Successional, spatial, and seasonal changes in seed rain in the Atlantic forest of southern Bahia, Brazil.

Seed arrival is a limiting factor for the regeneration of diverse tropical forests and may be an important mechanism that drives patterns of tree species' distribution. Here we quantify spatial and seasonal variation in seed rain of secondary forests in southern Bahia, Brazil. We also examine whether secondary forest age enhances seed dispersal and whether seed rain density and diversity in secondary forests decay with distance from mature forest. Across a chronosequence of 15 pairs of mature and secondary forests, 105 seed traps were installed and monitored for one year. We tested the effects of secondary forest age, distance from mature forest, and seasonality on monthly seed rain density, diversity, seed dispersal mode, and diaspore size. We found that secondary forest age had strong, positive effects on the diversity of seed rain, which was generally higher during the wet season. Moreover, contrasting patterns among diversity indices revealed that seeds of rare species occurred more often in 40 yr old secondary forests and mature forests. While the proportion of biotically and abiotically dispersed seeds did not change significantly with distance from mature forest across all forest age classes, we found that biotically dispersed seeds contributed disproportionately more to seed rain diversity. Our results emphasize the importance of biotic dispersal to enhance diversity during secondary succession and suggest that changes in secondary forest structure have the potential to enhance the diversity of tropical secondary forests, principally by increasing dispersal of rare species.

Ecologically asynchronous agricultural practice erodes sustainability of the Loess Plateau of China.

Sustainability of agricultural landscapes depends largely on land-use practices. As one of the most productive and widespread agricultural soils, loess is often deep and easily eroded, posing grand challenges for environmental sustainability around the world. One prime example is the Loess Plateau of China, which has been cultivated for more than 7500 years. Based on long-term data sets, this study demonstrates that the dominant agricultural practice, winter wheat cropping, continues to be the primary driver for the massive soil erosion and landscape modifications on the Loess Plateau. This traditional farming system is asynchronous with the dynamic rhythm between natural vegetation and climate in the region. In particular, the long summer fallow period for winter wheat fields is concurrent with the heavy-rainstorm season, which greatly accelerates soil erosion. Our finding indicates that common land-use practices that have lasted for thousands of years in China are not environmentally sustainable. Agriculture in this region has relied primarily on the continuous "mining" of the soil for the past several thousand years but does not have a one-thousand-year future because of myriad environmental and socioeconomic factors associated with soil erosion. To contain soil erosion and promote sustainability on the Loess Plateau, therefore, a change in the agricultural regime is needed to make sure that current and future agricultural practices follow the vegetation-climate rhythm. In addition, to achieve environmental, economic, and social sustainability in this region, multifunctional land-use planning is required to increase landscape diversity and functions (e.g., proper arrangement of crop fields, orchards, and protected areas).

Process-based models for forest ecosystem management: current state of the art and challenges for practical implementation.

Recent progress toward the application of process-based models in forestmanagement includes the development of evaluation and parameter estimation methods suitable for models with causal structure, and the accumulation of data that can be used in model evaluation. The current state of the art of process modeling is discussed in the context of forest ecosystem management. We argue that the carbon balance approach is readily applicable for projecting forest yield and productivity, and review several carbon balance models for estimating stand productivity and individual tree growth and competition. We propose that to develop operational models, it is necessary to accept that all models may have both empirical and causal components at the system level. We present examples of hybrid carbon balance models and consider issues that currently require incorporation of empirical information at the system level. We review model calibration and validation methods that take account of the hybrid character of models. The operational implementation of process-based models to practical forest management is discussed. Methods of decision-making in forest management are gradually moving toward a more general, analytical approach, and it seems likely that models that include some process-oriented components will soon be used in forestry enterprises. This development is likely to run parallel with the further development of ecophysiologically based models.