Trees in northern latitude ecosystems are projected to experience increasing drought stress as a result of rising air temperatures and changes in precipitation patterns in northern latitude ...ecosystems. However, most drought‐related studies on high‐latitude boreal forests (>50°N) have been conducted in North America, with few studies quantifying the response in European and Eurasian boreal forests. Here, we tested how daily whole‐tree transpiration (Q, Liters day−1) and Q normalized for mean daytime vapor pressure deficit (QDZ, Liters day−1 kPa−1) were affected by the historic 2018 drought in Europe. More specifically, we examined how tree species, size, and topographic position affected drought response in high‐latitude mature boreal forest trees. We monitored 30 Pinus sylvestris (pine) and 30 Picea abies (spruce) trees distributed across a topographic gradient in northern Sweden. In general, pine showed a greater QDZ control compared to spruce during periods of severe drought (standardized precipitation–evapotranspiration index: SPEI < −1.5), suggesting that the latter are more sensitive to drought. Overall, QDZ reductions (using non‐drought QDZ as reference) were less pronounced in larger trees during severe drought, but there was a species‐specific pattern: QDZ reductions were greater in pine trees at high elevations and greater in spruce trees at lower elevations. Despite lower QDZ during severe drought, drought spells were interspersed with small precipitation events and overcast conditions, and QDZ returned to pre‐drought conditions relatively quickly. This study highlights unique species‐specific responses to drought, which are additionally driven by a codependent interaction among tree size, relative topographic position, and unique regional climate conditions.
We studied whole‐tree transpiration (Q, L day−1) response of Pinus sylvestris and Picea abies to the severe drought registered in Northern Sweden in 2018. We analyzed the effects of species, tree size, and topographic position. We observed species‐specific response to drought, and a strong codependence between tree size and the relative topographic position of the trees, which highlights the need for three‐dimensional approaches for scaling up Q to stand transpiration. We discuss the important role of local climatic conditions in high‐elevation boreal forests (>55°N), which can reset drought spell length, directly influencing how different tree species respond to drought.
Models of forest energy, water and carbon cycles assume decreased stomatal conductance with elevated atmospheric CO₂concentration (CO₂) based on leaf‐scale measurements, a response not directly ...translatable to canopies. Where canopy–atmosphere are well‐coupled, CO₂‐induced structural changes, such as increasing leaf‐area index (LD), may cause, or compensate for, reduced mean canopy stomatal conductance (GS), keeping transpiration (EC) and, hence, runoff unaltered. We investigated GSresponses to increasing CO₂ of conifer and broadleaved trees in a temperate forest subjected to 17‐yr free‐air CO₂enrichment (FACE; + 200 μmol mol⁻¹). During the final phase of the experiment, we employed step changes of CO₂ in four elevated‐CO₂ plots, separating direct response to changing CO₂ in the leaf‐internal air‐space from indirect effects of slow changes via leaf hydraulic adjustments and canopy development. Short‐term manipulations caused no direct response up to 1.8 × ambient CO₂, suggesting that the observed long‐term 21% reduction of GSwas an indirect effect of decreased leaf hydraulic conductance and increased leaf shading. Thus, ECwas unaffected by CO₂ because 19% higher canopy LDnullified the effect of leaf hydraulic acclimation on GS. We advocate long‐term experiments of duration sufficient for slow responses to manifest, and modifying models predicting forest water, energy and carbon cycles accordingly.
Tropical forests are experiencing reduced productivity and will need restoration with suitable species. Knowledge of species-specific responses to changing environments during early stage can help ...identify the appropriate species for sustainable planting. Hence, we investigated the variability in whole-tree canopy conductance and transpiration (G
and E
) in potted saplings of common urban species in Thailand, viz., Pterocarpus indicus, Lagerstroemia speciosa, and Swietenia macrophylla, across wet and dry seasons in 2017-2018. Using a Bayesian modeling framework, G
and E
were estimated from sap flux density, informed by the soil, atmospheric and tree measurements. Subsequently, we evaluated their variations with changing vapor pressure deficit (VPD) and soil moisture across timescales and season. We found that G
and E
were higher and highly variable in L. speciosa across seasons than S. macrophylla and P. indicus. Our results implied that water-use in these species was sensitive to seasonal VPD. L. speciosa may be suitable under future climate variability, given its higher G
and E
across atmospheric and soil moisture conditions. With their lower G
and E
, P. indicus and S. macrophylla may photosynthesize throughout the year, maintaining their stomatal opening even under high VPD. These findings benefit reforestation and reclamation programs of degraded lands.
Gross primary production (GPP) is a key component of the forest carbon cycle. However, our knowledge of GPP at the stand scale remains uncertain, because estimates derived from eddy covariance (EC) ...rely on semi‐empirical modelling and the assumptions of the EC technique are sometimes not fully met. We propose using the sap flux/isotope method as an alternative way to estimate canopy GPP, termed GPPiso/SF, at the stand scale and at daily resolution. It is based on canopy conductance inferred from sap flux and intrinsic water‐use efficiency estimated from the stable carbon isotope composition of phloem contents. The GPPiso/SF estimate was further corrected for seasonal variations in photosynthetic capacity and mesophyll conductance. We compared our estimate of GPPiso/SF to the GPP derived from PRELES, a model parameterized with EC data. The comparisons were performed in a highly instrumented, boreal Scots pine forest in northern Sweden, including a nitrogen fertilized and a reference plot. The resulting annual and daily GPPiso/SF estimates agreed well with PRELES, in the fertilized plot and the reference plot. We discuss the GPPiso/SF method as an alternative which can be widely applied without terrain restrictions, where the assumptions of EC are not met.
Large-scale abandoned agricultural areas in Southeast Asia resulted in patches of forests of multiple successions and characteristics, challenging the study of their responses to environmental ...changes, especially under climatic water stress. Here, we investigated seasonal variation in leaf water status and drought tolerance of dominant tree species in three multi-aged tropical forests, ranging from 5 to > 200 years old, with contrasting soil moisture in Thailand. Seasonal variation in leaf water status differed among the forests with trees in young and intermediate sites demonstrating larger differences between seasons than the old-growth forest. Although vulnerability to embolism curves revealed that trees in old-growth forest were potentially more sensitive to declining leaf water status than others, they were predicted to lose < 5% of their hydraulic capacity as opposed to 13% for the trees in the younger sites. Our results suggest that the responses to water stress of tree species in different forest ages greatly vary with a tendency of trees in younger sites to be more resilience than those in older sites. Such information would benefit the selection of tree species that could adapt well to specific environments, thus improving the strategies for managing forests of different ages under a warmer future.
Changes in evapotranspiration (ET) from terrestrial ecosystems affect their water yield (WY), with considerable ecological and economic consequences. Increases in surface runoff observed over the ...past century have been attributed to increasing atmospheric CO2 concentrations resulting in reduced ET by terrestrial ecosystems. Here, we evaluate the water balance of a Pinus taeda (L.) forest with a broadleaf component that was exposed to atmospheric CO2 enrichment (ECO2; +200 ppm) for over 17 years and fertilization for 6 years, monitored with hundreds of environmental and sap flux sensors on a half‐hourly basis. These measurements were synthesized using a one‐dimensional Richard's equation model to evaluate treatment differences in transpiration (T), evaporation (E), ET, and WY. We found that ECO2 did not create significant differences in stand T, ET, or WY under either native or enhanced soil fertility, despite a 20% and 13% increase in leaf area index, respectively. While T, ET, and WY responded to fertilization, this response was weak (<3% of mean annual precipitation). Likewise, while E responded to ECO2 in the first 7 years of the study, this effect was of negligible magnitude (<1% mean annual precipitation). Given the global range of conifers similar to P. taeda, our results imply that recent observations of increased global streamflow cannot be attributed to decreases in ET across all ecosystems, demonstrating a great need for model–data synthesis activities to incorporate our current understanding of terrestrial vegetation in global water cycle models.
Here we evaluate the water balance of a Pinus taeda (L.) forest with a broadleaf component that was exposed to atmospheric CO2 enrichment (ECO2; +200 ppm) for over 17 years and fertilization for 6 years, monitored with hundreds of environmental and sap flux sensors on a half‐hourly basis. We found that ECO2 did not create significant differences in stand evapotranspiration (ET) or water yield under either native or enhanced soil fertility, despite a 20% and 13% increase in leaf area index, respectively. Given the global range of conifers similar to P. taeda, our results imply that recent observations of increased global streamflow cannot be attributed to decreases in ET across all ecosystems, demonstrating a great need for model‐data synthesis activities to incorporate our current understanding of terrestrial vegetation in global water cycle models.
Soil respiration (SR) in forests contributes significant carbon dioxide emissions from terrestrial ecosystems and is highly sensitive to environmental changes, including soil temperature, soil ...moisture, microbial community, surface litter, and vegetation type. Indeed, a small change in SR may have large impacts on the global carbon balance, further influencing feedbacks to climate change. Thus, detailed characterization of SR responses to changes in environmental conditions is needed to accurately estimate carbon dioxide emissions from forest ecosystems. However, data for such analyses are still limited, especially in tropical forests of Southeast Asia where various stages of forest succession exist due to previous land‐use changes. In this study, we measured SR and some environmental factors including soil temperature (ST), soil moisture (SM), and organic matter content (OM) in three successional tropical forests in both wet and dry periods. We also analyzed the relationships between SR and these environmental variables. Results showed that SR was higher in the wet period and in older forests. Although no response of SR to ST was found in younger forest stages, SR of the old‐growth forest significantly responded to ST, plausibly due to the nonuniform forest structure, including gaps, that resulted in a wide range of ST. Across forest stages, SM was the limiting factor for SR in the wet period, whereas SR significantly varied with OM in the dry period. Overall, our results indicated that the responses of SR to environmental factors varied temporally and across forest succession. Nevertheless, these findings are still preliminary and call for detailed investigations on SR and its variations with environmental factors in Southeast Asian tropical forests where patches of successional stages dominate.
We measured and analyzed soil respiration (SR) with environmental factors in three successional forests in Thailand. Results showed that SR was generally higher in the wet season and in older forests. Across all forest stages, soil moisture was the main factor contributing to variation in SR in the wet season while organic matter content played significant role in the dry season.
Canopy transpiration (EC) is a large fraction of evapotranspiration, integrating physical and biological processes within the energy, water, and carbon cycles of forests. Quantifying EC is of both ...scientific and practical importance, providing information relevant to questions ranging from energy partitioning to ecosystem services, such as primary productivity and water yield. We estimated EC of four pine stands differing in age and growing on sandy soils. The stands consisted of two wide-ranging conifer species: Pinus taeda and Pinus sylvestris, in temperate and boreal zones, respectively. Combining results from these and published studies on all soil types, we derived an approach to estimate daily EC of pine forests, representing a wide range of conditions from 35°S to 64°N latitude. During the growing season and under moist soils, maximum daily EC (ECm) at day-length normalized vapor pressure deficit of 1 kPa (KCm-ref) increased by 0.55 ± 0.02 (mean ± SE) mm/d for each unit increase of leaf area index (L) up to L = ~5, showing no sign of saturation within this range of quickly rising mutual shading. The initial rise of ECm with atmospheric demand was linearly related to ECm-ref. Both relations were unaffected by soil type. Consistent with theoretical prediction, daily EC was sensitive to decreasing soil moisture at an earlier point of relative extractable water in loamy than sandy soils. Our finding facilitates the estimation of daily EC of wideranging pine forests using remotely sensed L and meteorological data. We advocate an assembly of worldwide sap flux database for further evaluation of this approach.
Trees remove atmospheric carbon dioxide through photosynthesis, hereafter CO
absorption (A). Despite growing urban green areas, only a few studies have quantified A of urban trees and assessed their ...dynamical changes with varying atmospheric conditions. Hence, we investigated A in nine dominant tree species in a new park of Bangkok.
Results revealed that A of two tree species (Millingtonia hortensis and Afzelia xylocarpa) significantly increased with vapor pressure deficit (VPD) until it reached a maximum and declined when VPD decreased, with no seasonal difference. Five of them (Dalbergia cochinchinensis, Tabebuia rosea, Lagerstroemia floribunda, Dipterocarpus alatus and Bauhinia purpurea) exhibited different response patterns of A to VPD between wet and dry seasons. In contrast, the A of two tree species (Samanea saman and Homalium tomentosum) did not respond to changing VPD in either season.
Comparing planting scenarios of insensitive (i.e. no response to VPD) versus sensitive (i.e. significant response to VPD) species, we found that planting a mixture of sensitive and insensitive tree species would improve the park's capacity of A across seasons, allowing climate change adaptation to adverse environmental impacts such as droughts and the urban heat island effects, and would increase biodiversity. Additionally, planting insensitive tree species would significantly increase the capacity of the park for CO
mitigation. These findings are useful for those who design parks and expand urban green areas to fully benefit ecosystem services from trees.
BackgroundSoutheast Asia has experienced widespread deforestation and change in land use. Consequently, many reforestation projects have been initiated in this region. However, it is imperative to ...carefully choose the tree species for planting, especially in light of the increasing climate variability and the potential alteration of plantation on the watershed water balance. Thus, the information regarding water-use characteristics of various tree species and sizes is critical in the tree species selection for reforestation.MethodsWe estimated tree water use (T) of dominant species including Syzygium antisepticum and Adinandra integerrima, hereafter Sa and Ai, respectively, in a secondary tropical forest in Khao Yai National Park, Thailand, using sap flow data, and compared T between species and size classes. Additionally, we evaluated the responses of T of both species in each size class to environmental factors including soil moisture and vapor pressure deficit (VPD).ResultsResults showed consistently higher T in Sa compared to Ai across ranges of VPD and soil moisture. Under low soil moisture, T of Sa responded to VPD, following a saturating exponential pattern while Ai maintained T across different VPD levels, irrespective of tree size. No responses of T to VPD were observed in either species when soil water was moderate. When soil moisture was high, T of both species significantly increased and saturated at high VPD, albeit the responses were less sensitive in large trees. Our results imply that Ai may be suitable for reforestation in water-limited areas where droughts frequently occur to minimize reforestation impact on water availability to downstream ecosystems. In contrast, Sa should be planted in regions with abundant and reliable water resources. However, a mixed species plantation should be generally considered to increase forest resilience to increasing climate variation.
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