Where do conifers regenerate after selective harvest?

Forest Ecology and Management 253 (2007) 138–147 www.elsevier.com/locate/foreco

Where do conifers regenerate after selective harvest? A case study from a New Zealand conifer–angiosperm forest Fiona E. Carswell a,*, Sarah J. Richardson a, James E. Doherty b, Robert B. Allen a, Susan K. Wiser a b

a Landcare Research, P.O. Box 40, Lincoln 7640, New Zealand Tu¯hoe Tuawhenua Trust, P.O. Box 4, Murupara, Bay of Plenty, New Zealand

Received 30 March 2007; received in revised form 11 July 2007; accepted 11 July 2007

Abstract Ensuring the regeneration of selectively harvested canopy tree species remains challenging in mixed forests where species have different requirements for successful recruitment. Mature conifer trees (Podocarpaceae) have been selectively harvested from parts of New Zealand’s North Island conifer–angiosperm forests. Forest managers require guidance on podocarp restoration, given current dominance by the shade-tolerant angiosperm Beilschmiedia tawa. We surveyed seedling densities of podocarps and B. tawa ca. 40 years after harvesting and found that B. tawa seedlings outnumbered the combined total of podocarp seedlings by approximately 3:1. There were significant, positive associations between seedlings of most species suggesting that safe sites for establishment were similar, in part, for the suite of study species. These sites are the same as those where adult podocarp trees are reported to occur. We developed and tested candidate models predicting the influence of environmental factors on seedling regeneration; these focused on the roles of soil nutrients, landform, canopy openness, tree fern cover, ground cover by ferns and disturbance. We found the most support for models that used a combination of soil nutrients, canopy composition, landform index and disturbance type to predict seedling occurrence. A positive relationship was found between soil nitrogen (N) and seedling occurrence of all species surveyed, and this relationship alone had most support in explaining the occurrence of Prumnopitys taxifolia and Dacrycarpus dacrydioides seedlings. We found little difference in the current sites of young Prumnopitys ferruginea and B. tawa; both species occur in dense stands of adult B. tawa, although in contrast to B tawa, P. ferruginea seedling occurrence declined with increasing soil phosphorus (P). Dacrydium cupressinum also declined with increasing soil P. Given that the studied forest fits the apparent global trend for angiosperm ascendancy we suggest that manipulations will be required to restore podocarps. Possible interventions are planting seedlings and/or removing some of the B. tawa canopy. Given that podocarp seedlings were found on soils with low soil P, but high soil N concentrations, soil nutrient status should be taken into account during management. # 2007 Elsevier B.V. All rights reserved. Keywords: Conifer–angiosperm; Disturbance; Regeneration; Soil nutrients; Akaike’s Information Criterion (AICc); Prumnopitys ferruginea; Dacrydium cupressinum; Beilschmiedia tawa; Prumnopitys taxifolia; Dacrycarpus dacrydioides

1. Introduction ‘‘Selective harvest’’ of single trees or small groups of trees is increasingly viewed as more ecologically sustainable than clear-felling and single-age-class forestry (Attiwill, 1994; Coates and Burton, 1997). However, the issue of how to manage such forests for success of the more light-demanding canopy species remains challenging (Malcolm et al., 2001; Go¨tmark et al., 2005; Smith and Smith, 2005). New Zealand conifer–

* Corresponding author. Tel.: +64 3 321 9631; fax: +64 3 321 9998. E-mail address: [email protected] (F.E. Carswell). 0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2007.07.011

angiosperm forests are often composed of an upper Podocarpaceae conifer tier and a lower broadleaved angiosperm tier (Wardle, 1991). Of the lowland forests outside those primarily managed for conservation, an estimated 40% have experienced some level of harvesting (Ministry of Forestry, 1988), usually of the podocarp tier. Podocarps are of significant ecological, economic and cultural value such that forest managers require guidance on how best to regenerate them where the number of parent trees has been significantly reduced. Given the diminished seed source, harvesting modification of forest structure, and, in the face of an apparently global trend for angiosperms to outcompete conifers (Bond, 1989; Becker, 2000; Coomes et al., 2005), we investigate the sites of podocarp

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regeneration following harvest of mature trees ca. 40 years ago and identify factors that may be manipulated in the future to increase the relative success of seedlings. Variation in mature conifer–angiosperm forest composition has been linked to soil nutrient gradients with greater podocarp dominance at lower concentrations of total P and higher total N concentration whereas soils of higher P concentration are more likely to give rise to increased angiosperm dominance (Richardson et al., 2004; Coomes et al., 2005). However, densely stocked, even-aged podocarp stands regenerate on well-drained sites in the central North Island of New Zealand after infrequent catastrophic disturbances (McKelvey, 1963; Clarkson et al., 1992). At a given site, such disturbances generally increase soil nutrient availability, decrease soil organic matter and increase light transmission to the forest floor (Canham and Marks, 1985). Soil P, in particular, is thought to be increased by volcanic eruption (Vitousek et al., 2004), a major agent of catastrophic disturbance in central North Island forests. Subsequent continuous regeneration of lower density cohorts of podocarp species can occur in response to less devastating disturbances (Lusk and Ogden, 1992; Ogden and Stewart, 1995). Because podocarp species are long-lived, adult trees will likely occur on those sites to which they have been confined through ‘‘competitive sorting’’ during succession (see Peet, 1992). However, we expect that juveniles, which can establish after widespread canopy disturbance, could occur on a wider range of sites than the ‘‘steady state’’ adults (Peet, 1992). So far it is unclear whether podocarp seedlings occur on the same sites as adults, particularly after low intensity disturbance, such as selective harvest. In the forest studied, Prumnopitys taxifolia (D. Don) de Laub. is considered a relatively light-demanding podocarp, Dacrydium cupressinum Lamb is intermediate in its light requirements, while Prumnopitys ferruginea (D. Don) de Laub. is considered relatively shade-tolerant (Ogden and Stewart, 1995). Prolific regeneration of Dacrycarpus dacrydioides (A. Rich.) de Laub. after flood disturbance is thought to reflect a preference for high light and alluvial soils (Wardle, 1974; Smale, 1984; Duncan, 1993). While P. ferruginea may require some small-scale disturbance to attain canopy height, this species does appear to establish under intact canopies (Ogden and Stewart, 1995; Smale et al., 1997). The dominant competing angiosperm, Beilschmiedia tawa (A. Cunn.) is shade-tolerant (Smale and Kimberley, 1986), represents approximately 30% of the total current basal area of trees 10 cm dbh in the forest studied and casts dense shade. We expected to find more B. tawa and fewer podocarp seedlings under adult B. tawa trees. Podocarp regeneration may also be limited by shading and competition from tree and ground ferns, especially crown fern Blechnum discolor (G. Forst.) Keys (Beveridge, 1973; Coomes et al., 2005), as observed in other temperate conifer–angiosperm forests (Horsley, 1993; Donoso and Nyland, 2005). We test whether podocarp seedlings occur on the same sites reported for adult trees, namely on soils of lower P and higher N concentrations (Richardson et al., 2004; Coomes et al., 2005). Sites of lower P availability occur on ridge or upper-slope locations (Campbell, 1973). Given that harvesting disturbance

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occurred some 40 years ago, we use residual evidence of harvest disturbance as a proxy for whether canopy cover was lower at that time of podocarp establishment. We then expect light-demanding podocarps to be more commonly associated with harvesting disturbance and also where there is lower cover of the present canopy. We hypothesize that B. tawa seedlings will be most abundant on undisturbed sites with high canopy cover, particularly where B. tawa density is high. Further, we suggest that podocarp seedlings will be most abundant where adult B. tawa and fern densities are low. We hypothesize that the combination of soil nutrient concentrations, community structure and past harvesting disturbance will explain more of specific seedling occurrence than any single effect. Our literature-based hypotheses were used to formulate candidate statistical models for conifer–angiosperm regeneration patterns in our forest. Finally, we consider the management implications for podocarp regeneration in previously harvested New Zealand conifer–angiosperm forests. 2. Methods 2.1. Study area We studied privately owned forest bordering Te Urewera National Park (388360 4700 S, 1768570 3400 E) in central North Island, New Zealand. Soils are classified as Urewera steepland soils (NZ Soil Classification, Hewitt, 1992) and are described as well-drained (Rijkse, 1993). The parent material is greywacke (a fine-grained sandstone) overlain by rhyolitic ash (New Zealand Soil Bureau, 1954). Given the steep topography (generally slope is greater than 208) most of the tephra is likely to have been removed from ridges by erosion. Climate is cool temperate and humid with mean annual rainfall of 1600–2400 mm, equally distributed throughout the year. Mean annual temperature is 9 8C with a mid-summer (January) mean of 14 8C and a mid-winter (July) mean of 4 8C (New Zealand Meteorological Service, 1985). Altitude ranges from 530 to 890 m a.s.l. Before being selectively logged from 1956 to 1975, the podocarps and the angiosperm northern rata (Metrosideros robusta A. Cunn.) occurred as single or small-group emergents over a lower continuous tier of angiosperm trees (McKelvey, 1973). The lower canopy tier was dominated by B. tawa, although other evergreen angiosperm tree species and many tree ferns (Dicksonia squarrosa, Cyathea smithii, Dicksonia fibrosa, Cyathea medullaris, Cyathea dealbata) were present. Other ferns (Blechnum spp., Polystichum spp.) formed the dominant ground cover. Harvesting selectively removed almost all large podocarp trees, leaving B. tawa as the main canopy species (Table 1) with infrequent emergent northern rata. Remaining emergent podocarp trees tend to occur on the steepest slopes (greater than 358) or have obvious trunk defects. 2.2. Data collection Seedling occurrences were assumed to indicate regeneration and were therefore surveyed in each of three blocks of forest 115–450 ha in size. Within each block a 2-km2 (2 km  1 km)

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Table 1 Summary of study species in a harvested conifer–angiosperm forest in central North Island, New Zealand Species

Family

Common name

Lifespan (average for adult trees, in years)

Basal area in unharvested forest (m2/ha) a

Percentage of basal area removed through harvesting 1956–1975a

Beilschmiedia tawa Prumnopitys ferruginea Dacrydium cupressinum Prumnopitys taxifolia Dacrycarpus dacrydioides

Lauraceae Podocarpaceae Podocarpaceae Podocarpaceae Podocarpaceae

Tawa Miro Rimu Matai Kahikatea

400b 650c 650c 600c 600c

14.8 0.5 14.9 2.0 0.3

0 20 94 90 99

a b c

Tu¯hoe Tuawhenua Trust and Rod Scott, unpublished data. Knowles and Beveridge (1982). Enright and Ogden (1995).

unit was selected for sampling. Pasture incursions are present within the blocks so each 2-km2 unit was orientated to maximize forest cover. Transects were originated at twelve intercepts 500 m  330 m apart on a grid laid over the unit. These were 80 m in length and were directed along random compass bearings. Seedling plots, of 2-m radius, were located at 20-m intervals along each transect. In addition to this systematic sampling a further five transects were measured within each forest block. These were subjectively located to maximize the incidence of podocarp and B. tawa seedlings on five seedling plots subjectively placed at least 15 m apart along these transects. For each forest block the number of systematically sampled plots totalled 60 while the total number of subjectively placed plots was 25. Within each seedling plot, seedlings (0.15 m and <1.35 m in height) of D. dacrydioides, D. cupressinum, P. taxifolia, P. ferruginea and B. tawa were counted on all substrates. Densities of seedlings of the five study species were calculated for each block from the grid based plot data. Stand and species basal area for each seedling plot was quantified from the plot centre using angle-count sampling (Bitterlich, 1984). For B. tawa and tree ferns, diameter at breast height was measured for trees included in the angle-count to enable calculation of point densities of individual species (Beers and Miller, 1964). For analysis of ground fern association with seedling occurrence, B. discolor (crown fern) cover was ascribed to one of three categories (absent, low (50% cover) and high (>50% cover)). In addition, percentage of total canopy cover above 1.35 m in height was measured along a 4-m line using a GRS densitometer (Geographic Resource Solutions; Stumpf, 1993). For each of the seedling plots meso-scale landform index was measured. This is the mean of eight slope gradients from plot centre to topographic horizon and indicates the degree to which a given plot position most resembles a ridge, slope or gully (sensu McNab, 1993). Nutrient concentrations were subsequently measured for the upper 10 cm of mineral soil, sampled at four points systematically located within each seedling plot and bulked. Soils were oven dried to constant mass at 35 8C, then sieved to 2 mm, ground and analysed for total N and easily extractable P (Brookside Laboratories Inc., New Knoxville, OH). Extractions were carried out using the Mehlich III method (Mehlich, 1984).

Three categories were used to describe disturbance: no disturbance (10% or less of the plot area showed evidence of either harvesting or natural disturbance); disturbance caused by harvesting, e.g., more than 10% of the plot consisted of abandoned logging tracks or felled tree crowns; natural disturbance, e.g., more than 10% of the plot consisted of treefalls or landslides. 2.3. Data analysis First, we used a Chi-square contingency test to determine whether there were spatial associations among seedlings of all possible pairs of our five study species. Then, seedling occurrence was modelled in relation to a suite of predictor variables using generalised linear mixed effects models (GLMM) with a binomial distribution, using maximum likelihood (glmmML in R v 2.3.1). Seedling data were collected as counts per plot but as the majority of plots lacked any seedlings of the four podocarp species data were zeroinflated and seedlings were therefore modelled as presence/ absence data (binomial data). We found no differences in environmental factors between plots that contained seedlings growing on logs compared with those on the ground (P > 0.05), so we excluded the rare seedlings growing on logs to ensure that soil nutrient concentrations measured were a reasonable reflection of nutrient conditions in which seedlings were growing. Data from both systematic and subjective plots were combined but data for each species were analysed individually. Given the literature, we anticipated that seedling occurrence would reflect a combination of factors, namely soil nutrient predictors, canopy composition/cover predictors, landform index and disturbance. We therefore constructed a set of candidate models and tested this set for each species (Table 2). However, because of the low number of occurrences and high variability of P. taxifolia and D. dacrydioides it was not possible to fit candidate models for categorical variables to these species. Akaike’s Information Criterion (AICc) and Akaike’s weight ðwi Þ were used to evaluate the weight of evidence for each model (Burnham and Anderson, 2002). In addition, associations between variables were investigated in R v 2.3.1 using Pearson’s correlations for continuous predictor variables and analysis of variance (ANOVA) for categorical variables.

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Table 2 Candidate models for describing relationships between seedling occurrence and environmental factors in a conifer–angiosperm forest in central North Island, New Zealand Model terms

Justification

Soil N + P

The balance between N and P may be particularly important in determining regeneration success of the conifers relative to angiosperm species (Richardson et al., 2004; Coomes et al., 2005). Seedling growth is higher at higher soil macronutrient concentration (Hawkins and Sweet, 1989). Some New Zealand tree species appear particularly sensitive to individual nutrients (Allen et al., 1997; Carswell et al., 2003, 2005). Podocarp species regenerate after catastrophic disturbance events (McKelvey, 1963; Clarkson et al., 1992) that reduce canopy cover and podocarp species are thought to lie on a continuum of shade-tolerance (Ogden and Stewart, 1995). B. tawa is shade-tolerant (Smale and Kimberley, 1983, 1986). Ferns inhibit tree seedling regeneration (Wardle, 1984; Horsley, 1993; George and Bazzaz, 1999a,b; Coomes et al., 2005; Donoso and Nyland, 2005). B. tawa casts a deep shade that promotes self-replacement (Smale et al., 1986). D. cupressinum seedlings have been observed to preferentially regenerate on logging tracks (James and Franklin, 1978). Residual signs of harvest disturbance may be a better indicator of canopy openings at the time when seedlings established, rather than current canopy cover. Adult podocarp conifers tend to be found on mid-slope to ridge positions (Ogden and Stewart, 1995; Burns and Leathwick, 1996). Canopy tree seedling distributions are influenced by all the measured variables. Canopy tree seedling distributions are random with regard to the measured variables.

Each nutrient individually %Canopy cover

%Cover of crown fern and tree ferns Stem density of B. tawa Three categories of disturbance: none; disturbance caused by harvesting; natural disturbance Landform index Full model Null

canopy cover, composition, disturbance and landform index than for any of the components alone (Table 5). Of the factors included in the full model most support was given to B. tawa density for B. tawa and P. ferruginea seedlings although soil N plus P received as much support for P. ferruginea and was the best supported of the single factor models for D. cupressinum. Support for the nutrient influence on all species was derived from a strong positive relationship between seedling occurrence and soil N (Fig. 1A), and this model alone received the most support for predicting P. taxifolia and D. dacrydioides occurrence. This is likely a reflection of the high variability and low number of occurrences of these species. For P. ferruginea and D. cupressinum a negative relationship between seedling occurrence and soil P also played a role (Fig. 1B). In contrast, the model predicting B. tawa occurrence on the basis of soil P concentration received no support. For B. tawa and P. ferruginea, occurrence of both species increased with increasing B. tawa stem density (Fig. 2A). Occurrences of B. tawa and P. ferruginea also increased with increasing canopy cover (Fig. 2B). On the other hand, D. cupressinum and P. taxifolia occurrence decreased with increasing tree fern cover (Fig. 2C). Canopy cover appeared to be a more

3. Results 3.1. Seedling densities The number of B. tawa seedlings present on systematically located plots was five times that of P. ferruginea, the most numerous of the podocarp species (Table 3). Subjectively located plots suggest considerable contagion in the distribution of podocarp seedlings. 3.2. Spatial associations among seedling species Seven out of 10 pairwise comparisons were positive and significant indicating that the distribution of seedlings was clustered and that regeneration conditions for the five species were frequently similar (Table 4). There were no negative associations. 3.3. Candidate models predicting seedling occurrence For D. cupressinum, P. ferruginea and B. tawa there was more support for a model that combined soil nutrients with

Table 3 Mean densities (stems/ha) one standard error of seedlings (0.15 m and <1.35 m) of B. tawa and four podocarp species in three forest blocks in a conifer– angiosperm forest in central North Island, New Zealand Species

B. tawa P. ferruginea D. cupressinum P. taxifolia D. dacrydioides

Seedlings Density (stems/ha)

%Frequency (plots on grid)

%Frequency (subjective samples)

1667  322 323  115 128  18 44  12 35  35

42 17 11 4 2

100 40 44 17 24

Also given are the frequencies of plots on the grid in which seedlings occurred (180 plots). The frequencies of plots in which seedlings occurred in subjectively placed transects are also given (75 plots).

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Table 4 Spatial associations among seedling species in a conifer–angiosperm forest in central North Island, New Zealand B. tawa

P. ferruginea

D. cupressinum

P. ferruginea

Positive x2 = 12.82 P < 0.001

D. cupressinum

Positive x2 = 7.44 P = 0.006

Positive x2 = 18.89 P < 0.001

P. taxifolia

No association x2 = 0.31 P = 0.578

Positive x2 = 19.08 P < 0.001

Positive x2 = 18.00 P < 0.001

D. dacrydioides

Positive x2 = 8.71 P = 0.003

No association x2 = 1.13 P = 0.288

Positive x2 = 13.41 P < 0.001

P. taxifolia

No association x2 = 0.29 P = 0.587

These were assessed using Chi-square contingency tables and the direction of the association is given along with the Chi-square statistic and probability (P) value.

significant influence on B. tawa seedling occurrence than for the podocarp species although the model of canopy cover received more support than the null model in explaining P. ferruginea occurrence (Table 5). Although the model relating seedling occurrence to landform index received less support than other models, it still received greater support than the null model (Table 5). Seedlings of B. tawa, P. ferruginea and D. cupressinum were generally found at a lower landform index, indicating a preference for ridges rather than gullies (Fig. 3). Seedlings of B. tawa, P. ferruginea and D. cupressinum occurred more frequently on undisturbed plots (Fig. 4) and with moderate crown fern covers (Fig. 5). Most of the predictor variables in the candidate models are correlated (Tables 6–8), yet the full model still had the most support for three of the species. This indicates that each of the variables had some influence on seedling occurrence despite correlations between them. Soil P shows the least correlation with other continuous variables, only correlating with landform index. This correlation is positive indicating higher P concentrations in gullies. Landform index is uncorrelated with canopy cover (Table 6). Crown fern cover is most strongly correlated with soil P and landform index (Table 7). Crown fern

cover is lowest at the highest values of P, which are again in gullies. Disturbance covaries with most other variables but most significantly with soil N and P and canopy cover/B. tawa stem density (Table 8). These variables all tend to be higher where there is no disturbance except for soil P which is highest in plots that are naturally disturbed. 4. Discussion 4.1. Nutrients, canopy composition, landform, disturbance and seedling occurrence For D. cupressinum, P. ferruginea and B. tawa the model with most support for predicting seedling occurrence was that which combined soil nutrients with canopy composition, landform index and disturbance type. As hypothesized, there was strong support for soil nutrients playing a role in occurrence of both podocarp and angiosperm seedlings; the occurrence of all species increased with increasing N concentration (Fig. 1A). There was no evidence for P influencing B. tawa seedling occurrence. The increased occurrence of P. ferruginea and D. cupressinum seedlings with lower P concentrations is consistent with other

Table 5 Comparison of candidate models (from Table 2) predicting the occurrence of seedlings of five canopy species in a conifer–angiosperm forest in central North Island, New Zealand Model

B. tawa

P. ferruginea

D. cupressinum

P. taxifolia

D. dacrydioides

Full model B. tawa density (stems/ha) Canopy cover (%) Soil N + P (%N, mg/kg P) Soil N (%) Disturbance type Landform index Tree fern cover (%) Null model Crown fern cover (ordered factor) Soil P (mg/kg)

0 20 27 28 29 30 50 51 53 54 55

0 18 36 18 27 31 36 34 43 36 41

0 26 28 10 19 20 24 19 28 21 25

13 4 6 2 0 – 6 2 4 – 6

5 3 6 1 0 – 6 8 6 – 8

Species are ordered from left to right in apparent order of decreasing shade tolerance. Values shown are Di, the difference in AICc between the best model (model with smallest value of AICc) and the ith model. The best model has a Di value of 0. All models with Di  2 have substantial support (sensu Burnham and Anderson, 2002) and are shown in bold. AICc = Akaike’s Information Criterion. N = 255 for all species. Categorical models could not be fitted to P. taxifolia and D. dacrydioides (–).

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Fig. 1. The probability of seedling occurrence in relation to soil N and P in the full models fitted to four conifer species and one angiosperm species in a conifer–angiosperm forest in central North Island, New Zealand. Curves were fitted using binomial regression. (^) B. tawa; (&) P. ferruginea; (~) D. cupressinum; (*) D. dacrydioides; (*) P. taxifolia.

observations of increased adult podocarp dominance on such sites (Richardson et al., 2004; Coomes et al., 2005). Burns and Leathwick (1996) studied a far northern conifer–angiosperm forest and found that the podocarps occurred on ridges where the authors assumed the P concentration to be lower. South Island studies have reported greater adult podocarp abundance on sites of poor–moderate drainage (Norton and Leathwick, 1990) and this has been related both to concomitantly low P concentrations (Richardson et al., 2004; Wardle et al., 2004) and the poor drainage conferring vulnerability to windthrow (Veblen and Stewart, 1982; Stewart et al., 1998). Our results suggest that the former is the more likely determinant and, indeed, we measured lower P concentrations on well-drained ridges (Table 6). Peet (1992) outlines a model of ‘‘competitive sorting’’ of secondary succession whereby species present early in a succession may

Fig. 2. The probability of seedling occurrence in relation to canopy composition and cover variables in the full models fitted to four conifer species and one angiosperm species in a conifer–angiosperm forest in central North Island, New Zealand. Curves were fitted using binomial regression. (^) B. tawa; (&) P. ferruginea; (~) D. cupressinum; (*) = D. dacrydioides; (*) P. taxifolia.

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Fig. 3. The probability of seedling occurrence in relation to landform index in the full models fitted to four conifer species and one angiosperm species in a conifer–angiosperm forest in central North Island, New Zealand. Low values of landform index indicate ridges with higher values indicating gullies. Intermediate values indicate slopes. Curves were fitted using binomial regression. (^) B. tawa; (&) P. ferruginea; (~) D. cupressinum; (*) D. dacrydioides; (*) P. taxifolia.

occur across wider environmental gradients than during the thinning or steady state phases of the succession. Given that seedlings (this study) and adult podocarps (particularly P. ferruginea and D. cupressinum) occur in low P environments and/or near ridges (e.g., Burns and Leathwick, 1996; Richardson et al., 2004; Coomes et al., 2005) we suggest that competitive sorting during succession may not be an important mechanism in the study forest. Although decreased competition has been invoked as a mechanism for increased conifer abundance with low soil P in other conifer–angiosperm forests (Bond, 1989; Coomes et al., 2005) physiological studies show P. ferruginea seedlings respond positively to increasing N concentration and negatively

Fig. 4. Disturbance categories and expected occurrences of three seedling species in a conifer–angiosperm forest in central North Island, New Zealand. For each seedling species, the expected proportion of plots of each disturbance type is simply the proportion of all plots that are: undisturbed (59%), disturbed through harvesting (23%) or disturbed through natural processes (18%). The observed proportion of plots containing each species in each disturbance category is therefore plotted against the expected proportion. (^) B. tawa; (&) P. ferruginea; (~) D. cupressinum.

Fig. 5. Crown fern cover and three seedling species in a conifer–angiosperm forest in central North Island, New Zealand. The three bars per species represent different categories of crown fern cover.

to high concentrations of P (Carswell et al., 2003, 2005) so the response may be independent of interspecific competition. Further, B. tawa occurrence appears independent of soil P concentration in the current study. Although major disturbances such as volcanic eruptions can increase soil P (Vitousek et al., 2004), the dense podocarp regeneration that occurred in welldrained forests after such disturbance (McKelvey, 1963; Clarkson et al., 1992) seems unlikely to be caused by increased soil P per se. If we assume that high B. tawa stem density indicates a relatively closed canopy our results broadly support prevailing views of increasing shade tolerance from Podocarpus totara through to D. cupressinum, P. ferruginea and lastly B. tawa (Smale and Kimberley, 1986; Ogden and Stewart, 1995; Brodribb and Hill, 1997; Smale et al., 1997), as both P. ferruginea and B. tawa occurrence increased with stem density of B. tawa (Table 5, Fig. 2). Previous studies are equivocal on the relationship between podocarp seedling performance and light. June (1982) and McDonald and Norton (1992) found more D. cupressinum seedlings in gaps than under closed canopies, but Enright et al. (1993) and Ebbett and Ogden (1998) found no evidence for increased seedling growth with increased irradiance. Seedlings of D. cupressinum may be relatively shade-tolerant but require light to grow into saplings (Smale and Kimberley, 1986), or, light responses may be confounded with root competition (Coomes and Grubb, 2000). The relationship between seedling occurrence and B. tawa density received more support than that with canopy cover. Increases in canopy cover likely represent either high covers of B. tawa or tree ferns, with seedlings being largely negatively associated with the latter (Fig. 2C). This was most apparent for D. cupressinum and P. taxifolia seedling occurrence (Table 5). It has been suggested that decreasing podocarp abundance with increasing fern cover is a result of increasing soil P concentrations. In another conifer–angiosperm forest in the southern South Island, podocarps were more common on sites with low P and where tall tree ferns were absent (Coomes et al., 2005) and it was suggested that the relative infertility of the site

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Table 6 Pearson correlations between continuous predictor variables in a conifer–angiosperm forest in central North Island, New Zealand

Landform index B. tawa stem density (stems/ha) Tree fern cover (%) Soil P (mg/kg) Canopy cover (%)

Soil N

Landform index

B. tawa stem density

Tree fern cover

Soil P

S0.13 0.28 S0.15 0.25 0.23

S0.12 0.37 0.19 0.07

S0.18 0.07 0.24

0.02 0.17

0.05

N = 255, degrees of freedom = 253, critical r for 0.05 level of significance = 0.1229. All but four of the correlations are significant (significant correlations in bold).

Table 7 Analysis of variance between classes of crown fern cover and continuous predictor variables in a conifer–angiosperm forest in central North Island, New Zealand Crown fern cover

B. tawa stem density (stems/ha) Soil N (%) Canopy cover (%) Tree fern cover (%) Soil P (mg/kg) Landform index

Absent

50% cover

>50% cover

F

P

264.9 0.65 87.0 30.3 29.3 0.24

370.3 0.72 87.7 17.6 22.6 0.17

250.3 0.50 72.3 17.0 18.7 0.16

1.51 1.94 3.65 4.87 14.95 17.05

0.2229 0.0529 0.0273 0.0084 <0.0001 <0.0001

P-values for variables that varied significantly (P < 0.05) with crown fern cover are shown in bold. N = 255.

reduced fern competition and allowed podocarps to dominate. The current study found no evidence of a correlation between soil P and tree fern cover (Table 6) although P co-varied with crown fern cover (Table 7). There is clearly a need for experimental manipulations of soil P and fern densities to allow the effects of nutrients and competition on podocarp seedling growth to be separated. 4.2. Comparative regeneration success of B. tawa versus podocarps The densities of both B. tawa seedlings (1700 ha1) and podocarp seedlings (35–320 ha1) are particularly low at our study site compared with other central North Island conifer– angiosperm forests. Svavarsdo´ttir et al. (1999) observed B. tawa seedling densities of 41 000 ha1 and podocarp seedling densities ranging from 3000 to 9500 ha1 (different species to the current study). Smale et al. (1985) recorded B. tawa seedling densities of approximately 7000 ha1 and podocarp seedling densities of 2100–3200 ha1 for each of the same four species as measured in the current study. All three studies show

a larger number of B. tawa than podocarp seedlings. In the absence of major disturbance it seems likely that B. tawa will strongly dominate the canopy into the foreseeable future. Introduced browsing mammals play a significant role in regeneration of conifer–angiosperm forests (Smale et al., 1998). In forests encompassing our study area, Allen et al. (1984) observed B. tawa to be less frequent in browsed plots than those protected by exclosures. We observed P. ferruginea to be the most commonly browsed of the species studied (a species not considered by Allen et al., 1984), and suggest that the role of browsing should be examined in such forests. 4.3. Management implications Given the natural ascendancy of shade-tolerant B. tawa in New Zealand forests (McKelvey, 1963; Smale et al., 1985; West, 1995; Smale et al., 1998) management of the long-lived species of podocarps that tolerate less shade than B. tawa will be challenging. The goal of the owners of the study forest is to have sufficient podocarps recruited to allow a canopy to develop that is similar to the forest before it was harvested.

Table 8 Analysis of variance between disturbance type and continuous predictor variables in a conifer–angiosperm forest in central North Island, New Zealand

Tree fern cover (%) Landform index Soil N (%) B. tawa stem density (stems/ha) Soil P (mg/kg) Canopy cover (%)

No disturbance

Harvesting disturbance

Natural disturbance

F

P

27.4 0.21 0.74 394 27.0 93.1

17.2 0.19 0.52 193 20.4 76.8

29.0 0.23 0.60 113 32.2 75.3

2.53 2.83 6.36 8.97 15.80 18.32

0.0821 0.0607 0.0020 0.0002 <0.0001 <0.0001

Disturbance categories are as follows: no disturbance (10% or less of the plot area showed evidence of either harvesting or natural disturbance); disturbance caused by harvesting, i.e., more than 10% of the plot consisted of abandoned logging tracks or felled tree crowns; natural disturbance, i.e., more than 10% of the plot consisted of treefalls or landslides. P-values for variables that varied significantly (P < 0.05) with disturbance type are shown in bold. N = 255.

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Given the low density of podocarp seedlings, and the predominance of B. tawa, planting of podocarps and canopy thinning of B. tawa may now be required to increase the presence of podocarp trees. Canopy openings must be designed for the light requirements of even the most light-demanding conifers (Rogers, 1999; Malcolm et al., 2001). Clearly the understorey will also need to be managed in relation to its variability along soil fertility and compositional gradients as podocarp seedlings are not likely to occur in areas of high cover of either tree or crown ferns. The experience of Northern Hemisphere foresters is relevant here and suggests that artificial disturbance, such as fire, grubbing, thinning or partial cutting, is sometimes required to achieve a forest richer in lightdemanding species (Es¸en et al., 2004; Go¨tmark et al., 2005; Smith and Smith, 2005). Sites of high soil N and low P should be selected for promotion of podocarp species and it is possible that these sites are less favoured by B. discolor at least. As these sites appear to be on or near ridges, a variety of regeneration requirements can be met on the same sites. Because the sites where B. tawa seedlings are found appear very similar to those where P. ferruginea occur, intervention to reduce B. tawa competition will be required over a human inter-generational timeframe. Further investigation of the nature of this competition is required. Current New Zealand legislation stipulates that conifers must be harvested from conifer–angiosperm forests by extracting single trees, or small groups of trees. Evidence suggests that legal prescriptions may be appropriate for regeneration of two podocarp species that can regenerate in small canopy gaps, D. cupressinum and D. dacrydioides, given favourable soil conditions (e.g. Duncan, 1993; Stewart et al., 1998; Urlich et al., 2005). These two podocarp species have been observed to regenerate in small canopy openings at other New Zealand sites where there is evidence of poor soil drainage playing a role in regeneration (Norton and Leathwick, 1990; Stewart et al., 1998). At our study site there is no evidence either for a role of drainage (all sites were well-drained) or that small canopy openings have allowed adequate regeneration of D. cupressinum or D. dacrydioides. Clearly, legal prescriptions need to take account of site variability across all New Zealand conifer–angiosperm forests, unless extensive post-harvest management is envisaged. Acknowledgements This research was funded by the New Zealand Foundation for Research, Science and Technology. We thank the Tu¯hoe Tuawhenua Trust for providing access to their forest. We acknowledge the hard work of Larry Burrows, Laura Fagan and Tim McManus during their assistance with collection of field data. We also acknowledge the cheerful and competent assistance from students from Te Whare Wa¯nanga o Awanuia¯rangi, namely John Hauwaho, Frank Weko, Chris Clarke, Shay Houia and Antoni Nicholas. Guy Forrester and Richard Duncan provided statistical advice and Mark Smale and an anonymous reviewer provided helpful comments during the preparation of the manuscript.

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