Full Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=tphy20

Download by: [Eastern Washington University] Date: 15 June 2017, At: 10:38

Physical Geography

ISSN: 0272-3646 (Print) 1930-0557 (Online) Journal homepage: http://www.tandfonline.com/loi/tphy20

Annual growth zones in stems of Hypericumirazuense (Guttiferae) in the Costa Rican páramos

Matthew T. Kerr, Sally P. Horn, Henri D. Grissino-Mayer & Lauren A.Stachowiak

To cite this article: Matthew T. Kerr, Sally P. Horn, Henri D. Grissino-Mayer & Lauren A.Stachowiak (2017): Annual growth zones in stems of Hypericum irazuense (Guttiferae) in the CostaRican páramos, Physical Geography, DOI: 10.1080/02723646.2017.1340714

To link to this article: http://dx.doi.org/10.1080/02723646.2017.1340714

Published online: 15 Jun 2017.

Submit your article to this journal

View related articles

View Crossmark data

Physical GeoGraPhy, 2017https://doi.org/10.1080/02723646.2017.1340714

Annual growth zones in stems of Hypericum irazuense (Guttiferae) in the Costa Rican páramos

Matthew T. Kerra, Sally P. Horna, Henri D. Grissino-Mayera and Lauren A. Stachowiaka,b aDepartment of Geography, The University of Tennessee, Knoxville, TN, Usa; bDepartment of Geography and anthropology, eastern Washington University, cheney, Wa, Usa

ABSTRACTThe high peaks of the Cordillera de Talamanca in Costa Rica support shrub- and grass-dominated páramo ecosystems that experience stand-replacing wildfires. The dry season that facilitates these fires results in dormancy in plant growth and provides an opportunity to use dendrochronological analyses to determine ages of plants in burn sites to support studies of fire history and postfire vegetation recovery. This study investigates the formation of annual growth zones in stems of the common shrub Hypericum irazuense. Unlike other páramo shrubs, H. irazuense rarely resprouts following fire, instead recovering through seedling recruitment following seed dispersal from the unburned periphery. Laboratory analysis of 19 prepared cross sections from 15 stems shows that samples of H. irazuense from burned areas can provide an estimate of the minimum number of years since the previous fire, supporting earlier work based on field examination of stems. Including a time lag for seedling recruitment or resprouting refines that estimate. Counts of growth zones in most sections coincided with dates of the last known fires. The presence of annual growth zones in H. irazuense places the species within a relatively small group of woody Neotropical species for which annual rings or growth zones have been demonstrated.

Introduction

Understanding fire regimes and postfire vegetation recovery is important for science and for management of many vegetation types, including the high-elevation páramos of the north-ern Andes and the Cordillera de Talamanca of Costa Rica and Panama (Horn, 1998; Horn & Kappelle, 2009; Keating, 1998). Studies of lake-sediment cores and soil profiles in Costa Rican páramos have demonstrated histories of fire linked to climate change and human activity (Horn, 1993; Horn & Sanford, 1992; Islebe & Hooghiemstra, 1997; Lane & Horn, 2013; League & Horn, 2000), but these analyses lack the temporal resolution necessary for reconstructing fire history on annual to decadal timescales.

KEYWORDSFire history; growth zones; páramo; tree rings; tropical dendrochronology; vegetation recovery

ARTICLE HISTORYreceived 19 December 2016 accepted 7 June 2017

© 2017 informa UK limited, trading as Taylor & Francis Group

CONTACT Matthew T. Kerr mkerr6@vols.utk.edu

2 M. T. KERR ET AL.

Dendrochronology is widely used to reconstruct climate and fire history with annual resolution in temperate latitudes, where annual dormant periods allow the formation of unambiguous annual rings (Fritts, 1976; Grissino-Mayer & Swetnam, 2000; Speer, 2010; Villalba et al., 2011). Climates of many areas of the tropics and subtropics have low intra-an-nual variability in temperature and rainfall, and therefore do not lead to the formation of tree rings. Nevertheless, the field of tropical dendrochronology is well developed (Worbes, 1995, 2002). Several studies (Anchukaitis et al., 2012; Francisco, Hart, Li, Cook, & Baker, 2015; Harley, Grissino-Mayer, & Horn, 2011; Jacoby, 1989; Martin & Fahey, 2006; Speer, Orvis, Grissino-Mayer, Kennedy, & Horn, 2004; Stahle, 1999; Worbes & Fichtler, 2010) have demonstrated the potential for tree species in the tropics and subtropics to form annual rings that are useful for dendrochronology and several studies on Costa Rican trees (Fichtler, Clark, & Worbes, 2003; Fichtler, Helle, & Worbes, 2010; Worbes, 1989; Worbes, Blanchart, & Fichtler, 2013; Worbes & Junk, 1989) have shown annual periodicity in tree growth. For locations and plant species that do not form annual rings, the presence of “growth zones” can still allow an estimate of plant age (Fichtler et al., 2003; Schweingruber, 1992; Worbes & Fichtler, 2010).

In this study, we examine Hypericum irazuense Kuntze ex N. Robson (Robson, 1987), which is a common and locally dominant shrub in the Buenavista and Chirripó páramos of

Figure 1. Field photographs of H. irazuense (a) and horn measuring shrub heights (b).

PHYSICAL GEOGRAPHY 3

Costa Rica (Figures 1 and 2), along with the ericaceous shrubs Vaccinium consanguineum Klotzsch and Gaultheria myrsinoides Kunth (=Pernettya prostrata (Cav.) DC), and the dwarf bamboo Chusquea subtessellata Hitchc. (Kappelle & Horn, 2005, 2016). Macroscopic “rings” in páramo shrubs in Costa Rica have been examined in previous studies (Horn, 1989; Janzen, 1973; Schlachter, Grissino-Mayer, & Horn, 2007; Weberling & Furchheim-Weberling, 2005; Williamson et al., 1986); however, no published study has fully assessed the dendrochronological structure and potential of these shrub species. Our study provides a new examination of H. irazuense using established laboratory and dendrochronological methods and advances ecological knowledge of Costa Rican páramos, an ecosystem that is threatened by climate change and anthropogenic activity.

Seasonal drought in the Costa Rican páramos provides the potential for the formation of annual rings or growth zones in H. irazuense. Schweingruber (1992) defined growth zones as “latewood-earlywood boundaries [that] are not always or are never distinct and often do not go around the entire circumference of the stem.” In his study of wood anatomy of trees and shrubs in southern Australia, Schweingruber (1992) classified cross-sectional forms into cat-egories, from Class 1 (distinct, true annual growth rings) to Class 5 (no variation in growth, age cannot be determined). Macroscopic observation of cross sections of H. irazuense suggested to us that this plant conforms to Schweingruber’s Class 2–3: growth zones are distinct in one part of the stem, but are often difficult to follow around the circumference. The growth zones present in our cross sections likely represent annual formation, but also may include false rings or zones and intra-annual variation that cannot be differentiated

Figure 2. Map of study location in the cordillera de Talamanca, costa rica (modified from horn, 1989; used with permission of the california Botanical society). shading indicates the approximate distribution of páramo vegetation. Triangles are major peaks along the crest of the range. Based on field observations and the 1:50,000 topographic maps published by the instituto Geográfico Nacional.

4 M. T. KERR ET AL.

from true “rings.” In such cases the age of the stem may be estimated to within a few years, but the growth zones cannot be used for crossdating (Mucha, 1979; Schweingruber, 1992).

Our research on H. irazuense involved laboratory examination of growth zones in pre-pared samples from multiple páramo burn sites, building on field analysis at a single site by Williamson et al. (1986). Our purpose was to characterize patterns of wood growth and to test whether growth zones were annually formed by comparing counts of growth zones from samples with known cutting dates with the timing of the last known fires at sites. If H. irazuense forms annual rings or growth zones, then analyses of cross sections from living stems in areas of postfire recovery can provide estimates of the minimum number of years since the previous fire. Analyses of stems killed in the last fire or earlier fires, if present, can provide estimates of the number of years between previous fires (Horn, 1989, 1991; Janzen, 1973; Williamson et al., 1986). Knowing the recent history of fire is critical for studies of postfire vegetation recovery that can inform ecosystem management (Horn, 1998), but this information is often not available for remote sites in the Costa Rican páramos.

We asked three questions in our research: (1) do stems of H. irazuense show distinct annual growth zones that can be used to determine years of post-fire regeneration, and hence the time of the last fire at páramo burn sites, (2) are counts of growth rings or zones consistent between analysts (indicating replicability of observations), and (3) does staining samples with phloroglucinol improve visibility of growth rings or zones?

Methods

Study area

Our study made use of a collection of eleven live and four dead H. irazuense stems (originally ca. 1–2 m in length) that Horn cut with a handsaw from eleven plants at three páramo sites in 1984 and 1985. The stems we examined were from two sites (Asunción and Zacatales) in the Buenavista páramo along the Inter-American Highway route across the Cordillera de Talamanca and one site (Conejos) in the more remote páramo surrounding Cerro Chirripó, Costa Rica’s highest peak (Figure 2). Meteorological data show a mean annual temperature of 7.6 °C and total annual rainfall of ca. 2500 mm, with about 90% of precipitation falling during the May–November wet season (Horn, 1989; Figure 3). Low rainfall during the dry season, especially during February and March, allows ground litter to dry out and provides fuel for wildfires (Horn, 1989, 1991, 1997; Horn & Kappelle, 2009; Williamson et al., 1986).

The Asunción site is located on the steep south slope of Cerro Asunción at ca. 3300 m elevation in the Buenavista páramo. Janzen (1973) studied postfire vegetation recovery three years after a fire that occurred during the dry season of 1969. Horn collected living and dead stems of H. irazuense in November 1984 and March 1985, approximately 16 years after the fire.

The Zacatales site (Horn, 1989; Williamson et al., 1986) is located on the steep south-fac-ing slope of Cerro Zacatales, also in the Buenavista páramo. Horn collected living and dead stems of H. irazuense in December 1984 and April 1985 between 3340 and 3370 m elevation within the same area sampled by Williamson et al. (1986) in 1982. Williamson et al. (1986) reported that the previous fire at the site occurred between early February and late March of 1973. Following Williamson et al. (1986), we interpret the dead stems Horn sampled to have been killed by the penultimate fire at the site.

PHYSICAL GEOGRAPHY 5

The Conejos site (Horn, 1989) is located within a broad basin at the head of the glaciated Río Talari valley in the Chirripó páramo, the upper section of which is locally known as the Valle de los Conejos. The site lies on a south-facing slope near 3500 m elevation. The area last burned during a ca. 5000-ha fire that swept through the highlands in March 1976 (Chaverri, Vaughan, & Poveda, 1976). Horn collected stems from H. irazuense shrubs in February 1985, nine years after the fire.

Field sampling

In the field, Horn cut stems as close as practical to the base of shrubs. She then made additional cuts to examine cross sections at different points along the stems and to create shorter stem sections for easier transport (e.g. 0–30 cm, 30–60 cm). From each typically multi-stemmed shrub, she cut the largest accessible stems. Her intention was to use field counts of rings to determine fire recurrence intervals for a study of postfire regeneration (Horn, 1989) following Williamson et al. (1986). She was uncertain of her field counts and did not use them in her study, but archived the samples. The existence of these samples, unstudied since field collection, gave us an opportunity to investigate in more detail the dendrochronological potential of the species without need for additional destructive sam-pling in protected areas of Costa Rica.

Preparation and analysis of cross sections

In the lab, from all but the smallest stems, we cut cross sections 1–2 cm in height from the bottom end of each stem and then sanded each cross section with progressively finer grit sandpaper beginning with ANSI 120-grit (105–125 μm) and ending with ANSI 400-grit

Figure 3. climograph for cerro Páramo Meteorological station (3466 m), Buenavista highlands, costa rica. Bars represent mean monthly precipitation totals for the period 1971–1984 and dashed curve represents mean monthly temperatures for the period 1971–1979 (after horn, 1989).

6 M. T. KERR ET AL.

(20.6–23.6 μm) (Orvis & Grissino-Mayer, 2002). We processed smaller diameter stems with the same sanding technique, but did our analysis on the prepared end of the entire stem piece, rather than from a small cross section.

After sanding, we scanned each cross section using an Epson 10000XL flatbed scanner at 4800 dpi resolution to document the growth patterns in our samples. We next examined the prepared cross sections under a stereozoom microscope at 7× magnification to estimate the number of growth zones and at 45× magnification to assess cellular-level structure for identifying growth zones. Kerr first estimated the number of growth zones in all cross sections, but he had prior knowledge of sampling and fire dates. To ensure no bias in our counts, Grissino-Mayer and Stachowiak independently estimated the number of growth zones in the cross sections. Neither had knowledge of Kerr’s counts nor of sampling or fire dates, but both have extensive experience in dendrochronology. We then averaged our three counts to produce an estimate of the mean number of growth zones in each cross section. Following these initial counts, we treated the cross sections with phloroglucinol (Patterson, 1959; Phloroglucinol Increment Core Dye, No. 63485, Forestry Suppliers, Inc.) and recounted them to assess the effectiveness of staining on the visibility of growth zones in H. irazuense stems.

High-resolution scans of all cross sections along with photographs of study sites and plant habitats are archived in an online database at the University of Tennessee, Knoxville (Horn & Kerr, 2017).

Figure 4. cross sections of asunción Hypericum #1 from the asunción site: 10 cm (a), 30 cm (b), 60 cm (c), 90 cm (d), 120 cm (e), and dead stem (f ). see Table 1 for stem diameters.

PHYSICAL GEOGRAPHY 7

Results

Macroscopically, cross sections of H. irazuense exhibit alternating bands of light- and dark-colored cells that suggest periodic growth zones. The dark-colored, wider portion of the growth zone is likely earlywood, while the lighter-colored, narrower portion of the growth zone is latewood. These bands frequently form complete discernable “rings” around the circumference of the stem in the early years of growth, but generally become only partial rings or become faint and indistinct as H. irazuense ages (Figure 4; see Horn and Kerr (2017) and associated online files for additional figures and high-resolution scans). Microscopically, viewing at 7× magnification allowed an estimate of the number of growth zones in the cross sections (Table 1). At 45× magnification, however, no cellular-level differentiation was apparent beyond light/dark as we could not identify any clear terminal parenchyma. Additionally, at magnification greater than ca. 10×, the light/dark bands become less dis-tinguishable and the cross sections suggest that multiple narrower zones of light and/or dark cells can be formed throughout the growing season, analogous to intra-annual density fluctuations (i.e. false rings).

Estimates of the number of growth zones on basal cross sections and from positions higher up the stems corresponded with the dates of last known fires (Table 1). At the Asunción site, Hypericum #1 and #2 had means of 12 and 11 growth zones, respectively. Adding a recruitment lag of 4 years makes these counts compatible with an interval of 15–16 years between the 1969 fire and sample collection in 1984–1985. Asunción Hypericum #3, a shrub that survived the 1969 fire without complete crown loss, showed 24 growth zones, with evidence of damaged tissue between zones 15 and 16 that we interpret to be

Table 1. Growth zone counts in cross sections of Hypericum irazuense.

Notes: asunción Hypericum #4 was not within the 1969 burn site, but across the road from the site.initials below Growth Zones refer to coauthors Kerr, Grissino-Mayer, and stachowiak. sD = standard deviation of replicate

counts.

Site Sample IDDiameter

(cm)

Growth zones (MTK)

Growth zones (HGM)

Growth zones (LAS)

Mean number of

zones SDasunción asuncíon Hypericum #1              10 cm 3.15 12 13 12 12 0.58  30 cm 2.02 12 10 12 11 1.15  60 cm 1.77 12 9 10 10 1.53  90 cm 1.49 12 7 12 10 2.89  120 cm 1.11 5 4 7 5 1.53  Dead stem 1.73 20 19 20 20 0.58  asunción Hypericum #2 1.75 10 12 10 11 1.15  asunción Hypericum #3 3.60 24 25 24 24 0.58  asunción Hypericum #4 3.68 24 23 35 27 6.66Zacatales Zacatales Hypericum #1              35 cm 0.76 2 3 5 3 1.53  Dead stem 2.33 10 9 12 10 1.73  Zacatales Hypericum #2              Dead stem 2.54 10 8 10 9 1.15  Zacatales Hypericum #3              0 cm 1.64 7 6 8 7 1.00  5 cm 1.15 5 3 6 5 1.53  Dead stem 1.25 7 6 10 8 2.08  Zacatales Hypericum #4 1.03 4 3 5 4 1.00conejos conejos Hypericum #1 1.10 8 7 7 7 0.58  conejos Hypericum #2 0.94 7 5 7 6 1.15  conejos Hypericum #3 0.95 6 6 7 6 0.58

8 M. T. KERR ET AL.

a wound from the 1969 fire (Horn & Kerr, 2017). Asunción Hypericum #4 was collected across the highway from the 1969 burn zone and had 27 growth zones, suggesting the last fire there occurred in the 1950s.

The three stems from the Conejos burn site all had 6–7 growth zones. As with the Asunción samples, once 2–3 years are added for a recruitment lag, zone counts correspond with a fire date of 1976, nine years prior to the date of collection. Samples from the Zacatales site were problematic. The samples from Hypericum #1 and #4 were too small or too high on the plant to yield an accurate zone count. Cross sections from 0 and 5 cm on Zacatales Hypericum #3 have seven and five growth zones, respectively, suggesting that the stem had established many years after the 1973 fire. Williamson et al. (1986) reported a mean of 9.1 ± 0.7 rings in field examination of large living H. irazuense stems (n = 7) sampled at the site in 1982, a result in keeping with the past fire date of 1973. For the dead stems from Zacatales Hypericum #1, #2, and #3 that resprouted or established from seed following the penultimate fire, our average growth zone count was nine years, close to the mean count of 8.4 ± 1 rings reported by Williamson et al. (1986) for 9 large dead stems collected from seven plants in 1982. These results suggest that the site had previously burned in the early 1960s.

Stained samples

Recounts of growth zones in the samples stained with phloroglucinol were generally in agreement with counts from unstained samples (Figure 5, Table 2). In most cases, the mean numbers of zones in the stained samples were identical to the unstained samples or differed by one. Zone counts for three samples were plus or minus two, and for one sample the count was minus three. Due to the standard deviations of our replicate zone counts (Table 2), these differences did not affect our interpretations of years since previous fire.

Discussion

Growth patterns in stems of H. irazuense

Cross sections of H. irazuense stems from the Buenavista and Chirripó páramos of Costa Rica show annual growth zone formation based on the analysis of 19 cross sections from 15 stems in this study. Our study advances general and dendrochronological knowledge about one of the dominant species in Costa Rican páramo, a high-elevation ecosystem of lim-ited distribution in southern Central America that is threatened by anthropogenic climate change. Counts of the number of growth zones in the cross sections were consistent with the known dates of previous fires at the sample sites and expected recruitment dynamics. We conclude that counts of growth zones in H. irazuense can provide a reliable proxy for estimating past fire dates in páramo ecosystems, confirming the work of Williamson et al. (1986). However, we regard the structures that Williamson et al. (1986) and Weberling and Furchheim-Weberling (2005) termed “rings” to be growth zones.

The intra-annual variation in growth zones we observed in microscopic examination and the occurrence of what could be termed “false growth zones” analogous to false rings in ring-porous species may correspond to intra-annual climate variations, such as the severity of a secondary rainfall minimum (Figure 3). Our current data do not support speculation on what site conditions might lead to clearer growth zones or greater accuracy of zone counts

PHYSICAL GEOGRAPHY 9

at one site compared to another. However, we observed that larger and/or older stems tend to have growth zones that are easier to count. We think the clearer rings in the older/larger stems may be because of the presence of more heartwood, which is darker in color and has more distinct rings than the lighter sapwood in this species.

H. irazuense as an indicator of time since last fire

Our analysis of H. irazuense samples supports the use of growth patterns in stem cross sections to estimate the minimum time since the last fire. This technique can provide a way to determine the age of a fire for which no records or observations exist or to identify the burn perimeter for a fire of known age but uncertain extent. However, using this species to determine fire history requires consideration of a likely recruitment lag. H. irazuense generally shows low rates of resprouting ability following crown loss in fires and seed banks are absent or minimal. Postfire recolonization thus relies mainly on the dispersal of seeds from the unburned periphery (Horn, 1989, 1997; Williamson et al., 1986), resulting in a lag between wildfire events and recolonization of a burned area by H. irazuense. Shrubs that survive and resprout may also show this lag, as they continue to produce resprouts for many years following the fire, and stems selected years later for sampling may not be the oldest stems, even if they are the largest (Keeley, 2000). Our study indicates that the lag associated with seedling establishment or delayed sprouting is ca. 1–4 years for H. irazuense in the Costa Rican páramos.

Figure 5. cross sections of stained and unstained H. irazuense samples: asunción Hypericum #1 60 cm (a, b); asunción Hypericum #4 (c, d); conejos Hypericum #1 (e, f ). see Table 1 for stem diameters.

10 M. T. KERR ET AL.

Replicability of counts by different observers

We found close correspondence in zone counts by three observers, a correspondence long demonstrated in dendrochronology through skeleton plotting (Speer, 2010). While Kerr counted growth zones with prior knowledge of the dates for both sample collection and previous fires at the sites, Grissino-Mayer and Stachowiak examined samples blind. Our verification of growth zone counts by analysts expert in dendrochronology but unfamiliar with sampling and fire dates strengthened our conclusion that growth zones can be detected and have environmental meaning. In the vast majority of cases, growth zone counts by Grissino-Mayer and Stachowiak corresponded closely to growth zone counts made by Kerr, suggesting that observer bias was low in this study.

Phloroglucinol stain

Staining the Hypericum samples changed our growth zone counts in several instances (Figure 5, Table 2); however, our recounts of stained samples with standard deviations overlapped our replicate observations on unstained samples. Therefore, in no case did staining alter our interpretations of dates since previous fires. The close correspondence between our growth zone counts in stained and unstained samples suggests that staining with phloroglucinol should be a personal preference of the analyst. Staining added extra

Table 2. comparison of mean growth zone counts in stained and unstained H. irazuense samples.

Note: sD = standard deviation of replicate counts.

Site Sample ID Zones unstained SD unstained Zones stained SD stained ±asunción asunción Hypericum

#1         

  10 cm 12 0.58 12 0.58 0  30 cm 11 1.15 11 1.15 0  60 cm 10 1.53 10 2.52 0  90 cm 10 2.89 9 3.00 −1  120 cm 5 1.53 5 0.58 0  Dead stem 20 0.58 17 2.52 −3  asunción Hypericum

#211 1.15 9 1.53 −2

  asunción Hypericum #3

24 0.58 24 1.00 0

  asunción Hypericum #4

27 6.66 29 6.56 +2

Zacatales Zacatales Hypericum #1

         

  35 cm 3 1.53 3 0.58 0  Dead stem 10 1.73 10 1.00 0  Zacatales Hypericum

#2         

  Dead stem 9 1.15 8 1.00 −1  Zacatales Hypericum

#3         

  0 cm 7 1.00 7 0.58 0  5 cm 5 1.53 6 2.08 +1  Dead stem 8 2.08 7 0.58 −1  Zacatales Hypericum

#44 1.00 6 1.53 +2

conejos conejos Hypericum #1 7 0.58 7 0.58 0  conejos Hypericum #2 6 1.15 6 0.58 0  conejos Hypericum #3 6 0.58 6 0.58 0

PHYSICAL GEOGRAPHY 11

time to the analysis process, but it is simple and inexpensive and may enhance contrast between growth zones for some observers.

Recommendations

Despite the success of Williamson et al. (1986) in counting growth zones in the field on unprepared samples, we recommend laboratory examination of stem samples that have been sanded following standard protocols for H. irazuense and other tropical species in which true rings are not present or not consistently formed. We also recommend photo documentation of cross sections and sharing of photographs in publications and databases to help advance knowledge of the wood anatomy of species that are relatively new in den-drochronology. Finally, based on our results, we suggest that future analyses of growth zones or rings in tropical species with unproven dendrochronological potential include independent observers who have no prior knowledge of the dates of sample collection, fires, or other benchmark events used to determine whether growth zones or rings are annual.

Acknowledgements

The Hypericum irazuense samples we examined were collected by SPH with the permission of the Costa Rican National Park Service, during dissertation research supported by the Fulbright U.S. Student Program. Roger Horn assisted in the field and Carolyn Hall and the late Rodrigo Saborio pro-vided logistical and other support. Dendrochronological analyses were carried out in the Laboratory of Tree-Ring Science at the University of Tennessee. Alice Schoen assisted with sample preparation.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Lauren A. Stachowiak   http://orcid.org/0000-0001-6759-3072

References

Anchukaitis, K. J., Taylor, M. J., Martin-Fernandez, J., Pons, D., Dell, M., Chopp, C., & Castellanos, E. J. (2012). Annual chronology and climate response in Abies guatemalensis Rehder (Pinaceae) in Central America. The Holocene, 23, 270–277. doi:10.1177/0959683612455548

Chaverri, A., Vaughan, C., & Poveda, L. J. (1976). Informe de la gira efectuada al macizo de Chirripó al raiz del fuego ocurrido en Marzo de 1976. [Report on the visit to the Chirripó massif made as a result of the March 1976 fire.] Revista de Costa Rica, 11, 243–279.

Fichtler, E., Clark, D. A., & Worbes, M. (2003). Age and long-term growth of trees in an old-growth tropical rain forest, based on analyses of tree rings and 14C. Biotropica, 35, 306–317. doi:10.1646/03027

Fichtler, E., Helle, G., & Worbes, M. (2010). Stable-carbon isotope time series from tropical tree rings indicate a precipitation signal. Tree-Ring Research, 66, 35–49. doi:10.3959/2008-20.1

Francisco, K. S., Hart, P. J., Li, J., Cook, E. R., & Baker, P. J. (2015). Annual rings in a native Hawaiian tree, Sophora chrysophyla, on Maunakea, Hawai’i. Journal of Tropical Ecology, 31, 567–571. doi:10.1017/S026646741500036X

Fritts, H. C. (1976). Tree rings and climate. New York, NY: Academic Press.

12 M. T. KERR ET AL.

Grissino-Mayer, H. D., & Swetnam, T. W. (2000). Century scale climate forcing of fire regimes in the American Southwest. The Holocene, 10, 213–220. doi:10.1191/095968300668451235

Harley, G. L., Grissino-Mayer, H. D., & Horn, S. P. (2011). The dendrochronology of Pinus elliottii in the lower Florida keys: Chronology development and climate response. Tree-Ring Research, 67, 39–50. doi:10.3959/2010-3.1

Horn, S. P. (1989). Postfire vegetation development in the Costa Rican páramos. Madroño, 36, 93–114. Retrieved from http://www.jstor.org/stable/41424741

Horn, S. P. (1991). Fire history and fire ecology in the Costa Rican páramos. In S. C. Nodvin & T. A. Waldrop (Eds.), Fire and the environment: Ecological and cultural perspectives: Proceedings of an international symposium (Report SE-69) (pp. 289–296). Asheville, NC: USDA, Forest Service, Southeastern Forest Experiment Station.

Horn, S. P. (1993). Postglacial vegetation and fire history in the Chirripó Páramo of Costa Rica. Quaternary Research, 40, 107–116. doi:10.1006/qres.1993.1061

Horn, S. P. (1997). Postfire resprouting of Hypericum irazuense in the Costa Rican páramos: Cerro Asunción revisited. Biotropica, 29, 529–531. Retrieved from http://www.jstor.org/stable/2388948

Horn, S. P. (1998). Fire management and natural landscapes in the Chirripó Páramo, Chirripó National Park, Costa Rica. In K. S. Zimmerer & K. R. Young (Eds.), Nature’s geography: New lessons for conservation in developing countries (pp. 125–146). Madison: University of Wisconsin Press.

Horn, S. P., & Kappelle, M. (2009). Fire in the páramo ecosystems of Central and South America. In M. A. Cochrane (Ed.), Tropical fire ecology (pp. 505–539). Berlin: Springer.

Horn, S. P., & Kerr, M. T. (2017). Hypericum irazuense Kuntze ex N. Robson in the Buenavista and Chirripó páramos of Costa Rica: Photographs of stem cross sections, plants, habitats, and study sites. Knoxville: University of Tennessee. Retrieved from the Tennessee Research and Creative Exchange: http://trace.tennessee.edu/utk_geogpubs/15

Horn, S. P., & Sanford, R. L. (1992). Holocene fires in Costa Rica. Biotropica, 24, 354–361. Retrieved from http://www.jstor.org/stable/2388605

Islebe, G. A., & Hooghiemstra, H. (1997). Vegetation and climate history of montane Costa Rica since the last glacial. Quaternary Science Reviews, 16, 589–604. doi:10.1016/S0277-3791(96)00051-0

Jacoby, G. C. (1989). Overview of tree-ring analysis in tropical regions. IAWA Journal, 10, 99–108.Janzen, D. H. (1973). Rate of regeneration after a tropical high elevation fire. Biotropica, 5, 117–122.

Retrieved from http://www.jstor.org/stable/2989661Kappelle, M., & Horn, S. P. (Eds.). (2005). Páramos de Costa Rica [Páramos of Costa Rica]. Santo

Domingo de Heredia: INBio Press.Kappelle, M., & Horn, S. P. (2016). The páramo grasslands of Costa Rica’s highlands. In M. Kappelle

(Ed.), Costa Rican ecosystems (pp. 492–524). Chicago, IL: University of Chicago Press.Keating, P. L. (1998). Effects of anthropogenic disturbances on páramo vegetation in Podocarpus

National Park, Ecuador. Physical Geography, 19, 221–238. doi:10.1080/02723646.1998.10642648Keeley, J. E. (2000). Chaparral. In M. B. Barbour & W. D. Billings (Eds.), North American terrestrial

vegetation (2nd ed., pp. 203–253). New York, NY: Cambridge University Press.Lane, C. S., & Horn, S. P. (2013). Terrestrially derived n-alkane δD evidence of shifting holocene

paleohydrology in highland Costa Rica. Arctic, Antarctic, and Alpine Research, 45, 342–349. doi:10.1657/1938-4246-45.3.342

League, B. L., & Horn, S. P. (2000). A 10 000 year record of Páramo fires in Costa Rica. Journal of Tropical Ecology, 16, 747–752. Retrieved from http://www.jstor.org/stable/3068684

Martin, P. H., & Fahey, T. J. (2006). Fire history along environmental gradients in the subtropical pine forests of the Cordillera Central, Dominican Republic. Journal of Tropical Ecology, 22, 289–302. doi:10.1017/S0266467406003178

Mucha, S. B. (1979). Estimation of tree ages from growth rings of eucalypts in northern Australia. Australian Forestry, 42, 13–16.

Orvis, K. H., & Grissino-Mayer, H. D. (2002). Standardizing the reporting of abrasive papers used to surface tree-ring samples. Tree-Ring Research, 58, 47–50. Retrieved from http://hdl.handle.net/10150/262564

Patterson, A. E. (1959). Distinguishing annual rings in diffuse porous tree species. Journal of Forestry, 57, 129.

PHYSICAL GEOGRAPHY 13

Robson, N. K. B. (1987). Studies in the genus Hypericum L. (Guttiferae). 7. Section 29. Brathys (part 1). Bulletin of the British Museum (Natural History), 16, 1–106. Retrieved from http://www.biodiversitylibrary.org/item/19379#page/6/mode/1up

Schlachter, K. J., Grissino-Mayer, H. D., & Horn, S. P. (2007). Growth rings in stems of Vaccinium consanguineum in the high-elevation páramos of Costa Rica. Poster presented at the annual meeting of the Great Plains/Rocky Mountains Division of the Association of American Geographers, Denver, CO.

Schweingruber, F. H. (1992). Annual growth rings and growth zones in woody plants in Southern Australia. IAWA Journal, 13, 359–379. doi:10.1163/22941932-90001290

Speer, J. H. (2010). Fundamentals of tree-ring research. Tucson: University of Arizona Press.Speer, J. H., Orvis, K. H., Grissino-Mayer, H. D., Kennedy, L. M., & Horn, S. P. (2004). Assessing

the dendrochronological potential of Pinus occidentalis Swartz in the Cordillera Central of the Dominican Republic. The Holocene, 14, 563–569. doi:10.1191/0959683604hl732rp

Stahle, D. W. (1999). Useful strategies for the development of tropical tree-ring chronologies. IAWA Journal, 20, 249–253. doi:10.1163/22941932-90000688

Villalba, R., Luckman, B. H., Boninsegna, J., D’Arrigo, R. D., Lara, A., Villanueva-Diaz, J., … Pastur, G. M. (2011). Dendroclimatology from regional to continental scales: Understanding regional processes to reconstruct large-scale climatic variations across the western Americas. In M. K. Hughes, T. W. Swetnam, & H. F. Diaz (Eds.), Dendroclimatology: Progress and prospects (pp. 175–227). New York, NY: Springer.

Weberling, F., & Furchheim-Weberling, B. (2005). El mosaico de formas de crecimiento en los páramos de Costa Rica. [The mosaic of growth forms in the páramos of Costa Rica.] In M. Kappelle & S. P. Horn (Eds.), Páramos de Costa Rica [Páramos of Costa Rica] (pp. 437–473). Santo Domingo de Heredia: INBio Press.

Williamson, G. B., Schatz, G. E., Alvarado, A., Redhead, C. S., Stam, A. C., & Sterner, R. W. (1986). Effects of repeated fires on tropical paramo vegetation. Tropical Ecology, 27, 62–69.

Worbes, M. (1989). Growth rings, increment and age of trees in inundation forests, savannas and a mountain forest in the neotropics. IAWA Journal, 10, 109–122. doi:10.1163/22941932-90000479

Worbes, M. (1995). How to measure growth dynamics in tropical trees – A review. IAWA Journal, 16, 337–351. doi:10.1163/22941932-90001424

Worbes, M. (2002). One hundred years of tree-ring research in the tropics – A brief history and an outlook to future challenges. Dendrochronologia, 20, 217–231. doi:10.1078/1125-7865-00018

Worbes, M., Blanchart, S., & Fichtler, E. (2013). Relations between water balance, wood traits and phenological behavior of tree species from a tropical dry forest in Costa Rica – A multifactorial study. Tree Physiology, 33, 527–536. doi:10.1093/treephys/tpt028

Worbes, M., & Fichtler, E. (2010). Wood anatomy and tree-ring structure and their importance for tropical dendrochronology. In W. J. Junk, M. T. F. Piedade, F. Wittmann, J. Schöngart, & P. Parolin (Eds.), Amazonian floodplain forests: Ecophysiology, biodiversity, and sustainable management (pp. 329–346). Dordrecht: Springer.

Worbes, M., & Junk, W. J. (1989). Dating tropical trees by means of 14C from bomb tests. Ecology, 70, 503–507. doi:10.2307/1937554