Role of tree size in moist tropical forest carbon cycling and water deficit responses

Abstract

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.

The limited contribution of large trees to annual biomass production in an old‐growth tropical forest

“…Meakem et al. () also showed a decrease in the contribution to plot biomass gains with increasing tree size for trees larger than [10–50] cm diameter across three tropical forests in Panama. As they measured very small trees, with diameter ranging between 0 and 10 cm, they further noted that the relationships between stand‐level biomass contribution and tree size might be hump‐back, with a maximum around [10–50] cm diameter rather than a monotonic decrease as suggested by our results.…”

“…Yuan et al (2016) observed that carbon accumulation was almost entirely the result of the increase in stems of 30-70 cm diameter in the temperate Changbai forest (China) and Caspersen et al (2011) showed that dense hardwood stands dominated by small trees produced more wood than stands dominated by fewer large trees in Ontario (Canada). Meakem et al (2017) also showed a decrease in the contribution to plot biomass gains with increasing tree size for trees larger than [10-50] cm diameter across three tropical forests in Panama. As they measured very small trees, with diameter ranging between 0 and 10 cm, they further noted that the relationships between stand-level biomass contribution and tree size might be hump-back, with a maximum around [10-50] cm diameter rather than a monotonic decrease as suggested by our results.…”

“…), scaling up the individual processes (e.g., tree growth and mortality) at the forest level to assess their quantitative contribution to biomass dynamics remains to be further explored (Meakem et al. , Sheil et al. ).…”

“…Although previous studies have suggested that the role of large trees in forest biomass dynamics and carbon cycling could be predominant (Bastin et al 2015, Fauset et al 2015, scaling up the individual processes (e.g., tree growth and mortality) at the forest level to assess their quantitative contribution to biomass dynamics remains to be further explored (Meakem et al 2017, Sheil et al 2017. In other words, the expectation regarding the contribution of large trees to biomass dynamics may vary with the level under consideration, i.e., from individual or species levels to forest level.…”

Global importance of large‐diameter trees

“…The generally high proportion of biomass represented by the largest 1% of trees reinforces the importance of these individuals to carbon sequestration and productivity (e.g., Stephenson et al, ). Larger numbers of small‐ and medium‐diameter trees cannot provide equivalent biomass to a few large‐diameter trees, although small and medium sized trees can contribute significantly to carbon cycling (Fauset et al, ; Meakem et al, ). The implication from scaling theory (West et al, ) is that large‐diameter trees are taller and have heavier crowns, and occupy growing space not available to smaller trees (i.e., at the top of the canopy; Van Pelt et al, ; West et al, ).…”