Abstract: Extreme weather events are becoming more prevalent with increasing global temperatures. In the Northeastern U.S., spring frost events are destroying forest ecosystems by defoliating newly budded trees. In order to grasp a better understanding of community dynamics and carbon fluxes, it is imperative to understand more about species-specific phenological and physiological responses to these events. This study aimed to investigate the legacy effects of a spring frost event in Black Rock Forest on the specific photosynthetic and intrinsic water use efficiency responses within unaffected red maples and sugar maples alongside defoliated red oaks. A LI-6800 machine conducted gas exchange measurements in the north, south, valley, and headquarter sites for each species. The new flush of red oak leaves portrayed
the greatest amount of photosynthetic productivity and efficiency while red maples and sugar maples retained their original characteristics with increased sensitivities. Hence, the defoliated tree species had a competitive advantage with shifted phenological patterns. Future research can be conducted several growing seasons after the frost event to determine the extent to which these events impact species dynamics, including DBH tree growth. New predicative carbon models can also be formed to create new management for tree implantation’s that maximize sequestration rates.
Keywords: spring frost event, defoliation, photosynthetic productivity, water use efficiency, sequestration
I. Introduction
Due to changing climate conditions, extreme weather events including prolonged droughts, tropical storms, damaging spring frost events and heat waves are expected to become more common in forests of the northeastern United States (Richardson et al 2006, Diffenbaugh et al. 2018). In the northeastern United States, freezing events after bud break in the spring have had detrimental effects to deciduous forests by defoliating trees and consequently decreasing net carbon uptake (Vitasse et al. 2014, Nolè et al. 2018, Hufkens et al. 2012). Furthermore, research has suggested that a spring frost may have significant implications on the community composition of higher elevation hardwood forests in the northeast region (Hufkens et al. 2012). More specifically, they can tilt range margins and tree competition dynamics from sugar maples to other species (Hufkens et al. 2012). While the occurrence of this event is expected to increase, the understanding of the effects of a spring frost event on ecosystem-based carbon fluxes and the physiological sensitivities of co-existing tree species remains uncertain.
On May 8/9 of 2020, a late spring frost event occurred at higher elevations across the Hudson Highlands Region in southeastern New York, leading to temperatures declining from 25°C to just above -5°C. This region is cloaked with temperate broadleaf forests dominated by oak trees (Quercus sp.) and the hard freeze caused widespread defoliation of the newly emerged oak leaves. However, co-occurring red maple and sugar maple trees (Acer) leafed out after the frost event and were unaffected. The genus-specific influence of this spring frost event presents a unique opportunity to investigate species-specific physiological responses among red oaks (Q. rubra), red maples (A. rubrum) and sugar maples (A. saccharum). Doing so will increase knowledge of plant community dynamics and their effect on global biogeochemistry in the northeast, as these are three of the most common tree species in the region (Richardson et al. 2009) and there is evidence of an increase in the relative abundance of red maples across the eastern US (Abrams, 1998). Furthermore, although stochastic extreme climate events such as the spring frost have important carbon cycling implications (Príncipe et al. 2017), they are difficult to predict. Thus, through photosynthetic measurements, this research can help directly study the influence of these events on canopy carbon exchange.
Previous research has depicted the damaging effects that spring frost events can have on a forest ecosystem (Vitesse et al. 2014) and how variations in phenology and physiology could impart species-specific responses to such conditions (Hanninen & Tanino 2011, Kim et al. 2014). This study aimed to quantify the differential effects of this late spring frost event on the trends regarding photosynthetic capacity and water-use efficiency of red oaks, red maples and sugar maple trees in the Hudson Highlands Region of New York across the growing season. It was expected that at the start of the growing season, red oaks would have a significantly smaller photosynthetic capacity and efficiency as compared to red maples and sugar maples. Thus, the maples were predicted to increase in competitiveness.
II. Materials and Methodology
From late May through June 2020, leaf-level measurements were made every two weeks and monthly thereafter through September. Morning shotgun sampling was used to obtain branches <1.5 cm diameter from the upper canopy of each tree on each date. To maintain transpiration stream, the lower part of each branch was immediately snipped and submerged into a container of water that rested in the sunlight, allowing the leaves to acclimate to conditions prior to being measured. Gas exchange measurements were conducted within 45 minutes of initial leaf detachment from the tree to allow the leaves on excised branches to maintain constant gas exchange rates for at least one hour.
Leaf level gas exchange measurements, including carbon assimilation (i.e. photosynthesis; A) and stomatal conductance (gsw), were made using the LI-6800 (LI-COR Inc., Lincoln, Nebraska, USA) with the chamber set to saturating light conditions (i.e. 1400 µmoles/m2/s), a temperature of 24-26 °C and 60% relative humidity. This machine uses a mass balance approach and can find the photosynthetic rate, also known as the ‘A’ value, by calculating the net CO2 assimilation of a leaf placed in its chamber. Similarly, it also calculates the stomatal conductance, also known as ‘gsw’ of a leaf, which measures the amount of water vapor exiting through the stomata of a leaf. A and gsw were used to calculate the intrinsic WUE, the WUE of a tree at the leaf level, using the following equation: A/gsw (Medrano et al. 2015).
III. Results
Photosynthesis Rates
The defoliated red oaks had a mean rate of photosynthesis for the growing season that was significantly greater than that of the red maples and sugar maples by 3.8 µmol m-2 s-1 (Fig. 1). The defoliated red oaks and non defoliated red oaks alongside the red maples and sugar maples cannot be statistically compared to each other, as their error bars overlap each other and each other means (Fig. 1). Red maple and sugar maples had nearly the same mean photosynthetic rates (Fig. 1).
Intrinsic Water Use Efficiency Rates
The non defoliated red oaks, otherwise known as the control group, had the highest mean intrinsic water use efficiency of 120.1 µmol CO2/mol H20 across the growing season (Fig. 2). Due to the error bars of the defoliated red oaks, red maples and sugar maples overlapping each other and each other's means, they cannot be statistically compared to one another (Fig. 2). However, the defoliated red oaks did have the lowest, but comparatively moderate, mean water use efficiency of 78.84 µmol CO2/mol H20 (Fig. 2). Similar to their photosynthetic rates, the sugar maples and red maples had nearly identical mean intrinsic water use efficiencies (Fig. 2).
IV. Discussion
Photosynthesis
While the red maples and sugar maples initially had a competitive advantage by breaking bud after the hard freeze event, their photosynthetic rates were eventually overcome by that of the red oaks. Both species behaved nearly identically in the early and late growing season by being initially photosynthetically productive and exhibiting declining productivity by mid growing season.
Intrinsic Water Use Efficiency
Sugar maples and red maples tended to decrease their intrinsic water use efficiency later into the growing season. More specifically, within sites with more arid conditions, including the north site for red maples and headquarters for sugar maples, WUE was declining significantly, portraying the species inability to counteract their sensitivities to ecostress. This decline contradicts the basis that a plant would attempt to increase its water use efficiency when there was limited water availability for photosynthesis (Hatfield & Dold, 2019).
Reasons
The lack of competitiveness and productivity for red maples may have been due to the dry conditions they faced during the latter part of the growing season. Red maples typically decrease productivity when there are drought-like conditions and vapor pressure deficits (Anderson & Ryser, 2015). Hence, it is possible that the red maples shut down their photosynthetic processes by closing their stomata during arid conditions. For the sugar maples, their general inability to deal with the hotter temperatures in the growing season contradicts their typical conservative nature and self-developed resistant mechanisms which allow them to have a long life span (Goldblum & Kennett, 2010). This poses a question regarding the specific temperature sensitivity of photosynthesis within sugar maples, as they may have been focusing more heavily on cooling off then being productive. Overall, the maples chose to hinder photosynthetic processes during hotter temperatures and instead use their energy and water availability to cool down with transpiration.
V. Conclusion
This research demonstrates the wide effects that a spring frost event can have on certain species and overarching community dynamics. The second flush of leaves from the defoliated tree species, which in this case is the red oaks, have enhanced resistance mechanisms in regards to changing environmental conditions and thus are more photosynthetically productive. On the other hand, the species that are unaffected by the freeze event were not able to take proper advantage of their foliated conditions. They retained their original characteristics but were also more susceptible to arid conditions. In the case of this study, the idea that red maples are harmed by drought-like conditions is reinforced, while the idea of sugar maples maintaining relatively consistent physiological habits is variable. Naturally, species with more productivity maintain enhanced intrinsic WUE mechanisms.
There are certain legacy effects for this late spring frost event and others like it in the northeastern US. These events will likely change competition dynamics in an ecosystem, as the second flush of leaves from the defoliated tree species will be younger and stronger. Furthermore, these events will most likely have a complex change on the carbon sequestration of the affected region. There will be an initial significant decrease in net carbon intake due to the negative photosynthetic rates of the defoliated tree species. This may be counteracted, however, by the high rates of photosynthesis provided by the same defoliated tree species later on into the growing season.
References
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