Patrick H. Martin

Why forests appear resistant to exotic plant invasions: intentional introductions, stand dynamics, and the role of shade tolerance

Invasion ecology has traditionally focused on exotic plant species with early successional life-history traits, adapted to colonize areas following disturbance. However, the ecological importance of these traits may be overstated, in part because most invasive plants originate from intentional introductions. Furthermore, this focus neglects the types of plants most likely to invade established communities, particularly forests – namely shade-tolerant, late-successional species. In invasion ecology, it is generally assumed that undisturbed forests are highly resistant to plant invasions. Our review reveals that this assumption is not justified: in temperate and tropical regions around the world, at least 139 exotic plant species are known to have invaded deeply shaded forest understories that have not undergone substantial disturbance. These exotics present a particular management challenge, as they often increase in abundance during succession. While forest invasions may develop comparatively slowly under natural disturbance regimes, anthropogenic processes, including the spread of exotic pests and pathogens, can be expected to accelerate the rate of invasion.

Fire history along environmental gradients in the subtropical pine forests of the Cordillera Central, Dominican Republic

Fire history was reconstructed from fire-scarred individuals of the endemic pine (Pinus occidentalis )a long climatic gradients in the Cordillera Central, Dominican Republic. We analysed variation in fire frequency by climate, elevation and aspect (windward and leeward of the central massif). A high correspondence between known fires and fire-scar chronologies indicates that the primary rings of this species are annual. Evidence was found for 41 fire years since 1727 A.D.; 28 were landscape-scale fires, nine of which were exceptionally large and linked to El Ni ˜ no-triggered droughts. Mean fire return interval (FRI; the mean of individual samples) and mean fire interval (MFI; the composite mean of a group of samples) were used to estimate the upper and lower range in fire frequency. Mean FRI of the entire study area was 31.5 y (± 24.9 y SD) and MFI for landscape-scale fires was 5.6 y (± 4.1 y SD). The fire regime varied significantly with elevation and aspect. Mean FRI was 42.1 y (± 27.6 y SD) and MFI was 9.8 y (± 16.7 y SD) on moister windward zones, and mean FRI was 16.7 y (± 7.8 y SD) and MFI 4.2 y (± 1.9 y SD) in drier leeward zones. On windward slopes, high-elevation mean FRI (26.4 y) was significantly shorter than at middle (44.5 y) and low elevations (58.4 y). On leeward slopes, mean FRI did not vary significantly with elevation. The strong windward elevational patterning of the fire regime is driven by the trade wind inversion which traps moist air below 2000-2300 m. Such elevational patterns may be characteristic of montane fire regimes throughout the tropical trade wind belt.

Vegetation-environment relationships in forest ecosystems of the Cordillera Central, Dominican Republic1

Abstract Sherman, R. E., P. H. Martin, and T. J. Fahey (Department of Natural Resources, Cornell University, Ithaca, NY 14853) Vegetation-environment relationships in forest ecosystems of the Cordillera Central, Dominican Republic. J. Torrey. Bot. Soc. 132: 293–310. 2005.—We examined forest vegetation-environment relationships in the central mountain range of Hispaniola to improve general understanding of tropical montane forests. Forest inventory data were collected in 1999 and 2000 from 245 plots established in the Armando Bermúdez and Carmen Ramírez National Parks, Dominican Republic, over an elevation range of 1,100–3,075 m. Average tree density (≥ 10 cm dbh), basal area, and dbh were highly variable across the elevation gradient; species richness declined significantly with elevation; and the canopy height of broadleaved stands declined whereas the height of stands dominated by the endemic pine, Pinus occidentalis Sw., was relatively constant across the elevation gradient. Four major forest associations were identified using TWINSPAN: a low elevation broadleaved forest; a pine-broadleaved mixed forest; a mid-elevation cloud forest; and a largely monospecific pine forest that extends from the cloud forests to the summits of the highest peaks and dominates the leeward slopes of the mountains. Species composition varied continuously along the elevation gradient up to 2,250 m; however, above 2,250 m there was an abrupt shift from cloud forest to monospecific pine forests. Temperature, humidity, and fire history appear to regulate the position of this boundary, probably reflecting the position of the trade wind inversion. Ordination and logistic regression indicated that disturbance history and topo-edaphic factors influenced individual species distributions.

Fire and vegetation dynamics in high-elevation neotropical montane forests of the Dominican Republic.

Abstract In March and April 2005, severe fires burned over 1000 km2 of tropical montane forests in the Cordillera Central, Dominican Republic. The fire burned through our network of permanent vegetation plots, which were established in 1999 to examine interactions among environment, vegetation, and disturbance. We used QuickBird satellite imagery combined with field surveys to map the extent and severity of the fire across the landscape. The fire burned through 96% of the pine forest but quickly extinguished at the pine–cloud forest boundary along most of the ecotone. Topographic factors and fire severity had no influence on fire behavior at the ecotone. These observations support our original hypothesis that fire maintains the abrupt boundary between the pine and cloud forest vegetation in these mountains. Vegetation structure and composition played a direct role in regulating fire spread and behavior in this landscape.

Climate drivers of seed production in Picea engelmannii and response to warming temperatures in the southern Rocky Mountains

The increases in seed output observed in this study may promote population fitness of P. engelmannii in the face of changing climate regimes and increasing frequencies of fire- and insect-related tree mortality in the Rocky Mountains. Since this species lacks a persistent seed bank, re-colonization of disturbed areas or dispersal to shifting habitats depends on adequate production of seed by surviving trees, which according to these analyses may be moderately enhanced by current climate trends. However, some evidence also indicates that increases in seed output will ultimately be constrained by threshold high temperatures in the seed maturation year.

Climate and competition effects on tree growth in Rocky Mountain forests

Summary Climate is widely assumed to influence physiological and demographic processes in trees, and hence forest composition, biomass and range limits. Growth in trees is an important barometer of climate change impacts on forests as growth is highly correlated with other demographic processes including tree mortality and fecundity. We investigated the main drivers of diameter growth for five common tree species occurring in the Rocky Mountains of the western United States using nonlinear regression methods. We quantified growth at the individual tree level from tree core samples collected across broad environmental gradients. We estimated the effects of both climate variation and biotic interactions on growth processes and tested for evidence that disjunct populations of a species respond differentially to climate. Relationships between tree growth and climate varied by species and location. Growth in all species responded positively to increases in annual moisture up to a threshold level. Modest linear responses to temperature, both positive and negative, were observed at many sites. However, model results also revealed evidence for differentiated responses to local site conditions in all species. In severe environments in particular, growth responses varied nonlinearly with temperature. For example, in northerly cold locations pronounced positive growth responses to increasing temperatures were observed. In warmer southerly climates, growth responses were unimodal, declining markedly above a threshold temperature level. Net effects from biotic interactions on diameter growth were negative for all study species. Evidence for facilitative effects was not detected. For some species, competitive effects more strongly influenced growth performance than climate. Competitive interactions also modified growth responses to climate to some degree. Synthesis. These analyses suggest that climate change will have complex, species-specific effects on tree growth in the Rocky Mountains due to nonlinear responses to climate, differentiated growth processes that vary by location and complex species interactions that impact growth and potentially modify responses to climate. Thus, robust model simulations of future growth responses to climate trends may need to integrate realistic scenarios of neighbourhood effects as well as variability in tree performance attributed to differentiated populations.

Reconstructing Hurricane Disturbance in a Tropical Montane Forest Landscape in the Cordillera Central, Dominican Republic: Implications for Vegetation Patterns and Dynamics

Abstract Hurricanes are intense, frequent disturbances of Caribbean ecosystems and are important agents in structuring the regions ecological patterns and processes. The rugged topography and diverse vegetation of Caribbean tropical montane forests interact with hurricane forces to create complex disturbance patterns. In this study, we used a 158-yr historical database of hurricane tracks to calculate fine-scale hurricane return intervals across the Caribbean, and Landsat imagery and field inventory to reconstruct wind and rain disturbance from Hurricane Georges in the Cordillera Central, Dominican Republic. Spatial patterns of disturbance from Georges and the relationship of disturbance with hurricane meteorology, topography, and vegetation were analyzed for the landscape. The long-term hurricane return interval for the Cordillera Central was 14.4 yrs. Damage from hurricane Georges was distributed over a small portion of the study area (∼330 km2 study area); only 11.3% of the study area was disturbed by wind and 4.3% by flooding. Multiple logistic regression showed that hurricane meteorology, topography, and vegetation were all significant predictors of patterns of wind disturbance. In particular, wind disturbance was concentrated in high-elevation forest types, with 12% of the total disturbed area in cloud forest and 82% in pine forest. Within the wind-disturbed areas, the degree of mortality varied markedly by forest type, with the proportion of dead basal area 9.8% higher in cloud forest, 11.4% higher in transitional pine, and 50.5% higher in pine forest compared to controls. Although much of the landscape was undisturbed by Georges, the regularity of hurricanes in the region and the magnitude of change in disturbed patches suggest that hurricanes are an important driver of forest dynamics and demonstrates the importance of sampling across gradients of disturbance severity.

Mesoclimatic Patterns Shape the Striking Vegetation Mosaic in the Cordillera Central, Dominican Republic

Abstract A relationship between forest vegetation patterns and climate has been proposed for Caribbean mountains, but mesoscale temperature, precipitation (PPT), humidity, and cloud formation patterns are poorly documented. Half-hourly temperature and humidity observations were obtained from 2001 to 2011 from a network of 10 data-logging instruments ranging in elevation from 1500 to 2800 m on the windward slopes of the Cordillera Central, Dominican Republic. We report diurnal, seasonal, and annual patterns in temperature, PPT, humidity, and the trade wind inversion (TWI) along the elevation gradient. The elevational gradient in mean air temperature was non-linear during the dry season, with lapse rates decreasing to -0.5 °C km-1 between 1500 and 1900 m and -0.8 °C km-1 between 2100 and 2400 m. Relative humidity reached a maximum at 2100 m (mean of 91%), but remains above 85% over the entire gradient until 2600 m, above which it drops steeply. Relative humidity also showed marked seasonality but only at the highest elevations, dropping markedly above 2400 m and especially above 2600 m in the dry season, while remaining high at lower elevations throughout the year. PPT declined only slightly with elevation on windward slopes, but was markedly lower in leeward areas. Dry season PPT was lower on windward and leeward slopes at all elevations, except at ∼2400 m on windward slopes where it remained nearly as high as the rest of the year. Sub-zero temperatures occurred at elevations ≥2325 m and increased markedly in frequency ≥2600 m. These observations support the hypothesis that the discrete vegetation ecotone between the cloud forest and subalpine pine forest at ∼2200 m on windward slopes results from climatic discontinuities, especially during the dry season. In particular, the TWI effect on mesoclimatic patterns (especially moisture) regulates the elevational maximum of cloud forest flora and likely will represent a strong barrier to the future migration of cloud forest flora to higher elevations in response to warmer temperatures. Together with increased moisture stress due to higher temperatures, climate change in the high elevations of tropical mountains is therefore likely to disrupt the dynamics and distributions of tropical montane forests.

Patterns of growth, recruitment, mortality and biomass across an altitudinal gradient in a neotropical montane forest, Dominican Republic

We examined stand dynamics and biomass along an altitudinal gradient in a tropical montane forest (TMF) in the disturbance-prone Cordillera Central, Dominican Republic. We tested the general hypothesis that chronic disturbancebyfire,wind,floodsandlandslidesresultsinalandscapeofrelativelylowabove-groundbiomasswithhigh rates of mortality, recruitment and growth as compared with other TMFs. We also expected above-ground biomass to decrease with altitude in part due to declines in growth and increased biomass losses from mortality with increasing altitude. We resurveyed 75 0.1-ha plots distributed across the altitudinal gradient (1100-3100 m asl) 8 y after they wereestablished.Ourobservationsprovidedmixedevidenceonthesehypotheses.Turnoverrateswerehigh(>2%y −1 ) and significantly greater on windward slopes. Above-ground biomass (mean = 306 Mg ha −1 , 95% CI = 193-456 Mg ha −1 ) was highly variable but comparable to other TMFs. Altitudinal patterns of declining biomass and above-ground

Natural disturbance, vegetation patterns and ecological dynamics in tropical montane forests

Disturbance is a central process in forest dynamics, yet the role of natural disturbance in tropical montane forests (TMFs) has not been systematically addressed. We posit that disturbance in TMFs has a wider role than commonly acknowledged and its effects are distinctive because: (1) TMFs often have very low rates of productivity due to low resources, and so recovery from disturbance may be slow, (2) montane forests have marked environmental heterogeneity which interacts with disturbance, (3) a large percentage of TMFs are regularly exposed to high energy windstorms and landslides, and (4) TMFs contain a biogeographically rich mixture of tree species with divergent evolutionary histories that interact differently with different disturbance types. We reviewed the literature on natural disturbance in TMFs and found 119 peer-reviewed papers which met our search criteria. Our review shows that disturbance is widespread in TMFs with pronounced effects on structure, function, composition and dynamics. Disturbance is also evident in the ecology of TMF biota with clear examples of plant life-history traits adapted to disturbance, including disturbance-triggered germination, treefall gap strategies and resprouting ability. Important aspects of TMF disturbances are stochastic and site-specific, but there are broad patterns in disturbance type, frequency and severity along latitudinal, altitudinal and environmental gradients. Compared with the lowland tropics, TMF disturbances are more spatially structured, TMFs experience more disturbance types in a given area due to environmental complexity, and TMFs are much more prone to small-scale yet severe landslides as well the large and potentially catastrophic disturbances of cyclones, forest die-back and fire. On the whole, natural disturbance should assume a larger role in models of ecosystem processes and vegetation patterns in TMFs. An improved understanding of what creates variation in disturbance severity and post-disturbance recovery rates, how composition and diversity feedback on disturbance type and likelihood, and how global change will alter these dynamics are important priorities in future TMF ecology research.