SummaryDrought disproportionately affects larger trees in tropical forests, but implications for forest composition and carbon (C) cycling in relation to dry season intensity remain poorly understood.In order to characterize how C cycling is shaped by tree size and drought adaptations and how these patterns relate to spatial and temporal variation in water deficit, we analyze data from three forest dynamics plots spanning a moisture gradient in Panama that have experienced El Niño droughts.At all sites, aboveground C cycle contributions peaked below 50-cm stem diameter, with stems ≥ 50 cm accounting for on average 59% of live aboveground biomass, 45% of woody productivity and 49% of woody mortality. The dominance of drought-avoidance strategies increased interactively with stem diameter and dry season intensity. Although size-related C cycle contributions did not vary systematically across the moisture gradient under nondrought conditions, woody mortality of larger trees was disproportionately elevated under El Niño drought stress.Thus, large (> 50 cm) stems, which strongly mediate but do not necessarily dominate C cycling, have drought adaptations that compensate for their more challenging hydraulic environment, particularly in drier climates. However, these adaptations do not fully buffer the effects of severe drought, and increased large tree mortality dominates ecosystem-level drought responses.

Summary The climate sensitivity of forest ecosystem woody productivity (ANPPstem) influences carbon cycle responses to climate change. For the first time, we combined long‐term annual growth and forest census data of a diverse temperate broadleaf deciduous forest, seeking to resolve whether ANPPstem is primarily moisture‐ or energy‐limited and whether climate sensitivity has changed in recent decades characterised by more mesic conditions and elevated CO2. We analysed tree‐ring chronologies across 109 yr of monthly climatic variation (1901–2009) for 14 species representing 97% of ANPPstem in a 25.6 ha plot in northern Virginia, USA. Radial growth of most species and ecosystem‐level ANPPstem responded positively to cool, moist growing season conditions, but the same conditions in the previous May–July were associated with reduced growth. In recent decades (1980–2009), responses were more variable and, on average, weaker. Our results indicated that woody productivity is primarily limited by current growing season moisture, as opposed to temperature or sunlight, but additional complexity in climate sensitivity may reflect the use of stored carbohydrate reserves. Overall, while such forests currently display limited moisture sensitivity, their woody productivity is likely to decline under projected hotter and potentially drier growing season conditions.

Teixeira. 2016. Patterns of tree mortality in a temperate deciduous forest derived from a large forest dynamics plot.Ecosphere 7(12):e01595. 10. 1002/ecs2.1595 Abstract. Tree mortality is one of the most influential drivers of forest dynamics, and characterizing patterns of tree mortality is critical to understanding forest dynamics and ecosystem function in the present era of global change. Here, we use a unique data set of mortality in a temperate deciduous forest to characterize rates and drivers of mortality. At the 25.6-ha Center for Tropical Forest Science-Forest Global Earth Observatory forest dynamics plot at the Smithsonian Conservation Biology Institute (Virginia, USA), we conducted two full tree censuses in 2008 and 2013 and then tracked mortality over the next 2 years (2014 and 2015). Overall, the mortality rate, m, of stems ≥10 cm diameter was 1.3-2.1%/yr. Biomass mortality, M, was 1.9-3.4 MgÁha at the stand level (0.6-1.1%/yr of biomass), less than biomass gains from growth and recruitment, resulting in net live biomass accumulation. Small stems died at the highest rate; however, contributions to M increased toward larger size classes. Most species had m < 2%/yr and M < 0.25 MgÁha À1 Áyr À1 (<3%/yr of biomass), whereas two to four species had anomalously high mortality rates during each census period, accounting for 15-24% of m (n = 2, Cercis canadensis, Ulmus species) and 39-75% of M (n = 4 Quercus species). Stems that died, whether or not in association with mechanical damage, tended to grow more slowly in preceding years than surviving stems and, for certain shade-intolerant species, tended to be in neighborhoods with higher basal area. These findings show how relatively fine-scale mortality processes contribute to stand-level compositional change and carbon cycling. The mortality patterns reported here will provide a valuable basis for understanding future disturbance events within eastern deciduous forests and for comparing across forest types.

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