eucalypts planted in recent years compared to that obtained in the first eucalypt rotation. Further improvements are possible through adaptive management, guided by both experimental results and inventory, aimed at gains in operationally realisable productivity. The companies are committed to using only plantation grown wood for processing and are not expanding their own land base allocated for production. The imperative, therefore, is to focus on sustainably increasing the productivity per unit area on their existing land management units that are now under the fifth or sixth rotation.