Michael A. Nullet

Transpiration in a small tropical forest patch

A field study was conducted of microclimate and transpiration within a 12 ha patch of advanced secondary forest surrounded by active or recently abandoned swidden fields. Differences in microclimate among stations located within and near the patch, give evidence of the effects of the adjacent clearing on the environment in the patch. Volumetric soil moisture content at the end of the dry season was lowest at the two edge sites, suggesting greater cumulative dry season evapotranspiration (ET) there than at swidden and forest interior sites. Total evaporation, based on energy balance methods, was also higher at the two edge sites than at the swidden or forest interior sites. Spatial differences in evaporation decreased as conditions became wetter. Measurements of sap flow in nine trees near the southwestern edge of the patch and nine trees in the patch interior indicate considerable variability in transpiration among the three monitored tree species, Vernicia montana, Alphonsea tonkinensis, and Garcinia planchonii. Dry-period transpiration averaged about 39 and 43% of total evaporation for edge and interior trees, respectively, increasing to 60 and 68% after the start of rains. Transpiration in both zones was well-correlated with micrometeorological conditions in the adjacent clearing, implying that transpiration edge effect is greatest when conditions are favorable for high positive heat advection from the clearing to the forest edge. Transpiration rates of well-exposed trees were higher than poorly-exposed trees, and decreased with distance from the edge at a statistically significant rate of −0.0135 mm per day m −1 . Although the results on the strength of transpiration edge effect are somewhat equivocal due to variability within the small sample, there is clear evidence that ET within the patch is influenced by the surrounding clearings. If edges experience higher ET, greater fragmentation would result in higher regional evaporative flux, which would partly compensate for the reduction in regional ET due to deforestation.

Observations of Albedo and Radiation Balance over Postforest Land Surfaces in the Eastern Amazon Basin

Abstract Regional climatic change, including significant reductions in Amazon Basin evaporation and precipitation, has been predicted by numerical simulations of total tropical forest removal. These results have been shown to be very sensitive to the prescription of the albedo shift associated with conversion from forest to a replacement land cover. Modelers have so far chosen to use an “impoverished grassland” scenario to represent the postforest land surface. This choice maximizes the shifts in land surface parameters, especially albedo (fraction of incident shortwave radiation reflected by the surface). Recent surveys show secondary vegetation to be the dominant land cover for some deforested areas of the Amazon. The characteristics of secondary vegetation as well as agricultural land covers other than pasture have received little attention from field scientists in the region. This paper presents the results of field measurements of radiation flux over various deforested surfaces on a small farm in the...

Soil‐vegetation‐atmosphere processes: Simulation and field measurement for deforested sites in northern Thailand

In recent efforts to predict the climatic impacts of tropical deforestation an extreme scenario of impoverished grassland has been used to represent the future deforested landscape. Currently, deforested areas of the tropics are composed of a mosaic of crops, bare soil, grassland, and secondary vegetation of various ages. The dominant feature of deforested land is often secondary vegetation. Parameter values for important forest replacement land covers, including secondary vegetation, have been shown to differ from those of forest much less than that assumed in general circulation model (GCM) deforestation experiments. For this study, the biosphere-atmosphere transfer scheme (BATS) is run in uncoupled mode using measured input data in place of GCM forcing and using the same parameter settings employed in recent deforestation experiments. Model output is compared with measurements taken over seven different deforested land surfaces in northern Thailand. Comparisons reveal that the simulation of deforested land overestimates reflected shortwave radiation, the diurnal range of surface temperature for secondary vegetation, surface soil moisture loss during periods without rain, and surface soil moisture increase at the start of a rainy period and underestimates net radiation, the diurnal range of surface temperature on recently used land surfaces, and root zone soil moisture increase at the start of a rainy period at most sites. Most deforested land surfaces, especially intermediate and advanced secondary vegetation, are more similar, in terms of land surface-atmosphere interaction, to the model simulation of forest than of deforested land as depicted in GCM experiments. These comparisons suggest that modelers aspiring to make realistic simulations of deforestation should adopt parameter settings representative of the diverse range of forest replacement land covers, instead of again using the grassland scenario.

Compilation of climate data from heterogeneous networks across the Hawaiian Islands

Long-term, accurate observations of atmospheric phenomena are essential for a myriad of applications, including historic and future climate assessments, resource management, and infrastructure planning. In Hawai‘i, climate data are available from individual researchers, local, State, and Federal agencies, and from large electronic repositories such as the National Centers for Environmental Information (NCEI). Researchers attempting to make use of available data are faced with a series of challenges that include: (1) identifying potential data sources; (2) acquiring data; (3) establishing data quality assurance and quality control (QA/QC) protocols; and (4) implementing robust gap filling techniques. This paper addresses these challenges by providing: (1) a summary of the available climate data in Hawai‘i including a detailed description of the various meteorological observation networks and data accessibility, and (2) a quality controlled meteorological dataset across the Hawaiian Islands for the 25-year period 1990-2014. The dataset draws on observations from 471 climate stations and includes rainfall, maximum and minimum surface air temperature, relative humidity, wind speed, downward shortwave and longwave radiation data.