Suppression of transpiration due to cloud immersion in a seasonally dry Mexican weeping pine plantation

Alvarado-Barrientos, M.S., Holwerda, F., Asbjornsen, H., Dawson, T.E. and Bruijnzeel, L.A., 2014. Suppression of transpiration due to cloud immersion in a seasonally dry Mexican weeping pine plantation. Agricultural and Forest Meteorology, 186: 12-25.

ABSTRACT. Cloud immersion affects the water budget of fog-affected forests not only by introducing an additional source of water (via cloud water interception by the canopy), but also by suppressing plant transpiration. The latter effect is often overlooked and not routinely quantified, restricting a complete understanding of the net hydrological effect of cloud immersion and the possible consequences of projected reductions in cloud immersion under drier and warmer climates in tropical montane regions in the coming decades. This paper describes an approach to quantify the suppression of stand-level tree transpiration (Et) due to cloud immersion using measurements of sapflow, fog occurrence (visibility), leaf wetness, and near-surface climate. Estimates of fog-induced Et suppression in a 10-year-old Pinus patula plantation in the montane cloud belt of central Veracruz, Mexico, are presented for two contrasting dry seasons and a wet season. Fog occurred for 32% of the total study period, although showing pronounced seasonal variation (e.g. 44% during the second dry season). When fog occurred it was accompanied by rainfall during three quarters of the total time. Although the canopy was wet for almost a third of the time, fog-induced canopy wetness constituted only a very small portion of this total (2%). Relative to sunny conditions, Et was suppressed by 90 ± 7% under conditions of dense fog versus 83 ± 7% under light fog and 78 ± 10% during overcast conditions. Quantification of the potential change in annual Et associated with two scenarios for future cloud immersion at the study site revealed that: (i) when all fog occurrence is replaced by overcast conditions, mean annual Et (645 ± 50 mm) is likely to increase by only 2 ± 1%; and (ii) when sunny conditions replace all foggy conditions, the likely increase in annual Et is 17 ± 3%. As the rise in the regional lifting condensation level is likely to be on the order of only a couple of hundred meters and will probably result in a shift to overcast rather than clear-sky conditions, the present results suggest that the corresponding impact on Et may be relatively small. Consequently, a climate change-related reduction in dry-season precipitation, through the associated potential reductions in soil water reserves, presents a more worrisome prospect for plant–water relations and water yield from headwater catchments than diminishing cloud immersion alone. The present results highlight the need for better projections of climate change-related alterations in cloud cover and immersion, as well as rainfall patterns for tropical montane regions.

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Land use change effects on runoff generation in a humid tropical montane cloud forest region

Muñoz-Villers, L.E., and J.J. McDonnell. 2013. Land use change effects on runoff generation in a humid tropical montane cloud forest region. Hydrology and Earth System Sciences, 17, 3543–3560.

ABSTRACT. While tropical montane cloud forests (TMCF) provide critical hydrological services to downstream regions throughout much of the humid tropics, catchment hydrology and impacts associated with forest conversion in these ecosystems remain poorly understood. Here, we compare the annual, seasonal and event-scale streamflow patterns and runoff generation processes of three neighbouring headwater catchments in central Veracruz (eastern Mexico) with similar pedological and geological characteristics, but different land cover: old-growth TMCF, 20 yr-old naturally regenerating TMCF and a heavily grazed pasture. We used a 2 yr record of high resolution rainfall and stream flow data (2008–2010) in combination with stable isotope and chemical tracer data collected for a series of storms during a 6-week period of increasing antecedent wetness (wetting-up cycle). Our results showed that annual and seasonal streamflow patterns in the mature and secondary forest were similar. In contrast, the pasture showed a 10% higher mean annual streamflow, most likely because of a lower rainfall interception. During the wetting-up cycle, storm runoff ratios increased at all three catchments (from 11 to 54% for the mature forest, 7 to 52% for the secondary forest and 3 to 59% for the pasture). With the increasing antecedent wetness, hydrograph separation analysis showed progressive increases of pre-event water contributions to total stormflow (from 35 to 99% in the mature forest, 26 to 92% in the secondary forest and 64 to 97% in the pasture). At all three sites, rainfall-runoff responses were dominated by subsurface flow generation processes for the majority of storms. However, for the largest and most intense storm (typically occurring once every 2 yr), sampled under wet antecedent conditions, the event water contribution in the pasture (34% on average) was much higher than in the forests (5% on average), indicating that rainfall infiltration capacity of the pasture was exceeded. This result suggests that despite the high permeability of the volcanic soils and underlying substrate in this TMCF environment, the conversion of forest to pasture may lead to important changes in runoff generation processes during large and high intensity storms. On the other hand, our results also showed that 20 yr of natural regeneration may be enough to largely restore the original hydrological conditions of this TMCF.

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Foggy days and dry nights determine crown-level water balance in a seasonal tropical montane cloud forest

Gotsch SG, Asbjornsen H, Holwerda F, Goldsmith GR, Weintraub AE & Dawson TE. 2013. Foggy days and dry nights determine crown-level water balance in a seasonal tropical montane cloud forest. Plant, Cell and Environment. PDF

ABSTRACT: The ecophysiology of tropical montane cloud forest (TMCF) trees is influenced by crown-level microclimate factors including regular mist/fog water inputs, and large variations in evaporative demand, which in turn can significantly impact water balance. We investigated the effect of such microcli- matic factors on canopy ecophysiology and branch-level water balance in the dry season of a seasonal TMCF in Ver- acruz, Mexico, by quantifying both water inputs (via foliar uptake, FU) and outputs (day- and night-time transpiration, NT). Measurements of sap flow, stomatal conductance, leaf water potential and pressure–volume relations were obtained in Quercus lanceifolia, a canopy-dominant tree species. Our results indicate that FU occurred 34% of the time and led to the recovery of 9% (24 􏰀 9.1 L) of all the dry-season water transpired from individual branches. Capacity for FU was independently verified for seven additional common tree species. NT accounted for approximately 17% (46 L) of dry- season water loss. There was a strong correlation between FU and the duration of leaf wetness events (fog and/or rain), as well as between NT and the night-time vapour pressure deficit. Our results show the clear importance of fog and NT for the canopy water relations of Q. lanceifolia.

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Is nighttime transpiration enhanced after fog events?

Alvarado-Barrientos, M.S., Asbjornsen, H. and Holwerda, F. 2013. IS NIGHTTIME TRANSPIRATION ENHANCED AFTER FOG EVENTS?. Acta Hort. (ISHS) 991:133-139
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Fog occurrence has been shown to suppress transpiration (Et). On the other hand foggy conditions during which leaf wetness does not block stomatal gas exchange may enhance stomatal conductance, and so Et immediately after fog. Furthermore, although nighttime Et has been found to be prevalent for a wide range of species from cloud-affected forests, its magnitude relative to daytime Et has been reported to be generally small. Here, we report considerable variability in nighttime Et rates of Pinus patula trees associated to rapidly changing meteorological conditions typical for the dry season in the tropical montane cloud belt of the Eastern Sierra Madre, Mexico. Stand level tree Et was derived from sapflow measurements with the Heat Ratio Method in the stem of P. patula trees growing in contrasting stands and at different elevations within the cloud belt: 10-year-old reforestation at 2180 m a.s.l. and mature forest at 2470 m a.s.l. The dry-season range of nighttime Et for the young and mature forest was 0-0.08 and 0-0.06 mm h-1, respectively. Expressed as a proportion of dry-season daily totals, nighttime Et was high and variable (42±28 and 19±23% for the young and mature stand, respectively). This large variation was related to the wide range of air humidity, caused by the alternation of cold front intrusions bringing about fog events and high pressure weather characterized by dry nights with vapor pressure deficits up to 2 kPa. Shortly after the end of fog events without concurring rainfall, nighttime Et for the young stand was higher (although not significantly) and more variable than for fog-free nights. Climate change-related alterations in lifting condensation level that have been projected for tropical montane regions will also affect the dynamics of the inversion layer, and as shown here, nighttime Et may increase/decrease considerably depending on a lowering/rise of the cloud ceiling.
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The water and energy exchange of a shaded coffee plantation in the lower montane cloud forest zone of central Veracruz, Mexico

Holwerda, F., L.A. Bruijnzeel, V.L. Barradas, J. Cervantes. 2013. The water and energy exchange of a shaded coffee plantation in the lower montane cloud forest zone of central Veracruz, Mexico. Agricultural and Forest Meteorology 173: 1-13. PDF

Abstract: The water and energy fluxes of a shaded coffee plantation in the lower montane cloud forest (LMCF) zone of central Veracruz, Mexico, were measured over a two-year period (September 2006–August 2008) using the eddy covariance method. Complementary measurements of throughfall and stemflow were made to study rainfall interception. The sum of the observed sensible (H) and latent (E) heat fluxes was almost 95% of the net radiation (Rn) minus the canopy heat storage fluxes, indicating very good energy balance closure. The mean annual evapotranspiration was 1066 mm, and 95% of the corresponding FAO Penman-Monteith reference evapotranspiration (ET0) of 1117 mm yr−1. Interception loss was 8% of annual rainfall (1386 mm). Both the eddy covariance, and the throughfall and stemflow measurements showed average wet-canopy evaporation rate to be very low (0.05 mm h−1) compared to the corresponding rainfall rate (3.06 mm h−1). As a result, and despite the low canopy storage capacity of the coffee plantation (Cm, 0.50 mm), interception was dominated by post-event evaporation of intercepted water rather than by within-event evaporation. Comparing the results for the coffee plantation with interception data from mature and secondary LMCFs in the study area suggests that the conversion of LMCF to shade-coffee may lead to a decrease in interception loss of 8−18% of incident rainfall. This decrease is caused by a three to seven-fold decrease in Cm, probably due to the lower leaf area and smaller epiphyte biomass of the coffee plantation. The mean annual dry-canopy evaporation was 992 mm, and 89% of ET0. Comparing the eddy covariance-based estimate of dry-canopy evaporation for the coffee plantation with sapflow-based estimates of transpiration for the LMCFs did not show any clear differences.

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La niebla y la ecohidrología del Bosque Mesófilo de Montaña en México

Un articulo nuevo desde Profesores Friso Holwerda y Sybil Gotsch en el boletin del Centro de Ciencias de la Atmosfera (UNAM). (PDF)

La ecohidrología es un área de trabajo novedosa e interdisciplinaria, que busca entender las interacciones entre el ciclo hidrológico y los ecosistemas terrestres. La niebla es simplemente una nube en contacto con la superficie terrestre. Su presencia en forma de numerosas y muy pequeñas gotas de agua produce una visión limitada de los objetos. Por tanto, no es sorprendente que la densidad de la niebla se exprese en términos de la visibilidad, que es la distancia a la cual se puede observar claramente un objeto o una luz.

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Incidence and Implications of Clouds for Cloud Forest Plant Water Relations

Goldsmith, G.R., N.J. Matzke and T.E. Dawson. 2013. Incidence and Implications of Clouds for Cloud Forest Plant Water Relations. Ecology Letters 16: 307-314. (PDF)

Link: Press Release
Link: Science Magazine ScienceNow Article

Abstract: Although clouds are the most recognisable and defining feature of tropical montane cloud forests, little research has focussed on how clouds affect plant functioning. We used satellite and ground-based observations to study cloud and leaf wetting patterns in contrasting tropical montane and pre-montane cloud forests. We then studied the consequences of leaf wetting for the direct uptake of water accumulated on leaf surfaces into the leaves themselves. During the dry season, the montane forest experienced higher precipitation, cloud cover and leaf wetting events of longer duration than the pre-montane forest. Leaf wetting events resulted in foliar water uptake in all species studied. The capacity for foliar water uptake differed significantly between the montane and pre-montane forest plant communities, as well as among species within a forest. Our results indicate that foliar water uptake is common in these forest plants and improves plant water status during the dry season.

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