Julian Licata

Environmental sensitivity of gas exchange in different-sized trees.

The carbon isotope signature (δ13C) of foliar cellulose from sunlit tops of trees typically becomes enriched as trees of the same species in similar environments grow taller, indicative of size-related changes in leaf gas exchange. However, direct measurements of gas exchange in common environmental conditions do not always reveal size-related differences, even when there is a distinct size-related trend in δ13C of the very foliage used for the gas exchange measurements. Since δ13C of foliage predominately reflects gas exchange during spring when carbon is incorporated into leaf cellulose, this implies that gas exchange differences in different-sized trees are most likely to occur in favorable environmental conditions during spring. If gas exchange differs with tree size during wet but not dry conditions, then this further implies that environmental sensitivity of leaf gas exchange varies as a function of tree size. These implications are consistent with theoretical relationships among height, hydraulic conductance and gas exchange. We investigated the environmental sensitivity of gas exchange in different-sized Douglas-fir (Pseudotsuga menziesii) via a detailed process model that specifically incorporates size-related hydraulic conductance [soil–plant–atmosphere (SPA)], and empirical measurements from both wet and dry periods. SPA predicted, and the empirical measurements verified, that differences in gas exchange associated with tree size are greatest in wet and mild environmental conditions and minimal during drought. The results support the hypothesis that annual net carbon assimilation and transpiration of trees are limited by hydraulic capacity as tree size increases, even though at particular points in time there may be no difference in gas exchange between different-sized trees. Maximum net ecosystem exchange occurs in spring in Pacific Northwest forests; therefore, the presence of hydraulic limitations during this period may play a large role in carbon uptake differences with stand-age. The results also imply that the impacts of climate change on the growth and physiology of forest trees will vary depending on the age and size of the forest.

Belowground interactions for water between trees and grasses in a temperate semiarid agroforestry system

A fundamental hypothesis of agroforestry is the complementary use of soil resources. However, productivity of many agroforestry systems has been lower than expected due to net competition for water, highlighting the need for a mechanistic understanding of belowground interactions. The goal of this study was to examine root–root interactions for water in a temperate semiarid agroforestry system, based on ponderosa pines and a Patagonian grass. The hypotheses were: (a) A greater proportion of water uptake by pines is from deeper soil layers when they are growing with grasses than when they are growing alone; (b) Growth of grasses is improved by the use of water hydraulically lifted by pines. We used stable isotopes of O to analyze water sources of plants, and we measured sapflow direction in pine roots and continuous soil water content with a very sensitive system. We also installed barriers to isolate the roots of a set of grasses from pine roots, in which we measured water status, relative growth and water sources, comparing to control plants. The results indicated that pines and grasses show some complementary in the use of soil water, and that pines in agroforestry systems use less shallow water than pines in monoculture. We found evidence of hydraulic lift, but contradicting results were obtained comparing growth and isotope results of the root isolation experiment. Therefore, we could not reject nor accept that grasses use water that is hydraulically lifted by the pines, or that this results in a positive effect on grass growth. This information may contribute to understand the complex and variable belowground interactions in temperate agroforestry.

Bioenergy Development Policy and Practice Must Recognize Potential Hydrologic Impacts: Lessons from the Americas

Large-scale bioenergy production will affect the hydrologic cycle in multiple ways, including changes in canopy interception, evapotranspiration, infiltration, and the quantity and quality of surface runoff and groundwater recharge. As such, the water footprints of bioenergy sources vary significantly by type of feedstock, soil characteristics, cultivation practices, and hydro-climatic regime. Furthermore, water management implications of bioenergy production depend on existing land use, relative water availability, and competing water uses at a watershed scale. This paper reviews previous research on the water resource impacts of bioenergy production—from plot-scale hydrologic and nutrient cycling impacts to watershed and regional scale hydro-economic systems relationships. Primary gaps in knowledge that hinder policy development for integrated management of water–bioenergy systems are highlighted. Four case studies in the Americas are analyzed to illustrate relevant spatial and temporal scales for impact assessment, along with unique aspects of biofuel production compared to other agroforestry systems, such as energy-related conflicts and tradeoffs. Based on the case studies, the potential benefits of integrated resource management are assessed, as is the need for further case-specific research.

ENSO effects on the transpiration of eastern Amazon trees

Tree transpiration is important in the recycling of precipitation in the Amazon and might be negatively affected by El Niño–Southern Oscillation (ENSO)–induced droughts. To investigate the relative importance of soil moisture deficits versus increasing atmospheric demand (VPD) and determine if these drivers exert different controls over tree transpiration during the wet season versus the dry season (DS), we conducted sap flow measurements in a primary lowland tropical forest in eastern Amazon during the most extreme ENSO-induced drought (2015/2016) recorded in the Amazon. We also assessed whether trees occupying different canopy strata contribute equally to the overall stand transpiration (Tstand). Canopy trees were the primary source of Tstand. However, subcanopy trees are still important as they transpired an amount similar to other biomes around the globe. Tree water use was higher during the DS, indicating that during extreme drought trees did not reduce transpiration in response to low soil moisture. Photosynthetically active radiation and VPD exerted an overriding effect on water use patterns relative to soil moisture during extreme drought, indicating that light and atmospheric constraints play a critical role in controlling ecosystem fluxes of water. Our study highlights the importance of canopy and subcanopy trees to the regional water balance and highlights the resilience to droughts that these trees show during an extreme ENSO event. This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.