Abstract
Results from tropical planted forests have demonstrated that tree plantations can have variable growth and water use patterns in response to drought. Yet research on how specific species will perform during a drought and whether this response can be mediated through forest management is still poorly understood. We took advantage of the 2015-2016 El Nino drought in central Panama to test the effects of thinning on sap flux density, transpiration, and growth of planted Tectona grandis (teak), a non-native species introduced to Panama for timber production. Despite a reduction in growth of teak during drought for control and thinned sites, tree sap flux density of thinned sites significantly temporarily increased after the thinning. Sap flux density (Js) for teak is strongly driven by changes in vapor pressure deficit (VPD), temperature, and radiation; however, Js declines as temperature rises above 28C and VPD is above 0.5, suggesting a temperature threshold that could be problematic as droughts and temperatures increase in unison in the future. At the stand-level, all sites reduced transpiration during the drought. Although diameter growth and transpiration declined during drought, the leaf area index after the drought ended returned to pre-drought levels.
Highlights
Tectona grandis plantations have increased rapidly over the last few decades, especially in Panama, where an estimated 55,000 hectares is covered by planted teak (Kollert and Chrubini, 2012)
There are often species-specific thresholds where plant water use begins to decline in response to high vapor pressure deficit (VPD), radiation, or temperature (Sinacore et al, in review), which often leads to reduced tree water use and stand transpiration during droughts (Brum et al, 2018; Zhang et al, 2018)
Thinning had short-term effects on plant water use and growth that mediated the effects of drought
Summary
Tectona grandis (teak) plantations have increased rapidly over the last few decades, especially in Panama, where an estimated 55,000 hectares is covered by planted teak (Kollert and Chrubini, 2012). Changes in vapor pressure deficit (VPD), temperature, precipitation, and radiation have all been shown to affect plant water use at hourly, daily, and monthly timescales (Bretfeld et al, 2018; Brum et al, 2018). There are often species-specific thresholds where plant water use begins to decline in response to high VPD, radiation, or temperature (Sinacore et al, in review), which often leads to reduced tree water use and stand transpiration during droughts (Brum et al, 2018; Zhang et al, 2018). With predicted increases in temperature, in particular, understanding at what threshold tree water use begins to decline at high temperatures and VPDs is crucial for modeling future stand and landscape level responses to a changing climate. While precipitation and soil moisture are sometimes decoupled from transpiration on seasonal timescales (del Campo et al, 2019), deficits in soil moisture can still affect long term transpiration trends and growth of trees (Detto et al, 2018)
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