skip to main content


Title: Characterizing Growing Season Length of Subtropical Coniferous Forests with a Phenological Model
Understanding plant phenological change is of great concern in the context of global climate change. Phenological models can aid in understanding and predicting growing season changes and can be parameterized with gross primary production (GPP) estimated using the eddy covariance (EC) technique. This study used nine years of EC-derived GPP data from three mature subtropical longleaf pine forests in the southeastern United States with differing soil water holding capacity in combination with site-specific micrometeorological data to parameterize a photosynthesis-based phenological model. We evaluated how weather conditions and prescribed fire led to variation in the ecosystem phenological processes. The results suggest that soil water availability had an effect on phenology, and greater soil water availability was associated with a longer growing season (LOS). We also observed that prescribed fire, a common forest management activity in the region, had a limited impact on phenological processes. Dormant season fire had no significant effect on phenological processes by site, but we observed differences in the start of the growing season (SOS) between fire and non-fire years. Fire delayed SOS by 10 d ± 5 d (SE), and this effect was greater with higher soil water availability, extending SOS by 18 d on average. Fire was also associated with increased sensitivity of spring phenology to radiation and air temperature. We found that interannual climate change and periodic weather anomalies (flood, short-term drought, and long-term drought), controlled annual ecosystem phenological processes more than prescribed fire. When water availability increased following short-term summer drought, the growing season was extended. With future climate change, subtropical areas of the Southeastern US are expected to experience more frequent short-term droughts, which could shorten the region’s growing season and lead to a reduction in the longleaf pine ecosystem’s carbon sequestration capacity.  more » « less
Award ID(s):
1702996 1910811
NSF-PAR ID:
10251055
Author(s) / Creator(s):
; ; ; ;
Date Published:
Journal Name:
Forests
Volume:
12
Issue:
1
ISSN:
1999-4907
Page Range / eLocation ID:
95
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Climate models predict that water limited regions around the world will become drier and warmer in the near future, including southwestern North America. We developed a large-scale experimental system that allows testing of the ecosystem impacts of precipitation changes. Four treatments were applied to 1600 m2 plots (40 m × 40 m), each with three replicates in a piñon pine (Pinus edulis) and juniper (Juniper monosperma) ecosystem. These species have extensive root systems, requiring large-scale manipulation to effectively alter soil water availability.  Treatments consisted of: 1) irrigation plots that receive supplemental water additions, 2) drought plots that receive 55% of ambient rainfall, 3) cover-control plots that receive ambient precipitation, but allow determination of treatment infrastructure artifacts, and 4) ambient control plots. Our drought structures effectively reduced soil water potential and volumetric water content compared to the ambient, cover-control, and water addition plots. Drought and cover control plots experienced an average increase in maximum soil and air temperature at ground level of 1-4° C during the growing season compared to ambient plots, and concurrent short-term diurnal increases in maximum air temperature were also observed directly above and below plastic structures. Our drought and irrigation treatments significantly influenced tree predawn water potential, sap-flow, and net photosynthesis, with drought treatment trees exhibiting significant decreases in physiological function compared to ambient and irrigated trees.  Supplemental irrigation resulted in a significant increase in both plant water potential and xylem sap-flow compared to trees in the other treatments. This experimental design effectively allows manipulation of plant water stress at the ecosystem scale, permits a wide range of drought conditions, and provides prolonged drought conditions comparable to historical droughts in the past – drought events for which wide-spread mortality in both these species was observed.  Water potential measurements were used to monitor the water stress of the two target species across the four treatment regimes.  Sampling for water potentials occurred twice daily.  One set of samples was collected hours before dawn and another set was collected at mid-day.  The predawn readings provided the “least-stressed†tree water content values as they were collected after the trees had returned to equilibrium over the evening and had yet to start transpiring.  The mid-day values, collected after tree-level respiration had been occurring for hours and when the daily temperatures were highest, represented the opposite “most-stressed†scenario. To gauge the effect of the irrigation treatment on the water content of the trees, we sampled water potentials just before and just after irrigation events.    
    more » « less
  2. Abstract

    Cycles of plant growth, termed phenology, are tightly linked to environmental controls. The length of time spent growing, bounded by the start and end of season, is an important determinant of the global carbon, water, and energy balance. Much focus has been given to global warming and consequences for shifts in growing‐season length in temperate regions. In conjunction with warming temperatures, altered precipitation regimes are another facet of climate change that have potentially larger consequences than temperature in dryland phenology globally. We experimentally manipulated incoming precipitation in a semiarid grassland for over a decade and recorded plant phenology at the daily scale for 7 years. We found precipitation to have a strong relationship with the timing of grass greenup and senescence but temperature had only a modest effect size on grass greenup. Pre‐season drought strongly resulted in delayed grass greenup dates and shorter growing‐season lengths. Spring and summer drought corresponded with earlier grass senescence, whereas higher precipitation accumulation over these seasons corresponded with delayed grass senescence. However, extremely wet conditions diluted this effect and caused a plateaued response. Deep‐rooted woody shrubs showed few effects of variable precipitation or temperature on phenology and displayed consistent annual phenological timing compared with grasses. Whereas rising temperatures have already elicited phenological consequences and extended growing‐season length for mid and high‐latitude ecosystems, precipitation change will be the major driver of phenological change in drylands that cover 40% of the land surface with consequences for the global carbon, water, and energy balance.

     
    more » « less
  3. Climate models predict that water limited regions around the world will become drier and warmer in the near future, including southwestern North America. We developed a large-scale experimental system that allows testing of the ecosystem impacts of precipitation changes. Four treatments were applied to 1600 m2 plots (40 m × 40 m), each with three replicates in a piñon pine (Pinus edulis) and juniper (Juniper monosperma) ecosystem. These species have extensive root systems, requiring large-scale manipulation to effectively alter soil water availability.  Treatments consisted of: 1) irrigation plots that receive supplemental water additions, 2) drought plots that receive 55% of ambient rainfall, 3) cover-control plots that receive ambient precipitation, but allow determination of treatment infrastructure artifacts, and 4) ambient control plots. Our drought structures effectively reduced soil water potential and volumetric water content compared to the ambient, cover-control, and water addition plots. Drought and cover control plots experienced an average increase in maximum soil and air temperature at ground level of 1-4° C during the growing season compared to ambient plots, and concurrent short-term diurnal increases in maximum air temperature were also observed directly above and below plastic structures. Our drought and irrigation treatments significantly influenced tree predawn water potential, sap-flow, and net photosynthesis, with drought treatment trees exhibiting significant decreases in physiological function compared to ambient and irrigated trees. Supplemental irrigation resulted in a significant increase in both plant water potential and xylem sap-flow compared to trees in the other treatments. This experimental design effectively allows manipulation of plant water stress at the ecosystem scale, permits a wide range of drought conditions, and provides prolonged drought conditions comparable to historical droughts in the past – drought events for which wide-spread mortality in both these species was observed.A micrometeorological station was used to document the climatic conditions at the study site.  Monitoring the ambient environment in this way allowed us to more easily determine which tree growth responses were driven by changes in the native climate as opposed to those resulting from the rainfall manipulation treatments.  Environmental factors such as temperature, relative humidity, and photosynthetically active radiation (PAR) have a huge impact on the physiological processes that are being explored in this project.  The data collected by the station created a local climatic record which was needed to provide the context in which the treatment effects can be examined and sensor readings can be interpreted. 
    more » « less
  4. Climate models predict that water limited regions around the world will become drier and warmer in the near future, including southwestern North America. We developed a large-scale experimental system that allows testing of the ecosystem impacts of precipitation changes. Four treatments were applied to 1600 m2 plots (40 m × 40 m), each with three replicates in a piñon pine (Pinus edulis) and juniper (Juniper monosperma) ecosystem. These species have extensive root systems, requiring large-scale manipulation to effectively alter soil water availability.  Treatments consisted of: 1) irrigation plots that receive supplemental water additions, 2) drought plots that receive 55% of ambient rainfall, 3) cover-control plots that receive ambient precipitation, but allow determination of treatment infrastructure artifacts, and 4) ambient control plots. Our drought structures effectively reduced soil water potential and volumetric water content compared to the ambient, cover-control, and water addition plots. Drought and cover control plots experienced an average increase in maximum soil and air temperature at ground level of 1-4° C during the growing season compared to ambient plots, and concurrent short-term diurnal increases in maximum air temperature were also observed directly above and below plastic structures. Our drought and irrigation treatments significantly influenced tree predawn water potential, sap-flow, and net photosynthesis, with drought treatment trees exhibiting significant decreases in physiological function compared to ambient and irrigated trees. Supplemental irrigation resulted in a significant increase in both plant water potential and xylem sap-flow compared to trees in the other treatments. This experimental design effectively allows manipulation of plant water stress at the ecosystem scale, permits a wide range of drought conditions, and provides prolonged drought conditions comparable to historical droughts in the past – drought events for which wide-spread mortality in both these species was observed.  Soil temperature impacts both the abiotic and biotic processes at our site. The rate of evaporation, soil water content, VPD, and many other environmental factors are directly or indirectly affected by the temperature of the system. By monitoring the soil temperature at our site, we were able to determine its influence on the target trees and their associated physiological functions. Differences in soil temperature between plots can be impacted by the drought and cover-control structures used in our rainfall-manipulation treatments. Therefore, measuring soil temperatures in all three cover types and all four treatment regimes also allowed us to tease-out the temperature differences that were an artifact of the treatment structures as opposed to the actual treatments.  
    more » « less
  5. Climate models predict that water limited regions around the world will become drier and warmer in the near future, including southwestern North America. We developed a large-scale experimental system that allows testing of the ecosystem impacts of precipitation changes. Four treatments were applied to 1600 m2 plots (40 m × 40 m), each with three replicates in a piñon pine (Pinus edulis) and juniper (Juniper monosperma) ecosystem. These species have extensive root systems, requiring large-scale manipulation to effectively alter soil water availability. Treatments consisted of: 1) irrigation plots that receive supplemental water additions, 2) drought plots that receive 55% of ambient rainfall, 3) cover-control plots that receive ambient precipitation, but allow determination of treatment infrastructure artifacts, and 4) ambient control plots. Our drought structures effectively reduced soil water potential and volumetric water content compared to the ambient, cover-control, and water addition plots. Drought and cover control plots experienced an average increase in maximum soil and air temperature at ground level of 1-4° C during the growing season compared to ambient plots, and concurrent short-term diurnal increases in maximum air temperature were also observed directly above and below plastic structures. Our drought and irrigation treatments significantly influenced tree predawn water potential, sap-flow, and net photosynthesis, with drought treatment trees exhibiting significant decreases in physiological function compared to ambient and irrigated trees. Supplemental irrigation resulted in a significant increase in both plant water potential and xylem sap-flow compared to trees in the other treatments. This experimental design effectively allows manipulation of plant water stress at the ecosystem scale, permits a wide range of drought conditions, and provides prolonged drought conditions comparable to historical droughts in the past – drought events for which wide-spread mortality in both these species was observed. The focus of this study was to determine the effects of rainfall manipulation on our two target tree species.  Therefore, the analysis of the water relations of these trees was an essential component of the project.  Sap-flow within each individual target tree was monitored through the use of Granier probes.  These monitoring efforts provided a window on processes such as transpiration and the night-time re-filling of the xylem tissue.  Drought tolerance and adaptation strategies were also explored by comparing differences in sap-flow rates across treatment types and between species. 
    more » « less