The disturbance and destruction of the old-growth forests of the tropics continue to monopolize attention in international fora and the popular media, but a steadily growing land area is covered by secondary forest developing on sites which have been deforested and then abandoned by their owners. The natural process of secondary forest succession offers hope that the unique combination of goods and services provided by the original old-growth forests may be at least partially recovered. An enormous number of questions concerning secondary tropical forests and their potential role in sustainable land management and biodiversity conservation remain to be answered, however. Many of these questions are biological and ecological: What are the factors that bring about successional change in vegetation? How does biodiversity change during succession, does its similarity to the biodiversity of old-growth forests increase over time and why, or why not? How might secondary forests be manipulated to optimize their value for a given set of management objectives? Many more questions nevertheless concern people and their actions: what factors bring about land abandonment? How are secondary forests perceived and utilized by rural people? What market or policy changes may contribute to a more profitable and sustainable use of secondary forests? This volume contains 16 papers presented at a conference which brought together researchers concerned with biological, ecological, social/organizational, financial/economic and political aspects of secondary forests and their management with a strictly neo tropical focus. Although the biophysical side of secondary forest research dominated this conference, it is becoming clear that sound management of this resource will depend on interdisciplinary approaches.
Tag: Forest ecology
Positive feedbacks and alternative stable states in forest leaf types
The emergence of alternative stable states in forest systems has significant implications for the functioning and structure of the terrestrial biosphere, yet empirical evidence remains scarce. Here, we combine global forest biodiversity observations and simulations to test for alternative stable states in the presence of evergreen and deciduous forest types. We reveal a bimodal distribution of forest leaf types across temperate regions of the Northern Hemisphere that cannot be explained by the environment alone, suggesting signatures of alternative forest states. Moreover, we empirically demonstrate the existence of positive feedbacks in tree growth, recruitment and mortality, with trees having 4–43% higher growth rates, 14–17% higher survival rates and 4–7 times higher recruitment rates when they are surrounded by trees of their own leaf type. Simulations show that the observed positive feedbacks are necessary and sufficient to generate alternative forest states, which also lead to dependency on history (hysteresis) during ecosystem transition from evergreen to deciduous forests and vice versa. We identify hotspots of bistable forest types in evergreen-deciduous ecotones, which are likely driven by soil-related positive feedbacks. These findings are integral to predicting the distribution of forest biomes, and aid to our understanding of biodiversity, carbon turnover, and terrestrial climate feedbacks.
Ecological criteria and indicators for tropical forest landscapes: challenges in the search for progress
In the quest for global standards, “Criteria and Indicators” (C&I) are among the foremost mechanisms for defining and promoting sustainable tropical forest management. This paper examines some challenges posed by this approach, focusing on examples that reflect the ecological aspects of tropical forests at a management-unit level and assessments such as those required in timber certification. C&I can foster better forest management. However, there are confusions and tensions to reconcile between general and local applications, between the ideal and the pragmatic, and between the scientific and the democratic. To overcome this requires a sober appraisal of what can realistically be achieved in each location and how this can best be promoted. Good judgment remains the foundation of competent management. Data can inform this judgment, but an over-reliance on data collection and top-down bureaucratic interventions can add to problems rather than solving them. These arguments stress compromise, planning, guided implementation, and threat preparedness. Importance is also placed on skills and institutions: the building blocks of effective forest management. The authors suggest some options for improving forest management. Although a wider discussion of these issues is necessary, procrastination is harmful. Action is needed.
Tropical tree growth driven by dry-season climate variability
Interannual variability in the global land carbon sink is strongly related to variations in tropical temperature and rainfall. This association suggests an important role for moisture-driven fluctuations in tropical vegetation productivity, but empirical evidence to quantify the responsible ecological processes is missing. Such evidence can be obtained from tree-ring data that quantify variability in a major vegetation productivity component: woody biomass growth. Here we compile a pantropical tree-ring network to show that annual woody biomass growth increases primarily with dry-season precipitation and decreases with dry-season maximum temperature. The strength of these dry-season climate responses varies among sites, as reflected in four robust and distinct climate response groups of tropical tree growth derived from clustering. Using cluster and regression analyses, we find that dry-season climate responses are amplified in regions that are drier, hotter and more climatically variable. These amplification patterns suggest that projected global warming will probably aggravate drought-induced declines in annual tropical vegetation productivity. Our study reveals a previously underappreciated role of dry-season climate variability in driving the dynamics of tropical vegetation productivity and consequently in influencing the land carbon sink.